Patent Publication Number: US-7217978-B2

Title: SRAM memories and microprocessors having logic portions implemented in high-performance silicon substrates and SRAM array portions having field effect transistors with linked bodies and method for making same

Description:
TECHNICAL FIELD 
   The present invention generally concerns fabrication methods and device architectures for use in memory circuits, and more particularly concerns hybrid silicon-on-insulator (SOI) and bulk architectures for use in memory circuits. 
   BACKGROUND 
   There are several semiconductor memory types available for use in constructing memory components for incorporation in computers and other electronic devices. In addition, there are numerous semiconductor fabrication processes available for forming memory components operating in accordance with the several semiconductor memory types. Further, almost continual progress is being made in process and fabrication techniques, resulting in improvements to component speed and stability of operation and reductions in component size and power consumption. In such a situation it is therefore a continuing challenge to adapt emerging process, fabrication and device improvements to semiconductor memories in such a way that the maximum benefit is derived from the improvements. 
   For example, regarding semiconductor memory types two are most common—dynamic random access memories (hereinafter “DRAMs”) and static random access memories (hereinafter “SRAMs”). A DRAM memory is comprised of DRAM cells which essentially are capacitors for storing charge; the states of the capacitors constitute the memory states of the DRAM cell. DRAMs have relatively high memory densities in comparison to other memory technologies, for example SRAM memories, but this comes at a cost. For various reasons well-known to those skilled in the art, capacitors comprising the memory cells in DRAM devices cannot maintain their charge states in perpetuity and therefore have to be occasionally refreshed in order not to lose their memory state. 
   In contrast to DRAMs, SRAMs store information in bistable semiconductor circuits. More devices need to be fabricated in order to construct an SRAM memory cell in comparison to a DRAM memory cell, resulting in DRAMs generally achieving better memory density. On the other hand, SRAMs need not be refreshed in the manner of capacitive DRAMs. In addition, SRAMs generally have shorter read/write cycle times. Thus, SRAMs often are used in microprocessors in so-called “cache memory.” 
     FIG. 1  shows a cross section of a prior art SRAM through a pair of NFET transistors  142 ,  144 . The NFETs  142 ,  144  are formed in a thin silicon surface layer  130  that is isolated from an underlying silicon substrate  110  by a buried oxide (BOX) layer  120 . In a typically complex series of mask steps, silicon-on-insulator (“SOI”) regions are formed in the silicon surface layer  130  by etching shallow trenches through the surface layer  130  and filling the shallow trenches with oxide to isolate regions from one another. This type of isolation is normally referred to as shallow trench isolation (“STI”). STI is used to isolate circuits formed in the regions from each other and, also, isolate the FETs forming the circuits from each other. 
   After forming a gate oxide layer on the surface of the silicon regions, gates  116  are patterned and formed at the location of devices  142 ,  144 . Source/drain regions  132  are defined using a standard implant and diffusion step, after forming lightly doped diffusion regions  134  at the gate boundaries, if desired. Device channels  136  are completely isolated from other channels by source/drain diffusions  132  at either end, BOX layer  120  below, gate oxide above and STI (not shown) along the sides of the channel. Further, “halo” regions  133  have been formed between the source/drain regions  132  and channel  136  through a separate diffusion step of the same dopant type used to form the body regions but at a higher concentration. 
   Ideally, the thin silicon surface layer  130  is no thicker than what is necessary to form a channel  136  between a pair of source/drain diffusions  132 . However, in practice, the silicon surface layer can be thicker than the depth of the FET&#39;s channel inversion layer. So, when the channel inversion layer forms, i.e., when the FET is turned on, an uninverted layer can remain beneath the channel inversion layer. This uninverted layer remains isolated, resistively, from adjacent regions and any charge that is introduced into the uninverted channel region remains trapped there until it leaks out through junction leakage or is otherwise coupled out. This trapped charge can produce unwanted device channel bias resulting in what is referred to as body effects that are localized to an individual device. 
   So, these prior art SOI FETs  142 ,  144  have isolated floating channels (body regions) 136  that are not biased by any bias voltage. Thus, the channel bias of any device is dependent upon its current operating state and the device&#39;s history, i.e., any remaining charge that has been previously introduced through capacitive coupling or bipolar injection. For typical individual logic circuits such as, decoders, clock buffers, input or output drivers and array output drivers, variations in device characteristics resulting from floating device channels are predicted in device models and are accounted for in chip timing. 
   Localized body effects present significant problems for CMOS SOI SRAM arrays. This floating body effect allows the body potential and threshold voltage to vary from device to device within a single cell, introducing a use-dependent bias. There are several contributors to this variation or mismatch and body potential is a significant contributor. If the mismatch between devices is sufficiently large the cell will be disturbed during a read or a write operation and even in an idle state. Then data may be lost. 
   It is known that coupling the bodies of the devices reduces the body potential and threshold mismatch of the devices and so enhances the stability of SRAM cells. Known methods of doing this are to use so-called body-contacted SOI MOSFET transistors. When applied to CMOS SRAM cells, these methods significantly increase cell area and process complexity. The increase in area can be as much as two to three times for each transistor with a small dimension as used in SRAM cells and sums to form at least a two-times-larger SRAM cell. Another drawback is that the parasitic capacitance associated with the polysilicon gate and diffusions of the body-contacted transistor will degrade the SRAM array performance. 
