Abstract:
An apparatus and method for combating the effects of bias temperature instability (BTI) in a memory cell. Bit lines connecting to a memory cell contain two alternate paths criss-crossing to connect a lower portion of a first bit line to an upper portion of a second bit line, and to connect a lower portion of the second bit line to an upper portion of the first bit line. Alternative to activating transistors on the bit lines to read and write to the memory cell, transistors on the alternative paths may be activated to read and write to the memory cell from the opposite bit lines. The memory cell may be read through the bit lines to a sense amplifier, the transistors on the bit lines are subsequently deactivated and the transistors on the alternate paths are activated to write transposed bit values to the memory cell, thereby reversing the biases.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to improvements in lifetime reliability of semiconductor devices and, more particularly, to circuits and the use of the circuits to allow for devices within a memory cell to recover from variability, biases, and stresses. 
       BACKGROUND OF THE INVENTION 
       [0002]    A memory cell, comprised of a number of transistors, operates by holding a voltage value over a period of time. This value translates to either a “1” (high voltage) or a “0” (low voltage) during a read operation of the memory. Constant voltage stress can lead to rapid degeneration of transistor parameters, such as threshold voltage and source-drain current. These effects are known as bias temperature instability (BTI) and become more pronounced and destructive as device sizes are reduced. BTI can be either negative BTI (NBTI) when the bias voltage is negative on a metal-oxide-semiconductor (MOS) structure or positive BTI (PBTI) when the bias voltage is positive. The NBTI effect is more severe for P-channel MOS (PMOS) transistors and the PBTI effect is more severe for N-channel MOS (NMOS) transistors. BTI degrades the reliability of MOS transistors due to the change in threshold voltage, and the degradation increases at elevated temperatures. However, if the stress is periodically interrupted or reversed, the degradation may be significantly reduced, extending the projected life time of the MOS transistors. 
         [0003]    A conventional six-transistor static random access memory (SRAM) cell consists of two PMOS transistors and four NMOS transistors. With any data storage in the SRAM cell, the effects of NBTI degrade one of the PMOS transistors and the effects of PBTI degrade one of the NMOS transistors. If the same data is stored in the SRAM memory cell for an extended period of time, then there can be substantial degradation on the PMOS transistor and the NMOS transistor. During the operation of the SRAM, even if the data read and write operations are performed frequently with different data contents, there is a good possibility that some of the SRAM memory cells store the same high state or low state for an extended period of time. Again, these non-refreshed memory cells can have higher degradation in one of the PMOS transistors and one of the NMOS transistors which can decrease the reliability of the SRAM cell, as well as the life-time of the entire SRAM memory. 
         [0004]    To combat these effects, a recovery period where the polarity of the memory cell is reversed (and hence the biases in the transistors) can be successful. Current strategies to reverse the polarity include the use of at least one inverter and data drivers to re-write the opposite values to the memory cell. 
       SUMMARY 
       [0005]    One aspect of the present invention discloses an apparatus for reading and writing inverted bit values to a memory cell. The apparatus comprises a memory cell, which comprises a first node with one or more field-effect transistors (FETs) and capable of storing a first bit value, and a second node with one or more FETs and capable of storing a second bit value. The apparatus further comprises a first bit line coupled to the memory cell and corresponding to the first node of the memory cell, and a second bit line coupled to the memory cell and corresponding to the second node of the memory cell. A FET exists on the first bit line that, when activated, provides a path from a first section of the first bit line to a second section of the first bit line, the first section of the first bit line being closer to the memory cell than the second section of the first bit line. A FET exists on the second bit line that, when activated, provides a path from a first section of the second bit line to a second section of the second bit line, the first section of the second bit line being closer to the memory cell than the second section of the second bit line. A first alternative line connects the first section of the first bit line to the second section of the second bit line, the first alternative line being selectable by activating a FET on the first alternative line. A second alternative line connects the first section of the second bit line to the second section of the first bit line, the second alternative line being selectable by activating a FET on the second alternative line. 
