Abstract:
A system for reducing the recovery time for the second read in the double-bit cell of a semiconductor memory device. For reading the second bit of the double-bit cell, in addition to swapping the source and drain terminals of a core cell, the source and drain terminals of corresponding double-bit reference cells are also swapped. The system includes a circuit that effects the swapping by providing a path to enable reading the cells in the reverse direction for the second bit read. The swapping enables the bits of the core cell to be accurately determined over the life of the device while at the same time reducing the recovery time needed for execution of the read of the second bit of the double-bit cell.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to non-volatile semiconductor memory devices, and more particularly, to a system for reading the bits in a double-bit memory cell and associated reference cells. 
     BACKGROUND OF THE INVENTION 
     For conventional non-volatile semiconductor devices, such as flash memories, each memory cell stores only one bit of data. For a conventional single bit per cell flash memory architecture, each cell typically includes a metal oxide semiconductor transistor structure having a source, drain, and a channel in a substrate, and a stacked gate structure over a channel. The stacked gate structure typically includes a tunnel oxide formed on the surface of the channel, a floating gate on the tunnel oxide layer, a thin oxide layer over the floating gate, and a control gate overlying the oxide layer. Information is programmed into a flash memory by charging the floating gate for each individual core cell to a predetermined voltage threshold. For a single-bit memory cell, two threshold voltages are predefined such that the data bit is programmed at a threshold that is either a binary “0” or a binary “1”. 
     Modem devices having non-volatile memory are placing ever increasing demands for larger memory in a small profile package. To satisfy this demand for higher density memory, double-bit per cell (“double-bit cell”) flash memories have been developed. The double-bit cell refers to a cell that allows the storage of two bits of data into a single memory cell. 
     FIG. 1 is a schematic diagram of the double-bit core cell structure for the present invention. For each double-bit core cell  10 , two bits can be stored and programmed, a first bit identified as the normal bit and a second bit identified as the complementary (“comp”) bit. For a non-volatile memory such as flash memory, the integrity of the data must be maintained over the life of the device to avoid data errors that could adversely impact a user. Over the life of a flash memory cell there can be changes in the cell characteristics which can affect the data. One such change for a cell can occur due to the programming of an adjacent cell over time which disturbs the state of nearby cells. Charge loss over the life of the flash memory can also affect the cell characteristics. These changes in the cell characteristics can result in a shifting of the cell threshold voltages. This shifting can affect the state of the memory cells so as to create data errors. As a result, techniques are needed to assure the integrity of the data bits in a cell over time. 
     FIG. 2 a  shows two waveforms illustrating an example of the shifting in the distribution of the thresholds for a double-bit cell over time. Since there are two bits in the cell, four threshold distributions are used for the combinations of the two bits. Trace A shows an exemplary state of the voltage threshold distribution for a new memory cell (referred to in FIG. 1 as “before life”) that has not undergone any programming or read cycling. As memory cells in the array are cycled over the life of the cell array, charge loss, memory disturb, and other changes occur over time that can affect the device characteristics. The changing device characteristics can shift the threshold distribution into a different state. An example of the shifted state (referred to in FIG. 1 as “after life”) is shown in Trace B. For example, in the traces in FIG. 2 a , a (1,0) represents a normal bit equal to 1, and a second “complementary” bit equal to 0. Non-volatile memories such as flash memories might be programmed only once and then read intermittently over a long period of time. There is thus a need to determine the data bit values for the double-bit cell accurately for both threshold distributions shown in Trace A and Trace B. In one technique, two reference thresholds (REF 1  and REF 2 ) are provided and compared to the core cell threshold in order to determine the cell data. FIG. 2 b  illustrates how the technique determines the core cell data from the comparison of the reference thresholds to the normal and complementary bits. 
     As shown in FIG. 2 a  and the top and bottom data rows in FIG. 2 b , if the core cell threshold lies at either of the two extremes of the distribution, (1,1) or (0,0), only a comparison of the normal bit to the two reference thresholds is required in order to determine the data. For this case, since a comparison with the complementary bit is not needed for determining the cell data, the “comp bit” column in FIG. 2 b  is marked as an “x” representing a “don&#39;t care” state. For example, the core cell data should be “0” when the core cell threshold voltage is higher (identified as “0”) than the two reference thresholds, as shown in the bottom row in FIG. 2 b . Conversely, for a core cell threshold voltage lower than the REF 1  and REF 2  thresholds, the data should be “1”. 