   Problems have been encountered in other areas as well. Advances have been made in fundamental substrate fabrication techniques which now permit portions of a substrate to be fabricated in silicon with different crystal orientations. It has long been known that PFETs experience improved performance when fabricated in (110) crystal orientation silicon due to the increased mobility of the majority carrier (holes) in (110) crystal orientation silicon. It has only become possible recently to form (110) crystal orientation silicon regions no larger than the PFET devices themselves so that such regions can be incorporated in an otherwise (100) crystal orientation substrate without sacrificing device density. Although advantageous, the hybrid substrate technology must be used judiciously so that the potential improvements in device performance achievable with such technology are not blunted through sub-optimal decisions concerning other design issues, for example, overcoming the floating body effect. 
   Those skilled in the art thus desire CMOS SRAM cell architectures that overcome the problems associated with the floating body effect without sacrificing the gains made by fabricating the device in SOI, for example, improved read/write speeds and lower power consumption. In particular, such an improved CMOS SRAM cell architecture would have improved stability, and experience far fewer anomalies during read/write operations. 
   In addition those skilled in the art desire improved SRAM cell layouts that derive increased benefit from linked body technology; in particular, those skilled in the art desire SRAM cell layouts that reduce the resistance encountered in devices having linked bodies. 
   Further, those skilled in the art also desire the judicious use of state of the art device structures to improve the performance of the logic and memory portions of SRAM memories or microprocessors. In particular, those skilled in the art desire the application of state of the art device structures to improve the speed of logic operations in the logic portion of an SRAM memory or microprocessor and the stability of the memory portion of the SRAM memory or microprocessor. 
   SUMMARY OF THE PREFERRED EMBODIMENTS 
   A first embodiment of the present invention comprises an SRAM array comprising a plurality of SRAM cells, each of said SRAM cells comprising: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, where each cross-coupled inverter comprises an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters; and where at least two adjacent NFETs of the SRAM cell share a leakage path between body regions, and where the at least two adjacent NFETs have a source/drain diffusion region and a leakage path diffusion region under the source/drain diffusion region positioned between their respective body regions, wherein the source/drain diffusion region extends fractionally into the surface silicon layer and the leakage path diffusion region extends from a bottom of the source/drain diffusion down to the SOI buried-oxide layer, and where the leakage path diffusion region is counter-doped with the same dopant type as the source/drain diffusion but at relatively lower concentrations than the source/drain diffusion, thereby presenting a lower barrier to junction leakage than the source/drain regions. 
   A second embodiment of the present invention comprises a pair of adjacent SRAM cells in an SRAM array, the pair comprising a first SRAM cell and a second SRAM cell, each of the adjacent SRAM cells comprising: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried oxide layer, where each cross-coupled inverter comprises an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters; and where at least one of the NFETs from the first SRAM cell and at least one of the NFETs from the second SRAM cell share a leakage path between body regions, the respective NFETs sharing a leakage path being adjacent to one another and where the at least two adjacent NFETs have a source/drain diffusion region and a leakage path diffusion region under the source/drain diffusion region positioned between their respective body regions, wherein the source/drain diffusion region extends fractionally into the surface silicon layer and the leakage path diffusion region extends from a bottom of the source/drain diffusion down to the SOI buried-oxide layer, and where the leakage path diffusion region is counter-doped with the same dopant type as the source/drain diffusion but at relatively lower concentrations than the source/drain diffusion, thereby presenting a lower barrier to junction leakage than the source/drain regions. 
   A third alternate embodiment of the present invention comprises a pair of adjacent SRAM cells in an SRAM array, the pair comprising a first SRAM cell and a second SRAM cell, each of the adjacent SRAM cells comprising: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried oxide layer, where each cross-coupled inverter comprises an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled inverters; and where at least one of the PFETs from the first SRAM cell and at least one of the PFETs from the second SRAM cell share a leakage path between body regions, the respective PFETs sharing a leakage path being adjacent to one another and where the at least two adjacent PFETs have a source/drain diffusion region and a leakage path diffusion region under the source/drain diffusion region positioned between their respective body regions, wherein the source/drain diffusion region extends fractionally into the surface silicon layer and the leakage path diffusion region extends from a bottom of the source/drain diffusion down to the SOI buried-oxide layer, and where the leakage path diffusion region is counter-doped with the same dopant type as the source/drain diffusion but at relatively lower concentrations than the source/drain diffusion, thereby presenting a lower barrier to junction leakage than the source/drain regions. 
   A fourth alternate embodiment of the present invention comprises a pair of adjacent SRAM cells in an SRAM array, the pair comprising a first SRAM cell and a second SRAM cell, where each of the SRAM cells have two longitudinal and two lateral sides, the adjacent SRAM cells sharing a longitudinal side, each of the adjacent SRAM cells comprising: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, wherein the cross-coupled CMOS inverters each comprise an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters, and wherein one each of the pass gate NFETs and inverter NFETs are positioned along each of the lateral sides of the SRAM cell, whereby the pass gate NFET and inverter NFET positioned on the same lateral side of the SRAM cell comprise a pair and have body regions linked with leakage path diffusion regions formed beneath adjacent shallow source/drain diffusions wherein the shallow source/drain diffusion region extends fractionally into the surface silicon layer and the leakage path diffusion region extends from a bottom of the source/drain diffusion down to the SOI buried-oxide layer, and where the leakage path diffusion region is counter-doped with the same dopant type as the source/drain diffusion but at relatively lower concentrations than the source/drain diffusion, thereby presenting a lower barrier to junction leakage than the source/drain regions. 
