Compound semiconductor device and production method therefor

Abstract: A compound semiconductor device includes a plurality of compound semiconductor epitaxially grown layers including an InGaAs layer (6) as the uppermost layer, disposed on a semi-insulating compound semiconductor substrate (1); a recess groove (15) penetrating through prescribed layers of the compound semiconductor layers at least including the InGaAs layer; insulating side walls (10) disposed on the side surfaces of the recess groove (15), whose upper end parts extend outside the recess groove (15); a gate electrode (21) deposited on the bottom of the recess groove (15) and the internal surfaces of the insulating side walls (10); and source and drain electrodes (20) deposited on the InGaAs layer (6) at opposite sides of the recess groove (15) self-alignedly with the upper ends of the insulating side walls (10).

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a field-effect compound semiconductor device and, more particularly, to a high performance compound semiconductor device having reduced gate-to-source and gate-to-drain distance and a reduced gate resistance. BACKGROUND OF THE INVENTION In a field-effect transistor like a high electron mobility transistor (hereinafter referred to as HEMT) made of a compound semiconductor, such as gallium arsenide (GaAs), a gate electrode having a short length and a low resistance is required to enhance the characteristics of the transistor. A gate electrode having a T-shaped cross-section meets this requirement and various methods for producing such a gate electrode have been proposed. Document EP-A-0 394 590 discloses a respective method for producing a compound semiconductor device comprising the steps of epitaxially growing a plurality of compound semiconductor layers including an InGaAs layer as an uppermost layer. On the InGaAs layer an insulating film is deposited on which a photoresist pattern having an aperture at a gate electrode production position is formed. Using the photoresist pattern as a mask the insulating film is etched. Subsequently another insulating film is deposited on the first insulating film having a groove. The side walls of the groove of the second insulating film are covered by again depositing the material of the first insulating film. With a plurality of etching steps portions respective of the first and second insulating films are removed in such a way that only L-shaped insulating films remain on the InGaAs layer. However, this conventional method for producing a compound semiconductor device employs a great number of individual deposition and etching steps. Moreover the gate resistance and the gate-to-source resistance is not sufficiently low. Figure 3 is a cross-sectional view of a further prior art field-effect transistor, produced with reference to a method for producing a gate electrode disclosed in Japanese Published Patent Application No. Sho. 61-77370. In figure 3, reference numeral 31 designates a semi-insulating GaAs substrate. A high purity GaAs buffer layer 32 is disposed on the substrate 31. An n type AlGaAs layer 33 is disposed on the buffer layer 32. A high concentration n type (hereinafter referred to as n + type) GaAs layer 34 is disposed on the AlGaAs layer 33. Ohmic electrodes 35 serving as source and drain electrodes are disposed on the n + type GaAs layer 34 opposite to each other with a prescribed interval. A stripe-shaped recess groove 36 penetrates a prescribed portion of the n + type GaAs layer 34 between the ohmic electrodes 35 and a gate electrode 37 having a T-shaped cross-section is disposed on the AlGaAs layer 33 which is exposed on the bottom surface of the recess groove 36. A two-dimensional electron gas layer 38 is formed in the buffer layer 32 in the vicinity of the boundary with the AlGaAs layer 33, in which electrons supplied from the AlGaAs layer 33 are stored. Figures 4(a) to 4(f) are cross-sectional views in process steps for producing the conventional field effect transistor of figure 3. First of all, as shown in figure 4(a), a GaAs buffer layer 32, an n type AlGaAs layer 33, and an n + type GaAs layer 34 are successively epitaxially grown on the semi-insulating GaAs substrate 31. Then, ohmic electrodes 35 are formed on the n + type GaAs layer 34 opposite to each other with a prescribed interval, by deposition and lift-off technique using a photoresist, as shown in figure 4(b). Then, as shown in figure 4(c), on the n + type GaAs layer 34 having the ohmic electrodes 35, a lower resist film 40 approximately 200 nm (2000 angstroms) thick and an upper resist film 41 approximately 1 micron thick are successively deposited. The upper resist film 41 has a high sensitivity and the lower resist film 40 has a sensitivity lower than that of the upper resist film 41. Then, the lower and upper resist films 40 and 41 on the ohmic electrodes 35 are exposed in a stripe shape by an electron beam exposure method, forming a pattern of a gate electrode. If the amount and intensity of the