US20010033509A1 - Stacked integrated circuits - Google Patents

Abstract

System modules are described which include a stack of interconnected semiconductor dies. The semiconductor dies are interconnected by micro bump bonding of coaxial lines that extend through the thickness of the various dies. The coaxial lines also are selectively connected to integrated circuits housed within the dies. In one embodiment, a number of memory dies are interconnected in this manner to provide a memory module.

Description

TECHNICAL FIELD OF THE INVENTION
• The present invention relates generally to the field of integrated circuits and, in particular, to stacked integrated circuits.[0001]
BACKGROUND OF THE INVENTION
• Integrated circuits form the basis for many electronic systems. Essentially, an integrated circuit includes a vast number of transistors and other circuit elements that are formed on a single semiconductor wafer or chip and are interconnected to implement a desired function. The complexity of these integrated circuits requires the use of an ever increasing number of linked transistors and other circuit elements.[0002]
• Many electronic systems are created through the use of a variety of different integrated circuits; each integrated circuit performing one or more specific functions. For example, computer systems include at least one microprocessor and a number of memory chips. Conventionally, each of these integrated circuits is formed on a separate wafer or chip, packaged independently and interconnected on, for example, a printed circuit board.[0003]
• As integrated circuit technology progresses, there is a growing desire for a “system on a chip” in which the functionality of all of the integrated circuits of the system are packaged together without a conventional printed circuit board. Ideally, a computing system would be fabricated with all the necessary integrated circuits on one wafer, as compared with today's method of fabricating many chips of different functions and packaging them to assemble a complete system. Such a structure would greatly improve integrated circuit performance and provide higher bandwidth.[0004]
• In practice, it is very difficult with today's technology to implement a truly high-performance “system on a chip” because of vastly different fabrication processes and different manufacturing yields for the logic and memory circuits.[0005]
• As a compromise, various “system modules” have been introduced that electrically connect and package integrated circuit devices which are fabricated on the same or on different semiconductor wafers. Initially, system modules were created by simply stacking two semiconductor chips, e.g., a logic and memory chip, one on top of the other in an arrangement commonly referred to as chip-on-chip (COC) structure. Chip-on-chip structures most commonly use micro bump bonding technology (MBB) to electrically connect the working surfaces of two chips. Several problems, however, remain inherent with this design structure. For example, this approach is limited in the number of chips that can be interconnected as part of the system module.[0006]
• Some researchers have attempted to develop techniques for interconnecting a number of chips in a stack to form a system module. However, these modules suffer from additional problems. For example, some modules use chip carriers that make the packaging bulky. Further, others use wire bonding that gives rise to stray inductances that interfere with the operation of the system module.[0007]
• Thus, it is desirable to develop an improved structure and method for interconnecting integrated circuits on separate chips or wafers in a system module.[0008]
SUMMARY OF THE INVENTION
• The above mentioned problems with integrated circuits and other problems are addressed by the present invention and will be understood by reading and studying the following specification. System modules are described which include a stack of interconnected semiconductor chips, wafers or dies. The semiconductor dies are interconnected by micro bump bonding of coaxial conductors that extend through the thickness of the various dies. The coaxial lines also are selectively connected to integrated circuits housed within the dies. In one embodiment, a number of memory dies are interconnected in this manner to provide a memory module.[0009]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view of an embodiment of a system module according to the teachings of the present invention.[0010]
• FIG. 2 is a cross sectional view of a semiconductor chip or die of the system module of FIG. 1.[0011]
• FIG. 3 is a cross sectional view of an embodiment of a coaxial conductor.[0012]
• FIGS. 4, 5,[0013]6,7,8, and9 are elevational views of a semiconductor wafer at various points of an illustrative embodiment of a method according to the teachings of the present invention.
• FIG. 10 is a block diagram of an embodiment of an electronic system according to the teachings of the present invention.[0014]
DETAILED DESCRIPTION
