Patent Publication Number: US-7724565-B2

Title: Apparatus and method for small signal sensing in an SRAM cell utilizing PFET access devices

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application, Ser. No. 11/307,664, filed Feb. 16, 2006, now abandoned, which is a continuation of U.S. patent application, Ser. No. 10/708,713, filed Mar. 19, 2004, now U.S. Pat. No. 7,061,793. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates generally to integrated circuit memory devices and, more particularly, to an apparatus and method for small signal sensing in static random access memory (SRAM) cells utilizing PFET access devices. 
     The present invention also relates to a design structure embodied in a machine readable medium used in a design process. 
     A typical static random access memory (SRAM) cell includes an array of individual SRAM cells. Each SRAM cell is capable of storing a binary voltage value therein, which voltage value represents a logical data bit (e.g., “0” or “1”). One existing configuration for an SRAM cell includes a pair of cross-coupled devices such as inverters. With CMOS (complementary metal oxide semiconductor) technology, the inverters further include a pull-up PFET (p-channel) transistor connected to a complementary pull-down NFET (n-channel) transistor. The inverters, connected in a cross-coupled configuration, act as a latch that stores the data bit therein so long as power is supplied to the memory array. 
     In a conventional six-transistor cell  100  such as shown in  FIG. 1 , a pair of access transistors or pass gates T 1 , T 2  (when activated by a word line LWL) selectively couples the inverters to a pair of complementary bit lines BLT, BLC. Prior to a read operation, the bit lines BLT, BLC are precharged to the power supply voltage V DD . A read operation commences when a restore circuit (not shown) is turned off, and the word line LWL is driven high so as to activate NFET pass gates T 1 , T 2 . This in turn electrically connects the internal nodes A, B of the cell  100  to bit lines BLT, BLC, respectively. Whichever of the two bit lines is connected to the “low” (logic 0) cell node will begin to discharge to ground at a rate proportional to the current drive of the cell (i.e., the series connection of either T 2  and T 4 , or T 1  and T 3 ), and the capacitance of the bit lines. The bit line connected to the “high” (logic 1) cell node will be left floating high, since its respective word line access device is cut off (i.e., both the drain and source terminals thereof are at V DD  potential). The voltage difference between the bit line discharging to ground and the bit line left floating high is referred to as the bit line signal. Because the bit lines are typically highly capacitive, and cell current is typically relatively low, it generally takes a significant amount of time to generate an adequate amount of signal to reliably sense and amplify the cell data. 
     Accordingly, it would be desirable to be able to accurately sense the data in an SRAM cell at smaller level of signal differential than a conventional cell, and thus at an earlier time in the read cycle. 
     SUMMARY OF INVENTION 
     The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a method for small signal sensing during a read operation for a static random access memory (SRAM) cell. In an exemplary embodiment, the method includes coupling a pair of complementary sense amplifier data lines to a corresponding pair of complementary bit lines associated with the SRAM cell, and setting a sense amplifier so as to amplify a signal developed on the sense amplifier data lines, wherein the bit line pair remains coupled to the sense amplifier data lines at the time the sense amplifier is set. 
     In another embodiment, a method for implementing a read operation for a static random access memory (SRAM) cell includes activating a word line associated with the SRAM cell, and deactivating a precharge circuit configured for precharging a pair of complementary bit lines associated with the SRAM cell. A corresponding pair of complementary sense amplifier data lines is coupled to the pair of complementary bit lines associated with the SRAM cell, and a sense amplifier is set so as to amplify a signal developed on the sense amplifier data lines. The bit line pair remains coupled to the sense amplifier data lines at the time the sense amplifier is set. 
     In still another embodiment, an apparatus for small signal sensing during a read operation of a static random access memory (SRAM) cell includes a pair of complementary sense amplifier data lines selectively coupled to a corresponding pair of complementary bit lines associated with the SRAM cell, and a sense amplifier configured to amplify a signal developed on the sense amplifier data lines. The bit line pair is coupled to the sense amplifier data lines whenever the sense amplifier is set. 
