Patent Publication Number: US-6906962-B2

Title: Method for defining the initial state of static random access memory

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally to static random access memories, and more particularly to a method and apparatus for defining the initial state of the static random access memory cells. 
     BACKGROUND OF THE INVENTION 
     Random access memory arrays, including static random access memories (SRAMs), are well known in the art. Such arrays are comprised of a plurality of memory cells, each cell storing a single bit of information in the form of a binary 1 or a binary 0. Each cell is essentially a flip-flop positioned at the intersection of an array of row and column address lines. Specifically, each cell is positioned at the intersection of a word line, for selecting a row of the memory array, and a set of complimentary bit lines (i.e., a bit line and an inverse bit line) for selecting a column of the array. These memories provide random access in the sense that each cell can be individually addressed for read and write operations as determined by an address provided to a row and column address decoder, that in turn selects the intended cell at the intersection of the row and column address lines. Generally, the row or wordline is selected first, enabling all the cells on the selected row. The bitline and the inverse bitline select the individual column from within the selected cell row, for reading a bit from or writing a bit to the selected cell over the bitline and the inverse bitline. 
     SRAMs are often used to store program memory in electronic devices, such as computers and other devices that operate under control of a processor executing program commands. Since SRAMs are volatile memory devices, the program instructions and data items required during the start-up or boot-up operational phase are stored in non-volatile memory when no external power is supplied to the device. At start-up, the instructions and data are downloaded to the SRAM. During device operation the processor accesses instructions and data stored in the SRAM and writes resultant data back to the SRAM, as the SRAM provides faster access times than nonvolatile memory. However, initial operation of the device is delayed while program instructions and start-up data are loaded from the non-volatile memory into the SRAM. Included among the non-volatile memory types for storing executable programs and start-up data are: read only memories (ROM&#39;s), programmable read only memories (PROM&#39;s), erasable programmable read only memories (EPROM&#39;s), and optical media such as disk drives and magnetic media such as floppy disk drives. 
     BRIEF SUMMARY OF THE INVENTION 
     A memory array comprises a plurality of cross-connected CMOS inverter pairs, each one forming a memory cell. The CMOS inverters each comprise a plurality of MOSFET devices (metal-oxide semiconductor field-effect transistors). To predetermine the initial state of the array or individual memory cells in the array, a physical parameter affecting the start-up or initial state of one or more of the MOSFETs of a memory cell is identified. For example, the threshold voltage is such a parameter. During or following fabrication of the MOSFETs, the physical parameter is controlled so that the memory cell powers-up to the predetermined initial state. In another embodiment this process can be extended to all memory cells of the memory array to provide a predetermined initial state for the memory array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a schematic diagram of a typical static random access memory array; 
         FIGS. 2 and 3  are flowcharts setting forth the steps for achieving the desired initial state of a static random access memory according to the teachings of the present invention; and 
         FIGS. 4 through 6  are schematic diagrams of alternative embodiments of memory cells for a static random access memory. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing in detail the particular method for defining the initial state of an SRAM according to the teachings of the present invention, it should be observed that the present invention resides primarily in a novel combination of hardware elements and method steps. Accordingly, the elements and steps have been represented by conventional elements in the drawings, showing only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein. 
       FIG. 1  is an exemplary schematic of four static random access (SRAM) cells  20 ,  21 ,  22  and  23  constituting a SRAM memory array  18  to which the teachings of the present invention can be applied. The SRAM memory array  18  has n wordlines (wordline 0 to wordline n) and m bitlines (bitline 0 to bitline m). Each of the memory cells  20 ,  21 ,  22  and  23  comprises six metal-oxide field-effect transistors (MOSFETS) arranged as two cross-coupled complementary MOSFETS (i.e., CMOS) inverters. Each of the cells  20 ,  21 ,  22  and  23  includes the same basic components and functions in the same manner. Thus only the cell  20  is described in detail below. 
