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United States Patent 6,804,136
Forbes October 12, 2004

Write once read only memory employing charge trapping in insulators

Abstract

Structures and methods for write once read only memory employing charge trapping in insulators are provided. The write once read only memory cell includes a metal oxide semiconductor field effect transistor having a first source/drain region, a second source/drain region, a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator. A plug couples the first source/drain region to an array plate. A bitline is coupled to the second source/drain region. The MOSFET can be programmed by operation in a reverse direction trapping charge in the gate insulator adjacent to the first source/drain region such that the programmed MOSFET operates at reduced drain source current when read in a forward direction.


Inventors: Forbes; Leonard (Corvallis, OR)
Assignee: Micron Technology, Inc. (Boise, ID)
Appl. No.: 177077
Filed: June 21, 2002

Current U.S. Class: 365/94; 365/177
Intern'l Class: G11C 017/00
Field of Search: 365/94,104,103,174,177


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Primary Examiner: Phung; Anh
Attorney, Agent or Firm: Schwegman, Lundberg, Woessner & Kluth, P.A.

Claims



What is claimed is:

1. A write once read only memory cell, comprising:

a metal oxide semiconductor field effect transistor (MOSFET) in a substrate, the MOSFET having a first source/drain region, a second source/drain region, a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator;

a plug coupled to the first source/drain region, wherein the plug couples the first source/drain region to an array plate;

a transmission line coupled to the second source/drain region; and

wherein the MOSFET is a programmed MOSFET having a charge trapped in the gate insulator adjacent to the first source/drain region such that the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) and such that the programmed MOSFET operates at reduced drain source current.

2. The write once read only memory cell of claim 1, wherein the first source/drain region of the MOSFET includes a source region and the second source/drain region of the MOSFET includes a drain region.

3. The write once read only memory cell of claim 1, wherein the transmission line includes a bit line.

4. The write once read only memory cell of claim 1, wherein the charge trapped in the gate insulator adjacent the first source/drain region includes a trapped electron charge.

5. The write once read only memory cell of claim 1, wherein the second voltage threshold region (Vt2) in the channel region is adjacent the first source/drain region, and wherein the first voltage threshold region (Vt1) in the channel region is adjacent the second source/drain region.

6. The write once read only memory cell of claim 5, wherein the Vt2 has a higher voltage threshold than the Vt1.

7. The write once read only memory cell of claim 1, wherein the gate insulator has a thickness of at least 10 nanometers (nm).

8. The write once read only memory cell of claim 7, wherein the gate insulator includes a gate insulator selected from the group of silicon dioxide (SiO.sub.2) formed by wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO), and aluminum oxide (Al.sub.2 O.sub.3).

9. A write once read only memory cell, comprising:

a metal oxide semiconductor field effect transistor (MOSFET) in a substrate, the MOSFET having a source region, a drain region, a channel region between the source region and the drain region, and a gate seperated from the channel region by a gate insulator;

a wordline coupled to the gate;

an array plate coupled to the source region;

a bit line coupled to the drain region; and

wherein the MOSFET is a programmed MOSFET having a charge trapped in the gate insulator adjacent to the source region such that the channel region has a first voltage threshold region (Vt1) adjacent to the drain region and a second voltage threshold region (Vt2) adjacent to the source region, the Vt2 having a greater voltage threshold than Vt1 and such that the programmed MOSFET operates at reduced drain source current.

10. The write once read only memory cell of claim 9, wherein the gate insulator has a thickness of at least 10 nanometers (nm).

11. The write once read only memory cell of claim 10, wherein the gate insulator includes a gate insulator selected from the group of silicon dioxide (SiO.sub.2) formed by wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO), and aluminum oxide (Al.sub.2 O.sub.3).

12. The write once read only memory cell of claim 9, wherein the gate insulator includes a composite layer.

13. The write once read only memory cell of claim 12, wherein the composite layer includes a composite layer selected from the group of a silicon dioxide (SiO.sub.2) and silicon nitride composite layer and a silicon dioxide (SiO.sub.2) and aluminum oxide (Al.sub.2 O.sub.3) composite layer.

14. The write once read only memory cell of claim 9, wherein the gate insulator includes a multiple layer of oxide-nitride-oxide (ONO).

15. The write once read only memory cell of claim 9, wherein the gate insulator includes implanted oxides having traps.

16. A memory array, comprising:

a number of write once read only memory cells, wherein each write once read only memory cell includes a first source/drain region, a second source/drain region, a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator;

a number of bit lines coupled to the second source/drain region of each write once read only memory cell along rows of the memory array;

a number of word lines coupled to the gate of each write once read only memory cell along columns of the memory array;

an array plate, wherein the first source/drain region of each write once read only memory cell is coupled to the array plate by a conductive plug; and

wherein at least one of write once read only memory cells is a programmed MOSFET having a charge trapped in the gate insulator adjacent to the first source/drain region such that the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) and such that the programmed MOSFET operates at reduced drain source current.

17. The memory array of claim 16, wherein the charge trapped in the gate insulator includes a charge of approximately 100 electrons.

18. The memory array of claim 16, wherein the first source/drain region of the MOSFET includes a source region and the second source/drain region of the MOSFET includes a drain region.

