Recently, progress is being made in compound semiconductor devices, for example, semiconductor lasers and heterojunction bipolar transistors, which are key devices for today's highly advanced information society. The research and development for these compound semiconductor devices have been performed so that the devices have a finer structure, and are more complicated. This suggests that the a process combining fabrication of a fine structure by dry-etching and epitaxy should be a key technology essential in fabrications of compound semiconductor devices in the near future. In Si-LSI, which go on higher integration, the super clean technology utilizing complicated processing or the research on new device structures have top priority.
In the complicated process comprising dry-etching and epitaxy where a portion of a crystal is selectively etched and an epitaxial layer having different electrical and optical properties is regrown thereon control of the cleanness of the regrowth interface and maintenance of cleanliness of the regrowth interface remains as one of the most important problems to be solved. In particular, a compound semiconductor material including Al as constituent, such as AlGaAs, is easily oxidized on its surface when exposed to the atmosphere and once this happens, it is extremely difficult to clean the surface of AlGaAs oxidized thereby. Therefore, it is difficult to grow a high quality crystal on the oxidized surface of AlGaAs by epitaxial regrowth, resulting in a poorer quality of regrown semiconductor layer in terms of electrical and optical characteristics.
In regard to the situation described above, those who have invented the present invention have investigated the complicated process comprising dry-etching and epitaxial growth in many aspects, and they have proceeded the understanding of the degree of cleanliness of the regrowth interface and have provided improved techniques. For example, the inventors of the present invention disclosed, in pp. 35-42 of Journal of Crystal Growth 134 (1993), a method in which a GaAs cap layer is formed on AlGaAs, the sample is subjected to HCl gas etching at 750.degree. C. and thereafter, epitaxial regrowth is carried out thereon. This method aims at suppressing oxidation of AlGaAs surface to the utmost. This method includes forming GaAs cap layer formed on the AlGaAs, HCl gas etching is started at 750.degree. C. from the GaAs cap layer to reach the AlGaAs layer, and thereafter, regrowth in the same chamber so that accumulation of oxide on the regrowth interface is avoided. In the HCl gas etching of the AlGaAs oxidized surface at 750.degree. C., residual oxide remains on the surface after the etching therefore, although that regrowth by MOCVD is performed in the same chamber after etching, the crystal quality of the regrown GaAs layer could be poor. According to the method described above, however, residual oxide is reduced by the factor of 1/5 as compared with the oxidized AlGaAs layer being etched, and the crystalline quality of the regrown GaAs layer is also significantly improved. It was also pointed out that the flow rate of AsH.sub.3 used for the etching is important.
However, it is found that these methods described above are not sufficient to assure the complete removal of residual impurities from the regrowth interface. This is because the HCl gas etching at the high temperature of 750.degree. C. is not able to completely remove the oxide, thus leaving oxides on the surface after the etching.
As is made apparent from the above-described examination result, as described on pp. 35-42 of Journal of Crystal Growth 134 (1993), when a preferable epitaxial growth is to be performed in the same chamber on the AlGaAs layer on which the HCl gas etching has been performed, it is necessary to provide a GaAs cap layer on the AlGaAs layer. However, it is insufficient to adopt only the cap layer and it is indispensable to perform surface cleaning of the GaAs cap layer. In other words, it is quite difficult to maintain the degree of cleanliness of the regrowth interface only by performing the complicated process including dry-etching and epitaxial growth by successively performing each in the same chamber or by performing them in a system for transferring a wafer between two mutually connected chambers without exposing the wafer to the atmosphere. This means that it is indispensable to use the surface cleaning jointly with the above-described complicated process.
FIG. 11 illustrates a fabrication method that is intended to solve the above-described problems. This method was invented by the inventors of the present invention and is disclosed in Japanese Published Patent Application Hei. 5-44869. Reference numeral 1 designates a GaAs substrate, reference numeral 2 designates an AlGaAs layer, reference numeral 3 designates a GaAs cap layer, reference numeral 4 designates a regrown GaAs layer, reference numeral 5 designates a regrowth interface, reference numeral 6 designates an oxide film formed on the GaAs cap layer 3, reference numeral 8 designates a SiN film pattern, and reference numeral 9 designates a sulfur film.
The fabrication method of the prior art semiconductor device will be described as follows. First, the AlGaAs layer 2 2 .mu.m thick and the GaAs cap layer 3 0.1 .mu.m thick are successively grown by MOCVD on the GaAs substrate 1. Then the sample is taken out of the chamber and is kept in the atmosphere for several days, during which period a thin oxide film 6 is formed on the GaAs cap layer 3. FIG. 11(a) shows the sample thus prepared. Then, an SiN film pattern 8 of a desired configuration is formed on the sample surface as shown in FIG. 11(b). Next, the sample is treated in an ammonium sulfide solution. In this example, (NH.sub.4).sub.2 S is used as the ammonium sulfide solution and the sample is treated at 60.degree. C. for 3 hours. During that time, a portion of the GaAs cap layer 3 not covered with the SiN film pattern 8 is etched away and the sulfur film 9 is formed as shown in FIG. 11(c). Next, the sample is set in the MOCVD chamber and is treated in a hydrogen environment at 450.degree. C. for 30 minutes. Then, as shown in FIG. 11(d), the sample is etched for 1 .mu.m with a mixture of arsine (AsH.sub.3), HCl, and H.sub.2 using the SiN film pattern as the etching mask. Finally, the GaAs layer 4 is formed in the same chamber to produce the semiconductor device as shown in FIG. 11(e).
In the conventional method described above, the treatment with the ammonium sulfide removed the oxide film 6 and created the sulfur film 9 on the surface of the sample, thereby functioning to suppress further surface oxidation, and the sulfur film 9, the GaAs cap layer 3, and the portion of AlGaAs layer 2 are etched with HCl etching, and thereafter, the GaAs layer 4 is grown. This procedure does not cause oxidation on the regrowth interface 5, thereby improving both the degree of cleanliness of the regrowth interface 5 and the crystallinity of the regrown GaAs layer 4.
Another conventional method employing an ECR (Electron Cyclotron Resonance) plasma using hydrogen in which cleaning of the GaAs surface is performed at 300.degree. C. is disclosed by Kondo et. al., in Japanese Journal of Applied Physics, Vol. 28, No. 1, January, 1989, pp. L7-L9. This method is very effective in cleaning the GaAs surface.
As discussed above, when the prior art complicated process of dry-etching and recrystallization growth is to be performed, it is thought of combining the treatment with the ammonium sulfide and the treatment in an ECR plasma to clean the surface of semiconductor materials and to get rid of impurities such as oxide films. However, the treatment with the ammonium sulfide requires skilled expertise based on experience and, therefore, will not easily be applied to a mass production scheme. It is also hard to maintain the purity of the ammonium sulfide solution, resulting in the inability to achieve a desired result.
The problem for employing the ECR plasma treatment is that it requires the construction of a special apparatus which combines an ECR plasma chamber and an epitaxial growth chamber. It has been shown that this technology could not be easily applied to a typical mass production facility.