US20050232061A1 - Systems for writing and reading highly resolved domains for high density data storage - Google Patents

Abstract

Systems in accordance with the present invention can be include, in an embodiment, one or more tips and a media comprising an over-layer, an under-layer, and a phase change material disposed between the over-layer and under-layer. Portions of the phase change media which can be altered to have a resistance different from a resistance of the bulk material, thereby forming highly resolved bits. A distal end of the tip can have a substantially large radius of curvature relative to a width of the resolved portion. Other objects, aspects and advantages of the invention can be obtained from reviewing the figures, specification and claims. This description is not intended to be a complete description of, or limit the scope of, the invention.

Description

PRIORITY CLAIM
• This application claims priority to the following U.S. Provisional Patent Application:
• U.S. Provisional Patent Application No. 60/563,123, entitled “Super Resolution Writing and Reading for High Density Data Storage,” Attorney Docket No. LAZE-01024US0, filed Apr. 16, 2004.
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
• This application incorporates by reference all of the following co-pending applications and the following issued patent:
• • U.S. patent application Ser. No. 10/684,883, entitled “Molecular Memory Integrated Circuit Utilizing Non-Vibrating Cantilevers,” Attorney Docket No. LAZE-01011US1, filed Oct. 14, 2003;
  • U.S. patent application Ser. No. 10/684,661, entitled “Atomic Probes and Media for high Density Data Storage,” Attorney Docket No. LAZE-01014US1, filed Oct. 14, 2003;
  • U.S. patent application Ser. No. 10/684,760, entitled “Fault Tolerant Micro-Electro Mechanical Actuators,” Attorney Docket No. LAZE-01015US1, filed Oct. 14, 2003;
  • U.S. patent application Ser. No. 10/685,045, entitled “Phase Change Media for High Density Data Storage,” Attorney Docket No. LAZE-01019US1, filed Oct. 14, 2003;
  • U.S. patent application Ser. No. ______ entitled “Methods for Writing and Reading Highly Resolved Domains for High Density Data Storage,” Attorney Docket No. LAZE-01024US1, filed concurrently;
  • U.S. patent application Ser. No. ______ entitled “Methods for Erasing Bit Cells in a High Density Data Storage Device,” Attorney Docket No. LAZE-01031US0, filed concurrently;
  • U.S. patent application Ser. No. ______ entitled “High Density Data Storage Device Having Eraseable Bit Cells,” Attorney Docket No. LAZE-01031US1, filed concurrently;
  • U.S. patent application Ser. No. ______ entitled “Methods for Erasing Bit Cells in a High Density Data Storage Device,” Attorney Docket No. LAZE-01031US2, filed concurrently;
  • U.S. patent application Ser. No. 09/465,592, entitled “Molecular Memory Medium and Molecular Memory Integrated Circuit,” Attorney Docket No. LAZE-01000US0, filed Dec. 17, 1999; and
  • U.S. Pat. No. 5,453,970, entitled “Molecular Memory Medium and Molecular Memory Disk Drive for Storing Information Using a Tunnelling Probe,” issued Sep. 26, 1995 to Rust, et al.
COPYRIGHT NOTICE
• A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
TECHNICAL FIELD
• This invention relates to high density data storage using molecular memory integrated circuits.
BACKGROUND
• In 1965, Gordon Moore observed an exponential growth in the number of transistors in an integrated circuit and predicted that the trend would continue—and it has. Software developers have pushed each generation of integrated circuit to the limits of its capability, developing steadily more data intensive applications, such as ever-more sophisticated, and graphic intensive applications and operating systems (OS). Each generation of application or OS always seems to earn the derisive label in computing circles of being “a memory hog.” Higher capacity data storage, both volatile and non-volatile, has been in persistent demand for storing code for such applications. Add to this need for volume the confluence of personal computing and consumer electronics in the form of personal MP3 players, such as the iPod, personal digital assistants (PDAs), sophisticated mobile phones, and laptop computers, which has placed a premium on compactness and reliability.
• Nearly every personal computer and server in use today contains one or more hard disk drives for permanently storing frequently accessed data. Every mainframe and supercomputer is connected to hundreds of hard disk drives. Consumer electronic goods ranging from camcorders to TiVo® use hard disk drives. While hard disk drives store large amounts of data, they consume a great deal of power, require long access times, and require “spin-up” time on power-up. FLASH memory is a more readily accessible form of data storage and a solid-state solution to the lag time and high power consumption problems inherent in hard disk drives. Like hard disk drives, FLASH memory can store data in a non-volatile fashion, but the cost per megabyte is dramatically higher than the cost per megabyte of an equivalent amount of space on a hard disk drive, and is therefore sparingly used.
• Phase change media are used in the data storage industry as an alternative to traditional recording devices such as magnetic recorders (tape recorders and hard disk drives) and solid state transistors (EEPROM and FLASH). CD-RW data storage discs and recording drives use phase change technology to enable write-erase capability on a compact disc-style media format. CD-RWs take advantage of changes in optical properties (e.g., reflectivity) when phase change material is heated to induce a phase change from a crystalline state to an amorphous state. A “bit” is read when the phase change material subsequently passes under a laser, the reflection of which is dependent on the optical properties of the material. Unfortunately, current technology is limited by the wavelength of the laser, and does not enable the very high densities required for use in today's high capacity portable electronics and tomorrow's next generation technology such as systems-on-a-chip and micro-electric mechanical systems (MEMs). Consequently, there is a need for solutions which permit higher density data storage, while still providing the flexibility of current phase change media solutions.
BRIEF DESCRIPTION OF THE DRAWINGS
• Further details of the present invention are explained with the help of the attached drawings in which:
• FIG. 1A is an exemplary die for use with an embodiment of the present invention including sixteen cells, each cell having a tip platform comprising sixteen tips.
• FIG. 1B is a cell of the exemplary die ofFIG. 1A.
• FIG. 1C is an embodiment of a platform in accordance with the present invention including a sub-platform associated with a main platform, wherein a cantilever is connected with the sub-platform.
• FIG. 1D is a schematic showing hypothetical thermal expansion of four platforms within four cells.
• FIG. 1E is a schematic showing hypothetical thermal expansion of a single platform having a size similar to that of the four cells.
• FIG. 2 is a cross-section of a film stack for use in forming ultra-sharp tips for use in the exemplary die ofFIG. 1B.
• FIG. 3A is an exemplary media die corresponding to the exemplary die ofFIG. 1A for use with an embodiment of the present invention, the media die including sixteen cells, each cell having a media platform comprising sixteen media devices.
• FIG. 3B is a cell of the exemplary media die ofFIG. 3A.
• FIG. 4A is a cross-section of a media device in accordance with an embodiment of the present invention in an unwritten state.
• FIG. 4B is a cross-section of the media device ofFIG. 4A including a data bit written to a phase change layer of the media device.
• FIG. 5 is a phase change chart of an exemplary phase change material for use with systems and methods in accordance with the present invention.
• FIG. 6 illustrates heating characteristics of the media device ofFIGS. 4A and 4B.
• FIG. 7A is a top view, grayscale map of a sequence of bits written to a portion of a media device including a corresponding write waveform.
• FIG. 7B is a top view, grayscale map of the sequence of bits ofFIG. 7A including a corresponding analog read data signal and a digital conversion of the analog read data signal.
• FIG. 7C is a top view, grayscale map of an erase sequence to erase the sequence of bits ofFIGS. 7A and 7B, including a corresponding write waveform.
• FIG. 8 is a circuit diagram of a read/write circuit in accordance with an embodiment of the present invention.
• FIG. 9 is a circuit diagram of a read/write circuit in accordance with an alternative embodiment of the present invention.
• FIG. 10 is a circuit diagram of a read/write circuit in accordance with still another embodiment of the present invention including a plurality of active media regions.
• FIG. 11 is a circuit diagram of a read/write circuit in accordance with still another embodiment of the present invention including a plurality of active media regions.
• FIG. 12 is a circuit diagram of a read/write circuit in accordance with still another embodiment of the present invention including a plurality of active media regions and a processing element.
• FIG. 13 is a circuit diagram of an active media platform in accordance with an embodiment of the present invention.
• FIG. 14 is a circuit diagram of a die in accordance with an embodiment of the present invention having a plurality of the active media platforms ofFIG. 13.
• FIG. 15 is a circuit diagram of an active media platform in accordance with an alternative embodiment of the present invention, the active media platform including a plurality of dead spots.
• FIG. 16 is a circuit diagram of a die in accordance with an embodiment of the present invention having a plurality of the active media platforms ofFIG. 15.
DETAILED DESCRIPTION
• Read/Write Engine for Forming Indicia in a Media
• FIGS. 1A and 1B illustrate anexemplary die100 and anexemplary cell118 from theexemplary die100 for use with systems and methods in accordance with the present invention. Thedie100 comprises sixteencells118, eachcell118 including atip platform108 on which sixteencantilevers112 are connected. Theplatform108 is made positionable by a plurality of actuators124-127, each actuator being connected with thetip platform108 by a corresponding pull-rod120-123. As shown, eachtip platform108 includes four actuators124-127, an actuator being positioned on each side of theplatform108, allowing theplatform108 to be moved in any direction in a two-dimensional space within the physical limits of the actuators124-127. Thetips142 extending from thecantilevers112 and the actuators124-127 can be electrically coupled to a memory controller, or other outside source by a plurality ofinterconnects104, eachinterconnect104 electrically connecting arespective cantilever112 or actuator124-127 to aninterconnect node102. Theinterconnect104 can be made from any number of conductive materials.
• Interconnect nodes102 provide access to the die100 from sources outside of thedie100. For example, sense and control signals can be sent to, and read from, the actuators124-127 to determine a position of the actuators124-127 relative to a neutral state. Different signals can be sent to atip142 to read and/or write data to a media positioned in close proximity to thetip142. Many other signals can be sent through theinterconnect node102 and interconnect104 as desired in the design of thedie100, the design of the system incorporating thedie100, and/or as desired to meet other design goals. Control signals can be passed throughappropriate interconnect nodes102 andinterconnects104 to direct the actuators124-127 to perform some action. For instance, a stimulus can be sent to an actuator124-127 to actuate, causing the pull-rod120-123 to be drawn toward the outside of thecell118, thereby moving theplatform108. A control signal can be directed to multiple actuators124-127 frommultiple cells118 directingmultiple platforms108 to move in the same temporal space. In this way a plurality ofcells118 can be controlled simultaneously, individually, or they can be multiplexed. Ifcells118 are multiplexed, then additional multiplexing circuitry is required.
• A die100 can further comprise one ormore test structures114,116 positioned outside of thecells118.Test structures114,116 can be measured to ensure proper fabrication of arms of the actuators124-127 and/orinterconnects104, or other features of thedie100. For example, a test signal can be applied to atest circuit114 comprising an actuator arm and one or more test nodes, and measurements can be taken of the expansion rates of the arms without potentially damaging any of theinterconnect nodes102. Likewise, a test signal can be applied to atest actuator116 and a measurement taken to determine the maximum force that thetest actuator116 can apply to a pull-rod120-123. Other data can be collected as well, such as in situ process testing of the manufacturing process, quality assurance, or reliability testing (e.g., determining the stress limits of thetest actuator116 or the current requirements for inducing actuator movement). Any number of metrics can be measured using appropriate test structures.
• While theexemplary die100 described above includes an array of four by four (4×4)cells118, adie100 for use with systems and methods of the present invention can have any number of different arrangements ofcells118 within thedie100 having the same, fewer, ormore cells118. For example, thedie100 can comprise a single row of sixteencells118, or alternatively, an eight by sixteen (8×16) rectangular arrangement ofcells118. A die100 can include as few as asingle cell118 or asmany cells118 as the manufacturing process permits on a single wafer. As semiconductor manufacturing processes change so that greater die densities and larger wafers can be made, a greater number ofcells118 can be included on asingle die100. The number and arrangement ofcells118 incorporated into adie100 can be determined based on a target application. For example, where medium storage capacity is required in a very small volume of space,fewer cells118 can be used. One of ordinary skill in the art can appreciate the myriad different design factors that can be considered in determining adie100 configuration.
• Theexemplary die100 ofFIG. 1A can be associated with a media die (shown inFIG. 3A) comprising one or more media cells318 (shown inFIG. 3B), eachmedia cell318 corresponding to one ormore tips142. Theexemplary die100 and media die can be positioned in operative association to one another such that thetips112 can be electrically connected with the media surface.
• FIG. 1B is an illustration of acell118 from theexemplary die100 ofFIG. 1A. Thecell118 includes sixteentips142 associated with aplatform108 for writing and reading to a media. Eachtip142 extends, or is otherwise connected with a distal end of acantilever112. Thecantilever112 can be connected with theplatform108. In other embodiments, theplatform108 can be associated with fewer, ormore tips142. Theplatform108 can comprise a frame or lattice structure for supporting thecantilevers112, and can comprise some material or combination of materials having a thermal coefficient across a prescribed operating range substantially similar to a thermal coefficient of a corresponding media platform308 (shown inFIG. 3B) across the same operating range. It can be important for reading and writing that the thermal expansion of theplatform108 substantially match a thermal expansion of themedia platform308 so thatindividual tips142 arranged across theplatform108 can be properly positioned over a desired indicia or target location of a respective media device350 (also referred to herein as a media region). A disparity in a thermal expansion coefficient between themedia platform308 and theplatform108 that exceeds, for example, the platform's ability to compensate for slight drift over an operating range, can result in atip142 being improperly positioned over themedia device350, leading to read and/or write errors. For example, an indicia written to amedia device350 can be improperly indexed by a servo system of thedie100, or the wrong indicia can be erroneously read, corrupting a result. Thetip142 should be capable of reading an indicia that thetip142 intends to read, and write to a position to which thetip142 intends to write (and to which thedie100 is capable of correctly indexing). The ability of theplatform308 to compensate for slight drift over an operating range can determine how closely matched should be the coefficient of thermal expansion of aplatform108 and the coefficient of thermal expansion of amedia platform308, and can be dependent on the density of information storage, a size of a written indicia, a size and/or geometry of theplatform108, a size of thetip142, and other factors.