   A fifth alternate embodiment of the present invention comprises an SRAM array comprising a plurality of SRAM cells organized in rows and columns, wherein each of the SRAM cells have two longitudinal sides and two lateral sides, the SRAM cells further comprising: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, where each cross-coupled inverter comprises an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters; and where at least two adjacent NFETs of the SRAM cell share a leakage path between body regions, and where the at least two adjacent NFETs have a source/drain diffusion region and a leakage path diffusion region under the source/drain diffusion region positioned between their respective body regions, wherein the source/drain diffusion region extends fractionally into the surface silicon layer and the leakage path diffusion region extends from a bottom of the source/drain diffusion down to the SOI buried-oxide layer, wherein the leakage path diffusion region is counter-doped with the same dopant type as the source/drain diffusion but at relatively lower concentrations than the source/drain diffusion, thereby presenting a lower barrier to junction leakage than the source/drain regions, and where each of the SRAM cells arrayed in a particular row of the SRAM array share longitudinal sides with two other SRAM cells positioned in the same row, except for at least two of the SRAM cells one longitudinal side of each coincides with a termination point of the row, and where the at least two adjacent NFETs of each SRAM cell arrayed in the particular row of the SRAM array having body regions linked by the leakage path diffusion region have their body regions further linked to the body regions of NFETs contained in adjacent SRAM cells sharing longitudinal sides with the SRAM cell with leakage path diffusion regions positioned beneath adjacent shallow source/drain diffusion regions, except for the at least two of the SRAM cells having one longitudinal side coinciding with the termination point of the row which have at least one pair of NFETs having a body region linked to the body regions of NFETs positioned in one SRAM cell on a longitudinal side opposite from the termination point of the row with leakage path diffusion regions positioned beneath adjacent shallow source/drain diffusion regions; and whereby a continuous chain of NFETS having body regions linked with leakage path diffusion regions positioned beneath adjacent shallow source/drain diffusion regions exists across the particular row of the SRAM array. 
   A sixth alternate embodiment of the present invention comprises a microprocessor fabricated on a CMOS hybrid orientation substrate, wherein the microprocessor comprises a logic portion and a cache memory portion, wherein the cache memory portion further comprises at least one CMOS SRAM array and where: the logic portion comprises, in part, PFETs fabricated in (110) crystal orientation bulk silicon regions and NFETs fabricated in (100) crystal orientation SOI silicon regions, wherein the NFETs in the logic portion have floating body regions; and the CMOS SRAM array comprises a plurality of CMOS SRAM cells comprising, in part, PFETs fabricated in (110) crystal orientation silicon regions and NFETs fabricated in (100) crystal orientation SOI silicon regions, wherein at least a portion of the NFETs in the CMOS SRAM cells have body regions linked to body regions of adjacent NFETs with leakage path diffusion regions formed beneath adjacent shallow source/drain diffusions wherein the source/drain diffusion regions extend fractionally into a surface silicon layer and the leakage path diffusion regions extends from bottoms of the source/drain diffusions down to an SOI buried-oxide layer, and where the leakage path diffusion regions are counter-doped with the same dopant type as the source/drain diffusions but at relatively lower concentrations than the source/drain diffusions, thereby presenting a lower barrier to junction leakage than the source/drain regions. 
   A seventh alternate embodiment comprises a method of forming an SRAM array comprising a plurality of SRAM cells, each of the SRAM cells comprising a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, where each cross-coupled inverter comprises an NFET and a PFET and the SRAM cell further comprises a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters, the method comprising: forming a buried oxide layer in a silicon wafer, the buried oxide layer positioned between a surface silicon layer and a silicon substrate; forming a plurality of PFET and NFET gates above body regions in the surface silicon layer; forming a leakage path diffusion region between at least a pair of adjacent NFET body regions wherein the leakage path diffusion region is counter-doped with a same dopant type as a shallow source/drain diffusion to be formed in another step but at relatively lower concentration than the shallow source/drain diffusions, thereby presenting a lower barrier to junction leakage than the source/drain regions, the leakage path diffusion region extending to the buried oxide layer; and forming the shallow source/drain diffusions above the leakage path diffusion regions, the shallow source/drain diffusions extending fractionally into the surface silicon layer. 
   An eighth alternate embodiment of the present invention comprises an SRAM memory comprising: peripheral logic fabricated in a high-performance silicon substrate; an SRAM array comprised of a plurality of SRAM cells, wherein the SRAM cells are arrayed in rows and columns and further comprise: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, wherein the cross-coupled CMOS inverters each comprise an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters, where body regions of NFETs arrayed along a column of SRAM cells coinciding with a bit line are linked by leakage path diffusion regions beneath adjacent shallow source drain diffusion regions, thereby forming a chain of linked body regions. 
   In one variant of the eighth alternate embodiment, the high-performance silicon substrate of the peripheral logic comprises a strained silicon region. 
   In another variant of the eighth alternate embodiment, the high-performance silicon substrate of the peripheral logic comprises a hybrid orientation substrate, where the NFETs are fabricated in (100) crystal orientation silicon regions and PFETs are fabricated in (110) crystal orientation silicon regions. 
