Abstract:
A flash electrically-erasable, programmable read-only memory (EEPROM) with reduced area. The memory cells of the EEPROM are arranged into groups, and access to the groups is controlled by select transistors. In this manner the number of select transistors is reduced without requiring the entire array to be programmed or erased.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    NOT APPLICABLE  
         STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    NOT APPLICABLE  
         REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.  
         [0003]    NOT APPLICABLE  
         BACKGROUND OF THE INVENTION  
         [0004]    The present invention relates to flash electrically-erasable, programmable read-only memories (EEPROMs). In particular, the present invention relates to flash EEPROMs having selectable groups.  
           [0005]    [0005]FIG. 1A is a cross-sectional view of a standard floating gate tunnel oxide (FLOTOX) cell  20 . In the FLOTOX cell  20 , there is a polysilicon control gate  22  where a control voltage V CG  may be applied, a polysilicon floating gate  24 , a source terminal having a source voltage potential V S  coupled to an n-type region  32 , a drain terminal having a drain voltage potential V D , and another polysilicon  26  over two n-type regions,  28  and  30 , forming a select transistor on a p-type substrate  34 . An inter-poly dielectric region  38  is defined between the two polysilicon pieces  22  and  24  (control gate and floating gate); two gate-dielectric regions  40  and  44  are defined by the respective polysilicon areas; and a tunnel dielectric region is defined by the tunnel window region  36  of the polysilicon piece  24  (floating gate). Due to the specific shape of the control gate  22  and the floating gate  24 , a tunnel window region  36  is defined to allow the tunneling of electrons.  
           [0006]    [0006]FIG. 1B is a schematic representation of the FLOTOX cell  20 , in which a FLOTOX transistor  46  is coupled in series with a select transistor  48 . In operation, the select transistor  48  is turned on in order to operate the FLOTOX transistor  46 . A drain/source terminal  50  serves as the source terminal for the select transistor  48  and the drain terminal for the FLOTOX transistor  46 .  
           [0007]    Programming of the FLOTOX memory cell is carried out by applying a relatively high voltage pulse between the control gate  22  and the drain terminal  30  when there is a positive voltage applied at the select gate terminal  26 . The high voltage pulse initiates carrier generation in the substrate and causes electrons to penetrate the tunnel-dielectric region  42  and accumulate in the floating gate  24 . In a likewise manner, in erasing the memory cell, an inverse voltage is applied between the gate and drain terminals. Thus, the negative electrons in the floating gate are drawn to the drain through the thin tunnel oxide.  
           [0008]    The erase and program operations are achieved by taking advantage of the Fowler-Nordheim (F-N) tunneling mechanism occurring between the floating gate  24  and the silicon substrate  34  through a thin oxide called the tunnel oxide  42 . A tunnel window  36  defines the area of the tunnel oxide where a large tunnel window would improve the speed of the erase/program operation but would also increase the cell size. A thinner tunnel oxide region  42  would reduce the tunneling voltage requirement and reduce the erase/program operation time. However, such a memory cell is more difficult to manufacture and may have increased reliability kD concerns.  
           [0009]    Thus, attributes of the FLOTOX cell  20  include a relatively long tunneling time, a relatively large tunneling voltage, and a relatively long erase time.  
           [0010]    [0010]FIG. 2A is a cross-sectional view of another important type of non-volatile memory, the flash memory. In the flash memory cell  60 , there is a drain ( 62  or  64 ) and a source ( 64  or  62 ) region deposited on and within a substrate  76 . Over the substrate and the drain and source regions, insulating layers  66  and  68  are deposited. Over the insulating layers, a floating gate  70  is disposed in such a manner to partially overlap one of the drain and source regions. A second insulating layer  72  is then deposited over the floating gate  70 . A control gate  74  is then disposed over the floating gate  70  and partially overlapping the other region.  
           [0011]    [0011]FIG. 2B is the schematic representation of the memory cell  60  showing its circuit symbol.  
