JP2015227963A - Optical filter and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical filter which has a first transmission wavelength band and a second transmission wavelength band and which offers high transmittance by reducing haze and transmittance ripples in the transmission wavelength bands while suppressing the number of layers.SOLUTION: An optical filter 10 comprises a substrate 11 and an optical thin film portion 20 consisting of a plurality of layers formed on the substrate. The optical thin film portion 20 comprises, in order from the substrate 11 side: a first SWPF 22 consisting of low refractive index layers 13 made of a low refractive index material and high refractive index layers 12 made of a high refractive index material, which are alternately laminated; and a second SWPF 23 consisting of the low refractive index layers 13 and high refractive index layers 12 that are alternately laminated, where the layers have thickness that is different from that of the layers of the first SWPF 22.

Description

  The present invention relates to an optical filter having a first transmission wavelength band in the visible light region and a second transmission wavelength band in the infrared light region, and a method for manufacturing the same, and particularly to an optical filter that can be manufactured at low cost and has high transmittance. Etc.

  Patent documents 1 and 2 disclose day and night surveillance cameras. In this surveillance camera, an infrared cut filter is inserted into the optical axis during the daytime to remove the infrared rays, and at night, the infrared cut filter is taken out of the optical axis to include the infrared rays. That is, the infrared ray is cut off during the daytime to obtain a natural image by visible light, and the infrared ray is irradiated at night to obtain an image of the reflected light.

  Patent Document 3 proposes an optical filter having a first transmission wavelength band in the visible light region and a second transmission wavelength band in the infrared light region (paragraph 0047, FIG. 3B below). And FIG. 6C). This optical filter is made of a dielectric multilayer film, and detects infrared light in the same manner as visible light. Therefore, the visible light transmittance is set to about half of the infrared light transmittance.

  On the other hand, in an imaging apparatus such as a digital camera, an optical filter having a low-pass filter and an optical absorption filter is used. In that case, interference occurs when the spatial frequency of the subject is close to the pixel pitch of the image sensor, and this interference causes false colors, color moire, and the like. Further, the reflection of light on the front surface of the image sensor causes a red ghost. In order to reduce such deterioration in image quality, an optical filter that cuts ultraviolet rays (UV) and infrared rays (IR) is generally disposed on the front surface of the image sensor.

  For example, the optical filter described in Patent Document 4 forms a UV-IR cut coat in which TiO2 (titanium dioxide) and SiO2 (silicon dioxide) are alternately formed on an IR cut glass, and reflects both ultraviolet rays and infrared rays. It has a structure to let you. In addition, the spectral transmittance of the IR cut glass and the UV-IR cut coat is approximated to human visual sensitivity.

  Patent Documents 5 and 6 describe an IR cut filter configured by alternately forming a multilayer film of TiO 2 and SiO 2. Patent Document 5 describes a total of 40 layers, and Patent Document 6 describes an example in which a total of 48 layers or 60 layers are stacked. Non-Patent Document 1 (p199, chapter 5.5) shows a design example of a 42-layer UV-IR cut filter using a long wave pass filter (LWPF) and a short wave pass filter (SWPF). .

Japanese Patent No. 3861241 JP 2012-159658 A JP2013-085215A JP 2013-130886 A Japanese Patent No. 3937823 Japanese Patent No. 5126089

Mitsunobu Koyama, “Optical Thin Film Filter Design”, Optronics Inc., 2006

  The aforementioned day and night surveillance camera requires a mechanism for taking the infrared cut filter into and out of the optical axis, and thus it is difficult to reduce the size and cost. Moreover, since the optical filter having the first transmission wavelength band and the second transmission wavelength band described above does not disclose a specific example of the film configuration, it is unclear whether it can be actually realized.

  On the other hand, as described above, an optical filter having a structure in which a high refractive index material such as TiO 2 and a low refractive index material such as SiO 2 are alternately formed is already known. In such an optical filter, a transmission band is used. In order to eliminate fine fluctuations in light transmittance (so-called ripples), it is common practice to increase the number of layers in which a high refractive index material and a low refractive index material are alternately formed.

