Semiconductor memories

Abstract: In a stacked CMOS SRAM a power source line (25) connected to a load transistor (12, 13) of a flip-flop forming a memory cell extends above a boundary line between memory cells, and a power source line shunt (45) is formed by the same conductive layer as that of an earth line (33) or a gate electrode (12a, 13a) of the load transistor (12, 13).

Description: This invention relates to semiconductor memories and particularly to so-called stacked CMOS SRAMs in which a load transistor of a flip-flop forming a memory cell is formed of a semiconductor layer on a semiconductor substrate. In a resistance load MOS SRAM, which is the generally used form of MOS SRAM, it is difficult to maintain a high memory retention capacity with a low stand-by current. International Electron Devices Meeting (IEDM) Technical Digest, 11-14 December 1988, San Francisco, CA, US, pages 48-51 discloses a Poly-Si PMOS Load SRAM cell. A low stand-by current is achieved "using poly-Si PMOS loads and cross-coupled stacked capacitors formed between the NMOS and the stacked poly-Si PMOS". A full CMOS SRAM having a memory cell shown in Figure 1 has been proposed. Moreover, to reduce a chip area of the full CMOS SRAM to that of a resistance load MOS SRAM, there has been proposed a stacked CMOS SRAM wherein load PMOS transistors 12 and 13 of a flip-flop 11 forming a memory cell are formed by thin film transistors, and these thin film transistors are stacked on driver NMOS transistors 14 and 15 and transfer NMOS transistors 16 and 17 formed by bulk transistors (see, for example, "Nikkei Microdevice", September 1988, pages 123 to 130). Figure 2 shows an example of such a stacked CMOS SRAM. In this SRAM, impurity diffused regions 21a to 21g serving as source/drain regions of the driver NMOS transistors 14 and 15 and transfer NMOS transistors 16 and 17 are formed in a semiconductor substrate. Gate electrodes 14a to 17a of the transistors 14 to 17 are formed by a first poly-Si layer on an insulating film (not shown) on the semiconductor substrate. The gate electrodes 16a and 17a are parts of a word line 22. The gate electrode 14a is connected to the impurity diffused region 21d, and the gate electrode 15a is connected to the impurity diffused regions 21b and 21f. The gate electrodes 14a and 15a, the word line 22 and a surface of the semiconductor substrate are covered with an insulating interlayer (not shown). The gate electrodes 12a and 13a of the PMOS transistors 12 and 13 are formed by a second poly-Si layer on this insulating interlayer. As described above, the gate electrodes 12a and 13a are made of a poly-Si layer different from that of the gate electrodes 14a and 15a, and can have a length different from that of the gate electrodes 14a and 15a, as is apparent from Figure 2. The gate electrodes 12a and 13a are respectively connected to the gate electrodes 14a and 15a through contact holes 23 and 24 formed in the underlying insulating interlayer. The gate electrodes 12a and 13a are covered with a gate insulating film (not shown). On this gate insulating film, a power source line 25 and active layers 26 and 27 of the PMOS transistors 12 and 13 which are connected to the power source line 25 are made of a third poly-Si layer. Drain regions of the active layers 26 and 27 are respectively connected to the gate electrodes 15a and 12a through contact holes 31 and 32 formed in the underlying insulating film. The power source line 25 and the active layers 26 and 27 are covered with an insulating interlayer (not shown), and an earth line 33 made of a first Al layer is formed on this insulating interlayer. The earth line 33 is covered with an insulating interlayer (not shown). Bit lines 35 and 36 are made of a second Al layer on this insulating interlayer. The bit lines 35 and 36 are respectively connected to the impurity diffused regions 21g and 21e through contact holes 37 and 38 formed in the underlying insulating film. The impurity diffused regions 21g and 21e and the contact holes 37 and 38 are shared by two adjacent memory cells formed in a direction perpendicular to the word line 22, and are formed on the boundary line of these memory cells. As is apparent from the above description, in the stacked CMOS SRAM, it is most effective to form the active layers 26 and 27 of the PMOS transistors 12 and 13 and the power source line 25 by a single poly-Si layer for ease of fabrication. In such a stacked CMOS SRAM, it is very important to reduce OFF leakage currents of the PMOS transistors 12 and 13, to decrease current consumption and improve data retention characteristics. As a method of realizing this, a method of forming a thin poly-Si layer to form the active layers 26 and 27 of the PMOS transistors 12 and 13 is considered to be most promising. In the stacked CMOS SRAM, the power source line 25 is generally formed by the same poly-Si layer as that forming the active layers 26 and 27 of the PMOS transistors 12 and 13. When a thin poly-Si layer is formed as described above, the electrical resistance of the power source line 25 is increased, and high-speed operation and operational stability of the memory cells are disadvantageously degraded. On the other hand, when an additional conductive layer is formed to reduce the resistance of the power source