     The cell  20  includes NMOS switching transistors  30  and  32  having their gate terminals connected to a wordline 0. Source and drain terminals of the transistor  30  are connected between a bitline 0 and a node  34 . Source and drain terminals of the transistor  32  are connected between an inverse bitline 0 and a node  36 . A first source/drain terminal of NMOS (on n-channel MOSFETs) transistors  40  and  42  is connected to ground. A first source/drain terminal of PMOS (or p-channel MOSFETs) transistors  46  and  48  is connected to a supply voltage, V DD . A second source/drain terminal of the transistors  40  and  42  is connected to a second source/drain terminal of the transistors  46  and  48  at the nodes  34  and  36 , respectively. The node  34  is further connected to a gate terminal of each transistor  42  and  48 . The node  36  is further connected to a gate terminal of each of the transistors  40  and  46 . 
     In operation, the cross-coupling of the two CMOS inverters (where the first inverter comprises the transistors  40  and  46  with the node  34  operating as the output terminal, and the second inverter comprises the transistors  42  and  48  with the node  36  operating as the output terminal) creates a bistable device. If the output of the first inverter is high (that is, the transistor  46  is on, the transistor  40  is off and the voltage at the node  34  is high), that high voltage at the node  34  is provided as an input to the gate terminals of the transistors  42  and  48  that comprise the second inverter. The high voltage drives the second inverter low (that is, the transistor  48  is off, the transistor  42  is on and the voltage on the node  36  is low or at ground potential). 
     When the node  34  is high (i.e., the first inverter is high) the state of the cell  20  can be considered a “1” state. If the transistors  40 ,  42 ,  46  and  48  are in an opposite state to that described above, the first inverter output is low and the second inverter output is high. This state can be considered the “0” state for the cell  20 . In the “0” state the node  34  is low and the node  36  is high. 
     To write a bit to the cell  20 , the wordline 0 is selected, turning on the transistors  30  and  32 . The bitline 0 and the inverse bitline 0 are charged to opposite states by a writer-driver, not shown, to store the bit on the bitline 0 to the memory cell  20 . If a “1” on the bitline 0 is to be stored, the transistor  48  goes to an off state and the cross coupling drives the transistor  46  on. Thus the voltage at the node  34  goes high and a “1” is stored in the cell  20 . The inverse bitline 0 is low as the node  36  is effectively grounded when the transistor  48  is off. 
     Alternatively, a “0” is stored by placing a low voltage on the bitline 0 and a high voltage on the inverse bitline 0. These voltages drive the transistor  48  into conduction and the transistor  46  goes off, driving the node  34  to ground and the node  36  high. 
     The bit stored in the memory cell  20  is read by selecting the wordline 0 and determining the difference between the voltage on the bitline 0 and the inverse bitline 0. A sense amplifier (not shown in  FIG. 2 ) measures the voltage differential and provides an output bit representative of the stored bit. 
     During the design and fabrication processes, it is conventionally the intent to match the operational parameters of the two CMOS inverters comprising an SRAM cell, by matching their constituent MOSFETS. Thus quality fabrication process control seeks to match the two p-channel MOSFETs  46  and  48 , and the two n-channel MOSFETs  40  and  42 . 
     According to the teachings of the present invention, one or more process steps are employed to create mismatches and corresponding non-identical device characteristics in transistors comprising a memory cell, such as the memory cell  20 . In particular, in one embodiment it is desired to create devices with different threshold voltages. As a result, one of the CMOS inverters comprising the memory cell  20  turns on before the other and thus the memory cell  20  assumes a predictable initial power-on (or power-up) state. 
     In a second embodiment it is desired to create devices with different drive currents. As a result, even though both MOSFETs  46  and  48  have the same threshold voltage and thus turn on at the same time, one has a higher drive current and thus the memory cell  20  assumes a predictable initial power-on state. In a third embodiment it is desired to create devices with both different threshold voltages and drive currents. Again this will cause memory cell  20  to assume a predictable power-on state. 
     For example, if the two p-channel MOSFETS  46  and  48  are not precisely matched, such that one of the two MOSFETs  46  and  48  exhibits a lower threshold voltage than the other, the MOSFET with the lower threshold voltage turns on first when power is first applied to the memory array  18 . Thus if the MOSFET  46  turns on before the MOSFET  48  the voltage at the node  34  is high and the initial state of the memory cell  20  is a “1.” Conversely, if the MOSFET  48  has a lower threshold voltage, the initial state of the memory cell  20  is a “0.” 