19. The memory array of claim 16, wherein the second voltage threshold region (Vt2) in the channel region is adjacent the first source/drain region, and wherein the first voltage threshold region (Vt1) in the channel region is adjacent the second source/drain region, and wherein Vt2 has a higher voltage threshold than the Vt1.

20. The memory array of claim 16, wherein the gate insulator of each write once read only memory cell has a thickness of at least 10 nanometers (nm).

21. The memory array of claim 20, wherein the gate insulator includes a gate insulator selected from the group of silicon dioxide (SiO.sub.2) formed by wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO), and aluminum oxide (Al.sub.2 O.sub.3).

22. A memory device, comprising:

a memory array, wherein the memory array includes a number of write once read only memory cells, wherein each write once read only memory cell includes a source region, a drain region, a channel region between the source and the drain regions, and a gate separated from the channel region by a gate insulator;

a number of bitlines coupled to the drain region of each write once read only memory cell along rows of the memory array;

a number of wordlines coupled to the gate of each write once read only memory cell along columns of the memory array;

an array plate, wherein the source region of each write once read only memory cell is coupled to the array plate by a conductive plug;

a wordline address decoder coupled to the number of wordlines;

a bitline address decoder coupled to the number of bitlines;

a sense amplifier coupled to the number of bitlines; and

wherein at least one of write once read only memory cells is a programmed MOSFET having a charge trapped in the gate insulator adjacent to the source region such that the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) and such that the programmed MOSFET operates at reduced drain/source current.

23. The memory device of claim 22, wherein the charge trapped in the gate insulator includes a charge of approximately 100 electrons.

24. The memory device of claim 22, wherein the second voltage threshold region (Vt2) in the channel region is adjacent the source region, and wherein the first voltage threshold region (Vt1) in the channel region is adjacent the drain region, and wherein Vt2 has a higher voltage threshold than the Vt1.

25. The memory device of claim 22, wherein the gate insulator of each write once read only memory cell has a thickness of at least 10 nanometers (nm).

26. The memory device of claim 25, wherein the gate insulator includes a gate insulator selected from the group of silicon dioxide (SiO.sub.2) formed by wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO), and aluminum oxide (Al.sub.2 O.sub.3).

27. The memory device of claim 22, wherein the wordline address decoder and the bitline address decoder each include conventionally fabricated MOSFET transistors having thin gate insulators formed of silicon dioxide (SiO.sub.2).

28. The memory device of claim 22, wherein the sense amplifier includes conventionally fabricated MOSFET transistors having thin gate insulators formed of silicon dioxide (SiO.sub.2).

29. An electronic system, comprising:

a processor; and

a memory device coupled to the processor, wherein the memory device includes;

a memory array, wherein the memory array includes a number of write once read only memory cells, wherein each write once read only memory cell includes a source region, a drain region, a channel region between the source and the drain regions, and a gate separated from the channel region by a gate insulator;

a number of bitlines coupled to the drain region of each write once read only memory cell along rows of the memory array;

a number of wordlines coupled to the gate of each write once read only memory cell along columns of the memory array;

an array plate, wherein the source region of each write once read only memory cell is coupled to the array plate by a conductive plug;

a wordline address decoder coupled to the number of wordlines;

a bitline address decoder coupled to the number of bitlines;

a sense amplifier coupled to the number of bitlines; and

wherein at least one of write once read only memory cells is a programmed MOSFET having a charge trapped in the gate insulator adjacent to the source region such that the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2), wherein Vt2 is adjacent the source region of the programmed MOSFET and the programmed MOSFET operates at reduced drain/source current.

30. The electronic system of claim 29, wherein the charge trapped in the gate insulator includes a charge of approximately 100 electrons.

31. The electronic system of claim 29, wherein the gate insulator includes a gate insulator selected from the group of silicon dioxide (SiO.sub.2) formed by wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO), and aluminum oxide (Al.sub.2 O.sub.3).

32. The electronic system of claim 29, wherein the gate insulator includes a composite layer, and wherein the composite layer is selected from the group of a silicon dioxide (SiO.sub.2) and silicon nitride composite layer and a silicon dioxide (SiO.sub.2) and aluminum oxide (Al.sub.2 O.sub.3) composite layer.

33. The electronic system of claim 29, wherein the gate insulator includes a multiple layer of oxide-nitride-oxide (ONO).

34. The electronic system of claim 29, wherein the gate insulator includes implanted oxides having traps.

35. The electronic system of claim 29, wherein, in a read operation, the array plate is coupled to a ground potential, the drain region of an addressed write once read only memory cell is precharged to a fractional voltage of VDD, and the memory cell is selected by a wordline address.

36. The electronic system of claim 29, wherein, in a write operation, the array plate is biased to a voltage higher than VDD, the drain region of an addressed write once read only memory cell is couple to a ground, and the memory cell is selected by a wordline address.
Description



FIELD OF THE INVENTION

The present invention relates generally to semiconductor integrated circuits and, more particularly, to write once read only memory employing charge trapping in insulators.