         [0006]    A second aspect of the present invention discloses a method for extending lifetime reliability of a memory cell. The method comprises providing a first and second read and write (R/W) path connecting the memory cell to a sense amplifier, wherein one of the first and second R/W paths comprises a first conductive line connecting a first node of the memory cell to a first node of the sense amplifier and a second conductive line connecting a second node of the memory cell to a second node of the sense amplifier, and an other of the first and second R/W paths comprises a first conductive line connecting the first node of the memory cell to the second node of the sense amplifier and a second conductive line connecting the second node of the memory cell to the first node of the sense amplifier, wherein the respective first and second nodes of the memory cell and the sense amplifier are each capable of storing a bit value. The method further comprises activating the first R/W path and precharging the first and second conductive lines of the first R/W path. The method activates a word line corresponding to the memory cell to read the bit values of the first and second node of the memory cell onto the first and second conductive lines of the first R/W path. The method provides a voltage to the sense amplifier to latch in the bit values on the first and second conductive lines of the first R/W path into the first and second nodes of the sense amplifier. The method subsequently deactivates the word line and deactivates the first R/W path. The method keeps the sense amplifier activated to hold data. The method activates the second R/W path and reads the bit values on the first and second nodes of the sense amplifier onto the first and second conductive lines of the second R/W path. Finally, the method activates the word line again to write the bit values on the first and second conductive lines of the second R/W path into the respective first and second nodes of the memory cell to enter a recovery state. 
         [0007]    Once the cell is in recovery state (holding inverse data), it is possible to rely on the same infrastructure to read the true data by activating the second R/W path during the read cycle. In a similar fashion, if there is a need to write the inverse of data to a cell in the recovery mode group, then the second R/W path can be used to pass the inverse data to the cell. 
         [0008]    A third aspect of the present invention discloses a method for writing to a memory cell without the need for data drivers. The method provides a first and second read and write (R/W) path to the memory cell. The first R/W path comprises a first bit line coupled to the memory cell and corresponding to a first node of the memory cell, a second bit line coupled to the memory cell and corresponding to a second node of the memory cell, a FET on the first bit line that, when activated, provides a path from a first section of the first bit line to a second section of the first bit line, the first section of the first bit line being closer to the memory cell than the second section of the first bit line, and a FET on the second bit line that when activated provides a path from a first section of the second bit line to a second section of the second bit line, the first section of the second bit line being closer to the memory cell than the second section of the second bit line. The second R/W path comprises a first alternative line connecting the first section of the first bit line to the second section of the second bit line, the first alternative line being selectable by activating a FET on the first alternative line, and a second alternative line connecting the first section of the second bit line to the second section of the first bit line, the second alternative line being selectable by activating a FET on the second alternative line. The method provides a first conductive line connecting to the second section of the first bit line operational via a FET on the first conductive line and a second conductive line connecting the second section of the second bit line and operational via a FET on the second conductive line, wherein one of the first and second conductive lines accesses a high voltage when activated and an other of the first and second line accesses a low voltage when activated. The method determines a logic value to be written to the memory cell. The method activates the respective FETs on the first and second conductive lines. Subsequently, the method activates one of the first and second R/W paths based on the logic value. Finally, the method activates a word line corresponding to the memory cell to write values associated with the high and low voltages into the first and second node of the memory cell. 
         [0009]    A fourth aspect of the present invention discloses a method for selectively reading a node of a memory cell in a single-ended read. The method provides a column of one or more memory cells, each respective memory cell comprising a first and second node, each of the first and second nodes maintaining an opposite bit value. The method provides a bit line connecting each respective first node to a single-ended sense amplifier, the bit line being accessible via a FET on the bit line. The method provides an alternate line connecting each respective second node of the one or more memory cells to the single-ended sense amplifier, the alternate line being accessible via a FET on the alternate line. The method determines a read delay for each of the respective first nodes. If, for a given node from the respective first nodes, the read delay is greater than a threshold read delay, the method performs subsequent read and write requests via the alternate line to improve performance. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1  depicts a standard circuit configuration for a SRAM memory cell and sense amplifier in which embodiments of the present invention may be implemented. 
           [0011]      FIG. 2  illustrates a transistor circuit representation for a memory cell in accordance with an embodiment of the present invention. 
           [0012]      FIG. 3  illustrates a transistor circuit representation for a sense amplifier in accordance with an embodiment of the present invention. 
           [0013]      FIG. 4A  depicts additional circuitry to affect cell recovery in accordance with an embodiment of the present invention. 
           [0014]      FIG. 4B  depicts an alternative circuitry to affect cell recovery. 
           [0015]      FIGS. 5 and 6  illustrate exemplary data flow using the additional circuitry, wherein  FIG. 5  shows a standard read of values stored in a memory cell to a sense amplifier using column select transistors, and  FIG. 6  depicts the writing process that writes back the inverse values stored in the memory cell, allowing it to recover. 