line 25, the memory cell area is increased, and the fabrication process is complicated. According to the present invention there is provided a semiconductor memory of the stacked MOSFET SRAM type having a semiconductor substrate, memory cells each comprising a pair of bit lines, a word line, a flip-flop and a pair of transfer field-effect transistors, connected to respective ones of said bit lines and the gate electrodes of the transfer field-effect transistors forming parts of said wordline said flip-flop being formed by a pair of driver field-effect transistors, electrically connected to an earth line and a pair of load field-effect transistors, electrically connected to a power source line, wherein: a) gate electrodes of said driver transistors and said transfer transistors; b) said earth line; c) gate electrodes of said load transistors; and d) active layers of said load transistors and said power source line are formed of respective ones of four different conductive layers on the semiconductor substrate; said memory futher comprising a shunt of said power source line, said shunt being formed of the same conductive layer as that which forms said earth line or said gate electrodes of said load transistors. Although the power source line shunt is formed, a conductive layer corresponding to the shunt generally has a sufficient layout margin as compared with a conductive layer forming the gate electrodes of the driver and transfer transistors. The conductive layer corresponding to the power source line shunt serves as a conductive layer forming the earth line or the gate electrodes of the load transistors. The conductive layer corresponding to the power source line shunt is not an additional conductive layer. As described above, since the conductive layer corresponding to the power source line shunt has a sufficient layout margin, the memory cell area need not be increased. In addition, since the conductive layer corresponding to the power source line shunt is not an additional conductive layer, the fabrication process is not complicated. Also, a parasitic resistance of the power source line can be kept low even if the thickness of the conductive layer as the power source line is decreased. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is an equivalent circuit diagram of a full CMOS SRAM memory cell which can employ the present invention; Figure 2 is a plan view showing a previously proposed example; Figure 3 is a plan view of a memory cell given by way of background to the present invention; and Figures 4 and 5 are plan views showing embodiments of the present invention. Figure 3 shows a memory cell in which an earth line 33 is made of a second poly-Si layer, and conductive layers 41 and 42 extending from impurity diffused regions 21g and 21e to a word line 22 in a staggered manner are also made of the second poly-Si layer. Contact holes 37 and 38 are formed in an insulating film formed under the conductive layers 41 and 42. The conductive layers 41 and 42 are connected to the impurity diffused regions 21g and 21e through the contact holes 37 and 38, respectively. Gate electrodes 12a and 13a of PMOS transistors 12 and 13 are made of a third poly-Si layer, and a power source line 25 and active layers 26 and 27 of the PMOS transistors 12 and 13 are made of a fourth poly-Si layer. In the cell of Figure 3, unlike the cell shown in Figure 2, the power source line 25 does not extend above the word lines 22, but extends between the word lines 22, that is, above a boundary line between adjacent memory cells so as to share the impurity diffused regions 21g and 21e. Therefore, the power source line 25 is also shared by two adjacent memory cells. A contact hole 31 is formed in an insulating layer between a drain region of the active layer 26 and the gate electrode 13a. The drain region of the active layer 26 is connected to the gate electrode 13a through the contact hole 31. Bit lines 35 and 36 are made of an Al layer and are respectively connected to the conductive layers 41 and 42 through contact holes 43 and 44 formed in the underlying insulating interlayer above the word lines 22. Therefore, the bit lines 35 and 36 are connected to the impurity diffused regions 21g and 21e through the conductive layers 41 and 42, respectively. In the cell of Figure 3, since the power source line 25 extends above the boundary line between two adjacent memory cells, a sufficiently large gap S can be assured. Therefore, with a decrease in the gap S, the memory cell area can be reduced and the packing density of the memory can be increased. Figure 4 shows an embodiment of the present invention. The embodiment has substantially the same structure as that of the cell of Figure 3 except that a shunt 45 of a power source line 25 is formed by a third poly-Si layer which forms gate electrodes 12a and 13a of PMOS transistors 12 and 13, and that the power source line 25 is connected to the shunt 45 through contact holes 46 and 47 formed in an insulating interlayer formed under the power source line 25. A gate electrode 14a is connected to an impurity diffused region 21d through a contact hole 51 formed in an insulating film formed on a semiconductor substrate. A gate electrode 15a is connected to the impurity diffused regions 21b and 21f through contact holes 52 