  Patent Document 4 does not particularly define the number of layers, but generally, it is said that it will not withstand practical use unless at least 40 layers or more are laminated. Patent Document 6 describes design examples of 48 layers or 60 layers, and Non-Patent Document 1 describes 42 layers.

  However, as the number of layers increases, a longer time is required for film formation, and management in each process becomes difficult. In particular, when the number of film layers is large, scattering occurs due to a rough film surface, and when TiO2 is used as a high refractive index material, scattering occurs due to an increase in TiO2 crystal grains. This is called haze of the optical thin film. It becomes difficult to suppress the fogging of the optical thin film, which also greatly affects the manufacturing cost.

  Therefore, the present invention aims to realize an optical filter having a first transmission wavelength band in the visible light region and a second transmission wavelength band in the infrared light region, and transmits light while suppressing the number of layers. An object of the present invention is to provide an optical filter capable of achieving high transmittance by reducing occurrence of fogging and ripples in a band, and a method for manufacturing the same.

In order to achieve the above object, an optical filter according to the present invention is an optical filter having a first transmission wavelength band in a visible light region and a second transmission wavelength band in an infrared light region,
A substrate, and an optical thin film portion having a plurality of layers formed on the substrate,
The optical thin film portion is in order from the side closer to the substrate,
A first short wave pass filter configured by alternately laminating a low refractive index material and a high refractive index material, which are two kinds of materials having different refractive indices;
A second short wave pass filter configured by alternately laminating the low refractive index material and the high refractive index material with a film thickness different from those of the first short wave pass filter,
It is characterized by that.

In order to achieve the above object, an optical filter manufacturing method according to the present invention is a method of manufacturing an optical filter according to the present invention,
On the substrate, the first short wave pass filter is formed by alternately laminating the low refractive index material and the high refractive index material,
On the first short wave path filter, the low refractive index material and the high refractive index material are alternately laminated with a film thickness different from those of the first short wave path filter to form a second short wave path filter. Forming a pass filter,
It is characterized by that.

  According to the optical filter and the method for manufacturing the same according to the present invention, the first and second short wave path filters are formed with different film thicknesses, so that the first transmission wavelength band in the visible light range and the infrared light range are obtained. A filter characteristic having the second transmission wavelength band can be realized, and a high transmittance can be achieved by suppressing generation of fogging and ripples in the transmission band while suppressing the number of layers.

2 is an explanatory diagram illustrating a schematic configuration of an optical filter according to Embodiment 1. FIG. 3 is a graph showing a refractive index profile of the optical filter according to the first embodiment. 6 is a graph showing the transmittance with respect to the wavelength of incident light in the optical filter of the first embodiment. It is a graph which shows the transmittance | permeability with respect to the wavelength of incident light in the optical filter of a comparative example (the 1). It is a graph which shows the transmittance | permeability with respect to the wavelength of incident light in the optical filter of a comparative example (the 2). It is a graph which shows the transmittance | permeability with respect to the wavelength of incident light in the optical filter of a comparative example (the 3). 4 is a graph schematically showing transmittance with respect to a wavelength of incident light in each part and the whole of the optical filter according to the first embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as “embodiments”) will be described with reference to the accompanying drawings. In the present specification and drawings, the same reference numerals are used for substantially the same components. The shapes depicted in the drawings are drawn so as to be easily understood by those skilled in the art, and thus do not necessarily match the actual dimensions and ratios. In the following description and drawings, “short wave pass filter” is abbreviated as “SWPF”. The “filter characteristic” and the abbreviation “characteristic” refer to the relationship between the wavelength of incident light and the transmittance in the optical filter.

  The configuration of the first embodiment of the present invention will be described with reference to FIGS.

  The optical filter 10 according to the first embodiment includes a substrate 11 and an optical thin film portion 20 having a plurality of layers formed on the substrate. The optical thin film portion 20 is a first layer configured by alternately laminating a low refractive index layer 13 made of a low refractive index material and a high refractive index layer 12 made of a high refractive index material in order from the side closer to the substrate 11. It has SWPF22 and 2nd SWPF23 comprised by laminating | stacking alternately the low-refractive-index layer 13 and the high-refractive-index layer 12 with the film thickness different from those of 1st SWPF22.