   A ninth alternate embodiment of the present invention comprises an SRAM memory comprising: peripheral logic comprised of CMOS NFETs and PFETs, where the NFETs are fabricated in bulk silicon regions and the PFETs are fabricated in SOI silicon regions, where body regions of the PFETs are floating; and an SRAM array comprised of a plurality of SRAM cells, wherein the SRAM cells are arrayed in rows and columns and further comprise: a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, wherein the cross-coupled CMOS inverters each comprise an NFET and a PFET; a pair of NFET pass gates selectively coupling a pair of bit lines to said cross-coupled CMOS inverters; where body regions of NFETs along a column of SRAM cells coinciding with a bit line are linked by leakage path diffusion regions beneath adjacent shallow source drain diffusion regions, thereby forming a chain of linked body regions. 
   A tenth alternate embodiment of the present invention comprises a method of forming an SRAM memory comprised of an SRAM array portion and a peripheral logic portion, where the SRAM array portion is comprised of a plurality of SRAM cells, and where each of the SRAM cells further comprises a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, where each cross-coupled inverter comprises an NFET and a PFET and the SRAM cell further comprises a pair of NFET pass gates coupling a pair of bit lines to the cross-coupled CMOS inverters, the method comprising: forming a high-performance silicon substrate portion in a silicon wafer; forming circuits comprising the peripheral logic portion of the SRAM memory in the high-performance silicon substrate portion of the silicon wafer; forming the SRAM array portion of the SRAM memory by: forming a buried oxide layer in the silicon wafer, the buried oxide layer positioned between a surface silicon layer and a silicon substrate; forming a plurality of PFET and NFET gates above body regions in the surface silicon layer; forming a leakage path diffusion region between at least a pair of adjacent NFET body regions wherein the leakage path diffusion regions are counter-doped with a same dopant type as a shallow as a shallow source/drain diffusion to be formed in another step but at a relatively lower concentration than the shallow source/drain diffusions, thereby presenting a lower barrier to junction leakage than the source/drain regions, the leakage path diffusion region extending to the buried oxide layer; and forming the shallow source/drain diffusions above the leakage path diffusion regions, the shallow source/drain diffusions extending fractionally into the surface silicon layer. 
   In one variant of the tenth alternate embodiment the high-performance silicon substrate portion comprises a hybrid orientation substrate having (100) crystal orientation silicon regions and (110) crystal orientation silicon regions. 
   In another variant of the tenth alternate embodiment, the high-performance silicon substrate portion comprises a strained silicon region. 
   Thus it is seen that embodiments of the present invention overcome limitations of the prior art. Known device structures suitable for use in overcoming the floating body effect in SOI SRAM arrays suffer from a number of drawbacks. In particular, one known method comprising the use of body-contacted SOI MOSFET transistors significantly increases SRAM cell area and processing complexity. The present invention significantly reduces the need for increased SRAM cell area and further reduces the resistance of the leakage path used to join the body regions through an improved SRAM cell layout. 
   In addition, embodiments of the present invention improve the overall performance of SRAM memories by applying high-performance substrate technologies to peripheral logic portions of SRAM memories to improve the speed of operation of such portions, while applying linked body technology to the memory array portion of the SRAM memory to improve the stability of the array. This can be accomplished with differing high-performance substrate technologies; for example, strained silicon substrates or hybrid orientation substrates. Strained silicon substrates, which are relatively less expensive to fabricate when compared to hybrid orientation substrates, can be used in applications where improved performance is sought. In other applications where state-of-the-art performance is sought, hybrid orientation substrates can be used to achieve the fastest possible operation for the peripheral logic portions of SRAM memories. 
   Further, the present invention judiciously applies state of the art device structures to accomplish improved overall performance for microprocessors. In particular, the present invention applies hybrid orientation technology in combination with advances in overcoming the floating body effect to achieve an overall improvement in microprocessor performance. In particular, fabrication of the logic portion of a microprocessor in CMOS with NFETs in (100) crystal orientation SOI silicon where the NFETs have floating bodies and PFETs in (110) crystal orientation bulk silicon improves the operating speed of the logic portion of the microprocessor. 
