Method of producing a Schottky-barrier field effect transistor

Abstract: A method of forming a self-aligned gate electrode of a field effect transistor, in which a resist pattern is formed on a substrate by lithography, an ion-implanted layer in the substrate at the side of the resist is formed by ion implantation, an insulating film is formed on the substrate by electron cyclotron resonance plasma chemical vapor deposition, the resist pattern and a part of the insulating film on the resist pattern removed by lift-off to thereby form an insulating pattern with an opening, the substrate annealed to activate said ion-implanted layer and a gate electrode formed in the opening by a spacer lift-off method.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of producing a self-aligned gate electrode of a Schottky barrier gate field effect transistor according to the preamble of claim 1. Conventionally, substitutional and refractory metal techniques are used in forming gates in field effect transistors. The substitutional technique is more flexible than the refractory metal approach since the gate material can be selected at will. However, such process has disadvantages as descrived below. Figs. 1(a)-1(i) illustrate a part of the typical conventional process of producing a Schottky-gate field effect transistor. As shown in Fig. 1(a), a protective insulating film 12 such as a Si₃N₄ film is formed on a semiconductor substrate 10, and then a three-layered resist composed a resist 14, an insulating film 16 such as a SiO₂ film, and another resist 18 is formed. The insulating film is then etched while masked with a resist pattern 18A formed by patterning the uppermost resist 18 as shown in Fig. 1(b), to prepare an insulating film pattern 16A. Next, the resist 14 is etched by RIE while masked with the insulating film pattern 16A to thereby form a resist pattern 14A. While masked with the insulating film pattern 16A and the resist pattern 14A, ion implantation is carried out as shown by the arrow 20 of Fig. 1(c) to thereby form a source region 22A and a drain region 22B. After the implantation of the source and drain regions, an SiO₂ film 24 is formed on the surface of the substrate by sputtering or the like as shown in Fig. 1(d) After slight-etching, the resist pattern 14A and insulator 16A are removed by lift-off method to thereby form a reversal pattern 24A of the SiO₂ film as shown in Fig. 1(e). Further, as shown in Fig. 1(f), after ohmic electrodes 26A and 26B are respectively formed on the source region 22A and the drain region 22B, a three-layered resist 28 is formed again over the entire surface of the substrate. Next, the uppermost layer of the three-layered resist 28 is patterned to have an opening corresponding to a gate electrode. The insulating film formed at the middle layer of the three-layered resist 28 is masked with the resulting resist pattern formed at the uppermost layer. Further, the lowermost layer of the three-layered resist 28 is selectively removed while masked with the resulting insulating film pattern to thereby form a two layered resist pattern 28 as shown in Fig. 1(g). While masked with the pattern of the two-layered resist 28, a part of the insulating film 12 is removed by reactive-ion etching to thereby obtain an insulating film 12 with an opening 30 as shown in Fig. 1(h). Further, gate electrode material is deposited while masked with the pattern of the two-layered resist 28, and then the pattern of the two-layered resist 28 is removed by lift-off to thereby form a gate electrode 32 as shown in Fig. 1(i). In the conventional method, however, the gate region of the crystal surface is often damaged by ion radiation because the insulating film 12 in direct contact with the ssemiconductor substrate 10 is removed by reactive-ion etching. Further, contamination with impurities in the etching process and in the cleaning process after etching is a problem. A damaged and contaminated surface causes deterioration in the characteristics of the field effect transistor. Further, although a field effect transistor having a gate electrode self-aligned to source and drain regions can be attained by the convention producing process, a lot of steps are required. A method for producing the self-aligned gate electrode of a Schottky-barrier field effect transistor as mentioned at the beginning of the description is known from EP-0 057 605. In this method a pattern formed by a Si0₂-layer is provided on a substrate. As an insulating film formed on the pattern an SiN-film is deposited in a thickness of 0.2 µm by a plasma CVD-method. Annealing, to thus activate the ion implanted layer is carried out after deposition of the SiN-film. Finally, the gate electrode is formed by use of vapour deposition and lithographic technique. It is the object of the present invention to provide a method as mentioned at the beginning of the description which method is improved over the prior art. This object is solved by the features of the independant claim 1. The proposed inventive solutions provide an improved technology by which a short gate length can be obtained. Although the aforementioned method of forming an insulating pattern acording to the present invention can be applied to the process of manufacturing field effect transistors, it is to be understood that the invention is not limited thereto and that the method according to the invention may be applied to manufacturing other semiconductor devices as long as the insulating film pattern is formed of Si₃N₄, SiO₂, SiOxNy or