Abstract:
In the present invention a new method and circuit is disclosed to handle write data during CHE programming for a nonvolatile memory cell including cells created with MONOS technology. A plurality of bit lines are precharged to program inhibit all memory cells coupled to the bit lines. Then a selective bit line is discharged to program the selected memory cell. The number of bit lines selected to be precharged can be reduced to the bit line to be programmed to save power, and precharging a bit line can be done simultaneous with applying program data to a bit line to reduce the number of times a bit line is charged. The number of data latches may be reduced to the actual program data width, resulting in significant area savings and circuit simplification.

Description:
The instant application claims priority to U.S. Provisional Application Ser. No. 60/250,858 filed Dec. 4, 2000, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor memories and in particular to non-volatile floating gate and MONOS memories. 
     2. Description of the Related Art 
     A conventional two poly planar floating gate device structure is shown in FIG.  1 . The word select gate  10  is a polysilicon gate stacked above a floating gate  11 , which is also polysilicon. Two diffusions form the source  13  and drain  12 . In order to program the memory cell, electrons must cross the oxide barrier into the floating gate  11 . There are two main mechanisms for program, Channel Hot Electron injection (CHE) and Fowler-Nordheim tunneling (FN). Typical CHE voltages are as follows: The word gate  10  is biased to 11 V, and the program drain  12  is biased to 8V. The voltage of source diffusion  13  is grounded. In this device structure, CHE requires a high drain to source voltage and is characterized by program currents of greater than 100 uA per cell. Program data widths are usually limited to 8, 16 or 32 bits because of high voltage charge pump constraints. 
     In Fowler-Nordheim tunneling program, the word gate  10  is usually biased to a high voltage of 20V, and both the source  13  and drain  12  are grounded. Program currents are on the order of 10 nA/cell, so all cells sharing a single word line and can be selected and programmed together without needing a high current charge pump. For a conventional NAND array device that utilizes FN program, a page buffer is usually implemented in which there exists one program data latch for each bit diffusion line. The cells on a word line which are not to be programmed to a high threshold “0” state need to be program inhibited. Implementation of the page buffer presents a layout challenge and significant area penalty. 
     An implementation of program data latches in a page buffer is described in “A Dual Page Programming Scheme for High Speed Multi-Gb-Scale NAND Flash Memories”, by Ken Takeuchi and TomoharuTanaka, IEEE 2000 Symposium on VLSI Circuits Digest of Technical Papers, June 2000, pp 156-157. In this paper, two memory blocks share one page buffer, by taking advantage of the zero drain to source current during FN program. Bit line leakage was shown to be less than 0.1 mV. The values of the data latches could be coupled to the bit lines of one block and then safely floated, during program. The same data latches could then be used to program a second block, at the same time, reducing the number of data latches needed by half. However, this particular configuration could only be used by FN program, because the non-zero drainsource current in CHE program will not allow for floating bit lines. 
     In U.S. Pat. No. 6,275,415 B1 (Haddad et al) a memory device is directed to having multiple memory cells with a method of programming multiple memory cells wherein a bias voltage is applied to a common source terminal and a time varying voltage is applied to gates of cells to be programmed and using channel hot electrons (CHE). U.S. Pat. No. 5,753,951 (Geissler) is directed to CHE injection techniques where the memory cell has a floating gate structure that extends over a sharp edge of a memory cell trench and then into the trench. U.S. Pat. No. 5,874,337 (Geissler) is directed toward the method of creating a memory cell with a floating gate structure that extends over and into a memory cell ctiewzw trench. U.S. Pat. No. 6,166,410 (Lin et al.) is directed to a structure and method of manufacture of a split gate MONOS memory device having a MONOS transistor in series with a stacked polysilicon gate flash transistor. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a new method for writing data using CHE programming for non-volatile floating gate and MONOS memories. 
     Another objective of the present invention is to provide a new circuit to handle write data during CHE programming. 
     Yet another objective of the present invention is to provide a bit line precharge to inhibit all memory cells before a selective bit line discharge is done to program selected memory cells. 
     A further objective of the present invention is to reduce the number of sense amplifiers and write data circuits to be equal to the program data width. 
     Again a further objective of the present invention is to produce a write data circuit that is constructed with logic transistors. 
     Again a further objective of the present invention is to simplify circuitry required to write data for non-volatile floating gate and MONOS memories. 
     Still a further objective is to selectively precharge bit lines for low power and low current operations. 
     Yet a further objective of the present invention is produce circuit area savings through reduction in the complexity of circuitry associated with writing data to nonvolatile floating gate and MONOS memories. 
     Still yet a further objective of the present invention is to reduce the oxide thickness of decode transistors and reduce high voltage transistors. 
