Abstract:
A 2-bit non-volatile memory (NVM) transistor having a pair of isolated floating gate electrodes is provided. One of the floating gate electrodes is located over a first source/drain region, and a first adjacent end of a channel region. The other floating gate electrode is located over a second source/drain region and a second adjacent end of the channel region. A control gate extends over both floating gate electrodes and a centrally located portion of the channel region. The floating gate electrodes are independently programmed and independently read, thereby enabling the NVM transistor to effectively store 2-bits of data.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to non-volatile memory (NVM) transistors. More specifically, the present invention relates to a 2-bit split-gate non-volatile memory transistor.  
         RELATED ART  
         [0002]    [0002]FIG. 1 is a cross sectional view of a conventional 2-bit non-volatile semiconductor memory transistor  10  that utilizes asymmetrical charge trapping. 2-bit NVM transistor  10 , which is fabricated in p-type substrate  12 , includes n+ source region  14 , n+ drain region  16 , channel region  17 , silicon oxide layer  18 , silicon nitride layer  20 , silicon oxide layer  22 , and control gate  24 . Oxide layer  18 , nitride layer  20  and oxide layer  22  are collectively referred to as ONO layer  21 . NVM transistor  10  operates as follows. A first programming operation is performed by connecting source region  14  to ground, connecting drain region  16  to a programming voltage of about 5-8 Volts, and connecting control gate  24  to a voltage of about 10 Volts. As a result, electrons are accelerated from source region  14  to drain region  16 . Near drain region  16 , some electrons gain sufficient energy to pass through oxide layer  18  and be trapped in nitride layer  20  in accordance with a phenomenon known as hot electron injection. Because nitride layer  20  is non-conductive, the injected charge remains localized within charge trapping region  26  in nitride layer  20 . The charge stored by localized charge trapping region  26  represents a first bit of data stored by NVM transistor  10 .  
           [0003]    The first bit of NVM transistor  10  is read by applying 0 Volts to the drain region  16 , 2 Volts to the source region  14 , and 3 volts to the gate electrode. If charge is stored in charge trapping region  26  (i.e., the first bit of NVM transistor  10  is programmed), then NVM transistor  10  does not conduct current under these conditions. If there is no charge stored in charge trapping region  26  (i.e., the first bit of NVM transistor  10  is erased), then NVM transistor  10  conducts current under these conditions. The current, or lack of current, is sensed by a sense amplifier to determine the state of the first bit of NVM transistor  10 .  
           [0004]    Note that the polarity of the voltage applied across source region  14  and drain region  16  is reversed during the program and read operations. That is, the first bit of NVM transistor  10  is programmed in one direction (with source region  14  grounded), and read the opposite direction (with drain region  16  grounded). As a result, the read operation is referred to as a reverse read operation. NVM transistor  10  is described in more detail in U.S. Pat. No. 5,768,192.  
           [0005]    NVM transistor  10  also includes a second charge-trapping region in nitride layer  20 , which is located adjacent to source region  14 . FIG. 2 illustrates both the first charge trapping region  26  (described above in connection with FIG. 1), and the second charge-trapping region  28  in dashed lines. The second charge trapping region  28  is used to store a charge representative of a second bit. The second charge trapping region  28  is programmed and read in a manner similar to the first charge-trapping region  26 . More specifically, the second charge trapping region  28  is programmed and read by exchanging the source and drain voltages described above for programming and reading the first charge trapping region  26 . Thus, the second charge trapping region  28  is programmed by applying 0 Volts to drain region  16 , applying 5-8 Volts to source region  14  and applying  10  Volts to control gate  24 . Similarly, the second charge-trapping region  28  is read by applying 0 Volts to source region  14 , 2 Volts to drain region  16 , and 3 Volts to control gate  24 .  