the like. Further, the method according to the invention can be used even where the gate length is of the order of submicrons, and particularly, the method is suitable for manufacturing short-gate-length transistors having excellent characteristics. According to the present invention, the insulating film pattern can be easily formed by lift-off because of the use of an ECR plasma CVD method. Furthermore, it is unnecessary to use reactive-ion etching. The use of an ECR plasma CVD method has the advantage that the film can be formed at a low temperature so that a resist or the like is not changed in quality. Further, lift-off can be easily performed due to straight path shower of ions and radicals, compared to other film forming methods, such as a CVD, sputtering method and the like. Further, the film produced according to the invention is excellent in quality, and durability against etching. The invention provides an excellent film which is not peeled off even when annealed at about 800°C, and in which the diffusion of Ga and As can be suppressed. According to the method of forming an insulating film of the present invention, the substrate as a ground layer prepared for the formation of a gate electrode or the like is not damaged, so that excellent Schottky charactristics can be obtained. Further, there is no contamination with impurities at etching, so that excellent insulating characteristics can be attained. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a)-1(i) are views showing the steps in a conventional prior art method of producing a field effect transistor, Figs. 2(a)-2(h) are views showing the steps in a method of producing a field effect transistor according to an embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that the present invention is not limited to the embodiments. The ECR plasma CVD method used in the present invention employs an ECR plasma CVD apparatus as disclosed in Japanese Journal of Applied Physics, Vol. 22, page 210, 1983. This kind of ECR plasma CVD apparatus includes a plasma chamber and a reaction chamber (sample chamber), the plasma chamber being connected to a microwave waveguide through a separator and surrounded by an electromagnet to form a divergent magnetic field for establishing an ECR condition in the plasma chamber together with a microwave and for drawing out the plasma within the reaction chamber. The plasma chamber is connected to the reaction chamber through a plasma-drawing window so that the plasma is led toward the sample mounted on a sample stage by the divergent magnetic field. A gas, such as N₂, O₂, NH₃, a mixed gas of those described above, or the like, is introduced into the plasma chamber where the ECR condition is established by the microwave and magnetic field, and then a plasma gas formed in the plasma chamber is led to the reaction chamber by the divergent magnetic field. On the other hand, the reaction chamber includes a substrate mounted on the sample stage. A material gas, such as SiH₄, Si₂H₆, Si₃H₈ or the like, for forming an insulating film is introduced into the reaction chamber. The gas is excited or activated by the aforementioned plasma to thereby cause a reaction, so that a predetermined reaction product is deposited on the substrate. It is preferable that the insulating film formed by the ECR plasma method is selected from the group comprising an Si₃N₄ film, an SiO₂ film and a silicon nitride oxide film. For comparison, by the conventional method using reactive-ion etching as shown in Fig. 1, an Si₃N₄ insulating film and a gate electrode were formed on a GaAs substrate as follows. After forming a three-layered resist pattern on a substrate of GaAs as shown in Fig. 1(b), implurities were implanted into the substrate to form a source and drain regions as shown in Fig. 1(c). Next, an SiO₂ film was formed on the substrate by a sputtering method (Fig. 1(d)). The resist pattern and the insulating film formed thereon were removed by lift-off to form an insulating film reverse pattern (Fig. 1(e)). After forming Au/Ge/Ni ohmic electrodes, a three-layered resist pattern is further formed (Fig. 1(g)). The insulating film corresponding to the gate region was etched by reactive-ion etching using CF₄ gas (Fig. 1(h)). Then, a Ti/Pt/Au gate electrode was provided (Fig. 1(i)). The mutual conductance (gm) of the thus produced transistor was 170 mS/mm at gate length 1.0 µm. Fig. 2 shows a part of the steps in the method of producing a field effect transistor according to an embodiment of the invention. Fig. 2(a) shows a shallow active layer 6 formed in the substrate 1 by slight doping or epitaxial growth, and a resist pattern 44 corresponding to a gate electrode in the future formed on the active layer 6. As shown in Fig. 2(b), a first insulating film separated into two parts 46A and 46B is formed on the surface of the resulting substrate 1 by the ECR plasma CVD method. Only a small amount of insulating film is formed on the side of the resist pattern 44. As shown in Fig. 2(c), a second insulating film 48 is formed on the first insulating film by sputtering or the like. Next, a part of the second insulating film 48 is removed from the substrate and the horizontal portion of the resist pattern by a directional etching method such as reactive ion etching (RIE) to thereby leave a second insulating film pattern 50 only at the side of the resist pattern 44 as shown in Fig. 2(d). Under the aforementioned state, impurity ions are implanted in the substrate to thereby form an n⁺ source region 52 and an n⁺ drain