   In conclusion, the foregoing summary of the alternate embodiments of the present invention is exemplary and non-limiting. For example, one of ordinary skill in the art will understand that one or more aspects or steps from one alternate embodiment can be combined with one or more aspects or steps from another alternate embodiment to create a new embodiment within the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein: 
       FIG. 1  is a cross-section of pull down and pass gate devices in prior art SOI SRAM cells; 
       FIG. 2  depicts an M by N SRAM memory comprised of a plurality of SRAM cells with each SRAM cell supplied by a pair of independently coupled cell supply lines; 
       FIG. 3  is a schematic of a typical CMOS static RAM (“SRAM”) cell; 
       FIG. 4  is a cross-section of NFET pull down and pass gate NFET devices in an SOI CMOS SRAM cell made in accordance with the present invention; 
       FIG. 5  depicts the layout and interrelationships among a couple of SOI CMOS SRAM cells made in accordance with the present invention; 
       FIG. 6  depicts a larger portion of an SOI CMOS SRAM array made in accordance with the present invention; 
       FIG. 7  depicts simulation results showing SRAM cell access disturb margin increases as FET threshold voltage mismatch decreases; 
       FIG. 8  depicts a method for fabricating an SOI CMOS SRAM array having leakage path diffusion regions positioned beneath shallow source/drain diffusions; 
       FIG. 9  depicts dynamic stability of SRAM cells fabricated in an hybrid orientation substrate compared to prior art SOI CMOS SRAM cells; 
       FIG. 10  depicts in conceptual terms fabrication method selections made for an SRAM memory made in accordance with the present invention; 
       FIG. 11  depicts in a highly schematic form the architecture of a microprocessor made in accordance with the methods of the present invention; and 
       FIG. 12  depicts a method for fabricating an SOI CMOS SRAM memory having a peripheral logic portion fabricated in a high-performance silicon substrate. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  depicts a storage memory with each column of cells supplied by a pair of selectively asymmetric cell supply lines. Preferably, the storage circuit  150  includes an array  152 , sub-array or array of sub-arrays of static random access memory (SRAM) cells formed in CMOS. Normally, cell symmetry is maintained as a nominal supply voltage is commonly supplied on both column supply lines of each pair. During cell access the supply voltage is unbalanced for each column with accessed cells by applying an offset voltage to one column supply line and maintaining the nominal supply at the other. The unbalanced supply voltages favor the data state being written/read by making cells on each accessed column asymmetric during the access. 
   Supply asymmetry switches  154  selectively provide a higher, offset voltage, mutually exclusively, to one or the other of the cell supply line pairs. A bit decode circuit  156  decodes a bit address to select one of N columns  158  of cells in the array  152 . Each of the N columns  158  of cells in the array  152  is connected to one of N pairs of column supply lines. A word decoder  160  selects a row of cells by driving one of M local word lines  162 . So, in this example, the M by N array is addressed by coincidence of a selected row  162  with a selected column  158 . During a read, bit select  164 , which may include a sensing capability, selects one column  158  and buffers and re-drives data that is stored in the selected cells in that column  158 . An active/passive supply couple, e.g., located with supply asymmetry switches  154  or with the bit select  164 , passes a nominal supply voltage to the array  152 ; and when appropriate, allows the supply asymmetry switches  154  to mutually exclusively pass an offset voltage to one or the other of a pair of column supply lines. Examples of suitable supply couples include a resistor, diode or FET connected between the array supply and each of the column supply lines. Data input/output (I/O) drivers  166  receive input data and drive selected data from the bit select  164 , for example, off chip. Clock logic  168  provides local timing and glue logic  169  provides local control, for example, read/write select, address gating and buffering, etc. In the present application, “SRAM array” generally refers to array portion  152  of SRAM memory  150  and “peripheral logic” refers to the remaining portions of the SRAM memory. Further, the SRAM memory  150  depicted in  FIG. 2  is exemplary and the teachings of the present invention are applicable to CMOS SRAM memories having different designs than that depicted in  FIG. 2 . 
   Normally, the supply asymmetry switches  154  are open. Matched supply voltages (nominal) are provided to each pair of column supply lines to maintain cell symmetry. During accesses, supply asymmetric switches  154  switch the higher offset voltage onto one side of the cell in each column being accessed. Thus, the higher offset voltage unbalances the voltage on each pair of column supply lines, making the cells in each unbalanced column  158  asymmetric during the access. In particular, the offset voltage is switched such that the imbalance or asymmetry favors any data state being stored/read. So, the imbalance facilitates writing and reading data to and from preferred embodiment storage cells. 
     FIG. 3  is a schematic of a typical CMOS Static RAM (SRAM) cell  200  that would comprise the array portion  152  of SRAM memory  150 . The cell  200  is, essentially, an identical pair of cross coupled CMOS inverters  210 ,  220  and a pair of pass transistors  230 ,  240  between the cross coupled inverters  210 ,  220  and a pair of bit lines  250 ,  260 . A word line  270  is tied to the gate of pass transistors  230 ,  240 . Each CMOS inverter  210 ,  220  is, simply, an NFET  212 ,  222  and a PFET  214 ,  224 . The gate and drain of each PFET  214 ,  224  is tied to the gate and drain of corresponding NFET  212 ,  222 , respectively. The source of the PFETs  214 ,  224  are connected to supply voltage (V hi ) and the source of the NFETs  212 ,  222  are connected to GND. The channel body for each FET  212 ,  214 ,  222 ,  224 ,  230  and  240  is represented by node  212 C,  214 C,  222 C,  224 C,  230 C and  240 C, respectively. The state of the cross coupled inverter pair  210 ,  220  determines the state of data stored in the cell  200 . 
   Each SRAM cell  200  is written by pulling one of the bit line pair  250 ,  260  high and the other low while holding word line  270  high so that both access transistors  230 ,  240  are on; and, then pulling the word line  270  low to turn off the access transistors  230 ,  240 , trapping the state of the bit lines in the cross coupled inverters  210 ,  220 . The SRAM cell  200  is read by pre-charging the bit lines  250 , 260  to a known state; driving the word line  270  high which couples the cross coupled inverters  210 ,  220  through the access transistors  230 ,  240  to the bit line pair  250 ,  260 ; and, then, measuring the resulting voltage difference on the bit line pair  250 ,  260 . The signal on the bit line pair  250 ,  260  increases with time toward a final state wherein each one of the pair  250 ,  260  may be, ultimately, a full up level and a full down level. However, to improve performance, the voltage difference is sensed well before the difference reaches its ultimate value. 