• Themedia platform308 andtip platform108 need not be made of the same material to achieve the desired thermal expansion rates, or desired matching of thermal expansion rates. It is known that materials can be carefully doped so that such materials are tailored to thermally expand at a desired rate over a desired temperature range. For example, in one embodiment, aplatform108 for supporting a plurality oftips142 can comprise silicon, while acorresponding media platform308 can comprise an alloyed metal having an equivalent coefficient of thermal expansion as silicon over the desired operating range (e.g., 0-70° C.). Further, in some embodiments, theplatform108 can comprise a plurality of materials associated with one another so that a desired thermal drift is achieved as a result of the association of the plurality of materials. For example, referring toFIG. 1C, in an embodiment, one or more ofcantilevers112 associated with aplatform108 can be mounted on a sub-platform192 (in this case a “C”-shaped sub-platform) connected with amain platform190 the sub-platform comprising a material having a high thermal expansion coefficient (e.g., nickel or aluminum). Themain platform190 in this example comprises an oxide material. Each sub-platform192 is sized so that the sub-platform192 expands at a desired rate, causing thecantilever112 mounted on the sub-platform192 to drift with the thermal expansion of the sub-platform192, thereby tracking the thermal expansion of acorresponding media platform308, despite the low expansion of themain platform190. In such an embodiment, themain platform190 can comprise a material having a coefficient of thermal expansion substantially lower than that of a material of themedia platform308, and the sub-platform192 can comprise a material having a coefficient of thermal expansion higher than that of the material of themedia platform308, so that the drift of thecantilever112, and by extension thetip142, expands as desired over the operating temperature of thedie100. The sub-platforms192 compensate for the difference in thermal expansion of themain platform190 and themedia platform308, and the geometry as well as the material of the sub-platform192 can be chosen to achieve the desired result. The sub-platform192 need not be “C”-shaped as shown. In still other embodiments, aplatform108 in accordance with the present invention can be arrayed with a composite of metal and oxide structures in a grid, the geometries of the metal and oxide structures defining a net coefficient of thermal expansion of thecomposite platform108 that is substantially similar to a coefficient of thermal expansion of themedia platform308. One of ordinary skill in the art will appreciate the myriad different combinations of materials and geometries that can be employed to achieve aplatform108 wherein thetips142 drift with a change in temperature at substantially the same rate as the expansion of themedia platform308.
• In an embodiment as shown inFIG. 1B, theplatform108 can comprise a material having a low coefficient of thermal expansion, for example silicon dioxide. Having aplatform108 with a frame structure of a material having a low coefficient of expansion can limit the amount of drift of atip142 within a corresponding media, and the amount of drift of thetip142 relative to everyother tip142. Thetips142 can comprise silicon, or some other conductive material, or alternatively an insulator having a conductive coating. Thetips142 can be integrally formed with theplatform108 using a combination of well known semiconductor manufacturing processes. One of ordinary skill in the art can appreciate the means for forming cantilevers havingconductive tips112 on an insulated frame.
• Referring toFIGS. 1D and 1E, thermal drift betweentips142 can further be limited by separating thedie100 intomultiple cells118. Acell118 can include aplatform108 of a limited size (i.e., a platform having a small form factor). As shown inFIGS. 1A and 1B, eachplatform108 supports sixteencantilevers112, although in other embodiments fewer ormore cantilevers112 can be supported, depending on a size of aplatform108. The size of theplatform108 can be limited to a size that results in an acceptable level of drift during operation betweentips142 positioned at a maximum distance from one another on theplatform108. For example, as shown schematically inFIG. 1D fourcells118 are arranged in a 2×2 square, eachcell118 having aplatform108 with a platform center109. The die100 can heat because of a flow of current through thedie100, movement of theplatforms108 within thecells118, and environmental effects. As the die100 heats, theplatforms108 can expand. As can be seen inFIG. 1D, a first tip t1 and a second tip t2 are associated with afirst platform108a,and a third tip t3 is associated with asecond platform108b.The first tip at an initial position t1 can shift to a final position t1′ due to thermal expansion of thefirst platform108a.A second tip at an initial position t2 (a distance x from thecenter109aof thefirst platform108a) can shift to a final position t2′ due to thermal expansion. An initial distance d1 between the first tip and the second tip thus expands to a final distance d2 between the first tip and the second tip. Thermal drift can result in possible servo errors and misreading of data. The thermal expansion of aplatform108 is isolated relative to everyother platform108 and theplatform108 expands into empty space within thecell118. Such expansion can possibly cause slight displacement of the actuators124-127 and interconnects104, but does not cause theoverall cell118 size to increase, limiting the effects of thermal expansion on tip drift. While theplatform108 expands, the center109 of theplatform108 can remain in the same position, roughly. Thus for example, the third tip, also a distance x from thecenter109bof thesecond platform108b,shifts to a final position t3′ due to thermal expansion. An initial distance d3 between the first tip and the third tip expands to a final distance d4 between the first tip and the third tip. The increase in absolute distance between the first tip and the third tip (d4−d3) is roughly the same as the increase in absolute distance between the first tip and the second tip (d2−d1). Thus, d4−d3=d2−d1.
• As shown in the schematic ofFIG. 1E, asingle cell218 having a size similar to the space occupied by the fourcells118 ofFIG. 1D includes aplatform208 that expands at a compounded rate relative to the expansion of each of theplatforms108 ofFIG. 1D. As can be seen, a first tip at an initial position t1 can shift to a final position t1″ due to thermal expansion of theplatform208. The first tip, though located in the same initial position as the first tip ofFIG. 1D, is a further distance from thecenter209 of thecorresponding platform208 than the first tip ofFIG. 1D. As a result, the distance between the final position t1″ and the initial position t1 of the first tip of thesingle platform208 is greater than the distance between the final position t1′ and the initial position t1 of the first tip for each of thesmaller platforms108. A third tip at an initial position t3 can shift to a final position t3” due to thermal expansion, again with the distance between the final position t3″ and the initial position t3 of the third tip of thesingle platform208 being greater than the distance between the final position t3′ and the initial position t3 of the third tip for each of thesmaller platforms108. As a result, a distance D3 between the first tip and the third tip of thesingle platform208 expands to a distance D4 between the first tip and the third tip of thesingle platform208. The expansion in distance (D4−D3) across a singlelarge platform208 is greater than the expansion in distance (d4−d3) across two smaller platforms108 (D4−D3>d4−d3). The expansion of theplatform208 is compounded because theplatform208 must expand outward into thesingle cell218. Of course, theplatform208 ofFIG. 1E will have relatively smaller expansion acrossplatforms208 arranged in a 2×2 square (as thesmaller platforms108 ofFIG. 1D are arranged), when compared with a platform (not shown) yet four times the size of theplatform208 ofFIG. 1E. Systems and methods in accordance with the present invention can scale the size of atip platform108 so that thetip platform108 has appropriate expansion characteristics that are within the general error tolerance of the circuitry (e.g., positioning circuitry, servo circuitry, etc.) of thedie100. It has been demonstrated that adie100 as described above with reference to FIG. IA includes such atip platform108.
• Use of smallform factor platforms108 as described above can provide a memory device and system with thermal stability. Further, generally platforms having a relatively small number of cantilevers, wherein a small number of cantilevers is roughly defined as approximately one hundred cantilevers or less (i.e., an order of magnitude smaller than a platform supporting one thousand tips), can provide a memory device and system with thermal stability. Thermal stability can be defined as having thermal drift characteristics over a desired operating range that are within a tolerance of the system circuitry. One of ordinary skill in the art will note from the descriptions and benefits described herein that platforms sized to support fewer cantilevers can potentially enjoy such benefits to an increasing degree. In additional to thermal stability a smallform factor platform108 has a lower mass when compared with a largerform factor platform108. In general, aplatform108 having lower mass can be actuated at a higher speed when compared with aplatform108 comprising a similar material having a higher mass. Smallform factor platforms108, therefore, can provide a capability of faster access speed, improving overall performance of a memory device. Further, smallform factor platforms108 provide inherent fault tolerance. In the event that a fault tolerance scheme for a set of actuator arms (as described below) fails for asingle platform108, a loss of capacity of a memory device is limited to the portion of a media device accessed by thesingle platform108. Atip platform108 that supports a relatively small number of cantilevers112 (e.g., sixteen cantilevers) can have advantages over a tip platform that supports hundreds or thousands of cantilevers, for example. However, systems and methods in accordance with the present invention can include one or more platforms supporting hundreds or thousands of cantilevers, where desired.
• Referring again toFIG. 1B, aplatform108 is positionable using four bi-morph actuators: an X-left actuator124 coupled by a left pull-rod120 with thetip platform108, a Y-top actuator125 coupled by a top pull-rod121 with thetip platform108, anX-right actuator126 coupled by a right pull-rod122 with thetip platform108, and a Y-bottom actuator127 coupled by a bottom pull-rod123 with thetip platform108. Each actuator124-127 includes two sets of arms connected by acoupling bar141, with each set of arms including a plurality of bi-morph arms140. When a voltage is applied via aninterconnect104 to a bi-morph arm140, the bi-morph arm140 bends toward the outer edge of thecell118. Collectively, the two sets of arms draw the pull-rod120-123, which in turn pulls theplatform108, causing theplatform108 to shift in position toward the energized actuator. Theplatform108 can have relative movement typically in the range of plus or minus fifty microns, but this range can be extended or reduced as required by various design goals. Also, the actuators124-127 are not required to have an identical movement range in order to permit thecell118 to function. For example, theX-axis actuators124,126 could have a range of plus to minus fifty microns while the Y-axis actuators125,127 could have a range of plus to minus sixty-five microns, or vice versa. In other embodiments, the actuators124-127 can comprise structures other than bi-morph structures, for example, the actuators124-127 can comprise comb-electrode structures (for example as described in U.S. patent application Ser. No. 09/465,592, entitled “Molecular Memory Medium and Molecular Memory Integrated Circuit,” Attorney Docket No. LAZE-01000US0, filed Dec. 17, 1999). In still other embodiments, thetip platform108 need not include actuators, for example where acorresponding media platform308 is employed having sufficient range of movement.
• A plurality ofinterconnects104 are electrically coupled with theplatform108, for example inbundles103, with eachbundle103 includinginterconnects104 associated with a plurality oftips142. As shown, thecell118 includes fourbundles103, with eachbundle103 including fourinterconnects104 corresponding to fourtips142. In other embodiments, thecell118 can include one ormore bundles103, and eachbundle103 can comprise one ormore interconnects104. Eachbundle103 can be coiled or routed in an accordion-like fashion so that theinterconnect104 can expand—for example by partially unfolding, thereby preventing theinterconnect104 from restricting movement of thetip platform108 away from theinterconnect104—or collapse, for example by bending to accommodate a shorter distance where thetip platform108 is drawn toward theinterconnect104. A separate set of interconnects (not shown) is connected with each actuator124-127 to energize the actuator124-127. In some embodiments, theinterconnect104 can restrict undesired movement of thetip platform108. For example, in an embodiment aninterconnect104 can include a cross-section having a deep z dimension, i.e. having a dimension along a plane perpendicular to the plane of the die100 (also referred to herein as a z dimension). Such geometry can restrict movement of theplatform108 in the z dimension, thereby helping to ensure a desired z positioning of theplatform108 relative to themedia platform308. Where theinterconnect104 is configured, as described, to restrict z movement of theplatform108, it can be said that theinterconnect104 acts as a “suspension.” Such an arrangement can have an advantage that actuators124-127 need not hold theplatform108 as rigid in the z dimension.
• The actuators124-127 can further include a fault tolerant design so that the actuators124-127 will function so long as they are not completely destroyed, thereby increasing reliability and lifetime of a die. The fault tolerant design is described in more detail in U.S. patent application Ser. No. 10/684,760, entitled “Fault Tolerant Micro-Electro Mechanical Actuators,” filed Oct. 14, 2003. As can be seen inFIG. 1B, each actuator124-127 includes two sets of arms, each set having multiple bi-morph arms140. If one of the arms on an actuator124-127 breaks, that arm140 will form an open circuit. A broken arm140 will reduce the potential force that an actuator124-127 can exert upon theplatform108, thereby reducing the maximum range with which the actuator124-127 can move theplatform108. The actuator124-127 can be built so that the maximum range of movement of the actuator124-127 exceeds the usable range of movement of thetip142 so that where actuator124-127 performance degrades, no degradation incell118 performance results.FIG. 1B shows each actuator124-127 with a total of twenty arms140. Increasing the number of arms140 can increase the fault tolerance of the actuator124-127, but it will also increase the amount of physical space required for the actuator124-127. Likewise, fewer arms140, such as six arms140, can reduce the amount of physical space required for the actuator124-1276, but it will in turn increase the sensitivity that an actuator124-127 has to damage, thus reducing its efficiency for being fault tolerant.