     In yet another embodiment, a design structure embodied in a machine readable medium used in a design process, the design structure including an apparatus for small signal sensing during a read operation of a static random access memory (SRAM) cell, which includes a pair of complementary sense amplifier data lines selectively coupled to a corresponding pair of complementary bit lines associated with the SRAM cell, and a sense amplifier configured to amplify a signal developed on the sense amplifier data lines. The bit line pair is coupled to the sense amplifier data lines whenever the sense amplifier is set. Further included is a pair of PFET access transistors associated with the SRAM cell, the PFET access transistors configured to clamp one of the pair of complementary sense amplifier data lines to a logic high voltage upon activation of a word line associated with the SRAM cell. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a schematic diagram of a conventional SRAM storage cell; 
         FIG. 2  is a schematic diagram of an SRAM storage cell and associated sense amplifier, configured in accordance with an embodiment of the invention; 
         FIG. 3  is a timing diagram illustrating a method for small signal sensing in SRAM cells employing PFET pass gates, in accordance with a further embodiment of the invention; and 
         FIG. 4  is a graph comparing simulated results between a conventional SRAM read operation and the present method; and 
         FIG. 5  is a flow diagram of a design process used in semiconductor design, manufacturing and/or test. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is an apparatus and method for small signal sensing in SRAM cells, in which a bit switch pair couples a true and complement bit line pair to a corresponding pair of sense amp data lines both prior to and during activation of the sense amplifier. By configuring an SRAM cell with PFET access transistors, whichever of the bit lines (true or complement) that is coupled to the high node is clamped to V DD  through the corresponding PFET access transistor of the SRAM cell that is coupled to the word line. As a result of this clamping assistance provided by the memory cell PFET, the sense amplifier will be able to properly amplify with a smaller degree of signal on the bit lines. 
     Referring now to  FIG. 2 , there is shown a schematic diagram of an SRAM storage cell  200  and associated sense amplifier  202 , configured in accordance with an embodiment of the invention. In contrast to the cell  100  shown in  FIG. 1 , cell  200  utilizes PFET devices P 1 , P 2  as the access transistors thereto. Accordingly, the local word line LWL coupled to the gate terminals of P 1  and P 2  is active low.  FIG. 2  also illustrates a portion of the global word line circuitry  205 , the operation of which is well known to those skilled in the art. 
     The designations “S” and “L” associated with the cell transistors and the local word line inverter  204  refer to the relative strengths of the NFET and PFET devices. In a conventional CMOS inverter or SRAM cell, the relative strength of a PFET pull up device is weaker with respect to the NFET pull down device. Thus, the inverter  204  is labeled “S” (small) above the inverter symbol in  FIG. 2  and “L” (large) below the inverter symbol, meaning the PFET device therein is small compared to the NFET device therein. In contrast, the SRAM cell  200  in  FIG. 2  has large strength pull up PFETs compared to the cell NFETs, for purposes that will become apparent hereinafter. 
     The bit lines BLT, BLC of cell  200  are precharged to V DD  prior to a read operation by precharge circuitry including a series of pull up devices  206   a ,  206   b  and  206   c , controlled by an active low signal labeled “BL Restore”. The precharge circuitry operates in conjunction with a pair of read bit switches  208   a ,  208   b , controlled by an active low signal labeled “R Bitswitch”. The read bit switches selectively couple the cell bit line pair BLT, BLC, to a corresponding sense amplifier data line pair DLT, DLC, during a read operation. More specifically, during a read operation, the precharge circuitry is deactivated as the local wordline LWL is activated so as to couple the SRAM cell nodes to the bit line pair BLT, BLC. As this occurs, the bit line initially coupled to the logic high at the beginning of the read operation will begin to discharge to ground through one of the corresponding SRAM cell NFETs while the other bit line (left floating at high in a conventional cell) is actually clamped to V DD  due to the PFET access device (P 1  or P 2 ). Therefore, a voltage differential is now developed across the bit line pair. 