   As noted hereinabove, in a prior art bulk CMOS technology  214 C,  224 C,  230 C and  340 C were tied to GND and,  212 C and  222 C were tied to V hi  biasing the respective devices. However, in the prior art SOI process of  FIG. 1 , all of the FETs  212 ,  214 ,  222 ,  224 ,  230  and  240  in an SRAM cell  200  have floating channels, i.e.,  212 C,  214 C,  22 C,  224 C,  230 C and  240 C are not connected directly to any bias voltage and, at best, are capacitively coupled to underlying silicon substrate  110 . 
   The body potential mismatch problem is solved in this invention by linking the bodies of adjacent devices together with a leakage path diffusion positioned beneath a shallow source/drain diffusion. This avoids butting of the deep source and drain implants against the backside of the silicon film in an SOI device. It creates shallow source and drain implants with a leakage path diffusion region near the backside of the SOI silicon film to allow the two bodies of neighboring devices to electrically connect to each other via the leakage path. In some embodiments made in accordance with this invention, the body of a PFET is linked to an adjacent PFET in a neighbor cell using shallow source and drain implants and a leakage path diffusion region. The bodies of NFETs are linked along the bit line and further connected to ground at two sides of the bit line row. By doing this, the threshold voltage mismatches between pull down and pass gate NFETs and between PFETs are reduced. 
   The shallow source drain implant is performed by blocking the source/drain area of SRAM FETs from the normal deep source drain implant and defining separate lithographic regions in the SRAM arrays coinciding with the leakage path diffusion regions to receive less energy or smaller doses of implanted species. By opening contacts on source drain diffusions of NFETs at both sides of one bit line row, the bodies of N-type devices can be tied to a fixed potential, such as ground, for enhanced performance. Performance depends on the effectiveness of body linking. 
   To demonstrate the invention at work, a cross-section of the body linked and grounded pull down and pass gate devices is shown in  FIG. 4 .  FIG. 4  shows a cross section of a CMOS SOI SRAM through a pair of NFET transistors  342 ,  344 . The NFETs  342 ,  344  are formed in a thin silicon surface layer  330  that is isolated from an underlying silicon substrate  310  by a buried oxide (BOX) layer  320 . In a typically complex series of mask steps, silicon-on-insulator (“SOI”) regions are formed in the silicon surface layer  330  by etching shallow trenches through the surface layer  330  and filling the shallow trenches with oxide to isolate regions from each other. This type of isolation is normally referred to as shallow trench isolation (“STI”). STI is used to isolate circuits formed on the regions from each other and, also, isolate the FETs forming the circuits from each other. 
   After forming a gate oxide layer on the surface of the silicon regions, gates  316  are patterned and formed at the location of devices  342 ,  344 . Source/drain regions  332  are defined using a standard implant and diffusion step, after forming lightly doped diffusion regions  334  at the gate boundaries, if desired. In various embodiments halo regions  333  can be formed by a separate diffusion step of the same species as the body region. In other embodiments halo regions can be deleted. In contrast to the device architecture depicted in  FIG. 1  where device channels  136  are completely isolated from other channels by source/drain diffusions  132  at either end, in the device depicted in  FIG. 4  device channels  336  are linked by a leakage path diffusion region  338 . The leakage path diffusion region is counter-doped with the same dopant type as the source/drain diffusion but at relatively lower concentrations than the source/drain diffusion, thereby presenting a lower barrier to junction leakage than the source/drain regions. 
   Layouts of the body-linked SOI SRAM array/cells are illustrated in  FIGS. 5–6 . By connecting the bodies inter-cell (body links between pull down (inverter NFETs) and pass gates in  FIG. 5 ) and intra-cell (body links between pass gates and pull downs in  FIG. 5 ) using a leakage path diffusion region N+ implant mask “WN” in the region  614  depicted in  FIG. 6 , a connected body chain as shown in  FIG. 4  is formed along the bit line direction. At the end of each chain, a metal line ( 610  in  FIG. 6 ) can be used to connect each body chain together and tie to ground or a bias voltage for enhanced stability performance. 
   In  FIG. 5  each SRAM cell  400  has two lateral sides  401  and two longitudinal sides  402 . As used herein, “lateral” refers to the relatively short sides of the SRAM cell and “longitudinal” refers to the relatively long sides of the SRAM cell. The SRAM cells comprise cross-coupled inverters comprised of pull up inverter PFETs  414 ,  424  and pull down inverter NFETs  412 ,  422 . The SRAM cells further comprise two pass gate NFETs  430 ,  440  for selectively coupling the cross-coupled inverters to bit lines. As shown in the embodiment depicted in  FIG. 5 , pairs of pass gates and pull down inverter NFETs (( 412 ,  430 ) and ( 422 ,  440 )) are arrayed along the lateral sides  401  of the SRAM cells. The pull up inverter PFETs  414 ,  424  are positioned intermediate between the pair of pass gate and pull down inverter NFETs along the longitudinal sides  402  of the SRAM cells. In the embodiment depicted in  FIG. 5 , the pairs of pass gate and pull down inverter NFETs (( 412 ,  430 ) and ( 422 ,  440 )) have body regions linked by leakage path diffusion regions formed beneath adjacent shallow source/drain diffusions. Pass gate NFETs positioned in adjacent cells also have body regions linked by leakage path diffusion regions, for example, in the region  431 . Pull down inverter NFETs in adjacent SRAM cells  400  also have body regions linked with leakage path diffusion regions, for example, in the region  413 . In the embodiment depicted in  FIGS. 4–6 , bit lines coincide with the upper and lower portions of the SRAM cells  400  and run from left to right. All NFETs along a particular bit line have body regions linked to adjacent NFETS in the embodiment depicted in  FIG. 5 . Pull up inverter PFETS  414 ,  424  from adjacent SRAM cells also have body regions linked by leakage path diffusion regions. 