Here, L is a quarter optical film thickness of the low refractive index layer 13, H is a quarter optical film thickness of the high refractive index layer 12, m and n are the number of repetitions of the film, and M, N, and X are coefficients. Define each. At this time,
The film configuration of the first SWPF 22 is [M {(0.5 ± 0.05) L + (1 ± 0.05) H + (0.5 ± 0.05) L}] m,
The film configuration of the second SWPF 23 is [N {(0.7 ± 0.1) L + (1.4 ± 0.2) H + (0.7 ± 0.1) L}] n,
And,
M = 1 / {(1.033 ± 0.005) X},
N = (1.033 ± 0.005) X,
X = 1.0-1.10,
Is desirable. More preferably, the design wavelength of the 1/4 optical film thickness is 800 nm, and the repetition times m and n are m = 7 and n = 8.

  By providing the above configuration, the optical filter 10 can realize filter characteristics having the first transmission wavelength band 31 in the visible light region and the second transmission wavelength band 32 in the infrared light region shown in FIG. 3. In addition, while suppressing the number of layers to 40 layers or less, it is possible to obtain high transmittance by reducing occurrence of fogging and ripples in the transmission band.

Further, a reflection formed by alternately laminating two layers each of a high refractive index material and a low refractive index material with a film thickness thinner than those of the first SWPF 22 between the substrate 11 and the first SWPF 22. A prevention film 21 is interposed. The film configuration of the antireflection film 21 is [0.191H + 0.293L + 0.331H + 0.795L].
It is. Although four layers are shown as the film configuration of the antireflection film 21, it is only necessary to have the function of the antireflection film even in a configuration other than the four layers. By providing such an antireflection film 21, higher transmittance can be obtained.

  Hereinafter, this will be described in more detail.

  FIG. 1 is an explanatory diagram illustrating a schematic configuration of the optical filter 10 according to the first embodiment. The optical filter 10 has alternating low refractive index layers 13 having a relatively low refractive index and high refractive index layers 12 having a relatively high refractive index on a silicon dioxide crystal (crystal) or glass substrate 11. It is configured by laminating.

  Typically, the material of the high refractive index layer 12 is, for example, TiO2 (titanium dioxide, refractive index n = 2.30 to 2.40), and the material of the low refractive index layer 13 is, for example, SiO2 (silicon dioxide, refractive index). n = 1.46) can be used, but it is not necessary to follow this example.

  An optical thin film portion 20 is formed on the substrate 11 by the low refractive index layers 13 and the high refractive index layers 12 that are alternately stacked. The optical thin film portion 20 is composed of components such as an antireflection film 21, a first SWPF 22, and a second SWPF 23 for suppressing ripples in order from the substrate 11 side.

  The configuration of each part is expressed as follows. Here, the quarter optical film thickness (¼ of the design wavelength λ) of each material of the low refractive index layer 13 and the high refractive index layer 12 is defined as L and H, and the coefficients are defined as M, N, and X, respectively. To do.

Antireflection film 21 [0.191H + 0.293L + 0.331H + 0.795L]
First SWPF 22 [M {(0.5 ± 0.05) L + (1 ± 0.05) H + (0.5 ± 0.05) L}] m
Second SWPF 23 ... [N {(0.7 ± 0.1) L + (1.4 ± 0.2) H + (0.7 ± 0.1) L}] n
M = 1 / {(1.033 ± 0.005) X}
N = (1.033 ± 0.005) X
X = 1.0-1.10

  Here, m and n are the number of repetitions of the film in [], and m = 7 and n = 8 under the best conditions confirmed by experiments. With the above configuration, the number of layers of the film is 34 layers. That is, it has succeeded in reducing the number of layers compared with the existing techniques described in Patent Documents 4 to 6 in which 40 or more layers are stacked. In FIG. 1, priority is given to simply showing the concept of the present invention, so the number of layers does not match this best condition.