           [0012]    In operation, the flash memory cell  60  is erased when the drain and source terminals are connected to ground and a high voltage is applied at the control gate  74 , causing electrons in the floating gate  70  to tunnel to the control gate  74 . Comparing the tunneling process occurring in the FLOTOX memory cell  20  of FIG. 1A, the tunneling of electrons in the flash memory cell  60  of FIG. 2A is a faster process requiring lower voltage potential across the respective terminals. Additionally, the typical erase time for the flash memory cell is less than 1 ms with approximately 14 volts applied. The erase time and/or (lower) voltage potential can be further improved by modifying and optimizing the dimensions of the memory cell.  
           [0013]    To program the memory cell, the control gate  74  is set to be barely-on (around 2 volts), the terminal connected to the region  62 , away from the floating gate  74 , is connected to ground, and the terminal connected to the region  64  closer to the floating gate  74  is provided with a high voltage, generally around 12 volts. In this manner, an electric field is generated in the direction of the region  62  away from the floating gate  70 , causing electrons to travel through the channel region and be injected into the floating gate  70 , thereby charging the gate and programming the memory cell  60 . The flow of the electrons in this process is called hot carrier injection and is illustrated by the arrows.  
           [0014]    [0014]FIG. 3A shows a memory array  80  having a plurality of interconnected flash memory cells  60  arranged in rows and columns. The flash memory cells  60  are connected in such a manner that the terminals closer to the floating gates  70  are designated as the source terminals. The control gates  74  of the memory cells along the same row are connected to the same word line (e.g., WL 0 , WL 1 , etc.). The word lines are controlled and operated by a row address decoder  82  in response to a given row address. The source terminals of the memory cells along the same row are connected to the same source line (e.g. SL 0 , SL 1 , etc.). The source lines are also controlled and operated by the row address decoder  82 . In a similar manner, the drain terminals of the memory cells along the same columns are connected to the same bit line (e.g., BL 0 , BL 1 , etc.). The bit lines are controlled and operated by a column decoder  84  in response to a given column address (Y-MUX is a column address line multiplexer). In a read operation, the signals are amplified by a sense amplifier  86  and put into an output buffer  88 . In a program operation, data is first stored in an input buffer  90  before it is passed through the column decoder  84  for storage. In order to properly program data into the memory cells, data stored in the memory cells on the same row will have to be erased before the program operation. The reason here is that the control gate of the memory cells on the same row are connected to the same word line and hence will be affected as a group. In comparison, in such a memory array using flash memory cells, the memory cells have to be altered on a large block basis (here the entire row) while a memory array using FLOTOX memory cells can be altered on a byte to byte basis. When a memory array is altered on a block basis, invariably some of the data that does not need to be altered has to be rewritten back into the memory array which consumes time and power in the process.  
           [0015]    [0015]FIG. 3B is similar to FIG. 3A except that the flash memory cells  60  are connected in such a manner that the terminals farthest from the floating gates  70  are designated as the source terminals. The consequences of this difference are detailed below.  
           [0016]    The row address decoder  82  and column address decoder  84  control the bit lines, word lines and source lines during erase, program and read operations as detailed in Tables 1A and 1B. Table 1 A gives the operating characteristics for the memory cell of FIG. 2 using the flash memory array of FIG. 3A. Table 1B gives the operating characteristics for the memory cell of FIG. 2 using the flash memory array of FIG. 3B.  
