Patent Publication Number: US-2003235076-A1

Title: Multistate NROM having a storage density much greater than 1 Bit per 1F2

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] 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,” attorney docket no. 1303.052U.S. 1, Ser. No. ______, “Write Once Read Only Memory Employing Floating Gates,” attorney docket no. 1303.051U.S.1, Ser. No. ______, “Nanocrystal Write Once Read Only Memory for Archival Storage,” attorney docket no. 1303.054U.S.1, Ser. No. ______, “Write Once Read Only Memory with Large Work Function Floating Gates,” attorney docket no. 1303.055U.S.1, Ser. No. ______, “Ferroelectric Write Once Read Only Memory for Archival Storage,” attorney docket no. 1303.058U.S.1, Ser. No. ______, and “Vertical NROM Having a Storage Density of 1 Bit per 1F 2 ,” attorney docket no. 1303.057U.S.1, Ser. No. ______, which are filed on even date herewith and each of which disclosure is herein incorporated by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally to semiconductor integrated circuits and, more particularly, to multistate NROM having a storage density much greater than one bit for each 1.0 lithographic feature squared (1F 2 ) unit area.  
       BACKGROUND OF THE INVENTION  
       [0003] 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.  
       [0004] A requirement exists for memory devices which need only be programmed a limited number of times, 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.  
       [0005] Thus, there is a need for improved DRAM technology compatible multistate NROM having a storage density of much greater than one bit per for each 1.0 lithographic feature squared (1F 2 ) unit area. It is desirable that such a multistate NROM be fabricated on a DRAM chip with little or no modification of the DRAM process flow. It is further desirable that such a multistate NROM 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.  
       SUMMARY OF THE INVENTION  
       [0006] The above mentioned problems for creating DRAM technology compatible multistate 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 multistate memory cells in a DRAM integrated circuit. The structures and methods use the existing process sequence for MOSFET&#39;s in DRAM technology.  
       [0007] In particular, an illustrative embodiment of the present invention includes a vertical metal oxide semiconductor field effect transistor (MOSFET) extending outwardly from 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 sourceline is formed in a trench adjacent to the vertical MOSFET, wherein the first source/drain region is coupled to the sourceline. A transmission line is coupled to the second source/drain region. The can be programmed MOSFET to have one of a number of charge levels 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.  
       [0008] 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  
     [0009]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.  
     [0010]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.  
     [0011]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.  
     [0012]FIG. 2A is a diagram of a programmed MOSFET which can be used as a multistate cell according to the teachings of the present invention.  
     [0013]FIG. 2B is a diagram suitable for explaining the method by which the MOSFET of the multistate cell of the present invention can be programmed to achieve the embodiments of the present invention.  
     [0014]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).  
     [0015]FIG. 3 illustrates a portion of a memory array according to the teachings of the present invention.  
     [0016]FIG. 4 illustrates an electrical equivalent circuit for the portion of the memory array shown in FIG. 3.  
     [0017]FIG. 5 is another electrical equivalent circuit useful in illustrating a read operation on the novel multistate cell according to the teachings of the present invention.  
     [0018]FIG. 6 illustrates a portion of a memory array according to the teachings of the present invention.  
     [0019]FIG. 7A, illustrates one embodiment of the gate insulator for the present invention having a number of layers, e.g. an ONO stack, where the layer closest to the channel includes an oxide layer, and a nitride layer is formed thereon.  
     [0020]FIG. 7B aids to further illustrate the conduction behavior of the novel multistate cell of the present invention.  
     [0021]FIG. 8A illustrates the operation and programming the novel multistate cell in the reverse direction.  
     [0022]FIG. 8B illustrates the now programmed multistate cell&#39;s operation in the forward direction and differential read occurring in this differential cell embodiment, e.g. 2 transistors in each cell.  
     [0023]FIG. 9 illustrates a memory device according to the teachings of the present invention.  
     [0024]FIG. 10 is a block diagram of an electrical system, or processor-based system, utilizing a multistate cell constructed in accordance with the present invention.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0025] 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.  
     [0026] 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.  
     [0027]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.  
     [0028]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 .  
     [0029] 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 .  
     [0030] 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.  
