Abstract:
A D-Cache SRAM cell having a modified design in schematic and layout that exhibits increased symmetry from the circuit schematic and the physical cell layout perspectives. That is, the SRAM cell includes two read ports and minimizes asymmetry by provisioning one read port on a true side and one on the complement side. Asymmetry is additionally minimized in layout as cross coupling on both the true and complement sides rises up one level by providing from the local interconnect level a via connection to a M1 or metallization level. Moreover, the distance between the local interconnect (MC) and the gate conductor structure (PC) has been enlarged and equalized for each of the pFETs in the cross-latched SRAM cell. As a result, the SRAM cell has been rendered insensitive to overlay (local interconnect processing too close) by maximizing this MC-PC distance.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention broadly relates to an improvement in the design and structure of SRAM memory cells, and more particularly, it relates to a novel design and layout of a Data-cache (D-cache) cell that is more symmetric and insensitive to lithography alignments during manufacture. 
   2. Description of the Prior Art 
   In the field of microprocessor and cache memory designs, particularly, groups of SRAM cells making up a D-cache that is formed in close proximity to the microprocessor and in which speed of access is critical, it has been found that these D-cache cells were very sensitive to alignments—particularly between the transistor gate conductor regions (“PC”), e.g., of the pull-up transistor, and respective local interconnects (“MC”) connected to drain/source regions of the transistors, and, further, have asymmetries in both schematic and layout. Consequently, production yield has been very low. 
   For example,  FIG. 1  illustrates a circuit schematic  10  of the prior art D-cache comprising an SRAM cell  12  having a group of six (6) transistors, four of which indicated as P 0 , P 1 , N 4  and N 5  are of a cross-coupled latch configuration of typical SRAM design. As shown in  FIG. 1 , read and write access to the SRAM cell is accomplished via WBL (true side) and  WBL  (complement side) the respective bit line and bit line complement and are accessed by controlling respective access transistors N 12  and N 11  and the word line signal (WLW). In order to perform a very fast read (cache write-through) the SRAM cell design  10  further includes additional transistors N 16  and N 18  forming a first additional read port, and transistors N 15  and N 17  forming a second additional read port, both connected to the WBL at the intersection of the SRAM P 1  and N 5  transistors so that data can be quickly accessed and/or replicated. 
   As shown in  FIG. 1 , there is an asymmetry from the schematic perspective as no corresponding read port is provided for connection with the  WBL  line. This asymmetry is critical as it would be desirable on the complement side (  WBL  line) comprising transistors P 0  and N 4  to effortlessly store complementary bit data as it is designed. However, provision of first and second additional read port circuits  14   a  and  14   b  comprising read port transistors N 16  and N 18  provide a leakage path to ground, which has been shown to detrimentally affect operation of the P 1  pull-up transistor of the SRAM. In order to keep the cell high on the true side (e.g. logic “1”), P 1  has to supply additional currents to overcome the leakage to ground. There is no such leakage path if the complement side is high thus, rendering it harder to maintain a stored “1”. 
   As further shown in  FIG. 2(   a ), with like reference numbers corresponding to like elements depicted in the circuit schematic of  FIG. 1 , there additionally exists an asymmetry with respect to the physical cell layout. As shown in  FIG. 2(   a ), in the physical cell layout depicted, there is provided active silicon (“RX”) area  21  in which the physical read port transistors N 15 -N 17  reside; active RX area  31  in which the SRAM pull-up transistors P 0  and P 1  reside; and active RX area  41  in which the SRAM transistors N 4 , N 5  and bit line transistors N 11 , N 12  reside. The PC polysilicon gates for transistors N 15 , N 16  are shown as the horizontal areas  22 ,  32 , for example, and the PC polysilicon gates for pull-up transistor P 0  and transistor N 4  and the PC polysilicon gates for pull-up transistor P 1  and transistor N 5  are shown as the respective horizontal areas  42 ,  52  in  FIG. 2(   a ). Areas  22 ,  32  and  42  are connected. As shown in  FIG. 2(   a ), between arrows depicting a distance  30  between the polysilicon gate edge of P 1  and the edge of the L-shaped MC (local interconnect)  23 . This distance is the minimum allowable according to current lithography groundrules. It is noted that the corresponding distance between the PC polysilicon gate for pull-up transistor P 0  and its local interconnect depicted by irregular shaped region MC  33 , is increased due to the presence of a notch or jog that effectively increases the space between upper edge of the PC and the lower edge of the MC. 