and 53. In the embodiment, as is apparent from Figure 4, the shunt 45 extends almost above a boundary line between two memory cells adjacent to a direction perpendicular to the word lines 22. For this reason, even if both the gate electrodes 12a and 13a of the PMOS transistors 12 and 13 and the shunt 45 are formed by the third poly-Si layer, the gate electrodes 12a and 13a are perfectly isolated from the shunt 45, and therefore the shunt 45 can serve as a shunt for the power source line 25. Therefore, even if the thickness of active layers 26 and 27 and the power source line 25 is decreased to about 10 nm in order to reduce OFF leakage currents of the PMOS transistors 12 and 13, and even if a sheet resistance of these layers and line becomes about 10 4 ohm/square, a composite sheet resistance of the power source line 25 and the shunt 45 can be reduced to 2 x 10 2 ohm/square under the condition that the thickness of the shunt 45 is set to be about 100 nm. Even if the shunt 45 and the power source line 25 are almost present on the boundary line between the two memory cells, no problem occurs in connections between bit lines 35 and 36 and impurity diffused regions 21g and 21e because the bit lines 35 and 36 are connected to the conductive layers 41 and 42. In the embodiment of Figure 4, the gate electrodes 12a and 13a of the PMOS transistors 12 and 13 and the shunt 45 are formed of a third poly-Si layer, and the power source line 25 and the active layers 26 and 27 are formed of a fourth poly-Si layer. However, these layers may be reversed. Figure 5 shows another embodiment. The embodiment of Figure 5 is substantially the same as the embodiment of Figure 4, except that a shunt 45 of a power source line 25 is formed of a second poly-Si layer which also forms an earth line 33, that the shunt 45 and the power source line 25 extend on word lines 22, and that a shunt 54 of the word line 22 and a shunt 55 of the earth line 33 are formed of a second Al layer overlying the bit lines 35 and 36. The bit lines 35 and 36 are directly connected to impurity diffused regions 21g and 21e through contact holes 37 and 38, respectively. The power source line 25, the word lines 22, the earth line 36 and the shunts 45, 54 and 55 thereof are connected to every other several memory cell in a region between memory cells. In the embodiment of Figure 5, since the shunt 45 is formed for the power source line 25, a decrease in thickness of the active layers 26 and 27 and the power source line 25 to decrease OFF leakage currents of the PMOS transistors 12 and 13 allows a low composite sheet resistance of the power source line 25 and the shunt 45. The shunts 54 and 55 of the word lines 22 and the earth lines 33 can be made of a refractory metal layer, and the shunts 54 and 55 can be formed below the bit lines 35 and 36. The shunt 45 of the power source line 25 may be exchanged with the shunt 54 of the word line 22.

Claims: A semiconductor memory of the stacked MOSFET SRAM type having a semiconductor substrate, memory cells each comprising a pair of bit lines (35, 36), a word line (22), a flip-flop and a pair of transfer field-effect transistors (16, 17), connected to respective ones of said bit lines and the gate electrodes of the transfer field-effect transistors forming parts of said wordline, said flip-flop being formed by a pair of driver field-effect transistors (14, 15), electrically connected to an earth line (33) and a pair of load field-effect transistors (12, 13), electrically connected to a power source line (25), wherein: a) gate electrodes (14a, 15a) of said driver transistors (14, 15)-and said transfer transistors (16, 17); b) said earth line (33); c) gate electrodes (12a, 13a) of said load transistors (12, 13); and d) active layers (26, 27) of said load transistors (12, 13) and said power source line (25) are formed of respective ones of four different conductive layers on the semiconductor substrate; said memory further comprising a shunt (45) of said power source line (25), said shunt (45) being formed of the same conductive layer as that which forms said earth line (33) or said gate electrodes (12a, 13a) of said load transistors (12, 13). A memory according to claim 1, wherein: contact portions of said transfer transistors (16, 17) electrically connected to bit lines (35, 36) are located on a boundary line defined between two adjacent memory cells; said shunt (45) is formed of the same conductive layer as that which forms said gate electrodes (12a, 13a) of said load transistors (12, 13); said power source line (25) and said shunt (45) extend above the boundary line therealong; and the same conductive layer as that which forms said earth line (33) is electrically connected to said contact portion, extends from said contact portion to overlie said gate electrodes of said transfer transistor (16, 17), and is connected to said bit lines (35, 36) on said gate electrodes. A memory according to claim 1, wherein: said conductive layer which forms said earth line (33) also forms said shunt (45); and said power source line (25) and said shunt (45) extend on said gate electrodes of said transfer transistors (16, 17).

Inventors: SASAKI MASAYOSHI, SASAKI, MASAYOSHI, SASAKI, MASAYOSHI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H01L27
IPC codes: H01L27