METHOD OF ANNEALING A COMPOUND SEMICONDUCTOR SUBSTRATE

Abstract: An annealing method for activating ion-implanted layers (2, 3) of a compound semiconductor substrate (1) comprises a step of converting gas containing prescribed components into plasma through an electron cyclotron resonance process and making the same reaction with reactive gas to accumulate reaction products on the surface of the compound semi­ conductor substrate (1) having the ion-implanted layers (2, 3) thereby to form a protective film (10); and a step of perform­ ing heat treatment for activating the ion-implanted layers (2, 3). The gas converted into plasma through the electron cyclo­ tron resonance process is N₂, O₂, NH₃ or mixture gas thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of annealing a compound semiconductor substrate. More specifically, it relates to a method of annealing a substrate after ion implantation in the process of fabricating a compound semiconductor integrated circuit. Description of the Prior Art Electronic devices have rapidly matured in recent years with semiconductor devices being as the forerunners. Except for an amorphous semiconductor device which is the most recent of practical semiconductor devices, electronic and optical devices generally have main operational parts, i.e., active regions formed by semiconductor single crystals. A plurality of such active regions are provided in a semiconductor substrate single crystal as regions different in electrical and/or optical property from each other. In a general method of fabricating an integrated circuit through use of compound semiconductor, an epitaxial growth process, a thermal diffusion process, an ion implantation process or the like is employed for forming an active layer, a resistance layer, a contact layer etc. Within these processes, the ion implantation process is mainly employed nowadays since the same is capable of correctly controlling the dose and injected depth of impurities, operable under the room temperature, excellent in uniformity and reproducibility of ion implantation and short in process time. The ion implantation process is performed by ionizing target impurity atoms and accelerating the same at energies of 10 to several hundred KeV to implant the same in a semiconductor substrate, thereby to dope the impurities into the semiconductor substrate. However, since the impurity ions are accelerated at such high energies as hereinabove described to be implanted in the substrate crystal, the ion implantation process has various problems to be solved in application to a compound semiconductor single crystal. For example, device characteristics are significantly affected by types of the implanted impurity atoms, deviation in stoichiometry of the crystal after ion implantation, annealing conditions including the type and film quality of a protective film for preventing vaporization of high vapor pressure component atoms, crystal quality of ion-implanted regions and the like. With increase in demand for devices operable at very high speed and at high frequency such as a GaAs FET (field-effect transistor) and a GaAs IC (integrated circuit), on the other hand, the ion implantation process has been positively employed as the fabrication technique for such devices. Thus, it is of urgent necessity to solve the aforementioned problems of the ion implantation process. As one of the problems of the ion implantation process, ion-implanted regions are degraded in crystallinity since the impurities are accelerated at high energy to be implanted in the substrate crystal, while regions entering amorphous states are increased in the case of implanting the impurities in high concentration, whereby heat treatment is required to recover the degraded crystallinity and to substitute impurity atoms into lattice sites to obtain electrically activated layer. In a general compound semiconductor substrate, however, one kind of component atoms has such a high vapor pressure that the same are vaporized from the substrate during the heat treatment, whereby the surface of the substrate is remarkably disturbed in crystallinity (or in composition). As a result, implanted impurity ions cannot be constantly substituted with the substrate crystal causing variation in electrical property of devices, leading to a reduced production yield. In order to solve the aforementioned problems, generally employed are the following methods: (i) Method or applying a gas pressure: annealing ion-implanted layers in a gaseous atmosphere containing high vapor pressure component atoms of compound semiconductor, with employment of arsine (AsH₃) as an As pressure atmosphere for a GaAs substrate, for example. (ii) Method of facing surfaces: contacting an ion-implanted surface of a compound semiconductor oppositely with a crystal or crystal powder of a compound semiconductor which contains