  In the above equation, for example, “± 0.05” in “0.5 ± 0.05” is the maximum error that occurs during manufacturing, and “0.5 ± 0.05” is “0.45 to .0. 55 ". For example, “M {(0.5 ± 0.05) L +...” Means a layer having a film thickness of “(0.5 ± 0.05) × L × M” and “(1 ± 0. 05) × H × M ”means a three-layer structure of“ (0.5 ± 0.05) × L × M ”.

  FIG. 2 is a graph showing a refractive index profile of the optical filter 10. The horizontal axis in FIG. 2 indicates the position (nm) in the thickness direction when the substrate 11 side (left end side) is 0, and the vertical axis indicates the refractive index of the material at that position. The refractive index of the substrate 11 is about 1.55, the refractive index of the high refractive index layer 12 is about 2.30, and the refractive index of the low refractive index layer 13 is about 1.46. The right end of FIG. 2 is air, and its refractive index is 1. Further, the design wavelength λ = 800 nm.

  FIG. 3 is a graph showing the transmittance of the optical filter 10 with respect to the wavelength of incident light. The horizontal axis of FIG. 3 indicates the wavelength (nm) of incident light, and the vertical axis indicates the transmittance (%) of the incident light with respect to the wavelength. The incident angle is 0 degree, that is, a numerical value when the light enters the optical filter 10 perpendicularly.

  The transmittance of the optical filter 10 is 95 to 100% at a wavelength of 430 to 640 nm (first transmission wavelength band 31), rapidly decreases at 640 to 750 nm, becomes almost 0 at 750 nm or more, and again at 850 to 930 nm. It rises, becomes 90 to 100% at 930 to 940 nm (second transmission wavelength band 32), decreases again at 940 to 980 nm, and becomes almost 0 again at 980 nm or more. In the region where the wavelength is 430 to 640 nm, the transmittance varies within a range of 100 to 95%, and it is understood that the generation of ripples is small. Further, cloudiness (haze) was also measured with a haze meter, and a good value of about 0.08 (no unit amount) was obtained.

  The optical filter 10 is manufactured by sequentially forming the low-refractive index layer 13 and the high-refractive index layer 12 on the substrate 11 while changing the film thickness, so that the antireflection film is sequentially formed from the substrate 11 side. 21, the first SWPF 22 and the second SWPF 23 need only be formed with a refractive index profile as shown in FIG. 2 while increasing the film thickness per layer in this order.

  Therefore, no special manufacturing equipment is required compared to existing optical filters. Nevertheless, the optical filter 10 can be obtained with a smaller number of layers and less occurrence of fogging and ripples. In addition, since the number of layers is suppressed to 40 or less, the film formation time is shortened, and it becomes possible to manufacture at a low cost.

  4 to 6 are graphs showing the transmittance with respect to the wavelength of incident light in the optical filters of the respective comparative examples. In the optical filter of each comparative example, the coefficients M, N, and X are outside the above-described range, and the other configurations are the same as those of the optical filter 10.

FIG. 4 shows the case where M = 1 and N = 1, that is, M = 1> 1 / {(1.033 ± 0.05) X},
N = 1 <(1.033 ± 0.005) X
This is the case. In this case, since the second (long wavelength side) transmission wavelength band disappears, the characteristics of a normal UV-IR cut filter are obtained.

FIG. 5A shows a case where the maximum error of M and N is below the lower limit value −0.05 and X is lower than the lower limit value 1.0, that is, M = 1 / (1.027X) = 1 / {( 1.033-0.06) X},
N = 1.027X = (1.033−0.006) X,
X = 0.99 <1.0
This is the case. In this case, the transmittance in the second (long wavelength side) transmission wavelength band is 90% or less.

FIG. 5B shows a case where the maximum error of M and N exceeds the upper limit value +0.05 and X is lower than the lower limit value 1.0, that is, M = 1 / (1.039X) = 1 / {(1 .033 + 0.06) X},
N = 1.039X = (1.033 + 0.006) X,
X = 0.99 <1.0
This is the case. In this case, the transmittance in the second (long wavelength side) transmission wavelength band is 90% or less.

