Abstract:
In a static random access memory cell including two cross-coupled drive MOS transistors and two transfer MOS transistors connected to the drive MOS transistors, a plurality of gate electrodes of the drive MOS transistors and the transfer MOS transistors are formed over a semiconductor substrate, and a plurality of source/drain impurity diffusion regions of the transistors are formed within the semiconductor substrate. A plurality of pocket regions of the same conductivity type as the semiconductor substrate are formed within the semiconductor substrate. Each of the pocket regions is adjacent to the source of one of the drive MOS transistors and beneath the gate electrode thereof. The impurity concentration of the pocket regions is larger than that of the semiconductor substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device, and more particularly, a static random access memory (SRAM) cell. 
     2. Description of the Related Art 
     A prior art SRAM cell is constructed by a flip-flop formed by cross-coupled first and second inverters and transfer transistors connected to first and second nodes of the flip-flop. That is, the first inverter is formed by a first resistance element between a high power supply line and the first node and a first drive MOS transistor between the first node and a ground line. Similarly, the second inverter is formed by a second resistance element between the high power supply line and the second node and a second drive MOS transistor between the second node and the ground line. 
     Also, the first node is connected directly to a gate electrode of the second drive transistor, so that the second drive transistor is driven directly by the voltage at the first node. Similarly, the second node is connected directly to a gate electrode of the first drive transistor, so that the first drive transistor is driven directly by the voltage at the second node. 
     In the SRAM cell, the lower the threshold voltage of the transfer transistors, the higher the substantial write voltage. On the other hand, the higher the threshold voltage of the drive transistors, the higher the retention characteristics of the flop-flop against noise. In order to satisfy this requirement, the impurity concentration of a semiconductor substrate (well) of the drive transistors is higher than that of the substrate (well) of the transfer transistors. 
     The threshold voltage of the drive transistors is higher than the threshold voltage of the transfer transistors. 
     Additionally, in order to avoid the destruction of data during a write mode, the gate width of the drive transistors is increased while the gate length of the drive transistors is decreased. 
     In the above-mentioned prior art SRAM cell, however, since the threshold voltage of the drive transistors is set in a range where the threshold voltage greatly depends upon the gate length thereof, the set threshold voltage of the drive transistors greatly fluctuates. This will be explained later in detail. 
     Note that, in order to suppress the fluctuation of the threshold voltage of the drive transistors, the threshold voltage can be set to be a larger value; however, in this case, the gate length of the transfer transistors has to be larger, which degrades the integration. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to suppress the fluctuation of the threshold voltage of drive transistors in an SRAM cell without degrading the integration. 
     According to the present invention, in a static random access memory cell including two cross-coupled drive MOS transistors and two transfer MOS transistors connected to the drive MOS transistors, a plurality of gate electrodes of the drive MOS transistors and the transfer MOS transistors are formed over a semiconductor substrate, and a plurality of source/drain impurity diffusion regions of the transistors are formed within the semiconductor substrate. A plurality of pocket regions of the same conductivity type as the semiconductor substrate are formed within the semiconductor substrate. Each of the pocket regions is adjacent to the source of one of the drive MOS transistors and beneath the gate electrode thereof. The impurity concentration of the pocket regions is larger than that of the semiconductor substrate. 
     Thus, the short channel effect of the drive transistors is suppressed by the pocket regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein: 
     FIG. 1 is an equivalent circuit diagram illustrating a prior art SRAM cell; 
     FIG. 2 is a plan view of the SRAM cell of FIG. 1; 
     FIGS. 3A,  3 B and  3 C are cross-sectional views for explaining a method for manufacturing the SRAM cell of FIG. 2; 
     FIG. 4 is a graph showing the threshold voltage characteristics of the transistors of FIG. 2; 
     FIGS. 5A,  5 B and  5 C are cross-sectional views for explaining a first embodiment of the method for manufacturing the SRAM cell according to the present invention; 
     FIGS. 6A,  6 B and  6 C are cross-sectional views for explaining a second embodiment of the method for manufacturing the SRAM cell according to the present invention; 
     FIGS. 7A,  7 B and  7 C are cross-sectional views for explaining a third embodiment of the method for manufacturing the SRAM cell according to the present invention; 
     FIG. 8 is a graph showing the threshold voltage characteristics of the transistors according to the present invention; and 
     FIGS. 9A and 9B are equivalent circuit diagrams of SRAM cells to which the present invention is applied. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art SRAM cell will be explained with reference to FIGS. 1,  2 ,  3 A,  3 B,  3 C and  4 . 