     Exemplary process mismatches comprise a slight geometrical offset during an etch process step for forming the MOSFETs  46  and  48  such that one exhibits a shorter channel length than the other. This geometrical offset can be produced, for example, by slight modification of the one or more of the lithographic masks used to create the MOSFETs  46  and  48 . The geometrical offset causes one of the MOSFETS to have a larger drive current than the other, and thus the preferred one of the CMOS inverters turns on before the other. Thus the power-up state of the memory cell  20  is predetermined and predictable. 
     Another exemplary process mismatch comprises an implant adjust in one of the MOSFET channel regions. Since precise quantities of an impurity can be implanted, the process allows control over the threshold voltage. For example, if boron (a p-type material) is implanted through the gate oxide of a p-channel MOSFET, such that the peak implant occurs immediately below the surface of the channel region, the negatively charged boron acceptors reduce the effects of the positive depletion charge in the channel. Recognizing that the threshold voltage of a p-channel MOSFET is a negative value, the boron acceptors cause the threshold voltage to be less negative. Thus an implant adjustment performed on one of the two p-channel MOSFETs  46  and  48  causes the adjusted MOSFET to turn on before the other. Careful selection of the implant dose can set the initial state of the cells of the SRAM  18  without affecting normal operation of the SRAM  18 . 
     Among the many factors affecting the threshold voltage are the doping levels of the source and drain regions, the oxide capacitance, the various oxide interface trapped charges and geometrical offsets between adjacent layers. Any one or more of these parameters can be varied during the fabrication process to control the power on state of the memory cell  20 . 
     Additionally, one of the two n-channel MOSFETS  40  and  42  (having a threshold voltage that is a positive number) can undergo an impurity adjustment by the implantation of an n-type dopant to lower the threshold voltage thereof or an adjustment in the channel length to affect the drive current. 
     Since the power-on state of each memory cell of the SRAM is controllable, an executable program, which according to the prior art is stored in a non-volatile memory, can now be stored in the SRAM  18  such that the executable program is immediately available in the SRAM  18  after the device has been powered up. With the program code stored in the SRAM  18 , there is no need for a separate non-volatile memory device and the initialization time expended in transferring the program code from the non-volatile memory to the SRAM  18  is avoided. Also, the circuit board area devoted to memory elements is reduced. 
     In addition to storing program code in the SRAM  18 , mismatches in the memory cells can also permit the SRAM  18  to power up with known data stored therein, obviating the need for a separate non-volatile memory to store the data. Again, the circuit board area consumed by these separate memory devices and the device initialization time are reduced. 
       FIG. 2  illustrates a flow chart for setting the initial state of the SRAM  18  according to the teachings of the present invention. At a step  50 , the data or program code to be stored in the SRAM  18  at power-up is determined and individual binary bits of the code or data are assigned to memory cells within the SRAM  18 . At a step  52  the threshold voltage or drive current (or another operational characteristic that influences the turn-on state) of the MOSFETs comprising the memory cells of the SRAM  18  is established according to one or more of the aforementioned techniques for adjusting a physical characteristic of the MOSFET, to produce the desired operational effect that in turn determines the turn-on state of the MOSFET. Exemplary adjustable physical characteristics include an implant adjustment or a geometrical offset as described above. Other adjustable physical characteristics that can achieve a desired turn-on state are known to those skilled in the art. Thus when the SRAM  18  powers-up the appropriate MOSFET within each memory cell turns on first, and as a result the correct bit is present in the memory cell. At a step  54  the device, including the SRAM  18 , is powered-up and the SRAM  18  assumes the desired initial state values for proper operation of the device. 
     According to another embodiment of the invention, the physical or operational characteristics of the MOSFETs comprising the memory cell  20  are altered after fabrication of the SRAM  18 . Alteration of one or more such characteristics impacts the operational characteristics to predetermine the turn-on state of the effected MOSFETs, and thus the turn-on state of the memory cell comprising the effected MOSFET. 