• Atip142 and acorresponding cantilever112 can be formed so that the tip is in constant contact or near contact as thetip platform108 moves along the media surface. In one embodiment in accordance of the present invention, thecantilever112 can have curvature such that thecantilever112 curves away from a plane defined by theplatform108, and toward the media surface. Consequently, as a media platform308 (shown inFIG. 3A) is positioned in close proximity to thetip platform108, thetip142 will make first contact with themedia platform308. Thecantilever112 can be designed such that it has a spring-like response when pressure is applied to thecantilever112 from either thetip platform108 ormedia platform308. Hence, small changes in the distance between thetip platform108 and themedia platform308 will not necessarily cause thecantilever112 to break electrical contact with the media surface. Thetip142 of thecantilever112 can be positioned within the media through movement of thetip platform108 and/or themedia platform308 by the respective actuators124-127.
• In other embodiments, thetips142 can have independent directional control. Thus, cantilevers112 could be designed to be capable of moving along all three axes as defined by reference199 (x-axis, y-axis, and z-axis). Such a design would requireadditional interconnections104 in order to allow control signals todirect cantilevers112.
• Forming Ultra-Sharp Tips
• To form fine domains in a media surface, a tip having an extremely fine tip width can be formed. One method of forming a tip in accordance with the present invention can include forming a silicon nitride (SiN) hard mask on a silicon substrate and applying an isotropic etch to form a sharp tip. The method can include depositing SiN on a silicon wafer, for example using plasma enhanced chemical vapor deposition (PECVD) processing techniques or low pressure chemical vapor deposition (LPCVD) processing techniques. The wafer is then coated with photoresist, and exposed using standard lithography techniques to form a tip pattern comprising one or more small squares (also referred to herein as pads) sized according to a desired tip height. For example, where an approximately 2 μm tall tip is desired, a 3 μm×3 μm square can be formed. The SiN not protected by the photoresist mask can be removed from the surface using an anisotropic etch, for example in a plasma etcher. The photoresist can optionally be removed, leaving one or more SiN hard masks.
• FIG. 2 illustrates processing of a portion of awafer280 over a series of recipe steps. Ahard mask284 is formed over asilicon surface282 as described above. The wafer can then be isotropically etched, for example in a liquid chemical bath (i.e., wet etched) or in a plasma etcher using a sulfur hexaflouride (SF₆) chemistry. The isotropic etch can be applied such that the hard mask is undercut by about 1 μm, as shown by thesecond line286 inFIG. 2. The etch can be endpointed visually, for example by observing the structures under an optical microscope. If the photoresist has not been removed prior to performing the isotropic etch, the photoresist must then be removed. The wafers are cleaned to remove residual photoresist and other contamination prior to subsequent processing, for example in a wet chemical bath such as a piranha. Following these steps, the wafer is placed in an oxide growth furnace and oxidized so that a portion of the silicon is consumed during the oxide growth process, the portion corresponding to thethird line288. Approximately 1 μm of oxide can be grown to form a satisfactory tip. The nitride hard mask impedes oxide growth at the top of the tip, and the oxide grows steadily inward from the sides at least until the oxide completely undercuts the nitride pad. Once the wafer is removed from the furnace, the oxide can be stripped, for example in a chemical bath by hydrofluoric (HF) acid. The nitride pads fall off as the oxide is undercut by the etchant. The high etch selectivity between the oxide and the silicon will result in removal of the oxide with approximately no removal of silicon. The result of this step is the formation of ultrasharp silicon tips242. The wafers can further be reoxidized, if desired, to form oxide tips. Further, the tip height can be increased by performing an anisotropic silicon etch either before the isotropic etch or after.
• Storage Media for use with the Read/Write Engine
• FIGS. 3A and 3B illustrate an exemplary media die300 and anexemplary media cell318 from the exemplary media die300 for use with systems and methods in accordance with the present invention. The media die300 comprises sixteencells318, eachcell318 including amedia platform308 on which sixteenmedia devices350 are connected. Theplatform308 is made positionable by a plurality of actuators324-327, each actuator being connected with themedia platform308 by a corresponding pull-rod320-323. As shown, eachmedia platform308 includes four actuators324-327, an actuator being positioned on each side of themedia platform308, allowing theplatform308 to be moved in any direction in a two-dimensional space within the physical limits of the actuators324-327. Themedia device350 and the actuators324-327 can be electrically coupled to a memory controller, or other outside source by a plurality ofinterconnects304, eachinterconnect304 electrically connecting arespective memory device350 or actuator324-327 to an interconnect node302. Theinterconnect304 can be made from any number of conductive materials.
• As with thedie100, the media die300 includes interconnect nodes302 that provide access to the media die300 from sources outside of the media die300. Further, the media die300 can likewise further comprise one ormore test structures314,316 positioned outside of thecells318. While the media die100 shown inFIG. 3A includes an array of four by four (4×4)cells318, a media die100 for use with systems and methods of the present invention can have any number of different arrangements ofmedia cells318 within the media die300 configured and arranged to correspond to a configuration of acorresponding die100. As described above, theexemplary die100 and media die300 can be positioned in operative association relation to one another such that thetips142 can be electrically connected withcorresponding media devices350.
• FIG. 3B is an illustration of amemory cell318 for use with embodiments of the present invention. Thememory cell318 includes aplatform308 connected with sixteenmedia device350 which can be active or passive (as described in detail below). As with thetip platform208, themedia platform308 can comprise a frame or lattice structure for supporting themedia devices350, and comprise a material having a coefficient of thermal expansion substantial similar to the material comprising theplatform108. As above, in some embodiment themedia platform308 can comprise a composite structure. In one embodiment, themedia platform308 can comprise some material having a low coefficient of expansion, for example a silicon dioxide. Having aplatform308 with a frame structure of a material having a low coefficient of expansion can minify the amount of drift of eachmedia device350 relative to acorresponding tip142. Eachmedia device350 can be isolated from everyother media device350, or themedia cell318 can be a continuous surface. In other embodiments, theplatform308 can be connected with fewer ormore media devices350. A media die300 can include one ormore memory cells318, for example corresponding to a number oftip platforms108 on adie100.
• As with thetip platform208, themedia platform308 is positionable using four bi-morph actuators: anX-left actuator322 coupled by a left pull-rod320 with amedia platform308, a Y-top actuator326 coupled by a top pull-rod324 with themedia platform308, an X-right actuator328 coupled by a right pull-rod330 with themedia platform308, and a Y-bottom actuator332 coupled by a bottom pull-rod334 with themedia platform308. Each actuator324-327 includes two sets of arms connected by acoupling bar341, each set including a plurality ofbi-morph arms340. When a voltage is applied via an interconnect (not shown) to thebi-morph arms340, thebi-morph arms340 bends such that the attached pull-rod is pulled into the actuator. Collectively, the two sets of arms can draw the pull-rod320-323, which in turn pulls themedia platform308, causing themedia platform308 to shift in position toward the energized actuator. As with thetip platform208, themedia platform308 can have relative movement typically in the range of plus or minus fifty microns, but this range can be extended or reduced as required by various design goals. Also, the actuators324-327 are not required to have an identical movement range in order to permit thecell318 to function. In other embodiments, the actuators324-327 can comprise structures other than bi-morph structures, for example, the actuators324-327 can comprise comb-electrode structures. In still other embodiments, themedia platform308 need not include actuators, for example where acorresponding tip platform208 is employed having sufficient range of movement. As described above, the actuators324-327 can further include a fault tolerant design so that the actuators324-327 will function so long as they are not completely destroyed, thereby increasing reliability and lifetime of a die.
• A plurality ofinterconnects304 are electrically coupled with themedia platform308, for example in bundles with eachbundle including interconnects304 corresponding to a plurality ofmedia devices350. As shown, thecell318 includes four bundles, with each bundle including fourinterconnects304 corresponding to fourmedia devices350. In other embodiments, thecell318 can include one or more bundles303, and each bundle303 can comprise one ormore interconnects304. Each bundle303 can be coiled or routed in an accordion-like fashion so that theinterconnect304 can expand—for example by partially unfolding, thereby preventing theinterconnect304 from restricting movement of themedia platform308 away from theinterconnect304—or collapse, for example by bending to accommodate a shorter distance where themedia platform308 is drawn toward theinterconnect304. Additionally, a pair ofinterconnects304 are connected with each actuator to energize theactuator304, as described above. In some embodiments, theinterconnect304 can restrict undesired movement of themedia platform308. For example, as above, in an embodiment aninterconnect304 can include a cross-section having a deep z dimension, i.e. having a dimension along a plane perpendicular to the plane of the die300 (also referred to herein as a z dimension). Such geometry can restrict movement of themedia platform308 in the z dimension, thereby helping to ensure a desired z positioning of themedia platform308 relative to theplatform308. Where theinterconnect304 is configured, as described, to restrict z movement of thetip platform108, it can be said that theinterconnect304 acts as a “suspension.” Such an arrangement can have an advantage that actuators324-327 need not hold themedia platform308 as rigid in the z dimension.
• The actuators324-327 can further include a fault tolerant design so that the actuators324-327 will function as long as they are not completely destroyed, thereby increasing reliability and lifetime of a die. As can be seen inFIG. 3, each actuator324-327 includes two sets of arms, each set havingmultiple arms340. If one of thearms340 breaks, thatarm340 will form an open circuit. Abroken arm340 will reduce the potential force that an actuator324-327 can exert upon themedia platform308, thereby reducing the maximum range with which the actuator324-327 can move themedia platform308. The actuator324-327 can be built so that the maximum range of movement of the actuator324-327 exceeds the usable range of movement of thecorresponding tips242 so that where actuator324-327 performance degrades, no degradation inmedia cell318 performance results.FIG. 3 shows each actuator324-327 with a total of twentyarms340. Increasing the number ofarms340 can increase the fault tolerance of the actuator324-327, but it will also increase the amount of physical space required for the actuator324-327. Likewise,fewer arms340, such as sixarms340, can reduce the amount of physical space required for the actuator324-327, but it will in turn increase the sensitivity that an actuator324-327 has to damage, thus reducing its efficiency for being fault tolerant.
• FIG. 4A is a cross-section of an embodiment of amedia device450 in an unwritten state for use with systems and methods in accordance the present invention. Themedia device450 includes asubstrate452, for example comprising silicon, an under-layer454 formed over the silicon substrate, aphase change layer456 formed over the under-layer454, and optionally anover-layer458 formed over thephase change layer456. The under-layer454 can comprise a highly conductive material, thereby drawing heat away from thephase change layer456, facilitating fast cooling of thephase change layer456. In an embodiment, the under-layer454 can comprise tungsten, while in other embodiments the under-layer454 can comprise one or more of platinum, gold, aluminum, and copper. In still other embodiments, the under-layer454 can comprise some other material having high conductivity. One of ordinary skill in the art can appreciate the myriad different materials for forming the under-layer454. Where it is desired that the under-layer454 be insulated from thesubstrate452, there may be an inbetween layer of insulator between the under-layer454 and thesubstrate452. For example, in an embodiment the inbetween layer can comprise one of an oxide and a nitride material, thereby insulating themedia456 from thesubstrate452.
• Where anover-layer458 is included in themedia device450, theover-layer458 can comprise a material different from that of thephase change layer456, and may be selected to prevent physical damage to thephase change layer456 and/or thetip442 when thetip442 contacts theover-layer458. Theover-layer458 can comprise a material that is resistant to wear, thereby extending the lifetime of theover-layer458 and/or thetip442. Theover-layer458 typically includes a low conductance characteristic, and a high hardness characteristic. For example, in an embodiment theover-layer458 can comprise titanium nitride (TiN), a hard material that conducts poorly. However, it should be noted that it can be advantageous (as described in detail below) to employ an anisotropic columnar material that conducts current more readily through a film than across a film. TiN is one such anisotropic columnar material. In another embodiment, theover-layer458 can comprise diamond-like carbon (DLC). The conductivity of diamond-like carbon can be adjusted in the manufacturing process through a variety of techniques. One such technique includes using a dopant such as nitrogen in the formation of the diamond-like carbon. In still another embodiment, the over-layer include molybdenum nitride (MoN), another such substantially anisotropic columnar material. Many different metal nitrides can be used.
• In yet another embodiment of amedia device450, theover-layer458 can comprise an insulator. For example, theover-layer458 can comprise silicon nitride (SiN) or oxide. Where an insulator is used as anover-layer458, current applied to themedia device450 from the tip must tunnel through theover-layer458 before reaching thephase change layer456. Thus, in one embodiment, the insulator used forover-layer458 is thin (relative to the phase change layer456), thereby reducing the amount of tunneling required before a current can interact withphase change layer456. In another embodiment, the insulator forover-layer458 is an oxide.
