JP2001235709A - Optical low pass filter and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical low pass filter in which substrates are directly laminated without using an adhesive and which is made thin in a smaller number of production processes as compared with a conventional filter, and to provide a method of manufacturing that filter. SOLUTION: When the adhering faces of a plurality of birefringent plates (substrates) consisting of lithium niobate are subjected to hydrophilic treatment to turn into an activated state of surfaces, the birefringent plates can be joined without using an adhesive. Thus, a thinner optical low pass filter than a conventional one can be easily manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical low-pass filter formed by bonding a plurality of birefringent plates, and more particularly to an optical low-pass filter having a reduced overall thickness without using an adhesive for joining the birefringent plates. And its manufacturing method.

[0002]

2. Description of the Related Art
Most imaging devices including a CCD (solid-state imaging device), such as a video camera, are provided with an optical low-pass filter to prevent generation of moire-like pseudo signals. Typical examples of optical low-pass filters include:
There are a crystal, a lenticular micro lens, a noise filter, a phase rotation grating, and the like. Above all, the birefringent plate made of quartz has the characteristics that various characteristics can be realized depending on the bonding method and the performance is stable.
Recently, it has been used for most optical low-pass filters.

As shown in FIG. 4, when a crystal birefringent plate 2 is used as an optical low-pass filter, incident light is split into an ordinary ray and an extraordinary ray and emitted in parallel with the same intensity. Is made. At this time, one of the important factors in the function of the optical low-pass filter is an angle θ between the ordinary ray and the extraordinary ray. This angle θ is larger as the refractive index anisotropy is larger, but in the case of quartz, this value is as low as 0.009 (in the case of a wavelength of 632.8 nm) even for quartz having the highest value. And the element becomes large. Further, a manufacturing process such as polishing of quartz is required, which is not suitable for mass production, and when quartz is used, the raw materials themselves are expensive.

In order to solve such a problem, attention has been paid to a lithium niobate single crystal plate having a high birefringence as a birefringent plate. For example, Japanese Patent Application Laid-Open No. H11-218612 discloses an optical low-pass filter composed of a plurality of sheets in which a birefringent plate of lithium niobate having a high birefringence is bonded to both sides or one side of a quartz plate. In this optical low-pass filter, the thickness of the birefringent plate can be reduced by using lithium niobate having a crystal structure different from that of quartz, and the total thickness of the optical low-pass filter can be reduced by combination with a quartz plate.

However, an adhesive is used for bonding the substrates, and it is necessary to provide an antireflection film for preventing reflection between the substrates on the bonding surface.
There is a problem that the number of work steps increases.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical low-pass filter in which substrates are directly bonded to each other without using an adhesive. That is.

[0007]

Means for Solving the Problems In view of the above problems, as a result of diligent research, the present inventor has conducted a hydrophilization treatment on a bonding surface of a plurality of birefringent plates (substrates) made of lithium niobate to activate the surface. Then, it was found that the substrates could be joined together without using an adhesive, and an optical low-pass filter thinner than the conventional one could be easily manufactured, and the present invention was reached.

That is, the optical low-pass filter according to the present invention is characterized in that a plurality of birefringent plates are bonded to each other, and at least one surface of the plurality of birefringent plates is subjected to a hydrophilic treatment and bonded. And

In the optical low-pass filter of the present invention, antireflection films are provided on the upper and lower surfaces of a plurality of birefringent plates bonded to each other, and the birefringent plates are preferably made of lithium niobate.

In the method for manufacturing an optical low-pass filter according to the present invention, a surface to be bonded of the surfaces of the birefringent plate is subjected to a hydrophilic treatment, the surfaces subjected to the hydrophilic treatment are bonded, and a heat treatment is performed. It is characterized in that a refraction plate is joined.