     In FIG. 1, which is an equivalent circuit diagram illustrating the SRAM cell, one SRAM cell is provided at each intersection between word lines WL 1  , and WL 2  and complementary bit lines BL and {overscore (BL)}. This cell is constructed by a flip-flop formed by two cross-coupled inverters and transfer N-channel MOS transistors Q t   1  and Q t   2  connected between nodes N 1  and N 2  of the flip-flop and the bit lines BL and {overscore (BL)}. 
     The transfer transistors Q t   1  and Q t   2  are controlled by the voltages at the word line WL 1  and WL 2.  Note that the voltage at the word line WL 1  is the same as the voltage at the word line WL 2 . 
     Each of the inverters includes a load resistor R 1  (R 2 ) and a drive N-channel MOS transistor Q d   1  (Q d   2 ) between a power supply line V cc  and a ground line GND. The node N 1  is connected to the gate of the transistor Q d   2 , so that the transistor Q d   2  is driven by the voltage at the node N 1 . Also, the node N 2  is connected to the gate of the transistor Q d   1 , so that the transistor Q d   1  is driven by the voltage at the node N 2 . 
     The structure of the SRAM cell of FIG. 1 is explained next with reference to FIG.  2  and FIGS. 3A,  3 B and  3 C. Note that FIG. 2 is a plan view of two-bit SRAM cells, and FIGS. 3A,  3 B and  3 C are cross-sectional views taken along the lines I—I and II—II of FIG.  2 . 
     First, referring to FIGS. 2 and 3A, a photoresist pattern  102  is formed by a photolithography process on only an area of a P-type monocrystalline silicon substrate  101  corresponding to the gate electrodes of the transfer transistors Q t   1  and Q t   2  and their periphery. Then, boron ions are implanted into the substrate  101  by using the photoresist pattern  102  as a mask. As a result, a P-type impurity diffusion region  103  is formed within the substrate  101 . Then, the photoresist pattern  102  is removed. 
     Next, referring to FIGS. 2 and 3B, the substrate  101  is thermally oxidized to form a gate silicon oxide layer  104  on the substrate  101 . Then, gate electrodes  105   a  and  105   b  are formed on the gate silicon oxide layer  104 . Then, the gate silicon oxide layer  104  is etched out in self-alignment with the gate electrodes  105   a  and  105   b.  Note that the length L 1  of the gate electrodes  105   a  of the transfer transistors Q t   1  and Q t   2  is larger than the length L 2  of the gate electrodes  105   b  of the drive transistors Q d   1  and Q d   2 . Then, phosphorous or arsenic ions are implanted into the substrate  101  by using the gate electrodes  105   a  and  105   b  as a mask to form N-type impurity diffusion regions  106  of a lightly-doped drain (LDD) configuration. 
     Finally, referring to FIGS. 2 and 3C, a silicon nitride layer is formed on the entire surface, and an anisotropic etching operation is performed upon the silion nitride layer to form a protection layer  107  on the sidewalls and upper surfaces of the gate electrodes  105   a  and  105   b.  Then, arsenic ions are implanted into the substrate  101  by using the protection layer  107  as a mask to form N+-type impurity diffusion regions  108  of the LDD configuration. Note that, the lower the threshold voltage V th   1 (L) of the transfer transistors Q t   1  and Q t   2 , the higher the substantial write voltage. On the other hand, the higher the threshold voltage V th   2  (L) of the drive transistors Q d   1  and Q d   2 , the higher the retention characteristics of the flop-flop against noise. Therefore, the impurity concentration of the substrate  101  (the P-type type impurity diffusion region  103 ) of the drive transistors Q d   1  and Q d   2  is higher than that of the substrate  101  of the transfer transistors Q t   1  and Q t   2 . As a result, as shown in FIG. 4, which shows the threshold voltage characteristics of the transfer transistors Q t   1  and Q t   2  and the drive transistors Q d   1  and Q d   2 , the threshold voltage V th   2  (L) of the drive transistors Q d   1  and Q d   2  is higher than the threshold voltage V th   1  (L) of the transfer transistors Q t   1  and Q t   2 . 
