Abstract:
A Static Random Access Memory (SRAM) device includes at least a transfer transistor, a driving transistor and a load resistor which are commonly connected to a node. A well has a first conductive type, and is placed on a substrate. A first impurity region has a second conductive type opposite to the first conductive type, and is placed in the well. A second impurity region has the first conductive type and has higher impurity concentration than the well, and is placed at a lower portion of the first impurity region. The node is composed of at least the first impurity region and the second impurity region.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a semiconductor device such as SRAM (Static Random Access Memory), and a method of manufacturing the same. 
     Conventionally, a soft error (hereinafter, may be abbreviated as SER) caused by an α-ray often takes place with high-integration of a SRAM device in a semiconductor device of the type as described. 
     More specifically, when a memory cell is reduced in size in order to highly integrate the SRAM device, a current per a unit memory cell is reduced. On the other hand, the α-ray emitted from natural uranium or the like is irradiated into a semiconductor memory device. 
     Herein, it is noted that the natural uranium is slightly contained in a ceramic package or a cover for sealing the semiconductor memory device. 
     Thereby, a large number of electron-hole pairs are generated in a substrate. Consequently, the electron being generated moves in the substrate and destroys information (namely, electric charge) stored in the memory cell. This results to an error operation of the semiconductor memory cell. 
     Referring to FIG. 1, description will be made about a basic structure of a high-resistance load type memory cell serving as a main part of the related SRAM device. 
     The SRAM device includes a pair of transfer transistors ST 1  and ST 2 , a pair of driving transistors DT 1  and DT 2 , and a pair of load resistors L 1  and L 2 . 
     In the transfer transistor ST 1 , one terminal (source or drain) is connected to a bit line BL 1  while the other terminal (source or drain) is connected to a node N 1 . Further, the gate electrode terminal is connected to a word line WL 1 . 
     In the transfer transistor ST 2 , one terminal (source or drain) is connected to a bit line BL 2  while the other terminal (source or drain) is connected to a node N 2 . Further, the gate electrode terminal is connected to a word line WL 2 . 
     In the driving transistor DT 1 , one terminal (source or drain) is connected to a reference voltage Vss while the other terminal (source or drain) is connected to the node N 1 . Further, the gate electrode terminal is connected to the node N 2 . 
     In the driving transistor DT 2 , one terminal (source or drain) is connected to a reference voltage Vss while the other terminal (source or drain) is connected to the node N 2 . Further, the gate electrode terminal is connected to the node N 1 . 
     In the load resistor L 1 , one terminal is connected to a power supply voltage Vcc while the other terminal is connected to the node N 1 . 
     In the load resistor L 2 , one terminal is connected to the power supply voltage Vcc while the other terminal is connected to the node N 2 . 
     Further, a capacitor C 1  is coupled to the node N 1  while a capacitor C 2  is coupled to the node N 2 . 
     For example, a NMOS may be used as each of the transfer transistors ST 1 , ST 2 , and the driving transistors DT 1 , DT 2 . 
     Subsequently description will be made about a SER resistance in such high-resistance load type memory cell. 
     In the case of the resistance load type memory cell, the SER resistance is generally determined in dependence upon a current IL flowing through the load resistor L 1 , L 2  and node capacitance C 1 , C 2 . 
     When the node N 1  is put into a high state and a voltage is equal to V 1 h in the memory cell, the current IL flowing through the load resistor L 1  and the node capacitance C 1  has the following relationship with the SER resistance. 
     Namely, in the case where the bit line BL 1  is put into the power supply voltage Vcc, when the transfer transistor ST 1  is turned on, the voltage V 1 h of the node N 1  is reduced with about a threshold voltage Vt of the transfer transistor ST 1  from the power supply voltage Vcc to become Vcc−Vt. 
     Under this circumstance, if the current sufficiently flows through the load resistor L 1  from the power supply voltage Vcc, the voltage V 1 h is more increased to the power supply voltage Vcc. 
     In such a memory cell, when the transfer transistor ST 1  is turned on and the voltage V 1 h is reduced from the power supply voltage Vcc to Vcc−Vt, the probability of the occurrence of the decrease in the voltage V 1 h, in which the voltage is reduced from the power supply voltage Vcc to the Vc−Vt, may be lowered as the node capacitance C 1  becomes higher. 
     In addition, such a time that the voltage V 1 h further restores to the power supply voltage Vcc by the power supply voltage Vcc of the power supply becomes rapider, as the current IL flowing through the load resistor L 1  is higher and as the node capacitance C 1  is higher. 
     Hereinafter, description will be made about a method of manufacturing the high resistance load memory cell with reference to FIGS. 2 through 7. 