   A particular advantage of the embodiment depicted in  FIG. 5  is associated with the reduction in dimension of the lateral sides  401  of the SRAM cells associated with the rearrangement of the devices comprising the SRAM cell. This reduction reduces the path length of the linked body regions across a bit line of an SRAM cell, thereby reducing the resistance of the path. Reduction of the resistance enables charge to migrate more easily from the linked body regions and thereby improve the stability of the SRAM cells. 
   Bodies of P-type devices are linked in a similar way by using a leakage path diffusion region P+ implant mask “WP” in the regions  616  depicted in  FIG. 6 . One P-type device body is only linked to one neighbor P-type device body (as shown in  FIG. 5 ) because there is no continuous P-type device active region across the SRAM array. The two body linked P-type devices will have reduced threshold voltage mismatch. 
   Tying the connected body chain of N-type devices to ground is implemented by performing a normal P+ source drain implant on the outside diffusions of edge cells  339  ( FIG. 4 ) and opening contacts for the P+ regions and wiring the P+ contacts to a metal layer  610  ( FIG. 6 ). Two sides of metal layers can be wired together. 
   SRAM cell stability is studied using Access Disturb Margin approach. The stability dependence on threshold voltage mismatch is plotted in  FIG. 7 . The X-axis starts with 0 at right which represents current SRAM FET unit-sigma threshold mismatch read out from a typical robust process. With improving threshold voltage mismatch, the projected decreased mismatch goes from 0 to −40%. By varying the FET threshold voltage mismatch, simulation of the SRAM stability shows that the access disturb margin is improved by 66% as shown in  FIG. 7  for both 65 nm SOI SRAM cells. 
   A method for fabricating a CMOS SRAM array in accordance with the present invention is depicted in  FIG. 8 . The SRAM array made in accordance with this method generally comprise a plurality of SRAM cells further comprising (1) a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer, where each cross-coupled inverter comprises an NFET and a PFET and (2) a pair of NFET pass gate transistors selectively coupling a pair of bit lines to the cross-coupled inverters. In the first step at  710 , a buried oxide layer is formed between a surface silicon layer and a silicon substrate. Next, at step  720  a plurality of PFET and NFET gates are formed above body regions in the surface silicon layer. Then, at step  730  a leakage path diffusion region is formed between at least a pair of body regions wherein the leakage path diffusion regions are counter-doped with a same dopant type as a shallow source-drain to be formed in step  740  but at a relatively lower concentration than the shallow source/drain diffusion. The leakage path diffusion region extends to the buried oxide layer. Next at step  740  shallow source/drain diffusions are formed above leakage path diffusion regions. The shallow source/drain diffusions extend fractionally into the surface silicon layer. 
   In one variant of the method depicted in  FIG. 8 , the underlying silicon wafer comprises (100) crystal orientation silicon, and additional steps would be performed to form (110) crystal orientation silicon regions in the substrate. The PFETs would then be formed in the (110) crystal orientation silicon. Methods for fabricating hybrid orientation substrates are described in Min Yang et al., “On the Integration of CMOS with Hybrid Crystal Orientations”, 2004 IEEE Symposium on VLSI Technology Digest of Technical Papers, 2004, pp. 160–161, and U.S. patent application Ser. Nos. 10/725,850 and 10/830,347, all of which are hereby incorporated by reference in their entirety as if fully restated herein. 
   In another variant of the method depicted in  FIG. 8  the leakage path diffusion region links the body regions of adjacent pass gate and inverter NFETs in an SRAM cell. In a further variant of the method depicted in  FIG. 8 , the SRAM cell comprises a six-transistor circuit, and the body regions of the four NFETs comprising the cell are linked with leakage path diffusion regions positioned beneath adjacent shallow source/drain diffusions. In yet another variant of the method depicted in  FIG. 8 , the body regions of NFETs coinciding with a bit line row are linked with leakage path diffusion regions positioned beneath adjacent shallow source/drain diffusions. 
     FIG. 9  depicts the improvement in dynamic stability of an SRAM cell made in accordance with the present invention. “POR” refers to the read stability of a conventional CMOS SOI SRAM cell with floating bodies. “HOT” refers to the read stability of a CMOS SOI SRAM cell fabricated in a hybrid substrate with NFETs fabricated in (100) crystal orientation SOI silicon regions with body regions linked by leakage path diffusion regions and PFETs fabricated in (110) crystal orientation silicon regions. 
