1. Field of the Invention
The present invention relates to a parallel plate plasma CVD (Chemical Vapor Deposition) apparatus, and in particular, to a parallel plate plasma CVD apparatus used in production of a titanium oxide thin film, which is used for a microwave monolithic IC (hereinafter referred to as MMIC).
2. Description of the Related Art
Various methods for forming metal oxide thin films or dielectric thin films have been proposed. Examples of such methods are disclosed in, for example, Japanese Patent Publication (Kokoku) Nos. 59-37566, 60-12773, and 7-25545. Included in these publications is a report that a titanium oxide thin film can be formed using a CVD process. However, to carry out this method the substrate temperature must be high, for example, 500.degree. C. or more, during the film deposition step. This adversely affects the underlying substrate used for the device formation as well as other devices formed on the substrate. For example, if the temperature is high when MMIC capacitors are formed, characteristics of the underlying substrate such as GaAs and transistors deteriorate. To reduce this problem, a film deposition method which uses plasma generated in a deposition chamber during a CVD process (herein after referred to as a plasma CVD process) has been frequently used.
In a conventional parallel plate plasma CVD apparatus using a radio-frequency generator (13.56 MHz), a gaseous mixture is supplied into a glow discharge region between a top electrode (a radio-frequency input electrode) and a bottom electrode (a ground electrode), and activated for reaction in the mixture to deposit a film on a substrate located on the bottom electrode. The plasma must have a considerably high power density to decompose the gases sufficiently.
However, since the surface of the substrate on which the film is deposited is located near the plasma generation region, high-energy particles generated by plasma discharge frequently attack the substrate surface. This may cause deterioration of film characteristics, that is, the film may be damaged or deposited abnormally by the impact of particles having excessive energy.
In order to solve the problem, proposals have been made to moderate the particle impact by, for example, controlling the self-bias near the substrate. For example, J. W. Coburn et al. disclosed a method for controlling ions and radicals incident on the substrate by applying a direct current flow without grounding the bottom electrode supporting the substrate in J. Appl. Phys., 43, 4965 (1972). A. Matsuda et al. disclosed a triode configuration consisting of parallel plate electrodes and a mesh electrode interposed therebetween in Thin solid Films, 92, 171 (1982).
FIG. 3 is a schematic view illustrating the fundamental components of a conventional triode-type plasma CVD apparatus. A bottom electrode 2 and a top electrode 3 are arranged opposite to one other in a reaction chamber (not shown). The bottom electrode 2 is connected to an external DC electrical power source 4. A substrate S on which a dielectric thin film is to be formed is placed on the bottom electrode 2. The top electrode 3 is connected to a radio-frequency generator 5. A grooved mesh electrode 6 is located between the bottom electrode 2 and the top electrode 3 and a DC bias current is applied between the mesh electrode 6 and the bottom electrode 2.
In all of the above-mentioned methods for moderating particle impact, the bottom electrode must float electrically. However, as shown in FIG. 3, the bottom electrode of the conventional plasma CVD apparatus is generally connected to the body of the reaction chamber and grounded. Thus, the conventional plasma CVD apparatus cannot be used to carry out the foregoing methods.
Accordingly, there is a need for a parallel plate plasma CVD apparatus which moderates damage to the substrate during the film deposition process, forms high-quality dielectric thin films, and permits the use of a modified conventional plasma CVD apparatus.