Method for fabricating an insulated gate semiconductor device

Abstract: A semiconductor device and a fabrication is provided, which reduces the fabrication cost and effectively lowers the operating voltage. In a process for fabricating the semiconductor device comprising high-resistance polysilicon and a polysilicon gate CMOS transistor, introduction of ions into polysilicon gates uses ion implantation of forming source and drain regions. By using the above constitution, 2 masks and 30 processes are saved. Moreover, because both channels become the surface channel type, the operating voltage can be lowered. The minimum operating voltage of the voltage detector fabricated by the fabrication method of the present invention shows approx. 0.25 V at room temperature and 0.35 V or less even at -40°C.

Description: The present invention relates to a method for fabricating a semiconductor device and in particular, to a method allowing the lowering of the voltage of a power-supply semiconductor integrated circuit. An existing voltage detector or voltage regulator comprising a CMOS transistor uses an N-type gate electrode for the gate electrode of an NMOS transistor of the CMOS transistor and an N-type gate electrode for the gate electrode of a PMOS transistor of the CMOS transistor. Fig. 3 shows a circuit block diagram of a voltage detector. The voltage detector comprises a reference voltage circuit 301, a voltage division circuit 302, a voltage comparison circuit 303, and an output circuit 304. The reference voltage circuit 301 comprises a depression-type (also known as depletion-type) NMOS transistor and an enhancement-type NMOS transistor which mutually connect in series between a power supply line 202 and an earthing line 201 and has the function of outputting a constant voltage to the joint A 111. The voltage division circuit 302 comprises a plurality of series resistances connected between the power supply line 202 and the earthing line 201 and has the function of dividing a power supply voltage by using a joint B 112 as a separating point. The voltage comparison circuit 303 comprises PMOS transistors and NMOS transistors, which are connected between the power supply line 202 and the earthing line 201, and connect with the reference voltage circuit 301 at the joint A 111 and with the voltage division circuit 302 at the joint B 112, and compares the voltage difference between the joint A 111 and the joint B 112 and has the function of providing an output equal to the power supply voltage to a joint C 113 when the voltage at the joint B 112 is lower than the voltage at the joint A 111. The output circuit 304 comprises an inverter constituted by combining a PMOS transistor with an NMOS transistor, in which the source of the PMOS transistor connects with the power supply line 202, the drain of the PMOS transistor connects with the drain of the NMOS transistor, and the source of the NMOS transistor connects with the earthing line 201. The voltage comparison circuit 303 and the gate electrode of the inverter are connected to each other at the joint C 113. When the output of the voltage comparison circuit 303 is received by the gate electrodes of the PMOS and NMOS transistors, the drain joint between the PMOS and NMOS transistors has an output function equal to the power supply voltage. A detection voltage is determined by the reference voltage which is output by the reference voltage circuit 301 and the ratio of the resistances of high-resistance polysilicon, and a desired output voltage can be obtained by a polysilicon fuse built in. Moreover, high-resistance polysilicon with a high resistance value is used to decrease current consumption and the high-resistance polysilicon and a CMOS transistor are indispensable components for a CMOS power supply IC. The voltage detector has a function for decreasing an output voltage to O V when the power supply voltage comes to the detection voltage or lower (e.g. O. 8 V or lower). Fig. 6 shows a circuit block diagram of a voltage regulator. The voltage regulator comprises a reference voltage circuit 311, a voltage division circuit 312, a voltage comparison circuit 313, and an output circuit 314. The reference voltage circuit 311 comprises a depression-type NMOS transistor and an enhancement-type NMOS transistor which mutually connect in series between a power supply 212 and an earthing line 211 and has the function of providing a constant output voltage to the joint A 121. The voltage division circuit 312 comprises a plurality of series resistances connected between the output circuit 314 and the earthing line 211, and has a function for dividing a voltage difference by using a joint B 122 as a separating point. The voltage comparison circuit 313 comprises PMOS transistors and NMOS transistors, which are connected between the power supply line 212 and the earthing line 211, and connects with the reference voltage circuit 311 at the joint A 121 and with the voltage division circuit 312 at the joint B 122, and compares the voltage difference between the joint A 121 and the joint B 122 and has the function of outputting the gate controlled voltage to the output circuit 314. The output circuit 314 comprises a PMOS transistor or an NMOS transistor. When the circuit 314 comprises the PMOS transistor, it is constituted so as to connect the source with the power supply line 212 and the drain of the PMOS with the voltage division circuit 312. When the circuit 314 comprises the NMOS transistor, it is constituted so as to connect the drain with the power supply line 212 and the source of the NMOS transistor with the voltage division circuit 312. Moreover, the circuit 314 is constituted so as to connect the voltage comparison circuit 313 with the gate electrode of the PMOS or NMOS transistor at the joint C 123 and receive the output of the voltage comparison circuit 313 by the gate electrode of the PMOS or NMOS transistor, and has the function of providing an output voltage to an output terminal 133 connected to the drain of the PMOS transistor or the source of the NMOS transistor. The voltage regulator has the function of providing a continuously constant output voltage even if a power supply voltage or a load fluctuates. Fig. 21 shows an existing reference voltage circuit used for a voltage detector or voltage regulator. The reference voltage circuit comprises a depression-type NMOS transistor 401 and an enhancement-type NMOS transistor 402 which mutually connect in series between a power supply line 222 and an earthing line 221 and has the function of providing a constant output voltage to the joint A 161. The depression-type NMOS transistor 401 connects the gate with the source to equalise the potential with that of a substrate and connects the source with the drain of the enhancement-type NMOS transistor. The enhancement-type NMOS transistor 402 connects the gate with the drain to equalise the potential of the substrate with that of the drain. Moreover, the drain of the depression-type NMOS transistor 401 connects with the power supply line 222, the source of the enhancement-type NMOS transistor connects with the earthing line 221, and the joint A 161 between the depression-type transistor 401 and the enhancement-type transistor 402 is used as an output terminal. Existing example of a fabrication method In general, an existing voltage detector and a voltage regulator