the same high vapor pressure component atoms in order to prevent vaporization of the high vapor pressure component atoms from the ion-implanted surface by annealing both the surface and the crystal (powder) together. (iii) Protective film method: forming a protective film of silicon nitride, silicon oxide, silicon oxynitride or the like on the ion-implanted surface of a compound semiconductor substrate to be annealed through a CVD process, a sputtering process, a plasma CVD process or the like, to anneal the ion-implanted layers while preventing vaporization of high vapor pressure atoms from the substrate by the protective film. (iv) Heat treatment through combination of the methods (i) and (iii) or the methods (ii) and (iii). The method (i) employs extremely toxic gases such as AsH₃ as the atmospheric gas, which has to be diluted by hydrogen or inactive gas in consideration of safety and workability. Thus, a sufficient vapor pressure cannot be applied and hence vaporization of the high vapor pressure atoms cannot be sufficiently prevented. Such a problem similarly takes place in the method (ii) of facing surfaces, and hence it is difficult to prevent vaporization of the high vapor pressure component atoms from the substrate to be annealed only by the vapor pressure from the crystal or the crystal powder. Therefore, most generally employed is the method (iii) or (iv). However, the methods (iii) and (iv) commonly have such disadvantages that the protective film is inferior in quality and cannot sufficiently prevent vaporization of the high vapor pressure component atoms and that remaining low vapor pressure component atoms are diffused in the protective film. In the Japanese Journal of Applied Physics (vol. 22, 1983, pp. L100-L102) there is a comparison of plasma CVD silicon nitride films produced on heated substrates from SiH₄-N₂- and from SiH₄-NH₃-gas mixtures disclosing a higher density and tightly closed bonds in case of the ammonia-free films. As hereinabove described, the ion implantation process is indispensable as the process technique for fabricating various devices from compound semiconductor single crystal substrates, whereas the same has not yet been completely established, with various problems remaining to be solved. Although the aforementioned various methods have been proposed and carried out particularly in heat treatment for recovering crystallinity of regions degraded after ion implantation and for electrically activating implanted ions, the relatively effective methods (iii) and (iv) still have many problems to be solved. Thus, development of a novel annealing method capable of solving such problems is extremely useful not only for finally obtaining a semiconductor device of high efficiency and high reliability, but for improving the production yield thereof to save the cost. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of annealing a compound semiconductor single crystal substrate after ion implantation through a protective film method, which can effectively prevent vaporization of high vapor pressure component atoms while advantageously depressing diffusion of low vapor pressure component atoms into a protective film. In consideration of the aforementioned circumstances of the methods of heat treatment after ion implantation in compound semiconductor single crystal substrates, the inventors have made deep study on improvement in such heat treatment method, to find that a protective film obtained through a specific method is excellent in quality with no such disadvantages of the conventional protective film method and an advantageous result can be obtained through use of such a protective film. A method of annealing a compound semiconductor substrate according to the present invention is characterized in that a protective film is formed by a thin film forming method utilizing an electron cyclotron resonance ion source on the surface of the ion-implanted compound semiconductor substrate to be annealed, thereby to anneal the same at a temperature for activating atoms implanted through the ion implantation. The protective film effectively employed in the method according to the present invention is Si₃N₄ and is formed employing NH₃ to generate the N₂ plasma. Although this protective film is not particularly restricted in thickness which is varied with the material as employed, the same is preferably 10-200 nm (100 to 2000 Å) in thickness in consideration of a vaporization depressing property of the protective film and adhesion property to the substrate. This protective film is formed by a film forming apparatus having an electron cyclotron resonance (ECR) ion source, such as an ECR plasma CVD apparatus described in Japan Applied Physics, vol. 52-2, 1983, pp. 117 - 119. Another CVD apparatus utilizing an ECR process is described in the Japanese Journal of Applied Physics (Suppl. 