     [0031]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}{square root over (Ids)} versus VGS represents the change in the charge carrier mobility in the channel  106 .  
     [0032] In FIG. 1C, ΔVT represents the minimal change in the MOSFET&#39;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 1 represents the charge carrier mobility in the channel  106  for FIG. 1A having no electrons trapped in the gate oxide  110 . Slope 2 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 1 and slope 2 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 .  
     [0033] 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.  
     [0034] The inventors have previously described programmable memory devices and functions based on the reverse stressing of MOSFET&#39;s in a conventional CMOS process and technology in order to form programmable address decode and correction. (See generally, L. Forbes, W. P. Noble and E. H. Cloud, “MOSFET technology for programmable address decode and correction,” application Ser. No. 09/383804). That disclosure, however, did not describe multistate memory cell solutions, but rather address decode and correction issues.  
     [0035] 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. (See generally, L. Forbes, E. Sun, R. Alders and J. Moll, “Field induced re-emission of electrons trapped in SiO 2 ,” IEEE Trans. Electron Device, vol. ED-26, no. 11, pp. 1816-1818 (November 1979); S. S. B. Or, N. Hwang, and L. Forbes, “Tunneling and Thermal emission from a distribution of deep traps in SiO 2 ,” IEEE Trans. on Electron Devices, vol. 40, no. 6, pp. 1100-1103 (June 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., December 1975, pp. 35-38).  
     [0036] FIGS.  2 A- 2 C 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.  
     [0037]FIG. 2A is a diagram of a programmed MOSFET which can be used as a multistate cell according to the teachings of the present invention. As shown in FIG. 2A the multistate 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 .  
     [0038] As stated above, multistate 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 . According to the teachings of the present invention and as described in more detail below, the multistate cell can be programmed to have one of a number of charge levels trapped in the gate insulator adjacent to the first source/drain region  202  such that the channel region  206  will have a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) and such that the programmed multistate cell operates at reduced drain source current.  
     [0039]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 . Multiple bits can be stored on the multistate cell  201 .  
     [0040]FIG. 2B is a diagram suitable for explaining the method by which the MOSFET of the multistate 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.  
     [0041] 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.  
     [0042] 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 .  
     [0043]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 D1 represents the conduction behavior of a conventional MOSFET which is not programmed according to the teachings of the present invention. The curve D2 represents the conduction behavior of the programmed MOSFET, 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 (IDS 2 ) detected at the second source/drain region  204  for the programmed MOSFET (curve D2) is significantly lower than the current signal (IDS 1 ) detected at the second source/drain region  204  for the conventional MOSFET 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 .  
     [0044] 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. (See generally, 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)). Charge trapping in silicon nitride gate insulators was the basic mechanism used in MNOS memory devices (see generally, S. Sze, Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506), charge trapping in aluminum oxide gates was the mechanism used in MIOS memory devices (see generally, S. Sze, Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506), and the present inventors have previously disclosed charge trapping at isolated point defects in gate insulators (see generally, L. Forbes and J. Geusic, “Memory using insulator traps,” U.S. Pat. No. 6,140,181, issued Oct. 31, 2000).  
     [0045] In contrast to the above work, the present invention disclosures programming a MOSFET in a reverse direction to trap one of a number of charge levels near the source region and reading the device in a forward direction to form a multistate memory cell based on a modification of DRAM technology.  
     [0046] 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 multistate memory cell employs charge trapping in gate insulators such as, wet silicon oxide, silicon nitride, silicon oxynitride SON, silicon rich oxide SRO, aluminum oxide Al 2 O 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.  
     [0047]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 number of vertical pillars, or multistate cells,  301 - 1  and  301 - 2  formed according to the teachings of the present invention. As one of ordinary skill in the art will appreciate upon reading this disclosure, the number of vertical pillar are formed in rows and columns extending outwardly from a substrate  303 . As shown in FIG. 3, the number of vertical pillars,  301 - 1  and  301 - 2  are separated by a number of trenches  340 . According to the teachings of the present invention, the number of vertical pillars,  301 - 1  and  301 - 2 , serve as transistors including a first source/drain region,  302 - 1  and  302 - 2  respectively. The first source/drain region,  302 - 1  and  302 - 2 , is coupled to a sourceline  304 . As shown in FIG. 3, the sourceline  304  is formed in a bottom of the trenches  340  between rows of the vertical pillars,  301 - 1  and  301 - 2 . In one embodiment, according to the teachings of the present invention, the sourceline  304  is formed from a doped region implanted in the bottom of the trench. A second source/drain region,  306 - 1  and  306 - 2  respectively, is coupled to a bitline (not shown). A channel region  305  is located between the first and the second source/drain regions.  