comprising a CMOS transistor respectively are differently fabricated from each other by forming a high resistance and a gate electrode with a polysilicon film and changing the amount of impurities to be introduced into the polysilicon film. An existing method for fabricating high-resistance polysilicon and a polysilicon-gate CMOS transistor on the same substrate is described below by taking a P substrate as an example and referring to Figs. 19 and 20. An N well 2 is first formed on a P-type silicon substrate I through ion implantation and thermal diffusion and thereafter, a P-type region 3 and an N-type region 4 are formed through ion implantation to form separate devices. Then, a field oxide film 5 and a gate oxide film 6 are formed, see Fig. 20A, and ions are implanted into a channel region for transistor threshold control, see Fig. 20B and a polycrystalline silicon (hereinafter known as polysilicon) film 7 is deposited, see Fig. 20C. Then, ions are implanted into polysilicon film 7 to control the resistance value, see Fig. 20D. Then a CVD oxide film 9 wider than a portion of the polysilicon film 7 serving as the high-resistance polysilicon 8 is deposited, the pre-deposition of phosphorus is performed, and high-concentration phosphorus is diffused in the polysilicon film 7, see Fig. 20E. Then, the oxide film 9 is removed from the polysilicon film 7 and thereafter the high-resistance polysilicon 8 is formed through etching, see Fig. 20F, moreover polysilicon gate electrode 10 and 11 are formed through etching by using another mask, see Fig. 20G, and finally ions are implanted into an N-channel-side source and drain region 12 and a P-channel-side source and drain region 13, see Fig. 20H and provided with thermal annealing to complete the high-resistance polysilicon 8 and the polysilicon-gate CMOS transistor, see Fig. 20I. For an existing semiconductor device, the gate electrodes of the PMOS and NMOS transistors are the N type. Therefore, the PMOS transistor is necessarily the buried channel type. A buried-channel-type MOS transistor has disadvantages of inferior channel leak current characteristics, more leak current than the surface channel type, short-channel effect and the disadvantage that it is difficult to control the threshold. Particularly, the necessity for low power consumption under operation is larger than the case of a general IC because a voltage detector and voltage regulator continuously operate. Moreover, because energy is supplied to the buried-channel-type MOS transistor from a battery, low-voltage operation is necessary. Therefore, it is necessary to lower the threshold voltage of the MOS transistor to operate a semiconductor device at a low voltage. However, because the existing buried-channel-type MOS transistor has a large channel leak current, low-voltage operations are difficult to realise. Moreover, because channel leak current tends to increase at a high temperature, low-voltage operations are more difficuit than those at the room temperature. Furthermore, it is necessary to lower the reference voltage from the reference voltage circuits 301 and 311 in order to operate the semiconductor device at a low voltage. In the case of the existing reference voltage circuits 301 and 311, the reference voltage to be outputted is determined by the sum of the threshold voltage of the enhancement-type MOS transistor 402 and that of the depression-type MOS transistor 401 of the circuits. Therefore, the threshold voltage must be lowered to obtain a low reference voltage. However, lowering the reference voltage is limited due to the above channel leak current. Furthermore, the existing method for fabricating a semiconductor device comprising high-resistance polysilicon and a polysilicon-gate CMOS transistor has the following problems. That is, although introduction of impurities into the high-resistance polysilicon 8 uses ion implantation, step 2004 in Fig. 19, introduction of impurities into the polysilicon gates 10 and 11 uses the diffusion implantation method called pre-deposition, step 2006 in Fig. 19. Therefore, to perform the pre-deposition of phosphorus, high-concentration phosphorus must be prevented from entering a high-resistance polysilicon region by covering the high-resistance polysilicon region with a CVD oxide film. Moreover, because the etching rate differs due to the different phosphorus concentration, it is impossible to etch both the high resistance polysilicon 8 and the polysilicon gates 10 and 11 at the same time. Therefore, a problem occurs that the number of processes increases. Furthermore, as a method for solving the above problem, boron ions are implanted into the gate electrode of PMOS transistor when the gate electrode of an NMOS transistor uses an N-type gate electrode and the gate electrode of the PMOS transistor uses a P-type gate electrode. However, there are problems that boron ions penetrate into a channel region due to implantation of the ions into the gate electrode, the boron ions implanted into the gate electrode penetrate into the channel region, or the gate electrode is depleted due to insufficient thermal anneal. Moreover, to keep the thermal annealing temperature low, it is necessary to increase the phosphorus concentration of the inter-layer insulating Phosphosilicate glass (PSG) film 20. When increasing the phosphorus concentration, however, a problem occurs that the phosphorus in the inter-layer insulating PSG film 20 diffuses in the high-resistance polysilicon 8 to decrease the resistance value of the high-resistance polysilicon 8. Various semiconductor arrangements are disclosed in the following documents: EP-A-450503; J.C. Hsieh et al, IEEE Electron Device Letters, Vol 14 no.5 (1993) pages 222-224; DE-A-3107543; J.S. Cable et al, IEEE Electron Device Letters, Vol 12 no.3 (1991) pages 128-130; Patent Abstracts of JP-A-61-016562. According to the present invention, there is provided a method for fabricating a semiconductor device having high-resistance polysilicon and low-resistance polysilicon on the surface of a substrate, wherein a high-resistance polysilicon serves as a resistance and a low-resistance polysilicon serves as a gate electrode of a MOS transistor comprising the steps of: depositing a gate oxide film having a thickness of 40 nm or more; depositing a polysilicon film having a thickness of 300 to 400 nm; simultaneously forming the polysilicon film into a resistor, and a gate electrode through etching by using resist as a mask; completely removing said oxide film only from a source/drain region through etching after forming a gate electrode; depositing a further oxide film on the source/drain region up to a thickness of 10 to 20 nm through thermal oxidation after said removal of said oxide film on the source/drain region; introducing impurities for resistance value control into polysilicon serving as a resistor through ion implantation to form high-resistance polysilicon; introducing impurities serving as acceptors into polysilicon serving as the gate electrode of a PMOS transistor and the source and drain regions of the PMOS transistor through ion implantation such that the source and drain regions of the PMOS transistor have an acceptor concentration of 0.5 to 1.5 x 10 26 /m 3 ; and introducing ions serving as donors into polysilicon serving as the gate electrode of an NMOS transistor and the source and drain regions of the NMOS