   Referring to  FIG. 2(   b ) there is shown the transistors P 0  and P 1  and the approximate location of respective wide spacer structures  44 ,  45  as indicated. The spacer structures may be such as described in commonly-owned, co-pending U.S. patent application Ser. No. 10/277,907 (US Publication No. 2004/0075151). As shown in  FIG. 2(   b ), the P 1  spacer  45  cuts into the L-shaped local interconnect  23  while on the other side, the P 0  spacer  44  is separated from the jogged area of the interconnect  33  without encroachment of the MC. The consequence of this is as follows: the performance of P 1  is substantially weakened due to this smaller distance coupled with the read port transistors loading this transistor. This is a problem illustrated in more detail in  FIG. 3 , which depicts the pFET structure P 1 , through a cross-sectional view, including the spacer structure  45 . As shown in  FIG. 3 , the MC is depicted as encroaching the spacer  45  and is not separated from it. This is because underneath the MC is a layer of highly-conductive silicide (e.g., CoSi) formed above the drain/source region. However, it is clear that the left corner of the MC  23  encroaches before the end of the CoSi region, i.e., the MC  23  is etched into the spacer and cuts into the silicon closer than the CoSi is located which is detrimental to the device performance as it is closer to the shallow doped drain/source extensions. That is, it has been found that the MC material  23 , such as Tungsten or any conductive metal, for example, combines with the metallic surface formed in the shallow extensions and provides a recombination center for holes. This hole recombination mechanism at the source/drain extensions such as shown in  FIG. 3 , significantly reduces performance of the pFET P 0 . This is illustrated in  FIG. 4 , which depicts the performance of the odd wordlines, i.e., depicted in  FIG. 3 , that includes the P 1  transistor and subsequent odd numbered wordlines, etc. where a substantial number of high percent fails occurs on the odd wordlines (e.g., numbered 1, 3, 5 et seq.) due to the spacer encroachment problem. 
   Moreover, as shown in  FIG. 2(   b ), a further RX contact asymmetry exists which is depicted by a line  34 . This asymmetry is shown by a shortening of the RX region  41   b  above the N 11  transistor as compared to the RX region  41   a  that is extended at N 12 . 
   Thus, in sum, the prior art the transistors for CMOS  10 S (90 nm technology on SOI) were optimized for performance in the following way: nFETs received a thin spacer in order to ensure short extension regions and thus lower resistance; pFETs received a wide spacer to allow for high activation anneals in spite of larger boron (B) diffusion. As mentioned, there are six (6) transistors in an SRAM cell: 2 pass gates, 2 pulldowns, and 2 pull-ups, the latter being the pFETs. The multiport cell  10  of  FIG. 1  includes additional read or write ports and thus more transistors. In this specific case, there are 4 additional nFETs forming two additional read ports. The additional readports allow for fast read operation after the write operation (write through). This cell was designed with two goals: (1) High performance (fast read/write); and, (2) minimum area consumption. However, symmetry was not considered, neither from the schematic, nor from the layout perspective. The asymmetries in design (layout and schematic) combined with non optimized processing steps (MC etch too deep, too close), leads to substantial yield (and reliability) degradation for this cell, while other cells in the same technology yielded as expected. The mechanism for the yield degradation is understood as a hole recombination caused by the presence of the metallic surface (MC) in the lightly doped drain regions (e.g.,  FIG. 3 ). 
   Moreover, in the prior art designs, as the two pull-op and pull-downs are cross-coupled inverters, on each transistor side (e.g., drain region) the connection went up to one metallization (drain region-MC-CA-M1) and then coming back down on the other side through a CA-MC to another transistor. However, on the other side, the cross-coupling was accomplished by providing a large L-shaped MC interconnect that is flat. Thus, there is an asymmetry as one side goes up to a metallization level (M1 processing) and the other side remains flat at a lower level below. 
   It would thus be highly desirable to provide a SRAM cell design that addresses the performance and minimum area consumption considerations and, that considers the symmetry aspect in both schematic and layout perspectives. 
   It would thus be highly desirable to provide a SRAM cell design that is insensitive to overlay or misalignment in processing by maximizing the MC-PC distances. 