• In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.[0015]
• In the following description, the terms chip, die, wafer and substrate are interchangeably used to refer generally to any structure, or portion of a structure, on which integrated circuits are formed, and also to such structures during various stages of integrated circuit fabrication. The terms include doped and undoped semiconductors, epitaxial layers of a semiconductor on a supporting semiconductor or insulating material, combinations of such layers, as well as other such structures that are known in the art.[0016]
• The term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizonal as defined above. Prepositions, such as “on”, “side” (as in “sidewall”), “higher”, “lower”, “over” and “under” are defined with respect to the conventional plane or surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate.[0017]
I. System Module
• FIG. 1 is a perspective view of an embodiment of a system module, indicated generally at[0018]100, according to the teachings of the present invention.System module100 includes a plurality of semiconductor chips102-1, . . . ,102-N that are disposed and interconnected in a stack to provide “chip-sized” packaging. Each semiconductor chip102-1, . . . ,102-N includes integrated circuits106-1, . . . ,106-N, respectively. In one embodiment, semiconductor chips102-1, . . . ,102-N comprise semiconductor dies with memory circuits such as dynamic random access memory circuits. Thus, in this embodiment,system module100 is a “memory module” or “memory cube.” In other embodiments, integrated circuits106-1, . . . ,106-N comprise other appropriate circuits such as logic circuits.
• [0019]System module100 uses microbumps104 to interconnect the integrated circuits106-1, . . . ,106-N. In one embodiment, microbumps104 comprise controlled-collapse chip connections (C-4) solder pads. Other appropriate materials can be used to formmicrobumps104.Microbumps104 are formed on first sides108-1, . . . ,108-N and second sides110-1, . . . ,10-N of semiconductor chips102-1, . . . ,102-N. The microbumps104 are connected to coaxial conductors, described in more detail below, to carry signals between semiconductor wafers102-1, . . . ,102-N. Microbumps104 are selectively formed using, for example, a vacuum deposition through a mask. The deposited material is then reflowed to homogenize lead and tin as the microbumps. Selected microbumps104 are aligned and bonded, e.g., the microbumps indicated byarrow112, by bringing therespective microbumps104 into contact at an appropriate temperature.
II. Coaxial Conductors and Microbumps
• FIG. 2 is a cross sectional view of a semiconductor chip, indicated generally at[0020]102-I, according to an embodiment of the present invention. Semiconductor chip102-I includescoaxial conductors202 that are formed in high aspect ratio vias through the thickness of semiconductor chip102-I. Thecoaxial conductors202 have an aspect ratio in the range of approximately100 to200. Conventionally, a semiconductor wafer used to form an integrated circuit has a thickness in the range of approximately 500 to 1000 microns. Thus, the high aspect ratio vias can be fabricated with a width that is in the range from approximately 2.5 microns up to as much as approximately 10 microns.
• [0021]Coaxial conductors202 includecenter conductor204 that is surrounded by insulator, e.g., oxide,206. Further,outer conductor208surrounds insulator206.Coaxial conductor202 is shown in cross section in FIG. 3.Outer conductor208 comprises, for example, a metal layer that is deposited within the high aspect ratio via. Alternatively,outer conductor208 may comprise a portion of semiconductor chip102-I that has been doped with impurities to render it conductive.
• [0022]Coaxial conductor202 is selectively coupled to integrated circuit106-I using ametallization layer210. Further, microbumps104 are formed outwardly from themetallization layer210 to provide for interconnection with other semiconductor wafers in a stack to form a system module.
• Advantageously,[0023]coaxial conductors202 allow a number of semiconductor wafers to be interconnected in a stack with an increased density over other system modules. The space between semiconductor wafers insystem module100 is on the order of a few microns, e.g., the thickness of two bonded, solder microbumps. Assuming a wafer thickness on the order of 750 microns, eight semiconductor wafers can be stacked to form a system module with a thickness on the order of 6 millimeters. This compact system module can be readily mounted into a variety of system packages.
• The use of microbumps to interconnect the semiconductor wafers in a stack provides additional advantages. For example, stray capacitance, stray inductance and series resistance are all reduced over other system modules. This ultimately results in improved performance.[0024]
• The[0025]coaxial conductor202 shown in FIGS. 2 and 3 have, for simplicity, a geometry such that:$\frac{1}{\pi }\ln \left(\frac{r_2}{r_1}\right)=1$