           [0006]    Note that because nitride layer  20  is non-conductive, the charges stored in the first and second charge trapping regions  26  and  28  remain localized within nitride layer  20 . Also note that the state of the first charge-trapping region  26  does not interfere with the reading of the charge stored in the second charge-trapping region  28  (and vice versa). Thus, if the first charge trapping region  26  is programmed (i.e., stores charge) and the second charge trapping region  28  is not programmed (i.e., does not store charge), then a reverse read of the first charge trapping region will not result in significant current flow. However, a reverse read of the second bit will result in significant current flow because the high voltage applied to drain region  16  will result in unperturbed electronic transfer in the pinch off region adjacent to first charge trapping region  26 . Thus, the information stored in the first and second charge trapping regions  26  and  28  is read properly.  
           [0007]    Similarly, if both the first and second charge-trapping regions are programmed, a read operation in either direction will result in no significant current flow. Finally, if neither the first charge trapping region  26  nor the second charge trapping region  28  is programmed, then read operations in both directions will result in significant current flow.  
           [0008]    NVM transistor  10  has the following disadvantages. First, electrons and holes within the charge trapping regions  26  and  28  can migrate over time, thereby resulting in cycling/endurance degradation. Moreover, NVM transistor  10  is subject to over-programming and over-erase conditions. Furthermore, the nitride charge storage layer  20  structure cannot be erased by exposure to UV light, such that the threshold voltage of the NVM transistor cannot be reduced after manufacture. Finally, a relatively high current is required to program NVM transistor  10 .  
           [0009]    It would therefore be desirable to have a 2-bit non-volatile memory transistor that overcomes the above-described deficiencies of NVM transistor  10 .  
         SUMMARY  
         [0010]    Accordingly, the present invention provides a 2-bit split gate non-volatile memory (NVM) transistor, which is fabricated in a semiconductor region having a first conductivity type. First and second source/drain regions having a second conductivity type, opposite the first conductivity type, are located in the semiconductor region. In one embodiment, the first and second source/drain regions extend into diffusion bit lines. A channel region separates the first and second source drain regions in the semiconductor region. A gate dielectric layer is located over the channel region and portions of the first and second source/drain regions. In one embodiment, the gate dielectric layer is silicon oxide.  
           [0011]    A first floating gate electrode is located on the gate dielectric layer over portions of the channel region and the first source/drain region. The first floating gate electrode stores charge representative of a first data bit. A second floating gate electrode is located on the gate dielectric layer over portions of the channel region and the second source/drain region. The second floating gate electrode stores charge representative of a second data bit.  
           [0012]    The first and second floating gate electrodes are separated by a gap over the channel region. In one embodiment, the first and second floating gate electrodes are polysilicon.  
           [0013]    A dielectric layer is located over most of the first floating gate electrode and most of the second floating gate electrode. The dielectric layer also extends into the gap between the first and second floating gate electrodes, where the dielectric layer contacts the underlying gate dielectric layer. In one embodiment, the dielectric layer includes a silicon oxide layer and a silicon nitride layer formed over the silicon oxide layer. Oxide is thermally grown on exposed outer edges of the first and second floating gate electrodes, which are not covered by the dielectric layer. At this time, oxide is also thermally grown over exposed portions of the diffusion bit lines and the dielectric layer.  
           [0014]    A control gate is located over the dielectric layer. The control gate therefore extends over the first and second floating gate electrodes and the channel region. In one embodiment, the control gate is polycide or salicide.  
           [0015]    Advantageously, the process required to fabricate the NVM transistor of the present invention is relatively simple. Moreover, because the charge storage regions are formed by first and second floating gate electrodes that are isolated from each other, charge migration does not exist. Furthermore, the NVM transistor of the present invention is not subject to over-programming or over-erase conditions. In addition, the polysilicon structure allows the first and second floating gate electrodes to be erased by exposure to UV light, thereby reducing the threshold voltage of the NVM transistor after manufacture. Moreover, a relatively low current is required to program the NVM transistor of the present invention.  