transistor through ion implantation, such that the source and drain regions of the NMOS transistor have a donor concentration of 2.1 to 3.0 x 10 26 /m 3 , wherein the acceptor and donor ions are implanted in the source and drain regions through said further oxide film. Fig. 1 is a block diagram showing the process of the method for fabricating the sixth example; Figs. 2A and 2F are sectional views showing the process of the method for fabricating the sixth example; Fig. 3 is a block diagram showing the circuit of the voltage detector in the first example; Fig. 4 is a graph showing the comparison between the output voltage of the voltage detector of the first example and that of an existing voltage detector; Fig. 5 is a graph showing the relationship between the minimum operating voltage according to the voltage detector of the first example and the temperature; Fig. 6 is a block diagram showing the circuit of the voltage regulator in the second example; Fig. 7 is a block diagram showing the reference voltage circuit in the third example; Fig. 8 is a structural sectional view of the semiconductor device in the fourth example; Fig. 9 is a structural sectional view of the semiconductor device in the fifth example; Fig. 10 is a graph showing the comparison between the buried channel type and the surface channel type, and showing the relationship of a PMOS transistor fabricated by the sixth example method between channel leak current and threshold voltage; Fig. 11 is a graph showing the relationship between polysilicon gate film thickness and penetration density of boron into a channel region in the case of boron ion implantation used in the sixth example method; Fig. 12 is a graph showing the relationship between polysilicon gate film thickness and penetration density of boron into a channel region in the case of BF 2 ion implantation used in the sixth example method; Fig. 13 is a graph showing the relationship between boron ion implantation energy and penetration density of boron into a channel region, according to the sixth example method; Fig. 14 is a graph showing the relationship between BF 2 ion implantation energy and penetration density of boron into a channel region, according to the sixth example method; Fig. 15 is a graph showing the relationship between gate oxide film thickness and penetration density of boron (due to thermal diffusion) into a channel region, according to the sixth example method; Fig. 16 is a graph showing the relationship between thermal annealing temperature after forming a source and a drain and penetration density of boron (due to thermal diffusion) into a channel region, according to the sixth example method; Fig. 17 is a graph showing the comparison between penetration density of boron (due to thermal diffusion) into a channel region and anneal temperature in the sixth example method when a silicon nitride film is deposited on a gate oxide film or not; Figs. 18A to 18H are sectional views of the process in the embodiment method; Fig. 19 is a block diagram of the process in an existing semiconductor device fabrication method; Figs. 20A to 201 are sectional views of the process in an existing semiconductor device fabrication method; Fig. 21 is a diagram of the reference voltage circuit in an existing semiconductor device circuit; Fig. 22 is a graph showing the relationship between reflow temperature and breakdown voltage according to the donor density of the NMOS transistor; Fig. 23 is a graph showing the relationship between the gate capacity and gate voltage; and Fig. 24 is a graph showing the relationship between the depleting rate and the reflow temperature. Embodiment 1 A voltage detector which is the first example of the semiconductor device is described below by referring to Fig. 3. Each circuit of the voltage detector uses an N-type gate electrode for the NMOS transistor and a P-type gate electrode for the PMOS transistor, and the voltage detector comprises the reference voltage circuit 301, voltage division circuit 302, voltage comparison circuit 303, and output circuit 304. The reference voltage circuit 301 comprises a depression-type (also known as depletion-type) NMOS transistor and an enhancement-type NMOS transistor which mutually connect in series between the power supply line 202 and the earthing line 201, and at the joint A 111 are connected the source of the depression-type transistor and the drain of the enhancement-type transistor and it has the function of providing a constant output voltage to the joint A 111. The voltage division circuit 302 comprises a plurality of resistance values connected between the power supply line 202 and the earthing line 201 and has the function of dividing a power supply voltage by using the joint B 112 as a separating joint. The voltage comparison circuit 303 comprises PMOS transistors and NMOS transistors, which are connected between the power supply line 202 and the earthing line 201, and connects with the reference voltage circuit 301 at the joint A 111 and with the voltage division circuit 302 at the joint B 112, and compares the voltage difference between the joint A 111 and the joint B 112 and provides an output equal to the power supply voltage to the joint C 113 when the voltage at the joint B 112 is lower than the voltage at the joint A 111. The output circuit 304 comprises an inverter constituted by combining a PMOS transistor with an NMOS transistor, in which the source of the PMOS transistor connects with the power supply line 202, the drain of the PMOS transistor connects with the drain of the NMOS transistor, and the source of the NMOS transistor connects with the earthing line 201. The voltage comparison circuit 303 and the gate electrode of the inverter are connected to each other at the joint C 113, the gate electrodes of the PMOS and NMOS transistors receive the output of the voltage comparison circuit 303, and the the drains of both transistors are connected to the output terminal 103. The voltage detector outputs voltage applied to the power supply terminal 102 when the voltage applied to the power supply terminal 102 is higher than a preset detection voltage, and outputs the voltage of the earthing terminal 101 when the voltage applied to the power supply terminal 102 is lower than the detection voltage. Fig. 4 is a graph showing the output voltage of a voltage detector against the power supply voltage, in which line 3001 represents the output voltage of an existing voltage detector and line 3002 represents the output voltage of the voltage detector of the present invention. A desired voltage can be obtained by a polysilicon fuse built into the voltage division circuit 302. The minimum operating voltage of a voltage detector is the voltage when a voltage detector cannot output the voltage of the earthing terminal 101. This is because, when the voltage applied to the power supply terminal 102 lowers, the reference voltage circuit 301, voltage comparison circuit 303, and output circuit 304 cannot operate. As shown in Fig. 4, though the minimum operating voltage 3004 of the existing voltage detector is 0.4 V at the room temperature. the minimum operating voltage 3005 of the voltage detector of the present invention is 0.25 V under the same condition. This shows that each circuit operates even if the voltage applied to the supply terminal 102 is low, because in each circuit the gate electrode of the