           [0016]      FIG. 7  depicts the process and line charges to implement the recovery process in accordance with an embodiment of the present invention. 
           [0017]      FIG. 8  illustrates a standard data driver for writing to a memory cell. 
           [0018]      FIG. 9  demonstrates writing to a memory cell without the use of a data driver by making use of the additional circuitry of the present invention. 
           [0019]      FIG. 10  depicts the use of the additional circuitry as an alternate read path in a single-ended read configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention discloses a recovery method and design for semiconductor memories having at least one PMOS transistor and/or at least one NMOS transistor in its memory cell. A conventional six-transistor static random access memory (SRAM) is one such memory. 
         [0021]      FIG. 1  depicts a standard circuit configuration for a SRAM memory cell and sense amplifier in which embodiments of the present invention may be implemented. 
         [0022]    Circuit configuration  100  contains memory cell  102 . Memory cell  102  maintains two opposite logic values X and Y, each of which may be either a ‘1’ (high voltage) or a ‘0’ (low voltage). Memory cell  102  comprises two back-to-back inverters forming a flip-flop configuration, each inverter producing one of the two logic values. These inverters may also be referred to as nodes, with each node storing a value. The inner circuitry of memory cell  102  is shown in  FIG. 2 . 
         [0023]    Memory cell  102  is accessible by pass transistors within the memory cell  102  connected to word line  104 . Word line  104  may also be referred to as the row select line. While memory cell  102  is accessed by word line  104 , memory cell  102  can be written into or read out from bit lines  106  and  108 . A number of like memory cells may be arranged in an array of rows and columns. Typically, each row has a common word line and each column has common bit lines. “Lines” refer to any conductive path or circuitry. 
         [0024]    Input data to be written to memory cell  102  is placed on bit lines  106  and  108 . One bit line is set to true and the other as the inverse. For purposes of discussion, bit line  106  is assigned the true value and bit line  108  is assigned the inverse. To write ‘0’, bit line  106  is set to ‘0’ and bit line  108  is set to ‘1’. To write ‘1’, bit line  106  is set to ‘1’ and bit line  108  is set to ‘0’. When word line  104  is activated, logic values X and Y pick up the logic values from bit lines  106  and  108 . 
         [0025]    To read memory cell  102 , bit lines  106  and  108  must be pre-charged to logic value ‘1’. In one embodiment, precharge circuit  110  comprises a PMOS transistor for each of bit lines  106  and  108 . When the precharge signal is activated, bit lines  106  and  108  are connected to power source (Vdd)  112  to charge bit lines  106  and  108  up to Vdd. When word line  104  is activated, the bit line connected to the ‘0’ logic value within memory cell  102  will discharge to logic ‘0’ (referred to as pulling down to 0). 
         [0026]    To capture the logic values on bit lines  106  and  108 , sense amplifier  114  connects to bit lines  106  and  108  through column select transistors  116  and  118 . The circuits of sense amplifier  114  (described in  FIG. 3 ) implement a cross-coupled inverter arrangement similar to that used in memory cell  102 . Sense amplifier  114  latches in the required state until the memory cell  102  has been read. Following a successful read, sense amplifier  114  can be deactivated. 
         [0027]      FIG. 2  illustrates a transistor circuit representation for memory cell  102  in accordance with an embodiment of the invention. 
         [0028]    Pass transistors  202  and  204 , also referred to as access transistors, are connected to the word line and when activated provide access (reading and writing) to memory cell  102 . When memory  102  is not selected (pass transistors  202  and  204  are not activated), the two inverters, depicted as node X and node Y, feed back on each other to reinforce the stored data. Each inverter comprises a PMOS transistor and a NMOS transistor. In the depicted example, node X comprises PMOS  206  and NMOS  208 , and node Y comprises PMOS  210  and NMOS  212 . On a “write” operation, each respective node will pick up the logic value of the associated bit line. 
         [0029]      FIG. 3  illustrates a transistor circuit representation for sense amplifier  114  in accordance with an embodiment of the invention. 
         [0030]    NMOS  302  is activated by a high logic (‘1’) turning on sense amplifier  114 . As long as NMOS  302  is on, sense amplifier  114  latches (stores) the values received from the bit lines. Similar to circuitry in memory cell  102 , sense amplifier  114  stores values in an X and Y node comprised of a PMOS and a NMOS transistor, and the nodes feed back on each other to reinforce the stored data. 