           [0016]    The present invention also includes a method for operating the 2-bit non-volatile memory transistor, which includes the steps of (1) programming the first floating gate by hot electron injection using a first set of programming voltages, wherein the second floating gate is in an erased state when the first floating gate is programmed, and (2) programming the second floating gate by hot electron injection using a second set of programming voltages, wherein the first floating gate is in a programmed state when the second floating gate is programmed. The voltage applied to the control gate in the second set of programming voltages is greater than the voltage applied to the control gate in the first set of programming voltages. In a particular embodiment, the voltage applied to the control gate in the first set of programming voltages is about 1-2 Volts, and the voltage applied to the control gate in the second set of programming voltages is about 3-4 Volts.  
           [0017]    This method can further include the steps of (a) reading the state of the first floating gate by applying a first set of read voltages to the transistor, and (b) reading the state of the second floating gate by applying a second set of read voltages to the transistor, wherein the first floating gate is subjected to a reverse read operation in a first direction, and the second floating gate is subjected to a reverse read operation in a second direction, opposite the first direction.  
           [0018]    The present invention also includes a method for fabricating the NVM transistor. In one embodiment, this method includes the steps of (1) forming a gate dielectric layer over a semiconductor substrate having a first conductivity type, (2) forming floating gate layer over the gate dielectric layer, (3) removing a first portion of the floating gate layer, thereby creating an opening through the floating gate layer, (4) depositing a dielectric layer over the floating gate layer, wherein a portion of the dielectric layer extends into the opening and onto the gate dielectric layer, (5) removing a second portion of the floating gate layer, thereby creating first floating gate and a second floating gate, wherein a first opening is located adjacent to the first floating gate, and a second opening is located adjacent to the second floating gate, (6) implanting impurities having a second conductivity type, opposite the first conductivity type, into the substrate, through the first and second openings, (7) thermally growing oxide on the substrate and sidewalls of the first and second gate electrodes through the first and second openings, and (8) depositing a control gate over the dielectric layer and the oxide.  
           [0019]    The present invention will be more fully understood in view of the following description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIGS. 1 and 2 are cross sectional views of a conventional 2-bit non-volatile semiconductor memory transistor that utilizes asymmetrical charge trapping.  
         [0021]    [0021]FIG. 3 is a schematic diagram of a 2×2 array of 2-bit non-volatile memory (NVM) transistors in accordance with one embodiment of the present invention.  
         [0022]    FIGS.  4 - 12  are cross sectional views that illustrate the fabrication of the NVM transistors of FIG. 3 in accordance with one embodiment of the present invention.  
         [0023]    [0023]FIG. 13 is a circuit diagram illustrating an erase operation of the array of FIG. 3 in accordance with one embodiment of the present invention.  
         [0024]    [0024]FIGS. 14A and 14B illustrate read operations of the floating gates of an NVM transistor of the array of FIG. 3 in accordance with one embodiment of the present invention.  
         [0025]    [0025]FIGS. 15A and 15B illustrate programming operations of the floating gates of an NVM transistor of the array of FIG. 3 in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]    [0026]FIG. 3 is a schematic diagram of a 2×2 array  100  of 2-bit non-volatile memory (NVM) transistors  101 - 104  in accordance with one embodiment of the present invention. While a 2×2 array is shown, it is understood that arrays having other sizes can be implemented, and are considered to fall within the scope of the invention. Array  100  also includes row decoder  111 , column decoder  112 , word lines  121 - 122  and bit lines  131 - 133 . Word line  121  is coupled to the control gates of NVM transistors  101 - 102 , and word line  122  is coupled to the control gates of NVM transistors  103 - 104 . As described in more detail below, word lines  121 - 122  are polycide or salicide in the described embodiment. Word lines  121 - 122  are also coupled to row decoder  111 . Bit lines  131 - 133  are coupled to the source/drain regions of NVM transistors  101 - 104 , as illustrated. As described in more detail below, bit lines  131 - 133  are formed by doped diffusion regions in a semiconductor substrate. These doped diffusion regions can be coupled to other doped diffusion regions by metal strap lines. Bit lines  131 - 133  are also coupled to column decoder  112 .  
         [0027]    FIGS.  4 - 12  are cross sectional views that illustrate the fabrication of NVM transistors  101  and  102  in accordance with one embodiment of the present invention. NVM transistors  103  and  104  (not shown in FIGS.  4 - 12 ) are fabricated at the same time as NVM transistors  101  and  102 .  
