US20060273375A1 - Transistor with nanocrystalline silicon gate structure - Google Patents

Abstract

A memory is described which has memory cells that store data using hot electron injection. The data is erased through electron tunneling. The memory cells are described as floating gate transistors wherein the floating gate is fabricated using a conductive layer of nanocrystalline silicon particles. Each nanocrystalline silicon particle has a diameter of about 10 Å to 100 Å. The nanocrystalline silicon particles are in contact such that a charge stored on the floating gate is shared between the particles. The floating gate has a reduced electron affinity to allow for data erase operations using lower voltages.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)
• This patent application is a continuation of U.S. patent application Ser. No. 11/028,475, filed Jan. 3, 2005, which is a continuation of U.S. patent application Ser. No. 10/453,324, filed Jun. 3, 2003, now U.S. Pat. No. 6,912,158, which is a continuation of U.S. patent application Ser. No. 10/176,425, now U.S. Pat. No. 6,574,144, which is a division of U.S. patent application Ser. No. 09/145,722, filed on Sep. 2, 1998, now U.S. Pat. No. 6,407,424, which is a continuation of U.S. patent application Ser. No. 08/790,500, filed on Jan. 29, 1997, now U.S. Pat. No. 5,852,306, the specifications of which are hereby incorporated by reference.
TECHNICAL FIELD OF THE INVENTION
• The present invention relates generally to memory circuits and in particular the present invention relates to floating gate memory cells.
BACKGROUND OF THE INVENTION
• Non-volatile memory such as electrically programmable read-only memory (EPROM) and electrically-erasable programmable read-only memory (EEPROM) are extensively used for storing data in computer systems. EPROM and EEPROM comprise a large number of memory cells having electrically isolated gates, referred to as floating gates. Data is stored in the memory cells in the form of charge on the floating gates. Charge is transported to or removed from the floating gates by program and erase operations, respectively.
• Another type of non-volatile memory is flash memory. Flash memory is a derivative of EPROM and EEPROM. Although flash memory shares many characteristics with EPROM and EEPROM, the current generation of flash memory differs in that erase operations are done in blocks.
• A typical flash memory comprises a memory array which includes a large number of memory cells arranged in row and column fashion. Each of the memory cells include a floating gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.
• Flash memories have the potential of replacing hard storage disk drives in computer systems. The advantages would be replacing a complex and delicate mechanical system with a rugged and easily portable small solid-state non-volatile memory system. There is also the possibilities that given their very high potential densities that given more speed of operation particularity in the erase operation that flash memories might be used to replace DRAMs. Flash memories might then have the ability to fill all memory needs in future computer systems.
• One flash memory is described in S. Tiwari et al., “Volatile and Non-volatile Memories in Silicon with Nano-Crystal Storage,” Abstr. of IEEE Int. Electron Device Meeting, pp. 521-524 (1995), which uses confined nano-crystal particles in a floating gate memory cell. The individual nano-crystals are not in electrical contact with each other, and therefore cannot share a common charge. As referred to in the art, the memory has a thin gate oxide and uses a tunnel-tunnel process for writing and reading data. A memory designed to use a tunnel-tunnel process typically has a gate oxide thickness of about 15-20 Å which can be degraded over time resulting in a defective memory.
• For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a fast flash memory having a floating gate memory cell which in which the floating gate has a reduced electron affinity, can share a common charge, or does not use a tunnel-tunnel process.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a cross-sectional view of a prior art memory cell;
• FIG. 2 is the memory cell ofFIG. 1 during programming;
• FIG. 3 is a cross-section of a memory cell incorporating a film of nanocrystalline silicon as a floating gate;
• FIG. 4 is a graph of barrier height versus tunneling distance; and
• FIG. 5 is a simplified block diagram of a typical flash memory incorporating the present invention.
DETAILED DESCRIPTION OF THE INVENTION
• In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions 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 inventions. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present inventions is defined only by the appended claims.
• FIG. 1 is a cross-sectional view of a typical memory cell, such as a used in a flash memory.Memory cell100 comprises a region of asource102 and a region of adrain104.Source102 anddrain104 are constructed from an N+type of high impurity concentration which are formed in a P-type semiconductor substrate106 of low impurity concentration.Source102 anddrain104 are separated by a predetermined space of achannel region108.Memory100 further includes afloating gate110 formed by a first polysilicon (poly) layer, and acontrol gate114 formed by a second poly layer. Floatinggate110 is isolated fromcontrol gate114 by an interpoly dielectric layer112 and fromchannel region108 by athin oxide layer116 approximately 100 angstroms (Å) thick.