BACKGROUND OF THE INVENTION

Many electronic products need various amounts of memory to store information, e.g. data. One common type of high speed, low cost memory includes dynamic random access memory (DRAM) comprised of individual DRAM cells arranged in arrays. DRAM cells include an access transistor, e.g a metal oxide semiconducting field effect transistor (MOSFET), coupled to a capacitor cell. With successive generations of DRAM chips, an emphasis continues to be placed on increasing array density and maximizing chip real estate while minimizing the cost of manufacture. It is further desirable to increase array density with little or no modification of the DRAM optimized process flow.

A requirement exists for memory devices which need only be programmed once, as for instance to function as an electronic film in a camera. If the memory arrays have a very high density then they can store a large number of very high resolution images in a digital camera. If the memory is inexpensive then it can for instance replace the light sensitive films which are used to store images in conventional cameras.

Thus, there is a need for improved DRAM technology compatible write once read only memory. It is desirable that such write once read only memory be fabricated on a DRAM chip with little or no modification of the DRAM process flow. It is further desirable that such write once read only memory operate with lower programming voltages than that used by conventional DRAM cells, yet still hold sufficient charge to withstand the effects of parasitic capacitances and noise due to circuit operation.

REFERENCES

L. Forbes, W. P. Noble and E. H. Cloud, entitled "MOSFET Technology for Programmable Address Decode and Correction," U.S. Pat. No. 6,521,950;

L. Forbes, E. Sun, R. Adlers and J. Moll, "Field Induced Re-Emission of Electronics Trapped in SiO.sub.2," IEEE Trans. Electron Device, vol. ED-26, No. 11, pp. 1816-1818 (Nov. 1979);

S. S. B. Or, N. Hwang, and L. Forbes, "Tunneling and Thermal Emission from a Distribution of Deep Traps in SiO.sub.2," IEEE Trans. on Electron Devices, vol. 40, No. 6, pp. 1100-1103 (Jun. 1993);

S. A. Abbas and R. C. Dockerty, "N-Channel IGFET Design Limitations Due to Hot Electron Trapping," IEEE Int. Electron Devices Mtg., Washington D.C., Dec. 1975, pp. 35-38;

B. Eitan et al., "Characterization of Channel Hot Electron Injection by the Subthreshold Slope of NROM Device," IEEE Electron Device Lett., vol. 22, No. 11, pp. 556-558, (Nov. 2001);

B. Eitan et al., "NROM: A Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell," IEEE Electron Device Lett., vol. 21, No. 11, pp. 543-545, (Nov. 2000);

S. Sze, Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506);

L. Forbes and J. Geusic, "Memory Using Insulator Traps," U.S. Pat. No. 6,140,181.

SUMMARY OF THE INVENTION

The above mentioned problems for creating DRAM technology compatible write once read only memory cells as well as other problems are addressed by the present invention and will be understood by reading and studying the following specification. This disclosure teaches structures and methods using MOSFET devices as write once read only memory in a DRAM integrated circuit. The structures and methods use the existing process sequence for MOSFET's in DRAM technology.

In particular, an illustrative embodiment of the present invention includes a write once read only memory cell. The write once read only memory cell includes a metal oxide semiconductor field effect transistor (MOSFET). The MOSFET has a first source/drain region, a second source/drain region, a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator. A plug is coupled to the first source/drain region. The plug couples the first source/drain region to an array plate. A transmission line is coupled to the second source/drain region. The MOSFET is a programmed MOSFET having a charge trapped in the gate oxide adjacent to the first source/drain region. Accordingly, the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) such that the programmed MOSFET operates at reduced drain source current.

These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a metal oxide semiconductor field effect transistor (MOSFET) in a substrate according to the teachings of the prior art.

FIG. 1B illustrates the MOSFET of FIG. 1A operated in the forward direction showing some degree of device degradation due to electrons being trapped in the gate oxide near the drain region over gradual use.

FIG. 1C is a graph showing the square root of the current signal (Ids) taken at the drain region of the conventional MOSFET versus the voltage potential (VGS) established between the gate and the source region.

FIG. 2A is a diagram of a programmed MOSFET which can be used as a write once read only memory cell according to the teachings of the present invention.

FIG. 2B is a diagram suitable for explaining the method by which the MOSFET of the write once read only memory cell of the present invention can be programmed to achieve the embodiments of the present invention.

FIG. 2C is a graph plotting the current signal (Ids) detected at the drain region versus a voltage potential, or drain voltage, (VDS) set up between the drain region and the source region (Ids vs. VDS).

FIG. 3 illustrates a portion of a memory array according to the teachings of the present invention.

FIGS. 4A-4B illustrates the operation of the novel write once read only memory cell formed according to the teachings of the present invention.

FIG. 5 illustrates the operation of a conventional DRAM cell.

FIG. 6 illustrates a memory device according to the teachings of the present invention.

FIG. 7 is a block diagram of an electrical system, or processor-based system, utilizing write once read only memory constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.

