Production method of a semiconductor device
This application is a division of application Ser. No. 07/504,837, filed Apr. 5, 1990, now U.S. Pat. No. 5,187,112. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for producing a semiconductor device and, more particularly, to a method for producing a narrow gate and an offset gate of a refractory metal in a self-aligned gate GaAs field effect transistor. 
     BACKGROUND OF THE INVENTION 
     FIGS. 5(a) to 5(d) show process steps in a prior art method for producing a semiconductor device. As shown in FIG. 5(a), an active layer 2 is produced at a desired position in a semi-insulating GaAs substrate 1 by ion implantation. 
     A refractory metal 4, such as tungsten silicide (hereinafter referred to as WSi x ), is deposited on the entire surface of the semi-insulating GaAs substrate 1 by sputtering or vapor deposition, and a pattern of photoresist 11 is formed where a gate is to be prepared, as shown in FIG. 5(b). As shown in FIG. 5(c), part of the WSi x  4 is removed by reactive ion etching using the photoresist 11 as a mask, thereby producing a gate electrode 4&#39;. Finally, as shown in FIG. 5(d), ion implantation using the WSi x  4&#39; as a mask produces high dopant concentration regions 5 (hereinafter referred to as an n +   region) after annealing, and a drain electrode 6 and a source electrode 7 are respectively produced thereon. 
     In the prior art method, in order to produce a gate electrode 4&#39;, patterning of the photoresist 11 is required. The precision of gate pattern depends on the precision of the photolithographic alignment technique. Furthermore, because the WSi x  4&#39; is used as a mask for the ion implantation step, the n +   regions 5 below the source electrode 7 and the gate electrode 4&#39; are close to each other, reducing the source resistance, advantageously increasing the gain of the FET. However, since the n +   regions 5 below the drain electrode 6 and the gate electrode 4&#39; are close to each other, reducing the gate-drain breakdown voltage, this low gate-drain breakdown voltage makes it difficult to use this structure in high power analog ICs. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for producing a semiconductor device with a narrow refractory metal gate without using photolithography, increased gate-drain breakdown voltage, and low source resistance. 
     Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiments are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     In a first method according to the present invention, an insulating film is produced on an active layer and a refractory metal is deposited on the entire surface of the semiconductor substrate. The refractory metal is partially removed but left at a side wall of the insulating film to produce a gate electrode. A high dopant impurity concentration region is produced by ion implantation using the insulating film and the refractory metal as a mask, the insulating film is removed, and an intermediate dopant impurity concentration region is produced by ion implantation using the refractory metal as a mask. A source electrode is produced on the high dopant impurity concentration region, and a drain electrode is produced on the intermediate dopant impurity concentration region. 
     In this method, since the refractory metal at the side wall of the insulating film is used as a gate electrode, a narrow refractory metal gate can be produced without being restricted in width by the precision of photolithography. Furthermore, since a high dopant impurity concentration region is produced beneath the source electrode and an intermediate dopant impurity concentration region is produced beneath the drain electrode, the gate-drain breakdown voltage is increased and the source resistance is reduced. 
     In another method according to the present invention, after the refractory metal is partially removed but left at the side wall of the insulating film, as in the first described method, and an intermediate dopant impurity concentration region is produced by ion implantation using the first insulating film and the refractory metal as masks, a second insulating film is deposited on the entire surface of the semiconductor substrate and is partially removed but left at a side wall of the refractory metal, the first insulating film is removed, moved, and a high dopant impurity concentration region is produced by ion implantation using the refractory metal and the second insulating film remaining at the side wall as masks. A drain electrode is produced at the side of the refractory metal gate electrode where the intermediate dopant impurity concentration region is produced, and a source electrode is produced on the opposite side of the refractory metal gate electrode. 
     In still another method according to the present invention, after the refractory metal is partially removed but left at the side wall of the insulating film in the first described method, the insulating film is removed, an intermediate dopant impurity concentration region is produced by ion implantation using the refractory metal which is to become a gate electrode as a mask, and a second insulating film is deposited on the entire surface of semiconductor substrate. Masks having asymmetrical configurations are produced at both side walls of the gate electrode, and a high dopant concentration region is produced by ion implantation using the second insulating film as a mask. A drain electrode is produced at the side of the refractory metal electrode where a wide intermediate dopant impurity concentration region has been produced, and a source electrode is produced on the opposite side of the gate electrode. 
     In yet another method according to the present invention, after the second insulating film is deposited on the entire surface of semiconductor substrate in the third described method, the second insulating film is partially removed but left at both side walls of the refractory metal in asymmetrical configurations, and a high dopant impurity concentration region is produced by ion implantation using the second insulating film remaining at both side walls and the refractory metal as masks. A drain electrode is produced at the side of the refractory metal electrode where a wider intermediate dopant impurity concentration region has been produced, and a source electrode is produced on the opposite side of the refractory metal gate electrode. 