• In yet a further embodiment the over-layer can comprise a cermet-like material. Cermets are combinations of ceramic insulators (commonly dielectrics) and metal conductors that form a matrix. The matrix can have a concrete-like structure, where the metal is analogous to rocks in concrete and the dielectric is analogous to the “glue” that holds the rocks together. It can also have a columnar structure much like TiN. Either form will allow a relative anisotropic conductivity such that the current will preferably flow through the film rather than flow laterally across the film. In still other embodiments, the phase change material can be replaced with a cermet that comprises a phase change material as a conductor, surrounded by a matrix of an insulator. In still other embodiments, the phase change layer can consist of isolated dots, surrounded by an insulator. In alternative of such embodiments, the phase change layer can have discrete conductors over the dots, not electrically connected with adjacent dots. In still other embodiments, the over-layer can consist of a material that exhibits non-linear conductive properties with voltage, particularly those that have increasing conductivity with higher voltage potential. Such materials include tin oxide (SnO). In still another embodiment, the over-layer can comprise a material that exhibits non-linear conductive properties with temperature, particularly those that have increasing conductivity with higher temperature. Such materials include many semiconductors such as silicon. Many of these alternative materials can be used together such that the combination increases the anisotropic conductivity characteristic of the over-layer. Further, these over-layer materials can be used sequentially, rather than mixed together, to enhance the performance characteristics of the over-layer. For example, a very thin layer of carbon can be added over TiN to form a barrier to oxidization, as well as to improve lubricity of the surface.
• In some embodiments of amedia device450, thephase change layer456 comprises a phase change material. The phase change material can include, for example, germanium (Ge), antimony (Sb) and/or tellurium (Te) (such phase change materials are commonly referred to as chalcogenides). As a portion of the phase change material is heated beyond some threshold temperature and then cooled very quickly (i.e., quenched) the phase of the material changes from a crystalline state to an amorphous state. Conversely, if the phase change material is heated above some threshold and then allowed to cool slowly, the material will tend to re-crystallize. As a result of these phase changes, the resistivity of the material changes. This resistivity change is quite large in phase change materials and can be easily detected by a tip that is conductive or that includes a conductive coating by passing current through thetip442 and themedia device450. Phase change materials are well known in the art and can be found disclosed in numerous references, for example U.S. Pat. Nos. 3,271,591 and 3,530,441 both issued to Ovshinsky and incorporated herein by reference. In other embodiments of themedia device450 thephase change layer456 can be substituted by a magneto-optic material.
• In addition to anover-layer458, amedia device450 can optionally include alubricant451 that is formed, deposited, adhered, or otherwise placed, positioned or applied over theover-layer458. In some embodiments, thelubricant451 can be a liquid. In other embodiments, thelubricant451 can be a non-liquid, such as molybdenum disulfide. In another embodiment, thelubricant451 can be a form of carbon. Thelubricant451 can be applied to anover-layer458 using myriad different techniques. In an embodiment, thelubricant451 can be deposited on theover-layer458 using a deposition process. In another embodiment, thelubricant451 can be sprayed onto theover-layer458. One of ordinary skill in the art will appreciate the myriad different lubricants that can be employed to provide a desired relationship between a tip and amedia device450, and the myriad different techniques for applyingsuch lubricant451.
• Themedia device450 can be formed using traditional semiconductor manufacturing processes for depositing or growing layers of film in sequence using deposition chambers (e.g., chemical vapor deposition (CVD) chambers, plasma vapor deposition (PVD) chambers) and/or furnaces, for instance. Alternatively, themedia device450 can be formed using a shadow mask. Where a shadow mask is used, a mask wafer that contains at least one aperture is placed over a final wafer to form amedia device450. The mask wafer and final wafer are then subjected to a deposition process. During the deposition process, chemicals pass through the shadow mask and are deposited to form amedia device450. Additionally, the media and/or media stack can be deposited over a lift-off resist layer. The resist layer and excess media stack can be removed by placing a waver on which themedia device450 is formed in a solvent bath that dissolves the resist and allows excess material to be removed. One of ordinary skill in the art can appreciate the myriad different techniques for forming amedia device450.
• FIG. 4B is a cross-section of themedia device450 ofFIG. 4A in which an indicia460 (which can represent a data bit, and which for convenience is referred to herein as a data bit) has been formed. In an embodiment adata bit460 can be formed by passing current through thephase change layer456 from atip442 positioned in contact or near contact with theover-layer458, thereby heating thephase change layer456 near thetip442. As described above, when the temperature of thephase change layer456 exceeds a threshold temperature thephase change layer456 becomes semi-molten or molten, and can be quenched to form an amorphous bit. In other embodiments, the bulkphase change layer456 can have an amorphous structure and when heated can be more slowly cooled to form a crystalline structure. Quenching is defined as a rate of cooling that achieves an amorphous structure, or a partially non-crystalline structure, from a molten or semi-molten phase change material. Cooling, slow cooling, or simple cooling is defined as a rate of cooling that is slow enough that the phase change material forms a crystalline structure from a molten or semi-molten material. In an embodiment, quenching can be achieved by removing current from the heated portion, and allowing a conductive under-layer to remove heat from the heated portion, while simple cooling can be achieved by ramping down current from the heated portion and allowing the conductive under-layer to remove heat from the heated portion. In other embodiments, quenching can be achieved by not only removing current, but by diverting current from the heated portion via a clamp (described below), while simple cooling can include removing current from the heated portion. An exact technique for achieving quenching can depend on the phase change material, the conductivity of the under-layer, and the temperature to which the portion is heated, as well as environmental and other factors. Further, where multiple resistivity states are used (i.e., data is stored in a non-binary fashion), cooling and quenching can have varying cooling rates and can be combined with heating temperature to achieve multiple different resistivity states as desired and designed.
• In a binary system, the data bit460 has an incongruous resistance relative to the surrounding bulkphase change layer456, the incongruity representing data stored in themedia device450. To erase the data bit460 from themedia device450, a second current is applied to a portion of thephase change layer456 that includes the data bit460 to heat the portion and properly cool the portion to form the structure of the bulk phase change layer456 (whether amorphous or crystalline). The resistivity of the data bit460 is consequently changed to that of an unwritten state. For example, where the bulkphase change layer456 has an amorphous structure, acrystalline bit460 can be erased by heating a portion of thephase change layer456 containing thecrystalline bit460 to a second, higher temperature than was applied to form thecrystalline bit460. The portion is then quenched to ambient temperature, thereby causing the portion to form an amorphous structure having a resistivity similar to the original resistivity of the bulkphase change layer456.
• For example, in an embodiment of themedia device450 in accordance with the present invention thephase change layer456 can comprise a chalcogenide. The bulk of thephase change layer456 can have a crystalline structure, and can correspond to an unwritten state. To set the data bit460 to a written state, a first current can be applied to a target portion of thephase change layer456 causing the portion of thephase change layer456 to heat to a threshold temperature (which can be a melting temperature of a phase change material), which in one embodiment of a chalcogenide can be approximately 600° C. Thephase change layer456 can be quenched to ambient temperature, and the portion of thephase change layer456 heated to the threshold temperature will have a resistivity higher than the bulk, unwrittenphase change layer456, thereby forming an indicia that can be interpreted as adata bit460. In such an embodiment, quenching can be achieved by removing the first current at a rate ranging from 10 to 100 nanoseconds although the rate can vary substantially. To reset the data bit460 to an unwritten state (also referred to herein as a reset state, and an erased state), a second current can be applied so that the portion of thephase change layer456 is heated to a temperature approximately equal to a temperature ranging from 170° C. to 250° C. (or greater, including up to the threshold temperature). (The temperature range can depend on the composition of the chalcogenide, and in some embodiments can have some other range, such as from 100° C. to 250° C., or greater.) As the portion of thephase change layer456 cools to ambient temperature, adata bit460 forms having a crystalline structure, the crystalline structure having a resistivity that approximates the resistivity of the bulk, unwrittenphase change layer456. Different materials can be used for thephase change layer456 to adjust the operating range for writing and erasing adata bit460. Altering the proportions of the elements in a chalcogenide is one way of altering the written and erased temperatures.
• It should be noted that although temperatures have been described with some level of specificity, the state of the portion to which heat is applied is generally most influenced by a rate of cooling of the portion. A rate of cooling can be influenced by a rate at which the current through the heated portion is removed from the heated portion, and how quickly the heat can be carried away from the heated portion (i.e., the conductivity of the materials of themedia device450 stack). It is largely thought that where a minimum temperature is reached (i.e., the crystallization temperature, which in the embodiment described above is approximately 170° C.) and maintained, the material can be cooled slowly enough that the material can re-crystallize. Such cooling can be achieved using a number of different techniques, including ramping down a current applied to the heated portion. In some embodiments, the current can be ramped down in stages, and the heated portion can be maintained at desired temperature levels for desired times, so that crystallization is achieved across substantially the entire portion. One of ordinary skill in the art can appreciate the different applications ofphase change layer456 and the techniques for achieving changes in material properties of thephase change layer456.
• In other embodiments, thephase change layer456 can comprise a chalcogenide, the bulk of which includes an amorphous structure corresponding to an unwritten state. In such embodiments, targeted portions of thephase change layer456 can be heated and slowly cooled so that the portion crystallizes, forming an indicia that can be interpreted as adata bit460 having a written state. Systems and methods in accordance with the present invention should not be interpreted as being limited to the conventions disclosed herein or the temperature range or material characteristics described. Systems and methods in accordance with the present invention are meant to apply to all such applications ofphase change layer456 having indicia corresponding to material property.
• As described in the embodiment above, to erase anamorphous data bit460, a second current can be applied to the portion of thephase change layer456 including the data bit460. As the portion cools, the resistivity of the portion returns to a value approximately equal to the original value of the bulkphase change layer456, thereby erasing the data bit460.Multiple data bits460 can be reset to an unwritten state by applying heat to a large region of themedia device450. For instance, themedia device450 can apply a current to a buried heater under themedia device450. This heating can be applied to all of the memory locations in themedia device450 or a portion of themedia device450 such that the resistivity of heated portion of thephase change layer456 is returned to an unwritten value. For example, in an embodiment strip heaters can be positioned to heat up bands within themedia device450. In still other embodiments, a laser can be applied to at least a portion of themedia device450 to heat the portion. For example, where theplatform108 comprises a transparent material, such as silicon dioxide, a laser can be applied through theplatform108 to heat one ormore media devices450 on themedia platform308. In still other embodiments, a matrix of diode heaters can be formed to selectively heat portions of amedia device450. Such bulk erasing can add complexity to one or both dies100,300 but can potentially provide benefits such as reduced tip wear.
• In still another embodiment of amedia device450 in accordance with the present invention, thephase change layer456 is capable of having a plurality of resistivity states. For example, in the unwritten state, thephase change layer456 can have a first resistivity. Thephase change layer456 can then be heated to different temperatures and quenched, thereby changing the resistivity of thephase change layer456. In an embodiment, a read voltage can be applied across a tip andphase change layer456 to sense whether the resistivity of thephase change layer456 is at or near the initial, unwritten state for the bulkphase change layer456 or at some state that is sufficiently different to be measured as a state other than the unwritten state. Thephase change layer456 can have a first resistivity characteristic at an initial, or unwritten state. A first current can then be applied to thephase change layer456, heating thephase change layer456 to a first temperature. The first current can be removed from thephase change layer456 and thephase change layer456 cools to form a structure having a second resistivity characteristic. In an embodiment, the resistivity of thephase change layer456 in this second state can be measured. The second resistivity can vary depending on the temperature that thephase change layer456 is heated to by the first current, and the cooling time of thephase change layer456. A range of resistivity measurements can correspond to a data value, with different ranges corresponding to different data values. A plurality of resistivity ranges can be employed as a plurality of data values using a data storage scheme other than binary, for example. In an embodiment, a data storage scheme including three data values can utilize a base-3 system rather than a binary system for storing data. In another data storage scheme, where four different resistivity states are possible for each data bit, each data bit can correspond to two bits (e.g., each can correspond to 00, 01, 10 or 11). Alternatively, the precise value of the resistivity characteristic forphase change layer456 can be measured for more precise analog data storage. Measurements of the resistivity are preferentially obtained by taking measurements which are relative to a first state of the media, but can also be obtained by taking absolute value measurements. Another method of measurement extracts the data as the derivative of the measured data.