     A new method and circuit to handle write data during CHE program is described in the present invention. A sequence of first precharging memory bit lines and then discharging the bit lines is used to program selected memory cells. A bit line precharge is first implemented to program inhibit all memory cells, and then a selective bit line discharge is done to program selected memory cells. The number of data latches needed is reduced to the actual program data width resulting in significant area savings and circuit simplification. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     This invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 shows a two poly planar floating gate device structure of prior art, 
     FIGS. 2A and 2B show split gate flash memory devices of the present invention, 
     FIG. 3 shows a schematic diagram of a partial memory array of the present invention where precharge circuits for a plurality of bit lines share a same latch in a column decoder, 
     FIG. 4 shows a schematic diagram of a partial memory array of the present invention where precharge circuits connected to bit lines are individually controlled, 
     FIG. 5 shows a circuit diagram of a write data latch of the present invention, and 
     FIG. 6 shows a circuit diagram of a write data latch of the present invention having an optimized number of transistors. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 2A and 2B are shown two types of split gate NOR flash memory cells of the present invention. FIG. 2A shows a split gate, floating gate memory cell that is formed by sidewall spacers so that floating gate  21  is in series with word gate  20 . Electrons are stored in floating gate  21 . FIG. 2B shows a split gate MONOS memory cell that is formed by sidewall spacers and oxide-nitride-oxide (ONO) film. Electrons that control the data state of the memory cell are stored in the ONO film  27 , which lies under the sidewall control gate  28 , which is next to word gate  20 . For the purpose of description for FIGS. 3 and 4, the memory storage region  21  ( 27  for a MONOS cell) and word gate  20  are shown in series, the program diffusion  22  is defined as the diffusion on the same side as the memory storage region (either floating gate or ONO film), and the bit diffusion  23  is on the opposite side. 
     In FIG. 3 is shown the first embodiment of the present invention where a partial array of split-gate NOR flash memory cells is implemented. The program diffusions  22  of all of the memory cell transistors  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208  share a common source line SL 0 . The top row memory cells  201 ,  202 ,  203 ,  204  share a common word line WL 0 . The bottom row memory transistors  205 ,  206 ,  207   208  share word line WL 1 . The bit diffusions  23  are connected in common columns where memory cells in the same column share bit lines. Memory cells  201  and  205  share bit line BL 0 , memory cells  202  and  206  share bit line BL 1 , memory cells  203  and  207  share BL 2 , and memory cells  204  and  208  share BL 3 . Precharge transistors  220 ,  221 ,  222 ,  223  are each associated with a bit line. The precharge voltage is VBLHI. The gates of the precharge transistors  220 ,  221 .  222 ,  223  are connected by a single control signal BLH. One data line  299  is shared by a plurality of bit lines BL 0 -BL 3  through n-channel select transistors  230 ,  231 ,  232 ,  233 . The gates of the n-channel select transistors  230 ,  231 ,  232 ,  233  are controlled by y-decode signals Y 0 , Y 1 , Y 2 , Y 3 , respectively. The sense amplifier  270  and write data circuit  280  can be individually connected to the data line  299  line by switches  271  and  281 , respectively. 
     In the schematic diagram of FIG. 3, one sense amplifier  270  and one write data circuit  280  is shared between 4 bit lines. A decode of 4 bit lines was chosen to simplify explanation, and should not be considered as a limitation. The number of bit lines per sense amplifier and write data circuit can be any desired number. The source line SL 0  is connected to only 2 rows of memory cells. A single source line may be connected between as few as 1 row of memory cells to as many as  512  memory cell rows, or more. 
     Continuing to refer to FIG. 3, for ease of explanation, the voltages for a program operation on memory cell  201 , which is a floating gate split gate device, are assumed to be: program diffusion  22  biased to 5V, word gate  20  biased to 5V, and bit diffusion  23  biased to either 0V, or for program inhibit 5V. The bit line program inhibit voltage is chosen to be greater than the word gate voltage minus the word gate threshold voltage. It should be noted that for a split gate MONOS device, the word gate voltage can be as low as, or lower than, the logic power supply. Then the program inhibit voltage is not required to be a high voltage. As shown in FIG . 3 , several data bit lines share a single data latch by means of select transistors  230 ,  231 ,  232 ,  233 . Memory cell  203  is used as the target selected memory cell for explanation of FIGS. 3 and 4. Program data is latched into the write data circuit  280 . If the program data is a logical “0”, then the memory cell is to be written. If the program data is a logical “1”, then the memory cell is not to be written. Before the program sequence begins, assume that the bit lines BL 0 , BL 1 , BL 2 , BL 3  are at 0V, or floating around 0V. All of the word lines are unselected at 0V, and the source line SL 0  is 0V. Switches  271  and  281  are both open. The voltage of BLH is off, so precharge transistors  220 ,  221 ,  222 ,  223  are non-conducting. The voltage to the gate signals Y 0 , Y 1 , Y 2 , Y 3  of the select transistors  230 ,  231 ,  232 ,  233 , is 0V. The data line  299  is precharged to the power supply voltage (VDD) by an additional precharge transistor not shown. VBLHI is set to the program inhibit voltage of 5V. In the first step of the program sequence, a 5V bias is applied to SL 0 , which is connected to the program diffusions of all of the memory cells  201 - 208 . Next, all bit lines BL 0 -BL 3  are precharged to the 5V program inhibit voltage, by means of precharge transistors  220 ,  221 ,  222 ,  223 . 