16th Int. Conf. Solid State Devices and Materials, Kobe 1984). It allows the deposition of dense and high qualitiy thin films including Si₃N₄ films without substrate heating. The deposition reaction is enhanced by using a microwave ECR plasma excitation at low gas pressures employing N₂ to generate the N₂ plasma and an ion extraction in the form of plasma stream by a divergent magnetic field. After the protective film is thus formed, the compound semiconductor substrate is annealed to activate implanted ions. The substrate is annealed at a temperature for activating the implanted ions generally within a range of 700 to 1100°C, although the temperature is slightly varied with the types of the implanted ions. The heat treatment is performed through an electric furnace, a lamp anneal furnace or the like preferably in an atmosphere of unreactive gas such as nitrogen, hydrogen or gas mixture thereof. The method of the present invention can be advantageously applied particularly to a compound semiconductor substrate of group III - V compound semiconductor such as GaAs, InAs, GaP, InP, GaSb or InSb. The conventional ion implantation process for the compound semiconductor single crystal substrate has been particularly disadvantageous in that component atoms of the compound semiconductor such as those of a group V element are vaporized because of high vapor pressure unless appropriate means is applied in heat treatment for activation after ion implantation, whereby the surface of the substrate is remarkably disturbed in crystallinity. As the result, a semiconductor device obtained through such a substrate is extremely inferior in quality, and requires improvement. Although various means have been proposed to cope with such a problem, none of them can sufficiently solve the problem. For example, a protective film of high quality obtained through a reduced pressure plasma CVD process or a plasma CVD process is still insufficient in dense structure and cannot completely prevent vaporization of high vapor pressure atoms, while causing diffusion of low vapor pressure component atoms such as those of a group III element, to degrade crystallinity on the surface of the substrate. In the method according to the present invention, study has been made to improve a protective film in quality, to find that an Si₃N₄ film formed by an ECR plasma CVD process employing NH₃ to generate the N₂ plasma is optimumly employed for this purpose since the same is excellent in mechanical strength and density. A thin film forming method employing an apparatus having an ECR ion source is excellent in film characteristics. According to this method, plasma can be stably generated at a low gas pressure and the ionization factor of the implanted ions can be improved by two or three orders in comparison with the conventional case, while reaction can be facilitated by ion bombardment at appropriate energy, whereby a dense film can be obtained without damaging the substrate. In the annealing process according to the method of the present invention, the protective film employed is of high quality with dense structure and strong in composition with combined hydrogen quantity, whereby vaporization of high vapor pressure component atoms from the substrate and diffusion of low vapor pressure component atoms in the protective film can be efficiently prevented, to effectively and reproducibly recover crystallinity degraded by ion implantation and activate the implanted ions. As the result, various semiconductor devices of high reliability and high efficiency can be obtained from such ion-implanted substrates. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1D are sectional views showing steps of fabricating a compound semiconductor device; and Fig. 2 illustrates comparison of effects of protective films produced by a ECR plasma CVD apparatus (Fig. 2A) and by the general plasma CVD method (Fig. 2B). DESCRIPTION OF THE PREFERRED EMBODIMENTS Before an example of the present invention is described, brief description is made of an ECR plasma CVD apparatus as an example of a film forming apparatus having an electron cyclotron resonance ion source employed to form a protective film according to the present invention. Such an apparatus is described in detail in, e.g., Japan Applied Physics, vol. 52-2, 1983, pp. 117 - 119 and Japanese Journal of Applied Physics, vol. 22, No. 4, 1983, pp. L210 - L212. This apparatus includes a plasma chamber and a reaction chamber (specimen chamber). The plasma chamber is connected with a microwave waveguide through a wall plate, and is provided in its periphery with an electromagnet. This electromagnet is adapted to set ECR (electron cyclotron resonance) conditions in the plasma chamber with guided microwaves as well as to form a divergent magnetic field for extracting plasma in the specimen chamber. This plasma chamber is connected