The terms wafer and substrate used in the following description include any structure having an exposed surface with which to form the integrated circuit (IC) structure of the invention. The term substrate is understood to include semiconductor wafers. The term substrate is also used to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereupon. Both wafer and substrate include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures well known to one skilled in the art. The term conductor is understood to include semiconductors, and the term insulator is defined to include any material that is less electrically conductive than the materials referred to as conductors. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

FIG. 1A is useful in illustrating the conventional operation of a MOSFET such as can be used in a DRAM array. FIG. 1A illustrates the normal hot electron injection and degradation of devices operated in the forward direction. As is explained below, since the electrons are trapped near the drain they are not very effective in changing the device characteristics.

FIG. 1A is a block diagram of a metal oxide semiconductor field effect transistor (MOSFET) 101 in a substrate 100. The MOSFET 101 includes a source region 102, a drain region 104, a channel region 106 in the substrate 100 between the source region 102 and the drain region 104. A gate 108 is separated from the channel region 108 by a gate oxide 110. A sourceline 112 is coupled to the source region 102. A bitline 114 is coupled to the drain region 104. A wordline 116 is coupled to the gate 108.

In conventional operation, a drain to source voltage potential (Vds) is set up between the drain region 104 and the source region 102. A voltage potential is then applied to the gate 108 via a wordline 116. Once the voltage potential applied to the gate 108 surpasses the characteristic voltage threshold (Vt) of the MOSFET a channel 106 forms in the substrate 100 between the drain region 104 and the source region 102. Formation of the channel 106 permits conduction between the drain region 104 and the source region 102, and a current signal (Ids) can be detected at the drain region 104.

In operation of the conventional MOSFET of FIG. 1A, some degree of device degradation does gradually occur for MOSFETs operated in the forward direction by electrons 117 becoming trapped in the gate oxide 110 near the drain region 104. This effect is illustrated in FIG. 1B. However, since the electrons 117 are trapped near the drain region 104 they are not very effective in changing the MOSFET characteristics.

FIG. 1C illustrates this point. FIG. 1C is a graph showing the square root of the current signal (Ids) taken at the drain region versus the voltage potential (VGS) established between the gate 108 and the source region 102. The change in the slope of the plot of Ids versus VGS represents the change in the charge carrier mobility in the channel 106.

In FIG. 1C, .DELTA.VT represents the minimal change in the MOSFET's threshold voltage resulting from electrons gradually being trapped in the gate oxide 110 near the drain region 104, under normal operation, due to device degradation. This results in a fixed trapped charge in the gate oxide 110 near the drain region 104. Slope 103 represents the charge carrier mobility in the channel 106 for FIG. 1A having no electrons trapped in the gate oxide 110. Slope 105 represents the charge mobility in the channel 106 for the conventional MOSFET of FIG. 1B having electrons 117 trapped in the gate oxide 110 near the drain region 104. As shown by a comparison of slope 103 and slope 105 in FIG. 1C, the electrons 117 trapped in the gate oxide 110 near the drain region 104 of the conventional MOSFET do not significantly change the charge mobility in the channel 106.

There are two components to the effects of stress and hot electron injection. One component includes a threshold voltage shift due to the trapped electrons and a second component includes mobility degradation due to additional scattering of carrier electrons caused by this trapped charge and additional surface states. When a conventional MOSFET degrades, or is "stressed," over operation in the forward direction, electrons do gradually get injected and become trapped in the gate oxide near the drain. In this portion of the conventional MOSFET there is virtually no channel underneath the gate oxide. Thus the trapped charge modulates the threshold voltage and charge mobility only slightly.

The inventor, along with others, has previously described programmable memory devices and functions based on the reverse stressing of MOSFET's in a conventional CMOS process and technology in order to form programmable address decode and correction in U.S. Pat. No. 6,521,950 entitled "MOSFET Technology for Programmable Address Decode and Correction." That disclosure, however, did not describe write once read only memory solutions, but rather address decode and correction issues.

According to the teachings of the present invention, normal MOSFETs can be programmed by operation in the reverse direction and utilizing avalanche hot electron injection to trap electrons in the gate oxide of the MOSFET. When the programmed MOSFET is subsequently operated in the forward direction the electrons trapped in the oxide are near the source and cause the channel to have two different threshold voltage regions. The novel programmed MOSFETs of the present invention conduct significantly less current than conventional MOSFETs, particularly at low drain voltages. These electrons will remain trapped in the gate oxide unless negative gate voltages are applied. The electrons will not be removed from the gate oxide when positive or zero gate voltages are applied. Erasure can be accomplished by applying negative gate voltages and/or increasing the temperature with negative gate bias applied to cause the trapped electrons to be re-emitted back into the silicon channel of the MOSFET.

FIGS. 2A-2C illustrate are useful in illustrating the present invention in which a much larger change in device characteristics is obtained by programming the device in the reverse direction and subsequently reading the device by operating it in the forward direction.

FIG. 2A is a diagram of a programmed MOSFET which can be used as a write once read only memory cell according to the teachings of the present invention. As shown in FIG. 2A the write once read only memory cell 201 includes a MOSFET in a substrate 200 which has a first source/drain region 202, a second source/drain region 204, and a channel region 206 between the first and second source/drain regions, 202 and 204. In one embodiment, the first source/drain region 202 includes a source region 202 for the MOSFET and the second source/drain region 204 includes a drain region 204 for the MOSFET. FIG. 2A further illustrates a gate 208 separated from the channel region 206 by a gate oxide 210. A first transmission line 212 is coupled to the first source/drain region 202 and a second transmission line 214 is coupled to the second source/drain region 204. In one embodiment, the first transmission line includes a sourceline 212 and the second transmission line includes a bit line 214.