     The shifting of the distribution thresholds over time from Trace A to Trace B, as illustrated by example in FIG. 2 a , presents challenges in determining the data for various core cell thresholds. When a core cell threshold lies in the area between the two reference thresholds, the data cannot be determined solely by comparing the normal bit with the two reference thresholds. This is illustrated by FIG. 2 a  and FIG. 2 b , where for a normal bit of 1 for (1,0) in Trace A, and a normal bit of 0 in (0,1) in Trace B, the comparison to the two reference threshold voltages, REF 1  and REF 2 , yields the same result. In order to attempt to provide for proper determination of the data in the two cases, a technique has been developed that provides for two sequential data reads along with reading the two reference thresholds from two reference cells. If both the normal bit and complementary bit can be read along with the reference cells, then the cell data for either the trace A (before life) or Trace B (after life) distribution can be determined, as explained in more detail below. 
     The advantage of having the complementary bit in addition to the normal bit is illustrated in FIG. 2 b . For this technique, when the core cell threshold lies between two reference thresholds, identified as REF 1  and REF 2 , both the normal bit and the complementary bit are compared to REF 1  and REF 2 . For example, for the case above of a normal bit of 0 in (0,1) in Trace B, for this technique the complementary bit threshold lies under the (1,0) distribution, the opposite of the (0,1) distribution. Thus for a core cell threshold between REF 1  and REF 2 , the complementary bit threshold would not be between the two thresholds and thus can be determined. By illustration, the core cell threshold lies in the area under the (0,1) distribution in Trace B between REF 1  and REF 2 . This case is shown in the next to last row of FIG. 2 b . For the normal bit, the core cell threshold is higher than REF 1  (shown as “0” in the table) and lower than REF 2  (“1”). For Trace B, the complimentary bit corresponding to this core cell threshold lies under the (1,0) threshold. This complementary bit threshold is lower (“1”) than both REF 1  and REF 2 . This results in a cell data of 0 as shown in FIG. 2 b.    
     Similarly, as shown in the third row from the bottom of FIG. 2 b , the comparison is the same for the normal bit and REF 1  and REF 2  for the area between REF 1  and REF 2  under the (1,0) distribution in Trace A. Using the complementary bit results in determining the cell data for that case to be a “1”. 
     For the above technique, by adding a additional comparison of the complementary bit to the reference cells, REF 1  and REF  2 , the cell data can be accurately determined over the life of the cell, even for the case when the core cell threshold lies between the two reference cell thresholds. For the above method, each bit (normal and complementary) is read along with the two reference cells for the reference thresholds REF 1  and REF 2 . The combination of the data read for the reference cells determines the actual cell data. The system and method to provide for the reading of the complementary bit in order to provide the required comparison, is described in further detail below. 
     FIG. 3 shows the circuit diagram for the sense circuit architecture corresponding to the method described above. As can be seen from FIG. 3, a sense circuit  20  includes a data circuit  30  coupled to a sense amp  22 , and a reference circuit  50  coupled to a reference sense amp  24 . The outputs of the sense amp  22  and sense amp  24 , on lines (“SA”) and (“SAR”) respectively are coupled to the input of a comparator  26 . Comparator  26  is provided for comparing the output signal SA from the data sense amp  22  with the output signal SAR from the reference sense amp  24  in order to determine the data. 
     Data circuit  30  includes a core cell  10  having a control gate connected to the word line, a drain connected to a node  35  and a source connected to a node  33 . The memory cells in a cell array are typically organized by row and column. The common word line is provided by a control circuit (not shown) for selecting a row for the cell in the array. A VCC signal is also provided by the control circuit for selecting the column of the memory cell to be accessed. The details of the control circuit for the decoding and addressing of an individual cell in a memory array are well known to one of ordinary skill in the art. The present invention is described in further detail for a single core cell  10 . 