  
• In this case, the capacitance of coaxial conductor is:[0026]$C=2\frac{\pi }{\ln \left(\frac{r_2}{r_1}\right)}e_re_0d$

  
• The term e[0027]_re₀is the electric permittivity ofinsulator layer206 and d is the length ofcoaxial conductor202. Ifinsulator layer206 is silicon dioxide and the coaxial conductor has a length of approximately 750 microns, then the capacitance is approximately 50 femptofarads (fF). Likewise, the inductance can be calculated as follows:$L={\mu }_0\left(\frac{1}{2\pi }\right)\ln \left(\frac{r_2}{r_1}\right)d$

  
• In this equation, μ[0028]₀is the magnetic permeability of free space. Continuing with the same assumptions, this provides an inductance on the order of 0.5 nanohenries (nil). These values are less than those associated with a conventional wire bond, e.g., 500 to 1000 fF and 1-2 nH. Further, the large stray capacitances and inductances (1-7 picofarads (pF) and 2-35 nH) associated with a package and even larger capacitances and inductances associated with a printed circuit board are avoided.
• The microbumps of[0029]system100 have only a small stray capacitance, e.g.,100 to 500 fF, and a small inductance of less than 0.1 nH. The net result is that the interconnection between the semiconductor chips102-1, . . . ,102-N ofsystem100 can be made with about the same stray capacitance and inductance as that of a single wire bond. Alternative connection techniques would require significantly more wire bonds and huge stray inductances and capacitances associated with the packaging and even larger strays associated with interconnection of the packages.
• [0030]Coaxial conductors202 can be added to circuits using a conventional layout for the circuit without adversely affecting the surface area requirements of the circuit. Conventional circuits typically include pads formed on the top surface of the semiconductor wafer that are used to connect to various signals for the system. The bond wires of conventional circuits can be replaced bycoaxial conductors202 and microbumps104 to achieve the advantages described above.
III. Formation of Coaxial Conductors and Microbumps
• FIGS. 4, 5,[0031]6,7,8, and9 are elevational views of semiconductor chip102-I at various points of an illustrative embodiment of a method for forming an integrated circuit with coaxial conductors according to the teachings of the present invention.Functional circuit402 is formed in an active region ofsemiconductor wafer400. In one embodiment,semiconductor wafer400 comprises a monocrystalline silicon wafer. For purposes of clarity, the Figures only show the formation of two coaxial conductors throughsemiconductor wafer400. However, it is understood that with a particular functional circuit any appropriate number of vias can be formed throughsemiconductor wafer400. For example, the number of vias needed for a conventional dynamic random access memory (DRAM) may be on the order of100. Essentially, the coaxial conductors are formed in the same space on the surface ofsemiconductor wafer400 that is conventionally used to form bond pads to be connected to leads. The coaxial conductors replace the conventional bond wires which couple the bond pads to selected leads of a lead frame in the packaging of the semiconductor wafer.
• As shown in FIG. 4,[0032]photoresist layer404 is formed onsurface406 ofsemiconductor substrate400.Photoresist layer404 is patterned to provideopenings408 at points onsurface406 where high aspect ratio holes are to be formed throughsemiconductor wafer400.
• As shown in FIG. 5, etch pits[0033]410 are formed by conventional alkaline etching throughopenings408 inphotoresist layer404.Photoresist layer404 is then removed.
• FIG. 6 is a schematic diagram that illustrates an embodiment of a layout of equipment used to carry out an anodic etch that is used to form high aspect ratio holes[0034]450 of FIG. 7. Typically, holes450 have an aspect ratio in the range of100 to200.Bottom surface462 ofsemiconductor wafer400 is coupled tovoltage source434 bypositive electrode430. Further,negative electrode432 is coupled tovoltage source434 and is placed in a bath of 6% aqueous solution of hydrofluoric acid (HF) onsurface406 ofsemiconductor wafer400.
• In this example,[0035]illumination equipment436 is also included becausesemiconductor wafer400 is n-type semiconductor material. When p-type semiconductor material is used, the illumination equipment is not required.Illumination equipment436 assures that there is a sufficient concentration of holes insemiconductor wafer400 as required by the anodic etching process.Illumination equipment436 includeslamp438,IR filter440, andlens442.Illumination equipment436 focuses light onsurface462 ofsemiconductor wafer400.