     In addition, the signal developed on the bit line pair BLT, BLC, is also transferred to the sense amplifier data line pair, since the read bit switches are also activated at the beginning of the read operation. When a sufficiently large signal (i.e., voltage differential) is developed on the sense amplifier data line pair DLT, DLC, the sense amplifier  202  is set (i.e., activated) from an initial floating state by coupling the NFET devices T 1 , T 2  therein to ground through a pull down device activated by a set signal (SET). Once activated, the sense amplifier  202  amplifies the developed signal on DLT, DLC, to the full V DD  swing value. Because the sense amplifier  202  actually latches a data value opposite that stored in the SRAM cell  200 , a pair of inverters  212   a ,  212   b  is used to drive the correct data onto the data true and complement nodes DTN, DCN. 
     Ignoring momentarily the fact that the present embodiment of  FIG. 2  utilizes PFET access devices, in a conventional SRAM cell having NFET devices, the activation of the control signals BL Restore, R Bitswitch and SET are precisely coordinated with one another with regard to timing and interlocking. This is done so that the sense amplifier  202  is not set too early before there is adequate time to develop a sufficiently strong signal differential on DLT, DLC. If the sense amplifier  202  were to be activated too early, and if there were a voltage threshold mismatch in the sense amplifier pull down devices, then it is possible that the sense amplifier  202  could latch incorrect data therein. 
     [In contrast, the use of PFET access devices in the present invention allows the clamping of precharged bit line associated with the low cell node to V DD  through the corresponding PFET access transistor P 1 , P 2 . Moreover, unlike a conventional read operation, the bit switch pair  208   a ,  208   b , remains active during the setting of the sense amplifier  202 , thereby effectively clamping one of the data lines to V DD  through one of the “large” PFET pull up devices in the cell  200 . However, this would not be the case for an SRAM cell having NFET access devices, since they would not provide a clamp to V DD  when activated. Moreover, a coupling of the bit line pair to the data line pair during the sense amplifier set in a conventional scheme would also provide unwanted capacitive loading on the sense amplifier, further adding to the signal development time. Thus, it will be appreciated that with the configuration of  FIG. 2 , any problems with voltage threshold mismatch in the sense amplifier  202  will be easier to overcome. Therefore, the SET signal can be activated at an earlier point in time when the signal differential on DLT, DLC is smaller than conventionally required. 
       FIG. 3  is a timing diagram illustrating a read operation using the cell configuration of  FIG. 2 . As is shown, the read operation begins at time t 1 , at which point the local word line LWL is rendered active by transitioning from high to low. Concurrently, the R Bitswitch signal transitions from high to low in order to couple the sense amplifier data lines to the bit lines, while the bit line precharge circuitry is deactivated by signal BL Restore transitioning from low to high. Shortly thereafter, the voltage on one of the precharged bit lines BLT, BLC begins to drop (as shown at t 2 ) while the other bit line remains clamped to V DD . Upon development of a sufficient signal at time t 3 , the sense amplifier is activated, reflected by the SET signal transitioning from low to high in  FIG. 3 . 
     For purposes of illustration, the dashed transition in the R Bitswitch signal indicates the operation of a conventional cell, in which the sense amp data lines would be uncoupled from the bit lines when the sense amplifier is set, as discussed above. However, in the present embodiment, R Bitswitch remains active through the setting of the sense amplifier to maintain the clamping function. Thus, when the amplifier latches the cell data at time t 4 , the correct data is read from the cell and the SET signal can be triggered with a smaller amount of signal present on the bit lines. 
