Method for preparing an optical thin film

Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering

Reexamination Certificate

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Details

C204S192120, C204S192260, C204S192270, C204S192280

Reexamination Certificate

active

06635155

ABSTRACT:

DETAILED DESCRIPTION OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a method for preparing an optical thin film having multiple optical layers, particularly an optical thin film having multiple optical layers made of a dielectric material.
2. Prior Art
Heretofore, for the preparation of a dielectric optical thin film of a multi-layered film structure, it has been common to employ a film-forming method such as a vapor deposition method e.g. an electron beam evaporation, or a sputtering method e.g. an ion beam sputtering method. Particularly, in the field of optical communication, a dielectric optical thin film having multiple layers as many as 100 layers or more coated, is utilized as a narrow-band pass filter based on an interference effect of an optical multilayer film, to be used for high density transmission such as WDM (wavelength division multiplexing) or DWDM (dense wavelength division multiplexing), and a thin film-deposition process is desired to be operated in a stabilized condition over a long period of time during which a process for forming a multilayer film continues and whereby the film thickness and the optical properties are controlled very precisely.
In order to produce such a dielectric optical multilayer thin film, a production method employing a vacuum evaporation method, has been proposed, for example, in JP-A-5-196810 or JP-A-11-342354. However, in a vacuum evaporation method, the evaporation source is basically a point source, whereby it is difficult to obtain a uniform film thickness distribution over a wide region of a substrate. Further, it is difficult to maintain the temperature distribution or the molten state of the substance for vapor deposition always uniform and constant, and it is difficult to maintain the evaporation process in a stabilized condition over a long period of time. Accordingly, it is difficult to constantly obtain a dielectric optical multilayer thin film of high precision required for the preparation of a filter for such WDM or DWDM.
Further, an optical thin film prepared by a usual vacuum evaporation process has a low packing density of atoms and is porous, and thus, it has had a problem in durability and a problem in the reliability as a filter. Among them, the reliability of the performance of an optical film may be improved by combined use of ion irradiation or plasma irradiation. However, problems in the stability or controllability of the vacuum evaporation are derived from the evaporation source itself, and such problems can hardly be solved even by ion or plasma irradiation.
On the other hand, JP-A-12-178731, etc. propose a production method employing an ion beam sputtering method, as a method for forming a multilayer type optical thin film to improve the controllability or stability of the vacuum evaporation process. However, ion beam sputtering has a drawback that the sputtering rate of a dielectric film is usually slow, and a long time is required to prepare a multilayer type optical thin film, and the productivity is low. Further, a trouble of the apparatus is a factor to lower the yield of the product as the operation takes a long period of time, and there is a problem that the productivity will thereby be further lowered. Further, the ion source itself and its consumable parts are expensive, and it is accordingly a serious problem from the viewpoint of the production that the operation cost will be very high.
In a magnetron sputtering method, a thin film is usually formed under a relatively high discharge pressure (at a level of about 1 Pa) as compared with a vacuum evaporation method, whereby there has been a problem in reliability such that the thin film to be formed is porous and has a low packing density, and there may be a deterioration of optical characteristics attributable to this low packing density, such as a change in the optical characteristics due to a change with time of e.g. the refractive index. Further, in a magnetron sputtering method particularly for an oxide thin film among dielectric films, the film deposition rate is slow, and if the discharge power is increased to improve the film deposition rate, an electric charge will be accumulated on the insulating film of dielectric deposited on the target surface or on the inner wall of the sputtering chamber, whereby the glow discharge tends to become unstable, and an abnormal discharge such as arcing is likely to take place, thus leading to formation of a defective film.
Further, in a typical magnetron sputtering method, the distance between the substrate and the target is short, whereby it has been difficult to attach an optical monitor, so it has been difficult to control the sputtering process precisely. For these reasons, a conventional magnetron sputtering method has not been applied to a process for preparing a dielectric optical multi-layered thin film wherein the number of stacked layers is many and highly precise control of the film thickness is required.
Problems to be Solved by the Invention
It is an object of the present invention to overcome the above-mentioned drawbacks of the prior art and to provide a method for preparing an optical thin film having multiple optical layers having a high packing density with little inclusion of impurities. A further object is to provide a method for preparing an optical thin film having multiple layers with good precision by a stabilized process excellent in productivity. Further, it is another object of the present invention to provide a method for preparing an optical thin film having multiple optical layers, which is still more excellent in packing density and less susceptible to a change with time of the optical characteristic such as the refractive index and which has the improved durability.
Further, it is another object of the present invention to provide a method for preparing an optical thin film having multiple optical layers, which is suitable for the production of a multilayer optical filter or the like, and whereby a sufficiently high deposition rate of the optical layer can be obtained, stability of discharge can be secured over a long period of time at the time of forming multiple optical layers, and highly precise controllability can be obtained.
Means to Solve the Problems
The present invention provides (1) a method for preparing an optical thin film, which comprises forming an optical thin film having multiple optical layers on a substrate by a magnetron sputtering apparatus having cathodes, the substrate and targets disposed in a sputtering chamber, wherein an inert gas and a reactive gas are introduced into the sputtering chamber, and optical layers are successively formed on the substrate by a reactive magnetron sputtering method under a condition of a discharge pressure of 1.3×10
−1
Pa at most.
In the present invention, as the inert gas, it is common to employ Ar gas, and as the reactive gas, O
2
gas or O
2
gas containing an inert gas is employed when an optical layer of oxide type is to be formed, or N
2
gas or N
2
gas containing an inert gas is employed when an optical layer of nitride type is to be formed. As other reactive gas, an optional gas may be employed depending upon the type of the desired optical layer. Further, as the target to be used, a planar target may be mentioned.
Further, the present invention provides (2) a method for preparing an optical thin film, which comprises forming an optical thin film having multiple optical layers on a substrate by a magnetron sputtering apparatus having cathodes, the substrate and targets disposed in a sputtering chamber, wherein prior to forming the optical thin film, the sputtering chamber is evacuated to 1.0×10
−4
Pa at most, then, an inert gas and a reactive gas are introduced into the sputtering chamber, and optical layers are successively formed on the substrate by a reactive magnetron sputtering method under a condition of a discharge pressure of 1.3×10
−1
Pa at most.
Still further, the present invention provides (3) the method for preparing a

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