As stated above, write once read only memory cell 201 is comprised of a programmed MOSFET. This programmed MOSFET has a charge 217 trapped in the gate oxide 210 adjacent to the first source/drain region 202 such that the channel region 206 has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) in the channel 206. In one embodiment, the charge 217 trapped in the gate oxide 210 adjacent to the first source/drain region 202 includes a trapped electron charge 217.

FIG. 2A illustrates the Vt2 in the channel 206 is adjacent the first source/drain region 202 and that the Vt1 in the channel 206 is adjacent the second source/drain region 204. According to the teachings of the present invention, Vt2 has a higher voltage threshold than Vt1 due to the charge 217 trapped in the gate oxide 217 adjacent to the first source/drain region 202.

FIG. 2B is a diagram suitable for explaining the method by which the MOSFET of the write once read only memory cell 201 of the present invention can be programmed to achieve the embodiments of the present invention. As shown in FIG. 2B the method includes programming the MOSFET in a reverse direction. Programming the MOSFET in the reverse direction includes applying a first voltage potential V1 to a drain region 204 of the MOSFET. In one embodiment, applying a first voltage potential V1 to the drain region 204 of the MOSFET includes grounding the drain region 204 of the MOSFET as shown in FIG. 2B. A second voltage potential V2 is applied to a source region 202 of the MOSFET. In one embodiment, applying a second voltage potential V2 to the source region 202 includes applying a high positive voltage potential (VDD) to the source region 202 of the MOSFET, as shown in FIG. 2B. A gate potential VGS is applied to a gate 208 of the MOSFET. In one embodiment, the gate potential VGS includes a voltage potential which is less than the second voltage potential V2, but which is sufficient to establish conduction in the channel 206 of the MOSFET between the drain region 204 and the source region 202. As shown in FIG. 2B, applying the first, second and gate potentials (V1, V2, and VGS respectively) to the MOSFET creates a hot electron injection into a gate oxide 210 of the MOSFET adjacent to the source region 202. In other words, applying the first, second and gate potentials (V1, V2, and VGS respectively) provides enough energy to the charge carriers, e.g. electrons, being conducted across the channel 206 that, once the charge carriers are near the source region 202, a number of the charge carriers get excited into the gate oxide 210 adjacent to the source region 202. Here the charge carriers become trapped.

In one embodiment of the present invention, the method is continued by subsequently operating the MOSFET in the forward direction in its programmed state during a read operation. Accordingly, the read operation includes grounding the source region 202 and precharging the drain region a fractional voltage of VDD. If the device is addressed by a wordline coupled to the gate, then its conductivity will be determined by the presence or absence of stored charge in the gate insulator. That is, a gate potential can be applied to the gate 208 by a wordline 216 in an effort to form a conduction channel between the source and the drain regions as done with addressing and reading conventional DRAM cells.

However, now in its programmed state, the conduction channel 206 of the MOSFET will have a first voltage threshold region (Vt1) adjacent to the drain region 204 and a second voltage threshold region (Vt2) adjacent to the source region 202, as explained and described in detail in connection with FIG. 2A. According to the teachings of the present invention, the Vt2 has a greater voltage threshold than the Vt1 due to the hot electron injection 217 into a gate oxide 210 of the MOSFET adjacent to the source region 202.

FIG. 2C is a graph plotting a current signal (Ids) detected at the second source/drain region 204 versus a voltage potential, or drain voltage, (VDS) set up between the second source/drain region 204 and the first source/drain region 202 (Ids vs. VDS). In one embodiment, VDS represents the voltage potential set up between the drain region 204 and the source region 202. In FIG. 2C, the curve plotted as 205 represents the conduction behavior of a conventional MOSFET where the MOSFET is not programmed (is normal or not stressed) according to the teachings of the present invention. The curve 207 represents the conduction behavior of the programmed MOSFET (stressed), described above in connection with FIG. 2A, according to the teachings of the present invention. As shown in FIG. 2C, for a particular drain voltage, VDS, the current signal (IDS2) detected at the second source/drain region 204 for the programmed MOSFET (curve 207) is significantly lower than the current signal (IDS1) detected at the second source/drain region 204 for the conventional MOSFET (curve 205) which is not programmed according to the teachings of the present invention. Again, this is attributed to the fact that the channel 206 in the programmed MOSFET of the present invention has two voltage threshold regions and that the voltage threshold, Vt2, near the first source/drain region 202 has a higher voltage threshold than Vt1 near the second source/drain region due to the charge 217 trapped in the gate oxide 217 adjacent to the first source/drain region 202.

Some of these effects have recently been described for use in a different device structure, called an NROM, for flash memories. This latter work in Israel and Germany is based on employing charge trapping in a silicon nitride layer in a non-conventional flash memory device structure. Charge trapping in silicon nitride gate insulators was the basic mechanism used in MNOS memory devices, charge trapping in aluminum oxide gates was the mechanism used in MIOS memory devices, and the present inventor, along with another, disclosed charge trapping at isolated point defects in gate insulators in U.S. Pat. No. 6,140,181 entitled "Memory Using Insulator Traps,".