         [0031]      FIG. 4A  depicts additional circuitry to affect cell recovery in accordance with an embodiment of the present invention. 
         [0032]    While typically column select transistors  116  and  118  provide a direct path from one side of memory cell  102  to the corresponding side in sense amplifier  114 , additional BTI recovery (BTIR) transistors  402  and  404  are added as alternative paths from one side of sense amplifier  114  to the opposite bit line. 
         [0033]      FIG. 4B  depicts an alternative circuitry to affect cell recovery. In the depicted example, BTIR transistors  402  and  404  continue to provide an alternate transposing path. Additionally, standard path transistors  406  and  408  connect sense amplifier  114  to the traditional bit lines. In this manner, a selection may be made between the standard bit line path and the alternate transposing path independent of the column selection controlled by column select transistors  116  and  118 . This embodiment allows shared use of the BTIR transistors among different columns of cells. 
         [0034]      FIGS. 5 and 6  illustrate exemplary data flow using the additional circuitry.  FIG. 5  shows a standard read of values from X and Y of memory cell  102  to sense amplifier  114  using column select transistors  116  and  118 . BTIR transistors  402  and  404  are deactivated during the read.  FIG. 6  depicts a writing process that inverses the values stored in X and Y of memory cell  102 , allowing it to recover. Subsequent to reading the values into sense amplifier  114 , column select transistors  116  and  118  are deactivated and BTIR transistors  402  and  404  are selected to allow the latched values in sense amplifier  114  to write to the opposite bit lines and hence transpose the values of X and Y stored in memory cell  102  without the use of an inverter or data drivers. The transposed values on bit lines  106  and  108  are written to memory cell  102  by activating word line  104 . A memory cell where the values of X and Y have been transposed may be referred to as an “inverted” or “transposed” cell. In an alternative embodiment, when transposing the values of X and Y in memory cell  102 , the read may be done by selecting BTIR transistors  402  and  404 , thereby latching the opposite values in sense amplifier  114  and writing back to memory cell  102  on the standard path via column select transistors  116  and  118  and activating word line  104 . The standard lines and the transposing lines may be referred to as alternate read/write (R/W) paths. 
         [0035]    In a preferred embodiment, after memory cell  102  has been transposed, access to the cell, or perhaps the entire column of cells, is frozen, allowing memory cell  102 , and any other cells transposed in the column, to recover for a period of time. A person skilled in the art will recognize that if it became desirable or necessary to read or write to memory cell  102  while in this recovery state, the proper data may be inputted and received by reading and writing the data through BTIR transistors  402  and  404 , hence reading and writing true data out of the inverted cell. 
         [0036]      FIG. 7  depicts the process and line charges to implement the recovery process in accordance with an embodiment of the present invention. 
         [0037]    In a scenario where the X-value is ‘0’ and the Y-value is ‘l’, in the starting state, the word line (such as word line  104 ) would be off, and the precharge (such as precharge  110 ) would be on and charging both bit lines (such as bit lines  106  and  108 ) to a ‘1’ value. Sense (such as sense amplifier  114 ) would be off and both the standard access transistors to the sense amplifier (such as column select transistors  116  and  118 ) and the recovery transistors (such as BTIR transistors  402  and  404 ) would be off. 
         [0038]    After the bit lines are charged, precharge would be deactivated and the word line would be activated. This would cause the X-valued bit line to be pulled down to 0, with the resulting bit line values being ‘0’ and ‘1’. Precharge would remain off for the rest of the process. To latch the bit line values into sense amplifier  114 , sense would be turned on, and the standard access transistors would be activated. Sense amplifier  114  would now be storing values X=0 and Y=1 as long as the sense remains on. 
         [0039]    After the X- and Y-values are latched into sense amplifier  114 , the word line would be turned off and the standard access transistors would be turned off. The sense transistor would remain on to keep the X- and Y-values stored in sense amplifier  114 . 
         [0040]    Next, BTIR transistors would be turned on to provide the X- and Y-values in sense amplifier  114  to the opposite bit lines. The word line would be activated again, causing the X-value from the sense amplifier (‘0’) to write to the Y-value of memory cell  102 , and the Y-value from the sense amplifier (‘1’) to write to the X-value of memory cell  102 . Now that the values stored in memory cell  102  have been reversed, the word line, the recovery transistors, and the sense transistor may be deactivated. The same infrastructure can be used to rewrite the original content of the cell at the end of the recovery period. 