                                                                                           TABLE 1A                                               Source Line (SL)           Bit Line (BL) (Drain)   Word Line (WL) (Gate)   (Source)                Select   Non-Select   Select   Non-Select   Select   Non-Select                        Erase   Vs (˜0 V)   Vs (˜0 V)   Ve (˜15 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)       Program   Vdp0/Vdp1   Vcc (˜5 V)   Vcgp (˜2 V)   Vs (˜0 V)   Vp (˜12 V)   Vs (˜0 V)           (˜Vs/Vcc)           Read   Vdr (˜2 V)   Vs (˜0 V)   Vcgr (˜4 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)                  
 
           [0017]    [0017]                                                                                           TABLE 1B                                               Source Line (SL)           Bit Line (BL) (Drain)   Word Line (WL) (Gate)   (Source)                Select   Non-Select   Select   Non-Select   Select   Non-Select                        Erase   Vs (˜0 V)   Vs (˜0 V)   Ve (˜15 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)       Program   Vdp0/Vdp1   Vs (˜0 V)   Vcgp (˜2 V)   Vs (˜0 V)   Vp (˜0 V)   Vs (˜0 V)           (Vp/Vs)           Read   Vdr (˜2 V)   Vs (˜0 V)   Vcgr (˜4 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)                    
           [0018]    In Tables 1A and 1B, the abbreviations have the following meanings. Vs is the source voltage during read or erase. It has a low potential voltage of approximately 0 V. Ve is the control gate (CG) erasure voltage required for F-N tunneling. It is approximately +15 volts depending upon the fabrication process used. Vp is the programming voltage applied to the drain (or source near the floating gate side) of the storage transistors. It is approximately +12 volts depending upon the fabrication process used. Vcgp is the CG programming voltage. It should be higher than the threshold voltage of the storage transistors to be erased (generally approximately 1.5 volts) in order to turn on the selected transistors. Vcgp is thus approximately +2 volts. Vdp 0  is the drain programming voltage to be applied to the selected storage transistors in order to program them with a logical input data of “0” (i.e., to store electrons into the floating gate (FG)). It can be approximately either 12 volts (Vp) or +0 volts (Vs) depending upon the array and memory cell configurations. Vdp 1  is the drain programming voltage to be applied to the selected storage transistors in order to program them with a logical input data of “1” (i.e., not to store electrons into the FG), which is the same as program-inhibit, which is also the same as for unselected bit lines or storage transistors. It can be approximately either 0 volts (Vs) or 5 volts (Vcc) depending upon the array and memory cell configurations. Vcgr is the CG read bias voltage, which may be approximately +4 volts depending upon the fabrication process and design. Vdr is the drain read bias voltage, which may be approximately between +1.5 to +2 volts. Vcc is the power supply voltage, which may be approximately +5 volts depending upon the fabrication process and design.  
           [0019]    One of the problems of the memory array structure shown in FIGS. 3A and 3B is that the memory cells have to altered on a large block (i.e., row) basis even when part of the data does not need to be altered. Consequently, even unaltered data is required to be rewritten back to the memory cells, consuming power and time in the process.  
           [0020]    U.S. Pat. No. 5,812,452 (which is incorporated herein by reference in its entirety for all purposes) provides one potential solution to the above problem. Each memory cell in this patent includes two transistors: a select transistor and a storage transistor. The select transistor is connected in series with the storage transistor. When placed in a memory array, a predefined number of memory cells can be grouped into blocks. By using a block select transistor, the memory cells can be accessed and altered on a block-by-block basis.  
           [0021]    However, the solution described in U.S. Pat. No. 5,812,452 requires two transistors per bit stored. This increases the size of the memory array.  
           [0022]    There is a need to reprogram less than large blocks of an EEPROM array without requiring so much additional area occupied by requiring two transistors per bit stored.  
         BRIEF SUMMARY OF THE INVENTION  
         [0023]    According to one embodiment of the present invention, a flash EEPROM includes a plurality of groups of memory cells, one or more group select lines, a plurality of first select transistors, a plurality of second select transistors, a plurality of first word lines, and a plurality of second word lines. The group select lines provide an erasure voltage. The first and second word lines control the first and second select transistors to couple the erasure voltage to a selected group of memory cells.  
           [0024]    In this manner, a group of memory cells may be erased while requiring only two select transistors per group. If each group is an eight-bit byte, the number of required select transistors is only one-fourth that required in the prior art such as U.S. Pat. No. 5,812,452.  