     For the reading of the complementary bit, the drain and source of the core cell  10  must be swapped, as compared to the reading of the normal bit. Thus there is one path to the DATA bit line provided for the reading of the normal bit and a different path provided for reading the complementary bit. As shown in FIG. 3, additional transistors are typically provided as pass transistors for providing the two conduction paths to the DATA bit line, as will be described in more detail below. 
     In the data circuit  30  in FIG. 3, the source terminal of the core cell  10  connects, at node  33 , to the drain terminal of a pass transistor  32 . Pass transistor  32  has a gate connected to VCC and a source connected to a node  31 . A transistor  38  has a drain connected to node  31 , a source connected to the input of sense amp  22  at node  37 , and a gate connected to a “2 nd ” line. This 2 nd  line is provided by a control circuit (not shown) and provides control for the selection of the path for the reading of the second of the two bits in the double-bit cell (the complementary bit). Thus, if the second bit of core cell  10  is to be read, the gate inputs of the transistors in data circuit  30  will be controlled so as to provide for a connection path from the source terminal of core cell  10  at node  33 , through transistors  32  and  38 , to node  37  which connects to the DATA bit line at the input to the data sense amp  22 . Node  31  is also connected to the drain of a transistor  44 . The source of transistor  44  is connected to ground and the gate is connected to a “2 nd  bar line” as shown in FIG.  3 . Transistor  44  provides a path to ground that is needed for the reading of the normal (first) bit. 
     Node  35 , at the drain terminal of core cell  10 , connects to a source terminal of a pass transistor  34  having a gate connected to VCC and a drain connected to the source of a transistor  36  at node  39 . Transistor  36  has a drain connected to the DATA bit line at the input of sense amp  22  at node  37 . The gate for transistor  36  is connected to a “1st” line. This 1 st  line provides control for reading the first of the two bits (the normal bit). Node  39  is also connected to the drain of a transistor  42 . The source of transistor  42  is connected to ground and the gate is connected to a “1st bar line”. Transistor  42  provides a path to ground for the drain of cell  10  for the reading of the complementary (second) bit. The 1 st and “2 nd  bar” signal are thus active for the reading of the normal bit, and the 2 nd  and “1 st  bar” are active for the reading of the complementary bit. The VCC and word line signals as described above, are set active when core cell  10  is selected. 
     Reference circuit  50  will now be described with reference to FIG.  3 . Although only one reference circuit  50  (e.g. for REF 1 ) and sense amp  24  and comparator  26  is shown if FIG. 3, sense circuit  20  would include an identical circuit (not shown) associated with the second reference cell for the second reference threshold, REF 2 . Reference circuit  50  includes a reference cell  52  having a control gate connected to the reference word line, a source connected to ground, and a drain connected to a node  55 . Two transistors  54 ,  56  are connected in series between node  55  and the input of the sense amp  24 . The gates of transistor  54 ,  56  are shown connected to VCC. The source of transistor  54  is connected to node  55  at the drain terminal of reference cell  52 . The drain of transistor  54  and source of transistor  56  are both connected at node  57 . The drain of transistor  56  connects to the input of the sense amp  24  as shown at the reference signal DATAR in FIG.  3 . 
     The operation of the sense circuit  20  will now be described with reference to FIGS.  3  and  4 . For the method corresponding to FIG. 3, each bit (normal and complementary) is read along with the two reference cells. The comparisons of the voltages for the bit and reference cells for the two reads determines the actual core cell data. For the circuit in FIG. 3, the normal bit and complementary bit cannot be read at the same time, because the reading of the second (complementary) bit requires the swapping of the source and drain of core cell  10 . In order to obtain both data bits in the double-bit cell for circuit  20  there must be two separate reads: one read for the normal bit (and reference cells), and a second read for the complementary bit (and reference cells). Because of the need for two reads, the access time required to complete both reads is critical in determining how fast both bits are read accurately. 