electron beam are made large at the center of the gate pattern and small at the both sides of the gate pattern, both of the upper and lower resist films 41 and 40 are exposed at the center of the gate pattern while only the upper resist film 41 is exposed at the both sides of the gate pattern, resulting in the lower resist 40 and the upper resist 41 shown in figure 4(d). Then, the n + type GaAs layer 5 is etched by wet etching using the lower resist film 40 as a mask to form a recess groove 36. Thereafter, a gate metal 45 is deposited on the entire surface of the wafer as shown in figure 4(e). Then, the gate metal 45 on the upper resist film 41 is removed together with the resist films 41 and 40 by a lift-off technique, resulting in a field effect transistor shown in figure 4(f). According to this conventional method, the high sensitivity upper resist film and the low sensitivity lower resist film are patterned by the electron beam exposure whose amount and intensity are varied on the resist films and then the gate metal is formed by metal deposition and lift-off technique. Therefore, a fine T-shaped gate is formed in a simple process and an increase in the gate resistance due to a reduced gate length is suppressed, resulting in a high performance field-effect transistor even in a high frequency region. However, when the photoresist is deposited on the n + type GaAs layer 34 in the step of figure 4(c), since the ohmic electrodes 35 are present, the step difference causes a non-uniformity of the photoresist, deteriorating the patterning precision of the photoresist. Further, in the step of figure 4(d), the gate-to-source distance varies due to a mask alignment error or the like, inviting variation in the characteristics of the completed devices. Figures 5(a) to 5(h) are cross-sectional views in process steps for producing a conventional field effect transistor using a conventional production method of a gate electrode disclosed in Japanese Published Patent Application No. Hei. 3-21032. First of all, a semiconductor substrate having an active region is prepared. Then, as shown in figure 5(a), a photoresist film 51 approximately 200 nm (2000 angstroms) thick is formed on the substrate 50 and a polyimide film 52 approximately 1000 nm (10000 angstroms) thick is formed thereon. As shown in figure 5(b), an aluminum film 53 approximately 50 nm (500 angstroms) thick having a stripe-shaped aperture 54 whose width corresponds to a gate length is formed on the polyimide film 52 using deposition and lift-off technique. Then, the polyimide film 52 and the photoresist film 51 are etched by dry etching using O 2 plasma and using the aluminum film 53 as a mask to form an aperture 55 as shown in figure 5(c). Thereafter, the wafer is soaked in a mixed solution comprising ethylenediamine and hydrazine to etch away the polyimide film 52, whereby the aperture 55 of the polyimide film 52 is widened as shown in figure 5(d). Then, the aluminum film 53 is etched using a mixed solution comprising H 3 PO 4 and H 2 O as shown in figure 5(e). As shown in figure 5(f), a gate metal 56, such as Ti/Pt/Au, is deposited on the entire surface of the wafer. Thereafter, the gate metal on the polyimide film 52 is removed together with the polyimide film 52 and the photoresist film 51 by a lift-off technique using acetone solution, resulting in a T-shaped gate comprising base part 57 and head part 58 shown in figure 5(g). Since the head part 58 of the gate is formed utilizing the selective etching of the polyimide film 52, the head part 58 does not deviate from the base part 57. Finally, as shown in figure 5(h), an Au system metal 59 is deposited on the entire surface of the wafer and then the metal 59 on the semiconductor substrate 50 is alloyed with the semiconductor by annealing to form source and drain electrodes which ohmic contact with the semiconductor. According to this conventional method, since the source and drain electrodes are formed self-alignedly with the gate electrode, the gate-to-source distance does not vary, so that the characteristics of the device do not deteriorate. In addition, the gate-to-source distance can be shorter than the precision of the mask alignment, resulting in a high performance device having a reduced gate-to-source resistance. Further, since the metal 59 deposited on the gate electrode has a lower resistivity than that of the T-shaped gate comprising the base part 57 and the roof part 58, this metal 59 reduces the resistance of the gate electrode resistance, enhancing the high frequency characteristic of the device. When a field effect transistor is manufactured in accordance with the process steps shown in figures 5(a) to 5(h), in order to further reduce the gate resistance to enhance the characteristics, the metal layer 59, which is deposited on the lower T-shaped gate electrode, may be thicker. In this case, the ohmic electrodes 35, which are deposited with the metal layer 59, unfavorably become thick and may be short-circuited with the lower gate electrode 57 as shown in figure 6. As a result, the gate resistance cannot sufficiently be reduced. If the photoresist film 51 is formed thick to heighten the base part 57 of the T-shaped gate, the thick ohmic electrodes 35 may not be short-circuited with the gate electrode. However, since a metal is accumulated vertically to the plane surface of the substrate during vapor deposition, it is impossible to form the photoresist film 51 thicker than the deposited gate metal 56. That is, there is a limitation to the reduction in gate resistance according to this method. In addition, the metal deposition is required twice in each method of figure 4 and figure 5, thereby complicating the production process. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing a compound semiconductor device that can reduce a gate-to-source resistance and that can sufficiently suppress an increase in gate resistance caused by a reduction in gate length. According to the invention this object is achieved by a method for producing a compound semiconductor device comprising the steps of epitaxially growing a plurality of compound semiconductor layers including an InGaAs layer as an uppermost layer; successively depositing first and second insulating films on said InGaAs layer, forming a photoresist pattern having an aperture at a gate electrode production position, on said second insulating film; etching said first and second insulating films using said photoresist pattern as a mask; selectively etching the second insulating film so that the aperture of the second insulating film is larger than the aperture of the first insulating film; forming a recess groove by etching away a portion of the compound semiconductor epitaxially grown layers using the first insulating film as a mask; forming a third insulating film on the side surfaces of said first and second insulating films and said recess groove; removing first and second insulating films and depositing gate, source and drain electrodes. The advantages of the invention will become apparent and obvious to those skilled in the pertinent art upon referring to the following description provided in connection with the accompaning drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view showing a compound semiconductor device fabricated in accordance with an embodiment of the present invention; Figures 2(a) to 2(h) are cross-sectional views in process steps according to the present invention for producing the compound semiconductor device of figure 1; Figure 3 is a cross-sectional view showing a prior art compound semiconductor device; Figures 4(a) to 4(f) are cross-sectional views in process steps for producing the compound semiconductor device of figure 3; Figures 5(a) to 5(h) are cross-sectional views in process steps for producing another prior art compound semiconductor device; and Figure 6 is a cross-sectional view for explaining problems in the prior art devices. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is a cross-sectional view showing a compound semiconductor device fabricated in accordance with an embodiment of the present invention. In figure 1, a GaAs buffer layer 2 approximately 3000 angstroms thick is disposed on the semi-insulating GaAs substrate 1. An n type AlGaAs layer 4 having a thickness of approximately 400 angstroms and a carrier concentration of approximately 2 X 10 18 cm -3 is disposed on the buffer layer 2. A two-dimensional electron gas layer 3 is formed in the buffer layer 2 in the vicinity of the boundary with the n type AlGaAs layer 4. An n type GaAs layer 5 having a thickness of approximately 1500 angstroms and a carrier concentration of approximately 8 X 10 18 cm -3 is disposed on the n type AlGaAs layer 4. An n type InGaAs layer 6 having a thickness of approximately 500 angstroms and a carrier thickness of approximately 4 X 10 19 cm -3 is disposed on the n type GaAs layer 5. A recess groove 15 0.5 micron width penetrates the n type InGaAs layer 6 and the n type GaAs layer 5. SiN films 10 are disposed on the opposite side walls of the recess groove 15. The upper end parts of the SiN films 10 extend outside the recess groove over the n type InGaAs layers 6. A gate electrode 21 is disposed on the bottom surface of the recess groove 15 and on the internal surfaces of the SiN films 10. Source and drain electrodes 20 are disposed on the n type InGaAs layers 6 at both sides of the recess groove 15, alignedly with the upper end parts of the SiN films 10. Figures 2(a) to 2(h) are cross-sectional views illustrating process steps according to the present invention for producing the semiconductor device of figure 1. First, as shown in figure 2(a), a GaAs buffer layer 2 having a thickness of 3000 angstroms, an n type AlGaAs layer 4 having a thickness of 400 angstroms and a carrier concentration of 2 X 10 18 cm -3 , an n type GaAs layer 5 having a thickness of 1500 angstroms and a carrier concentration of 8 X 10 18 cm -3 , and an n type InGaAs layer 6 having a thickness of 500 angstroms and a carrier