     Herein, only a region around the node N 1  of the memory cell in FIG. 1 is illustrated in FIGS. 2 through 7, and the illustration of the peripheral circuit portion is omitted. 
     Referring to FIG. 2, a thick device isolation silicon oxide film  2  is formed to a thickness of 400 nm by the use of Local Oxidation of Silicon (LOCOS) method on a principal surface of a silicon substrate  1 . 
     Thereafter, only region for forming a memory cell region, a transfer transistor, and a driving transistor (namely, NMOS) are opened by the use of the photolithography technique. 
     Subsequently, impurity (boron) is implanted so as to form a P-type well region  21  by the ion-implanting technique. 
     In this event, the ions are implanted within a concentration range between 1×10 13  and 2×10 13  [cm −2 ] and within an accelerating voltage range between 250 and 350 [Kev]. 
     Although not illustrated, the ions are implanted to form the device isolation region at the same time, and a P-type impurity region is formed under the device isolation silicon oxide film  2 . Further, the ions are also implanted so as to control the voltage Vt. 
     Thereafter, the silicon substrate  1  is thermally oxidized to form a gate silicon oxide film  3  to a thickness of about 8 nm. Successively, a polysilicon film is deposited to a thickness of about 100 nm of the gate silicon oxide film  3  by the use of CVD technique. 
     Subsequently, compound (namely, silicide) between Ti or W serving vas a high-melting point metal and silicon is deposited to a thickness of about 100 nm by thermally diffusing phosphorus to form a polyside. 
     Further, the gate electrode  4  is patterned by the use of the photolithography technique. 
     Referring to FIG. 3, only a region for forming the memory cell region, the transfer transistor and the driving transistor (namely, NMOS) is opened by the use of the photolithography technique. 
     Thereafter, impurity (phosphorus) is implanted in a self-alignment. manner using the gate electrode  4  as a mask by the ion implanting technique to form an N-type low concentration impurity region  5 . 
     In this case, the ions are implanted within the concentration range between 1×10 13  and 3×10 13  [cm −2 ] and within the accelerating voltage range between 15 and 25 [Kev]. 
     Next, the silicon oxide film  6  is formed within the thickness range between 100 and 150 nm on the device isolation silicon oxide film  2 , the gate silicon oxide film  3 , and the gate electrode  4  by the use of the CVD technique. 
     Successively, referring to FIG. 4, the silicon oxide film  6  is etched-back by the use of the etching technique to form a sidewall silicon oxide film  7  at the sidewall of the gate electrode  4 . 
     Thereafter, only a region for forming the memory cell region, the transfer transistor and the driving transistor (namely, NMOS) is opened by the use of the photolithography technique. 
     Subsequently, impurity (phosphorus) is implanted in a self-alignment manner using the gate electrode  4  and the sidewall silicon oxide film  7  as a mask by the ion implanting technique to form a N-type high concentration impurity region  8 . 
     In this case, the ions are implanted within the concentration range between 1×10 15  and 5×10 15  [cm −2 ] and within the accelerating voltage range between 30 and 40 [Kev]. 
     Next, the silicon oxide film  9  is formed within the thickness range between 100 and 150 nm on the device isolation silicon oxide film  2 , the gate silicon oxide film  3 , and the gate electrode  4  by the use of the CVD technique. 
     Further, a TEOS.BPSG film  10  having excellent reflow characteristic is deposited to a thickness of about 500 nm on the silicon oxide film  9  by the use of the CVD technique. 
     Thereafter, a reflow is performed for about 30 to 60 minutes within the temperature range between 800 and 900° C., and the surface of the TEOS.BPSG film  10  is flattened. In this event, the flattening process is carried out such that a wiring layer of a polysilicon film  14  (will be formed later) is not short-circuited. 
     Subsequently, referring to FIG. 5, a contact hole  11  is opened for the silicon oxide film  9  and the TEOS.BPSG film  10  by the etching technique. Thereinafter, impurity (phosphorus) is partially implanted into the N-type high concentration impurity region  8  under the contact hole  11  by the use of the ion implanting technique to form the N-type high impurity region  12 . 
     In this case, the ions are implanted within the concentration range between 1×10 14  and 1×10 15  [cm −2 ] and within the accelerating voltage range between 40 and 60 [Kev]. 
     Through the contact hole  11 , the diffusion layers of the driving transistor DT 1  and the transfer transistor ST 1 , the load resistor L 1 , and the gate electrode of the driving transistor DT 2  illustrated in FIG. 1 are connected to each other. 
