Static memory cell design

Abstract: A novel layout performing SRAM cell is disclosed wherein conductive straps (36) connect first and second driver gates (22,24) to second and first drains (33,31) respectively without connecting the moat of one cell with the moat of another cell such that the conductive straps are never in a DC current path.

Description: FIELD OF THE INVENTION This invention generally relates to a novel memory cell design, particularly to, a SRAM cell (static random access memory cell), and to a method of forming such a memory cell. BACKGROUND OF THE INVENTION Two types of memory cell, the resistive load cell or the stacked 4T-2R cell, and the CMOS (complementary metal oxide semiconductor) cell or stacked 6T (transistor) cell, have been widely used in the VLSI SRAMS. Schematics of a stacked 6 T cell and a stacked 4T-2R cell are shown in Figures 1 and 2 respectively. Both the stacked 6T cell and the stacked 4T-2R cell of Figures 1 and 2 comprise two pass transistors 30 and two driver transistors 31. Specific characteristics of these driver and pass transistors in addition to layout dimensions, such as those seen in Figure 3, can in part determine the performance of the SRAM cell. For example, the β ratio of a memory cell, which is one of the factors determining the stability of the cell, is a function of the width and length of the driver and pass transistor gates as shown below: β= W D W P . L P L D A high β ratio reduces the soft error rate thereby improving the stability of the SRAM cell. The soft error rate is a measure of how easily the data stored in the cell can be altered in the presence of ionizing radiation. In other words, the soft errors refer to an upset or change of state of the cell typically due to ionizing alpha particles. The length of the driver gate 22,24 in Figure 3 is determined by length L D and the patterned driver gate electrode and is usually a minimum for a given technology. The width of the pass transistor Wp is determined by the width of the patterened moat region and is also usually a minimum for a given technology. To maximise β, it is then desired to maximise either W D or Lp. However, Lp is constrained since the pass transistor must provide for adequate current carrying capabitity. Thus, a large value of driver moat width WD is desired. In Figure 4, a large value driver moat width WD can be achieved for a given cell size for which no intra-cell moat-to-moat limitation is apparent. This is unlike prior art in Figure 3 for which the width of the driver moat WD must be reduced to simultaneously maintain adequate moat-to-moat spacing 19. SRAM cells of conventional design, such as the cell shown in Figure 3, are described in US-A-4 408 385 and US-A-4 416 049, for example. Both documents disclose resistive load memory cells, wherein the resistor elements are made in polycrystalline silicon ion implanted to provide the desired resistivity which method is compatible with a self-aligned N-channel silicon-gate process. In the fabrication of high density SRAMs, for example, the one shown in Figure 3, there are always concerns regarding intra-cell moat-to-moat spacing 19 due to isolation requirements. Moat-to-moat spacing 19 may be critical to maintain leakage currents at a minimum and to reduce overall the amount of moat-to-moat interaction. Therefore, the driver moat width WD is limited to meet the spacing requirements, having an adverse effect on the β ratio. As cell area is reduced to, for example, 20µm2, the moat-to-moat spacing 19 is typically reduced to ≤0.6µm for conventional SRAM designs. SUMMARY OF THE INVENTION To alleviate strict moat-to-moat spacing 19, or n+-n+ minimum isolation width, requirements while maintaining a high β ratio, a novel SRAM cell is designed which comprises the features defined in claim 1. The method of the invention comprises the steps defined in claim 9. As such, high density SRAM cells can be fabricated with relaxed isolation technology. Another advantage of Applicant's invention is that the interconnects connecting a driver moat region, e.g. 21, to an opposite driver gate (e.g. 240 are not in a DC current path. Thus improving cell stability. Another advantage of Applicant's invention is the flexibility of SRAM cell layout provided by using two word lines per cell. Another advantage of Applicant's invention is the ease of fabrication of a complete 4T-2R SRAM cell, including resistors, requiring only two polysilicon levels. Other advantages of this structure include symmetry, reduced topology, high β ratio and formation of an interconnect which need not have extremely low contact or strap resistance. BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic diagram of a stacked 6T (transistor) SRAM cell; Figure 2 is a schematic diagram of a stacked 4T (transistor) -2R (resistor) SRAM cell; Figure 3 is a structure layout of a prior art SRAM cell; Figure 4 is a structure layout of a SRAM cell according to a preferred embodiment of Applicant's invention; Figure 5 is a structural layout of the next process step of the structure of Figure 4 which shows the formation of the conductive straps with which the driver gates are connected to nodes a and b; Figure 6 is a cross-sectional view of Figure 5 along the lines A-A', showing the conductive strap using one method of introducing an insulator; Figure 7 is a cross-sectional view of Figure 5 along the lines A-A', showing the conductive strap using a planarized insulation layer; Figure 8 is a structural layout of the next process step of the structure of Figure 5 if you are forming the SRAM cell in Figure 1. Figure 9 is a structural layout of another prior art conventional cell; Figure 10 is a structural layout an alternative process step of the structure Figure 5 which includes the resistors in the same material as the conductive straps. Figure 11 is a structural layout of another alternative process step of the structure of Figure 5 which forms the gate contacts beyond the moat. Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A preferred embodiment of Applicants invention is shown in Figure 4. The process flow is summarised in Figure 7. An SRAM continuous moat cell is shown generally at 10. The field oxide 20 is formed using a LOCOS process, thereby defining the moat 21. A gate insulator 23 is then formed. next, the polysilicon gates 22,24 are deposited, patterned and etched and implantation of n+, using the poly Si gates as a mask, forms lightly doped drain and source regions or LDDs 25. Sidewall oxide spacers 27 are next formed on either side of the poly Si gates 22,24 to mask the LDD regions 25 from the heavy implantations of n+ in the source 29 and drain 31,33 regions as the source and drain regions are formed. The two source regions 29 adjacent to the driver gates 22,24 are connected together and later will be connected to ground through a contact. The two word lines 28 can be deposited, patterned and etched in the same process step that the poly Si gates 22,24 are deposited, patterned and etched. The bit line and bit line bar are often formed of metal later in the formation of the SRAM cell. Although polysilicon is the conductive material used in describing the process flow of the device, it is well known in the art that other appropriate conductive materials could be used in the alternative. All of the above processing steps describe the formation of the pass and driver transistors as well as the word lines of Figure 1 or 2, without the connections between them. Therefore, Figure 4, although it only shows the moat 21, the poly Si driver gates 22,24 and the word lines 28, a source 29, drain 31,33 and LDD regions 25 shown in Figure 7 have been formed and exist by this step in the flow. Now, according to Figure 2, the driver gates 22,24 must be connected to the desired nodes a and b. Figure 5 illustrates the straps, 36 through which a contact hole connects driver gate 22 to node point b and driver gate 24 to node point a. The cross sectional view of strap 36 is shown in Figure 6. An insulator 40 is deposited, masked and etched to allow access to the gate 24 and the moat 21 at the node region a. Then the contacts through the insulating layer will be formed. If the insulator 40 is planarized prior to etching of contacts through the insulator, a two mask process can be used to form a contact hole to region 31 and separately, a contact hole to region 24. Another alternative method is the formation of the gate contact beyond the moat region 21 as shown in Figure 11. This removes the potential for inadvertently connecting one moat to another and also shorting the gate to both moats. Finally, a material layer is deposited, masked and etched and to form the interconnect strap 36 which connects the driver gate 24 to moat node point a. A second cross sectional view of the interconnect strap 36 and the contact holes within that strap is shown in Figure 7 using a planarized insulator. If a stacked 6T cell is formed as shown in Figure 1, the next step in the process flow is the formation of the PMOS gates 35. Figure 8 will show this step from the strap up. Another layer of insulator is deposited, patterned and etched. Then, again the contact hole through another insulator connects the PMOS gate 35 to an underlying strap 36. Finally, the PMOS gate material 50 is deposited, patterned and etched to overlay a small area of one interconnect strap and a larger area of the second strap as shown in Figure 8. The filled in squares represent the contact hole through the last applied insulator which connects the PMOS gates 35 to the underlying interconnect straps 22,24. In a second embodiment, at the same process step that the interconnect straps are formed, the resistors of Figure 2 could also be formed as shown in Figure 10. Instead of patterning and etching for the interconnect straps alone, extend the polysilicon beyond one end of the moat to the very edge of the cell. On the other end of the moat, a stretch of polysilicon, which overlies the word line 28, can be deposited, patterned and etched simultaneously with the interconnect straps to form a power bus line, with polysilicon runners lying in line with the stretch of polysilicon extended out towards the other end of the cell, as shown in Figure 10. The polysilicon which overlies the word line and the interconnect straps can then be heavily doped via implantation, as represented with dark cross-hatching, for maximum conductivity. In addition, the lightly cross-hatched areas are lightly doped to provide adequate resistance. In this way, the second level of polysilicon serves as the interconnect as well as the resistor load. This layout of SRAM cell is able to be arrayed in the vertical or the lateral direction. This cell is compatible with the needs for an ASIC SRAM cell of approximately 40µm 2 . Additionally, this cell allows for much reduced complexity in regards to other cell designs. One advantage of a preferred embodiment of Applicant's invention is that the interconnect straps formed never connect one moat to any other moat and thus the straps are never in a DC current path. For example, Figure 9 is the layout of a conventional cell. The moat regions 21 are shown as clear areas in the figure. The driver gates 22,24 and pass gates 28 are designated by the lightly cross-hatched areas and the heavily cross-hatched areas are the interconnect straps 36. As is evident from the layout, the path from bit line to ground must traverse two interconnects which connect two different moats. Keeping these interconnect straps from being in DC current paths static noise margin and improves cell symmetry and helps inverse the cell stability. A few preferred embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims.