     Continuing to refer to FIG. 3, after all of the bit lines have been charged to the program inhibit voltage of 5V, BLH is turned off and the bit lines float at 5V. Switch  281  is then closed, connecting the write data circuit to the data line  299 . The write data circuit  280  contains a pull-down transistor to ground which conducts or does not conduct, depending upon whether the program data value is a “0” or “1”. If the data is a “0”, then data line  299  will fall to 0V, otherwise, the data line  299  will be remain precharged near VDD. Next, the bit line BL( 2 ) associated with the target memory cell  203  is selected, by biasing the decode signal Y( 2 ) to VDD. It should be noted that if data line  299  is 0V, the pull-down transistor in the write data circuit will force the bit line BL 2  to 0V. If the data line  299  is VDD and the decode signal Y 2  is also VDD, then the gate to source voltage of transistor  232  is 0V, and transistor  232  will not conduct. The precharged program inhibit value of 5V will remain on bit line BL 2  if the program data is “1”, and the memory cell  203  will not be programmed. After the program diffusion and bit diffusion voltages have been setup, the word line WL 0  is activated and CHE program injection will begin for the cell  203 , if the program data is “0”. 
     The advantages of first embodiment are: 1) The number of sense amplifiers and write data circuits is equal to the program bandwidth, not the number of bit lines. 2) The write data circuit can be simply constructed with logic transistors. No high voltage transistors or level shifters are necessary due of a unique sequence of data line precharge and selective discharge. 3) The decode signals Y 0 , Y 1 , Y 2 , and Y 3  can be logic levels. In the conventional method, they would have to be boosted high in order to pass 5V to the bit line for program inhibit. Thus, the oxide thickness of the decode transistors can be reduced and high voltage transistors can be reduced. 
     In the second embodiment of this invention, the voltage setup sequence differs from the first embodiment in that, during bit line precharge through the precharge transistors  220 ,  221 ,  222 ,  223 , the decode transistor  232  is also selected. Thus, data line  299  is precharged high via the n-channel decode device  232 , to the voltage of Y 2  (VDD) minus the threshold of the n-channel decode device  232 . This data line  299  precharge occurs at the same time as the bit line precharge and no extra precharge device is required to specifically recharge data line  299 . In a third embodiment of this invention, the p-channel precharge transistors  220 ,  221 ,  222 ,  223  are replaced with n-channel devices, if the desired program inhibit voltage is less than the n-channel gate voltage minus the n-channel threshold. 
     FIG. 4 shows a fourth embodiment of this invention in which the signals to the gates of the precharge transistors are separated, for individual transistor control. The control line BLH shown in FIG. 3 becomes BLH 0  to BLH 3  in FIG.  4 . This capability to selectively precharge bit lines is useful during the read operation, for low power, low current applications. During program and considering the same power and current constraints as well as the program inhibit voltage charge pump limitations, it may be impractical to precharge all bit lines to 5V at once. Instead, the bit lines may be precharged singularly, or grouped, in any sequence, as long as all of the bit lines that are associated with the memory cells on a given word line are precharged to the program inhibit voltage, prior to accessing the memory cell. 
     Two variations of circuit implementation for the write data latch  280  in FIG. 3 are given in FIG.  5  and FIG.  6 . In FIG. 5, the data line  299  is connected to the drain of pull down transistor  403 , the source of transistor  403  is grounded. The gate of the pull down transistor  403  is controlled by a logical AND of DATA_ENABLE  405  and NOT PGM_DATA  404 . The signal PGM_DATA is the program data from the PGM data latch  401 . DATA_ENABLE is a control logic signal which has the functional equivalent of the switch  281  in FIG.  3 . When DATA_ENABLE and NOT PGM_DAT A are high, then the pull-down transistor  403  is activated and the data line  299  is pulled down to ground. 
     The circuit of FIG. 6 has the same equivalent function as the write data circuit  280  in FIG.  4 . However, the number of transistors has been optimized. The drain of pull-down transistor  504  is also data line  299 . But instead of placing an AND circuit at the gate of the pull-down transistor  503 , the PGM_DATA”  504  is connected to the source of transistor  503 . The gate of the pull-down transistor  503  is simply DATA_ENABLE. When PGM_DATA=0 and the DATA_ENABLE is high, then the pull-down transistor  503  is activated and data line  299  is connected to ground. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.