• In operation, the anodic etch etches high aspect ratio holes through[0036]semiconductor wafer400 at the location of etch pits410.Voltage source434 is turned on and provides a voltage across positive andnegative electrodes430 and432. Etching current flows frompositive electrode430 tosurface406. This current forms the high aspect ratio holes throughsemiconductor wafer400. Further, illumination equipment illuminatessurface462 ofsemiconductor wafer400 so as to assure a sufficient concentration of holes for the anodic etching process. The size and shape of the high aspect ratio holes throughsemiconductor wafer400 depends on, for example, the anodization parameters such as HF concentration, current density, and light illumination. An anodic etching process is described in V. Lehmann,The Physics of Macropore Formation in Low Doped n-Type Silicon,J. Electrochem. Soc., Vol. 140, No. 10, pp. 2836-2843, Oct. 1993, which is incorporated herein by reference.
• FIG. 7 illustrates the formation of the[0037]outer conductor454 of a coaxial conductor.Outer conductor454 can be formed in at least one of two ways. First,outer conductor454 can be formed using a low pressure chemical vapor deposition of tungsten in a self-limiting process which provides a tungsten film oninner surface452 ofholes450 by silicon reduction. Accordingly, silicon material withinholes450 is replaced by tungsten atoms in a WF₆reaction gas. A reaction product, SiF₄is pumped out or otherwise removed from the reaction chamber. This can be followed by a silane or polysilane reduction of the WF₆until a desired thickness is achieved. Deposition rates for this process are dependent on temperature and reaction gas flow rate. Deposition rates of 1 micron per minute have been observed at temperatures of 300° C. and with a flow rate of WF₆at 4 sccm in a cold wall CVD reactor.
• Alternatively,[0038]outer conductor454 can be formed as diffusion regions withinsemiconductor wafer400 alonginner surface450. To accomplish this, surfaces406 and462 are masked by a masking layer and conductivity enhancing impurities are introduced. The impurities formouter conductor454 as, for example, n+ regions.
• FIGS. 8 and 9 illustrate the completion of the coaxial conductors. First, an insulator material, e.g., silicon dioxide, is formed in[0039]holes450 along the length ofouter conductor454 to forminsulator layer455.Insulator layer455 is formed so as to leave an opening extending through the thickness ofsemiconductor wafer400.
• Next, a process of aluminum/polysilicon substitution is used to fill the remaining portion of[0040]holes450 with aluminum. Such a process is described in H. Horie et al.,Novel High Aspect Ratio Aluminum Plug for Logic/DRAM LSIs Using Polysilicon-Aluminum Substitute,Dig. IEEE Int. Electron Device Meeting, San Francisco, pp. 946-948, 1996, which is incorporated herein by reference. First,hole450 is filled with a layer ofpolysilicon456 by a process of chemical vapor deposition (CVD). It is noted that, conventionally, such a deep trench cannot be filled directly with aluminum using a direct chemical vapor deposition technique. However, conventionally, polysilicon has been deposited in holes with such high aspect ratios, e.g., deep trenches for trench capacitors. Once the holes are filled with polysilicon, excess polysilicon onsurface406 is removed by, for example, chemical/mechanical polishing. Aluminum layers458 and460 are deposited onsurfaces406 and462 using, for example, a sputtering technique used to coat optical disks.Layers458 and460 have a thickness on the order of a few microns. The structure shown in FIG. 8 is then annealed at 500 degrees Celsius in Nitrogen ambient. This allows the aluminum material oflayers458 and460 to be substituted for thepolysilicon456 inholes450. The displaced polysilicon and any residual aluminum fromlayers458 and460 are removed by, for example, chemical/mechanical polishing. By depositing a thin, e.g, 0.1 μm, layer of titanium on top oflayers458 and460 the above mentioned anneal can be reduced from 500° Celsius to 450° Celsius. The structure is now as shown in FIG. 9 withcoaxial conductors464 extending throughsemiconductor wafer400.
IV. Electronic System
• FIG. 10 is a block diagram of an embodiment of an electronic system, indicated generally at[0041]500, and constructed according to the teachings of the present invention.System500 includesprocessor504 andmemory module502.Memory module502 includes a number of memory circuits that are fabricated on separate semiconductor chips or dies. These dies include a plurality of coaxial conductors that are formed through the thickness of their respective dies as described in more detail above. These coaxial conductors are interconnected with other semiconductor chips using a microbump bonding, e.g., C-4 type microbumps.
Conclusion
• Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. For example, the high aspect ratio vias can be applied in a wide variety of circuits including but not limited to dynamic random access memory devices, logic circuits, and other appropriate circuits. Further, other techniques can be used to form and fill the high aspect ratio holes to form the coaxial conductors. Further, the high aspect ratio vias can be filed with a conductive material without forming a coaxial conductor.[0042]