     Herein, the ions are implanted so as to reduce contact resistance between the load resistor L 1 , the diffusion layers of the driving transistor DT 1  and the transfer transistor ST 1  and the gate electrode of the driving transistor DT 2 . 
     Further, referring to FIG. 6, the polysilicon film  14  is deposited to a thickness within the range between 100 and 150 nm on the N-type high concentration impurity region  12  and the TEOS.BPSG film  10  by the CVD technique. 
     Thereafter, impurity (phosphorus) is implanted for the entire surface of the polysilicon film  14  by the use of the ion implanting technique. 
     In this case, the ions are implanted within the concentration range between 5×10 12  and 3×10 13  [cm −2 ] and within the accelerating voltage range between 50 and 70 [Kev]. 
     The ion implantation serves to determine the resistance value of the load resistor L 1  illustrated in FIG.  1 . This implanting condition is important for manufacturing the SRAM device because the resistance value of the load resistor L 1  is a factor for determining consuming current during a standby mode in the SRAM device. 
     Thereafter, the polysilicon film  14  is patterned by the photolithography technique. Successively, impurity (phosphorus) is implanted onto the polysilicon film  14  and the TEOS.BPSG film  10  patterned by the photolithography technique and the ion implanting technique. 
     In this case, the ions are implanted within the concentration range between 1×10 15  and 1×10 18  [cm −2 ] and within the accelerating voltage range between 50 and 70 [Kev]. 
     Herein, the polysilicon film  14  serves as the load resistor L 1  illustrated in FIG. 1 while the ion implantation serves to form the wiring pattern for the power supply voltage Vcc in FIG.  1 . 
     Further, a silicon oxide film  15  is deposited to a thickness within the range between 100 and 150 nm on the TEOS.BPSG film  10  and the polysilicon film  14  by the CVD technique. 
     Thereafter, the TEOS.BPSG film  16  is deposited to a thickness within the range between 500 and 1500 nm by the CVD technique. 
     In addition, the TEOS.BPSG film  16  is polished by the Chemical Mechanical Polishing (CMP) technique in order to flatten the surface. The flattening process is conducted so that the wiring layer is not short-circuited. 
     Finally, referring to FIG. 7, a contact hole  17  is opened for the silicon oxide film  9 , the TEOS.BPSG film  10 , the silicon oxide film  15 , and the TEOS.BPSG film  16  by the use of the etching technique. 
     Thereafter, the contact hole  17  is buried with W (tungsten) serving as the high-melting point metal by sequentially depositing a titanium film and a titanium nitride serving as the high-melting point metal. 
     Subsequently, W serving as the high-melting point metal is etched-back by the etch-back technique to deposit Al (aluminum). 
     At the same time, an Al wiring layer  18  is patterned by the use of the photolithography technique. 
     Through the above-mentioned steps, the main part of the high-resistance load type memory cell for the SRAM device is completed. 
     The related technique with respect to such a semiconductor device is, for example, disclosed in Japanese Unexamined Patent Publication (JPA) No. Sho. 62-31155 and Japanese Unexamined Patent Publication (JPA) No. Hei. 8-23037. 
     In the high-resistance load type memory cell of the SRAM device, when the memory is reduced in size to realize the high-integration, the node capacitance is reduced also. 
     Thereby, the ratio, in which the voltage V 1 h is reduced to Vcc−Vt by the power supply voltage Vcc, becomes high. Further, the time, in which the voltage V 1 h restores to the power supply voltage Vcc by the power supply voltage Vcc, also become slow. As a result, the SER resistance is deteriorated. 
     To avoid the deterioration of the SER resistance, a P-type impurity region having higher concentration than the P-well region may be formed on the entire surface of the memory cell region. 
     However, this method deteriorates substrate bias characteristic of the transfer transistor. Consequently, it is difficult to actually apply this method for the SRAM device because the high-speed of the SRAM device can not be readily realized. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a semiconductor device in which substrate bias characteristic is not deteriorated in a transfer transistor even when a memory is reduced in size. 
     It is another object of this invention to provide a semiconductor device which is capable of enhancing a SER resistance by increasing node capacitance of a memory cell. 
     According to this invention, a SRAM device includes at least a transfer transistor, a driving transistor and a load resistor which are commonly connected to a node. 
     With this structure, a well has a first conductive type, and is placed on a substrate. 
     Further, a first impurity region has a second conductive type opposite to the first conductive type, and is placed in the well. 
     Moreover, a second impurity region has the first conductive type and has higher impurity concentration than impurity concentration of the well, and is placed at a lower portion of the first impurity region. 
     Herein, the node is composed of at least the first impurity region and the second impurity region. 
     For example, the first conductive type is a P-type while the second conductive type is an N-type. 