concentration of 4 X 10 19 cm -3 are successively epitaxially grown on the semi-insulating GaAs substrate 1 by MBE. Then, as shown in figure 2(b), a SiO film 7 serving as a first insulating film and a SiON film 8 serving as a second insulating film are successively deposited onto the n type InGaAs layer 6 to a thickness of 2000 angstroms each by plasma CVD. Then, a photoresist film 9 is formed on the SiON film 8 and exposed and developed to form an aperture pattern of a gate electrode region. Thereafter, the SiON film 8 and the SiO film 7 are anisotropically etched by reactive ion etching using a mixed gas comprising, for example CHF 3 and O 2 . Thereafter, as shown in figure 2(c), only the SiON film 8 is selectively etched by about 0.15 to 0.2 microns by reactive ion etching using a mixed gas comprising CF 4 and O 2 . After removing the photoresist 9 by selective etching, the n type InGaAs layer 6 and the n + type GaAs layer 5 are etched by reactive etching using SiCl 4 gas and a mixed gas comprising CF 4 and O 2 , respectively, to form a recess groove 15. The width of the recess groove 15 (w 1 in figure 1) is about 0.5 micron. Thereafter, as shown in figure 2(e), a SiN film 10 serving as a third insulating film is deposited on the entire surface of the wafer to a thickness of approximately 3000 angstroms by plasma CVD. Then, as shown in figure 2(f), the SiN film 10 is anisotropically etched by electron cyclotron plasma etching using a mixed gas of SF 6 and CHF 3 leaving portions on the side surfaces of the SiON and SiO films 8 and 7 and on the side walls of the recess groove. The width of the SiN film 10 left on the side wall of the recess groove 15 (w 3 in figure 1) is about 0.2 micron and the interval between opposite SiN films 10 at the bottom of the recess groove 15 (w 4 in figure 1) is about 0.1 micron (= w 1 - 2w 3 ). This interval w 4 will be a gate length. Then, the SiON film 8 and the SiO film 7 are removed by wet etching using a solution comprising water and ammonium fluoride mixed in a ratio of 30 : 1 as shown in figure 2(g). The SiN films 10 extend outside the recess groove 15 over the InGaAs layers 6, and the width of the upper end part of each SiN film 10 (w 2 in figure 1) is about 0.15 to 0.2 micron, which is equal to the width of the selectively etched portion of the SiON film 8 in the step of figure 2(c). Finally, a Ti/Mo/Au layer 11 is deposited on the wafer so as to have a directionality vertical to the wafer surface, whereby source and drain electrodes and a gate electrode are formed at the same time, resulting in the field-effect transistor of figure 1. In this embodiment, the gate-to-source distance is equal to the sum of the width w 3 of the SiN film 10 in the recess groove and the width w 2 of the upper end part. That is, the gate-to-source distance is not limited by the mask alignment precision and it is precisely controlled by the selective etching of the SiON film 8 and the anisotropic etching of the SiN film 10. Although the gate-to-source distance is about 0.35 to 0.4 micron in this embodiment, it can be easily reduced by the above-described control. In a compound semiconductor device like HEMT, a gate electrode produces Schottky junction with a semiconductor layer and source/drain electrodes produce ohmic contact with the semiconductor layer. When the source/drain electrodes are formed on a GaAs layer, the ohmic contact between the electrodes and the semiconductor layer is not achieved by only depositing the electrode metal, so that annealing is carried out to alloy the metal thereby to achieve the ohmic contact. In a case where the gate electrode has already been formed in the annealing process, the gate metal may be deteriorated by the annealing. In order to avoid this, in the conventional production process shown in figure 4, the source/drain electrodes are formed in the step 4(b) and then the gate electrode is formed in the later step 4(e). In the embodiment of the present invention, the uppermost layer of the epitaxially grown layers, i.e., the layer on which source/drain electrodes are to be formed, comprises InGaAs. Since InGaAs has a small energy band gap, the metal deposited on the n type InGaAs layer 6 becomes an ohmic electrode without being alloyed by annealing. Therefore, the annealing process can be omitted, resulting in no deterioration of the gate metal. When the gate electrode metal is deposited thick to reduce the gate electrode resistance, the source/drain electrodes, which are deposited at the same time, are thick. However, since the SiN films 10 are present on the rear surface of the upper end parts of the gate electrode, the short-circuiting of the source/drain electrodes and the gate electrode as shown in figure 6 is avoided. If the source/drain electrodes are deposited thick exceeding the SiN film 10 on the rear surface of the gate electrode, the source/drain electrodes may be short-circuited with the gate electrode. However, the distance between the surface of the n type InGaAs layer 6 and the upper