     In these second, third, and fourth described methods, similar to the first described method, since the refractory metal remaining at the side wall of the insulating film is used as a gate electrode, a narrow refractory metal gate electrode is produced without being restricted in width by the precision of photolithography. Furthermore, the high dopant impurity concentration region below the source electrode is adjacent the gate electrode and the distance between the gate electrode and the high dopant impurity concentration region below the drain electrode is lengthened by producing an intermediate dopant impurity concentration region between the gate electrode and the high dopant impurity concentration region, thereby increasing the gate-drain breakdown voltage while holding the source resistance at a low value. Furthermore, a narrow interval is provided between the gate electrode and the high dopant impurity concentration region below the source electrode by producing an intermediate dopant impurity concentration region that is narrower than the intermediate impurity dopant concentration region and is disposed between the gate electrode and the high dopant impurity concentration region below the drain electrode, thereby preventing current leakage between the gate electrode and the high dopant impurity concentration region below the source electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(a) to 1(g) are cross-sectional views illustrating process steps of a method for producing a semiconductor device according to a first embodiment of the present invention; 
     FIGS. 2(a) to 2(e) are cross-sectional views illustrating process steps of a method for producing a semiconductor device according to a second embodiment of the present invention; 
     FIGS. 3(a) to 3(c) are cross-sectional views illustrating process steps of a method for producing a semiconductor device according to a third embodiment of the present invention; 
     FIGS. 4(a) to 4(c) are cross-sectional views illustrating process steps of a method for producing a semiconductor device according to a fourth embodiment of the present invention; and 
     FIGS. 5(a) to 5(d) is a diagram showing major process steps of a prior art method for producing a semiconductor device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1(a) to 1(g) are cross-sectional views illustrating process steps for producing a semiconductor device according to a first embodiment of the present invention. First, as shown in FIG. 1(a), silicon ions are implanted in the semi-insulating GaAs substrate 1 at an energy of 50 KeV and a flux of 1.5×10 12  /cm 2 , thereby producing an active layer 2 that is 600 angstroms thick. A SiO 2  film 3 is deposited on the entire surface of the substrate to a thickness of about 3000 angstroms, and, thereafter, as shown in FIG. 1(b), the SiO 2  film 3 is patterned by reactive ion etching using a mixture of CHF 3  +O 2 . 
     Next, as shown in FIG. 1(c), WSi x  4 is anisotropically etched by reactive ion etching using a mixture of CHF 4  +O 2 , leaving a WSi x  gate electrode 4&#39; at the side wall of the SiO 2  film 3. In this case, the width of the WSi x  gate electrode 4&#39;, that is, the gate length, has an upper limit of about 0.2 microns. The gate length is controlled by the thickness of the WSi x  4. 
     Next, as shown in FIG. 1(e), silicon ions are implanted at a flux of 3×10 13  /cm 2  using the SiO 2  film 3 and WSi x  gate electrode 4&#39; as masks, thereby producing a high dopant impurity concentration n +   region 5 that is 1000 angstroms thick. 
     As shown in FIG. 1(f), the SiO 2  film 3 is removed using a hydrofluoric acid solution having a ratio of 1:30 of HF:H 2  O, and silicon ions are implanted at a flux of 7×10 12  /cm 2  using the gate electrode 4&#39; as a mask, thereby producing an intermediate dopant impurity concentration n&#39; region 8 that is 800 angstroms thick. In this case, silicon ions are also implanted into the n +   region 5, but the dopant impurity concentration of the n +   region 5 is quite high relative to the concentration of n&#39; layer 8, thereby resulting in essentially no change in the dopant impurity concentration. 
     As shown in FIG. 1(g), after annealing at 800° C. for about 45 minutes, AuGe/Ni/Au is deposited and partially lifted off, thereby producing a source electrode 6 on the n +   region 5 and a drain electrode 7 on the n&#39; region 8. Thus, a field effect transistor having a narrow gate is completed. 
     In this method, since the refractory metal remaining at the side wall of the insulating film after the etching is used as the gate electrode, a narrow refractory metal gate can be produced without being restricted in width by the precision of photolithography. Furthermore, since the dopant impurity concentration at the drain electrode side of the gate electrode is lower than at the source electrode side of the gate electrode and the depth of the drain region 8 is shallower than the depth of the source region 5, the gate-drain breakdown voltage is increased while the source resistance remains low. 
