# Patent

## Title
Write once read only memory with large work function floating gates

## Abstract

Structures and methods for write once read only memory employing floating gates are provided. The write once read only memory cell includes a floating gate transistor formed in a modified dynamic random access memory (DRAM) fabrication process. The floating gate transistor has a first source/drain region, a second source/drain region, a channel region between the first and the second source/drain regions, a large work function floating gate separated from the channel region by a gate insulator, and a control gate is separated from the floating gate by a gate dielectric. A plug is coupled to the first source/drain region and couples the first source/drain region to an array plate. A transmission line is coupled to the second source/drain region. The floating gate transistor can be programmed in two directions to trap charge in the high work function floating gate.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to the following co-pending, commonly assigned U.S. patent applications: “Write Once Read Only Memory Employing Charge Trapping in Insulators,” Ser. No. 10/177077, “Write Once Read Only Memory Employing Floating Gates,” Ser. No. 10/177083, “Nanocrystal Write Once Read only Memory for Archival Storage,” Ser. No. 10/177214, “Vertical NROM Having a Storage Density of 1 Bit per 1 F2,” Ser. No. 10/177208, “Ferroelectric Write Once Read Only Memory for Archival Storage,” Ser. No. 10/177082, and “Multistate NROM Having a Storage Density Much Greater than 1 Bit per 1 F2,” Ser. No. 10/177211, which are filed on even date herewith and each of which disclosure is herein incorporated by reference.


FIELD OF THE INVENTION
The present invention relates generally to semiconductor integrated circuits and, more particularly, to write once read only memory with large work function floating gates.
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. Another type of high speed, low cost memory includes floating gate memory cells. A conventional horizontal floating gate transistor structure includes a source region and a drain region separated by a channel region in a horizontal substrate. A floating gate is separated by a thin tunnel gate oxide. The structure is programmed by storing a charge on the floating gate. A control gate is separated from the floating gate by an intergate dielectric. A charge stored on the floating gate effects the conductivity of the cell when a read voltage potential is applied to the control gate. The state of cell can thus be determined by sensing a change in the device conductivity between the programmed and un-programmed states.
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. And, if the retention time is long then the memory can also be used in place of microfilm for archival storage.
REFERENCES


    L. Forbes, W. P. Noble and E. H. Cloud, “MOSFET Technology for Programmable Address Decode and Correction,” application Ser. No. 09/383804, now U.S. Pat. No. 6,521,950;
    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, (November 2001);
    B. Etian et al., “NROM: A novel localized Trapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Lett., Vol. 21, No. 11, pp. 543-545, (November 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, issued Oct. 31, 2000;