     Additionally, in order to avoid the destruction of data during a write mode, the gate width W of the drive transistors Q d   1  and Q d   2  is increased while the gate length L of the drive transistors Q d   1  and Q d   2  is decreased. 
     In order to satisfy the above-mentioned conditions, the threshold voltage V th   1  (L) of the transfer transistors Q t   1  and Q t   2  is set to be a value V th   1  (L 1 ) as indicated by P 1  in FIG. 4, and the threshold voltage V th   2  (L) of the drive transistors Q d   1  and Q d   2  is set to be a value V th   2  (L 2 ) as indicated by P 2  in FIG.  4 . 
     In FIG. 4, however, since the threshold voltage V th   2  (L) of the drive transistors Q d   1  and Q d   2  is set in a range where the threshold voltage V th   2  (L) greatly depends upon the gate length L thereof, the set threshold voltage V th   2  (L 2 ) of the drive transistors Q d   1  and Q d   2  greatly fluctuates. 
     In order to suppress the fluctuation of the threshold voltage V th   2  (L) of the drive transistors Q d   1  and Q d   2 , the threshold voltage V th   2  (L) can be set to be a value V th   2  (L 2 ′) as indicated by P 2 ′ in FIG. 4; however, in this case, the gate length L of the transfer transistors Q t   1  and Q t   2  has to be larger (L&gt;L 1 ), which degrades the integration. 
     FIGS. 5A,  5 B and  5 C are cross-sectional views for explaining a first embodiment of the method for manufacturing an SRAM cell according to the present invention. Note that an equivalent circuit and a plan view of this SRAM cell are also illustrated in FIGS. 1 and 2. 
     First, referring to FIG. 5A, a P-type monocrystalline silicon substrate  1  having an impurity concentration of about 1×10 16  to 1×10 18  cm 3  is thermally oxidized to form a gate silicon oxide layer  2  on the substrate  1 . Then, gate electrodes  3   a  and  3   b  are formed on the gate silicon oxide layer  3 . Then, the gate silicon oxide layer  2  is etched out in self-alignment with the gate electrodes  3   a  and  3   b.  Note that the length L 1  of the gate electrodes  3   a  of the transfer transistors Q t   1  and Q t   2  is larger than the length L 2  of the gate electrodes  3   b  of the drive transistors Q d   1  and Q d   2 . Then, a photoresist pattern  4  having an opening corresponding to the sources of the drive transistors Q d   1  and Q d   2  is formed by a photolithography process. Then, about 1×10 13  to 1×10 14  boron ions per cm 2  are implanted at an energy of about 5 to 30 keV and at an incident angle of 0′ to about 60′ into the substrate  1  by using the photoresist pattern  4  as a mask. As a result, P-type impurity diffusion regions  5  are formed within the substrate  1 . Then, the photoresist pattern  4  is removed. 
     Next, referring to FIG. 5B, about 1×10 13  to 1×10 14  phosphorous or arsenic ions per cm 2  are implanted at an energy of about 5 to 30 keV into the substrate  1  by using the gate electrodes  3   a  and  3   b  as a mask to form N-type impurity diffusion regions  6  of an LDD configuration. 
     Finally, referring to FIG. 5C, a silicon nitride layer is formed on the entire surface, and an anisotropic etching operation is performed upon the silion nitride layer to form a sidewall silicon nitride layer  7  on the sidewalls of the gate electrodes  3   a  and  3   b  . Then, about 1×10 15  to 1×10 16  arsenic ions per cm 2  are implanted at an energy of about 10 keV to 100 keV into the substrate  1  by using the sidewall silicon nitride layer  7  as a mask to form N+-type impurity diffusion regions  8  of the LDD configuration. 
     Thus, in the first embodiment, the N-type impurity diffusion regions  6  having an impurity concentration of about 1×10 19  cm −3  and the N+-type impurity diffusion regions  8  having an impurity concentration of about 2×10 20  cm −3  form the sources/drains of the transistors, and the P-type impurity diffusion regions  5  having an impurity concentration of about 1×10  18  cm −3  are formed as P-type pocket regions adjacent to only the sources of the drive transistors Q d   1  and Q d   2  beneath the gate electrodes  3   a  and  3   b  thereof. 