Claims: An SRAM cell comprising: a moat area (21) formed in a substrate, said moat (21) including first and second drains (31, 33) located within said moat area (21) and a source (29) located between and separated from said first and second drains (31, 33); a first elongated driver gate (22) located between said first drain (31) and said source (29) and a second elongated driver gate (24) parallel to said first elongated driver gate (22) and located between said second drain (33) and said source (29); a first word line (28) outside of said moat area (21) and extending parallel to said first elongated driver gate (22), and a second word line (28) outside of said moat area (21) extending parallel to said first word line (28) and wherein said first and second driver gates (22, 24) are located between said first word line (28) and said second word line (28); a first conductive strap (36) connecting said first driver gate (22) to a first selected region of said moat area (21) and a second conductive strap (36) connecting said second driver gate (24) to a second selected region of said moat area (21); and an insulator (40) separating said first and second conductive straps (36) from said first and second driver gates (22, 24) and said first and second selected regions. The SRAM cell of claim 1 and further including a first set of contacts connecting said first driver gate (22) to said first selected region of said moat area (21) via said first conductive strap (36), and a second set of contacts connecting said second driver gate (24) to said second selected region of said moat area (21) via said second conductive strap (36), wherein both sets of contacts lie within said moat area (21). The SRAM cell of claim 1 or claim 2, wherein said first selected region is said second drain (33) and said first conductive strap (36) connects said first driver gate (22) to said second drain (33). The SRAM cell of any preceding claim, wherein said second selected region is said first drain (31) and said second conductive strap (36) connects said second driver gate (24) to said first drain (31). The SRAM cell of any preceding claim and further including an insulating layer covering said first and second conductive straps and first and second PMOS gates (35) formed over said insulating layer, said first PMOS gate (35) in electrical contact with said first conductive strap (36) and said second PMOS gate (35) in electrical contact with said second conductive strap (36) by means of apertures defined in said insulating layer, said first PMOS gate (35) overlying a portion of said second conductive strap (36) and said second PMOS gate (35) overlying a portion of said first conductive strap (36). The SRAM cell of any of claims 1 to 4, wherein said first and second conductive straps (36) are polysilicon straps and said straps overlying said moat area (21) are heavily doped, such that said straps are conductive. The SRAM cell of claim 6 and further including a conductive power bus line overlying at least one of said word lines. The SRAM cell of claim 7, wherein said polysilicon straps (36) extend beyond said moat area (21) and cross over said second word line so as to contact said power bus line, and said straps beyond said moat area (21) are not or only lightly doped, such that said straps are resistive. A method of forming an SRAM cell comprising the steps of: defining a moat area (21) in a substrate having one region and another region, at opposite ends of said moat (21); depositing an oxide layer (23) to at least cover selected portions of said moat area (21); depositing, patterning and etching of polysilicon to form a first and second word line (28) outside of said moat area (21), and over said selected portion to form a first and second gate (22, 24) to define drain and source regions, said first and second gates (22, 24) located between said first word line (28) and said second word line (28); doping said source and drain regions (29, 31, 33) with a dopant of a first conductivity type; depositing, patterning, etching and planarizing an insulator material (40) defining a first hole extending between said one region to said first gate (22), and a second hole extending between said another region to said second gate (24); and depositing, patterning and etching a material layer to form a first interconnect strap (36) connecting said one region to said first gate (22) and a second interconnect strap (36) connecting said another region to said second gate (24).

Inventors: RODDER MARK S, RODDER, MARK S.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: H01L27
IPC codes: H01L27, H01L21