Further, in the hydrophilic treatment in the method for manufacturing an optical low-pass filter of the present invention, the birefringent plate is immersed in a mixed solution of hydrogen peroxide, ammonia water and pure water, washed with pure water, and dried. It preferably comprises a step. Further, the heat treatment is preferably performed at 100 to 400 ° C. in a nitrogen atmosphere. Further, it is preferable to provide an anti-reflection film on the upper and lower surfaces of the plurality of birefringent plates bonded together.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [1] Optical Low-Pass Filter The optical low-pass filter of the present invention is formed by laminating a plurality of birefringent plates. By utilizing the birefringence of the birefringent plate, in an optical low-pass filter that separates an image of an object into two or more and cuts a high-frequency image, in order to obtain a sufficient low-pass effect, the birefringent plates of each other are required. It is necessary to change the direction of the crystal direction and attach them. Therefore, at least 2
Two birefringent plates are required.

FIG. 1 shows an example of an optical low-pass filter 1 according to the present invention. The optical low-pass filter 1 shown in FIG.
It has a configuration in which two birefringent plates 2 are directly joined without using an adhesive. The optical low-pass filter 1 shown in FIG. 1B has antireflection films 3 on the upper and lower surfaces of two joined birefringent plates.

As the birefringent plate 2, a crystal substrate of lithium niobate, lithium tantalate, quartz, lithium tetraborate, calcite or the like is used. Above all, when lithium niobate is used as the substrate, the generation of pseudo signals can be reduced as compared with a generally used quartz substrate, even if the thickness is small.

As shown in FIG. 3, a conventional optical low-pass filter 5 utilizing a lithium niobate birefringent plate is provided with an antireflection film 3 on the upper and lower surfaces of the birefringent plate 2. The adhesive 4 is applied and bonded to both surfaces of a birefringent plate 6 made of quartz. Lithium niobate birefringent plates have a very high refractive index of 2.2, so there is no equivalent with current adhesives. for that reason,
An antireflection film 3 is provided to prevent reflection between the substrates. Therefore, in the conventional optical low-pass filter 5 in which such an anti-reflection film 3 must be provided, there is a limit in reducing the thickness, and the operation process becomes complicated.

On the other hand, since the optical low-pass filter of the present invention does not bond the substrates via an adhesive, it is not necessary to provide the antireflection film 3 on the bonding surface as in the conventional case. Therefore, the thickness can be reduced, and the size of the element can be easily reduced. Further, the working process can be shortened.

In the case where three or more birefringent plates 2 are bonded, if the surfaces to be bonded are subjected to a hydrophilic treatment and bonded, the birefringent plates 2 can be easily directly bonded to each other while maintaining high adhesion. Can be joined.

[2] Manufacturing method of optical low-pass filter The bonding surface of the birefringent plate 2 is sufficiently washed and immersed in a mixed solution of hydrogen peroxide, ammonia water and pure water for 10 to 15 minutes. Here, the concentration of the hydrogen peroxide solution in the mixed solution is 1 to 20%.
And the ammonia concentration is preferably 1 to 10%.
After immersing the bonding surface of the birefringent plate 2, rinsing with pure water,
After drying by spin drying or the like, the birefringent plates 2 are bonded together.

By this hydrophilic treatment, the surface of the birefringent plate 2 is brought into a state of high activity. Although the state of this high activity is not necessarily clear, it is considered that a highly active atomic layer is generated on the surface of the birefringent plate 2 by the hydrophilic treatment. Then, when a plurality of birefringent plates 2 having the surface active atomic layer are superimposed, it is considered that the birefringent plates 2 are bonded to each other by bonding between atoms.

Next, heat treatment is performed at 100 to 400 ° C. for 60 to 120 minutes. The heat treatment is performed in a nitrogen atmosphere to prevent inactivation of the substrate surface due to oxidation. By this processing, the bonding between the birefringent plates 2 becomes stronger.