FIG. 6A shows the case where the maximum error of M and N is below the lower limit value −0.05 and X is higher than the upper limit value 1.10, that is, M = 1 / (1.027X) = 1 / {( 1.033-0.06) X},
N = 1.027X = (1.033−0.006) X,
X = 1.11> 1.10
This is the case. In this case, the transmittance in the second (long wavelength side) transmission wavelength band is 90% or less.

FIG. 6B shows a case where the maximum error of M and N exceeds the upper limit value +0.05 and X exceeds the upper limit value 1.10, that is, M = 1 / (1.039X) = 1 / {(1 .033 + 0.06) X},
N = 1.039X = (1.033 + 0.006) X,
X = 1.11> 1.10
This is the case. In this case, the transmittance in the second (long wavelength side) transmission wavelength band is 90% or less.

  As described above, when M = 1 and N = 1, the second (long wavelength side) transmission wavelength band disappears, and in any other case, the transmittance of the second (long wavelength side) transmission wavelength band Is 90% or less. Therefore, it is desirable that the coefficients M, N, and X are within the above-described range. In other words, by setting the coefficients M, N, and X within the above-described range, the transmittance in the visible wavelength region is 95% or more, and the transmittance in the infrared wavelength region is 90%. That is all.

  FIG. 7 is a graph schematically showing the transmittance of each part and the whole of the optical filter according to the first embodiment with respect to the wavelength of incident light.

  FIG. 7A is a graph when the antireflection film 21 is used as a single optical filter. The antireflection film 21 reduces the transmittance on the shortest wavelength side.

  FIG. 7B is a graph when the first SWPF 22 is used as a single optical filter. The filter characteristics of the first SWPF 22 include a transmission wavelength band 22a on the short wavelength side of the center wavelength λ1 and a transmission wavelength band 22b on the long wavelength side of the center wavelength λ1. The center wavelength λ1 is the aforementioned design wavelength λ. Then, the filter characteristic is shifted to the long wavelength side by applying the coefficient M to the film thickness of each layer. As the coefficient M is increased, the filter characteristics are shifted to the longer wavelength side.

  FIG. 7C is a graph when the second SWPF 23 is used as a single optical filter. The filter characteristics of the second SWPF 23 include a transmission wavelength band 23a on the short wavelength side of the center wavelength λ2 and a transmission wavelength band (not shown) on the long wavelength side of the center wavelength λ2. The center wavelength λ2 is the aforementioned design wavelength λ × 1.4. The filter characteristic is shifted to the short wavelength side by multiplying the film thickness of each layer by the coefficient N. As the coefficient N is decreased, the filter characteristics are shifted to the shorter wavelength side.

  FIG. 7D is a graph of the optical filter 10 in which the antireflection film 21, the first SWPF 22 and the second SWPF 23 having the characteristics shown in FIGS. 7A to 7C are stacked. The first transmission wavelength band 31 includes a portion where the transmission wavelength band 22a and the transmission wavelength band 23a overlap. The second transmission wavelength band 32 includes a portion where the transmission wavelength band 22b and the transmission wavelength band 23a overlap. Since the optical filter 10 is formed by laminating the antireflection film 21, the first SWPF 22, and the second SWPF 23, the transmittance in FIG. 7D at a certain wavelength is from FIG. 7A to FIG. C] is approximately equal to the product of transmittance.

  In the first embodiment, in the film configuration of a normal UV-IR cut filter (FIG. 4), the characteristics of the first SWPF 22 are shifted to the longer wavelength side by applying a coefficient M to each layer of the first SWPF 22 (FIG. 7). [B]), a coefficient N is applied to each layer of the second SWPF 23 to shift the characteristics of the second SWPF 23 to the short wavelength side (FIG. 7C). At this time, by optimizing the coefficients M and N, the second transmission wavelength band 31 (FIG. 7D) consisting of the overlapping part of the transmission wavelength band 22b and the transmission wavelength band 23a is generated.

  Therefore, the second transmission wavelength band 32 can be added to a normal UV-IR cut filter by changing only the coefficients M and N without changing the entire film design. An optical filter 10 having a (Filter) function can be easily realized. That is, the difference in film design is small compared to a normal UV-IR cut filter. The optical filter 10 can be immediately produced with a high yield by using the film forming apparatus for which the UV-IR cut filter condition setting has been completed.