     Finally,  FIG. 4  is a graph  400  comparing simulated results between a conventional SRAM read operation and a read operation in accordance with the apparatus and method shown in  FIGS. 2 and 3 . The upper portion of the graph demonstrates both a successful data read operation and an unsuccessful data read operation for a conventional SRAM cell coupled to a sense amplifier having a +30 mV voltage threshold mismatch on one pull down transistor and a −30 mV voltage threshold mismatch on the other pull down transistor. With the conventional cell with NFET access transistors, the read operation commences at t 1  with the word line signal (WL) transitioning from low to high. In the simulation of  FIG. 4 , it is assumed that the SRAM cell node coupled to the true bit line has a logic 1 stored therein. 
     Time t 2  in  FIG. 4  represents an “early” set of the sense amplifier; in other words, the amount of time indicated by “WL to SET # 1 ” is considered to be too early for an appropriate amount of signal to be developed on the bit line pair (i.e., a relatively small voltage differential between BLT and BLC). As a result of both a voltage threshold mismatch in the sense amplifier and an early set signal at time t 2 , the sense amplifier has incorrectly latched the true data line DLT to low and the complementary data line DLC to high. As such, time t 3  (WL to SET # 2 ) may be considered a sufficiently long time for a signal to be developed on BLT and BLC. Thus, if the set signal is activated at t 3  for the conventional SRAM cell, the sense amplifier will properly latch DLT to high and DLT to low. 
     In contrast, the lower portion of  FIG. 4  illustrates that the sense amplifier will properly latch DLT and DLC to the correct logic levels, even if the set signal is activated at the earlier time t 2 . Although the signal developed on the bit line pair is smaller at t 2  than at t 3 , the clamping function provided by the PFET access transistor combined with keeping the sense amplifier data line pair coupled to the bit line pair prevents the sense amplifier from latching an incorrect cell state. Thus configured, the read operation timings are less susceptible to voltage threshold mismatches in the sense amplifier, and can allow for shorter read times. 
       FIG. 5  shows a block diagram of an example design flow  500 . Design flow  500  may vary depending on the type of IC being designed. For example, a design flow  500  for building an application specific IC (ASIC) may differ from a design flow  500  for designing a standard component. Design structure  520  is preferably an input to a design process  510  and may come from an IP provider, a core developer, or other design company or may be generated by the operator of the design flow, or from other sources. Design structure  520  comprises circuit  200  in the form of schematics or HDL, a hardware-description language (e.g., Verilog, VHDL, C, etc.). Design structure  520  may be contained on one or more machine readable medium. . For example, design structure  520  may be a text file or a graphical representation of circuit  200 . Design process  510  preferably synthesizes (or translates) circuit  200  into a netlist  580 , where netlist  580  is, for example, a list of wires, transistors, logic gates, control circuits, I/O, models, etc. that describes the connections to other elements and circuits in an integrated circuit design and recorded on at least one of machine readable medium. This may be an iterative process in which netlist  580  is resynthesized one or more times depending on design specifications and parameters for the circuit. 
     Design process  510  may include using a variety of inputs; for example, inputs from library elements  530  which may house a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications  540 , characterization data  550 , verification data  560 , design rules  570 , and test data files  585  (which may include test patterns and other testing information). Design process  510  may further include, for example, standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. One of ordinary skill in the art of integrated circuit design can appreciate the extent of possible electronic design automation tools and applications used in design process  510  without deviating from the scope and spirit of the invention. The design structure of the invention is not limited to any specific design flow. 
     Ultimately, design process  510  preferably translates circuit  200 , along with the rest of the integrated circuit design (if applicable), into a final design structure  590  (e.g., information stored in a GDS storage medium). Final design structure  590  may comprise information such as, for example, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, test data, data for routing through the manufacturing line, and any other data required by a semiconductor manufacturer to produce circuit  200 . Final design structure  590  may then proceed to a stage  595  where, for example, final design structure  590 : proceeds to tape-out, is released to manufacturing, is sent to another design house or is sent back to the customer. 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.