     A key determining factor in the total read access time is how fast the second (complementary bit) read is completed after the first (normal bit) read. A drawback of the sense circuit  20  and method in FIG. 3, is the bit line undershoot that delays the execution of the second read. FIG. 4 illustrates this undershoot drawback. The voltage traces in FIG. 4 show the voltages over time as the first read is completed and the second read is executed. Trace C in FIG. 4 shows a waveform for the sense amp signal (SA) from FIG. 3 for the output of the data sense amp  22 . Trace D is a waveform for the voltage for the corresponding data (DATA) at the bitline. Trace E is a waveform for the voltage for the SAR signal from the reference sense amp  24 ; and Trace F represents the reference cell data voltage (DATAR) input to the sense amp  24 . In operation, when the second bit is to be read, the source and drain are swapped. As a result, the drain of core cell  10  starts to charge and the source begins to be discharged. This transition period causes a huge voltage drop in the bit line (DATA) which becomes a huge voltage drop “undershoot”), in the corresponding sense signal SA in circuit  20  as shown in Trace C in FIG.  4 . 
     As shown in FIG. 4, after this undershoot in SA occurs, there is a significant time interval before the SA signal can return to a desirable level in order to perform the second read. This time interval is the “recovery time” identified in FIG.  4 . In operation, the SA signal must recover sufficiently so that there is the required margin between the SA and SAR signals in order for the second read to proceed. This large recovery time is a drawback of the circuit  20  of FIG. 3 since it results in a significant time loss in the total access time to complete the two required reads. 
     Modern devices increasingly are requiring faster memory access times in order to process the data at a rate that can provide the response that users demand. Access times for a flash memory must be optimized while increasing the density. There is therefore a need for providing optimized memory access times for a double-bit flash memory while maintaining the integrity of the memory data over the life of the memory. 
     Therefore, it would be desirable to have a system and method for reducing the recovery time for the second read in order to provide faster access time for execution of the two reads for a double-bit memory cell in a nonvolatile memory. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for reference cell swapping for reducing the recovery time for the second read for a double-bit cell in a non-volatile memory, such as a flash memory. The reduction in recovery time reduces the overall read time for the two reads substantially compared to other techniques. For the present invention, two consecutive reads are performed with each bit in the double-bit cell being read along with two reference cells. The system includes a circuit to provide for the swapping of the source and drain terminals of both the reference circuit and the data circuit in order to efficiently and correctly read the data. The system of the present invention includes a circuit and method that accounts for changes in the memory cell characteristics over time and thereby eliminates data errors. 
     In an embodiment of the present invention, an apparatus for providing for reading two data bits and two reference cells. The threshold voltages of each data bit and two reference cells are sensed. The two reference cell threshold voltages are each separately compared to the data bit sensed voltage threshold. A second complementary bit of the double-bit cell is read after a normal bit by swapping the core cell drain and source terminals. In order to substantially reduce recovery time for the second read, the reference cell drain and source are also swapped for the second read such that the sense reference threshold voltage has a drop (undershoot) that occurs due to the swapping, and that this undershoot tracks with a corresponding undershoot for the sensed data threshold voltage. The system of the present invention thereby reduces recovery time for the second read by enabling the sense signal for the data and sense signal for the reference cells to track each other. The tracking of the voltage undershoots that is provided permits the sense data voltage signal to reach the desired level in order to enable the second read to proceed and be executed much faster than for the known systems and methods. 
     Thus, the system of the present invention achieves fast read access times since the two bits in a double-bit memory cells are read with minimized recovery time between reads while also obtaining the desired accuracy in reading the data values despite characteristic changes of the memory cell over time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and related advantages and features of the present invention will become apparent upon review of the following detailed description of the invention, taken in conjunction with the following drawings, where like numerals represent like elements, in which: 
     FIG. 1 is a schematic diagram of the double-bit core cell structure for the present invention; 
     FIG. 2 a  is a timing diagram showing two exemplary waveforms illustrating the shifting in the distribution of the thresholds for a double-bit cell over time; 
     FIG. 2 b  is a table showing the use of the complementary bit and reference cells to determine the cell data; 
     FIG. 3 is a schematic diagram of a sense circuit architecture; 
     FIG. 4 is a timing diagram showing voltages over time for execution of the read of the second bit; 
     FIG. 5 is a schematic diagram of an embodiment of the circuit of the present invention; and 
     FIG. 6 is a timing diagram showing voltages over time illustrating the reduced recovery time for the second read for the embodiment of the present invention in FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     The present invention includes a system for reference cell swapping for reducing the recovery time for the second read in the double-bit cell. The present invention has the advantage of significantly reducing the total access time to execute both reads for a double-bit cell. The present invention has the further advantage of reducing this read time while enabling the data in the double-bit cell to be read accurately over the life of the memory cell, accounting for changes in the cell characteristics over the life of the memory due to disturb, charge loss and other changes. 