     The SRAM device further comprises a bit line and a word line. The transfer transistor includes a first terminal, a second terminal and a third terminal. 
     In this condition, the first terminal is connected to the bit line, the second terminal is connected to the node, and the third terminal is connected to the word line. 
     More specifically, the transfer transistor includes a source, a drain and a gate, the first terminal and the second terminal is any one of the source and the drain, and the third terminal is the gate. 
     Further, the SRAM device comprises a reference voltage terminal. The driving transistor includes a first terminal and a second terminal, the first terminal is connected to the node, and the second terminal is coupled to the reference voltage terminal. 
     More specifically, the driving transistor includes a source and a drain, and the first terminal and the second terminal is any one of the source and the drain. 
     Moreover, the SRAM device comprises a power supply voltage terminal. The load resistor includes a first terminal and a second terminal, the first terminal is connected to the node, and the second terminal is coupled to the power supply voltage terminal. 
     Further, a node capacitor is coupled to the node. In this event, the node capacitor has a capacitance, and the transfer transistor has a substrate bias characteristic. 
     Under this circumstance, the second impurity region serves to increase the capacitance without deterioration of the substrate bias characteristic. 
     In addition, the SRAM device has a soft error resistance, and the second impurity region serves to enhance the soft error resistance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a basic structure of a high-resistance load type memory cell serving as a main part of a related SRAM device; 
     FIG. 2 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG. 1; 
     FIG. 3 is cross sectional side view showing a method of manufacturing the high resistance load type memory cell around one node in FIG. 1; 
     FIG. 4 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG. 1; 
     FIG. 5 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG. 1; 
     FIG. 6 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG. 1; 
     FIG. 7 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG. 1; 
     FIG. 8 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node according to an embodiment of this invention; 
     FIG. 9 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG. 8; and 
     FIG. 10 is cross sectional side view showing a method of manufacturing the high-resistance load type memory cell around one node in FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 8 through 10, description will be made about a method of manufacturing a high-resistance load memory according to an embodiment of this invention. 
     In this embodiment, only the manufacturing steps illustrated in FIGS. 5 through 7 have been improved in the manufacturing steps illustrated in FIGS. 2 through 7 with respect to the above-mentioned related SRAM device. 
     In other words, the initial manufacturing steps illustrated in FIGS. 2 through 4 are substantially equivalent to the manufacturing steps in this embodiment. Therefore, description thereof will be omitted in this embodiment. 
     Herein, only a region around the node N 1  of the memory cell in FIG. 1 is illustrated in FIGS. 8 through 10, and the illustration of the peripheral circuit portion is omitted. 
     Referring to FIG. 8, a contact hole  11  is opened for the silicon oxide film  9  and the TEOS.BPSG film  10  by the etching technique. 
     Thereinafter, impurity (phosphorus) is partially implanted into the N-type high concentration impurity region  8  under the contact hole  11  by the use of the ion implanting technique to form the N-type high impurity region  12 . 
     In this case, the ions are implanted within the concentration range between 1×10 14  and 1×10 15  [cm −2 ] and within the accelerating voltage range between 40 and 60 [Kev]. 
     Through the contact hole  11 , the diffusion layers of the driving transistor DT 1  and the transfer transistor ST 1 , the load resistor L 1 , and the gate electrode of the driving transistor DT 2  illustrated in FIG. 1 are connected to each other. 
     Herein, the ions are implanted so as to reduce contact resistance between the load resistor L 1 , the diffusion layers of the driving transistor DT 1  and the transfer transistor ST 1  and the gate electrode of the driving transistor DT 2 . 
     Subsequently, a P-type impurity region  13  is formed so as to contact with the lower portion of the N-type high concentration impurity region  12  by injecting impurity (boron) using the ion-implanting technique. 
     In this case, the ions are implanted within the concentration range between 1×10 12  and 1×10 13  [cm −2 ] and within the accelerating voltage range between 60 and 80 [Kev]. 
     Further, referring to FIG. 9, the polysilicon film  14  is deposited to a thickness within the range between 100 and 150 nm on the N-type high concentration impurity region  12  and the TEOS.BPSG film  10  by the CVD technique. 
     Thereafter, impurity (phosphorus) is injected for the entire surface of the polysilicon film  14  by the use of the ion implanting technique. 
     In this case, the ions are implanted within the concentration range between 5×10 12  and 3×10 13  [cm −2 ] and within the accelerating voltage range between 50 and 70 [Kev]. 
     The ion implantation serves to determine the resistance value of the load resistor L 1  illustrated in FIG.  1 . This implanting condition is important for manufacturing the SRAM device because the resistance value of the load resistor L 1  is a factor for determining consuming current during a standby mode in the SRAM device. 