• Thephase change layer456 can posses a large dynamic range for resistivity states, thereby allowing analog data storage. The dynamic range for the resistivity characteristic of thephase change layer456 can be approximately 3 to 4 orders of magnitude (i.e., 1000-10,000×). For example, the resistivity can range from lower than 0.1 ohm-centimeters to 1000 ohm-centimeters or more. In one embodiment, however, heating from the probe on the phase change material can cause only a very small area ofmedia456 to undergo a change in its resistivity. In this form a smaller dynamic range may be observed, as only a small region of the media is altered. Media systems typically display a range of values in the initially deposited state, such that the resistance values measured vary at different locations. Additionally, variations in the thickness of the phase change material and the over-layer can form differences in the measured resistance as sensed through a tip. These differences manifest as noise in a signal read from the tip. One method of reducing noise uses the analog nature of the recording medium. The state of the media under the tip can be detected by means described elsewhere. A voltage waveform is then applied to the tip to heat and cool the media such that the media changes state. The media under the tip is then read again. If the value is not within the desired noise tolerance for the location, another voltage waveform is applied to change the value to within the desired tolerance range. The waveform can consist of a crystalline pulse or an amorphizing pulse, or some combination of such pulses. Multiple cycles of reading and writing can be used to drive the value to the desired tolerance range. In this way, the media can be adaptively written to reduce noise in the subsequent read back signal. Alternatively, the waveforms used to drive the recording medium to a desired state can operate during the heating and cooling process itself by measuring the resistance state while heating and cooling.
• In other embodiments, themedia456 can be a material other than a phase change material. For example, themedia device450 can include a charge storage-type media. Charge storage media store data as trapped charges in dielectrics. Thus, for charge storage media, themedia456 would be a dielectric material that traps charges when in a written state. Changing themedia456 back to an unwritten state simply requires the removal of the trapped charges. For instance, a positive current can be used to store charges in themedia456. A negative current can then be used to remove the stored charges from themedia456.
• Super Resolution Writing and Reading
• A tip formed as described above can include a distal end having a radius of curvature of about 25 nm, in one embodiment. As the tip moves across the media surface, in contact or near contact with the surface, the tip wears such that after some initial period the nominal radius of curvature of the distal end ranges from 50 to 100 nm (or more), in one embodiment. A voltage is applied across the media to form domains of low (or high) resistivity. The distal end of the tip is typically not completely flat, therefore the distal end is likely not in uniform contact or near-contact with the phase change material (or the over-layer where present). The portion of the distal end in contact or near-contact with the phase change layer is limited by the radius of curvature of the distal end. The portion of the tip in contact or near contact is also referred to herein as the terminus of the tip. It should be noted that while the distal end is described as having a radius of curvature, the distal end need not be shaped so that the terminus lies along a perfect arc. The radius of curvature can be thought of as an increase in width of the distal end of the tip from the terminus, and as referred to herein is not meant to be limited to geometries wherein a distal end includes a smooth, arced shape. The distal end can, for example, have a parabolic shape, a trapezoidal shape, or a non-uniform shape. The tip is electrically conductive, and when a voltage potential is applied between the tip and the media, current passes from the tip, through the over-layer and media to the underlying substrate (in the case where the tip is a voltage source rather than a voltage sink). The current flowing between the media and tip varies across the radius of curvature as the electric field between the tip and the media decays inversely with distance from the surface of the phase change layer.
• The current passing from the tip to the media heats the phase change layer near the tip. The phase change layer, the over-layer, the phase change layer/over-layer interface and the tip/over-layer interface act as resistors. As the voltage potential across the media increases, the current increases, and the temperature of the phase change layer increases.FIG. 6 is a first order model of the heating characteristics of an exemplary media as a voltage potential is applied across the media in accordance with an embodiment of the present invention. The exemplary media includes a film stack comprising atitanium nitride over-layer458 deposited over aphase change layer456. The heat generated by the current can be distributed in a substantially parabolic fashion from the contact or near contact point of thetip542 and the media surface. A small portion of themedia450 near the surface of the film stack (the first isovolume664) is heated above 780 K, and the material surrounding thefirst isovolume664 to the second isovolume662 ranges from 780 K to 500 K. The portion of thephase change layer456 heated above about 575 K, in one embodiment, becomes molten. If the bulkphase change layer456 is amorphous, the molten portion can be cooled slowly to form a crystalline structure having a relative resistivity orders of magnitude lower than a resistivity of the bulkphase change layer456. If the bulkphase change layer456 has a crystalline structure, the molten portion can be quenched quickly, causing the molten portion to become predominantly amorphous and to have a relative resistivity orders of magnitude higher than the resistivity of the bulkphase change layer456. The temperature achieved during heating, and the cooling characteristics depend on the composition of thephase change layer456, and can vary greatly.
• As can be seen inFIG. 6, the portion of thephase change layer456 heated to a molten state, and thereafter properly cooled to form a domain having a resistivity substantially different than the bulk material can be substantially small in width relative to the radius of curvature of thetip542. For example, where methods in accordance with the present invention are applied to create a voltage potential between thephase change layer456 and thetip542, it has been demonstrated that atip542 having an approximate radius of curvature ranging from 50 nm to 100 nm can produce a domain having a width of approximately 15 nm. The domain can be said to be “super resolved.” Such super resolution can result in part from properties of the over-layer, which can be an anisotropic columnar material (e.g., TiN, microcrystalline silicon) that conducts better through the film rather than across the film. This property can focus electron flow near the center of the tip. Further, a portion of thephase change layer456 near the center of thetip542 is heated first, the portion consequently exhibiting lower resistance than the surrounding media, even the unheated crystalline material. Electron flow follows the lowest resistance, and thus the electron flow is further focused.
• The amount of focusing of the current through thephase change layer456—and thus the size of the domain that results—can vary with the voltage potential across thephase change layer456 and the pressure between thetip542 and the surface of the media. The voltage potential can determine the size of an air gap across which the current can arc, and current may or may not flow between thetip542 and thephase change layer456 where an air gap exists (i.e. where the tip is not in direct contact with the media due to curvature). The pressure applied by thetip542 against the surface can likewise affect the portion of thetip542 in direct contact with the surface and a size of the air gap where the tip curves away from the surface.
• Once a domain has been defined within thephase change layer456, the resistivity of the domain can be measured by applying a smaller voltage potential across the portion of the media including the domain (e.g., in one embodiment less than 1 volt) and measuring the current through the portion. The small voltage potential drives a small current, insufficient to heat the portion to a crystallization or threshold temperature. Thus, the resistance (and resistivity) of the portion including the domain can be measured without substantially heating thephase change layer456 and causing the electrical characteristics of thephase change layer456 to be altered.
• Methods and systems in accordance with embodiments of the present invention can be applied to form a plurality of bit cells comprising one or more domains in aphase change layer456. The plurality of bit cells can be arranged in predefined proximity to one another and in any order, so long as a technique can be employed to locate a desired bit. For example, in one embodiment the bit cells can be arranged in rows.FIG. 7A illustrates one embodiment of a method for forming and arranging bit cells. As shown, a “1” is represented by a domain having a high resistance. Thebulk phase material456 preferably has an initially crystalline structure. When a command is received to write a string of bits, in this example “011011”, thetip542 is moved across the media surface (from left to right in the figure) and moved within each bit cell to apply an appropriate current to form the desired value within the bit cell. The bulkphase change material456 can have a nominal “0” or “1” state depending on the structure of the phase change layer and the convention of indicating a “0” or “1”. Assuming, for this example, that a domain having a low resistance indicates a “0”, and that the bulkphase change material456 is in a crystalline state, if the series of bit cells have not been used to store data, each bit cell will have a uniformly low resistance, indicating “0” values. However, more likely the bit cells will have been used to store data.
• To overwrite the bit cells, thetip542 can perform overlap writing to ensure that where a “0” is written thephase change material456 is sufficiently phase changed, which can allow the bit cells to be positioned more closely together. In part, an overlap strategy for writing a “0” can reduce concern that an insufficiently large crystalline domain is formed. A write “0” voltage potential is typically smaller than a write “1” voltage potential, and when applied across the media less current flows through the targeted portion of the media device450 (this can result also from a high resistance state of the phase change material). Thus, where overlap writing is not performed, an insufficiently large domain can result because less phase change material is phase-changed. A read voltage applied to thetip542 is smaller still than the “0” write voltage and has a narrow current path (relative to the domain), allowing a finer resolution during read (resulting, effectively, in a write wide/read narrow scheme).
• Also, in part or in whole, overlap writing can be performed to improve crystallization of a targeted portion of thephase change layer456. An amorphous domain has high resistance, causing current to follow an electrical path around the edge of the amorphous domain. As a result, the amorphous domain is prevented from sufficiently heating, and crystallization and erasure of the amorphous domain does not desirably occur. Such a result can be advantageously avoided by “dragging” thetip542 from an at least partially crystalline region and through the amorphous domain. Thetip542 can be arranged at the edge of the amorphous domain in the crystalline region and repositioned along a path through the amorphous domain while applying a voltage across themedia device450. For example, in an embodiment a plurality of pulses can be applied to themedia device450 as thetip542 is repositioned from the crystalline region across the amorphous domain. It is believed that applying a series of pulses perpetuates a heat wave front, thereby improving crystallization of the amorphous domain. It is known that starting a crystallization process in an amorphous structure can require an unacceptably long time delay before nucleation occurs and crystallization subsequently follows. Such delay can be unacceptable where media surrounding the targeted domain is undesirably heated, or where write times are unacceptably long. Positioning thetip542 over a location where nucleation has been, or can easily be, achieved can allow crystallization to occur more rapidly. Repositioning thetip542 while applying a voltage across themedia450 can propagate nucleation sites and/or the crystallized region within the phase change layer. Such propagation can be said to result from one or both of “pushing” the wave front of crystallization away from thetip542, effectively pushing crystallization along ahead of thetip542, or “pulling” the crystal structure formed at nucleation sites through the amorphous domain along with thetip542. It should be noted that other mechanisms may be involved in crystallizing the amorphous domain, and embodiments of the present invention are not meant to exclusively apply the mechanism described herein. Rather, embodiments of the present invention are meant to capture all such methods wherein an amorphous domain is crystallized by actively moving atip542 across the amorphous domain beginning from an at least partially crystallized region, while applying a voltage potential across themedia device450. It should also be noted that in alternative embodiments of a method in accordance with the present invention, a plurality of pulses need not be applied across themedia device450. For example, in some embodiments a constant voltage potential can be applied to themedia device450 while thetip542 is repositioned. In still other embodiments, a waveform other than a pulse can be applied to themedia device450 as thetip542 is repositioned. For example, the waveform can be a ramp, a saw-tooth, a trailing edge, etc. One of ordinary skill in the art can appreciate the myriad different methods of applying voltage to themedia device450 to heat themedia device450 as thetip542 moves over the surface of themedia device450.
• Referring toFIG. 7A, the tip moves across the surface continuously from left to right writing a “1” or a “0”. The write waveform plot indicates an action of the tip as a function of tip position. For example, in bit cell N, a “0” is written in the first part of the bit cell to crystallize any amorphous material. To write the “0”, a voltage is ramped across the phase change layer to a first potential, heating the phase change layer proximate to the tip to at least the crystallization temperature. The voltage is ramped down, allowing the phase change layer to crystallize. The “0” is followed and overlapped by a “1”, written by ramping the voltage to a second potential, higher than the first potential, heating the phase change layer proximate to the tip to at least the threshold temperature. The voltage is removed so that the molten material is quenched to form a high resistance, amorphous domain. The tip then continues moving to the center of bit cell N+1, and again writes a “1”. The tip then moves to bit cell N+2 and writes two consecutive “0” domains to ensure that the phase change layer in bit cell N+2 is crystallized, and a third consecutive “0” domain preceding a “1” written to bitcell N+3. The tip is then repositioned to bit cell N+4, where the final “1” is written.
• The final written series of bits can be discerned inFIG. 7A from a grayscale pattern of resistance, with darker portions corresponding to higher resistance and a gradient corresponding to a gradient of resistance. Positioning the tip over a written bit and applying a voltage potential across the media can produce an analog resistance measurement correlated to a digital value. Referring toFIG. 7B, a tip applying a low voltage potential between the tip and media can produce analog read data of measured resistance as the tip moves along the media surface and over the bits. The digital equivalent, as read for example by a PRML channel interprets the string “011011”, with resistance peaks corresponding to “1” measurements. A media can be formatted or cleared by erasing all data written to the media, in a similar fashion as described above. To wit, a string of overlapped “0” domains can be written by ramping the voltage to the first voltage and ramping down the voltage to slowly cool the phase change layer, forming a crystalline structure having low resistance, as shown inFIG. 7C.