     FIGS. 6A,  6 B and  6 C are cross-sectional views for explaining a second embodiment of the method for manufacturing an SRAM cell according to the present invention. Note that an equivalent circuit and a plan view of this SRAM cell are also illustrated in FIGS. 1 and 2. 
     First, referring to FIG. 6A, a P-type monocrystalline silicon substrate  1  having an impurity concentration of about 1×10 16  to 1×10 18  cm 3  is thermally oxidized to form a gate silicon oxide layer  2  on the substrate  1 . Then, gate electrodes  3   a  and  3   b  are formed on the gate silicon oxide layer  3 . Then, the gate silicon oxide layer  2  is etched out in self-alignment with the gate electrodes  3   a  and  3   b.  Note that the length L 1  of the gate electrodes  3   a  of the transfer transistors Q t   1  and Q t   2  is larger than the length L 2  of the gate electrodes  3   b  of the drive transistors Q d   1  and Q d   2 . Then, about 1×10 13  to 1×10 14  phosphorous or arsenic ions per cm 2  are implanted at an energy of about 5 to 30 keV into the substrate  1  by using the gate electrodes  3   a  and  3   b  as a mask to form N-type impurity diffusion regions  6  of an LDD configuration. 
     Next, referring to FIG. 6B, a photoresist pattern  4 ′ having an opening corresponding to the sources of the drive transistors Q d   1  and Q d   2  is formed by a photolithography process. Then, about 1×10 13  to 1×10 14  boron fluoride ions per cm 2  are implanted at an energy of about 20 to 120 keV and at an incident angle of 0° to about 60° into the substrate  1  by using the photoresist pattern  4 ′ as a mask. As a result, P-type impurity diffusion regions  5  are formed within the substrate  1 . 
     Then, the photoresist pattern  4 ′ is removed. 
     Finally, referring to FIG. 6C, in the same way as in FIG. 5C, a silicon nitride layer is formed on the entire surface, and an anisotropic etching operation is performed upon the silion nitride layer to form a sidewall silicon nitride layer  7  on the sidewalls of the gate electrodes  3   a  and  3   b.  Then, about 1×10 15  to 1×10 16  arsenic ions per cm 2  are implanted at an energy of about 10 keV to 100 keV into the substrate  1  by using the sidewall silicon nitride layer  7  as a mask to form N+-type impurity diffusion regions  8  of the LDD configuration. 
     Thus, even in the second embodiment, the N−-type impurity diffusion regions  6  having an impurity concentration of about 1×10 19  cm −3  and the N+-type impurity diffusion regions  8  having an impurity concentration of about 2×10 20 cm −3  form the sources/drains of the transistors, and the P-type impurity diffusion regions  5  having an impurity concentration of about 1×10 18  cm −3  are formed as P-type pocket regions adjacent to only the sources of the drive transistors Q d   1  and Q d   2  beneath the gate electrodes  3   a  and  3   b  thereof. 
     FIGS. 7A,  7 B and  7 C are cross-sectional views for explaining a third embodiment of the method for manufacturing an SRAM cell according to the present invention. Note that an equivalent circuit and a plan view of this SRAM cell are also illustrated in FIGS. 1 and 2. 
     First, referring to FIG. 7A in the same way as in FIG. 5A, a P-type monocrystalline silicon substrate  1  having an impurity concentration of about 1×10 16  to 1×10 18  cm 3  is thermally oxidized to form a gate silicon oxide layer  2  on the substrate  1 . Then, gate electrodes  3   a  and  3   b  are formed on the gate silicon oxide layer  3 . Then, the gate silicon oxide layer  2  is etched out in self-alignment with the gate electrodes  3   a  and  3   b.  Note that the length L 1  of the gate electrodes  3   a  of the transfer transistors Q t   1  and Q t   2  is larger than the length L 2  of the gate electrodes  3   b  of the drive transistors Q d   1  and Q d   2 . Then, about 1×10 13  to 1×10 14  phosphorous or arsenic ions are implanted at an energy of about 5 to 30 kev into the substrate  1  by using the gate electrodes  3   a  and  3   b  as a mask to form N-type impurity diffusion regions  6  of an LDD configuration. 