         [0028]    As illustrated in FIG. 4, array  100  is fabricated in a semiconductor region  401 . In the described embodiment, semiconductor region  401  is a p-type well formed in a monocrystalline silicon substrate. Semiconductor region  401  has a dopant concentration of about 5e16-2e17 atoms/cm . In other embodiments, semiconductor region  401  can be a p-type silicon substrate. Field oxide  402  is thermally grown at the upper surface of substrate  401  using a conventional local oxidation of silicon (LOCOS) process. In the described embodiment, field oxide  402  is grown to a thickness in the range of about 4000 to 8000 Å. In the described embodiment, the field oxide is grown to a thickness of about 6000 Å. In an alternate embodiment, field oxide  402  can be replaced with a shallow trench isolation (STI) structure.  
         [0029]    After field oxide  402  has been grown, a sacrificial oxide layer (not shown) is grown and then removed (etched) with a diluted hydrofluoric acid (HF). In one embodiment, a blanket threshold voltage implant is performed through the sacrificial oxide. However, in the described embodiment, no threshold voltage implant is performed through the sacrificial oxide. Rather, the required threshold voltage implant is performed as described in more detail below.  
         [0030]    Field oxide  402  defines the perimeter of the area where the NVM transistors  101 - 104  of array  100  are fabricated. However, field oxide  402  is not used to provide isolation between NVM transistors  101 - 104 . For this reason, the resulting configuration of NVM transistors is referred to as a fieldless array of NVM transistors.  
         [0031]    Gate dielectric layer  403  is then thermally grown or deposited on the upper surface of semiconductor region  401 . In the described embodiment, gate dielectric layer  403  is silicon oxide, which is thermally grown to a thickness in the range of about 50 to 150 Å over the upper surface of semiconductor region  401 . In a particular embodiment, gate dielectric layer  403  has a thickness of about 70 Å.  
         [0032]    Polysilicon layer  404  is then deposited over field oxide  402  and gate oxide layer  403 . As described in more detail below, polysilicon layer  404  is used to create the charge storage regions of NVM transistors  101 - 104 . In the described embodiment, polysilicon layer  404  is undoped polysilicon deposited to a thickness in the range of 1000 to 3000 Å. In a particular embodiment, polysilicon layer  404  has a thickness of about 2000 Å. In an alternate embodiment, polysilicon layer  404  can be doped.  
         [0033]    A photoresist mask  405 , having openings  406 , is formed over polysilicon layer  404 . As will become apparent in view of the following disclosure, openings  406  define certain edges of the floating gate electrodes of the NVM transistors. Each of openings  406  has a width that corresponds with the minimum line width of the process used to fabricate the array. In the described embodiment, the process has a minimum line width of 0.18 microns, such that each of openings  406  has a width of 0.18 microns.  
         [0034]    As illustrated in FIG. 5, polysilicon layer  404  is etched through openings  406 , thereby creating polysilicon regions  404   1 ,  404   2  and  404   3 . This etch is controlled to leave underlying gate oxide layer  403  substantially intact.  
         [0035]    As illustrated in FIG. 6, photoresist layer  405  is stripped, and a thin dielectric layer  407  is deposited over the resulting structure. In the described embodiment, dielectric layer  407  includes a silicon oxide layer and a silicon nitride layer, which is deposited over the silicon oxide layer. In one embodiment, the silicon oxide layer has a thickness in the range of about 50 to 200 Å, and the silicon nitride layer has a thickness in the range of about 50 to 200 Å. In a particular embodiment, both the silicon oxide layer and the silicon nitride layer have a thickness of about 70 Å, such that dielectric layer  407  has a thickness of about 140 Å. In an alternate embodiment, the silicon nitride layer can be replaced with a silicon oxynitride (SiON) layer. In the described embodiment, the upper surface of dielectric layer  407  cannot be silicon oxide, for reasons that will become apparent in view of the following disclosure.  