In contrast to the above work, the present invention disclosures programming a MOSFET in a reverse direction to trap charge near the source region and reading the device in a forward direction to form a write once memory based on a modification of DRAM technology.

Prior art DRAM technology generally employs silicon oxide as the gate insulator. Further the emphasis in conventional DRAM devices is placed on trying to minimize charge trapping in the silicon oxide gate insulator. According to the teachings of the present invention, a variety of insulators are used to trap electrons more efficiently than in silicon oxide. That is, in the present invention, the write-once-read-only-memory (WOROM) employs charge trapping in gate insulators such as, wet silicon oxide, silicon nitride, silicon oxynitride SON, silicon rich oxide SRO, aluminum oxide Al.sub.2 O.sub.3, composite layers of these insulators such as oxide and then silicon nitride, or oxide and then aluminum oxide, or multiple layers as oxide-nitride-oxide. While the charge trapping efficiency of silicon oxide may be low such is not the case for silicon nitride or composite layers of silicon oxide and nitride.

FIG. 3 illustrates a portion of a memory array 300 according to the teachings of the present invention. The memory in FIG. 3, is shown illustrating a pair of write once read only memory cells 301-1 and 301-2 formed according to the teachings of the present invention. As one of ordinary skill in the art will understand upon reading this disclosure, any number of write once and read only memory cells can be organized in an array, but for ease of illustration only two are displayed in FIG. 3. As shown in FIG. 3, a first source/drain region, 302-1 and 302-2 respectively, is coupled to an array plate 304. A second source/drain region, 306-1 and 306-2 respectively, is coupled to a bitline, 308-1 and 308-2 respectively. Each of the bitlines, 308-1 and 308-2, couple to a sense amplifier, shown generally at 310. A wordline, 312-1 and 312-2 respectively, is couple to a gate, 314-1 and 314-2 respectively, for each of the write once read only memory cells, 301-1 and 301-2. Finally, a write data/precharge circuit is shown at 324 for coupling a first or a second potential to bitline 308-1. The illustrated write data/precharge circuit 324 is connected to a write data/precharge control line 325. As one of ordinary skill in the art will understand upon reading this disclosure, the write data/precharge circuit 324 is adapted to couple either a ground to the bitline 308-1 during a write operation in the reverse direction, or alternatively to precharge the bitline 308-1 to fractional voltage of VDD during a read operation in the forward direction. As one of ordinary skill in the art will understand upon reading this disclosure, the array plate 304 can be biased to a voltage higher than VDD during a write operation in the reverse direction, or alternatively grounded during a read operation in the forward direction.

As shown in FIG. 3, the array structure 300, including write once read only memory cells 301-1 and 301-2, has no capacitors. Instead, according to the teachings of the present invention, the first source/drain region or source region, 302-1 and 302-2, are coupled via a conductive plug directly to the array plate 304. In order to write, the array plate 304 is biased to voltage higher than VDD and the devices stressed in the reverse direction by grounding the data or bit line, 308-1 or 308-2. If the write once read only memory cell, 301-1 or 301-2, is selected by a word line address, 312-1 or 312-2, then the write once read only memory cell, 301-1 or 301-2, will conduct and be stressed with accompanying hot electron injection into the cells gate insulator adjacent to the source region, 302-1 or 302-2. During read the write once read only memory cell, 301-1 or 301-2, are operated in the forward direction with the array plate 304 grounded and the bit line, 308-1 or 308-2, and respective second source/drain region or drain region, 306-1 and 306-2, of the cells precharged to some fractional voltage of Vdd. If the device is addressed by the word line, 312-1 or 312-2, then its conductivity will be determined by the presence or absence of stored charge and so detected using the DRAM sense amplifier 310. The operation of DRAM sense amplifiers is described, for example, in U.S. Pat. Nos. 5,627,785; 5,280,205; and 5,042,011, all assigned to Micron Technology Inc., and incorporated by reference herein. The array would thus be addressed and read in the conventional manner used in DRAM's, but programmed as write once read only memory cells in a novel fashion.

In operation the devices would be subjected to hot electron stress in the reverse direction by biasing the array plate 304, and read while grounding the array plate 304 to compare a stressed write once read only memory cell, e.g. cell 301-1, to an unstressed dummy device/cell, e.g. 301-2, as shown in FIG. 3. The write and possible erase feature could be used during manufacture and test to initially program all cells or devices to have similar or matching conductivity before use in the field. The sense amplifier 310 can then detect small differences in cell or device characteristics due to stress induced changes in device characteristics during the write operation.

As one of ordinary skill in the art will understand upon reading this disclosure such arrays of write once read only memory cells are conveniently realized by a modification of DRAM technology. According to the teachings of the present invention a gate insulator of the write once read only memory cell includes gate insulators selected from the group of thicker layers of SiO.sub.2 formed by wet oxidation, SON silicon oxynitride, SRO silicon rich oxide, Al.sub.2 O.sub.3 aluminum oxide, composite layers and implanted oxides with traps (L. Forbes and J. Geusic, "Memory using insulator traps," U.S. Pat. No. 6,140,181. Conventional transistors for address decode and sense amplifiers can be fabricated after this step with normal thin gate insulators of silicon oxide.