NMOS transistor comprises an N-type gate electrode and the gate electrode of the PMOS transistor comprises a P-type gate electrode. Moreover, at high temperature the operation of the existing voltage detector is indeterminate because the threshold voltage of the PMOS transistor lowers and channel-leak current increases. Fig. 5 shows a relationship between the minimum operating voltage and temperature. In the case of the voltage detector, however, the operation is stable even in a temperature range of -40°C to 80°C and the error of a detected voltage only ranges between 0.05 and 0.2 mV/°C as shown in Fig. 5. Also, the minimum operating voltage only ranges between 0.18 and 0.32 V in the temperature range of -40°C to 80°C. Therefore, the voltage detector operates at 0.35 V or higher in the temperature range of -40°C to 80°C. Moreover, for a voltage detector, the voltage applied to the power supply terminal 102 is not constant but it ranges between 0 and several volts. Therefore, the breakdown voltage of a voltage detector requires several volts or more. Thus, because the breakdown voltage of NMOS transistor and PMOS transistor ranges between 11 and 13 V in the temperature range of -40°C to 80°C, the voltage detector operates at a voltage between 0.35 and 10 V (both included) by considering that IC operations are assured in the temperature range of -40°C to 80°C. Moreover, as another example, it is possible to constitute a voltage regulator so that each circuit of the regulator uses an N-type gate electrode for the NMOS transistor and a P-type gate electrode for the PMOS transistor. Fig. 6 shows a block diagram of a voltage regulator. The voltage regulator comprises the reference voltage circuit 311, voltage division circuit 312, voltage comparison circuit 313, and output circuit 314. The reference voltage circuit 311 comprises a depression-type NMOS transistor and an enhancement-type NMOS transistor which mutually connect in a series between the power supply line 212 and the earthing line 201, and the source of the depression-type transistor and the drain of the enhancement-type transistor are connected to the joint A 121, and it has the function of providing a constant output voltage to the joint A 121. The voltage division circuit 312 comprises a plurality of series resistance connected between the output circuit 314 and the earthing line 211 and has a function for dividing a voltage difference by using a joint B 122 as a separating point. The voltage comparison circuit 313 comprises PMOS transistors and NMOS transistors, which are connected between the power supply line 212 and the earthing line 211, and connects with reference voltage circuit 311 at the joint A 121 and with the voltage division circuit 312 at the joint B 122, and compares the voltage difference between the joint A 121 and the joint B 122 and has the function of outputting the gate controlled voltage to the output circuit 314. The output circuit 314 comprises a PMOS transistor or an NMOS transistor. When the circuit 314 comprises the PMOS transistor, it is constituted so as to connect the source of the PMOS with the power supply line 212 and the drain of the PMOS with the voltage division circuit 312. When the circuit 314 comprises the NMOS transistor, it is constituted so as to connect the drain of the NMOS with the power supply line 212 and the source of the NMOS transistor with the voltage division circuit 312. Moreover, the circuit 314 is constituted so as to connect the voltage comparison circuit 313 with the gate electrode of the PMOS or NMOS transistor at the joint C 123 and receive the output of the voltage comparison circuit 313 by the gate electrode of the PMOS or NMOS transistor, and has the function of outputting a voltage to an output terminal 133 connected to the drain of the PMOS transistor or the source of the NMOS transistor. The voltage regulator has the function of providing a continuous constant output voltage even if the voltage of the power supply terminal 132 or a load connected to the output terminal 133 fluctuates. The voltage of the output terminal 133 is divided in the voltage division circuit 312. The comparison circuit 313 compares the divided voltage with the reference voltage outputted from the reference voltage circuit 311, and it outputs the gate controlled voltage. The gate controlled voltage keeps the voltage of the output terminal 133 constant by controlling the gate voltage of a MOS transistor. Because the voltage regulator enables the threshold voltage of the MOS transistor in each circuit to lower, low-voltage operations are realised and the operating voltage range is between 0.35 and 10 V in the temperature range of -40°C to 80°C. Example 2 The second example of the semiconductor device is described below. To operate the semiconductor device at a low voltage, it is also necessary to lower the reference voltage that is output by a reference voltage circuit. In the case of an existing reference voltage circuit, the reference voltage is determined by the sum of the threshold voltage of an enhancement-type MOS transistor of the circuit and that of a depression-type MOS transistor of the circuit. Therefore, to lower the reference voltage, a threshold voltage must be lowered. However, when the threshold voltage is lowered, off-leak current increase and other circuits do not operate normally. Therefore, lowering the reference voltage by decreasing the threshold voltage is limited. However, the reference voltage circuit is constituted as shown in Fig. 7. For the depression-type NMOS transistor 401, the gate is connected with the earthing line 221 and the potential of the substrate is equalised with that of the source and the source is connected with the drain of the enhancement-type NMOS transistor 402. For the enhancement-type NMOS transistor 402, the gate is connected with the drain and the potential of the substrate is equalised with that of the source. The drain of the depression-type NMOS transistor 401 is connected with the power supply line 222 and the source of the enhancement-type NMOS transistor 402 is connected with the earthing line 221. The joint A 151 between the depression-type NMOS transistor 401 and the enhancement-type NMOS transistor 402 is an output terminal of the reference voltage circuit. In an additional example, it is also possible to connect a resistance between the gate of the depression-type NMOS transistor 401 and the earthing line 221 or between the drain of the depression-type NMOS transistor 401 and the power supply line 222. It is desirable to set the threshold voltage V TND of the depression-type NMOS transistor 401 on -0.2 to -0.4 V and the threshold voltage V TN of the enhancement-type NMOS transistor 402 on 0.2 to 0.3 V. Furthermore, it is most desirable to set V TND to -0.3 V and V TN to 0.2 V. By using the above constitute, it makes it possible to operate the voltage detector and voltage regulator at a low voltage, because the reference voltage is half of the existing reference voltage even if using the MOS transistors with the same size and the same threshold voltage as the existing reference voltage circuit shown in Fig. 23. Example 3 The third example of the semiconductor device is described below. For the semiconductor device, the gate electrode of the NMOS transistors are N-type and the gate electrode of the PMOS transistors are P-type. However, if a gate