• FIG. 2 is the memory cell ofFIG. 1 during a programming operation. During programming, a positive programming voltage of about12 volts is applied tocontrol gate114. This positive programming voltage attractselectrons120 from P-type substrate106 and causes them to accumulate at the surface ofchannel region108. A voltage ondrain104 is increased to about6 volts, and thesource102 is connected to ground. As the drain-to-source voltage increases,electrons120 flow fromsource102 to drain104 viachannel region108. Aselectrons120 travel towarddrain104, they acquire substantially large kinetic energy and are referred to as hot electrons.
• The voltages atcontrol gate114 anddrain104 creates an electric field inoxide layer116, this electric field attracts the hot electrons and accelerates them toward thefloating gate110. At this point, floatinggate110 begins to trap and accumulate the hot electrons and starts a charging process. Gradually, as the charge on the floating gate increases, the electric field inoxide layer116 decreases and eventually loses it capability of attracting any more of the hot electrons to floatinggate110. At this point, floatinggate110 is fully charged. The negative charge from the hot electrons collected in thefloating gate110 raises the cell's threshold voltage (Vt) above a logic 1 voltage. When a voltage oncontrol gate114 is brought to a logic 1 during a read operation, the cell will barely turn on. Sense amplifiers are used in the memory to detect and amplify the state of the memory cell during a read operation. Thus, data is read from a memory cell based upon its “on” characteristics.
• Electrons are removed from the floating gate to erase the memory cell. Many memories, including flash memories, use Fowler-Nordheim (FN) tunneling to erase a memory cell. The erase procedure is accomplished by electrically floating the drain, grounding the source, and applying a high negative voltage (−12 volts) to the control gate. This creates an electric field across the gate oxide and forces electrons off of the floating gate which then tunnel through the gate oxide. For a general description of how a flash memory having floating gate memory cells operates see B. Dipert et al., “Flash Memory Goes Mainstream,” IEEE Spectrum, pp. 48-52 (October 1993), and incorporated herein by reference.
• One of the difficulties with flash memories has been the erase operation using Fowler-Nordheim tunneling. The erase operation requires high voltages, and is relatively slow. Further, an erratic over erase can be induced as a result of the very high erase voltages used. These very high erase voltages are a fundamental problem arising from the high electron affinity of bulk silicon or large grain polysilicon particles used as the floating gate. This creates a very high tunneling barrier. Even with high negative voltages applied to the gate, a large tunneling distance is experienced with a very low tunneling probability for electrons attempting to leave the floating gate. This results in long erase times since the net flux of electrons leaving the gate is low. Thus, the tunneling current discharging the gate is low. In addition, other phenomena result as a consequence of this very high negative voltage. Hole injection into the oxide is experienced which can result in erratic over erase, damage to the gate oxide itself, and the introduction of trapping states.
• The solution to these problems is to use a floating gate having a lower electron affinity for electrons. Thus, a lower barrier is provided for electrons to escape over, or through by tunneling. Lower barriers require lower voltages and result in smaller tunneling distances for the electrons during the erase operation. This results in much faster erase times and much less damage. The possibility of a secondary problem occurring in the gate oxide are also reduced, such as electron traps and hole injection.
• The present invention describes the use of nanocrystalline silicon films as a floating gate in flash memories rather than the large bulky and thick polysilicon normally used. The nanocrystalline silicon films form in silicon rich oxide after silicon implantation into the oxide and appropriate anneal conditions. The silicon crystals can be made in a variety of sizes with a uniform distribution in particle sizes by appropriate anneal conditions. Although the particles may not be formed in a uniform sphere, they can be described as having a general diameter of approximately 10 Å to 100 Å. They can also be formed by chemical vapor deposition, by rapid thermal anneal of amorphous silicon layers or by other known techniques.
• The primary advantage of a nanocrystalline film floating gate is that these nano-scale particles have a larger bandgap than bulk silicon due to confinement in the small particles. A quantum mechanical effect is experienced which results in a different band structure and widening of the bandgap. A wider bandgap results in a lower barrier for the electrons at the surface of the silicon particle and a much larger tunneling probability when negative potentials are applied during the write operation. The barrier of course can not be made too small or electrons might be thermally excited over the barrier at high operating temperatures and the stored charge leak off of the floating gate in long time periods at high temperature. Normally this is not a problem in flash memories, the barrier is so high as to result in extremely long retention times, far in excess of any requirement. The problem is that the barrier is too high and results in FN tunneling discharge times which are too long and require voltages which are too high. Reducing the barrier between the silicon gate particles and the gate oxide serves to significantly improve flash memories. These nanocrystalline silicon films are quite conductive even when intrinsic and can be made more conductive by the appropriate doping as is done with larger grain polysilicon films currently used as the gate structure in flash memories. These floating gate films, however, need not be very conductive since they are not used elsewhere for wiring and need only be conductive enough to allow for a redistribution of carriers in the floating gate structure itself.
