Antireflection Coating for Infrared Light

1. Field of the Invention

The present invention relates to an antireflection coating for infrared light which can be formed on silicon or germanium substrates.

2. Description of Related Art

Infrared light in a specific wavelength range of 3 to 5 .mu.m, which is a wavelength range that is rarely absorbed by the atmosphere, is used for various devices such as an infrared camera, an infrared detector, and an infrared light measuring device. The kinds of materials which can transmit the above-mentioned infrared light is considerably less than those materials which can transmit visible light. Therefore, an optical element, such as a lens, which is formed from a silicon or germanium substrate, is generally used for the above-mentioned devices. However, silicon and germanium have high refractive indices, so that problems can occur in that the resulting reflectance on a surface of the substrate is high (silicon: approximately 30%, germanium: approximately 36%), and the corresponding transmittance therethrough is low. Accordingly, for achieving a higher transmittance and for preventing any ghost and a flare from occurring in a lens system, it is indispensable to form an antireflection coating on the surface.

As such, an antireflection coating in which a layer of zinc sulfide (ZnS) is contiguously formed on a substrate is known (for example, Japanese Laid-Open Patent Application No. Sho. 64-15703, Japanese Laid-Open Patent Application No. Hei. 2-135401, Japanese Examined Patent Publication No. Hei 2-11121, and Japanese Examined Patent Publication No. Hei. 2-13761).

However, zinc sulfide (ZnS) has a relatively bad adhesion to the substrate, so that the layer of zinc sulfide (ZnS) can easily be removed. Thus, there is a problem in that conventional antireflection coatings will be lacking in endurance and can be stripped from the substrate.

An object of the present invention is to provide an antireflection coating which is formed on a silicon or germanium substrate used as a substrate of optical parts for infrared light in the wavelength range of 3 to 5 .mu.m, and which has a good antireflection characteristic and endurance.

Conventionally, when an antireflection coating is formed on a silicon or germanium substrate, a layer of zinc sulfide (ZnS) is formed contiguously to the substrate. The above-mentioned object is achieved in the present invention by forming a layer of silicon dioxide (SiO.sub.2) instead of the layer of zinc sulfide (ZnS).

The present invention relates to an anti-reflection coating for a silicon or a germanium substrate, which comprises a multilayered coating including a layer of silicon dioxide (SiO.sub.2) formed contiguously to the substrate.

An antireflection coating having a layer of silicon dioxide (SiO.sub.2) which is formed contiguously to the silicon or germanium substrate has a good optical characteristic and good endurance.

FIG. 1 is a sectional view of a basic structure of an antireflection coating formed on a silicon or germanium substrate in the present invention;

FIG. 2 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 1 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 3 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 2 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 4 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 3 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 5 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 4 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 6 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 5 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 7 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 6 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 8 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 7 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 9 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 8 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 10 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 9 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 11 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 10 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 12 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 11 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 13 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 12 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 14 is a graph showing a reflectance characteristic of an antireflection coating of the embodiment 13 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 15 is a graph showing a reflectance characteristic of an antireflection coating of the comparative embodiment 1 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 16 is a graph showing a reflectance characteristic of an antireflection coating of the comparative embodiment 2 to the infrared light in the wavelength range of 3 to 5 .mu.m;

FIG. 17 is a graph showing a reflectance characteristic of an antireflection coating of the comparative embodiment 3 to the infrared light in the wavelength range of 3 to 5 .mu.m; and

FIG. 18 is a graph showing a reflectance characteristic of an antireflection coating of the comparative embodiment 4 to the infrared light in the wavelength range of 3 to 5 .mu.m.

Silicon dioxide, which is formed as the N-th layer (counted from the air side) on the silicon or germanium substrate, will actually absorb a certain percentage of infrared light in the wavelength range of 3 to 5 .mu.m. So, it has not conventionally been used as a material for comprising an antireflection coating within this wavelength range. However, in the present invention, the predetermined thickness of the silicon dioxide (SiO.sub.2) layer selected is so slight that any bad influence to an optical characteristic by its absorption can be ignored. The thickness of the layer of silicon dioxide (SiO.sub.2), however, still has good adhesion to both silicon and germanium substrates and to a contiguous layer of germanium (Ge). Therefore, the layer of silicon dioxide (SiO.sub.2) can contribute to an improvement of endurance of the coating much more than it would adversely influence the optical characteristic.

A basic structure of an antireflection coating of the present invention is shown in FIG. 1. In the present invention, from the air side, the first layer, the second layer, . . . and the N-th layer are all formed above a substrate made of silicon or germanium so that the N-th layer can be formed contiguously to the silicon or germanium substrate. The first layer is made of fluoride, the second layer is made of zinc sulfide (ZnS), the third layer is made of germanium (Ge), and from the fourth layer, a layer of zinc sulfide (ZnS), and a layer of germanium (Ge) are formed alternately. The [N-1]th layer is made of germanium (Ge) and the Nth layer is made of silicon dioxide (SiO.sub.2).

In the basic structure of an antireflection coating of the present invention, a layer of a material having a tensile stress (fluoride or germanium) and a layer of a material having a compressive stress (zinc sulfide) are piled alternately. Therefore, the two stresses are compensated to lessen each other, and further, these two materials have good adhesion, so that the resultant layers are hard to split. Generally, an antireflection coating is comprised of piled layers of materials having, respectively, a different refractive index. By carefully adjusting the thickness and the refractive index, and antireflection effect can be obtained. The number of the piled layers can be subjectively determined by the optical designer. But too many piled layers make the coating thick and, as a result, not only is the transmittance lowered, but also, the piled layers may become easy to split. Especially, since the thickness of the layer is determined on the basis of the wavelength of the infrared light (whose wavelength is in a range of 3 to 5 .mu.m), the above-described tendency will be strengthened. From this aspect, it is best for an antireflection coating formed on a silicon or germanium substrate to be comprised of about four layers.