     [0048] As shown in FIG. 3, a gate  307  is separated from the channel region  305  by a gate insulator  307  in the trenches  340  along rows of the vertical pillars,  301 - 1  and  301 - 2 . In one embodiment, according to the teachings of the present invention, the gate insulator  307  includes a gate insulator  307  selected from the group of silicon dioxide (SiO 2 ) formed by wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO), and aluminum oxide (Al 2 O 3 ). In another embodiment, according to the teachings of the present invention, the gate insulator  307  includes a gate insulator  307  selected from the group of silicon rich aluminum oxide insulators, silicon rich oxides with inclusions of nanoparticles of silicon, silicon oxide insulators with inclusions of nanoparticles of silicon carbide, and silicon oxycarbide insulators. In another embodiment, according to the teachings of the present invention, the gate insulator  307  includes a composite layer  307 . In this embodiment, the composite layer  307  includes a composite layer  307  selected from the group of an oxide-aluminum oxide (Al 2 O 3 )-oxide composite layer, and oxide-silicon oxycarbide-oxide composite layer. In another embodiment, the composite layer  307  includes a composite layer  307 , or a non-stoichiometric single layer, of two or more materials selected from the group of silicon (Si), titanium (Ti), and tantalum (Ta). In another embodiment, according to the teachings of the present invention, the gate insulator  307  includes an oxide-nitride-oxide (ONO) gate insulator  307 .  
     [0049]FIG. 4 illustrates an electrical equivalent circuit  400  for the portion of the memory array shown in FIG. 3. As shown in FIG. 4, a number of vertical multistate cells,  401 - 1  and  401 - 2 , are provided. Each vertical multistate cell,  401 - 1  and  401 - 2 , includes a first source/drain region,  402 - 1  and  402 - 2 , a second source/drain region  406 - 1  and  406 - 2 , a channel region  405  between the first and the second source/drain regions, and a gate  409  separated from the channel region by a gate insulator  407 .  
     [0050]FIG. 4 further illustrates a number of bit lines,  411 - 1  and  411 - 2 , coupled to the second source/drain region  406 - 1  and  406 - 2  of each multistate cell. In one embodiment, as shown in FIG. 4, the number of bit lines,  411 - 1  and  411 - 2 , are coupled to the second source/drain region  406 - 1  and  406 - 2  along rows of the memory array. A number of word lines, such as wordline  413  in FIG. 4, are coupled to the gate  409  of each multistate cell along columns of the memory array. And, a number of sourcelines, such as common sourceline  415 , are coupled to the first source/drain regions, e.g.  402 - 1  and  402 - 2 , along columns of the vertical multistate cells,  401 - 1  and  401 - 2 , such that adjacent pillars containing these transistors share the common sourceline  415 . In one embodiment, column adjacent pillars include a transistor which operates as a vertical multistate cell, e.g.  401 - 1 , on one side of a shared trench, the shared trench separating rows of the pillars as described in connection with FIG. 3, and a transistor which operates as a reference cell, e.g.  401 - 2 , having a programmed conductivity state on the opposite side of the shared trench. In this manner, according to the teachings of the present invention and as described in more detail below, at least one of multistate cells can be programmed to have one of a number of charge levels trapped in the gate insulator, shown generally as  417 , adjacent to the first source/drain region, e.g.  402 - 1 , such that the channel region  405  will have a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) and such that the programmed multistate cell operates at reduced drain source current.  
     [0051]FIG. 5 is another electrical equivalent circuit useful in illustrating a read operation on the novel multistate cell  500  according to the teachings of the present invention. The electrical equivalent circuit in FIG. 5 represents a programmed vertical multistate cell. As explained in detail in connection with FIG. 3, the programmed vertical multistate cell  500  includes a vertical metal oxide semiconductor field effect transistor (MOSFET)  500  extending outwardly from a substrate. The MOSFET has a source region  502 , a drain region  506 , a channel region  505  between the source region  502  and the drain region  506 , and a gate  507  separated from the channel region  505  by a gate insulator, shown generally as  517 .  