with the reaction chamber through a plasma extracting window, so that the plasma is accelerated by the divergent magnetic field and guided to a specimen placed on a specimen table. According to this apparatus, N₂, O₂, NH₃, gas mixture thereof or the like is introduced into the plasma chamber in which the ECR conditions are set by the microwaves and the magnetic field, so that the plasma gas is induced by the divergent magnetic field to be introduced into the reaction chamber. On the other hand, the reaction chamber contains a substrate placed on the specimen table and is supplied with raw gas for forming a protective film such as SiH₄, Si₂H₆ or SiF₄, which is excited and activated by the plasma to cause reaction, whereby prescribed reaction products are accumulated on the substrate. The protective film formed by this ECR plasma CVD process can be removed by a well-known etching process, generally through wet etching employing buffered fluoric acid (BHF). Detailed description is now made on examples relating the state of art (example 1) and the present invention (example 2). Example 1 Figs. 1A to 1D are sectional views showing steps of fabricating a compound semiconductor device. In example 1 GaAs was employed to prepare a compound semiconductor substrate to be annealed after ion implantation, to fabricate a MES-FET (metal-semiconductor field-effect transistor) in the structure as shown in Fig. 1D. The fabricating method is now described with reference to Figs. 1A to 1D. A resist film was formed on the surface of a cleaned and etched GaAs substrate 1 and patterned through photolithography and etching technique to define an ion-implanted layer serving as an active layer. Then 28Si was ion-implanted as impurities at acceleration energy of 60 KeV in a dose of 2 x 10¹² cm⁻² with the patterned resist film employed as a mask, to form an active layer (n layer) 2. Similar ion implantation was performed at acceleration energy of 50 KeV in a dose of 2 x 10¹³ cm ⁻² to form a contact layer (n⁺ layer) 3 of low resistance (Fig. 1A). Then a silicon nitride film was formed on the ion-implanted surface of the GaAs substrate 1 through a thin film forming apparatus having an electron cyclotron resonance ion source as a protective film 10 for heat treatment. This process was performed by introducing N₂ gas of 10 SCCM into the plasma chamber and applying microwaves of 2.45 GHz and a magnetic field of 0.0875T (875 gauss) in magnetic flux density to cause electron cyclotron resonance and generate nitrogen plasma. The plasma thus generated was induced in the reaction chamber through the divergent magnetic field while 6 SCCM of silane (SiH₄) was introduced as reactive gas, to accumulate a silicon nitride film of 140 nm (1400 Å) in thickness on the GaAs substrate 1 (Fig. 1B). Then the ion-implanted surface of the GaAs substrate 1 was faced to a cleaned and etched surface of another GaAs substrate to be annealed in an electric furnace at 820°C for 20 minutes in a nitrogen atmosphere, to activate the implanted ions (Fig. 1C). After the heat treatment, the crystal was partially cracked to obtain a specimen for Auger spectrum analysis. As to the remaining part, the silicon nitride film was removed by hydrofluoric acid to form source and drain electrodes 4 and 5 by Au/Ge/Ni and a gate electrode 6 by Ti/Au through a lift-off method (Fig. 1D). The gate width of the FET thus obtained was 5 µm and the source-to-drain distance was 5 µm. This FET was fabricated at the pitch of 200 µm, to measure electrical properties. Auger analysis was performed through sputtering by argon, to measure distributions of Ga, As, Si and N in the depth direction. Another FET was similarly fabricated as a reference example by forming a protective film by mixture gas of SiH₄, NH₃ and N₂ through a general plasma CVD process and annealing the same. The crystal and the FET thus obtained were also subjected to Auger analysis and measurement of electrical properties. Film forming conditions in the plasma CVD process were a temperature of 300°C and RF output of 0.26 W/cm². Other process was performed in a similar manner to the above. Fig. 2 shows distributions of respective elements in the depth direction through Auger analysis. Graphic in Fig. 2A shows, that there was no detection of As in the Si₃N₄ film produced by the ECR plasma CVD apparatus. In contrast to this outcome there were in the case of the reference example Ga and As atoms detectable over 40-60 nm (400 to 600 Å) from the crystal side in the film (Fig. 2B) as a result of diffusion. In the reference example Ga was also detected from the surface side, which shows penetration of Ga from the opposite GaAs crystal. Consequently, it is understood that the protective film of silicon nitride according to the ECR plasma CVD method is of dense structure which is excellently applicable to heat treatment. Table 1 