• FIG. 3 is a cross-sectional view of atransistor300 of the present invention.Transistor300 comprises a region of asource302 and a region of adrain304.Source302 and drain304 are constructed from an N+type of high impurity concentration which are formed in a P-type semiconductor substrate306 of low impurity concentration.Source302 and drain304 are separated by a predetermined space of achannel region308.Transistor300 further includes a floatinggate310 formed as a silicon nanocrystalline film. Acontrol gate314 is formed by a polysilicon layer. Floatinggate310 is isolated fromcontrol gate314 by aninterpoly dielectric layer312 and fromchannel region308 by a thingate oxide layer316. The floating gate silicon nanocrystalline film is comprised of nanocrystalline particles which can be embedded in eitherdielectric layer312 or316. These particles have a diameter in the size range of approximately 10 Å to 100 Å and are in a uniform size distribution for a particular set of processing conditions. The particles are in contact with each other and form as a result of annealing the silicon rich oxide which follows silicon implantation into the gate oxide or deposition of silicon and appropriate anneal conditions to grow nanocrystalline particles of silicon which then form a film. This film is patterned using standard techniques known to those skilled in the art to form the floating gates. When used as a memory cell, the drain of the transistor is typically coupled to a bit line, and the control gate is coupled to a word line.
• Using these silicon nanocrystals in a conductive film is distinctly different than other techniques of using isolated silicon nanocrystals to observe trapping of single electrons on these isolated crystals, as described above. Here the nanocrystals are used as a conductive film to replace the coarse grain polysilicon floating gate structure in a conventional flash memory structured with hot electron injection as the write mechanism and tunneling as the erase mechanism.
• FIG. 4 illustrates how the smaller barrier at the surface of the floating gate silicon storing electrons results in a shorter tunneling distance through a gate oxide insulating layer and consequently a much higher tunneling probability. Tunneling distance “do” represents the tunneling distance experienced in typical transistor having a polysilicon floating gate. Tunneling distance “dn” represents the tunneling distance experienced in a transistor having a floating gate as described inFIG. 3. The tunneling probability is an exponential function of the reciprocal tunneling distance and even a small reduction in the tunneling distance results in tunneling probabilities which are orders of magnitude higher and tunneling time which are orders of magnitudes smaller.
• The result then is a memory cell with a much faster and much easier erase operation due to the lower barrier height for the electrons to tunnel through in leaving the floating gate structures. Also, the thick polysilicon floating gate structure is replaced by a much thinner film of nanocrystals of silicon particles embedded in an oxide region. This results in a much more compact device, and a memory cell with a planar structure having less stray sidewall capacitive coupling.
• FIG. 5 is a simplified block diagram of a typical system having a flash memory incorporating the present invention.Memory200 comprises amemory array202 having memory cells. Arow decoder204 and acolumn decoder206 are designed to decode and select addresses provided onaddress lines208 to access appropriate memory cells. Command andcontrol circuitry210 is designed to control the operation ofmemory200 in response to incoming command and control signals from aprocessor201, including a write enable signal212 (WE*),control lines216 and data communication lines218. Furthermore, avoltage control switch214 is provided to apply appropriate voltages to the memory cells during programming operation. It will be appreciated by those skilled in the art that the memory ofFIG. 5 has been simplified for the purpose of illustrating the present invention and is not intended to be a complete description of a flash memory.
CONCLUSION
• A memory device is described which has fast read, write and erase operations. The memory uses memory cell floating gate transistors with a floating fabricated of a conductive layer of nano-size silicon crystals. Although the floating gate is conductive, it need only be conductive enough to allow for a redistribution of carriers in the floating gate structure itself. The memory cell has a lower electron affinity than conventional memory cells having a floating gate fabricated from polysilicon.
• 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. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims (22)