end of the SiN film 10 on the rear surface of the gate electrode can be controlled by the thicknesses of the SiO film 7 and the SiON film 8, so that the gate electrode resistance can be sufficiently reduced by depositing those films 7 and 8 thick. In the present invention, the insulating side walls 10 whose upper end parts extend outside the recess groove 15 are formed on the side walls of the recess groove 15 and then the metal layer 11 is deposited using the insulating side walls 10 as a mask, whereby the gate electrode is formed on the bottom surface of the recess groove 15 and on the the insulating side walls while the source and drain electrodes are formed on the InGaAs layers 6 self-alignedly with the upper ends of the insulating side walls 10. As a result, a high performance HEMT having a short gate length, a low gate resistance and a low gate-to-source resistance can be achieved. While in the above-described embodiment the HEMT having epitaxially grown layers in which the two-dimensional electron gas layer is formed is described, the present invention can be also applied to other semiconductor devices having epitaxially grown layers in which no two-dimensional electron gas layer is formed, for example MESFETs, with the same effects as described above. As is evident from the foregoing description, according to a method for producing a compound semiconductor device of the present invention, a plurality of compound semiconductor layers including an InGaAs layer as an uppermost layer are epitaxially grown on a semi-insulating compound semiconductor substrate, first and second insulating films are deposited on the InGaAs layer, a photoresist pattern having an aperture at a gate electrode production position is formed on the second insulating film, the first and second insulating films are etched using the photoresist pattern as a mask, the second insulating film is selectively etched so that the aperture of the second insulating film is larger than the aperture of the first insulating film, a recess groove is formed by etching away a portion of the compound semiconductor epitaxially grown layers using the first insulating film as a mask, a third insulating film is deposited on the side surfaces of the first and second insulating films and on the recess groove, the first and second insulating films are removed, and a metal deposition is performed so that a gate electrode is formed on the bottom of the recess groove and on the third insulating film while source/drain electrodes are formed on the InGaAs layer. Therefore, the source/drain electrodes produce ohmic contact with the semiconductor layer in its non-alloyed state. As a result, a compound semiconductor device having reduced gate-to-source resistance and reduced gate electrode resistance can be produced with good yield in a simple process.

Claims: A method for producing a compound semiconductor device comprising the steps of: epitaxially growing a plurality of compound semiconductor layers including an InGaAs layer (6) as an uppermost layer; successively depositing first and second insulating films (7, 8) on said InGaAs layer (6); forming a photoresist pattern (9) having an aperture at a gate electrode production position, on said second insulating film (8); etching said first and second insulating films (7, 8) using said photoresist pattern (9) as a mask; selectively etching the second insulating film (8) so that the aperture of the second insulating film (8) is larger than the aperture of the first insulating film (7); forming a recess groove (15) by etching away a portion of the compound semiconductor epitaxially grown layers using the first insulating film (7) as a mask; forming a third insulating film (10) on the side surfaces of said first and second insulating films (7, 8) and said recess groove (15); removing the first and second insulating films (7, 8); and depositing gate, source and drain electrodes (11). A method for producing a compound semiconductor device in accordance with claim 1 wherein said compound semiconductor layers are grown by MBE, said first and second insulating films (7, 8) are deposited by plasma CVD and anisotropically etched by reactive ion etching using a mixed gas of CHF3 and 02, said second insulating film (8) is selectively etched by reactive ion etching using a mixed gas of CF4 and 02, said recess groove (15) is formed using a mixed gas of SiC14 and SF6, said third insulating film (10) is formed by plasma CVD and anisotropically etched by electron cyclotron plasma etching using a mixed gas of SF6 and CHF3, and said first and second insulating films (7, 8) are removed by wet etching using a solution comprising water and amonium fluoride in a ratio of 30:1. A method in accordance with claim 1 or 2 wherein said third insulating film (10) is made of SiN and said gate, source and drain electrodes (11) are made of Ti/Mo/Au.

Inventors: KOHNO YASUTAKA, KOHNO, YASUTAKA, Kohno, Yasutaka, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L29
IPC codes: H01L21, H01L29