• Read/Write Circuitry
• FIG. 8 is a circuit diagram of a read/write circuit800 in accordance with an embodiment of the present invention which can deliver an amount of power (or energy) to apassive media450 to heat themedia450, thereby changing the phase of a portion of themedia450 from a crystalline state to an amorphous state (or vice-versa). As described above, thepassive media450 comprises a substrate and a film stack deposited or grown over the substrate. The film stack can include a conductor layer contacting the substrate, a phase change layer, an over-layer to protect the phase change layer and tip(s) from damage. Atip542 is placed in near or actual contact with the media surface when reading and/or writing from the media surface. In an embodiment, one of the tip platform and the media platform is physically positioned below the other of the tip platform and the media platform so that the tips do not contact the media surface until current is applied to the actuators of the lower platform to create tension in the actuators and remove slack (such embodiments are loosely analogous to a hammock that hangs slack, but can be raised when the end ropes are placed in tension). Once current is passed through the positioning actuators of the lower platform, tension is produced in the pull-rods as the pull-rods are drawn to the periphery of the cell. This causes the platform to rise up to meet an opposite platform, contactingtip542 tomedia450. By taking advantage of passive non-contact between the tip platform and the media platform, the complexity of read/write circuitry on a chip-wide scale can be reduced, as well as the complexity of chip packaging (as is described in more detail below). The addressing scheme can be connected with multiple platforms, yet can address one platform at a time by completing a circuit between tip and media platforms.
• In still other embodiments, as will be discussed in more detail, the platform can be positioned such that the cantilevers continuously contact the media surface. With the tip platform in near or actual contact, atip542 is selected from the tip platform and activated so that current flows through thetip542. Where all tips of a platform continuously contact a media surface, it can be advantageous to apply current through one of a plurality of tips mounted to the platform. Applying current through multiple tips from a single platform during a write operation, for example, can cause unintended bits to be written or erased. However, the invention is equally applicable to platforms wherein multiple tips can be accessed at a single time.
• The activatedtip542 allows current to flow, completing a circuit between a voltage source and the groundedpassive media450. A WRITE signal S6 selectively sets aswitch812 completing the circuit with either a read voltage source which is defined by a read voltage digital/analog converter (DAC)802, or awrite ramp generator806. Current passes through the circuit and through thepassive media450 at a rate determined by the voltage potential between the voltage source and the groundedpassive media450. Asense resistor816 is placed in series with the completed circuit between two inputs connected with afirst amplifier818. Thefirst amplifier818 measures a drop in voltage across thesense resistor816 and outputs a current measurement to apower calculator822. Asecond amplifier820 measures the voltage drop across the passive media. The output of thesecond amplifier820 is fed to both the power calculator and an analog read data channel, such as for example a PRML channel. The two amplifiers together can approximately measure the voltage and current through the passive media. Thepower calculator822, in one embodiment a high bandwidth multiplier, calculates the product of the tip current and the tip voltage to determine the power applied to thepassive media450. The product of the current and the tip voltage is further multiplied by a constant, K, and the output is supplied as input to apower comparator826, along with a power reference signal824 (i.e., a threshold set by a digital to analog converter, or other means). Thepower reference signal824 is determined based on prior knowledge of the media phase change characteristics. Thecomparator826 compares the output of thepower calculator822 to thepower reference signal824 and outputs a digital signal to a flip-flop circuit828. In other embodiments, a desired energy, voltage or current can be targeted. To determine the energy across the circuit, the output of the calculator is provided to a resetable integrator, the output of which is applied to the comparator.
• The flip-flop circuit828 can be a standard bistable flip-flop circuit with “set”, “reset” and “clear” inputs and a digital output. For example, in one embodiment the flip-flop828 can be a commercially available flip-flop circuit, such as an SN7474 circuit. If a pulse is provided to the set input, the flip-flop828 will be “1” at its output, and if a pulse is provided to the reset input, the flip-flop828 will be “0” at its output. The flip-flop828 will ignore all input in a clear state. The clear input is a false term input. The WRITE signal S6 selectively activates a switch setting the voltage source to be either thewrite ramp generator806 or the readvoltage source802. When a WRITE signal S6 is false—i.e., the circuit is performing a read—then the flip-flop828 is in a clear state and the output is “0”. When the WRITE signal S6 is true the switch configures the circuit in series with thewrite ramp generator806, and the flip-flop828 is no longer in a clear state. The WRITE CLOCK signal S2 defines the bit cell time and sets the flip-flop828, initiating the circuit to write. The flip-flop828 outputs to thewrite ramp generator806 and causes thewrite ramp generator806 to generate a waveform for writing. The maximum voltage of thewrite ramp generator806 is limited by aWRITE Vmax DAC808. When the calculated power through thepassive media450 exceeds thepower reference signal824, thecomparator826 outputs a “1” to the flip-flop828, triggering the flip-flop828 to reset.
• The digital output of the flip-flop828 is sent to a one-shot timer830 as a clock input. The one-shot timer830 is a standard circuit having “clock input”, “enable” and “clear” inputs. The one-shot timer830 can be a commercially available circuit, such as an SN74123 circuit. If a WRITE DATA signal S4 is false, then the one-shot timer output is “0”, and the clock input is ignored. The clear input is a false term input, as is the clear input for the flip-flop circuit, and when the WRITE signal S6 is true the one-shot timer830 is no longer in a clear state. Both the WRITE DATA signal S4 and WRITE signal S6 must be true to enable the one-shot timer830.
• When both the WRITE signal S6 and the WRITE DATA signal S4 are true, the voltage source switch is set so that thewrite ramp generator806 is in series with a writecurrent limiting resistor812, thesense resistor816, and the groundedpassive media450. Theactive tip542 can be positioned or moved over the media surface, without drawing significant current. To write a bit, the WRITE CLOCK signal S2 is pulsed, causing the flip-flop828 to set, outputting a “1” to the one-shot timer830 and thewrite ramp generator810. Thewrite ramp generator810 ramps the write voltage starting from 0 volts. Ramping the write voltage provides an advantage in controlling the power delivered to the media. For example, ramping can compensate for stray capacitance resulting from a step in sensed current due to the product of the capacitance and the time integral of voltage. The step in sensed current can be calibrated for by adding an offset to thecurrent sense amplifier818,820. Another advantage of ramping is that thecurrent sense amplifiers818,820 have lower bandwidths since the power is changing at the ramp rate, resulting in smaller errors in power calculation and consequently in resetting the flip-flop828 when the power exceeds thepower reference signal824. In other embodiments, the maximum and minimum voltage, and the ramp rate can vary. Note that ramping is not required. In other embodiments the write voltage can be pulsed across the circuit. Similarly, the wave form produced by the wave form generators can vary. For example, the waveform can be triangular or saw-tooth.
• As the voltage increases, the current increases and thepassive media450 begins to heat. The calculated power increases, and at some time t, the calculated power is equal to thepower reference signal824. When thepower reference signal824 is reached, the power comparator sends a pulse resetting the flip-flop circuit828. The flip-flop828 sends a signal ramping down thewrite ramp generator806 and opening aswitch814 between thewrite ramp generator806 and thewrite limit resistor810, and provides a pulse as clock input to the one-shot timer830. The one-shot timer830, enabled by the true value of the WRITE DATA signal S4 and WRITE signal S6, sends a signal activating aclamp834, which in one embodiment can be a high speed analog switch having a turn off time on the order of 10 ns. Theclamp834 short-circuits the voltage source to thetip542 through the conductive layer of thepassive media450. With the tip voltage source shorted, current no longer flows through thephase change layer456, but rather through the grounded conductor layer. The substrate draws heat away from the phase change layer via the conductor, quenching the phase change layer. The speed with which thepassive media450 cools leaves the phase bit in a high resistance amorphous state, thus creating a “1”. As described above, in an amorphous state a phase change material does not have free carriers, causing resistance to increase by as much as 1000:1 ratio. Clamping the circuit can decrease cooling time from, in an embodiment, approximately 200 ns to under 5 ns.
• When the WRITE signal S6 is “1” and the WRITE DATA signal S4 is “0”, the circuit is configured to erase or write a “0”. When the WRITE CLOCK signal S2 provides a pulse and sets the flip-flop828, the flip-flop828 sends a “1” output signal to the one-shot timer830 and thewave ramp generator806. Thewave ramp generator806 increases the voltage across thepassive media450 and thesense resistor816 and the writecurrent limiting resistor810. The current begins to increase and thepassive media450 begins to heat. Erasing a previously written “1” requires that the phase change layer be heated well into the media's “dynamic on state,” and then slowly cooled to form a crystalline state having low resistance.FIG. 5 is a chart illustrating the characteristics of a chalcogenide media device. As can be seen, the voltage ramped across an amorphous region of a chalcogenide must exceed a threshold voltage before the amorphous region can cooled to form crystalline structure. As the current increases, the calculated power increases, and at some time t2, the calculated power is equal to thepower reference signal824. When thepower reference signal824 no longer exceeds the power applied to thepassive media450, thepower comparator826 sends a pulse resetting the flip-flop828. The flip-flop828 sends a signal ramping down thewrite ramp generator806 and provides a pulse as clock input to the one-shot circuit830. The output of the one-shot circuit830, disabled by the false value of the WRITE DATA signal S4, continues to be “0”. As the phase change layer region cools—more slowly with the clamp remaining open absent a pulse from the one-shot circuit output—the heated medium crystallizes, creating a “0”.
• When the WRITE signal S6 is false, the flip-flop828 is in a clear state, ignoring all inputs. Themode switch812 configures the circuit so that the readvoltage source802 is in series with aread limiting resistor804, thesense resistor816, and the grounded,passive media450. The voltage across thepassive media450 draws a relatively small amount of current, insufficient to change the material properties of the phase change layer, but sufficient to allow thesecond amplifier820 to measure the voltage drop across thepassive media450 and send the output as ANALOG READ DATA signal832 to a read circuit (not shown). In an embodiment, the read voltage is approximately 1 volt or less, acting as a current source with a voltage ceiling. Reading the media at lower voltages can limit a “tunneling effect” to a smaller tip region close to the media and can provide better resolution. In other embodiments the read voltage can be higher than 1 volt. Where the phase change layer is amorphous, the resistivity of the phase change layer is higher (i.e., in some embodiments four orders of magnitude (10⁴) higher than the crystalline material) resulting in a larger, detectable voltage drop between the voltage source and thepassive media450. Where the phase change layer is crystalline, the resistivity of the phase change layer is lower.
• FIG. 9 is a circuit diagram of an alternative embodiment of a circuit in accordance with the present invention. The circuit includes separate wave form generators and write power reference signals for writing “1”s and “0”s. The WRITE DATA signal S4 further selectively configures the write power reference and wave form generator. When the WRITE DATA signal S4 and the WRITE signal S6 are “1”, a wave form generator selector switch and themode switch812 configure the circuit in series with the write “1”wave form generator906, and a power reference selection switch924 configures thecomparator826 to receive awrite1 power reference signal924. When the WRITE CLOCK signal S2 is “1”, a pulse is sent setting the flip-flop circuit828. The flip-flop828 sends a “1” output signal to the one-shot timer830 and thewrite1wave form generator906. The write “1”wave form generator906 increases the voltage across thepassive media450 and thesense resistor816 and thewrite1 current limitingresistor910. As the current increases the phase change layer begins to heat. The power calculated by thepower calculator822 increases, and at some time t, the calculated power is equal to the write “1” power reference signal924. When the power reference924 no longer exceeds the power applied to thepassive media450, thepower comparator826 sends a pulse resetting the flip-flop828. The flip-flop sends a signal ramping down the write “1” wave form generator and provides a pulse as clock input to the one-shot circuit. The one-shot circuit, enabled by the true value of the WRITE DATA signal S4 and WRITE signal S6, sends a signal activating aclamp834 which short circuits the voltage source to the tips through the circuit. By shorting the tip voltage source, current no longer flows through thepassive media450, and the phase change layer quickly cools. The speed with which the phase change layer cools leaves the phase bit in an amorphous state, thus creating a “1”. Note that for circuits ofFIGS. 9-12, as withFIG. 8, ramping is not required. In other embodiments the voltage can be pulsed across the circuit. Similarly, the wave form produced by he wave form generators can vary. For example, the waveform can be triangular or saw-tooth.
• When the WRITE signal is “1” and the WRITE DATA input signal S4 is “0”, the circuit is configured so that the write “0”wave form generator940 is in series with awrite0 current limitingresistor944, thepassive media450, and thetip542. When the WRITE CLOCK signal S2 provides a pulse and sets the flip-flop828, the flip-flop circuit sends a “1” output signal to the one-shot timer and the wave form generators. Thewrite0 wave form generator increases the voltage across the media and thesense resistor816 and thewrite0 current limitingresistor944. In one embodiment, thewrite0 wave form generator can be a higher voltage source than thewrite1 wave form generator, creating a larger voltage potential across the circuit and increasing current. As the current increases thepassive media450 begins to heat. When the phase change layer is in a high resistance state (i.e., an amorphous state), the voltage across the phase change layer must exceed a threshold voltage, whereby the phase change layer enters a negative resistance mode, as shown inFIG. 5. The resistance of the phase change layer continues to change as it heats. When the phase change layer reaches a semi-molten state, the voltage current curve merges with the voltage-current curve of the low resistance, crystalline material (once the material is semi-molten or molten, the phase change layer is neither crystalline or amorphous). The calculated power increases as the voltage and current increase, and at some time t2, the calculated power is equal to thewrite0power reference946. When thepower reference signal946 no longer exceeds the power applied to thepassive media450, thepower comparator826 sends a pulse resetting the flip-flop828. The flip-flop828 sends a signal slowly ramping down the write “0”wave form generator940 and provides a pulse as clock input to the one-shot circuit830. The output of the one-shot circuit830, disabled by the false value of the WRITE DATA signal S4, continues to be “0”. As the phase change layer slowly cools, the heated medium crystallizes, creating a “0”. When writing a “0”, a lower power reference is used, and once the power level is reached the voltage is ramped down slowly so that the media still has power in a diminishing amount with time, allowing the phase change layer to cool at a sufficiently slow rate to crystallize.
