1. Field of the Invention
The present invention relates to an optical thin film, such as an antireflective film and a dielectric multilayer mirror, used in optical components for visible and ultraviolet regions, and a method for manufacturing the same.
2. Related Background Art
Heretofore, when an optical thin film such as an antireflective film or a mirror is formed, a vacuum vapor deposition method has been mainly used in which a film-forming material is heated by means of an electron beam or the like in a vacuum to be deposited on a substrate. In general, the antireflective film, mirror or the like is composed of either one of a material having a low refractive index such as magnesium fluoride (MgF2), and a material having a high refractive index such as zirconium oxide (ZrO2), tantalum oxide (Ta2O5) and titanium oxide (TiO2), or a multilayer film formed by combining these materials with each other, and the layer constitution, film thickness or the like is adjusted in various ways depending on required optical performance.
Although the vapor deposition method is a film-forming method that can form films rapidly on a large-area substrate using a simple-structure apparatus and is excellent in productivity, the method has posed the problems that highly accurate control of the film thickness and development of an automatic production machine are difficult, and that if a film is formed in a state of a low substrate temperature, the obtained film will have an insufficient strength and is liable to be scratched, and that the adhesion between the film and the substrate is poor.
In recent years, since a higher efficiency of production has been demanded, there has been increased for these optical thin films a need for coating using a sputtering method, which is advantageous in labor saving, stabilization of quality, and improvement in film quality (adhesion and film strength).
By using the sputtering method for formation of an oxide dielectric thin film such as of zirconium oxide (ZrO2), tantalum oxide (Ta2O5), titanium oxide (TiO2) and aluminum oxide (Al2O3), thin films of a low absorption and a high refractive index can easily be formed. However, fluorides such as MgF2 and AlF3, which are important materials for thin films having such a low refractive index as 1.45 or less and having a large influence on the optical performance of multilayer optical thin films, have a problem that low-absorption thin films cannot easily be formed.
As a method for forming the fluoride thin film using a sputtering method, for example, the method disclosed in Japanese Patent Application Laid-Open No. H04-289165 has been known. This is a method in which a film of an alkaline-earth metal fluoride such as MgF2 is formed by sputtering using a mixed gas of an inert gas such as Ar and a fluorine-based gas such as CF4.
Also, as shown in Japanese Patent Application Laid-Open No. H07-166344, there has been known a method in which DC sputtering is performed using a metal target and a mixed gas of an inert gas such as Ar and a fluorine-based gas such as CF4.
However, when a film of MgF2 used as the most typical material having a low refractive index is formed using a sputtering method, F is dissociated during sputtering, so that the composition of the film deviates out of the stoichiometric composition to form an Mg-rich film, whereby the film will cause absorption in the ultraviolet and visible regions.
In order to solve this problem, Japanese Patent Application Laid-Open No. H04-289165 discloses a sputtering method using a fluorine-based gas to complement F.
Further, according to Japanese Patent Application Laid-Open No. H07-166344, DC sputtering using a metal target enables substrate sheath voltage to be lowered, and cation damage to de reduced.
However, even if a fluorine-based gas is introduced to complement fluorine atoms, as compared with the quantity of metal atoms that are sputtered from the target and impinge on the substrate, active fluorine atoms formed by dissociation in a plasma in the vicinity of the target and fluorine-containing gas molecules are consumed to fluorinate the target and the metallic atoms deposited on a portion other than the substrate in the vicinity of the target and only a small quantity thereof reaches the surface of the substrate, resulting in shortage of fluorine in the film on the substrate. This tendency becomes remarkable when the sputtering rate is high.
Moreover, if sputtering is performed in a state in which fluorine is adsorbed to the surface of the target, and the surface of the target is fluorinated, there are posed the problems of occurrence of abnormal discharge or significant dust generation.
Therefore, when the above-mentioned sputtering method is used, since the quantity of fluorine atoms that impinge on the substrate is insufficient, and the bonding state is disturbed, only MgF2 thin films with a large absorption can be formed. Similarly, only thin films having insufficient quantities of fluorine can be formed for metal fluorides such as AlF3, LiF and LaF3.