Patent Application
20110310472
The present invention relates to an infrared optical filter and a manufacturing method thereof.
It has been known an optical filter composed of a dielectric multilayer film in which two kinds of dielectric thin films having dissimilar refractive indices from each other but an identical optical film thickness (optical thickness of film) are alternately stacked. Examples of materials of the dielectric films include, for instance, TiO2, SiO2, Ta2O5, Nb2O5, Al2O3, Si3N4, ZrO2, MgF2, CaF2 and the like.
There has been proposed a solid-state imaging device provided with an optical filter 200 that has a plurality of kinds of filter parts 21, 22, 23 each of which is configured to selectively transmit incident light (for instance, WO 2005/069376: referred to as Patent document 1), as illustrated in
The filter parts 21, 22, 23 of the above-described optical filter 200 comprise: a first λ/4 multilayer film 21; a second λ/4 multilayer film 22; and a wavelength selection layer 231, 232, 233, respectively. In the first λ/4 multilayer film 21, two kinds of thin films 21a, 21b having identical optical film thickness but formed of dielectric materials having mutually dissimilar refractive indices are alternately stacked. The second λ/4 multilayer film 22 is formed on the opposite side of the first λ/4 multilayer film 21 from the n-type semiconductor substrate 101 side. In the second λ/4 multilayer film 22, the two kinds of thin films 21a, 21b are alternately stacked. Each the wavelength selection layer 231, 232, 233 is interposed between the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, and that has an optical film thickness different from the optical film thickness of the thin films 21a, 21b according to a desired selection wavelength. As regards the materials of the two kinds of thin films 21a, 21b, TiO2 is used as a high refractive index material having a relatively high refractive index, and SiO2 is used as a low refractive index material having a relatively low refractive index. In the example illustrated in
Transmission spectra of the filter parts 21, 22, 23 are explained next based on
A stacked film (laminated film) in which the two kinds of thin films 21a, 21b having dissimilar refractive indices are periodically stacked, as illustrated in
In contrast, according to the above-described filter part 21, 22, 23, a wavelength selection layer 23 (231, 232, 233) having dissimilar optical film thickness is provided in the periodic refractive index structure, as illustrated in
The range over which the transmission peak wavelength can shift through modulation of the optical film thickness of the wavelength selection layer 23 depends on the reflectance bandwidth set by the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22. The wider the reflectance bandwidth is, the wider becomes the range over which the transmission peak wavelength can shift. As is known (Reference document: Mitsunobu Kobiyama, “Optical Thin-Film Filters”, The Optronics Co., Ltd., p. 102-106), a reflectance bandwidth Δλ can be worked out approximately on the basis of Equation (1) below, in which nH denotes the refractive index of the above-described high refractive index material, nL denotes the refractive index of the low refractive index material, λ0 denotes a set wavelength that is equivalent to four times the optical film thickness, which is common to the thin films 21a, 21b.
Equation (1) indicates that the reflectance bandwidth Δλ depends on the refractive index nL of the low refractive index material and the refractive index nH of the high refractive index material, such that, in order to widen the reflectance bandwidth Δλ, it is important to increase the value of the refractive index ratio nH/nL, i.e. it is important to increase the refractive index difference between the high refractive index material and the low refractive index material.
The optical filter 200 in the solid-state imaging device illustrated in
Various technologies are known in which gases and/or flame are sensed by using infrared ray detection elements and infrared optical filters (for instance, Japanese Patent Application Publication No. S58-58441: referred to as Patent document 2; Japanese Patent Application Publication No. 2001-228086: referred to as Patent document 3; Japanese Patent Application Publication No. H5-346994: referred to as Patent document 4).
Patent document 3 proposes an infrared optical filter in the form of a multi-wavelength selection filter that transmits infrared rays of dissimilar selection wavelengths depending on an in-plane position, as shown in
In the infrared optical filter having the configuration illustrated in
The optical filter 200 having the configuration illustrated in
In a case where TiO2 is used as the high refractive index material and SiO2 is used as the low refractive index material, the refractive index of TiO2 is 2.5 and the refractive index of SiO2 is 1.5. Therefore, the refractive index ratio nH/nL is 1.67, and the reflectance bandwidth Δλ is 0.3 times the set wavelength λ0, as indicated by point Q1 in
Specific wavelengths for detecting (sensing) various gases and flame that can occur, for instance, in houses, include 3.3 μm for CH4 (methane), 4.0 μm for SO3 (sulfur trioxide), 4.3 μm for CO2 (carbon dioxide), 4.7 μm for CO (carbon monoxide), 5.3 μm for NO (nitrogen monoxide) and 4.3 μm for flame. Therefore, a reflection band in the infrared region of about 3.1 μm to 5.5 μm is required for selective detection of all the above-listed specific wavelengths, and thus the reflectance bandwidth Δλ must be 2.4 μm or greater.