   It would thus be highly desirable to provide a SRAM cell design that addresses the asymmetry problems by accomplishing that both connections on all pull-up and pull-down transistors be connected up to the M1 level. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to provide a D-Cache cell having a modified design in schematic and layout that accomplishes the following:
         1) Asymmetry minimized in schematic: One read port on true side and one on the complement side.   2) Asymmetry minimized in layout: (a) Cross coupling goes through MC-CA-M1-CA-MC on both sides, i.e., (b) MC-PC distance for pFETs equalized.   3) Cell made insensitive to overlay (MC processing too close) by maximizing MC-PC distance. For example, in one embodiment the outer MC-PC distance for pFETs increased from about worst case 70 nm, for example, to about 114 nm.       

   Specifically, according to a first aspect of the invention, there is provided a SRAM cell comprising: 
   first and second cross-coupled inverter devices; 
   a bit line connected to the first of the cross-coupled inverter devices through a first access transistor, and a complement bit line connected to the second of the cross-coupled inverter devices through a second access transistor; and, 
   first read port means connected at an internal node of the first of the cross-coupled inverter devices enabling read access to the cell, and, second read port means connected at an internal node of the second of the cross-coupled inverter devices enabling read access to the complement bitline, wherein increased symmetry exists thereby improving SRAM cell performance. 
   According to another aspect of the invention, there is provided a D-cache memory device comprising: first and second cross-coupled inverter devices defining a respective true bitline node and complement bitline node, the first cross-coupled inverter device including a first pull-up pFET transistor device comprising of a gate conductor structure and a drain and source region upon one of which a first conductive interconnect layer is formed in a semiconductor structure, and the second cross-coupled inverter device includes a second pull-up pFET transistor device comprising a gate conductor structure and a drain and source region upon one of which a second conductive interconnect layer is formed in the semiconductor structure, wherein a distance between the gate conductor structure of the first pull-up pFET transistor device and the first conductive interconnect layer is substantially equal to the distance between the gate conductor structure of the second pull-up pFET transistor device and the second conductive interconnect layer, wherein increased symmetry exists thereby improving D-cache memory device performance. 
   Further to this other aspect of the invention, the distance between the gate conductor of the first pull-up pFET transistor device and the first conductive interconnect layer and, the distance between the gate conductor of the second pull-up pFET transistor device and the second conductive interconnect layer is maximized, and thus desensitized to lithographic misalignment between gate conductor and interconnect layer. 
   Moreover, according to this other aspect of the invention, the cross-coupling of the first cross-coupled inverter devices comprises a connection formed at the first conductive interconnect layer through a first via connection to a metal structure formed at a metallization level lying above the first conductive interconnect layer and from the metal structure formed at a metallization level down through a second via connection connecting the gate conductor structure of the second pull-up pFET transistor device. 
   Likewise, the cross-coupling of the second cross-coupled inverter devices comprises a connection formed at the second conductive interconnect layer through a first via connection to a metal structure formed at a metallization level above the second conductive interconnect layer and from the metal structure formed at a metallization level down through a second via connection connecting the gate conductor structure of the first pull-up pFET transistor device. 
   Additionally, according to this other aspect of the invention, there is provided a bit line coupled to the true bitline node of the first cross-coupled inverter device through a first access transistor; and, a complement bit line coupled to the complement bitline node of the second cross-coupled inverter device through a second access transistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in more detail by referring to the drawings that accompany the present application. It is noted that in the accompanying drawings like reference numerals are used for describing like and corresponding elements thereof. 
       FIG. 1  illustrates a circuit schematic  10  of the prior art D-cache comprising an SRAM cell  12 ; 
       FIGS. 2(   a ) and  2 ( b ) depict the additionally asymmetries that exist in the physical cell layout of the D-cache SRAM circuit design  10  depicted in the circuit schematic of  FIG. 1 ; 
       FIG. 3  depicts the pFET structure P 1 , through a cross-sectional view, including the MC depicted as encroaching a spacer structure  45 ; 
       FIG. 4  depicts the performance of wordlines, embodied as pFET structure of  FIG. 3 , that includes the P 1  transistor and illustrates a substantial number of high percent fails occurs on those wordlines; 
       FIG. 5  depicts a new circuit schematic of a D-cache SRAM cell  100  according to the present invention; 
       FIG. 6  depicts a schematic and layout capture of the D-cache SRAM cell design according to the invention; 
       FIG. 7  depicts a schematic and layout capture of the D-cache SRAM cell design of  FIG. 6  and includes the M1 metallization layer; and, 
       FIG. 8  depicts a schematic and layout capture of the D-cache SRAM cell design according to an alternative embodiment of the invention shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the description, the following referred to terms will have the following associated meanings: MC is a local interconnect layer in 90 nm, or like, CMOS semiconductor technology which may be Tungsten and is used for contacting metallization lines or contact at low levels; PC is a transistor gate conductor region, e.g., a gate conductor layer comprising any conductive material, such as doped polysilicon; RX is the active silicon area or level where devices are formed, e.g., may comprise Silicon on Insulator (SOI); and, CA are contact vias that are formed to land on an MC local interconnect and represent a connection up to a M1 or metallization level. 