Then, if necessary, bonding (joining)
When the antireflection film 3 is provided on the upper and lower surfaces of the later birefringent plate 2 as shown in FIG. 1, the optical low-pass filter 1 of the present invention is manufactured.

With this method, the birefringent plates 2 maintain high adhesion. Further, according to the method of the present invention, since no adhesive is used, it is not necessary to provide the antireflection film 3 on the joint surface, and the number of working steps can be reduced.

[0023]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example Two birefringent plates (thickness: 0.4 mm) made of lithium niobate were thoroughly washed and then subjected to a hydrophilic treatment. In the hydrophilization treatment, a 30% aqueous hydrogen peroxide solution, a 25% aqueous ammonia solution, and pure water are prepared at a ratio of 1: 1: 6 (volume ratio), and the bonded surface of the birefringent plate is immersed in the mixed solution for 10 minutes. went.

Thereafter, the birefringent plate was rinsed with pure water. After spin-drying, the lithium niobate birefringent plates were bonded together and heat-treated at 300 ° C. in a nitrogen atmosphere to produce an optical low-pass filter.

The transmittance of the optical low-pass filter manufactured as described above was measured by a spectrophotometer. For comparison, the transmittance of only one birefringent plate that was not bonded was also measured. The results are shown in FIG. FIG. 2 (a)
FIG. 2 (b) shows the result of a birefringent plate that has not been bonded, which is a bonded birefringent plate (optical low-pass filter manufactured in this example). When these were compared, no difference was found in the transmittance. Therefore, in the optical low-pass filter of the present invention, it was found that there was no reflection between the substrates and bonding occurred at the interface between the substrates.

[0027]

As described above in detail, the optical low-pass filter of the present invention has a simple structure and can be miniaturized since the birefringent plates are directly bonded to each other without using an adhesive. Since there is no need to provide an anti-reflection film between the refraction plates, it is possible to shorten the working process.

[Brief description of the drawings]

FIG. 1 is a side view showing a layer configuration of an optical low-pass filter according to the present invention, wherein (a) shows an example in which birefringent plates are directly joined to each other, and (b) shows reflection on upper and lower surfaces of the birefringent plate. An example in which a prevention film is provided is shown.

FIGS. 2A and 2B are diagrams showing results of transmittance measurement of the optical low-pass filter of the present invention, wherein FIG. 2A shows a bonded birefringent plate, and FIG. It is a refraction plate.

FIG. 3 is a side view showing an example of a conventional optical low-pass filter.

FIG. 4 is an explanatory diagram showing the optical characteristics of a conventionally used optical low-pass filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical low-pass filter 2 ... Birefringent plate 3 ... Antireflection film

Claims (7)

[Claims] 1. An optical low-pass filter comprising a plurality of birefringent plates bonded to each other, wherein at least one surface of the plurality of birefringent plates is subjected to a hydrophilic treatment and bonded. filter. 2. The optical low-pass filter according to claim 1, wherein an anti-reflection film is provided on upper and lower surfaces of the plurality of bonded birefringent plates. 3. The optical low-pass filter according to claim 1, wherein the birefringent plate is made of lithium niobate. 4. The method for manufacturing an optical low-pass filter according to claim 1, wherein a surface to be bonded among the surfaces of the birefringent plate is subjected to a hydrophilic treatment, and the surface subjected to the hydrophilic treatment is bonded. Heat treating to join the plurality of birefringent plates. 5. The method for producing an optical low-pass filter according to claim 4, wherein the hydrophilizing treatment includes immersing the birefringent plate in a mixed solution of hydrogen peroxide, ammonia water, and pure water, and adding pure water. A method comprising the steps of washing and drying. 6. The method for manufacturing an optical low-pass filter according to claim 4, wherein the heat treatment is performed at 100 to 400 ° C. in a nitrogen atmosphere. 7. The method for manufacturing an optical low-pass filter according to claim 4, wherein an antireflection film is provided on upper and lower surfaces of the plurality of bonded birefringent plates.