oxide film is thinner than 20 nm, ions may penetrate the film to reach a channel region due to ion implantation and thermal annealing. Moreover, to assure insulation of an oxide film for 10 years, an electric field applied to the oxide film must be set to 3 MV/cm or less. Therefore, the gate oxide film thickness of MOS transistors constituting a semiconductor device having a wide operating voltage, such as a voltage detector or voltage regulator, must be determined on that conditions. Therefore, the gate oxide film thickness is set to 20 nm or more. However, if the gate oxide film is thicker than 100 nm, it is impossible to decrease the threshold voltage less than 1.0 V, without against type channel doping. To use against type channel doping is no use for making P-type gate on the PMOS transistor, because it makes buried channel. Therefore, the gate oxide film thickness of the semiconductor device is set to 20 to 100 nm. Also, if too many boron ions are implanted into the polysilicon gate, ions may penetrate the gate and the gate oxide to reach a channel region. Therefore, the number of boron ions to be implanted in PMOS transistor is set to 1.5 x 10 26 /cm 3 or less. But, decreasing of the concentration of ions implanted into the source/drain region, less than 0.5 x 10 10 26 /cm 3 causes high increasing of the source/drain resistance, that causes bad effects on the integrated circuit. Therefore, for the semiconductor device, the acceptor concentration of the source/drain region of the PMOS transistor is set to 0.5 to 1.5 x 10 26 /cm 3 . Moreover, for an NMOS transistor, if few ions are implanted, the drain breakdown voltage may lower due to defects which are caused when activating impurities through thermal annealing. Fig. 22 shows a graph of the yield of NMOS transistor drain breakdown voltage to the reflow temperature, when setting the donor concentration of the source/drain region to 1.5 x 10 26 /cm 3 4001 and 2.1 x 10 26 /cm 3 4002. From the graph, it is found that the donor concentration of the source/drain region of the NMOS transistor should be set to 2.1 x 10 26 /cm 3 or more in order to keep the high yield. But, it is difficult to make the concentration more than 3.0 x10 26 /cm 3 by the present use of ion implantation method. Therefore, for the semiconductor device the donor concentration of the source/drain region of the NMOS transistor is set to 2.0 to 3.0 x 0 26 /cm 3 . Example 4 The fourth example of the semiconductor device is described below. In the case of a semiconductor device comprising PMOS transistors and NMOS transistors, in which the gate electrode of the PMOS transistors are made of P-type polysilicon containing boron and the gate electrode of the NMOS transistors are made of N-type polysilicon containing phosphorus or arsenic, there is a problem that boron ions implanted into the polysilicon gate electrode of the PMOS transistor penetrate into the channel region due to the thermal annealing. That changes the threshold voltage of the PMOS transistors. Fig. 8 shows a structural sectional view of the present semiconductor device of the fourth example. However, boron cannot penetrate into the channel region because of the presence of a silicon nitride film 17. Therefore, the threshold voltage of PMOS transistors will not change, under the structure that the insulating film is constituted by a three-layer film which consists of gate oxide film 6, silicon nitride film 17, and top oxide film 18, arranged in order from the substrate surface toward the P-channel polysilicon gate electrode 11. When the insulating film of the NMOS transistor is the same as in the PMOS transistor, NMOS transistors have a problem that hot carriers are trapped at the interface between the silicon nitride film 17 and the gate oxide film 6 to change the threshold voltage. Therefore, the insulating film of PMOS transistors consist of the gate oxide film 6, silicon nitride film 17, and top oxide film 18. And, the insulating film of NMOS transistors consist of the gate oxide film 6. Example 5 The fifth example of the semiconductor device is described below by referring to Fig. 9. There is a method for lowering a thermal annealing temperature as the method for preventing boron in the polysilicon gate electrode of PMOS transistors from penetrating into the channel regions. In this case, the phosphorus concentration of the inter-layer insulating PSG (Phosphosilicate Glass) film 20 must be increased to secure the coverage. However, when increasing the phosphorus concentration, a problem occurs that the phosphorus in the inter-layer insulating PSG film 20 diffuses in the high-resistance polysilicon 8 to decrease the resistance value of the high-resistance polysilicon 8. However, it is possible to prevent phosphorus diffusing into the high-resistance polysilicon 8 from the PSG 20 by forming a film 19 of NSG (Nondoped Silicate Glass) contacting the high-resistance polysilicon 8 and flatly deposited at the opposite side to the silicon substrate 1, and the PSG 20 contacting the NSG 19 and flatly deposited at the opposite side to the high-resistance polysilicon 8. Example 6, Gate oxide film 200A The example is described below by referring to the accompanying drawings. Fig. 1 is a block diagram of the process of the sixth example of the semiconductor device. Figs. 2A to 2F are sectional views of the process of the sixth example of the semiconductor device. First, an N well 2 is formed in a P-type (20 to 30 Ωcm) silicon substrate 1 through ion implantation (phosphorus: 100 to 150 KeV, 3 to 4 E12/cm 2 ) and thermal diffusion (in a 1175°C oxidation atmosphere for 19 hr) and thereafter the P-type region 3 and N-type region 4 for device separation are formed through ion implantation (P-type region: Ion implanting conditions Boron 25 to 35 KeV 5.0 to 8.0 E13/cm 2 , N-type region: Ion implanting conditions Phosphorus 60 to 120 KeV 1.0 to 2.0 E12/cm 2 ). Then, a field oxide film 5 (approx. 1µm) and a gate oxide film 6 (approx. 20 nm) are formed through thermal oxidation as shown in Fig. 2A. Then ion implantation (N-channel enhancement-type boron 25 to 40 KeV 1 to 4 E11/cm 2 , N-channel depression-type phosphorus 40 to 400 KeV 2 to 6 E11/cm 2 , P-channel enhancement-type phosphorus 40 to 100 KeV 1 to 4 E11/cm 2 ) to a channel region for transistor threshold voltage control is performed as shown in Fig. 2B. Next, the polysilicon film 7 (300 to 400 nm) is deposited, see Fig. 2C. Then, a high-resistance polysilicon 8, polysilicon wiring, and polysilicon gate electrodes 10 and 11 are simultaneously formed through etching, see Fig. 2D. Then, ion implantation (phosphorus 40 to 60 KeV 2 to 8 E14/cm 2 ) for control of the resistance of high-resistance polysilicon 8 and ion implantation (N-channel side phosphorus 40 to 80 KeV 7.0 to 10.0 E15/cm 2 P-channel side boron 20 to 35 keV 2.0 to 5.0 E15/cm 2 ) for introduction of impurities into a gate electrode and formation of source and drain regions at the N-channel side (polysilicon gate electrode 10 and source/drain region 12) and the P-channel side (polysilicon gate electrode 11 and source/drain region 13) are performed, see Fig. 2E. In this case, ions are also implanted into wiring polysilicon. Then, an NSG (Nondoped Silicate Glass) film 19 is deposited up to a thickness of 200 to 300 nm and moreover a PSG (Phosphosilicate Glass) 20 film containing 8 to 12 wt% of phosphorus is deposited up to thickness of 400 to 600 nm to perform the reflow of an inter-layer insulating film at a temperature of 800 to 900°C for approx. 