shows results of measurement of electrical properties of MES-FETs annealed through protective films formed according to both the methods described in example 1. It is understood from Table 1 that the FET fabricated by the use of a ECR plasma CVD apparatus was small in variation of electrical properties, and the crystal was effectively protected. Example 1 Reference Example mean value of threshold voltage (V) -0.5 -0.4 distribution of threshold voltage (mV) 23 35 measured FET number 17500 17350 Example 2 A protective film was formed according to the present invention through the following method: 15 SCCM of ammonia (NH₃) gas was introduced into a plasma chamber to apply microwaves of 2.45 GHz and a magnetic field of 0.0875T (875 gauss) in magnetic flux density, thereby to cause electron cyclotron resonance for generating nitrogen plasma. This N₂ plasma was introduced into a specimen chamber through a divergent magnetic field. On the other hand, 6 SCCM of silane (SiH₄) gas was introduced into the specimen chamber as reactive gas, to deposit a silicon nitride film of 140 nm (1400 Å) in thickness on a GaAs substrate. When NH₃ gas is employed to generate N₂ plasma, a protective film obtained has smaller flakes and higher density in comparison with that in the case of generating plasma through N₂ gas. In the case of employing NH₃ gas, further, film forming parameters can be properly adjusted to form an insulating film excellent in etching resistance in comparison with that formed through N₂ gas. In formation of a protective film through NH₃ gas, the range of film forming conditions can be widened in comparison with the case of employing N₂ gas, whereby film formation can be easily controlled in the process of forming the protective film. After a protective film of silicon nitride was formed on a GaAs substrate, ion-implanted layers were annealed through a method similar to that of Example 1. The result of Auger analysis on the protective film and the GaAs substrate after annealing was similar to that shown in Fig. 2A, with no evaporation of high vapor pressure component atoms and diffusion of low vapor pressure component atoms in the protective film. According to the method of the present invention as hereinabove described in detail, the protective film for annealing the compound semiconductor substrate is formed by a film forming apparatus having an electron cyclotron resonance ion source, whereby a highly dense film can be obtained to substantially completely prevent vaporization of high vapor pressure component atoms of the crystal substrate and diffusion of low vapor pressure component atoms in the film, thereby to effectively anneal the ion-implanted surface without degrading the crystal composition. A semiconductor element or device obtained through such ion implantation and annealing according to the present invention is excellent in electrical property, while the production yield is widely improved.

Claims: A method of annealing a compound semiconductor substrate (1) having ion-implanted layers (2, 3) to serve as active layers in predetermined regions on its surface, said method comprising the steps of: converting ammonia gas (NH₃) into plasma through an electron cyclotron resonance process and making said converted gas react with reactive gas to accumulate reaction products on said compound semiconductor substrate thereby to form a protective film of Si₃N₄ (10); and performing heat treatment for activating said ion-implanted layers (2,3). A method of annealing a compound semiconductor substrate in accordance with claim 1, wherein said protective film (10) is 10-200 nm (100 to 2000 Å) in thickness. A method of annealing a compound semiconductor substrate in accordance with claim 1, wherein said heat treatment is performed in an electric furnace. A method of annealing a compound semiconductor substrate in accordance with claim 1, wherein said heat treatment is performed in a lamp anneal furnace. A method of annealing a compound semiconductor substrate in accordance with claim 1, wherein said heat treatment is performed through contacting an ion-implanted surface of a compound semiconductor oppositely with a crystal or crystal powder of a compound semiconductor which contains the same high vapour pressure component atoms and annealing both the surface and the crystal (powder) together. A method of annealing a compound semiconductor substrate in accordance with claim 1, wherein said compound semiconductor substrate (1) is a group III - V compound semiconductor substrate. A method of annealing a compound semiconductor substrate in accordance with claim 6, wherein said group III - V compound semiconductor substrate (1) is one of a group of GaAs, InAs, GaP, InP, GaSb and InSb.

Inventors: SHIKATA, SHINICHI C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1986
Main classification: H01L21
IPC codes: H01L21