           [0025]    These and other features of the present invention will become well understood from an examination of the following drawings and detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1A is a cross-sectional side view of a conventional FLOTOX memory cell;  
         [0027]    [0027]FIG. 1B is a schematic symbol for the FLOTOX cell of FIG. 1A;  
         [0028]    [0028]FIG. 2A is a cross-sectional side view of a conventional flash memory cell;  
         [0029]    [0029]FIG. 2B is a schematic symbol for the flash cell of FIG. 2A;  
         [0030]    [0030]FIG. 3A is a schematic diagram of a first conventional memory array;  
         [0031]    [0031]FIG. 3B is a schematic diagram of a second conventional memory array;  
         [0032]    [0032]FIG. 4A is a schematic diagram of a memory array according to an embodiment of the present invention; and  
         [0033]    [0033]FIG. 4B is a schematic diagram of a memory array according to another embodiment of the present invention  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    [0034]FIG. 4A shows one embodiment of a memory array  100  according to the present invention. The memory array  100  includes a number of groups  102  of memory cells  60 . Each group  102  is shown in FIG. 3A as being an eight-bit byte and thus includes eight storage transistors. Other group sizes are possible according to design criteria. The groups  102  are organized into rows and columns. The memory array  100  can include any number of rows and columns according to design criteria.  
         [0035]    The memory array  100  has numerous similarities to the memory array  80  of FIG. 3A. The word lines WL 0 , WL 1 , etc. control horizontal access to the groups  102 . The bit lines BL 0 , BL 1 , etc. provide access to each bit in the groups  102 . The source lines SL 0 , SL 1 , etc. provide a source connection. The functions of the sense amplifier  86 , the output buffer  88 , and the input buffer  90  are the same as described above.  
         [0036]    However, there are numerous differences from the memory array  80  of FIG. 3A. One difference is the addition of block select transistors (e.g., M 00 A, M 01 A, etc.; collectively MA) and block deselect transistors (e.g., M 00 B, M 01 B, etc.; collectively MB). A second difference is the addition of word deselect lines (e.g., WL 0 \, WL 1 \, etc.). A third difference is the addition of block select lines (e.g., BS 0 , BS 1 , etc.). These differences also involve changes to the row address decoder  104  and the column address decoder  106 .  
         [0037]    The block select transistors MA are coupled between the block select lines BS and the control gate sublines  108  for each group  102 . Each control gate subline  108  is coupled to the control gates of the flash memory cells in a corresponding group  102 .  
         [0038]    The block deselect transistors MB are coupled between the block select transistors MA and the source lines SL. Thus, the block deselect transistors can couple the control gate sublines  108  to the source lines SL.  
         [0039]    The word lines WL control the block select transistors MA, and the word deselect lines WL\ control the block deselect transistors MB. The word lines WL provide horizontal (row) access to the memory array  100 . In operation, a particular word deselect line WL\ is provided a logic level complementary to that of its associated word line WL. For example, if WL 0  is at a logic level of “high” then WL 0 \ is at a logic level of “low.” Therefore, each control gate subline  108  is coupled either to its corresponding block select line BS or to its corresponding source line SL.  
         [0040]    The block select lines BS provide vertical (column) access to the memory array  100 . Therefore, by a particular combination of one word line WL and one block select line BS, a particular block  102  may be accessed for reading, programming or erasing.  
         [0041]    [0041]FIG. 4B shows another embodiment of a memory array  100   b  according to the present invention. The memory array  100   b  is similar to the memory array  100  of FIG. 4A except that the drain (connected to the bit line BL) is defined to be the region near the floating gate side of the memory cell  60 . The memory cells  102   b  are similar to the memory cells  102 , as are the sense amplifier  86 , output buffer  88  and input buffer  90 . The major consequence of redefining the drain of the memory cells  60  is that the row address decoder  104   b  and column address decoder  106   b  provide different control voltages to the word lines WL, word deselect lines WL\, block select lines BS, and source lines SL than the row address decoder  104  and the column address decoder  106  of FIG. 4A.  