30 min. Finally, contact hole and metallic electrodes 21 are formed to obtain the circuit, comprising high-resistance polysilicons and surface-channel-type polysilicon gate CMOS transistors, see Fig. 2F. A main feature of this example lies in the fact that impurities are introduced into polysilicon gate electrodes through implantation of ions into source and drain regions by using an ion implantation technique. This can be done simultaneously with implanting ions into high resistance polysilicon regions when fabricating a semiconductor device comprising high-resistance polysilicon regions and polysilicon gate CMOS transistors. When using phosphorus pre-deposition for introduction of impurities into polysilicon gate electrodes according to the existing method, it is necessary to form and remove a mask oxide and etch polysilicons one by one with different phosphorus concentrations. In the case of the present example, however, one-time etching is enough because the polysilicon is formed through etching while in a non-doped state and moreover pre-deposition of phosphorus is unnecessary because introduction of impurities into polysilicon gate electrodes are performed through implantation of ions into source and drain regions. Moreover, the existing method cannot lower the threshold voltage of PMOS transistors, because the PMOS transistors made by the existing method are buried-channel-type, therefore the off characteristic is inferior to the surface-channel-type (off-leak current strongly increased), the present example improves the off characteristic at lower threshold voltage because PMOS transistors become the surface channel type similarly to NMOS. Fig. 10 is a graph showing the characteristics of the semiconductor device of an example, which shows the relation between threshold voltage and PMOS channel leak current. from Fig. 10, it is found that the surface channel type has smaller leak current and much better off characteristic than the buried-channel-type. The PMOS is surface channel type, the same as the NMOS. Therefore, PMOS transistors have small leak current even for low threshold voltage. This makes the operating voltage lower. The present example is also useful for decreasing the production costs of the semiconductor devices. For example, when a voltage detector which is one of the semiconductor devices is produced by the existing fabrication method, approx. 20 masks and 300 processes are required. However, when the same devices are produced by the fabrication method of the present example, 2 masks and 30 processes can be saved. Fig. 4 shows the comparison between the characteristic of the semiconductor device (voltage detector) of the present invention and that of an existing semiconductor device (voltage detector) In Fig. 4, X axis shows power supply voltage and Y axis shows output voltage. A voltage detector has the function of providing an output equal to a power supply voltage when the power supply voltage becomes equal to or higher than a detection voltage 3003 (0.8 V in Fig. 4). Fig. 4 shows the comparison of the output voltage curve 3001 of the voltage detector fabricated by the existing fabrication method and the output voltage curve 3002 of the voltage detector fabricated by the fabrication method of the present example in the same graph. When the power supply voltage is very small, the voltage detectors of the existing method and the present example produce an output equal to the power supply voltage because their internal circuits cannot be operated. However, the voltage detector fabricated by the fabrication method of the present example starts operating at low voltage and the minimum operating voltage is approx. 0.25 V (3005). However, the minimum operating voltage of the voltage detector fabricated by the existing fabrication method is approx. 0.4 V (3004). Figs. 11 to 14 are illustrations showing the characteristics of the semiconductor device of the present example. Figs. 11 and 12 are graphs in which X axis shows polysilicon concentration and Y axis shows channel surface boron concentration immediately after implantation of B + and BF 2 + respectively. Figs. 13 and 14 are graphs in which X axis shows B + and BF 2 + ion implantation energies respectively and Y axis shows channel surface boron concentration immediately after ion implantation. Because boron has a small atomic weight, boron ions are easily implanted deeply. From Figs. 11 to 14, it is found that boron may reach up to a channel region (penetration of boron into channel region) when the polysilicon film thickness is thin or the ion implantation energy is high. The present example prevents the penetration of boron into the channel region by limiting the polysilicon gate film thickness to 300 nm or more and boron and BF 2 ion implantation energies to 35 KeV or less and 100 KeV or less respectively. The lower limit of the polysilicon film thickness is determined by penetration of boron into the channel region. However, the upper limit of the polysilicon film thickness is set to 400 nm in order to escape depletion of the gate electrode and decrease in the sheet resistivity of high-resistance polysilicon. Though it is possible to use a polysilicon film with a thickness of 400 nm or more, the polysilicon film thickness of 300 to 400 nm is proper as a use level. Also, the upper limits of B + and BF 2 + ion implantation energies are determined by penetration of boron into a channel region as described above. However, very low implantation energy is not better. Unless impurities are sufficiently introduced into a source and drain or a gate electrode, a parasitic resistance increases or, in the worst case, no operation may be performed. Therefore, the lower limits of B + and BF 2 + ion implantation energies are set to 20 KeV and 50 KeV respectively by selecting a condition in which 60 to 80% of implanted impurities are introduced into source and drain surfaces even if an oxide film with a thickness of approx. 20 nm is present on the surface. Fig. 15 is a graph in which X axis shows gate oxide film thickness and Y axis shows channel surface boron concentration after thermal annealing. Fig. 16 is a graph in which X axis shows reflow temperature of an inter-layer insulating film and Y axis shows channel surface boron temperature after thermal annealing. From Figs. 15 and 16, it is found that high-concentration boron in a gate electrode may pass through an oxide film and diffuse in a channel region (penetration of boron into channel region). Therefore, the present invention prevents the penetration of boron into a channel region due to thermal annealing by limiting the gate oxide film thickness to 20 nm or more and the thermal annealing temperature after forming source and drain regions to 900°C or lower. The upper limit of reflow temperature is et to a value in which penetration of boron into a channel region does not occur as described above. However, the lower limit of reflow temperature is set to a value in which depleting of a gate electrode does not occur. The depleting of a gate electrode is a phenomenon in which a depletion layer is formed when the impurity concentration in the gate electrode is low. Depleting of a gate electrode deteriorates the on-characteristic (changing a lot of