1. Field of the Invention
This invention relates to a plasma treatment apparatus adapted to treating the surface of an optical part having a non-planar surface to be treated such as a convex lens, a concave lens or a concave mirror. More broadly, it relates to a technological field of manufacturing optical parts, using such an apparatus.
2. Related Background Art
There is a demand for improved surface treatment techniques to be used for cleaning and/or forming a film coat on objects having a non-planar surface. An anti-reflection film formed on a convex lens may be a typical example of film coat of the type under consideration.
Japanese Patent Application Laid-Open No. 2-232367 describes a typical known film forming method employing PVD, which may be sputtering. However, because PVD is not highly effective for forming a film coat on an object having an undulated or otherwise non-planar surface, the inventors of the present invention have been trying to form a film coat on objects of treatment by CVD.
Since the technique of thermal CVD has a disadvantage of producing thermal deformations, it is not adapted to forming a film coat of the type under consideration. On the other hand, the technique of optical CVD is not satisfactory in terms of throughput.
Meanwhile, even the technique of plasma excitation CVD (PECVD) using an RF power source with a frequency of 13.56 MHz is not satisfactory for producing an anti-reflection film having a higher transmission coefficient (lower absorption coefficient) than those currently obtainable by PVD and having improved light resistance and environment resistance. Therefore, a PECVD technique that can produce a higher plasma density will have to be used.
A technique of electrodeless PECVD using a microwave such as electron-cyclotron-resonance CVD (ECR-PECVD) is known to produce a high density plasma exceeding the density level of 1010 cm−3.
FIG. 24 of the accompanying drawings is a schematic cross sectional view of a plasma treatment apparatus disclosed in Japanese Patent Application Laid-Open No. 6-216047.
Referring to FIG. 24, this apparatus comprises a plasma generator chamber 2 and a treatment chamber 4, of which the plasma generator chamber 2 contains therein microwave power introducing means 5, 6, 8 and a magnetic field applying means 10 and is connected to a plasma source gas introduction system 20 while the treatment chamber 4 is connected to a chemically reactive material gas introduction system 22, sample table 14 of the apparatus being connected to an RF power introducing means 18. Additionally, a control unit 27 is provided to modulate the power output of the microwave power generator 8 and that of the RF power generator 18. With this arrangement, the RF power and the microwave power are modulated synchronously and the film forming conditions are alternately modified in favor of film forming and in favor of sputtering/etching to produce a CVD film.
While an anti-reflection film formed on a convex lens by means of an apparatus as shown in FIG. 24 is dense and fine in average, it is relatively poor in terms of intra-surface uniformity. More specifically, reaction by-products can frequently remain on and near the treated surface to give rise to areas showing different compositional ratios. Such by-products can adhere to the surface, and, if they are removed during or after the film-forming process, pin holes can be produced in the film.