     FIG. 5  depicts a circuit schematic of a D-cache SPAM cell  100  according to the present invention. As shown in  FIG. 5 , the modified SPAM Cell design  100  includes the 6T SRAM cell  120  as in the prior art, however, includes a readport  150  on the internal node of the cross-coupled latch configuration of the bitline “true” side and a readport  160  on the internal node of the cross-coupled latch configuration on the bitline complement side. Particularly, as compared with the prior art SPAM cell design  10  of  FIG. 1 , the circuit layout for the SRAM cell  120  includes the group of six (6) transistors, four of which indicated as P 0 , P 1 , N 4  and N 5  from a cross-coupled latch configuration of typical SRAM design. Such semiconductor structures include pFETs P 0 , P 1  and nFETs N 4 , N 5  and requisite spacer structures can be manufactured, for example, according to techniques such as described in commonly-owned, co-pending U.S. patent application Ser. No. 10/277,907 (US Publication No. 2004/0075151). 
   As shown in  FIG. 5 , reading and write access to the SRAM cell  120  is accomplished via WBL and  WBL  the respective bit line and bit line complement and are accessed by controlling respective access transistors N 12  and N 11  and the word line signal (WLW). In order to perform a very fast read (cache write-through) the SRAM cell design  100  further includes additional transistors N 15  and N 17  forming the read port connected to the WBL at the intersection of the SRAM P 1  and N 5  transistors so that data can be quickly accessed and/or replicated. The SRAM cell design  100  further includes additional transistors N 16  and N 18  forming the read port  160  connected to the  WBL  at the intersection of the SRAM P 0  and N 4  transistors so that data can be quickly accessed and/or replicated. 
   As shown in  FIG. 5 , the SRAM cell design  100  minimizes the asymmetry from the schematic perspective as corresponding read ports are provided for connection with each of the WBL and  WBL  lines. 
     FIG. 6  depicts a layout capture of the D-cache SRAM cell design corresponding to the SRAM cell design circuit schematic  100  as shown in  FIG. 5 . As shown in  FIG. 6 , islands  210 ,  310  and  410  indicate the active area (RX) which may comprise monocrystalline silicon, silicon-on-insulators (SOI), or SiC-on-insulator (SiCOI) or silicon germanium-on-insulators (SGOI) prepared in accordance with conventional SOI fabrication techniques. It is understood that between the active RX islands resides shallow trench isolation (STI) fill comprising dielectric materials such as an oxide, nitride, oxynitride or like materials formed in accordance with conventional STI fabrication techniques. Horizontally extending areas, such as areas  242 ,  252 , represent the PC gate conductor regions, with region  242  indicating transistors N 15 , P 1 , N 5  and region  252  indicating transistors N 16 , P 0 , N 4  in accordance with the schematic of  FIG. 5 . Regions  230 ,  330  indicate MC regions, i.e., local interconnects, formed of a metal such as copper, aluminum, tungsten, or like conductive material. The square regions, such as region  350  indicated in  FIG. 6 , represent CA or contact vias that are formed to land on an MC local interconnect and represent the connection up to a M1 or metallization level. As known, these contact vias may include steps such as patterning and RIE etching according to well-known damascene or dual damascene process techniques and may initially include the deposition of a thin refractory metal liner, e.g., on the sidewalls and bottom of an etched via (contact) opening. The refractory metal liner may comprise a refractory material, e.g., tantalum, tantalum nitride, chromium/chromium oxide, titanium, titanium nitride, tungsten, or the like, deposited using any of the known deposition methods, such as, for example, CVD, Atomic Layer deposition, hollow cathode magnetron sputtering, deposit-etch (dep.-etch) process, or any combination of these or, other similar methods. Further, as known to skilled artisans, subsequent to the formation of the liner is the deposition of a contact “plug” (usually by chemical vapor deposition or electroplating) of conductive material, e.g., Copper or Tungsten (or alloys thereof). 