• As in the previous embodiment, when the WRITE signal S6 is false the flip-flop828 is in a clear state, ignoring all inputs. Themode switch812 configures the circuit so that the readvoltage source802 is in series with aread limiting resistor804, thesense resistor816, and the grounded,passive media450. The voltage across thepassive media450 draws a relatively small amount of current, insufficient to change the material properties of the phase change layer, but sufficient to allow thesecond amplifier820 to measure the voltage drop across thepassive media450 and send the output as ANALOG READ DATA signal832 to a read circuit (not shown). The read voltage should be sufficiently low such that the portion through which the voltage is applied does not undergo a phase change as a result of the application of the voltage. In an embodiment, the read voltage is typically less than 1 volt, acting as a current source with a voltage ceiling (the read voltage can vary with stack composition of themedia device450, and generally can be less than 0.8 volts across the chalcogenide material). Reading the media at lower voltages can limit a “tunneling effect” to a smaller tip region close to the media and can provide better resolution. In other embodiments the read voltage can be higher than 1 volt. Where the phase change layer is amorphous, the resistivity of the phase change layer is higher (i.e., in some embodiments on the order of four orders of magnitude higher than the crystalline material) resulting in a larger, detectable voltage drop between the voltage source and thepassive media450. Where the phase change layer is crystalline, the resistivity of the phase change layer is lower.
• FIG. 10 is a circuit diagram of still another embodiment of a circuit in accordance with the present invention. In such an embodiment, the read/write circuitry is associated with the media platform andpassive tips442 provide a ground path for current flowing through the circuit1000. The media is “active” rather than “passive” and media islands are isolated from one another, as shown in the media cell ofFIG. 3. The media regions can be isolated from one another by insulating regions using standard semiconductor processing techniques (e.g., trench etch and deposition, etc.). Use of active media regions allows all tips to be connected in common, thus potentially reducing the number of leads necessary to the tip platforms. As described below, use of active media regions also permits platform bussing, and thus further reductions in interconnects and read/write circuitry. Theactive media1050 comprises a film stack including an insulator deposited or grown on the substrate, a conductor, a phase change layer, and optionally an over-layer grown or deposited on the insulator. The conductor, phase change layer, and over-layer (where present) are isolated media regions, electrically connected to the media platform read/write circuitry (e.g., by etching contacts or vias prior to forming the conductive layer). The read/write circuitry can selectively activate a media region, for example by a transmission gate, completing a circuit between a voltage source and apassive tip442 in contact or near contact with the media. The read/write circuitry is arranged, for example, as described inFIGS. 8 and 9, however when the one-shot timer830 activates the clamp, an electrical path is formed between the voltage source and the grounded tip platform and stored charge is drawn sharply from the phase change layer through the conductor and to the substrate. This allows the media to cool rapidly through conduction and convection and leaves the region in an amorphous state.
• The circuit diagrams illustrated and described above are exemplary and can include myriad different variations. The scope of the present invention is not intended to be limited to exemplary circuits described herein. For example,FIG. 11 is a circuit diagram of a circuit in accordance with still another embodiment, the circuit including an active media and read/write circuitry associated with the media platform, as described in reference toFIG. 10; however, a voltage is commonly applied to tips connected with one or more platforms. In this embodiment the writewave form generators940,906 and the read voltage DAC sink rather than source the current. When the clamp is applied the tip is directly shorted to the media substrate.
• In still more embodiments, the circuit can be simplified by integrating portions of the circuit into a single processing element, for example such as application specific integrated circuit (ASIC), and enabling an adaptive feedback circuit (rather than an open loop circuit, as described above).FIG. 12 illustrates such an embodiment, wherein a power calculator, comparator, flip-flop circuit, and one-shot timer can be integrated into a single processing element. A WRITE signal can configure the circuit such that a writewave form generator806 is in series with a writecurrent limiting resistor810, anactive media region1050, and atip542. A WRITE DATA signal S4 is received by theprocessing element1252, which configures the writewave form generator806 to apply an increase in voltage in accordance with an action of the circuit. When theprocessing element1252 determines that sufficient power (or energy) has been applied to achieve a phase change in thephase change layer450, the processing element provides an output signal to aclamp834 to short circuit theactive media region1050. Theprocessing element1252 can constantly monitor the resistance of thephase change layer450 and adjust the write waveform and power levels. Should a bit cell already contain a data bit equivalent to the one to be written, theprocessing element1252 could optionally not initiate any current flow through the tip for that bit cell.
• In one embodiment, the processing element can be a very fast processor that, for example, performs a table lookup and generates a write waveform that is a function of the voltage across the media tip or the current through the tip, rather than generating a fixed wave form for writing a “1” or a “0”. The processor could apply a complex transfer function. The phase change layer is described in simple, uniform terms above, however, the phase change layer characteristics can vary across the film with grain boundary regions, nonuniformity, etc. By generating a write waveform that is a function of the characteristics of the media, the processor provides the ability to read the current voltage or resistance before writing so that the write waveform generated is suited to the state of the media prior to writing. Further, the write waveform can be modified during the write depending on what it experiences. The processor can measure the effectiveness of the write afterwards and memorize a running history of how the active media region measures so that, for example, the tip can attempt to write to a portion of the media with a given set of characteristics. If the desired results are not achieved, the process can be reiterative, with the processing element modifying current, and/or wave shapes in order to optimize the final result. Such a circuit is adaptive to characteristics of the media and/or a tip, and can be particularly beneficial where such characteristics include a great deal of variability.
• Active Media Region Selection/Interconnect Reduction
• FIG. 13 is a circuit diagram of an exemplary media platform in accordance with one embodiment of the present invention. The exemplary media platform comprises64 active media regions. Each active media region is associated with a corresponding tip from a tip platform, the corresponding tip being positionable within the active media region by actuating one or both of the media and tip platform. The active media region can be sized, in one embodiment, as a 100 μm×100 μm square. If the tip is capable of writing a 20 nm bit, the active media region is capable of storing 25×10⁶bits—i.e., approximately 3 MB—packed side-by-side. A portion of each active media region can be dedicated to servos, headers and gap, for example 10%. The tip platform can rely on clocking and a servo scheme, for example as known in the art, to position a tip over a desired bit.
• The active media regions of the exemplary platform are each associated with one of 16 bit-lines1372 and one of four banks1370. Each bit-line1372 is associated with a read/write circuit, and is connected with the exemplary platform when atransmission gate836 is closed and the media platform is active. The transmission gate controls the current source or the ground plane underneath each media region. This is made possible by a discontinuous ground plane. Rather than using a common ground plane, for example as in the embodiment illustrated inFIGS. 8 and 9, the ground plane is selectably activated. Each bank1370 is associated with one active media region from each of the 16 bit-lines1372. As can be seen inFIG. 1, a die havinginterconnect nodes102 for each required signal can become extremely dense with wiring. As can be seen, each platform of the exemplary die includes sixteen tips. Each tip is individually addressed, therefore each platform includes sixteen wires. Further, each actuator includes a dual wire ground created eight additional wires for each platform. In adie100 having sixteen platform, this results in384separate interconnect nodes102 in an approximately 10 mm space and/or384 separate read/write circuits—a complicated package and/or chip to manufacture. The problem is exacerbated with platforms having 64 tips, and1152 interconnects are required for a sixteen cell die.
• Methods and systems in accordance with the present invention can comprise selectable banks1370 for accessing groups of (or individual) tips. The banks1370 can reduce the complexity of read/write circuitry, and the number of interconnects between the platform and the read/write circuitry by activating only one of the active media regions for each bit-line. A supervisor processor controls the memory system and provides a signal to one of the banks1370 closing the media region select switch (i.e., the transmission gate)836 associated with a corresponding active media region for each of the bit-lines1372, completing the circuit between the bit-line1372 and the active-media region, as described above in reference toFIG. 10-12. Thetransmission gates836 thus allow electronic tip selection by controlling the current flow or the ground plane beneath the media region. The tips can be connected with a common voltage source or sinked to a common ground, reducing the complexity of the read/write engine (the die comprising tip platforms). Thetransmission gates836 can be formed, for example, beneath the media regions on the media die.
• FIG. 14 is a circuit diagram of a single memory chip comprising16 platforms. Each media platform is connected with the16 bit-lines, but only one platform is active at a time. The active platform is activated by a decoder (i.e., a multiplexer or demultiplexer) that receives input from four platform select bits, and selects the appropriate media platform. A plurality of chips can be bussed (connected in common with the 16 bit-lines) and controlled by a single supervisor processor. Further, the bank select and platform select signals from the supervisor processor can be bussed, so that only a corresponding chip select signal need by uniquely routed to each chip. Thus, the supervisor processor can select the bank and platform, and select the chip to determine which of the 16 tips from which to receive data. Further, signals from the supervisor processor are sent to the platform to position the media platform relative to the tip platform, thereby selecting a desired bit within the active media region. Each exemplary chip includes16 platforms having four actuators, therefore each exemplary chip includes64 actuators. The actuators signals are multiplexed, and based on the bank and platform select signals.
• FIG. 15 is a circuit diagram of an alternative embodiment of a media platform in accordance with the present invention. Each active media region includes an insulated portion, or dead spot isolation region. The dead spots can be formed in the film stack of the active media using different techniques. For example, the dead spot can be formed by partially etching the film stack, filling the etched portion with an oxide, and polishing the film stack using chemical-mechanical polishing (CMP). Alternatively, nitride can be deposited on the film stack, a portion of the film stack can be etched to form dead spots, a thermal oxide can be grown in the etched portions, and the nitride can be stripped. Myriad different manufacturing techniques can be applied to form an insulated isolation region within or on a conductive material. One of ordinary skill in the art can appreciate the different methods for forming features in a film stack. In one embodiment, the dead spot is an indentation in the film stack surface, such that a tip rests in the spot. The tip can rest in the shallow depression until the actuators of an associated tip platform (or a corresponding media platform) are activated. A platform positioned such that the tips rest in dead spots in the active media region is isolated from the bit-lines, therefore a platform select signal is not required, and the circuitry is simplified. The actuator signals are separately controlled, and either the media platform or the tip platform is controlled. When the platform is repositioned off-center a circuit is completed between the tip and the bit-line.
• Each insulated portion can be a relative small portion within the active media region. The insulated portion is sized according to manufacturing tolerances of the platforms and/or tips, thermal expansion of each platform relative to one another, etc., and in one embodiment can comprise a 5 μm×5 μm square. An isolation region of that size within a 100 μm×100 μm active media region consumes only 0.25% of the active media region surface.
• FIG. 16 is a circuit diagram of an alternative embodiment of a single memory chip comprising 16 platforms, with each platform including active media regions having dead spots. Only one platform is active at a time, because only one platform is positioned off of the dead spot. The decoder ofFIG. 14 and the chip select signal is no longer required. A plurality of chips can be bussed, connecting the 16 bit-lines and bank select signals in common. Further, the bank select and platform select signals from the supervisor processor can be bussed, so that only a corresponding chip select signal need by uniquely routed to each chip. Thus, the supervisor processor can select the bank and platform, and select the chip to determine which of the 16 tips from which to receive data. Further, signals from the supervisor processor are sent to the platform to position the media platform relative to the tip platform, thereby selecting a desired bit within the active media region. Each exemplary chip includes 16 platforms having four actuators, therefore each exemplary chip includes 64 actuators. The actuators signals are multiplexed, and based on the bank and platform select signals.
• The foregoing description of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