Claims (31)

What is claimed is:

1. A system module, comprising:

a plurality of stacked semiconductor chips each including an integrated circuit;

each semiconductor chip including a plurality of vias formed through the thickness of the semiconductor chip;

a plurality of conductors, each conductor having first and second opposite ends and formed in one of the vias;

each conductor selectively interconnected with the integrated circuit of its semiconductor chip; and

a plurality of microbumps, each microbump formed on an end of a selected conductor so as to interconnect the integrated circuits of the plurality of stacked semiconductor chips.

2. The system module of

claim 1

, wherein the integrated circuits comprise memory circuits.

3. The system module of

claim 1

, wherein each conductor comprises a coaxial conductor.

4. The system module of

claim 1

, wherein the microbumps comprise controlled-collapse chip connections (C-4) solder pads.

5. The system module of

claim 1

, wherein each conductor comprises:

an outer conductive layer formed along a wall of a selected via;

an insulator layer; and

an inner conductive layer substantially parallel to and separated from the outer conductive layer by the insulator layer.

6. The system module of

claim 1

, wherein the semiconductor chips each comprise a die having a random access memory circuit.

7. A memory module, comprising:

a plurality of stacked semiconductor chips each including a memory circuit;

each semiconductor chip including a plurality of vias formed through the thickness of the semiconductor chip;

a plurality of coaxial conductors, each having first and second opposite ends and formed in one of the vias;

each coaxial conductor selectively interconnected with the memory circuit of its semiconductor chip; and

a plurality of microbumps, each microbump formed on an end of a selected conductor so as to interconnect the memory circuits of the plurality of stacked semiconductor chips.

8. The memory module of

claim 7

, wherein the microbumps comprise controlled-collapse chip connections (C-4) solder pads.

9. The memory module of

claim 7

, wherein each conductor comprises:

an outer conductive layer formed along a wall of a selected via;

an insulator layer; and

an inner conductive layer substantially parallel to and separated from the outer conductive layer by the insulator layer.

10. The memory module of

claim 9

, wherein the outer conductive layer comprises a layer of doped semiconductor material.

11. The memory module of

claim 9

, wherein the outer conductive layer comprises a metal layer that lines a surface of the via.

12. A method for interconnecting integrated circuits to form a system module, the method comprising:

selectively forming microbumps on first and second opposite surfaces of a plurality of semiconductor chips, each semiconductor chip having an integrated circuit that is formed in at least one working surface of the semiconductor chip;

selectively aligning the plurality of semiconductor chips to form a stack; and

for each interface between adjacent semiconductor chips in the stack, bonding the microbumps on the surface of one semiconductor chip with the microbumps on the surface of the other, adjacent semiconductor chip.