     In this embodiment, each memory cell  20 ,  21 ,  22  and  23  of the SRAM array  18  is placed into the inverse of the desired state. That is, if it is desired to store a binary “1” in a memory cell to represent a bit of executable code or start-up data, the inverse of “1”, that is, a “0”, is stored in the memory cell. Assume the memory cell  20  is placed in the “0” state. The MOSFETs  48  and  40  are on and the MOSFETs  42  and  46  are off. The MOSFETs are then stressed, for example, by raising the supply voltage (V DD ) above its nominal value until hot carriers are formed in the channel of the “on” MOSFETs  48  and  40 . Specifically, hot carrier holes are formed in the channel of the MOSFET  48 . If the supply voltage is raised to a sufficiently high level, the holes will gain enough kinetic energy to surmount the potential barrier between the channel and the gate oxide. Some of these hot holes become trapped in the gate oxide as fixed charges and raise the threshold voltage for the MOSFET  48 . Since the threshold voltage for the MOSFET  48  has increased, the MOSFET  46  turns on before the MOSFET  48  at power-up. Thus after stressing the memory cells the MOSFETs are mismatched in such a way that the desired program code or data will be “loaded” into the memory array on power-up. 
     Hot carriers can also be created for n-channel MOSFETs (such as n-channnel MOSFETs  40  and  42 ). The hot carrier effect for electrons in n-channel devices is more pronounced than for holes in p-channel devices. The hole mobility is about half the mobility of electrons, hence for the same electric field created by raising V DD , the number of hot holes is about half the number of hot electrons. Also, the potential barrier that the holes must surmount to enter the gate oxide is greater than the potential barrier for electrons. Careful selection of the supply voltage value for creating the hot carriers, both hot holes and hot electrons, results in a shift of the threshold voltage without significantly affecting the normal operation of the memory array  18 . 
     Also as a result of the formation of hot carriers, the output current of the stressed device increases. This current increase results in an imbalance in the memory cell  20  when it is powered up and the memory cell  20  therefore goes into a predictable initial state. 
     To stress the MOSFETs as described above, the supply voltage may need to be increased significantly above its nominal value, three or four times the nominal value, for example. Also, the formation and impact of hot carriers on MOSFET performance can be influenced by modifying the tub bias of the MOSFET. The tub refers to the doped semiconductor well or region in which the source, drain and channel regions are formed for a CMOS inverter device. The NMOS device is formed in a p-type well and the PMOS device is formed within an n-type well. The wells are also insulated from each other. 
       FIG. 3  is a flowchart of anther embodiment according to the teachings of the present invention, illustrating the steps for controlling the power-up state of the SRAM  18 . At a step  60  the inverse of the program code or data that is to appear in memory at power-up is loaded into the SRAM  18 . At a step  62  the MOSFET devices comprising the memory cells storing the program code or data are stressed as described above. When the device is later powered-up (step  64 ), the stressed memory cells of the SRAM  18  go to the desired initial state. 
     The process of stressing the SRAM  18  to predetermine the initial state of the memory cells  20 ,  21 ,  22  and  23  can also be performed after the SRAM  18  has been placed into service. Thus according to this method, the SRAM  18  can be “reprogrammed” during service. This embodiment is particularly useful when the SRAM  18  is operative with a device that is placed into service with initial state values for start-up data and executable program code, but later it is required to change the start-up data or the code. Restressing the SRAM  18  after initial service allows the device to be reprogrammed by changing the code or the start-up data. 
     In other embodiments of the present invention, NMOS, PMOS and bipolar transistor configurations, including their associated resistors, as illustrated in  FIGS. 4 ,  5  and  6 , form the memory cells  20 ,  21  and  22  and  23 , a plurality of which form the memory array  18 . As is known to those skilled in the art, operation of these embodiments is similar to the cross-coupled CMOS embodiment described above. These devices can also be fabricated with different physical parameters or stressed to control the start-up state as described above. 
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. The scope of the present invention further includes any combination of the elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. 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.