     The present invention will now be described with reference to FIGS. 5 and 6. FIG. 5 is a schematic diagram of an embodiment of the system of the present invention. A sense circuit  100  in FIG. 5 includes a data circuit  30  coupled to a sense amp  22 , and a reference circuit  70  coupled to a reference sense amp  24 . As in circuit  20  in FIG. 3, in FIG. 5, the outputs of the sense amp  22  and sense amp  24 , on lines (“SA 1 ”) and (“SAR 1 ”) respectively are coupled to the input of a comparator  26 . For circuit  100 , the data circuit  30  is as described above for FIG.  3 . 
     In contrast to the circuit  20  in FIG. 3, circuit  100  provides for swapping the source and drain terminals of the reference cell in the reference circuit  70 , just like the drain and source are swapped for the core cell  10  in the data circuit  30 . As a result of swapping the source and drain for both the reference circuit  70  and the data circuit  30  for this system in FIG. 5, the voltage drop during the transition to the second read is applicable to both the data cell and the reference cell, as will be described further below. For the embodiment in FIG. 5, the data circuit  30  and the swapping of the associated core cell  10  drain and source therein are as described above for FIG.  3 . The reference cell swapping for the circuit in FIG. 5 will now be described further. 
     As shown in FIG. 5, reference circuit  70  includes a reference cell  72  having a control gate connected to the reference word line, a drain connected to a node  75 , and a source connected to the drain of a pass transistor  54  at node  73 . Although only one reference circuit  70  (for a first threshold, e.g. REF 1 ) and sense amp  24  and comparator  26  are shown if FIG. 5, circuit  100  would include an identical circuit (not shown) associated with the second reference cell for the second reference threshold, REF 2 . Thus, the each data bit can be separately compared two sensed reference thresholds in order to determine the cell data. In the reference circuit  70 , two transistors  56  and  76  are connected in series between node  75  and the input of the sense amp  24 . The gates of transistor  54 ,  56  are connected to VCC. Transistors  54  and  78  are connected in series between node  73  and the input of the sense amp  24 . The source of transistor  54  is connected to node  71  at the drain of transistor  78 . The source of transistor  78  connects to the input of the sense amp  24  at node  77  as shown at the reference signal DATAR 1  in FIG.  5 . Transistor  78  has a gate connected to a “2 nd ” line. This 2 nd  line is provided by a control circuit (not shown) as described for the data circuit  30  and provides control for the selection of the path for the reading of the second of the two bits in the double-bit reference cell (the reference complementary bit). When active this 2 nd  line provides for the selection of the reference complementary bit by providing a connection path from the source terminal of reference cell  72 , through transistors  54  and  78  to node  77 . This provides a connection to provide the second reference threshold bit to the DATAR bit line at the input to the data sense amp  24 . Node  71  is also connected to the drain of a transistor  74 . The source of transistor  74  is connected to ground and the gate is connected to a “2 nd  bar line”. As described, the 1 st  and “2 nd  bar” signal are thus active for the reading of the normal bit, and the 2 nd  and “1 st  bar” are active for the reading of the complementary bit. The VCC and reference word line (“WL”) signals are set active when core cell  10  is selected. Transistor  74  provides a path to ground for the reading the first (normal) reference threshold. 
     As shown in FIG. 5, node  77  connects to the DATAR 1  bit line at the input to the data sense amp  24 . Transistor  76  has a drain connected to node  77 . The gate for transistor  76  is connected to a “1st ” line. This 1st line provides control for the selection of the first (normal) of the two reference threshold bits. When the 1 st  line is set, the transistor  76  provides a connection to provide a path (through transistor  56 ) to enable the first (normal) reference threshold bit from reference cell  72  to be sensed at the data sense amp  24  for comparison with the data bits. The source of transistor  76  is connected to the drain of transistor  56  at a node  79 . Node  79  is also connected to the drain of a transistor  82 . The source of transistor  82  is connected to ground and the gate is connected to a “1st bar line”. Transistor  82  provides the connection to ground for the drain of reference cell  72  required for reading the second (complementary) reference threshold. 