region 54 as shown in Fig. 2(e). Next, the second insulating film 50 is removed from the side of the resist pattern 44 by wet etching as shown in Fig. 2(f). There are enough etching selectivity between 46B and 50. Then, impurity ions are slightly implanted in the substrate again. Further, the resist pattern 44 and the first insulating film pattern 46A on the resist pattern 44 are removed by a lift-off method so as to form an ECR plasma CVD first insulating film pattern 46B having an opening 56 as shown in Fig. 2(g). After forming ohmic electrodes, as shown in Fig. 2(h), a gate electrode 5 is formed on the substrate within the opening 56. As an example, an insulating film pattern and a gate electrode for a field effect transistor were formed on a substrate according to the method shown in Fig. 2 as follows. A GaAs substrate 1 was used as a semiconductor substrate. The resist pattern that is a dummy gate was formed by lithography. Next, as shown in Fig. 2(b), an Si₃N₄ film 46 with a thickness of about 2,000 Å was formed from a mixed gas of SiH₄, NH3 and N₂ by an ECR plasma CVD method. Next, as shown in Fig. 2(c), an SiO₂ film 48 with a thickness of about 2,000 Å were made by bias sputtering method so that the side of the resist pattern 44 was also effectively coated. Next, a part of the SiO₂ film 48 was removed by RIE from the substrate and the horizontal portion to thereby leave an SiO₂ film pattern 50 at the side of the resist pattern 44 as shown in Fig. 2(d). Under the aforementioned condition, ²⁸Si⁺ ions were implanted into the substrate at an acceleration voltage of 150 to 200 KV to thereby form an n⁺ source region 52 and an n⁺ drain region 54 with the implantation dose of 3 x 10¹³/cm² as shown in Fig. 2(e). Next, as shown in Fig. 2(f), the SiO₂ film 50 was removed from the side of the resist pattern 44 by etching using a hydrofluoride buffer solution diluted with NH₄F. Then, ²⁸Si⁺ ions were slightly implanted into the substrate. Further, the structure comprising a first portion 46A of the Si₃N₄ film and the resist pattern 44 was removed by lift-off using acetone to thereby form an ECR plasma CVD Si₃N₄ film pattern 46B having an opening 56 as shown in Fig. 2(g). Next, the thus prepared substrate was annealed in an AsH₃ atmosphere at about 800°C for 30 minutes. The annealing must be carried out in such an AsH₃ atmosphere to prevent As evaporation from the GaAs substrate. After forming AuGe/Ni ohmic electordes, a gate electrode of Ti/Pt/Au was formed by lift-off as shown in Fig. 2(h). As the result of measurement, the transconductance (gm) was found to be 230 mS/mm at gate length 1.0µ. In the aforementioned method of producing a field effect transistor according to the embodiment of the present invention, the first insulating film is formed by an ECR plasma CVD method and then the second insulating film is formed thereon. The insulating film formed by the ECR plasma CVD method and the insulating film formed by a conventional method such as a sputtering method or the like, are different from each other in deposition conditions, etching characteristics and the like. The use of the ECR plasma CVD method makes it possible to form a film at a low temperature, so that a resist or the like does not deteriorate. If a conventional method such as a plasma CVD or the like is used to form an insulating film, the resist just under the insulating film deteriorates and becomes difficult to dissolve in solvent. Further, only a small amount of the insulating film is formed on the side of the resist pattern due to the use of ECR plasma CVD, and accordingly, in the ECR plasma CVD method, lift-off can be easily performed compared to other film forming methods, such as a CVD, sputtering method and the like. Further, by changing etching conditions, it is possible to remove only the insulating film formed by the ECR plasma CVD method or to remove only the insulating film formed by the conventional method such as sputtering or the like. Accordingly, any one of the aforementioned first and second insulating films can be selectively removed, wether the films were annealed or not. Further, the film formed by the ECR plasma CVD method is excellent in quality and durability against etching. In short, the ECR plasma CVD method provides an excellent film which is not peeled off even when annealed at about 800°C, and which can suppress the diffusion of components, such as Ga, As and the like, of a compound semiconductor. Accordingly, when an etching method is suitably selected after the second insulating film is formed on the first insulating film, the second insulating film can be partly removed while leaving a part of the second insulating film at the side of the resist pattern. Under this state, source and drain regions can be formed by implanting ions into the semiconductor substrate. When the resist pattern and the first insulating film on the resist pattern are removed by lift-off, a first insulating film pattern having an opening corresponding to the resist pattern can be formed. When a gate electrode is formed in the opening, the gate electrode is automatically aligned to the implanted regions. That is, a self-aligned gate electrode can be formed. Further, when the source and drain regions are formed by ion implantation in the state in which the second insulating film is left at the side of the resist pattern, an LDD (lightly doped drain) MESFET structure can be formed. In this embodiment of the invention, the second insulating film is formed by bias sputtering, and the removal of