The present invention is described further in detail with an example of an antireflecting coating comprised of four piled layers hereinafter.

The first layer is made of fluoride and has a refractive index (n.sub.1) of 1.25 to 1.60. As the first layer, the following fluoride can be applied: yttrium fluoride (YF.sub.3), magnesium fluoride (MgF.sub.2), aluminum fluoride (AlF.sub.3), lithium fluoride (LiF.sub.2), barium fluoride (BaF.sub.2), calcium fluoride (CaF.sub.2), lanthanum fluoride (LaF.sub.3), sodium fluoride (NaF), aluminum sodium fluoride (Na.sub.3 AlF.sub.6), and so on.

The second layer is formed under the first layer and made of zinc sulfide (ZnS) having a refractive index less than that of the first layer.

The third layer is formed under the second layer and made of germanium (Ge) having a refractive index greater than that of the second layer.

The fourth layer is formed under the third layer and made of silicon dioxide (SiO.sub.2) having a refractive index less than that of the third layer.

The first to the fourth layers can be formed by vacuum evaporation, sputtering, or CVD, and the refractive index of each layer can be adjusted by changing the forming condition (for example, vacuum degree or heating temperature).

The optical thickness of each layer is formed within the range as follows:

When the optical thickness of each layer is formed to be out of the above-mentioned range, the expected antireflection effect cannot be obtained.

In the case of forming the first layer out of fluoride whose refractive index (n.sub.1) is from 1.35 up, it is suitable to form the optical thickness of the first layer approximately 0.25.lambda. and to form the total optical thickness of the second, third, and fourth layers approximately 0.25.lambda..

In the case of forming the first layer out of fluoride whose refractive index (n.sub.1) is not exceeding 1.45, it is suitable to form the respective optical thicknesses of the first and second layers approximately 0.25.lambda. and to form the total optical thickness of the third and fourth layers approximately 0.10.lambda..

The layer of silicon dioxide is selected to be of a predetermined optical thickness sufficient to improve adhesion of subsequent coating layers without significantly adsorbing the design wavelength of infrared radiation. A ratio of the optical thickness of the layer of silicon dioxide to the final effective coating layer in the antireflecting coating design is within the following range: ##EQU1##

The present invention is further described with specifications of embodiments hereinafter.

Tables 1 to 4 show structures of antireflection coatings formed on germanium or silicon substrates, according to the embodiments 1 to 4 of the present invention, respectively.

Tables 5 to 13 show structures of antireflection coatings in which the second to the fourth layers are the same as the structure in the embodiments 1 to 4, and the thickness and the refractive index of the first layer are variably changed, according to the embodiments 5 to 13 of the present invention, respectively.

Tables 14 to 17 (the comparative examples 1 to show structures of antireflection coatings which are formed on germanium or silicon substrates and in which the fourth layer is made not of silicon dioxide (SiO.sub.2), but zinc sulfide (ZnS). The comparative examples 1 to 4 are for comparison of the endurance with the embodiments 1 to 4.

The reflectance characteristic of the embodiments 1 to 13 and the comparative examples 1 to 4 are shown in FIGS. 2 to 18, respectively.

The reflectance characteristic of this embodiment is shown in FIG. 2.

The reflectance characteristics of this embodiment is shown in FIG. 3.

The reflectance characteristic of this embodiment is shown in FIG. 4.

The reflectance characteristic of this embodiment is shown in FIG. 5.

The reflectance characteristics of this embodiment is shown in FIG. 6.

The reflectance characteristic of this embodiment is shown in FIG. 7.

The reflectance characteristic of this embodiment is shown in FIG. 8.

The reflectance characteristic of this embodiment is shown in FIG. 9.

The reflectance characteristic of this embodiment is shown in FIG. 10.

The reflectance characteristic of this embodiment is shown in FIG. 11.

The reflectance characteristic of this embodiment is shown in FIG. 12.

The reflectance characteristic of this embodiment is shown in FIG. 13.

The reflectance characteristic of this embodiment is shown in FIG. 14.

The reflectance characteristic of this example is shown in FIG. 15.

The reflectance characteristic of this example is shown in FIG. 16.

The reflectance characteristic of this example is shown in FIG. 17.

The reflectance characteristic of this example is shown in FIG. 18.

For a reliability test of the antireflection coatings shown in Tables 1 to 4 and 41 to 17, a tape peel test and an organic solvent test has been utilized.

After adhering a tape on a surface of a multilayered coating of an antireflection coating, the tape is peeled perpendicularly off the surface of the multilayered coating. This operation is performed five times.

The surface of a multilayered coating of an antireflection coating is rubbed 40 times with a cloth dipped in alcohol at a 2 kilogram weight pressure.

The result of the above-mentioned tests is shown in Table 18.

In the antireflecting coatings of the comparative embodiments, the multilayered coatings had been partially peeled off substrates, while in those of the embodiments of the present invention, peeling had not occurred.

Concerning an environmental test (70.degree. C., 80% humidity, leave 24 hours) and a heat cycle test (-20.degree. C. to +85.degree. C.), no abnormality had been recognized.

Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.