     [0052] As shown in FIG. 5 a wordline  513  is coupled to the gate  507 . A sourceline  504 , formed in a trench adjacent to the vertical MOSFET as described in connection with FIG. 3, is coupled to the source region  502 . A bit line, or data line  511  is coupled to the drain region  506 . The multistate cell  500  shown in FIG. 5 is an example of a programmed multistate cell  500  having one of a number of charge levels trapped in the gate insulator, shown generally as  517 , adjacent to the first source/drain region,  502 , such that the channel region  505  will have a first voltage threshold region (Vt1) and a second voltage threshold region (Vt2) and such that the programmed multistate cell  500  operates at reduced drain source current. According to the teachings of the present invention, the second voltage threshold region (Vt2) is now a high voltage threshold region which is greater than the first voltage threshold region (Vt1).  
     [0053]FIG. 6 illustrates a portion of a memory array  600  according to the teachings of the present invention. The memory in FIG. 6, is shown illustrating a pair of multistate cells  601 - 1  and  601 - 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 multistate cells can be organized in an array, but for ease of illustration only two are displayed in FIG. 6. As shown in FIG. 6, a first source/drain region,  602 - 1  and  602 - 2  respectively, is coupled to a sourceline  604 . A second source/drain region,  606 - 1  and  606 - 2  respectively, is coupled to a bitline,  608 - 1  and  608 - 2  respectively. Each of the bitlines,  608 - 1  and  608 - 2 , couple to a sense amplifier, shown generally at  610 . A wordline,  612 - 1  and  612 - 2  respectively, is couple to a gate,  614 - 1  and  614 - 2  respectively, for each of the multistate cells,  601 - 1  and  601 - 2 . According to the teachings of the present invention, the wordlines,  612 - 1  and  612 - 2 , run across or are perpendicular to the rows of the memory array  600 . Finally, a write data/precharge circuit is shown at  624  for coupling a first or a second potential to bitline  608 - 1 . As one of ordinary skill in the art will understand upon reading this disclosure, the write data/precharge circuit  624  is adapted to couple either a ground to the bitline  608 - 1  during a write operation in the reverse direction, or alternatively to precharge the bitline  608 - 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 sourceline  604  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.  
     [0054] As shown in FIG. 6, the array structure  600 , including multistate cells  601 - 1  and  601 - 2 , has no capacitors. Instead, according to the teachings of the present invention, the first source/drain region or source region,  602 - 1  and  602 - 2 , are coupled directly to the sourceline  604 . In order to write, the sourceline  604  is biased to voltage higher than VDD and the devices stressed in the reverse direction by grounding the data or bit line,  608 - 1  or  608 - 2 . If the multistate cell,  601 - 1  or  601 - 2 , is selected by a word line address,  612 - 1  or  612 - 2 , then the multistate cell,  601 - 1  or  601 - 2 , will conduct and be stressed with accompanying hot electron injection into the cells gate insulator adjacent to the source region,  602 - 1  or  602 - 2 . As one of ordinary skill in the art will understand upon reading this disclosure, a number of different charge levels can be programmed into the gate insulator adjacent to source region such that the cells is used as a differential cell and/or the cell is compared to a reference or dummy cell, as shown in FIG. 6, and multiple bits can be stored on the multistate cell.  
     [0055] During read the multistate cell,  601 - 1  or  601 - 2 , is operated in the forward direction with the sourceline  604  grounded and the bit line,  608 - 1  or  608 - 2 , and respective second source/drain region or drain region,  606 - 1  and  606 - 2 , of the cells precharged to some fractional voltage of Vdd. If the device is addressed by the word line,  612 - 1  or  612 - 2 , then its conductivity will be determined by the presence or absence of the amount of stored charge trapped in the gate insulator as measured or compared to the reference or dummy cell and so detected using the sense amplifier  610 . 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&#39;s, but programmed as multistate cells in a novel fashion.  
