Interference Film Filter and an Optical Waveguide and a Method for Producing the Same

The present invention relates to an interference film filter used such as for an optical multiplexer and demultiplexer in optical communication systems and optical microscopes and to an optical waveguide used such as for optical measurement systems and optical integrated circuits and relates to a method for producing the interference film filter and the optical waveguide.

Japanese Patent Laid-Open No. 60-87302 (1985) discloses an interference film filter including alternative lamination of a thin Ti0.sub.2 film having a high refractive index of 2.2 and a thin Si0.sub.2 film having a low refractive index of 1.46.

Japanese Patent Laid-Open No. 60-114811 (1985) (U.S. Ser. No. 06/674,770, filed on Nov. 26, 1984 and assigned to the present assignee , now U.S. Pat. No. 4,737,015 disclosed a stepped and a graded optical waveguide including an oxynitride film composed of silicon oxide and silicon nitride having a high refractive index of 1.72 at a wavelength of 0.633 .mu.m, the wavelength generated by He-Ne gas laser and forming a core portion and an oxynitride film composed of silicon oxide and silicon nitride having a low refractive index of 1.65-1.69 at a wavelength of 0.633 .mu.m and forming a clad portion. The oxynitride films are deposition-formed by sputtering a polycrystalline silicon in a sputter gas containing 0.sub.2 and N.sub.2 gases, and refractive indexes of the oxynitride films are controlled by adjusting the 0.sub.2 gas content in the sputter gas.

Japanese Patent Laid-Open No. 60-37504 (1985) discloses a stepped and a graded optical waveguide comprising a core portion formed of an alternate lamination of a thin Ti0.sub.2 film having a high refractive index of 2.3 and a thin Si0.sub.2 film having a low refractive index of 1.477 at a wavelength of 0.633 .mu.m. The laminate is then heat-treated to form a core portion having a uniform composition and a refractive index between 1.477 and 2.3.

In the interference film filter and the optical waveguide, when a thin film having a high refractive index n with thickness d exhibits a local maximum transmittance at a wavelength .lambda., the following equation generally holds;

The above equation indicates that the film thickness be decreased with the increase in refractive index of the film. On one hand, loss of light in general decreases with the decrease in thickness of the film.

Loss of light in the interference film filter and the optical waveguide is further reduced in dependence upon extinction coefficient specific to a material of thin film. The extinction coefficient represents a degree of light absorption in the thin film material, in that, the smaller the extinction coefficient, the smaller the loss of light in the thin film. Until now, it was considered that the extinction coefficient of an optical material in general increases in dependence upon an increase of the refractive index thereof.

Refractive index and extinction coefficient of the conventionally used Ti0.sub.2 thin film for the interference film filter and the optical waveguide are respectively 2.3 and 0.5-1.0, which are not necessarily satisfactory for the reduction of loss of light in the interference film filter and the optical waveguide.

One object of the present invention is to provide an interference film filter and an optical waveguide with a small loss of light.

Another object of the present invention is to provide an interference film filter and an optical waveguide which comprise a high refractive index thin film or a core having a higher refractive index and a smaller extinction coefficient than those of the conventional Ti0.sub.2 thin film.

Further object of the present invention is to provide a method for producing an interference film filter and an optical waveguide with reduced production time and production cost.

An interference film filter and an optical waveguide of the present invention includes a substrate such as glass, plastic and silicon wafer and a laminate supported on the substrate, the laminate is formed of at least one alternative lamination of a first thin film of amorphous Si.sub.x C.sub.y O.sub.z having a high refractive index of up to 3.4, wherein x >o, y >z and z .gtoreq.o and a second thin film having a low refractive index in comparison with that of the first thin film. The amounts of y and z in the amorphous Si.sub.x C.sub.y O.sub.z are preferably in a range of 0.6-1 and 0-0.5 respectively when assumed x is 1. The second thin film is preferably to be amorphous Si.sub.x O.sub.y C.sub.z, wherein x >o, y >z, and z .gtoreq.o, and the amounts of y and z thereof are preferably in a range of 1.8-2 and 0-0.5 respectively when assumed x is 1.