         [0041]    In a preferred embodiment, all memory cells enter a recovery period for at least one minute a day or according to a preferred technology-specific ratio (of recovery time to operation time) that would be sufficient to recover most of the stress. In one embodiment, sections of memory are refreshed together. For example, if there is a plurality of memory banks, each holding a plurality of memory cells, an entire memory bank may undergo the recovery process. An array keeping track of each memory bank may be implemented, storing a ‘1’ if the bank has been accessed or a ‘0’ if the memory bank has not been accessed. After a predefined period of time, all memory banks that have not been accessed (bit=0) are refreshed and the bit array is reset. Periodically, all memory banks may be refreshed regardless of a special bit value. Other methods for determining when to refresh a memory cell or cells may be used. 
         [0042]      FIG. 8  illustrates a standard data driver for writing to a memory cell. As previously discussed, to write to memory cell  102 , the values to be written are placed on the bit lines  106  and  108 . Data driver  800  depicts a traditional method for writing an input value. A “true” logic value comes in on a single line  802 . The logic value is divided to traverse two lines. One line retains the true value, while the other line uses inverter  804 . The two lines access the bit lines through column select transistors  806 . 
         [0043]      FIG. 9  demonstrates writing to a memory cell without the use of a data driver by making use of the additional circuitry of the present invention. In this embodiment, high logic  902  and low logic  904  access the bit lines through write transistors  906  and  908 . Though the values provided by accessing write transistors  906  and  908  are fixed, any value may be written to memory cell  102  by accessing either column select transistors  116  and  118  or BTIR transistors  402  and  404  for the inverse. In the depicted embodiment, sense amplifier  114  is also connected to high logic  902  and low logic  904 . In such an embodiment, sense amplifier  114  may temporarily latch the values from high logic  902  and low logic  904  and/or strengthen the signal of those values. For example, if column select transistors  116  and  118  and BTIR transistors  402  and  404  are all deactivated, sense amplifier  114  may be turned on, and write transistors  906  and  908  may be activated to latch the high and low value into sense amplifier  114 . Write transistors  906  and  908  may be subsequently deactivated and the values stored in sense amplifier  114  may be written to memory cell  102  using either column select transistors  116  and  118  or BTIR transistors  402  and  404  depending on the value to be written. 
         [0044]      FIG. 10  depicts the use of the additional circuitry as an alternate read path in a single-ended read configuration. In addition to BTI, as device size continuously scales down, the increasing leakage current, systematic process variations, and local random variation lead to a large spread in read SNM (static noise margin) and cause weak performance and/or destructive read errors at the tail of the distribution. The previously described circuitry presents an opportunity for yield improvement. 
         [0045]    Single ended read circuit  1000  typically always reads from the same (default) side. In the depicted example, the default side is the right-hand side via column select transistor  118 . If it is determined that the read delay for a cell in the column is greater than some threshold value and/or that the delay is reduced or the delay numbers on the opposite side are more tolerable, it may be desirable to rely on the opposite side of the column for the read. Though technically, a single bad cell can be read from the opposite side, the preferred embodiment reads the entire column from the opposite side. The column may be labeled as inverted, meaning that all data going in to or out of the column, will actually be going in to or out of the opposite side of the column. Column select transistor  118  is deactivated and BTIR transistor  402  is activated so that single ended read circuit  1000  may read and write to the opposite side (true data) of the inverted column. One example of a threshold value that the read-delay should stay within is a 4-sigma delay (a delay meeting or exceeding four standard deviations from the average). Typically, few columns will have cells with a delay this substantial and those columns will not typically have more than one slow cell. Flipping the read to the opposite side on these few columns helps to avoid degraded performance. 
         [0046]    The functioning of activating, deactivating, charging and discharging circuits and devices is preferably implemented in whole or in part by computer circuits and other hardware. Alternatively, the functioning can be written in various programming languages including low-level, high-level, object-oriented or non object-oriented languages. 
         [0047]    Based on the foregoing, an apparatus and methods have been disclosed for combating the effects of BTI and other variability in a memory cell. However, numerous modifications and substitutions can be made without deviating from the scope of the present invention. Therefore, the present invention has been disclosed by way of example and not limitation.