drain current) of a MOS transistor and affects operations of a semiconductor device. In general, impurities introduced into polysilicon through ion implantation uniformly diffuse in a gate electrode through thermal annealing after introduction. However, because diffusion is not adequately performed when the reflow temperature is low, the impurity concentration in the gate electrode is not uniform and the impurity concentration near a channel is kept low. Therefore, the gate electrode is depleted. In this case, to measure how a gate electrode is depleted, there is a method of measuring the gate capacity-voltage characteristic. Fig. 23 shows the gate capacity-voltage characteristic. When comparing the maximum capacity value Cmax 5001 with the gate capacity at maximum power supply voltage value Cvdd 5002 in this graph, it can be said that the gate electrode is less depleted as the ratio between them approaches 1. Fig. 24 shows a graph of Cmax/Cvdd to reflow temperature. According to the graph, the lower limit of the reflow temperature in which a gate electrode is not depleted is determined as 800°C. The fabrication method of the present example limits the reflow temperature to 900°C or lower in order to prevent the penetration of boron into a channel region. In such a case, the coverage of an inter-layer insulating film set between polysilicon and a metallic electrode is deteriorated. Then it may happen that problems of deterioration of the reliability of wires such as electromigration will occur. Therefore, for the present example, a PSG (Phosphosilicate Glass) film is selected as an inter-layer insulating film which contains high-concentration (8 to 12 wt%) phosphorus which is completely softening even at a temperature of 800 to 900°C. Moreover, because, if high-resistance polysilicon directly contacts a high-concentration PSG film, the phosphorus in the PSG film diffuses in the high-resistance polysilicon to change the resistance value of the high-resistance polysilicon and increase the fluctuation of the resistance value, NSG (Nondoped Silicate Glass) is deposited between the high-resistance polysilicon and the high-concentration PSG film. Fig. 17 is a graph in which X axis shows reflow temperature and Y axis shows channel surface boron concentration after thermal annealing. From Fig. 17, it is found that reflow at 900°C or higher is possible because boron hardly diffuses in a silicon nitride film. Therefore, boron does not penetrate even if the reflow temperature is higher than 900°C when the silicon nitride film is contained in a gate insulating film (e.g. ONO structure). Moreover, because boron hardly diffuses for RTA (Rapid Thermal Annealing), a fabrication method is considered which is added with RTA at 900°C or higher for a short time (total thermal annealing time of approx. 10 mins or less) after source and drain regions are formed in the present example. RTA has an advantage of activating impurities contained in a gate electrode and not activated yet which is decreasing a gate resistance. If the gate resistance can be decreased, operations of a semiconductor device can be accelerated. Moreover, to decrease the gate resistance, it is devised to connect a gate electrode with a metallic wire every certain interval (e.g. 50µm)(backing of gate electrode by metallic wire) or form a two-layer structured gate electrode of polysilicon and refractory metal. Embodiment, Gate oxide film 1000A The embodiment of the present invention is described below by referring to the accompanying drawings. Figs. 18A to 18H are sectional views showing the fabrication process of the embodiment of the method of the semiconductor device of the present invention. First, an N well 2 is formed in a P-type substrate 1 (20 to 30 Ωcm) through ion implantation (phosphorus 100 to 150 KeV 3 to 4 E12/cm 2 ) and thermal diffusion (1175° C oxidation atmosphere 19 hr). Thereafter, a P-type region 3 and an N-type region 4 for device separation are formed through ion implantation (P-type region: boron 25 to 35 KeV 5.0 to 8.0 E13/cm 2 , N-type region: phosphorus 60 to 120 KeV 1.0 to 2.0 E12/cm 2 ). Then, a field oxide film 5 (approx. 1µm) and a gate oxide film 6 (approx. 100 nm) are formed through thermal oxidation, see Fig. 18A and ion implantation (N-channel enhancement-type boron 40 to 45 KeV 1 to 4 E11/cm 2 , N-channel depression-type phosphorus 120 to 140 KeV 2 to 6 E11/cm 2 , P-channel enhancement-type phosphorus 120 to 140 KeV 1 to 4 E11/cm 2 ) to a channel region for threshold voltage control is performed, see Fig. 18B, and a polysilicon film 7 (300 to 400 nm) is deposited, see Fig. 18C. Then, high-resistance polysilicon 8 and polysilicon gate electrode 10 and 11 and wiring polysilicon (not shown) are simultaneously formed through etching, see Fig. 18D, and the oxide film is completely removed from the source and drain region through etching, see Fig. 18E, and an oxide film is formed again on the source and drain up to a thickness of 10 to 20 nm, see Fig. 18F. Then, ion implantation for the resistance value control of high-resistance polysilicon 8 (phosphorus 40 to 60 KeV 1 to 6 E14/cm 2 ) and ion implantation (N-channel side phosphorus 40 to 80 KeV 7.0 to 10.0 E15/cm 2 , P-channel side boron 20 to 35 KeV 2.0 to 5.0 E15/cm 2 ) for introduction of impurities into gate electrodes and formation of source and drain regions at the N-channel side (polysilicon gate electrode 10 and source and drain region 12) and the P-channel side (polysilicon gate electrode 11 and source and drain region 13) are performed, see Fig. 18G. In this case, ions are also implanted into wiring polysilicon. Then, an NSG (Nondoped Silicate Glass) film 19 is deposited up to a thickness of 200 to 300 nm and moreover a PSG (Phosphosilicate Glass) film 20 containing 8 to 12 wt% of phosphorus is deposited up to a thickness of 400 to 600 nm to perform the reflow of an inter-layer insulating film at a temperature of 800 to 900°C for 30 mins. Finally, a contact hole and a metallic electrode 21 are formed to obtain the circuit comprising high-resistance polysilicon and surface-channel-type polysilicon gate CMOS transistors, see Fig. 18H. A main feature of this embodiment lies in the fact that a thick gate oxide film is completely removed from the surface of source and drain after forming polysilicon gate electrodes and an oxide film is formed again up to a thickness of 10 and 20 nm through thermal oxidation. That is useful for fabricating semiconductor devices comprising MOS transistors with a thick gate oxide film. In general, high concentration impurities are introduced into the source and drain of MOS transistor in order to decrease the parasitic resistance of the source and drain regions. When the gate oxide film thickness increases, ion implantation energy must be increased to introduce impurities into the source and drain at a high concentration. To increase the implantation energy, however, there are problems such as limitation of machine performance, charge-up, and penetration of boron into channel region. Therefore, the oxide film thickness is set to 10 to 20 nm on the surface of source and drain without changing the thickness of a gate oxide film under a polysilicon gate electrode by using the gate electrode as an etching mask, so that impurities can be implanted into the source and drain at a high concentration even at a low energy. The 10 to 20 nm thickness oxide film of the source and drain is used for protection against external contamination. Oxidation of polysilicon slightly increases the drain breakdown voltage (by approx. 