A method of the present invention for producing an interference film filter and an optical waveguide including a substrate and a laminate supported on the substrate, the laminate being formed by sputtering deposition of at least one alternative lamination of a first thin film having a high refractive index and a second thin film having a low refractive index in comparison with that of the first thin film comprises a step of depositing an amorphous Si.sub.x C.sub.y O.sub.z film, wherein y is 0.6-1 and z is 0-0.5 when assumed x is 1, by sputtering an SiC sinter target and controlling the ratio of 0.sub.2 gas content in a mixture sputter gas of an inert gas such as argon (Ar), helium (He), neon (Ne), Krypton (Kr) and xenon (Xe) and oxygen gas in a range of 0-5 wt % so as to constitute the first thin film, and a step of depositing an amorphous Si.sub.x O.sub.y C.sub.z film, wherein y is 1.8-2 and z is 0-0.05 when assumed x is 1 by sputtering the same common SiC sinter target and controlling the ratio of 0.sub.2 gas content in the mixture sputter gas in a range of 10-20wt % so as to constitute the second thin film.

The resultant thin film of amorphous Si.sub.x C.sub.y O.sub.z exhibits a refractive index of upto 3.4 at a wavelength of 0.633 .mu.m, which is very higher than the refractive index 2.3 of the Ti0.sub.2 thin film, and an extinction coefficient below 0.1, which is very smaller than the extinction coefficient 0.5 to 1.0 of the Ti0.sub.2 thin film.

The resultant thin film of amorphous Si.sub.x O.sub.y C.sub.z exhibits substantially the equivalent properties of the Si0.sub.2 thin film in that a refractive index of 1.4 to 1.5 at a wavelength of 0.633 .mu.m and an extinction coefficient of substantial zero.

The thin film of amorphous Si.sub.x O.sub.y C.sub.z, which constitutes the low refractive index thin film or the clad portion of the present invention, may be replaced by the conventional thin film forming materials by sputtering having a low refractive indexes such as Si0.sub.2 and oxynitride composed of silicon oxide and silicon nitride, wherein the sputtering target and the sputter gas have to be changed for the film formation, which prolongs the production time and increases the production cost, as well as interrupts the continuous operation, moreover during exchange of the sputter target, dust is likely to deposit on the surface of the previously formed film to reduce the quality of the resultant product.

FIG. 1 shows a sectional view of one embodiment of an interference film filter according to the present invention.

FIG. 2 shows a sectional view of another embodiment of an interference film filter according to the present invention and

FIG. 3 shows a sectional view of one embodiment of an optical waveguide according to the present invention.

The interference film filter shown in FIG. 1 was produced as follows,

On the glass substrate 1, an amorphous Si.sub.x C.sub.y film 2 (x=0.6,y=0.4) having thickness of 490.ANG. was deposited by sputtering in an argon gas with an SiC sinter target for 2.5 minutes, wherein sputter power was 200W and sputter gas pressure was 1.times.10.sup.-2 Torr. Then, the same SiC sinter target was sputtered in a mixture sputter gas consisting of argon gas and 15wt % of oxygen for 5 minutes in order to deposit an amorphous Si.sub.x O.sub.y film 3 (x=0.4, y=0.8) having thickness of 1000 .ANG.. By repeating the above operation a laminate having alternate lamination of the films 2 and 3 in a number of several tens of films supported on the glass substrate 1 was produced.

The amorphous Si.sub.x C.sub.y film 2 (x=0.6, y=0.4) and the amorphous Si.sub.x O.sub.y film 3 (x=0.4, y=0.8) exhibited refractive indexes 3.2 and 1.4 respectively.

Arrows indicated by numerals 6, 7 and 8 respectively show the directions of an incident light, a transmitted light and a reflected light to the resultant interference film filter.