     FIG. 10  depicts in highly conceptual terms fabrication method selections made for an SRAM memory made in accordance with the present invention. The SRAM memory  750  as in the case of the memory depicted in  FIG. 2  is comprised of a peripheral logic portion  752  and an SRAM array portion  760 . In the example depicted in  FIG. 10 , the NFETs  754  of the peripheral logic portion are fabricated in SOI with floating body regions, and the PFETs  756  are fabricated in (110) crystal orientation bulk regions. The SRAM array portion  760  has NFETs  762  fabricated in (100) crystal orientation silicon SOI regions with linked bodies and PFETs  764  fabricated in (110) crystal orientation bulk silicon regions. 
     FIG. 11  depicts in simplistic terms the structure of a microprocessor. In general terms, the microprocessor  800  comprises a logic portion  810  and cache memory portion  820 . The cache memory portion typically comprises at least one CMOS SOI SRAM array. In one aspect of this invention, the logic portion  810  of the microprocessor  800  has NFETs fabricated in (100) crystal orientation SOI silicon regions with floating body regions and PFETs fabricated in (110) crystal orientation bulk silicon regions. The cache memory portion  820  comprises at least one CMOS SRAM array where the NFETs are fabricated in (100) crystal orientation SOI silicon regions with body regions linked by leakage path diffusion regions beneath shallow source/drain diffusions and PFETs fabricated in ( 110 ) crystal orientation silicon regions. 
   In other embodiments of the invention improved overall performance for SRAM memories is achieved by applying high-performance substrate technologies to peripheral logic portions of an SRAM memory in combination with the linked body technology applied to the memory array portion of the SRAM memory. For example, with reference to  FIG. 2 , in one embodiment of the present invention strained silicon substrate technology can be applied to the non-array portions of the SRAM memory. U.S. Pat. Nos. 5,906,951 and 6,603,156 describe methods for fabricating strained silicon substrates and are hereby incorporated by reference in its entirety as if fully restated herein. This would improve the speed of operation of the NFETs and PFETs comprising the peripheral logic portion of the SRAM memory. In another embodiment, hybrid orientation technology can be applied to the peripheral logic portion of the SRAM array. In hybrid orientation embodiments, the NFETs would be fabricated in (100) crystal orientation silicon regions and the PFETs in (110) crystal orientation silicon regions. In further hybrid orientation embodiments, the NFETs are fabricated in (100) crystal orientation SOI regions and the PFETs are fabricated in (110) crystal orientation bulk regions. In still further variants where speed of operation is desired, the NFETs fabricated in (100) crystal orientation SOI regions have floating body regions. In applications where other device characteristics are sought, the peripheral NFETs can be fabricated in bulk regions and the PFETs in SOI regions, where body regions of the PFETs are floating. 
   An exemplary method for fabricating an SRAM memory having a peripheral logic portion implemented in a high-performance silicon substrate is depicted in  FIG. 12 . The SRAM memory created in the method of  FIG. 12  comprises an SRAM array portion and the peripheral logic portion. The SRAM array portion comprises a plurality of SRAM cells, where each of the SRAM cells further comprises a pair of cross-coupled CMOS inverters in a surface silicon layer disposed on an SOI buried-oxide layer. The cross-coupled inverters each comprise an NFET and a PFET, and each SRAM cell has a pair of NFET pass gates coupling the cross-coupled inverters to bit lines. In the method, a high-performance silicon substrate portion is formed in a silicon substrate at step  910 . Then, at step  912  circuits comprising the peripheral logic portion of the SRAM memory are formed in the high-performance silicon substrate portion of the silicon wafer. Next, at step  914 , the SRAM array portion of the SRAM memory is formed. This comprises forming at step  916  a buried oxide layer in the silicon wafer, the buried oxide layer positioned between a surface silicon layer and a silicon substrate. Then, at step  918 , a plurality of PFET and NFET gates are formed above body regions in the surface silicon layer. Next, at step  920 , a leakage path diffusion region between at least a pair of adjacent NFET body regions wherein the leakage path diffusion regions are counter-doped with a same dopant type as a shallow source/drain diffusion to be formed in another step but at a relatively lower concentration than the shallow source/drain diffusions, thereby presenting a lower barrier to body-to-body leakage than the source/drain regions, the leakage path diffusion region extending to the buried oxide layer. Then, at step  922 , shallow source/drain diffusions are formed above the leakage path diffusion regions, the shallow source/drain diffusions extending fractionally into the surface silicon layer. 
   One of ordinary skill in the art will understand that the ordering of the steps in  FIG. 12  and other methods described herein is exemplary and that certain of the steps can be reordered. Any such reordering of the method in  FIG. 12  is within the scope of Applicants&#39; invention as herein described. Further, the steps of one method described herein can be combined with steps of another method described herein; all such variations are within the scope of the present invention. 
   In one variant of the method of  FIG. 12 , the high-performance silicon substrate portion comprises a hybrid orientation substrate having (100) crystal orientation silicon regions and (110) crystal orientation silicon regions. 
   In another variant of the present invention, the high-performance silicon substrate portion comprises a strained silicon region. 
   Thus it is seen that the foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the best method and apparatus presently contemplated by the inventors for creating CMOS SOI SRAM arrays having body regions of FETs linked with leakage path diffusion regions. One skilled in the art will appreciate that the various embodiments described herein can be practiced individually; in combination with one or more other embodiments described herein; or in combination with SOI CMOS SRAM architectures differing from those described herein. Further, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments; that these described embodiments are presented for the purposes of illustration and not of limitation; and that the present invention is therefore limited only by the claims which follow.