2 V) by rounding the edge of gate polysilicon. The oxide film thickness is set to 10 to 20 nm in order to prevent contamination atoms from entering from the outside of the substrate, and with boron and BF 2 ion implantation energies limited to 35 KeV and 100 KeV respectively, no boron penetrates into the channel region. This specification provides a semiconductor device, wherein a reference voltage circuit comprising a depression-type NMOS transistor and an enhancement-type NMOS transistor which connects a source with a drain in series between a power supply line and an earthing line is included, and the gate electrode of the depression-type NMOS transistor contacts a ground line and the gate electrode of the enhancement-type NMOS transistor contacts a joint between the source of the depression-type NMOS transistor and the drain of the enhancement-type NMOS transistor. The following advantages are obtained. The first advantage is that the MOS transistors channel leak current are decreased by using the surface channel type and the operating voltage is lowered (e.g. a minimum operating voltage of a voltage detection is 0.35 V). The second advantage is that a voltage detector and a voltage regulator are operated at a low voltage by using the low reference voltage circuit. The third advantage is that setting the gate oxide film thickness to 20 to 100 nm and the acceptor concentration of the source/drain region of the MOS transistor to 0.5 to 1.5 x 10 26 /m 3 , and the donor concentration of the source/drain region of an NMOS transistor to 2.1 to 3.0 x 10 26 /m 3 prevents the penetration of boron into channel regions and the increasing of the source/drain resistance. The fourth advantage is that the capacity of an insulating film increases, a high conductance is obtained, high current CMOS transistors is obtained, and boron in a gate electrode is prevented from penetrating into a channel region by using a silicon nitride film as the insulating film. The fifth advantage is that the resistance value of high-resistance polysilicon is prevented from fluctuating due to thermal annealing by forming an NSG film between high-resistance polysilicon and the PSG film. The sixth advantage is that 2 masks and 30 processes are saved. This corresponds to approx. 10% of all processes for existing fabrication of a voltage detector and a voltage regulator. The seventh advantage is that boron is prevented from penetrating into a channel region by setting the polysilicon gate film thickness to 300 to 400 nm and the boron ion implantation energy to 20 to 35 KeV. The eighth advantage is that boron is prevented from penetrating into a channel region by setting the polysilicon gate film thickness to 300 to 400 nm and the BF 2 ion implantation energy to 50 to 100 KeV. The ninth advantage is that even if a gate oxide film has a large thickness, ions can be implanted into a source/drain region because ion implantation energy is controlled by forming a gate electrode through etching, thereafter completely removing an oxide film from source and drain regions through etching, then forming an oxide film up to a thickness of 20 nm through thermal oxidation, and setting boron and BF 2 ion implantation energies for the gate electrode to 20 to 35 KeV and 50 to 100 KeV respectively. The tenth advantage is that boron is prevented from penetrating into a channel region due to thermal diffusion by setting the gate oxide film thickness to 20 to 100 nm and every thermal annealing temperature after forming source and drain to 800 to 900°C. The eleventh advantage is that the resistance values of source/drain region and polysilicon gate electrodes are decreased by activating the impurities in silicon through RTA (Rapid Thermal Annealing). Boron in a gate electrode is prevented from penetrating into the channel region de to thermal diffusion.

Claims: A method for fabricating a semiconductor device having high-resistance polysilicon and low-resistance polysilicon on the surface of a substrate, wherein a high-resistance polysilicon (8) serves as a resistance and a low-resistance polysilicon serves as a gate electrode (10, 11) of a MOS transistor comprising the steps of: depositing a gate oxide film (6) having a thickness of 40 nm or more; depositing a polysilicon film (7) having a thickness of 300 to 400 nm; simultaneously forming the polysilicon film into a resistor, and a gate electrode through etching by using resist as a mask; completely removing said oxide film only from a source/drain region through etching after forming a gate electrode; depositing a further oxide film on the source/drain region up to a thickness of 10 to 20 nm through thermal oxidation after said removal of said oxide film on the source/drain region; introducing impurities for resistance value control into polysilicon serving as a resistor through ion implantation to form high-resistance polysilicon; introducing impurities serving as acceptors into polysilicon serving as the gate electrode of a PMOS transistor and the source and drain regions of the PMOS transistor through ion implantation such that the source and drain regions of the PMOS transistor have an acceptor concentration of 0.5 to 1.5 x 10 26 /m 3 ; and introducing ions serving as donors into polysilicon serving as the gate electrode of an NMOS transistor and the source and drain regions of the NMOS transistor through ion implantation, such that the source and drain regions of the NMOS transistor have a donor concentration of 2.1 to 3.0 x 10 26 /m 3 , wherein the acceptor and donor ions are implanted in the source and drain regions through said further oxide film. A method for fabricating a semiconductor device according to claim 1, comprising the step of implanting boron ions into the polysilicon film at an energy of 20 to 35 KeV. A method for fabricating a semiconductor device according to claim 1 or claim 2, comprising the step of implanting BF 2 ions into the polysilicon film at an energy of 50 to 100 KeV. A method for fabricating a semiconductor device according to any one of the preceding claims comprising the steps of forming the gate oxide film up to a thickness of 40 to 100 nm and performing the reflow of an inter-layer insulating film over said gate oxide film and said polysilicon in a temperature range of 800 to 900°C. A method for fabricating a semiconductor device according to any one of the preceding claims wherein impurities are introduced into the polysilicon gate electrode and thereafter impurities in silicon are activated through RTA (Rapid Thermal Annealing). A method of operating a semiconductor device fabricated by a method according to any one of the preceding claims wherein the semiconductor device is operated in a temperature range of -40°C to 80°C and a voltage range of 0.35 to 10.0 V.

Inventors: OSANAI JUN, SAITOH YUTAKA, YOSHIDA SHINICHI, YOSHIDA YOSHIFUMI, OSANAI, JUN, SAITOH, YUTAKA, YOSHIDA, SHINICHI, YOSHIDA, YOSHIFUMI, OSANAI, JUN, C/O SEIKO INSTRUMENTS INC., SAITOH, YUTAKA, C/O SEIKO INSTRUMENTS INC., YOSHIDA, SHINICHI, C/O SEIKO INSTRUMENTS INC., YOSHIDA, YOSHIFUMI, C/O SEIKO INSTRUMENTS INC.
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1995
Main classification: H01L27
IPC codes: H01L21, H01L27