Claims (61)

1. A data storage device, comprising:

a media, including:

a conductive under-layer;

an over-layer; and

a phase change layer disposed between the conductive under-layer and the over-layer;

a tip positioned in proximity to the media such that a current can flow between the tip and the media;

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to a threshold temperature;

wherein the circuit and over-layer are adapted to focus the current so that the portion is narrower in width than the tip.

2. The data storage device ofclaim 1, wherein the circuit includes a bypass to direct the current to an electrical path not including the phase change layer so that information forms in the portion.

3. The data storage device ofclaim 1, wherein the over-layer is a conductive material that conducts better through the over-layer than across the over-layer.

4. The data storage device ofclaim 1, wherein the over-layer is a conductive material that has a higher conductivity along a plane substantially perpendicular to a surface of the media than along a plane substantially parallel to a surface of the media.

5. The data storage device ofclaim 1, wherein the over-layer is an anisotropic columnar material.

6. The data storage device ofclaim 1, wherein the over-layer is one of titanium nitride and microcrystalline silicon.

7. The data storage device ofclaim 1, wherein the phase change layer has one of a crystalline and amorphous structure, the phase change layer having a bulk resistivity.

8. The data storage device ofclaim 7, wherein the phase change layer is a chalcogenide.

9. The data storage device ofclaim 1, wherein the circuit is adapted to apply a current between the tip and the media by applying a first voltage potential across the tip and the media.

10. The data storage device ofclaim 9, wherein the first voltage potential is applied as a waveform.

11. The data storage device ofclaim 10, wherein the waveform is one of a pulse, a triangle, a saw-tooth, and a trailing edge.

12. The data storage device ofclaim 1, wherein:

the tip includes a radius of curvature; and

to focus the current, the circuit:

applies the current so that a portion of the current flowing between the tip and the media decreases as the radius of curvature curves away from the media; and

allows the current to be at least partially confined by the over-layer so that a flow of the current is substantially columnar.

13. A data storage device, comprising:

a media, including:

a conductive under-layer;

an over-layer; and

a phase change layer disposed between the conductive under-layer and the over-layer;

a tip;

a computer readable media having instruction to perform the steps of:

positioning the tip in proximity to the media such that a current can flow between the tip and the media;

applying the current between the tip and the media so that a portion of the phase change layer is heated to a threshold temperature;

removing the current from between the tip and the media so that information is formed within the portion;

wherein the current is focused by one or both of the over-layer and the computer readable medium so that the portion is narrower than a width of the tip.

14. The data storage device ofclaim 13, wherein:

the tip includes a radius of curvature; and

the computer readable medium can focus the current by limiting a magnitude of the current so that a portion of the current flowing between the tip and the media decreases as the radius of curvature curves away from the media.

15. The data storage device ofclaim 14, wherein the over-layer is a conductive material that conducts better through the over-layer than across the over-layer.

16. The data storage device ofclaim 14, wherein the over-layer is a conductive material that has a higher conductivity along a plane substantially perpendicular to a surface of the media than along a plane substantially parallel to a surface of the media.

17. The data storage device ofclaim 14, wherein the over-layer is an anisotropic columnar material.

18. The data storage device ofclaim 17, wherein the over-layer is one of titanium nitride and microcrystalline silicon.

19. The data storage device ofclaim 18, wherein the phase change layer has one of a crystalline and amorphous structure, the phase change layer having a bulk resistivity.

20. The data storage device ofclaim 19, wherein the phase change layer is a chalcogenide.

21. The data storage device ofclaim 14, wherein applying a current between the tip and the media includes applying a voltage potential across the tip and the media.

22. The data storage device ofclaim 21, wherein the voltage potential is applied as a waveform.

23. The data storage device ofclaim 22, wherein the waveform is one of a pulse, a triangle, a saw-tooth, and a trailing edge.

24. A memory apparatus, comprising:

a moveable media including:

a moveable media platform, and

a plurality of media devices connected with the moveable media platform, at least one of the media devices having a conductive under-layer, an over-layer, and a phase change layer disposed between the under-layer and the over-layer;

a moveable read/write mechanism, including:

a moveable read/write platform; and

a plurality of tips connected with said moveable read/write platform;

a media movement mechanism operably attached to said moveable media and configured to move said media platform in response to media control signals; and

a read/write platform movement mechanism operably attached to said read/write platform and configured to move said read/write platform in response to read/write platform control signals; and

a computer readable media having instruction to perform the steps of:

positioning the at least one tip in proximity to the at least one media such that a current can flow between the at least one tip and the at least one media;

applying the current between the at least one tip and the at least one media so that a portion of the phase change layer is heated to a threshold temperature;

removing the current from between the at least one tip and the at least one media so that information is formed within the portion;

wherein the current is focused by one or both of the over-layer and the computer readable medium.

25 The memory apparatus ofclaim 24, wherein:

the at least one tip includes a radius of curvature; and

the computer readable medium can focus the current by limiting a magnitude of the current so that a portion of the current flowing between the tip and the media decreases as the radius of curvature curves away from the media.

26. The memory apparatus ofclaim 24, wherein the over-layer is a conductive material that conducts better through the over-layer than across the over-layer.

27. The memory apparatus ofclaim 24, wherein the over-layer is a conductive material that has a higher conductivity along a plane substantially perpendicular to a surface of the media than along a plane substantially parallel to a surface of the media.

28. The memory apparatus ofclaim 24, wherein the over-layer is an anisotropic columnar material.

29. The memory apparatus ofclaim 28, wherein the over-layer is one of titanium nitride and microcrystalline silicon.

30. The memory apparatus ofclaim 24, wherein the phase change layer is a chalcogenide.

31. The memory apparatus ofclaim 24, wherein applying a current between the tip and the media includes applying a voltage potential across the tip and the media.

32. The memory apparatus ofclaim 31, wherein the voltage potential is applied as a waveform.

33. The memory apparatus ofclaim 32, wherein the waveform is one of a pulse, a triangle, a saw-tooth, and a trailing edge.

34. A data storage device, comprising:

a media, including:

a quench layer;

a tunneling layer; and

a phase change layer disposed between the quench layer and the tunneling layer;

a tip positioned in proximity to the media such that a current can flow between the tip and the media;

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to a threshold temperature;

wherein the circuit and over-layer are adapted to focus the current so that the portion is narrower in width than the tip; and

wherein the circuit includes a bypass to direct the current to an electrical path not including the phase change layer so that information forms in the portion.

35. The data storage device ofclaim 34, wherein the tunneling layer is a conductive material that conducts better through the tunneling layer than across the tunneling layer.

36. The data storage device ofclaim 34, wherein the tunneling layer is one of titanium nitride and microcrystalline silicon.

37. The data storage device ofclaim 34, wherein the phase change layer has one of a crystalline and amorphous structure, the phase change layer having a bulk resistivity.

38. The data storage device ofclaim 37, wherein the phase change layer is a chalcogenide.

39. The data storage device ofclaim 34, wherein the circuit is adapted to apply a current between the tip and the media by applying a first voltage potential across the tip and the media.

40. The data storage device ofclaim 39, wherein the first voltage potential is applied as a waveform.

41. The data storage device ofclaim 40, wherein the waveform is one of a pulse, a triangle, a saw-tooth, and a trailing edge.

42. A data storage device including a media having a phase change layer and a tip including a radius of curvature and positioned in proximity to the media such that a current can flow between the tip and the media, the current being adapted to heat a portion of the phase change layer to a threshold temperature, the improvement comprising:

an over-layer disposed between the tip and the phase change layer, the over-layer being adapted to focus the current so that the portion is narrower in width than the tip.

43. A data storage device, comprising:

a media including an over-layer and a phase change layer;

a tip being positioned in proximity to the media such that a current can flow between the tip and the media; and

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to at least a threshold temperature;

wherein the circuit includes a mechanism to direct the current along an electrical path not including the phase change layer so that an indicia forms in the portion.

44. The data storage device ofclaim 43, wherein the over-layer is a conductive material that conducts better through the over-layer than across the over-layer.

45. The data storage device ofclaim 43, wherein the over-layer is a conductive material that has a higher conductivity along a plane substantially perpendicular to a surface of the media than along a plane substantially parallel to a surface of the media.

46. The data storage device ofclaim 43, wherein the over-layer is an anisotropic columnar material.

47. The data storage device ofclaim 46, wherein the over-layer is titanium nitride.

48. The data storage device ofclaim 46, wherein the over-layer is microcrystalline silicon.

49. The data storage device ofclaim 43, wherein:

the phase change layer has a crystalline structure; and

the bit has an amorphous structure.

50. The data storage device ofclaim 43, wherein:

the phase change layer has an amorphous structure; and

the bit has a crystalline structure.

51. The data storage device ofclaim 50, wherein the phase change layer is a chalcogenide.

52. The data storage device ofclaim 43, wherein the circuit is adapted to apply a first current between the tip and the media by applying a first voltage potential across the tip and the media.

53. The data storage device ofclaim 52, wherein the first voltage potential is applied as a waveform.

54. The data storage device ofclaim 53, wherein the waveform is one of a pulse, a triangle, a saw-tooth, and a trailing edge.

55. A data storage device, comprising:

a media including an over-layer and a phase change layer;

wherein the over-layer is a conductive material that conducts better through the over-layer than across the over-layer;

a tip being positioned in proximity to the media such that a current can flow between the tip and the media; and

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to at least a threshold temperature.

56. A data storage device, comprising:

a media including an over-layer and a phase change layer;

wherein the over-layer is a conductive material that has a higher conductivity along a plane substantially perpendicular to a surface of the media than along a plane substantially parallel to a surface of the media;

a tip being positioned in proximity to the media such that a current can flow between the tip and the media; and

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to at least a threshold temperature.

57. A data storage device, comprising:

a media including an over-layer and a phase change layer;

wherein the over-layer is an anisotropic columnar material;

a tip being positioned in proximity to the media such that a current can flow between the tip and the media; and

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to at least a threshold temperature.

58. A data storage device including a media having a phase change layer and a tip positioned in proximity to the media such that a current can flow between the tip and the media, the current being adapted to heat a portion of the phase change layer to a threshold temperature, the improvement comprising:

an over-layer disposed between the tip and the phase change layer, the over-layer being adapted to focus the current;

wherein the over-layer is a conductive material that conducts better through the over-layer than across the over-layer.

59. A data storage device including a media having a phase change layer and a tip positioned in proximity to the media such that a current can flow between the tip and the media, the current being adapted to heat a portion of the phase change layer to a threshold temperature, the improvement comprising:

an over-layer disposed between the tip and the phase change layer, the over-layer being adapted to focus the current;

wherein the over-layer is a conductive material that has a higher conductivity along a plane substantially perpendicular to a surface of the media than along a plane substantially parallel to a surface of the media.

60. A data storage device including a media having a phase change layer and a tip positioned in proximity to the media such that a current can flow between the tip and the media, the current being adapted to heat a portion of the phase change layer to a threshold temperature, the improvement comprising:

a circuit adapted to apply a current between the tip and the media so that a portion of the phase change layer is heated to at least a threshold temperature;

wherein the circuit includes a mechanism to direct the current along an electrical path not including the phase change layer so that an indicia forms in the portion.

61. The memory apparatus ofclaim 24, wherein focusing the current includes:

applying the current so that a portion of the current flowing between the tip and the media decreases as the radius of curvature curves away from the media; and

allowing the current to be at least partially confined by the over-layer so that a flow of the current is substantially columnar.

Images