     [0056] In operation the devices would be subjected to hot electron stress in the reverse direction by biasing the sourceline  604 , and read while grounding the sourceline  604  to compare a stressed multistate cell, e.g. cell  601 - 1 , to an unstressed dummy device/cell, e.g.  601 - 2 , as shown in FIG. 6. 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. Likewise, the transistors in the reference or dummy cells, e.g.  601 - 2 , can all initially be programmed to have the same conductivity states. According to the teachings of the present invention, the sense amplifier  610  can then detect small differences in cell or device characteristics due to stress induced changes in device characteristics during the write operation.  
     [0057] As one of ordinary skill in the art will understand upon reading this disclosure such arrays of multistate cells are conveniently realized by a modification of DRAM technology. According to the teachings of the present invention a gate insulator of the multistate cell includes gate insulators selected from the group of thicker layers of SiO 2  formed by wet oxidation, SON silicon oxynitride, SRO silicon rich oxide, Al 2 O 3  aluminum oxide, composite layers and implanted oxides with traps (L. Forbes and J. Geusic, “Memory using insulator traps,” Micron disclosure 97-0049, U.S. Pat. No. 6,140,181 Oct. 31, 2000). Conventional transistors for address decode and sense amplifiers can be fabricated after this step with normal thin gate insulators of silicon oxide.  
     [0058] FIGS.  7 A- 7 B and  8 - 9  are useful in illustrating the use of charge storage in the gate insulator to modulate the conductivity of the multistate cell according to the teachings of the present invention. That is, FIGS.  7 A- 9  illustrates the operation of the novel multistate cell  701  formed according to the teachings of the present invention. As shown in FIG. 7A, the gate insulator  707  has a number of layers, e.g. an ONO stack, where layer  707 A is the oxide layer closest to the channel  705  and a nitride layer  707 B is formed thereon. In the embodiment shown in FIG. 7A the oxide layer  707 A is illustrated having a thickness of approximately 6.7 nm or 67 Å (roughly 10 −6  cm). In the embodiment shown in FIG. 7A a multistate cell is illustrated having dimensions of 0.1 μm (10 −5  cm) by 0.1 μm. For purposes of illustration, the charge storage region near the source can reasonably have dimensions of 0.1 micron (1000 Å) by 0.02 micron (200 Å) in a 0.1 micron technology. If the gate oxide  707 A nearest the channel  705  is 67 Å then a charge of 100 electrons will cause a threshold voltage shift in this region of 1.6 Volts since the oxide capacitance is about 0.5 micro-Farad (μF) per square centimeter. If the transistor has a total effective oxide thickness of 200 Å then a change in the threshold voltage of only 0.16 Volts near the source, corresponding to 10 electrons, is estimated to change the transistor current by 4 micro Amperes (μA). The sense amplifier described in connection with FIG. 6, which is similar to a DRAM sense amplifier, can easily sense this charge difference on the data or bitlines. In this embodiment, the sensed charge difference on the data or bitlines will be 40 femto Coulombs (fC) over a sense period of 10 nano seconds (nS).  
     [0059] To illustrate these numbers, the capacitance, Ci, of the structure depends on the dielectric constant, ε 1  (which for silicon dioxide SiO 2  equates to 1.06/3×10 −12  F/cm), and the thickness of the insulating layers, t, (given here as 6.7×10 −7  cm), such that Ci=εi/t=((1.06×10 −12 F/cm/(3×6.7×10 −7  cm))=0.5×10 −6  Farads/cm 2  (F/cm 2 ). This value taken over the charge storage region near the source, e.g. 20 nm×100 nm or 2×10 −11  cm 2 , results in a capacitance value of Ci=10 −17  Farads. Thus, for a change in the threshold voltage of ΔV=1.6 Volts the stored charge must be Q=C×ΔV=(10 −17  Farads×1.6 Volts)=1.6×10 −17  Coulombs. Since Q=Nq, the number of electrons stored is approximately Q/q=(1.6×10 −17  Coulombs/1.6×10 −19  Coulombs) or 100 electrons. In effect, the programmed multistate 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 ΔQ=100 electrons in the dimensions given above, if the transistor has a total effective oxide thickness of 200 Å then a change in the threshold voltage of only 0.16 Volts near the source, corresponding to 10 electrons, is estimated to change the transistor current by 4 micro Amperes (μA). As stated above, the sense amplifier described in connection with FIG. 6, which is similar to a DRAM sense amplifier, can easily sense this charge difference on the data or bitlines. And, the sensed charge difference on the data or bitlines will be 40 femto Columbs (fC) over a sense period of 10 nano seconds (nS) for this representative one of a number of stored charge levels according to the teachings of the present invention. Again, a number of different charge levels can be programmed into the gate insulator adjacent to source region such that the cell is used as a differential cell and/or the cell is compared to a reference or dummy cell, as shown in FIG. 6, and multiple bits can be stored on the multistate cell of the present invention.  