FIGS. 4A-B and 5 are useful in illustrating the use of charge storage in the gate insulator to modulate the conductivity of the write once read only memory cell according to the teachings of the present invention. That is, FIGS. 4A-4B illustrates the operation of the novel write once read only memory cell 401 formed according to the teachings of the present invention. And, FIG. 5 illustrates the operation of a conventional DRAM cell 501. As shown in FIG. 4A, the gate insulator 410 is made thicker than in a conventional DRAM cell. For example, an embodiment of the gate insulator 410 has a thickness 411, equal to or greater than 10 nm or 100 .ANG. (10.sup.-6 cm). In the embodiment shown in FIG. 4A a write once read only memory cell has dimensions 413 of 0.1 .mu.m (10.sup.-5 cm) by 0.1 .mu.m. The capacitance, Ci, of the structure depends on the dielectric constant, .di-elect cons..sub.i, and the thickness of the insulating layers, t. In an embodiment, the dielectric constant is 0.3.times.10.sup.-12 F/cm and the thickness of the insulating layer is 10.sup.-6 cm such that Ci=.di-elect cons.i/t, Farads/cm.sup.2 or 3.times.10.sup.-7 F/cm.sup.2. In one embodiment, a charge of 10.sup.12 electrons/cm.sup.2 is programmed into the gate insulator of the write once read only memory cell. This produces a stored charge .DELTA.Q=10.sup.12 electrons/cm.sup.2.times.1.6.times.10.sup.-19 Coulombs. In this embodiment, the resulting change in the threshold voltage (.DELTA.Vt )of the write once read only memory cell will be approximately 0.5 Volts (.DELTA.Vt=.DELTA.Q/Ci or 1.6.times.10.sup.-7 /3.times.10.sup.-7 =1/2 Volt). In effect, the programmed write once read only memory cell, or modified MOSFET is a programmed MOSFET having a charge trapped in the gate insulator adjacent to a first source/drain region, or source region, such that the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2), where Vt2 is greater than Vt1, and Vt2 is adjacent the source region such that the programmed MOSFET operates at reduced drain source current. For .DELTA.Q=10.sup.12 electrons/cm.sup.3 in an area of 10.sup.-10 cm.sup.2, this embodiment of the present invention involves trapping a charge of approximately 100 electrons in the gate insulator of the write once read only memory cell. In this embodiment, an original V.sub.T is approximately 1/2 Volt and the V.sub.T with charge trapping is approximately 1 Volt.

FIG. 4B aids to further illustrate the conduction behavior of the novel write once read only memory cell of the present invention. As one of ordinary skill in the art will understand upon reading this disclosure, if the write once read only memory cell is being driven with a gate voltage of 1.0 Volt (V) and the nominal threshold voltage without the gate insulator charged is 1/2 V, then if the gate insulator is charged the transistor of the present invention will be off and not conduct. That is, by trapping a charge of approximately 100 electrons in the gate insulator of the write once read only memory cell, having dimensions of 0.1 .mu.m (10.sup.-5 cm) by 0.1 .mu.m, will raise the threshold voltage of the write once read only memory cell to 1.0 Volt and a 1.0 Volt gate potential will not be sufficient to turn the device on, e.g. Vt=1.0 V, I=0.

Conversely, if the nominal threshold voltage without the gate insulator charged is 1/2 V, then I=.mu.C.sub.ox.times.(W/L).times.((Vgs-Vt).sup.2 /2), or 12.5 .mu.A, with .mu.C.sub.ox =.mu.C.sub.i =100 .mu.A/V.sup.2 and W/L=1. That is, the write once read only memory cell of the present invention, having the dimensions describe above will produce a current I=100 .mu.A/V.sup.2.times.(1/4).times.(1/2)=12.5 .mu.A. Thus, in the present invention an un-written, or un-programmed write once read only memory cell can conduct a current of the order 12.5 uA, whereas if the gate insulator is charged then the write once read only memory cell will not conduct. As one of ordinary skill in the art will understand upon reading this disclosure, the sense amplifiers used in DRAM arrays, and as describe above, can easily detect such differences in current on the bit lines.

By way of comparison, in a conventional DRAM cell 550 with a 30 femtoFarad (fF) storage capacitor 551 charged to 50 femto Coulombs (fC), if these are read over 5 nS then the average current on the bit line 552 is only 10 .mu.A (I=50 fc/5ns=10 .mu.A). Thus, storing a 50 fC charge on the storage capacitor shown in FIG. 5 equates to storing 300,000 electrons (Q=50fc/(1.6.times.10.sup.-19)=30.times.10.sup.4 =300,000 electrons).