1. A memory cell, comprising:

a source region;

a drain region;

a channel region located between the source region and the drain region;

a control gate located over the channel region;

a floating gate located between the control gate and the channel region and separated from both the control gate and the channel region by insulator portions, the floating gate including:

a plurality of nanocrystalline silicon particles in physical and electrical contact with each other; and

wherein the nanocrystalline silicon particles are embedded in one of the insulator portions.

2. The memory cell ofclaim 1, wherein the nanocrystalline silicon particles have a general diameter of approximately 10 Å to 100 Å.

3. The memory cell ofclaim 1, wherein the insulator portions include a first insulator layer and a second insulator layer and at least one insulator layer includes silicon oxide.

4. The memory cell ofclaim 1, wherein the nanocrystalline silicon particles are conductively doped.

5. The memory cell ofclaim 5, wherein the gate insulator portion between the floating gate and the channel region is thicker than 20 Å.

6. A memory cell, comprising:

a source region;

a drain region;

a channel region located between the source region and the drain region;

a control gate located over the channel region;

a floating gate located between the control gate and the channel region and separated from both the control gate and the channel region by insulator portions, the floating gate including a plurality of nanocrystalline silicon particles embedded between the insulator portions; and

wherein the floating gate is configured to share a common charge in a plane substantially parallel to the channel region.

7. The flash memory cell ofclaim 6, wherein the nanocrystalline silicon particles have a general diameter of approximately 10 Å to 100 Å.

8. The memory cell ofclaim 6, wherein the nanocrystalline silicon particles are conductively doped.

9. A memory cell, comprising:

a source region;

a drain region;

a channel region located between the source region and the drain region;

a control gate located over the channel region;

a floating gate located between the control gate and the channel region and separated from both the control gate and the channel region by insulator portions, the floating gate having a lower electron affinity than a polysilicon gate, the floating gate including a plurality of nanocrystalline silicon particles in physical and electrical contact with each other.

10. The memory cell ofclaim 9, wherein the nanocrystalline silicon particles have a general diameter of approximately 10 Å to 100 Å.

11. The memory cell ofclaim 9, wherein the control circuitry is configured to utilize Fowler-Nordheim tunneling mechanisms to store data on the gate.

12. A flash memory device, comprising:

an array of memory cells, the cells including:

a source region;

a drain region;

a channel region located between the source region and the drain region;

a gate, including:

a plurality of nanocrystalline silicon particles in physical contact and in electrical contact with each other;

wherein the nanocrystalline silicon particles are at least partially embedded in an insulator layer;

a gate insulator separating the gate from the channel region; and

control circuitry to select cells in the array of memory cells.

13. The flash memory device ofclaim 12, wherein the nanocrystalline silicon particles have a general diameter of approximately 10 Å to 100 Å.

14. The flash memory device ofclaim 12, wherein the gate insulator is thicker than 20 Å.

15. The flash memory device ofclaim 14, wherein the control circuitry is configured to utilize Fowler-Nordheim tunneling mechanisms to store data on the gate.

16. A flash memory device, comprising:

an array of memory cells, the cells including:

a source region;

a drain region;

a channel region located between the source region and the drain region;

a gate, including a plurality of nanocrystalline silicon particles in an insulator layer configured to allow a shared common charge in a plane substantially parallel to the channel region;

a gate insulator separating the gate from the channel region; and

control circuitry to select cells in the array of memory cells.

17. The flash memory device ofclaim 16, wherein the plurality of nanocrystalline silicon particles includes clusters of nanocrystalline particles.

18. The flash memory device ofclaim 16, wherein the nanocrystalline silicon particles are conductively doped.

19. An electronic system, comprising:

a flash memory, wherein memory cells in the flash memory include:

a source region;

a drain region;

a channel region located between the source region and the drain region;

a gate having a lower electron affinity than a polysilicon gate, the gate including a plurality of nanocrystalline silicon particles in physical contact and in electrical contact with each other;

a gate insulator separating the gate from the channel region; and

a processor coupled to the flash memory to process data stored in the flash memory.

20. The electronic system ofclaim 19, wherein the control circuitry is configured to utilize Fowler-Nordheim tunneling mechanisms to store data on the gate.

21. The electronic system ofclaim 19, wherein the gate insulator includes silicon oxide.

22. The electronic system ofclaim 19, wherein the nanocrystalline silicon particles are conductively doped.

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