     [0060]FIG. 7B aids to further illustrate the conduction behavior of the novel multistate cell of the present invention. The electrical equivalent circuit shown in FIG. 7B illustrates a multistate cell  701  having an equivalent oxide thickness of 200 Å. The charge storage region near the source  702  can reasonably have a length dimension of 0.02 micron (20 nm) in a 0.1 micron technology with a width dimension of 0.1 micron (100 nm). Therefore, for a change in the drain source voltage (ΔV DS ) in this region an electric field of E=(0.1V/2×10 −6  cm)=0.5×10 5  V/cm or 5×10 4  V/cm is provided. The drain current is calculated using the formula I D =μC OX ×(W/L)×(Vgs−Vt)×ΔV DS . In this example, μC OX =μC 1  is taken as 50 μA/V 2  and W/L=5. Appropriate substitution into the drain current provides I D =(50 μA/V 2 ×5×0.16 Volts×0.1 Volts)=2.5×1.6 μA=4 μA. As noted above this drain current I D  corresponds to 10 electrons trapped in the gate insulator, or charge storage region  707  near the source  702 . Sensed over a period of 10 nanoSeconds (nS) produces a current on the bitline of 40 fC (e.g. 4 μA×10 nS=40×10 −15  Columbs).  
     [0061]FIGS. 8A and 8B, illustrate the operation and programming the novel multistate cell as described above. However, FIGS. 8A and 8B also help illustrate an alternative array configuration where adjacent devices are compared and one of the devices on the opposite side of a shared trench is used as a dummy cell transistor or reference device. Again, the reference devices can all be programmed to have the same initial conductivity state. FIG. 8A illustrates the operation and programming the novel multistate cell in the reverse direction. As shown in FIG. 8A, a transistor  801 - 1  on one side of the trench (as described in connection with FIG. 3) is stressed by grounding its respective drain line, e.g.  811 - 1 . As shown in FIG. 8A, the drain line  811 - 2  for the transistor  801 - 2  on the opposite side of the trench is left floating. A voltage is applied to the shared sourceline  804  located at the bottom of the trench (as described in connection with FIG. 3) which now acts as a drain. As shown in this electrical equivalent circuit, the neighboring (shared trench)/column adjacent transistors,  801 - 1  and  801 - 2 , share a gate  807  and the wordline  813 , e.g. polysilicon gate lines, coupling thereto run across or are perpendicular to the rows containing the bit and source lines, e.g.  811 - 1 ,  811 - 2 , and  804 . A gate voltage is applied to the gates  807 . Here the multistate cell  801 - 1  will conduct and be stressed with accompanying hot electron injection into the cells gate insulator  817  adjacent to the source region  802 - 1 .  
     [0062]FIG. 8B illustrates the now programmed multistate cell&#39;s operation in the forward direction and differential read occurring in a this differential cell embodiment, e.g. 2 transistors in each cell. To read this state the drain and source (or ground) have the normal connections and the conductivity of the multistate cell is determined. That is, the drain line,  811 - 1  and  811 - 2 , have the normal forward direction potential applied thereto. The shared sourceline  804  located at the bottom of the trench (as described in connection with FIG. 3) is grounded and once again acts as a source. And, a gate voltage is applied to the gates  807 . As one of ordinary skill in the art will understand upon reading this disclosure, a number of different charge levels can be programmed into the gate insulator  817  adjacent to source region  802 - 1  and compared to the reference or dummy cell,  802 - 2 . Thus, according to the teachings of present invention multiple bits can be stored on the multistate cell.  