         [0036]    As illustrated in FIG. 7, a photoresist mask  408 , having openings  409 - 411 , is formed over dielectric layer  407 . As will become apparent in view of the following disclosure, openings  409 - 411  define certain edges of the floating gates of the NVM transistors. Each of openings  409 - 411  has a width that corresponds with the minimum line width of the process being used to fabricate the array. In the described embodiment, each of openings  409 - 411  has a width of 0.18 microns. Openings  409 - 411  are offset with respect to openings  406  (FIGS.  4 - 5 ). In the described embodiment, openings  409 - 411  are offset by 0.18 microns with respect to openings  406 .  
         [0037]    As illustrated in FIG. 8, a first etch is performed through openings  409 - 411 , thereby removing the exposed portions of dielectric layer  407 . A second etch is then performed through openings  409 - 411 , thereby removing the exposed portions of polysilicon regions  4041 - 4043 . This second etch is controlled to leave the underlying portions of gate dielectric layer  403  substantially intact. The remaining portions of polysilicon regions  404   1 - 404   3  are labeled as polysilicon regions  404   11 ,  404   21 ,  404   21 ,  404   22 ,  404   31 , and  404   32 . As described in more detail below, polysilicon regions  404   11  and  404   21  form the floating gate electrodes of NVM transistor  101 , and polysilicon regions  404   22  and  404   32  form the floating gate electrodes of NVM transistor  102 .  
         [0038]    Openings  409 - 411  also define the diffusion bit lines of array  100 . More specifically, openings  409 ,  410  and  411  define the locations of diffusion bit lines  131 ,  132  and  133 , respectively. After the above-described etching steps are completed, high angle implants are performed through openings  409 - 411 . More specifically, a P-type impurity, such as boron, is implanted through openings  409 - 411   411  at high angles with respect to the upper surface of semiconductor substrate  401 , such that the dopant extends under the edges of photoresist mask  408 . In accordance with one embodiment of the present invention, the high angle implants are performed by implanting P-type impurities with a dopant density in the range of 1e13 to 5e13 ions/cm 2 , and an implantation energy in the range of 40 to 100 KeV. In a particular embodiment, the high angle implants are performed with a dopant density of about 2.2e13 ions/cm 2  and an implantation energy of about 25 KeV. In one embodiment, the high angle implants are performed at angles in the range of 15 to 45 degrees from the vertical axis of FIG. 8, which extends perpendicular to the upper surface of substrate  401 . In the described embodiment, the high angle implants are performed at angles approximately 25 degrees from the vertical axis of FIG. 8. The implanted boron serves to adjust the threshold voltages of NVM transistors  101 - 104 . The implanted p-type impurities are illustrated as regions  412 - 414  in FIG. 8. In an alternate embodiment, the p-type impurities can be implanted along the vertical axis of FIG. 8. Note that in the described example, the pocket implant (and other process features) are less critical than in the NVM transistor  10  of FIGS. 1 and 2, because a simpler process technology is being used in the present invention.  
         [0039]    In an alternative embodiment, an additional counter-doping implant can be implemented. The counter doping implant is performed by implanting an n-type impurity, such as phosphor, using parameters similar to the parameters of the above-described high angle implants. The n-type impurity provides improved junction edge optimization. In yet another embodiment, counter-doping is achieved by performing a blanket low energy implant of an n-type impurity over the entire array, after the formation of field oxide  402 .  
         [0040]    After performing the high angle implants, an N-type impurity, such as arsenic, is implanted through openings  409 - 411  of photoresist mask  408 . In one embodiment, arsenic is implanted with a dopant density in the range of 1e15 to 1e16 ions/cm 2  and an implantation energy in the range of 30 to 100 KeV. In a particular embodiment, arsenic is implanted with a dopant density of about 3e15 ions/cm 2  and an implantation energy of about 60 KeV. The implanted N-type impurities are illustrated as regions  422 - 424  in FIG. 9.  