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 flash memory cells, yet still hold sufficient charge to withstand the effects of parasitic capacitances and noise due to circuit operation.
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 floating gate 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 floating gate transistor formed in a modified dynamic random access memory (DRAM) fabrication process. The floating gate transistor has a first source/drain region, a second source/drain region, a channel region between the first and the second source/drain regions, a large work function floating gate separated from the channel region by a gate insulator, and a control gate is separated from the floating gate by a gate dielectric. A plug is coupled to the first source/drain region and couples the first source/drain region to an array plate. A transmission line is coupled to the second source/drain region. The floating gate transistor can be programmed in two directions to trap charge in the high work function floating gate.
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.
FIGS. 6 and 7 illustrate the dependence of tunneling current on barrier height as applicable to the present invention.
FIG. 8 illustrates a memory device according to the teachings of the present invention.
FIG. 9 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 √{square root over (Ids)} versus VGS represents the change in the charge carrier mobility in the channel 106.
In FIG. 1C, Δ 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. The inventor also describes write once read only memory cells employing charge trapping in gate insulators for MOSFETs and charge trapping in floating gates, programmable in either direction, for flash cells. The same are described in co-pending, commonly assigned U.S. patent application, entitled “Write Once Read Only Memory Employing Charge Trapping in Insulators,” Ser. No. 10/177077, and “Write Once Read Only Memory Employing Floating Gates,” Ser. No. 10/177083. The present application, however, describes write once read only memory cells, programmable in either direction, formed from flash memory device structures, but having high work function floating gates.
According to the teachings of the present invention, flash memory cells can be programmed, or written to, and read from in two directions and include high work function material floating gates. The novel write once read only memory cells are programmed in either a first or a second mode, by operation in the reverse direction and utilizing avalanche hot electron injection to trap electrons on the high work function floating gate of the floating gate transistor. When the programmed high work function floating gate of the floating gate transistor is subsequently operated in the forward direction the electrons trapped on the high work function floating gate cause the channel to have a different threshold voltage. According to the teachings of the present invention, the high work function floating gates reduce leakage and provide even greater retention times for the write once read only memory. The novel programmed floating gate transistors of the present invention conduct significantly less current than conventional flash cells which have not been programmed. These electrons will remain trapped on the floating gate unless negative control gate voltages are applied. The electrons will not be removed from the floating gate when positive or zero control gate voltages are applied. Erasure can be accomplished by applying negative control gate voltages and/or increasing the temperature with negative control gate bias applied to cause the trapped electrons on the floating gate to be re-emitted back into the silicon channel of the MOSFET.
FIG. 2A is a diagram of a programmed floating gate transistor 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 floating gate transistor 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 floating gate transistor and the second source/drain region 204 includes a drain region 204 for the floating gate transistor. FIG. 2A further illustrates a high work function floating gate 208 separated from the channel region 206 by a floating gate insulator 210. A control gate 216 is further separated from the high work function floating gate 208 by a gate dielectric 218. An array plate 212 is coupled to the first source/drain region 202 and a transmission line 214 is coupled to the second source/drain region 204. In one embodiment, the transmission line 214 includes a bit line 214.
As stated above, write once read only memory cell 201 is comprised of a programmed floating gate transistor. This programmed floating gate transistor has a charge 217 trapped on the high work function floating gate 208. In one embodiment, the charge 217 trapped on the high work function floating gate 208 includes a trapped electron charge 217.
FIG. 2B is a diagram suitable for explaining the method by which the high work function floating gate 208 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 floating gate transistor. Programming the floating gate transistor includes applying a first voltage potential V1 to a drain region 204 of the floating gate transistor and a second voltage potential V2 to the source region 202.
In one embodiment, applying a first voltage potential V1 to the drain region 204 of the floating gate transistor includes grounding the drain region 204 of the floating gate transistor as shown in FIG. 2B. In this embodiment, applying a second voltage potential V2 to the source region 202 includes biasing the array plate 212 to a voltage higher than VDD, as shown in FIG. 2B. A gate potential VGS is applied to the control gate 216 of the floating gate transistor. 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 floating gate transistor 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 floating gate transistor creates a hot electron injection into the high work function floating gate 208 of the floating gate transistor 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 high work function floating gate 208 adjacent to the source region 202. Here the charge carriers become trapped.
In an alternative embodiment, applying a first voltage potential V1 to the drain region 204 of the floating gate transistor includes biasing the drain region 204 of the floating gate transistor to a voltage higher than VDD. In this embodiment, applying a second voltage potential V2 to the source region 202 includes grounding the array plate 212. A gate potential VGS is applied to the control gate 216 of the floating gate transistor. In one embodiment, the gate potential VGS includes a voltage potential which is less than the first voltage potential V1, but which is sufficient to establish conduction in the channel 206 of the floating gate transistor between the drain region 204 and the source region 202. Applying the first, second and gate potentials (V1, V2, and VGS respectively) to the floating gate transistor creates a hot electron injection into the high work function floating gate 208 of the floating gate transistor adjacent to the drain region 204. 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 drain region 204, a number of the charge carriers get excited into the high work function floating gate 208 adjacent to the drain region 204. Here the charge carriers become trapped as shown in FIG. 2A.