     [0063] As stated above, these novel multistate cells can be used in a DRAM like array. Two transistors occupy an area of 4F squared (F=the minimum lithographic feature size) when viewed from above, or each memory cell consisting of one transistor has an area of 2F squared. Each transistor can now, however, store many bits so the data storage density is much higher than one bit for each 1F squared unit area. Using a reference or dummy cell for each memory transistor where the reference transistor is in close proximity, e.g. the embodiment shown in FIGS. 8A and 8B vs. that shown in FIG. 4, results in better matching characteristics of transistors, but a lower memory density.  
     [0064] In FIG. 9 a memory device is illustrated according to the teachings of the present invention. The memory device  940  contains a memory array  942 , row and column decoders  944 ,  948  and a sense amplifier circuit  946 . The memory array  942  consists of a plurality of multistate cells  900 , formed according to the teachings of the present invention whose word lines  980  and bit lines  960  are commonly arranged into rows and columns, respectively. The bit lines  960  of the memory array  942  are connected to the sense amplifier circuit  946 , while its word lines  980  are connected to the row decoder  944 . Address and control signals are input on address/control lines  961  into the memory device  940  and connected to the column decoder  948 , sense amplifier circuit  946  and row decoder  944  and are used to gain read and write access, among other things, to the memory array  942 .  
     [0065] The column decoder  948  is connected to the sense amplifier circuit  946  via control and column select signals on column select lines  962 . The sense amplifier circuit  946  receives input data destined for the memory array  942  and outputs data read from the memory array  942  over input/output (I/O) data lines  963 . Data is read from the cells of the memory array  942  by activating a word line  980  (via the row decoder  944 ), which couples all of the memory cells corresponding to that word line to respective bit lines  960 , which define the columns of the array. One or more bit lines  960  are also activated. When a particular word line  980  and bit lines  960  are activated, the sense amplifier circuit  946  connected to a bit line column detects and amplifies the conduction sensed through a given multistate cell, where in the read operation the source region of a given cell is couple to a grounded array plate (not shown), and transferred its bit line  960  by measuring the potential difference between the activated bit line  960  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.  
     [0066]FIG. 10 is a block diagram of an electrical system, or processor-based system,  1000  utilizing multistate memory cells  1012  constructed in accordance with the present invention. That is, the multistate memory cells  1012  utilizes the modified DRAM cell as explained and described in detail in connection with FIGS.  2 - 4 . The processor-based system  1000  may be a computer system, a process control system or any other system employing a processor and associated memory. The system  1000  includes a central processing unit (CPU)  1002 , e.g., a microprocessor, that communicates with the multistate memory  1012  and an I/O device  1008  over a bus  1020 . It must be noted that the bus  1020  may be a series of buses and bridges commonly used in a processor-based system, but for convenience purposes only, the bus  1020  has been illustrated as a single bus. A second I/O device  1010  is illustrated, but is not necessary to practice the invention. The processor-based system  1000  can also includes read-only memory (ROM)  1014  and may include peripheral devices such as a floppy disk drive  1004  and a compact disk (CD) ROM drive  1006  that also communicates with the CPU  1002  over the bus  1020  as is well known in the art.  
     [0067] It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device  1000  has been simplified to help focus on the invention. At least one of the multistate cell in NROM  1012  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.  
     [0068] It will be understood that the embodiment shown in FIG. 10 illustrates an embodiment for electronic system circuitry in which the novel memory cells of the present invention are used. The illustration of system  1000 , as shown in FIG. 10, 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  1000  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.  
     [0069] 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.  
     [0070] Conclusion  
     [0071] 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&#39;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 a limited number of times, 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. The novel multistate cells can be used in a DRAM like array. Two transistors occupy an area of 4F squared (F=the minimum lithographic feature size) when viewed from above, or each memory cell consisting of one transistor has an area of 2F squared. Each transistor can now, however, store many bits so the data storage density is much higher than one bit for each 1F squared unit area. Using a reference or dummy cell for each memory transistor where the reference transistor is in close proximity, e.g. the embodiment shown in FIGS. 8A and 8B vs. that shown in FIG. 4, results in better matching characteristics of transistors, but a lower memory density.  
     [0072] 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.