     Thereafter, the polysilicon film  14  is patterned by the photolithography technique. Successively, impurity (phosphorus) is injected onto the polysilicon film  14  and the TEOS.BPSG film  10  patterned by the photolithography technique and the ion implanting technique. 
     In this case, the ions are implanted within the concentration range between 1×10 15  and 1×10 16  [cm −2 ] and within the accelerating voltage range between 50 and 70 [Kev]. 
     Herein, the polysilicon film  14  serves as the load resistor L 1  illustrated in FIG. 1 while the ion implantation serves to form the wiring pattern for the power supply voltage Vcc in FIG.  1 . 
     Further, the silicon oxide film  15  is deposited to a thickness within the range between 100 and 150 nm on the TEOS.BPSG film  10  and the polysilicon film  14  by the CVD technique. 
     Thereafter, the TEOS.BPSG film  16  is deposited to a thickness within the range between 500 and 1500 nm by the CVD technique. 
     In addition, the TEOS.BPSG film  16  is polished by the Chemical Mechanical Polishing (CMP) technique in order to flatten the surface. The flattening process is conducted so that the wiring layer is not short-circuited. 
     Finally, referring to FIG. 10, the contact hole  17  is opened for the silicon oxide film  9 ; the TEOS.BPSG film  10 , the silicon oxide film  15 , and the TEOS.BPSG film  16  by the use of the etching technique. 
     Thereafter, the contact hole  17  is buried with W (tungsten) serving as the high-melting point metal by sequentially depositing a titanium film and a titanium nitride serving as the high melting point metal. 
     Subsequently, W serving as the high melting point metal is etched-back by the etch-back technique to deposit Al (aluminum). 
     Further, the Al wiring layer  18  is patterned by the use of the photolithography technique. 
     Through the above-mentioned steps, the main part of the high-resistance load type memory cell for the SRAM device is completed. 
     In the above-mentioned method of the high-resistance load type memory of the SRAM device, the first contact hole  11  is opened for the second silicon oxide film  9  and the first TEOS.BPSG film  10  by the use of the etching technique in order to connect the diffusion layers of the first driving transistor DT 1  and the first transfer transistor ST 1 , the first load resistor L 1 , and the gate electrode of the second driving transistor DT 2 . 
     Thereafter, the second N-type high impurity region  12  is formed by injecting impurity (phosphorus) for a part of the first N-type high impurity region  8  under the first contact hole  11 . 
     Subsequently, the P-type impurity region  13  having higher concentration than the P-type well region  21  is formed by injecting impurity (boron) with suitable energy so as to contact with the lower portion of the second N-type high impurity region  12 . 
     Actually, the node N 2  is simultaneously formed in addition to the node N 1  illustrated in FIG.  1 . In consequence, the node N 2  has the same structure as the node N 1  illustrated in FIG.  1 . 
     In such a high-resistance load memory cell, the P-type high concentration region  13  having the higher concentration than the P-type well region  21  is formed so as to contact with the second N-type high impurity region  12  only at the lower portion of the first contact hole  11 . 
     Consequently, even when the memory cell is reduced in size to realize the high-integration, the transistor characteristics of the first driving transistor DT 1 , the second driving transistor DT 2 , the first transfer transistor ST 1 , and the second transfer transistor ST 2  are not deteriorated. 
     Further, the node capacitance of the node N 1  and the node capacitance C 2  of the node N 2  in the memory cell can be increased, and the SER resistance can be enhanced without the deterioration of the substrate bias characteristic of the transfer transistor ST 1 , ST 2 . 
     Moreover, if impurity (boron) injecting concentration for forming the P-type impurity region  13  positioned at the lower portion of the second N-type high impurity region  12  is suitably selected, the SER resistance can be improved with 2˜5 times in comparison with the high-resistance load type memory cell produced by the above-mentioned related manufacturing method. 
     As described above, in the manufacturing method according to this invention, the P-type impurity region having higher concentration than the P-type well region is formed at the lower portion of the node of the memory cell in order to increase the node capacitance. 
     As a result, even when the memory is reduced in size to realize the high-integration, the substrate bias characteristic of the transfer transistor is not degraded, and the SER resistance can be enhanced also. 
     Therefore, the node capacitance in the semiconductor device produced by such a manufacturing method is increased as compared with the conventional case. 
     Further, the SER resistance is excellent, and the highly integrated semiconductor device can be obtained with high-performance and high quality. 
     While this invention has been thus far been disclosed in conjunction with an embodiment thereof, it will be readily possible for those skilled in the art to put this invention into practice in various other manners.