In one embodiment of the present invention, the method is continued by subsequently operating the floating gate transistor in the forward direction, shown in FIG. 2A, 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 control gate 216, then its conductivity will be determined by the presence or absence of stored charge in the high work function floating gate 208. That is, a gate potential, VGS, can be applied to the control gate 216 in an effort to form a conduction channel between the source and the drain regions, 202 and 204 respectively, as done with addressing and reading conventional DRAM cells. However, now in its programmed state, the conduction channel 206 of the floating gate transistor will have a higher voltage threshold
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 floating gate transistor where the transistor 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 floating gate transistor (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 floating gate transistor (curve 207) is significantly lower than the current signal (IDS1) detected at the second source/drain region 204 for the conventional floating gate cell (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 floating gate transistor of the present invention has a different voltage threshold.
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 inventors have previously disclosed charge trapping at isolated point defects in gate insulators. However, none of the above described references addressed forming write once read only memory cells, having high work function floating gates and programmable in either direction in a first and second mode of operation, for flash memory cell device structures.
That is, in contrast to the above work, the present invention discloses programming a floating gate transistor, in either a first or a second direction, to trap charge in high work function floating gates and reading the device to form a write once read only memory (WOROM) based on a modification of DRAM technology.
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 floating gate 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 floating gate 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 transmission line, or 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 control gate, 318-1 and 318-2 respectively, for each of the write once read only floating gate memory cells, 301-1 and 301-2. According to the teachings of the present invention a high work function floating gate, 320-1 and 320-2 respectively, is separated from a channel region, 322-1 and 322-2, in the write once read only floating gate memory cells, 301-1 and 301-2, beneath the control gate, 318-1 and 318-2.
According to the teachings of the present invention, in one embodiment the high work function floating gate, 320-1 and 320-2 respectively, is formed of a refractory metal selected from the group of molybdenum (Mo) and tungsten (W). In another embodiment, the high work function floating gate, 320-1 and 320-2 respectively, is formed of a large work function material which includes a large work function material selected from the group of p-type silicon germanium gates, p-type polycrystalline gate of silicon carbide, p-type polycrystalline gate of silicon oxycarbide, gallium nitride (GaN), and aluminum gallium nitride (AlGaN). In still other embodiments, the high work function floating gate, 320-1 and 320-2 respectively, includes a heavily doped p-type polysilicon with a vacuum work function of 5.3 eV.
A write data/precharge circuit is shown at 324 for coupling a first or a second potential to transmission line, or 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 a first program 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 first program direction, or alternatively grounded during a read operation in the forward direction.
As shown in FIG. 3, the array structure 300 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 a first program 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 high work function floating gate, 320-1 and 320-2 respectively, adjacent to the source region, 302-1 or 302-2. Alternatively, the array plate 304 can be grounded and the data or bit line, 308-1 or 308-2 driven to some voltage higher than VDD to stress the device in a second program direction. Again, 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 high work function floating gate, 320-1 or 320-2, adjacent to the drain region, 306-1 or 306-2. During read, the write once read only floating gate memory cell, 301-1 or 301-2, is 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 on the cells high work function floating gate, 320-1 and 320-2 respectively, 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 can be subjected to hot electron stress in either a first or a second program direction, e.g. first or second mode. In a first program direction, the array plate 304 is biased, and then 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.
It is important to note that according to the teachings of the present invention, the write once read only memory cell, e.g. cell 301-1, can be written to or programmed in two directions. That is, writing to the one or more the floating gate transistors having high work function floating gates, 320-1 and 320-2 respectively, includes writing to the one or more floating gate transistors in a first and a second direction. Writing in a first and a second direction includes applying a first voltage potential to the first source/drain region, 302-1 or 302-2, of the floating gate transistor, applying a second voltage potential to the second source/drain region, 306-1 or 306-2, of the floating gate transistor, and applying a gate potential to the control gate, 312-1 and/or 312-2 of the floating gate transistor. As one of ordinary skill in the art will appreciate upon reading this disclosure, applying the first, second and gate potentials to the one or more floating gate transistors, e.g. cell 301-1, includes creating a hot electron injection into the large work function floating gate, e.g. 320-1, of the one or more floating gate transistors such that a programmed floating gate transistor operates at a reduce drain source current.
For purposes of illustration herein, programming cell 301- 1 is described using 301-2 as a reference or dummy cell. Thus, in one embodiment as described above, when writing in a first direction, applying a first voltage potential to the first source/drain region 302-1 of the floating gate transistor includes grounding the first source/drain region 302-1 of the floating gate transistor, applying a second voltage potential to the second source/drain region 306-1 includes applying a high voltage potential (VDD) to the second source/drain region, and applying a gate potential to the control gate creates a conduction channel between the first and the second source/drain regions, 302-1 and 306-1 respectively, of the floating gate transistor 301-1. And, when writing in a second direction, applying a first voltage potential to the first source/drain region 302-1 of the floating gate transistor includes applying a high voltage potential (VDD) to the first source/drain region 302-1 of the floating gate transistor, applying a second voltage potential to the second source/drain region 306-1 includes grounding the second source/drain region 306-1, and applying a gate potential to the control gate creates a conduction channel between the first and the second source/drain regions of the floating gate transistor 301-1.