  In addition, when the optical filter 10 is used for a day and night surveillance camera, the optical filter 10 can be kept on the optical axis, and therefore a mechanism for taking the optical filter 10 in and out of the optical axis is not necessary. Lower prices can be achieved. This is because by optimizing the coefficients M and N, the bandwidth of the second transmission wavelength band 32 can be set to a minimum, so that infrared rays that affect daytime photography can be minimized.

  The present invention has been described with reference to the specific embodiments shown in the drawings. However, the present invention is not limited to the embodiments shown in the drawings, and any known hitherto provided that the effects of the present invention are achieved. Even if it is a structure, it is employable.

  Although it can be widely used in general equipment including an optical filter, it is particularly suitable for use in an imaging apparatus, particularly an imaging apparatus that is required to be small, light, and low cost. More specifically, it is an optical filter or the like mounted on a monitoring camera for day and night as described above.

DESCRIPTION OF SYMBOLS 10 Optical filter 11 Board | substrate 12 High refractive index layer 13 Low refractive index layer 20 Optical thin film part 21 Antireflection film 22 1st SWPF
22a Transmission wavelength band on short wavelength side 22b Transmission wavelength band on long wavelength side 23 Second SWPF
23a Transmission wavelength band on short wavelength side 31 First transmission wavelength band 32 Second transmission wavelength band

Claims (6)

An optical filter having a first transmission wavelength band in the visible light region and a second transmission wavelength band in the infrared light region,
A substrate, and an optical thin film portion having a plurality of layers formed on the substrate,
The optical thin film portion is in order from the side closer to the substrate,
A first short wave pass filter configured by alternately laminating a low refractive index material and a high refractive index material, which are two kinds of materials having different refractive indices;
A second short wave pass filter configured by alternately laminating the low refractive index material and the high refractive index material with a film thickness different from those of the first short wave pass filter,
An optical filter characterized by the above. When L is defined as 1/4 optical film thickness of the low refractive index material, H is defined as 1/4 optical film thickness of the high refractive index material, m and n are the number of repetitions of the film, and M, N, and X are defined as coefficients. ,
The film configuration of the first short wave pass filter is [M {(0.5 ± 0.05) L + (1 ± 0.05) H + (0.5 ± 0.05) L}] m,
The film configuration of the second short wavepass filter is [N {(0.7 ± 0.1) L + (1.4 ± 0.2) H + (0.7 ± 0.1) L}] n,
And,
M = 1 / {(1.033 ± 0.005) X},
N = (1.033 ± 0.005) X,
X = 1.0-1.10
Is,
The optical filter according to claim 1. The design wavelength of the ¼ optical film thickness was 800 nm, and m and n as the number of repetitions were m = 7 and n = 8.
The optical filter according to claim 2. Between the substrate and the first short wave pass filter, the high refractive index material and the low refractive index material are alternately arranged in two layers each having a thickness smaller than those of the first short wave pass filter. With an antireflection film laminated on the
The film configuration of the antireflection film is [0.191H + 0.293L + 0.331H + 0.795L]
Is,
The optical filter according to claim 3. A method for producing the optical filter according to any one of claims 1 to 3,
On the substrate, the first short wave pass filter is formed by alternately laminating the low refractive index material and the high refractive index material,
On the first short wave path filter, the low refractive index material and the high refractive index material are alternately laminated with a film thickness different from those of the first short wave path filter to form a second short wave path filter. Forming a pass filter,
An optical filter manufacturing method characterized by the above. A method for producing the optical filter according to claim 4, comprising:
On the substrate, the antireflective film is formed by alternately laminating the high refractive index material and the low refractive index material by two layers each with a film thickness thinner than those of the first short wave pass filter,
On the antireflection film, the low refractive index material and the high refractive index material are alternately laminated to form the first short wave pass filter,
On the first short wave path filter, the low refractive index material and the high refractive index material are alternately laminated with a film thickness different from those of the first short wave path filter to form a second short wave path filter. Forming a pass filter,
An optical filter manufacturing method characterized by the above.

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