         [0041]    Photoresist mask  408  is then stripped, and a thermal oxidation step is performed, thereby creating bit line oxide regions  442 - 444  and sidewall oxide regions  442 A- 444 A, as illustrated in FIG. 10. This thermal oxidation step also results in the formation of a thin silicon oxide layer over the exposed silicon nitride of dielectric layer  407 . The growth of bit line oxide regions  442 - 444  causes the portions of polysilicon regions  404   11 - 404   11 ,  404   21 - 404   22  and  404   31 - 404   32 , which are adjacent to bit line oxide regions  442 - 444  to bend upward. In one embodiment, the bit line oxide is thermally grown using a wet oxidation process at a temperature in the range of 800 to 1000° C. to a thickness in the range of 100 to 500 Å. In a particular embodiment, the bit line oxide is thermally grown using a wet oxidation process at a temperature of about 900° C. to a thickness of about 200 Å. This oxidation step also activates and diffuses the implanted impurities  412 - 414  and  422 - 424 , thereby forming diffusion bit lines  432 - 434 . Note that diffusion bit lines  432 - 434  diffuse under polysilicon regions  404   11 - 404   11 ,  404   21 - 404   22  and  404   31 - 404   32 , as illustrated. (Subsequent high temperature processing steps complete the activation of the implanted impurities in regions  412 - 414  and  422 - 424 ). Normally, the relatively low temperature of 700° C. would result in very slow oxidation of silicon. However, the heavy doping of diffusion bit lines  432 - 434  increases the rate of silicon oxidation by approximately a factor of four. Consequently, low temperature oxidation at 700° C., which provides better control, can be used.  
         [0042]    As illustrated in FIG. 11, a blanket layer of polysilicon  451  is then deposited over the upper surface of the resulting structure. In some embodiments, phosphorus oxychloride (POCl 3 ) is used to dope polysilicon layer  451  to increase the conductivity of polysilicon layer  451 . Other embodiments may implant impurities such as phosphorus ions to increase the conductivity of polysilicon layer  451 . A layer of metal silicide, such as tungsten silicide, is deposited directly on polysilicon layer  451  to form metal silicide layer  452 . In an alternate embodiment, a blanket layer of a refractory metal, such as tungsten, titanium, or cobalt, is sputtered over the upper surface of polysilicon layer  451 , and subsequently reacted to form a metal silicide.  
         [0043]    A photoresist mask (not shown) is then formed over the resulting structure. This photoresist mask is patterned to define the control gates and word lines of the NVM transistors  101 - 104 . An etch is then performed to remove the portions of metal silicide layer  452  and polysilicon layer  451  that are exposed by the photoresist mask. In one embodiment, this polycide etch is a dry etch. Tungsten silicide layer  452  is etched with a gas mixture of HBr, SF 6  and He. Polysilicon layer  451  is etched with a gas mixture of HBr and Cl 2 .  
         [0044]    After the polycide etch is completed, the photoresist mask is stripped and a tungsten silicide anneal is then performed at 900° C. with low oxygen flow. (This anneal adheres the tungsten silicide to the underlying polysilicon and is part of the activation of the impurities in the buried diffusion bit lines  432 - 434 ). A boron implant is then performed to prevent current leakage between diffusion bit lines at the locations between adjacent gates electrodes in the fieldless array. This boron implant is a blanket implant, with no mask protection provided on the wafer. In one embodiment, boron is implanted at a dopant density in the range of 1e12 to 6e12 ions/cm 2  and an energy in the range of 20 to 60 KeV. In a particular embodiment, boron is implanted at a dopant density of about 3e12 ions/cm 2  and an energy of about 30 KeV.  
         [0045]    [0045]FIG. 12 is a top view of NVM transistors  101 - 104 . NVM transistors  101 - 102  are labeled with the reference numbers described above in FIGS.  4 - 11 . Each of NVM transistors  101 - 104  has a horizontal dimension of 0.72 microns (between the centers of the adjacent diffusion bit lines), and a vertical dimension of 0.5 microns. These dimensions are shown on NVM transistor  104  in FIG. 12. The area of each NVM transistor is therefore 0.36 u 2 , with a per bit area of 0.18 u 2 .  