In the invention, reading one or more floating gate transistors in the DRAM array includes operating an addressed floating gate transistor, e.g. 301-1 in a forward direction. In one embodiment, operating the floating gate transistor in the forward direction includes grounding the array plate 304, precharging the transmission line 308-1 to a fractional voltage of VDD, and applying a control gate potential of approximately 1.0 Volt to the gate of the addressed floating gate transistor.
In one embodiment as described in more detail below reading the one or more floating gate transistors includes using a sense amplifier 310 to detect a change in an integrated drain current of the addressed floating gate transistor 301-1 as compared to a reference or dummy cell, e.g. 301-2. In one read embodiment, the floating gate transistor will exhibit a change in an integrated drain current of approximately 12.5 μA when addressed over approximately 10 ns when no charge is programmed in the high work function floating gate. According to the teachings of the present invention, the floating gate transistors in the DRAM array as active devices with gain, and wherein reading a programmed flash cell includes providing an amplification of the stored charge in the floating gate from 100 to 800,000 electrons over a read address period of approximately 10 ns.
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. That is, the transfer devices in the DRAM arrays are replaced by flash memory type devices with high work function floating gates. 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 high work function floating gate to modulate the conductivity of the write once read only floating gate 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 floating gate 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 Å (10−6 cm). In the embodiment shown in FIG. 4A a write once read only floating gate memory cell has dimensions 413 of 0.1 μm (10−5 cm) by 0.1 μm. The capacitance, Ci, of the structure depends on the dielectric constant, ∈i, and the thickness of the insulating layers, t. In an embodiment, the dielectric constant is 0.3×10−12 F/cm and the thickness of the insulating layer is 10−6 cm such that Ci=∈i/t, Farads/cm2 or 3×10−7 F/cm2. In one embodiment, a charge of 1012 electrons/cm2 is programmed into the high work function floating gate 408 of the write once read only floating gate memory cell 401. This produces a stored charge ΔQ=1012 electrons/cm2×1.6×10−19 Coulombs. In this embodiment, the resulting change in the threshold voltage (Δ Vt) of the write once read only floating gate memory cell will be approximately 0.5 Volts (Δ Vt=ΔQ/Ci or 1.6×10−7/3×10−7=1⁄2 Volt). For ΔQ =1012 electrons/cm3 in an area of 10−10 cm2, this embodiment of the present invention involves trapping a charge of approximately 100 electrons in the high work function floating gate 408 of the write once read only floating gate memory cell 401. In this embodiment, an original VT is approximately 1⁄2 volt and the VT with charge trapping is approximately 1 Volt.
FIG. 4B aids to further illustrate the conduction behavior of the novel write once read only floating gate 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 floating gate memory cell is being driven with a control gate 416 voltage of 1.0 Volt (V) and the nominal threshold voltage without the high work function floating gate 408 charged is 1⁄2 V, then if the high work function floating gate 408 is charged the floating gate transistor 401 of the present invention will be off and not conduct. That is, by trapping a charge of approximately 100 electrons in the high work function floating gate 408 of the write once read only floating gate memory cell, having dimensions of 0.1 μm (10−5 cm) by 0.1 μm, will raise the threshold voltage of the write once read only floating gate memory cell to 1.0 Volt and a 1.0 Volt control 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 high work function floating gate 408 charged is 1⁄2 V, then I=μCox×(W/L)×((Vgs−Vt)2/2), or 12.5 μA, with μCox=μCl=100 μA/V2 and W/L=1. That is, the write once read only floating gate memory cell 401 of the present invention, having the dimensions describe above will produce a current I=100 μA/V2×(1⁄4)=( l/2)=12.5 μA. Thus, in the present invention an un-written, or un-programmed high work function floating gate 408 of the write once read only floating gate memory cell 401 can conduct a current of the order 12.5 μA, whereas if the high work function floating gate 408 is charged then the write once read only floating gate memory cell 401 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 a bit line 552 is only 10 μA (I=50 fC/5 ns=10 μA). Thus, storing a 50 fC charge on the storage capacitor shown in FIG. 5 equates to storing 300,000 electrons (Q=50 fC/(1.6×10−19)=30×104=300,000 electrons).
According to the teachings of the present invention, the floating gate 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 floating gate transistor “off,” requires only a stored charge in the high work function floating gate 408 of about 100 electrons if the area is 0.1 μm by 0.1 μm. And, if the write once read only floating gate memory cell 401 is un-programmed, e.g. no stored charge trapped in the high work function floating gate 408, and if the floating gate transistor is addressed over 10 nS a current of 12.5 μA 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 floating gate transistors in the array as active devices with gain, rather than just switches, provides an amplification of the stored charge, in the high work function floating gate 408, from 100 to 800,000 electrons over a read address period of 10 nS.
The unique aspect of this disclosure is the use of floating gates with large work functions to increase the tunneling barriers with the silicon oxide gate insulators on each side of the floating gate, as shown in FIG. 6. Current flash memories utilize a floating polysilicon gate over a silicon dioxide gate insulator of thickness of the order 100 Å or 10 nm or less in a field effect transistor. This results in a high barrier energy, as shown in FIG. 7, or around 3.2 eV for electrons between the silicon substrate and gate insulator and between the floating polysilicon gate and silicon oxide gate insulators. FIG. 7 provides a chart showing the dependence of tunneling current on barrier height. FIG. 7 illustrates a number of different electric fields E1, E2, and E3 plotted for the log of various tunneling current density (A/cm2) versus various barrier energy, Φ, (eV). This combination of barrier height and oxide thickness results in long retention times even at 250 degrees Celsius. The simple idea would be that retention times are determined by thermal emission over the 3.2 eV barrier, however, these are extremely long so the current model is that retention is limited by thermally assisted tunneling off of the charged gate. This produces a lower “apparent” activation energy of 1.5 eV as has been observed and shorter retention times. For archival storage in a write once mode of operation with no requirement to erase the longest possible retention times will be achieved with floating gates with work functions larger than 3.2 eV.
According to the teachings of the present invention, retention times are increased by using:

    
    
        (i) thick gate insulators between the silicon substrate and floating gates, since there is no requirement for erase lower electric fields result in lower tunneling currents and longer retention, see FIG. 6
        (ii) thick gate insulators between the floating gate and address or control gate; since there is no requirement for erase lower electric fields result in longer retention times
        (iii) low read voltages on the address or control gates; since the DRAM sense amplifiers can sense small differences in conductivity states smaller biases can be applied to the devices resulting in lower electric fields and longer retention times

This disclosure then describes the use of:

        (i) refractory metal floating gates, Mo and W, with vacuum work functions of around 4.7 eV which is larger than that of conventional n-type polysilicon floating gates with a vacuum work function of 4.1 eV, larger barriers result in lower tunneling currents and longer retention times, see FIG. 7
        (ii) heavily doped p-type polysilicon floating gates with a vacuum work function of 5.3 eV, p-type poly silicon-germanium gates, or p-type polycrystalline gates of other semiconductors as silicon carbide, silicon oxycarbide, GaN or AlGaN, with vacuum work functions greater than conventional n-type polysilicon floating gates.
    
    


In FIG. 8 a memory device is illustrated according to the teachings of the present invention. The memory device 840 contains a memory array 842, row and column decoders 844, 848 and a sense amplifier circuit 846. The memory array 842 consists of a plurality of write once read only floating gate memory cells, formed according to the teachings of the present invention, whose word lines 880 and bit lines 860 are commonly arranged into rows and columns, respectively. The bit lines 860 of the memory array 842 are connected to the sense amplifier circuit 846, while its word lines 880 are connected to the row decoder 844. Address and control signals are input on address/control lines 861 into the memory device 840 and connected to the column decoder 848, sense amplifier circuit 846 and row decoder 844 and are used to gain read and write access, among other things, to the memory array 842.
The column decoder 848 is connected to the sense amplifier circuit 846 via control and column select signals on column select lines 862. The sense amplifier circuit 846 receives input data destined for the memory array 842 and outputs data read from the memory array 842 over input/output (I/O) data lines 863. Data is read from the cells of the memory array 842 by activating a word line 880 (via the row decoder 844), which couples all of the memory cells corresponding to that word line to respective bit lines 860, which define the columns of the array. One or more bit lines 860 are also activated. When a particular word line 880 and bit lines 860 are activated, the sense amplifier circuit 846 connected to a bit line column detects and amplifies the conduction sensed through a given write once read only floating gate memory cell and transferred to its bit line 860 by measuring the potential difference between the activated bit line 860 and a reference line which may be an inactive bit line. Again, in the read operation the source region of a given cell is coupled to a grounded array plate (not shown). 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. 9 is a block diagram of an electrical system, or processor-based system, 900 utilizing write once read only floating gate memory 912 constructed in accordance with the present invention. That is, the write once read only memory (WOROM) 912 utilizes the modified flash cell as explained and described in detail in connection with FIGS. 2-7. The processor-based system 900 may be a computer system, a process control system or any other system employing a processor and associated memory. The system 900 includes a central processing unit (CPU) 902, e.g., a microprocessor, that communicates with the write once read only floating gate memory 912 and an I/O device 908 over a bus 920. It must be noted that the bus 920 may be a series of buses and bridges commonly used in a processor-based system, but for convenience purposes only, the bus 920 has been illustrated as a single bus. A second I/O device 910 is illustrated, but is not necessary to practice the invention. The processor-based system 900 can also includes read-only memory (ROM) 914 and may include peripheral devices such as a floppy disk drive 904 and a compact disk (CD) ROM drive 906 that also communicates with the CPU 902 over the bus 920 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 900 has been simplified to help focus on the invention. At least one of the write once read only floating gate memory cell in WOROM 912 includes a programmed flash cell, programmable in a first and second direction and having a high work function floating gate.
It will be understood that the embodiment shown in FIG. 9 illustrates an embodiment for electronic system circuitry in which the novel memory cells of the present invention are used. The illustration of system 900, as shown in FIG. 9, 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 900 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 for archival storage. 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 and retained in archival storage.
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.