     The operation of the sense circuit  100  will now be further described with reference to FIGS. 5 and 6. For the method corresponding to FIG. 5, each bit (normal and complementary) is read along with the two reference cells. The comparisons of the voltages for the bit and reference cells for the two reads determines the actual core cell data. For the circuit in FIG. 5, the normal bit and complementary bit cannot be read at the same time, because the reading of the second (complementary) bit after the normal bit requires swapping of the source and drain. In order to obtain both data bits in the double-bit cell for circuit  100  there must be two separate reads: one read for the normal bit (and reference cells), and a second read for the complementary bit (and reference cells). 
     FIG. 6 is a timing diagram showing voltages over time illustrating the reduced recovery time for the second read for the circuit  100  of the present invention in FIG.  5 . Trace G in FIG. 6 shows the sense amp signal (SA 1 ) for the circuit in FIG. 5 for the output of the data sense amp  22 . Trace H shows the voltage for the corresponding data (DATA 1 ) at the bitline. Trace I is a waveform showing the voltage for the SAR signal from the reference sense amp  24 . Trace J shows a waveform for the reference cell data voltage (DATAR) input to the sense amp  24 . 
     For this embodiment shown in FIG. 5, for the second read, in addition to swapping the source and drain of the data cell  10 , the source and drain for the reference cell  72  are also swapped. As shown in FIG. 6, during the transition from the first to second read for the reference bits in circuit  70  a huge voltage drop undershoot happens to the reference side. As described above, a similar huge voltage drop undershoot occurs during the transition from the first to second read for the core cell side since the drain and source are swapped for circuit  30 . As seen in FIG. 6, the sense signals SA 1  and SAR 1  both exhibit this undershoot after the first read and track much closer than the respective sense signals SA and SAR in FIG.  4 . The recovery time required before the second read can be completed depends on whether there is sufficient margin between the SA 1  and SAR 1  signals. Once this margin appears, the second read can be completed. As shown in FIG. 6, because SAR 1  had the same relative voltage drop as occurs for signal SA 1 , the margin appears much faster for the circuit  100  in FIG. 5 as compared to the circuit  22  in FIG.  3 . Thus, for the embodiment in FIG. 5, the recovery time for the second read is minimized. As shown in FIG. 4, the sense circuit  20  does not provide such tracking which results in increased recovery time and much slower read times for the second read. For the present invention, there will be two reference cells, one reference cell programmed for a first predetermined reference threshold (identified as REF 1 ) and one reference cell programmed for a second predetermined reference threshold (identified as REF 2 ). Each reference cell is a double-bit cell having a normal and complementary bit. In order for the comparisons with the core cell thresholds to be made with either bit, the normal and complementary bit are both programmed within a cell to the same threshold (e.g. threshold REF 1  for one cell and threshold REF 2  for the second reference cell). As noted, although only one reference circuit  70  (for a first threshold, e.g. REF 1 ) and sense amp  24  and comparator  26  are shown if FIG. 5, circuit  100  includes an identical circuit (not shown) associated with the second reference cell for the second reference threshold, REF 2 . For the second reference circuit the reference normal and complementary bits must be set to the same predetermined threshold (e.g. REF 2 ). Details regarding the control of addressing and selection of a particular memory cell and a particular bit in a double-bit cell are not shown; such details being known to one of ordinary skill in the art. 
     This reference cell swapping system in FIG. 5 of the present invention has the advantage of reducing the second read time significantly by providing for better tracking between the reference sense signal SAR 1  and the core cell sense signal SA 1  during the transition to the second read while accounting for changes in the cell characteristics over time. 
     While the present invention has been described in connection with exemplary embodiments thereof, those skilled in the art will appreciate that the invention is not limited to the embodiments described, and that modifications of the exemplary embodiments may be made without departing from the spirit and scope of the appended claims. Thus, the foregoing description is to be regarded as illustrative instead of limiting.