         [0042]    The row address decoder  104  ( 104   b ) and column address decoder  106  ( 106   b ) control the bit lines BL, word lines WL, word deselect lines WL\, block select lines BS, and source lines SL as set forth in Tables  2 A and  2 B. Table 2A gives the operating characteristics for the memory cell of FIG. 2 using the flash memory array of FIG. 4A. Table 2B gives the operating characteristics for the memory cell of FIG. 2 using the flash memory array of FIG. 4B.  
         [0043]    In Tables  2 A and  2 B, the abbreviations have the same meanings as discussed above regarding Tables  1 A and  1 B. In addition, Vwle is the word line erasure voltage. It should be greater than or equal to Ve plus the threshold voltage of the select transistor Vt. Assuming a Vt of approximately +2 volts, Vwle is approximately +17 volts. Vwle and Ve can both be +17 volts such that the actual voltage on the CG of the storage transistors is adequate for F-N tunneling (e.g., approximately +15 volts).  
                                                                                                               TABLE 2A                                       Bit Line (BL) (Drain)   Word Line (WL) (Gate)   Block Select Line (BS)   Source Line (SL) (Source)                Select   Non-Select   Select   Non-Select   Select   Non-Select   Select   Non-Select                        Erase   Vs (˜0 V)   Vs (˜0 V)   Vwle   Vs (˜0 V)   Ve (˜15 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)                   (˜17 V)       Program   Vdp0/Vdp1   Vcc (˜5 V)   Vcc (˜5 V)   Vs (˜0 V)   Vcgp (˜2 V)   Vs (˜0 V)   Vp (˜12 V)   Vs (˜0 V)           (Vs/Vcc)       Read   (Vdr (˜2 V)   Vs (˜0 V)   Vcc (˜5 V)   Vs (˜0 V)   Vcgr (˜4 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)                  
 
         [0044]    [0044]                                                                                                               TABLE 2B                                       Bit Line (BL) (Drain)   Word Line (WL) (Gate)   Block Select Line (BS)   Source Line (SL) (Source)                Select   Non-Select   Select   Non-Select   Select   Non-Select   Select   Non-Select                        Erase   Vs (˜0 V)   Vs (˜0 V)   Vwle   Vs (˜0 V)   Ve (˜15 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)                   (˜17 V)       Program   Vdp0/Vdp1   Vs (˜0 V)   Vcc (˜5 V)   Vs (˜0 V)   Vcgp (˜2 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)           (Vp/Vs)       Read   (Vdr (˜2 V)   Vs (˜0 V)   Vcc (˜5 V)   Vs (˜0 V)   Vcgr (˜4 V)   Vs (˜0 V)   Vs (˜0 V)   Vs (˜0 V)                    
         [0045]    As detailed above, a group  102  ( 102   b ) of memory cells  60  may be erased while requiring only two select transistors (MA and MB) per group. If each group is an eight-bit byte, the number of required select transistors is only one-fourth that required in the prior art such as U.S. Pat. No. 5,812,452. This reduces the space occupied by the memory array  100  ( 100   b ).  
         [0046]    According to another embodiment, the source lines SL of two adjacent rows of memory cells  102  ( 102   b ) are combined into one source line. For example, in the memory array  100  of FIG. 4A, the source lines SL 0  and SL 1  may be combined into a single source line. This combining further reduces the space occupied by the memory array  100  ( 100   b ). Referring to TABLE 2B, note that this combining has no negative consequences to the memory array  100   b  because the source line SL voltage is always Vs (0 V). Referring to TABLE 2A, note that the programming voltage Vp differs from the source voltage Vs, so this will introduce more write disturbance to the non-selected adjacent row during programming for the memory array  100 . This combining involves changes to the control logic of the row address decoder  104  ( 104   b ).  
         [0047]    According to yet another embodiment, the source lines SL may all be combined into a single common source that is always at Vs. Noting the above mention of write disturbance, this embodiment is most applicable to the memory array  102   b . In such an embodiment, the row address decoder  104   b  would not be required to control the common source. This further reduces the control logic requirements and reduces the space occupied by the memory array  100   b.    
         [0048]    Although the above description has focused on specific embodiments, other alternatives and their equivalents are considered to be within the scope of the following claims.