         [0046]    The operation of NVM transistors  101 - 104  in accordance with one embodiment of the present invention will now be described.  
         [0047]    [0047]FIG. 13 is a circuit diagram illustrating an erase operation of array  100 . In the described embodiment, array  100  is operated as a flash memory, such that all of the NVM transistors  101 - 104  in the array are erased as a block. To accomplish this, column decoder  112  is controlled to apply an erase voltage of 4 to 6 Volts to bit lines  131 - 133 , and row decoder  111  is controlled to apply an erase voltage of −3 to −6 Volts to word lines  121 - 122 . Under these conditions, electrons are drawn out of the floating gate electrodes (e.g., floating gate electrodes  404   11 ,  404   21 ,  404   22  and  404   32 ) of NVM transistors  101 - 104 , thereby leaving these floating gate electrodes substantially uncharged. Note it is not possible to over-erase NVM transistors  101 - 104  because of the portion of the channel region located between the floating gates of each transistor. For example, even if floating gates  404   11 , and  404   21  are over-erased (and thereby exhibit a positive charge), the portion of the channel region located between these floating gates will not be significantly affected by the positive charge on these floating gates. Thus, NVM transistor  101  will turn on in response to the over-erased floating gates  404   11 , and  404   21 . Although FIG. 13 illustrates all of the NVM transistors  101 - 104  being erased at the same time, in other embodiments, these NVM transistors can be erased in sections.  
         [0048]    [0048]FIGS. 14A and 14B illustrate read operations of floating gates  404   11  and  404   21 , respectively, of NVM transistor  101 . As illustrated in  14 A, floating gate  404   11  is read as follows. Row decoder  111  is controlled to apply a voltage of about 3-4 Volts to the control gate of NVM transistor  101  via word line  121 . Column decoder  112  is controlled to apply a voltage of 0 Volts to bit line  131  and a voltage of about 1.5 to 2 Volts to bit line  132 . Under these conditions, relatively large read current will flow through NVM transistor  101  if floating gate  40411  is not programmed. Conversely, a relatively small current will flow through NVM transistor  101  if floating gate  404   11  is programmed. Column decoder  112  further couples a sense amplifier (not shown) to bit lines  131 - 132  in order to sense the read current. In response, the sense amplifier provides an amplified signal representative of the current flow through NVM transistor  101 . As will become apparent in view of the following described programming operations, floating gate  404   11  is read using a reverse read operation.  
         [0049]    As illustrated in  14 B, the state of floating gate  404   21  is read as follows. Row decoder  111  is again controlled to apply a voltage of about 3-4 Volts to the control gate of NVM transistor  101  via word line  121 . Column decoder  112  is controlled to apply a voltage of 0 Volts to bit line  132  and a voltage of about 1.5 to 2 Volts to bit line  131 . Under these conditions, a relatively large read current will flow through NVM transistor  101  if floating gate  404   21  is not programmed. Conversely, a relatively small read current will flow through NVM transistor  101  if floating gate  404   21  is programmed. Column decoder  112  further couples a sense amplifier (not shown) to bit lines  131 - 132  in order to sense the read current. In response, the sense amplifier provides an amplified signal representative of the current flow through NVM transistor  101 . As will become apparent in view of the following described programming operations, floating gate  404   21  is read using a reverse read operation.  
         [0050]    Table 1 below summarizes the read currents for the possible read operations of NVM transistor  101 .  