According to the teachings of the present invention, the transistors in the array are utilized not just as passive on or off switches as transfer devices in DRAM arrays but rather as active devices providing gain. In the present invention, to program the transistor "off," requires only a stored charge in the gate insulator is only about 100 electrons if the area is 0.1 .mu.m by 0.1 .mu.m. And, if the write once read only memory cell is un-programmed, e.g. no stored charge trapped in the gate insulator, and if the transistor is addressed over 10 nS a of current of 12.5 .mu.A is provided. The integrated drain current then has a charge of 125 fC or 800,000 electrons. This is in comparison to the charge on a DRAM capacitor of 50 fC which is only about 300,000 electrons. Hence, the use of the transistors in the array as active devices with gain, rather than just switches, provides an amplification of the stored charge, in the gate insulator, from 100 to 800,000 electrons over a read address period of 10 nS.

The retention of the memory devices depends on mobility degradation, which is for all intensive purposes probably permanent and trapped charge which won't decay with zero or positive gate bias. There are some design considerations involved in that the easier programming with SON and/or SRO insulators will result in shorter retention times.

In FIG. 6 a memory device is illustrated according to the teachings of the present invention. The memory device 640 contains a memory array 642, row and column decoders 644, 648 and a sense amplifier circuit 646. The memory array 642 consists of a plurality of write once read only memory cells 600, formed according to the teachings of the present invention whose word lines 680 and bit lines 660 are commonly arranged into rows and columns, respectively. The bit lines 660 of the memory array 642 are connected to the sense amplifier circuit 646, while its word lines 680 are connected to the row decoder 644. Address and control signals are input on address/control lines 661 into the memory device 640 and connected to the column decoder 648, sense amplifier circuit 646 and row decoder 644 and are used to gain read and write access, among other things, to the memory array 642.

The column decoder 648 is connected to the sense amplifier circuit 646 via control and column select signals on column select lines 662. The sense amplifier circuit 646 receives input data destined for the memory array 642 and outputs data read from the memory array 642 over input/output (I/O) data lines 663. Data is read from the cells of the memory array 642 by activating a word line 680 (via the row decoder 644), which couples all of the memory cells corresponding to that word line to respective bit lines 660, which define the columns of the array. One or more bit lines 660 are also activated. When a particular word line 680 and bit lines 660 are activated, the sense amplifier circuit 646 connected to a bit line column detects and amplifies the conduction sensed through a given write once read only memory cell, where in the read operation the source region of a given cell is couple to a grounded array plate (not shown), and transfered its bit line 660 by measuring the potential difference between the activated bit line 660 and a reference line which may be an inactive bit line. The operation of Memory device sense amplifiers is described, for example, in U.S. Pat. Nos. 5,627,785; 5,280,205; and 5,042,011, all assigned to Micron Technology Inc., and incorporated by reference herein.

FIG. 7 is a block diagram of an electrical system, or processor-based system, 700 utilizing write once read only memory 712 constructed in accordance with the present invention. That is, the write once read only memory (WOROM) 712 utilizes the modified DRAM cell as explained and described in detail in connection with FIGS. 2-4. The processor-based system 700 may be a computer system, a process control system or any other system employing a processor and associated memory. The system 700 includes a central processing unit (CPU) 702, e.g., a microprocessor, that communicates with the write once read only memory 712 and an I/O device 708 over a bus 720. It must be noted that the bus 720 may be a series of buses and bridges commonly used in a processor-based system, but for convenience purposes only, the bus 720 has been illustrated as a single bus. A second I/O device 710 is illustrated, but is not necessary to practice the invention. The processor-based system 700 can also includes read-only memory (ROM) 714 and may include peripheral devices such as a floppy disk drive 704 and a compact disk (CD) ROM drive 706 that also communicates with the CPU 702 over the bus 720 as is well known in the art.

It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device 700 has been simplified to help focus on the invention. At least one of the write once read only memory cell in WOROM 712 includes a programmed MOSFET having a charge trapped in the gate insulator adjacent to a first source/drain region, or source region, such that the channel region has a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2), where Vt2 is greater than Vt1, and Vt2 is adjacent the source region such that the programmed MOSFET operates at reduced drain source current.

It will be understood that the embodiment shown in FIG. 7 illustrates an embodiment for electronic system circuitry in which the novel memory cells of the present invention are used. The illustration of system 700, as shown in FIG. 7, is intended to provide a general understanding of one application for the structure and circuitry of the present invention, and is not intended to serve as a complete description of all the elements and features of an electronic system using the novel memory cell structures. Further, the invention is equally applicable to any size and type of memory device 700 using the novel memory cells of the present invention and is not intended to be limited to that described above. As one of ordinary skill in the art will understand, such an electronic system can be fabricated in single-package processing units, or even on a single semiconductor chip, in order to reduce the communication time between the processor and the memory device.

Applications containing the novel memory cell of the present invention as described in this disclosure include electronic systems for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems, such as a clock, a television, a cell phone, a personal computer, an automobile, an industrial control system, an aircraft, and others.

CONCLUSION

Utilization of a modification of well established DRAM technology and arrays will serve to afford an inexpensive memory device which can be regarded as disposable if the information is later transferred to another medium, for instance CDROM's. The high density of DRAM array structures will afford the storage of a large volume of digital data or images at a very low cost per bit. There are many applications where the data need only be written once, the low cost of these memories will make it more efficient to just utilize a new memory array, and dispose of the old memory array, rather than trying to erase and reuse these arrays as is done with current flash memories.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

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