     FIGS. 2(a) to 2(e) are cross-sectional views illustrating process steps for producing a semiconductor device according to a second embodiment of the present invention. Beginning from the structure of FIG. 1(d), silicon ions are implanted at a flux of 7×10 12  /cm 2  using the SiO 2  film 3 and WSi x  gate electrode 4&#39; as a mask, thereby producing an n&#39; region 8. An Si 3  N 4  film 9 is deposited on the entire surface of the semi-insulating GaAs substrate 1 in a chemical vapor deposition (CVD) process, thereby producing the structure of FIG. 2(a). 
     Next, as shown in FIG. 2(b), the Si 3  N 4  film 9 is anisotropically etched by reactive ion etching, leaving a remaining portion of the Si 3  N 4  film 9 at the side of the WSi x  gate electrode 4&#39;. 
     Next, as shown in FIG. 2(c), the SiO 2  film 3 is removed using a hydrofluoric acid solution having a ratio of 1:30 of HF:H 2  O. In this case, since the etching rate of the remaining portion of the Si 3  N 4  film 9 is less than 1/10 of the etching rate of the SiO 2  film 3, the remaining Si 3  N 4  film 9 is hardly etched. 
     As shown in FIG. 2(d), silicon ions are implanted at a flux of 3×10 13  /cm 2  using the WSi x  gate electrode 4&#39; and the remaining portion of the Si 3  N 4  film 9 as a mask, thereby producing n +   regions 5 and leaving the part of the n&#39; region 8 below the remaining portion of the Si 3  N 4  film 9 in place. 
     Next, as shown in FIG. 2(e), after annealing at 800° C. for about 45 minutes, the remaining portion of the Si 3  N 4  film 9 is removed, and an AuGe/Ni/Au film is deposited and partially lifted off, thereby producing a drain electrode 7 on the n +   region 5 at the side of the gate electrode where the n&#39; region 8 is left and a source electrode 6 is produced on the n +   region on the opposite side of the gate electrode 4&#39;, thereby completing a field effect transistor. 
     In this method, similar to the first embodiment, since the refractory metal remaining at the side wall of the insulating film 3 is used as the gate electrode, a narrow refractory metal gate electrode is produced without being restricted in width by the precision of photolithography. Furthermore, since the n +   region 5 below the source electrode 6 and the gate electrode 4&#39; are close to each other and the n +   region 5 below the drain electrode 7 and the gate electrode 4&#39; are spaced from each other by the remaining part of the n&#39; region 8 between the n +   region and the gate electrode 4&#39;, the source resistance is reduced and the gate-drain breakdown voltage is increased. 
     FIGS. 3(a) to 3(c) are cross-sectional side views illustrating process steps for producing a semiconductor device according to a third embodiment of the present invention. 
     Beginning from the structure of FIG. 1(d), the SiO 2  film 3 is removed using a hydrofluoric acid solution having a ratio of 1:30 of HF:H 2  O, and, thereafter silicon ions are implanted at a flux of 3×10 13  /cm 2  using the WSi x  gate electrode 4&#39; as a mask, thereby producing n&#39; regions 8 as shown in FIG. 3(a). 
     As shown in FIG. 3(b), an SiO 2  film 10 is deposited on the entire surface of the semi-insulating GaAs substrate 1 to a thickness of about 2000 angstroms. In this case, since the configuration of the WSi x  gate electrode 4&#39; is asymmetrical, the SiO 2  film 10 is about 1000 angstroms thick at the side wall of the WSi x  gate electrode 4&#39; in FIG. 3(b) and about 2000 angstroms thick at the opposite, curved side wall of the WSi x  gate electrode 4&#39;. Therefore, the SiO 2  film 10 has an asymmetrical configuration at left and right sides of the WSi x  gate electrode 4&#39;. Accordingly, when n +   regions 5 are produced by implanting silicon ions at a flux of 3×10 13  /cm 2  using the SiO 2  film 10 as a mask, the distances between the n +   regions 5 and the WSi x  gate electrode 4&#39; are also asymmetrical. Furthermore, the distances between the n +   regions 5 and the WSi x  gate electrode 4&#39; are controlled by the thicknesses of the SiO 2  film 10 at the sides of the gate electrode 4&#39;. This process leaves residual portions of the n&#39; regions 8 at both sides of the gate electrode 4&#39;. The residual n&#39; regions are asymmetrical because of the asymmetry of the SiO 2  film at the sides of the gate electrode 4&#39;. 
     Next, after annealing, the SiO 2  film 10 is removed, and AuGe/Ni/Au is deposited and partially lifted off, thereby producing a drain electrode 7 on the n +   region 5 on the side of the gate electrode 4&#39; where the wider n&#39; region 8 remains and a source electrode 6 on the opposite side of the gate electrode 4&#39;. 