   According to the layout depicted in  FIG. 6 , symmetry is improved, particularly between the MC-PC. For instance, the outer MC-PC distance for both pFETs P 0  and P 1  is increased. That is as shown in  FIG. 6 , the MC-PC distance was made symmetrical as a result of providing a bend or jog at the MC  230 , as indicated by the arrows, resulting in an increase in the outer MC-PC distance, labeled “a”, and an increase in the inner MC-PC distance, labeled “b”, for both pFETS P 0 , P 1 . Thus, as indicated in  FIG. 6 , PC-MC distances  300   a  and  300   b  match for respective pFETS P 0 , P 1 . In one embodiment, the outer MC-PC distance has increased from a worst case 70 nm to 114 nm, and the inner MC-PC distance for pFETs has increased from 70 nm to 79 nm. Moreover, the RX regions has been enlarged at where each of the MC regions  230 ,  330  cross. The enlargement of the RX areas at the cross-section with the MC results in improved contact area for the devices, which renders the devices less sensitive to current degradations. Further, as shown in  FIG. 6  at locations labeled “c” in  FIG. 6 , unnecessary RX “cut-outs” or concave corners have been removed from the RX islands which decreases the likelihood of forming dislocations. 
   Further as shown at location labeled “d” in  FIG. 6 , it is the case that processing now renders the cross-coupling through MC-CA-M1-CA-MC on both the WBL (true) and  WBL  (complement) side. As shown in the prior art embodiments depicted in  FIGS. 2(   a ) and  2 ( b ), what was the short arm of the L-shaped MC metallization  23  now has been broken into additional separate MC regions  230   a ,  230   b  in  FIG. 6 . Then, CA contact regions of Copper or Tungsten (or alloys thereof) labeled “g” in  FIG. 6 , are formed at each MC region  230   a,b . As shown in  FIG. 7 , an M1 metallization layer  399  forming a bridge, connects each of the contact vias “g” so that cross-coupling on the true and complement sides are each propagated all the way up to M1 for symmetry purposes. This ensures no remnant charging exists in the finished wafer. 
   Referring back to  FIG. 6 , the PC conductor region  242  through P 1  has been extended horizontally as compared to the PC  52  depicted in prior art embodiment of  FIGS. 2(   a ) and  2 ( b ). This results in the ability to fabricate the second read port at the complement side (e.g.,  FIG. 5)  thereby improving the symmetry of the SRAM cell layout. As a result the second read port is separated enabling a data read at each of the true and complement sides. In one embodiment, an inverter device may be provisioned in order to read out data at the complement side. Thus, although asymmetry is eliminated in the modified design, there is a compromise with respect to efficiency since an additional inverter is required, as the readout becomes inverted (i.e., a read port on complement side and this needs to be converted to true). 
   Further as shown at locations labeled “f” in  FIG. 6 , it is the case that contact areas of MC landing on RX are equalized and maximized. That is, the contact asymmetry existing as depicted by a line  34  in  FIGS. 2(   a ) and  2 ( b ) have been eliminated thereby improving overall symmetry of the SRAM cell. 
   Furthermore, as depicted in  FIG. 6 , every formed MC region is contacted with CA-M1 (reduced charging during plasma processing). That is, M1 is provided for cross-coupling and ridding of any MC that do not have CA landing on top of them. Thus, all of MC regions will be subject to the plasma processing coming from the CA contact being open thus preventing any asymmetric charging for the devices that would result. 
     FIG. 8  depicts a schematic and layout capture of the D-cache SRAM cell design  500  according to an alternative embodiment of the invention. This D-cache SRAM cell design  500  comprises a subset of the improvements depicted in the embodiments depicted in the SRAM cell design  100  of  FIG. 6 . For example, the MC-PC distance at P 1  adjusted to 79 nm (same as P 0 , symmetrical, but not quite as much as in the embodiment depicted in  FIG. 6 ; the MC-RX landing pads have been enlarged and provide more symmetry, but not quite as much symmetry as in the embodiment depicted in  FIG. 6 . It is noted also that the direction of the extended MC-RX landing pads are in the opposite direction than the embodiment depicted in  FIG. 6 . Further, the MC local interconnect  23 ′ retains its L-shape structure as in the prior art structure, however, has the bend or jog that effectively spaces out the MC-PC distance, as indicated at the arrows, in accordance with the invention. 
   While this invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms described and illustrated, but fall within the scope of the appended claims.