13. The method of

claim 12

, wherein selectively forming microbumps comprises selectively forming controlled-collapse chip connection (C-4) solder pads.

14. The method of

claim 12

, wherein selectively aligning the plurality of semiconductor chips comprises aligning corresponding microbumps on adjacent surfaces of the plurality of semiconductor chips in the stack.

15. The method of

claim 12

, wherein bonding the microbumps comprises bringing the microbumps into contact with each other.

16. A method for interconnecting memory circuits to form a memory module, the method comprising:

selectively forming microbumps on first and second opposite surfaces of a plurality of semiconductor chips, each semiconductor chip having a memory circuit that is formed in at least one working surface of the semiconductor chip;

selectively aligning the plurality of semiconductor chips to form a stack; and

for each interface between adjacent semiconductor chips in the stack, bonding the microbumps on the surface of one semiconductor chip with the microbumps on the surface of the other, adjacent semiconductor chip.

17. The method of

claim 16

, wherein selectively forming microbumps comprises selectively forming controlled-collapse chip connection (C-4) solder pads.

18. The method of

claim 16

, wherein selectively aligning the plurality of semiconductor chips comprises aligning corresponding microbumps on adjacent surfaces of the plurality of semiconductor chips in the stack.

19. The method of

claim 16

, wherein bonding the microbumps comprises bringing the microbumps into contact with each other.

20. A system, comprising:

a processor circuit;

a memory module that is communicatively coupled to the processor circuit; and

wherein the memory module includes a plurality of semiconductor memory chips that are coupled in a stack by microbump bonding and coaxial conductors that extend through the thickness of the semiconductor chips.

21. The system of

claim 20

, wherein the memory module comprises:

a plurality of vias formed through the thickness of each semiconductor memory chip;

a plurality of coaxial conductors, each having first and second opposite ends and formed in one of the plurality of vias;

each coaxial conductor selectively interconnected with a memory circuit of its semiconductor memory chip; and

a plurality of microbumps, each microbump formed on an end of a selected conductor so as to interconnect the memory circuits of the plurality of stacked semiconductor memory chips.

22. The system of

claim 21

, wherein the microbumps comprise controlled-collapse chip connections (C-4) solder pads.

23. The system of

claim 21

, wherein each conductor comprises:

an outer conductive layer formed along a wall of a selected via;

an insulator layer; and

an inner conductive layer substantially parallel to and separated from the outer conductive layer by the insulator layer.

24. The system of

claim 21

, wherein the outer conductive layer comprises a layer of doped semiconductor material.

25. The system of

claim 21

, wherein the outer conductive layer comprises a metal layer that lines a surface of the via.

26. A memory cube, comprising:

a plurality of semiconductor dies;

each semiconductor die including a memory circuit formed in a working surface of the semiconductor die;

a plurality of coaxial conductors formed through the thickness of each semiconductor die and having first and second opposite ends;

a metallization layer formed on the working surface of each semiconductor die to selectively interconnect the coaxial conductors with the memory circuit;

a plurality of microbumps on each semiconductor die, each microbump coupled to an end of a selected coaxial conductor; and

wherein the semiconductor dies are disposed in a stack with microbumps on surfaces of adjacent semiconductor dies being bonded together to interconnect the memory circuits in the memory cube.

27. The memory cube of

claim 26

, wherein the microbumps comprise controlled-collapse chip connections (C-4) solder pads.

28. The memory cube of

claim 26

, wherein each conductor comprises:

an outer conductive layer formed along a wall of a selected via;

an insulator layer; and

an inner conductive layer substantially parallel to and separated from the outer conductive layer by the insulator layer.

29. The memory cube of

claim 26

, wherein the outer conductive layer comprises a layer of doped semiconductor material.

30. The memory cube of

claim 26

, wherein the outer conductive layer comprises a metal layer that lines a surface of the via.

31. The memory cube of

claim 26

, wherein the microbumps are formed around a periphery of the semiconductor die.

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