     In this method, similar to the first and second embodiments, a narrow gate electrode is produced and a field effect transistor having a high gate-drain breakdown voltage is obtained while keeping the source resistance low. Since a narrow interval is provided between the gate electrode 4&#39; and the n +   region 5 below the source electrode 6 by the n&#39; region 8, which is narrower than the n&#39; region 8 between the gate electrode 4&#39; and the n +   region 5 below the drain electrode 7, leakage current flow between the gate electrode and the n +   region below the source electrode is prevented, and an improved efficiency field effect transistor is obtained. 
     FIGS. 4(a) to 4(c) are cross-sectional side views illustrating process steps for producing a semiconductor device according to a fourth embodiment of the present invention. 
     Beginning from the structure of FIG. 3(a), an SiO 2  film 10 is deposited on the entire surface of the semi-insulating GaAs substrate 1 to a thickness of about 2000 angstroms, thereby resulting in the structure of FIG. 4(a). The SiO 2  film 10 is anisotropically etched by reactive ion etching using a mixture of CHF 3  to O 2 , leaving portions of the film 10 at both sides of the WSi x  gate electrode 4&#39;. Since the WSi x  gate electrode 4&#39; has an asymmetrical configuration at the left and right sides of FIG. 4(b), the thicknesses of the SiO 2  films 10&#39; remaining at the sides of WSi x  gate electrode 4&#39; are also asymmetrical. An SiO 2  film 10&#39; about 1000 angstroms wide is produced at the vertical side wall of the WSi x  gate electrode 4&#39; and another film 10&#39; about 2000 angstroms wide is produced at the curved side wall of the WSi x  gate electrode 4&#39;. 
     As shown in FIG. 4(b), silicon ions are implanted at a flux of 3×10 13  /cm 2  using the SiO 2  films 10&#39; at both sides of the WSi x  gate electrode 4&#39; as a mask, thereby producing n +  regions 5. In this case, the distances between the n +   regions and the WSi x  gate electrode 4&#39; are asymmetrical. Furthermore, the distances between the n +   regions 5 and the WSi x  gate electrode 4&#39; are controlled by the thicknesses of the SiO 2  films 10. 
     After annealing, the SiO 2  films 10&#39; are removed and AuGe/Ni/Au is deposited and partially lifted off, thereby producing a drain electrode 7 on the n +   region 5 at the side of the gate electrode 4&#39; where the wider n&#39; region 8 remains and a source electrode 6 on the n +   region 5 at the opposite side of the gate electrode 4&#39;. 
     In this method, similar to the first to third embodiments, since photolithography is not required for the production of the gate electrode, a narrow gate electrode is precisely produced. Furthermore, since a narrow interval is provided between the gate electrode 4&#39; and the n +   region 5 below the source electrode, leakage current flow between the gate electrode 4&#39; and the n +   region 5 is effectively prevented and the gate-drain breakdown voltage is increased while suppressing the source resistance. 
     In the first to fourth embodiments WSi x  is used for the refractory metal of the gate electrode 4&#39;, WSiN or WN may also be used with the same effects as described above. In the second, third, and fourth embodiments the insulating films at the side walls of the gate electrode 4&#39; are removed after the ion implantation, these films may be left in place with the same effects as described above. 
     As is evident from the foregoing description, according to the present invention, a refractory metal is deposited on the entire surface of a semiconductor substrate covering an insulating film deposited on an active layer in the semiconductor substrate. Thereafter, the refractory metal is removed, leaving a residual portion at the side wall of the insulating film, thereby producing a gate electrode. Accordingly, patterning by photolithography is not required to produce the gate electrode and the gate length is not restricted by the precision of photolithography, whereby a field effect transistor having a narrow gate is precisely and reproducibly made. 
     Furthermore, an n +   region having a high dopant impurity concentration is disposed below the source electrode and an n&#39; region having a lower dopant impurity concentration is disposed below the drain electrode. In order to obtain an offset structure gate, the distance between the gate electrode and the n +   region below the drain electrode is lengthened by the insertion of an n&#39; region between the gate electrode and the n +   region while maintaining a short distance between the gate electrode and the n +   region below the source electrode. Therefore, the source resistance is reduced and the gate-drain breakdown voltage is increased. Furthermore, since a narrow spacing is provided between the gate electrode and the n +   region below the source electrode by inserting an n&#39; region having a narrower width than the n&#39; region between the gate electrode and the n +   region below the drain electrode, leakage current flow between the gate electrode and the n +   region below the source electrode is prevented, and an improved efficiency field effect transistor is obtained.