the second insulating film is by reactive-ion etching. In spite of using reactive-ion etching for removing the second insulating film, the substrate is not at all damaged because the substrate is protected by the first insulating film. Also, it is preferable that the first insulating film is a silicon nitride film and the second insulating film is selected from the group comprising silicon nitride silicon oxide and silicon oxynitride films. The semiconductor substrate may be made of a Group III - Group V compound semiconductor such as GaAs, InP etc. Further, the method of producing a field effect transistor according to the embodiment of the invention can be used even in the case where the gate length is of the order of submicrons, so the method is suitable for manufacturing short-gate-length transistors having excellent characteristics. In the case where the gate length is of the order of submicrons, the short-channel effect can be effectively suppressed if a buried p layer is implanted under the lightly-doped drain regions to form a DI-LDD-MESFET the sturcture according to the method of the invention. As described above, according to the present invention, not only the surface of the substrate on which a gate electrode will be formed is not damaged, but also the gate electrode is self-aligned to the source and drain regions. Accordingly, a field effect transistor having excellent electrical characteristics can be produced. Further, according to the present invention, a field effect transistor having a gate electrode self-aligned to source and drain regions can be produced by a small number of steps compared to those of the conventional method. Also, according to the present invention, an LDD structure having a self-aligned gate electrode can be obtained. By using such an LDD structure, the operation speed can be increased without causing the short-channel effect even if the gate length is scaled down to the Submicron level.

Claims: A method of producing a Schottky-barrier gate field effect transistor having a self-aligned gate electrode, comprising the steps of: forming an active layer (6) at the surface of a semiconductor substrate (1); forming a patterned layer (44) on said active layer at a first location, corresponding to a gate region; forming an insulating film (46A,46B) on said active layer and on said patterned layer by plasma chemical vapour deposition; removing said patterned layer and said insulating film on said patterned layer by lifting off to thereby form a pattern of said insulating film having an opening (56), and forming a self-aligned gate electrode (5) in said opening, wherein additionally a source region (52) and a drain region (54) of said field effect transistor are formed by ion implantation into said substrate, characterised by forming said patterned layer as a resist pattern (44) by a lithographic process; forming said insulating film (46A,46B) on said substrate and on said resist pattern (44) by an electron cyclotron resonance plasma chemical vapour deposition, thereby leaving vertical edges of said resist pattern exposed; after forming said insulating film and before removing said resist pattern and said insulating film on said resist pattern, forming a second insulating film (48) on said first insulating film, and on exposed edges of said resist pattern; removing said second insulating film except for the parts (50) of said second insulating film on said exposed edges of said resist pattern; forming said source region and said drain region by ion implantation into said substrate through said insulating film, said resist pattern and said parts of said second insulating film masking said implantation; then removing said parts of said second insulating film; then applying a shallow ion implantation to said substrate through said insulating film, said resist pattern masking said implantation, to thereby form shallow doped regions, aligned to said resist pattern, acting as source and drain extensions; and forming said self-aligned gate electrode (5) in said opening (56) by a lift-off process. A method of producing a Schottky-barrier gate field effect transistor according to claim 1, in which said second insulating film (48) is removed by reactive-ion etching. A method of producing a Schottky-barrier gate field effect transistor according to claim 1, further comprising the step of annealing said substrate after the formation of said pattern of said first insulating film (46A,46B) to activate the ion-implanted regions (52,54) in which said gate electrode is refractory. A method of producing a Schottky-barrier gate field effect transistor according to claim 1, in which said first insulating film (46A,46B) is a silicon nitride film. A method of producing a Schottky-barrier gate field effect transistor according to claim 1, in which said second insulating film (48) is selected from the group consisting of a silicon nitride film, a silicon oxide film and a silicon nitride oxide film. A method of producing a Schottky-barrier gate field effect transistor according to claim 1, in which said semiconductor substrate (1) is made of a group III - V compound semiconductor. A method of producing a Schottky-barrier gate field effect transitor according to claim 6, in which said group III - V compound semiconductor is GaAs.

Inventors: HAYASHI HIDEKI C O YOKOHAMA WO, SHIKATA SHINICHI C O YOKOHAMA, HAYASHI, HIDEKI C/O YOKOHAMA WORKS, SHIKATA, SHINICHI C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1987
Main classification: H01L21
IPC codes: H01L21