## Claims

1. A write once read only memory cell, comprising:
a floating gate transistor, the floating gate transistor including; 
a first source/drain region; 
a second source/drain region; 
a channel region between the first and the second source/drain regions; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate is formed of a material having a vacuum work function larger than 4.1 eV; and 
a control gate separated from the floating gate by a gate dielectric; 

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 floating gate transistor is a programmed floating gate transistor having a charge trapped in the floating gate such that the programmed floating gate transistor operates at reduced drain source current.

2. A write once read only memory cell, comprising:
a floating gate transistor, the floating gate transistor including; 
a first source/drain region; 
a second source/drain region; 
a channel region between the first and the second source/drain regions; 
a floating gate separated from the channel region by a gate insulator; 
wherein the floating gate is formed of a refractory metal selected from the group of molybdenum (Mo) and tungsten (W) having a vacuum work function approximately equal to 4.7 eV;and 
a control gate separated from the floating gate by a gate dielectric; 
a plug coupled to the first source/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 floating gate transistor is a programmed floating gate transistor having a 
charge trapped in the floating gate such that the programmed floating gate transistor operates at reduced drain source current.

3. A write once read o nly memory cell, comprising:
a floating gate transistor, the floating gate transistor including; 
a first source/drain region; 
a secon source/drain region; 
a channel region between the first and the second source/drain regions; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate is formed of a material having a vacuum work function larger than 4.1 eV; and 
a control gate separated from the floating gate by a gate dielectric; 
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 floating gate transistor is a programmed floating gate transistor having a charge trapped in the floating gate such that the programmed floating gate transistor operates at reduced drain source current; 
wherein the floating gate material includes a large work function material selected from the group of p-type silicon germanium gates, p-type polycrystalline gate of silicon carbide, p- type polycrystalline gate of silicon oxycarbide, gallium nitride (GaN), and aluminum gallium nitride (AlGaN).

4. A write once read only memory cell, comprising:
a floating gate transistor, the floating gate transistor including; 
a first source/drain region; 
a second source/drain region; 
a channel region between the first and the second source/drain regions; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate is formed of a material having a vacuum work function largr than 4.1 eV; and 
a control gate separated from the floating gate by a gate dielectric; 
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 floating gate transistor is a programmed floating gate transistor having a charged trapped in the floating gate such that the programmed floating gate transistor operates at reduced drain source current; 
wherein the floating gate includes a heavily doped p-type polysilicon with a vacuum work function of 5.4 eV.

5. The write once read only memory cell of claim 1, wherein the write once read only memory (WOROM) cell is formed in a modified dynamic random access memory (DRAM) fabrication process.

6. The write once read only memory cell of claim 2, wherein the gate insulator has a thickness of at least 10 nanometers (nm).