The method of producing the interference film filter shown in FIG. 2 is substantially the same as of FIG. 1, in that, on the glass substrate 1, an amorphous Si.sub.x C.sub.y O.sub.z film 4 (x=0.5,y=0.4,z=0.2) having thickness of 630 .ANG. was deposited by sputtering in a mixture sputter gas consisting of argon gas and 5 wt % of oxygen using the same SiC sinter target for 3 minutes, wherein sputter power was 200 W and sputter gas pressure was 1.times.10.sup.-2 Torr . Then, an amorphous Si.sub.x O.sub.y C.sub.z film 5 (x=0.4, y=0.8, z=0.1) having thickness of 980 .ANG. was deposited on the film 4 using the same SiC sinter target and a mixture sputter gas consisting of argon gas and 10 wt % of oxygen for 5 minutes. By repeating the above operation, the amorphous Si.sub.x C.sub.y O.sub.z films 4 and the amorphous Si.sub.x O.sub.y C.sub.z films 5 were alternatingly laminated upon one another in a number of several tens of films.

The amorphous Si.sub.x C.sub.y O.sub.z film 4 (x=0.5, y=0.4, z=0.2) and the amorphous Si.sub.x O.sub.y C.sub.z film 5 (x=0.4, y=0.8, z=0.1) exhibited refractive indexes 2.5 and 1.6 respectively.

Arrows indicated by numerals 6.7 and 8 respectively show the directions of an incident light, a transmitted light and a reflected light to the resultant interference film filter.

The optical waveguide shown in FIG. 3 was produced in a manner substantially the same as of the interference film filter shown in FIGS. 1 and 2, in that, on a glass substrate 30, an amorphous Si.sub.x O.sub.y C.sub.z film 20 (x:y:z=1:1.8-2:0-0.5) having thickness of 5-30 .mu.m, which constitutes a clad portion, was deposited by sputtering in a mixture sputter gas consisting of argon gas and 10-20 wt % of oxygen for 250-1500 minutes, wherein sputter power was 200 W and sputter gas pressure was 1.times.10.sup.-2 Torr. Then the same common SiC sinter target was sputtered in a mixture sputter gas consisting of argon gas and 0-5 wt % of oxygen for 50-500 minutes in order to deposit an amorphous Si.sub.x C.sub.y O.sub.z film 10 (x:y:z =1:0.6-1:0-0.5) having thickness of 1-10 .mu.m, which constitutes a core position, on the amorphous Si.sub.x O.sub.y C.sub.z film 20 formed previously. Finally the same common SiC sinter target again was sputtered to deposit an amorphous Si.sub.x O.sub.y C.sub.z film 20 (x:y:z =1:1.8-2:0-0.5) on the amorphous Si.sub.x C.sub.y O.sub.z film 10 formed previously under the same condition for the formation of the amorphous Si.sub.x O.sub.y C.sub.z film 20 formed previously.

The resultant amorphous Si.sub.x C.sub.y O.sub.z film 10 and the amorphous Si.sub.x O.sub.y C.sub.z film 20 exhibited refractive indexes 1.8-3.2 and 1.4-1.8 respectively.

Arrows indicated by numerals 6 and 9 respectively show the directions of an incident light and a guided light to the resultant optical waveguide.

Although, the embodiment of FIG. 3 shows a stepped type optical waveguide, however a core portion having a predetermined refractive index distribution is easily formed to constitute a graded type optical waveguide in that by using the common SiC sinter target and controlling non-stepwisely the oxygen concentration ratio in the mixture sputter gas, amorphous Si.sub.x C.sub.y O.sub.z film or amorphous Si.sub.x O.sub.y C.sub.z film having refractive index ranging from 1.4 to 3.4 is freely deposition-formed through the method of the present invention.

Whether the resultant deposition films explained above are in the forms of amorphous Si.sub.x C.sub.y O.sub.z and amorphous Si.sub.x O.sub.y C.sub.z was analyzed and confirmed by an ion micro analyzer, an electron microscope and an infrared absorption spectrum.