     Next, referring to FIG. 7B, a silicon nitride layer is formed on the entire surface, and an anisotropic etching operation is performed upon the silion nitride layer to form a protection layer  7 ′ on the sidewalls of the gate electrodes  3   a  and  3   b.  Then, about 1×10 15  to 1×10 16  arsenic ions per cm 2  are implanted at an energy of about 10 kev to 100 keV into the substrate  1  by using the protection layer  7 ′ as a mask to form N+-type impurity diffusion regions  8  of the LDD configuration. 
     Finally, referring to FIG. 7C, a silicon oxide layer  4 A is deposited on the entire surface. Then, a photoresist pattern  4 B having an opening corresponding to the sources of the drive transistors Q d   1  and Q d   2  is formed by a photolithography process. Then, the silicon oxide layer  4 A is etched out by using the photoresist pattern  4 B as a mask. Then, about 1×10 13  to 1×10 14  boron ions per cm 2  are implanted at an energy of about 20 to 150 keV and at an incident angle of about 15° to 60° by using the photoresist pattern  4 B and the silicon oxide layer  4 A. As a result, P-type impurity diffusion regions  5  are formed within the substrate  1 . Then, the photoresist pattern  4 B is removed. 
     In FIG. 7C, boron fluoride ions instead of boron ions can be implanted into the substrate  1 . In this case, the implanting conditions are a doze amount of about 1×10 13  to 1×10 14  ions per cm 2  at an energy of about 80 to 400 kev and at an incident angle of about 15° to 60° . Thus, even in the third embodiment, the N-type impurity diffusion regions  6  having an impurity concentration of about 1×10 19  cm −3  and the N-type impurity diffusion regions  8  having an impurity concentration of about 2×10 20  cm −3  form the sources/drains of the transistors, and the P-type impurity diffusion regions  5  having an impurity concentration of about 1×10 18  cm −3  are formed as P-type pocket regions adjacent to only the sources of the drive transistors Q d   1  and Q d   2  beneath the gate electrodes  3   a  and  3   b  thereof. 
     In the above-described embodiments, since the P-type pocket regions (P-type impurity diffusion regions)  5  are provided on only the sides of the sources of the drive transistors Q d   1  and Q d   2 , the short channel effect of the drive transistors Q d   1  and Q d   2  is suppressed. As a result, as shown in FIG. 8, which shows the threshold voltage characteristics of the transfer transistors Q t   1  and Q t   2  and the drive transistors Q d   1  and Q d   2 , the threshold voltage,V th   2 ′ (L) of the drive transistors Q d   1  and Q d   2  is higher than the threshold voltage V th   2  (L) of the transfer transistors Q d   1  and Q d   2 . In this case, the flat range of the threshold voltage V th   2 ′ (L) of the drive transistors Q d   1  and Q d   2  is larger than that of the threshold voltage V th   1  (L) of the transfer transistors Q t   1  and Q t   2 . 
     Additionally, in order to increase the gate width W of the drive transistors Q d   1  and Q d   2  and decrease the gate length L of the drive transistors Q d   1  and Q d   2 , the threshold voltage V th   1  (L) of the transfer transistors Q t   1  and Q t   2  is set to be a value V th   1  (L 1 ) as indicated by P 1  in FIG. 8, and the threshold voltage V th   2 ′ (L) of the drive transistors Q d   1  and Q d   2  is set to be a value V th   2 ′ (L 2 ) as indicated by P 2  in FIG.  8 . That is, in FIG. 8, since the threshold voltage V th   2 ′ (L) of the drive transistors Q d   1  and Q d   2  is set in a range where the threshold voltage V th   2 ′ (L) hardly depends upon the gate length L thereof, the set threshold voltage V th   2 ′ (L 2 ) of the drive transistors Q d   1  and Q d   2  hardly fluctuates. 
     The present invention can be applied to other SRAM cells such as a thin film transistor (TFT) load type SRAM as illustrated in FIG. 9A where P-channel TFTs Q p   1  and Q p   2  are provided as loads, and a non-load type SRAM cell as illustrated in FIG.  9 B. 
     As explained hereinabove, according to the present invention, the fluctuation of the threshold voltage of drive transistors in an SRAM can be suppressed without degrading the integration.