7. A write once read only memory cell, comprising:
a floating gate transistor formed according to a modified DRAM fabrication process, the floating gate transistor including; 
a source region; 
a drain region; 
a channel region between the first and the second source/drain regions; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate is formed of a refractory metal selected from the group of molybdenum (Mo) and tungsten (W); and 
a control gate separated from the floating gate by a gate dielectric; 

a wordline coupled to the control gate; 
an array plate coupled to the source region; 
a bit line coupled to the drain region; and 
wherein the floating gate transistor is a programmed floating gate transistor having a charge trapped in the floating gate.

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

9. A memory array, comprising:
a number of write once read only floating gate memory cells, wherein each write once read only floating gate 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; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate includes a large work function material selected from the group of p-type silicon germanium gates, p-type polycrystalline gate of silicon carbide, p-type polycrystalline gate of silicon oxycarbide, gallium nitride (GaN), and aluminum gallium nitride (AlGaN); and 
a control gate separated from the floating gate by a gate dielectric 

a number of bit lines coupled to the second source/drain region of each write once read only floating gate memory cell along rows of the memory array; 
a number of word lines coupled to the control gate of each write once read only floating gate memory cell along columns of the memory array; 
an array plate, wherein the first source/drain region of each write once read only floating gate memory cell is coupled to the array plate by a conductive plug; and 
wherein at least one of write once read only floating gate memory cells is a programmed floating gate transistor having a charge trapped in the floating gate such that the programmed floating gate transistor operates at reduced drain source current.

10. The memory array of claim 9, wherein the charge trapped in the floating gate includes a charge of approximately 100 electrons.

11. The memory array of claim 9, wherein the first source/drain region of the floating gate cell includes a source region and the second source/drain region of the floating gate cell includes a drain region.

12. The memory array of claim 9, wherein the number of write once read only memory (WOROM) cell are formed in a modified dynamic random access memory (DRAM) fabrication process.

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

14. A memory device, comprising:
a memory array, wherein the memory array includes a number of write once read only floating gate memory cells, wherein each write once read only floating gate 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; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate includes a heavily doped p-type polysilicon with a vacuum work function of 5.3 eV; and 
a control gate separated from the floating gate by a gate dielectric; 

a number of bitlines coupled to the drain region of each write once read only floating gate memory cell along rows of the memory array; 
a number of wordlines coupled to the control gate of each write once read only floating gate memory cell along columns of the memory array; 
an array plate, wherein the source region of each write once read only floating gate 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 floating gate memory cells is a programmed flash cell having a charge trapped in the floating gate such that the programmed flash cell operates at reduced drain/source current.

15. The memory device of claim 14, wherein the charge trapped in the floating gate includes a charge of approximately 100 electrons.

16. The memory device of claim 14, wherein the number of write once read only memory (WOROM) cell is formed in a modified dynamic random access memory (DRAM) fabrication process.

17. The memory device of claim 14, wherein the gate insulator of each write once read only floating gate memory cell has a thickness of at least 10 nanometers (nm).

18. The memory device of claim 14, wherein the wordline address decoder and the bitline address decoder each include conventionally fabricated transistors having thin gate insulators formed of silicon dioxide (SiO2).

19. The memory device of claim 14, wherein the sense amplifier includes conventionally fabricated transistors having thin gate insulators formed of silicon dioxide (SiO2).

20. An electronic system, comprising:
a processor; and 
a memory device coupled to the processor, wherein the memory device including a memory array, wherein the memory array includes a number of write once read only floating gate memory cells, wherein each write once read only floating gate 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; 
a floating gate separated from the channel region by a gate insulator; wherein the floating gate is formed of a refractory metal selected from the group of molybdenum (Mo) and tungsten (W); and 
a control gate separated from the floating gate by a gate dielectric; 
a number of bitlines coupled to the drain region of each write once read only floating gate memory cell along rows of the memory array; 
a number of wordlines coupled to the control gate of each write once read only floating gate memory cell along columns of the memory array; 
an array plate, wherein the source region of each write once read only floating gate 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 floating gate memory cells is a programmed flash cell having a charge trapped in the floating gate such that the programmed flash cell operates at reduced drain/source current.

21. The electronic system of claim 20, wherein the charge trapped in the floating gate includes a charge of approximately 100 electrons.

22. The electronic system of claim 20, wherein, in a read operation, the array plate is coupled to a ground potential, the drain region of an addressed write once read only floating gate memory cell is precharged to a fractional voltage of VDD, and the memory cell is selected by a wordline address.

23. The electronic system of claim 20, 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.