                                                                     TABLE 1                                   State of   State of                       Floating   Floating   Read       WL 121   BL 131   BL 132   Gate 404 11      Gate 404 21     Current                                3-4 V   0   V   1.5-2   V   Programmed   Programmed   Low       3-4 V   0   V   1.5-2   V   Programmed   Erased   Low       3-4 V   0   V   1.5-2   V   Erased   Programmed   High       3-4 V   0   V   1.5-2   V   Erased   Erased   High       3-4 V   1.5-2   V   0   V   Programmed   Programmed   Low       3-4 V   1.5-2   V   0   V   Programmed   Erased   High       3-4 V   1.5-2   V   0   V   Erased   Programmed   Low       3-4 V   1.5-2   V   0   V   Erased   Erased   High                  
 
         [0051]    [0051]FIGS. 15A and 15B illustrate programming operations of floating gates  404   11  and  404   21  , respectively, of NVM transistor  101 . In general, each programming operation is preceded by a read operation, such that the appropriate programming voltages can be determined. Thus, to program floating gate  404   11 , a read operation is first performed on floating gate  404   21 , in the manner illustrated in FIG. 14B. The read state of floating gate  404   21  is used to determine the appropriate programming voltages required to program floating gate  404   11 .  
         [0052]    If the read operation determines that floating gate  404   21  is in an erased state, then the subsequent programming of floating gate  404   11  can be performed as follows. Row decoder  111  is controlled to apply a voltage of about 1-2 Volts to the control gate of NVM transistor  101  via word line  121 . Column decoder  112  is controlled to apply a voltage of 0 Volts to bit line  132  and a voltage of about 5 to 8 Volts to bit line  131 . Under these conditions, electrons are transferred into floating gate  404   11  by hot electron injection.  
         [0053]    However, if the read operation of floating gate  404   21  determines that floating gate  404   21  is in a programmed state, then the subsequent programming of floating gate  404   11  is performed as follows. Row decoder  111  is controlled to apply a voltage of about 3-4 Volts to the control gate of NVM transistor  101  via word line  121 . Note that the voltage applied to the control gate of NVM transistor  101  must be higher if floating gate  404   21  is programmed. Column decoder  112  is controlled to apply a voltage of 0 Volts to bit line  132  and a voltage of about 5 to 8 Volts to bit line  131 . Under these conditions, electrons are transferred into floating gate  404   11  by hot electron injection.  
         [0054]    Advantageously, the read operation and the following program operation require similar voltages. Thus, reading floating gate  404   21  and programming floating gate  404   11  both require: 0 Volts on bit line  132 , a positive voltage on bit line  131 , and a positive voltage on word line  121 . As a result, the transition between the read operation and the subsequent program operation does not require large signal swings on word line  121  or bit lines  131 - 132 .  
         [0055]    Note that row decoder  111  allows word line  122  to float, such that NVM transistors  103  and  104  are not programmed. Also note that column decoder  112  allows bit line  133  to float, such that NVM transistor  102  is not programmed. In addition, over-programming is suppressed in NVM transistor  101  because as the floating gate potential increases, the hot electron channeling is suppressed.  
         [0056]    [0056]FIG. 15B illustrates the programming of floating gate  404   21 , which is programmed using the same read-then-program method described above in connection with FIG. 15A.  
         [0057]    Other advantages of the 2-bit NVM transistor of the present invention are listed below.  
         [0058]    Because floating gates  404   21  and  404   11  are electrically isolated from each other, the programmed/erased charges are easily maintained in the desired locations. That is, charge migration is not possible. Because there is no charge migration over time, there is no degradation in cycling/endurance.  
         [0059]    Moreover, because there is no over-erase or over-programming, the program/erase algorithm may be made relatively simple compared to conventional 2-bit non-volatile memory transistors. That is, a wider program/erase window is allowed because there is no over-program and no over-erase.  
         [0060]    Furthermore, the split-gate structure of NVM transistors  101 - 104  allows the required word line voltages and the required programming current to be relatively low. As a result, these NVM transistors can be scaled relatively easily.  
         [0061]    In addition, the polysilicon construction of the floating gates in the present invention enables the NVM transistors to be erased by exposure to ultraviolet light. Thus, after manufacturing, it is possible to use an ultraviolet light to initially reduce the threshold voltages of NVM transistors  101 - 104 . This option is not available in the conventional 2-bit NVM transistor  10  of FIGS. 1 and 2.  
         [0062]    Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. For example, although the invention has been described in connection an n-channel NVM transistor, it is understood that the described conductivity types can be reversed to provide a p-channel NVM transistor. Thus, the invention is limited only by the following claims.