As illustrated in
In the field of infrared optical filters, combination of Ge and ZnS, both transparent in the infrared region, are ordinary adopted as the combination of a high refractive index material and a low refractive index material. However, the refractive index of Ge is 4.0 and the refractive index of ZnS is 2.3. Therefore, the refractive index ratio nH/nL is 1.74, and the reflectance bandwidth Δλ is of about 1.5 μm. In this case as well, it is not possible to set all the above-described selection wavelengths. Patent document 3 discloses the feature of using Si as the high refractive index material, but this entails a further narrower reflectance bandwidth Δλ.
In order to use the optical filter 200 having the configuration illustrated in
In case of using the combination of Ge and ZnS as the combination of a high refractive index material and a low refractive index material in an infrared optical filter as described above, a thin film formed of a semiconductor material of Ge or ZnS is to be exposed at the surface of the infrared optical filter. Thus, the properties of the thin film at the topmost layer may change thereupon on account of, for instance, adhesion or adsorption of impurities and/or reactions with moisture, oxygen and the like in air, all of which is deemed to impair filter performance. Therefore, in a case where the infrared optical filter is disposed for instance on the light-receiving face of an infrared ray detection element, the sensitivity and stability of infrared ray detection may be impaired at the infrared ray detection element. Besides, if the thin film at the topmost layer is formed of Ge, which is the high refractive index material, the reflective component at the surface becomes larger, and it becomes difficult to enhance filter characteristics.
In the light of the above-described issues, it is an object of the present invention to provide an infrared optical filter that affords a high degree of freedom in the design of selection wavelengths and that exhibits better filter performance, and to provide a method for manufacturing that infrared optical filter.
The infrared optical filter of the present invention comprises: a substrate formed of an infrared transmitting material; and a plurality of filter parts arranged side by side on one surface side of the substrate. Each filter part includes: a first λ/4 multilayer film in which two kinds of thin films having mutually different refractive indices but an identical optical film thickness are alternately stacked; a second λ/4 multilayer film in which the two kinds of thin films are alternately stacked, said second λ/4 multilayer film being formed on the opposite side of the first λ/4 multilayer film from the substrate side; and a wavelength selection layer interposed between the first λ/4 multilayer film and the second λ/4 multilayer film, said wavelength selection layer having an optical film thickness different from the optical film thickness of each the thin film according to a desired selection wavelength. A low refractive index material of the first λ/4 multilayer film and the second λ/4 multilayer film is an oxide, and a high refractive index material thereof is a semiconductor material of Ge. A material of the wavelength selection layer is identical to a material of the second top thin film of the first λ/4 multilayer film.
In this case, using an oxide as the low refractive index material and Ge of a semiconductor material as the high refractive index material in the first λ/4 multilayer film and the second λ/4 multilayer film allows increasing the refractive index difference between the high refractive index material and the low refractive index material, as compared with a case where both the high refractive index material and the low refractive index material are semiconductor materials. And thus, there can be widened the reflectance bandwidth and the selection wavelength range at which selection is enabled through setting of the film thickness of wavelength selection layer. The degree of freedom in the design of the selection wavelengths is increased as a result. The material of the wavelength selection layer is identical to the material of the second top thin film of the first λ/4 multilayer film. Therefore, the etching selectivity in a case where the wavelength selection layer is patterned through etching can be increased, and thereby a decrease of the optical film thickness of the thin film of the topmost layer of the first λ/4 multilayer film during the above-mentioned patterning can be prevented, so that filter performance can be enhanced.
In the second λ/4 multilayer film, preferably, the thin film furthest from the substrate is formed of the low refractive index material.
This allows preventing changes in the properties of the thin films that are furthest from the substrate in the filter parts, caused by for instance reactions with moisture, oxygen and the like in air or adhesion and/or adsorption of impurities. The filter performance in stability is thus improved. Besides, reflection at the surfaces of the filter parts can be reduced. The filter performance can be enhanced accordingly.
Preferably, the low refractive index material is Al2O3 or SiO2.
In this case, a filter performance can be achieved such that the filter has a reflection band at an infrared region of about 3.1 μm to 5.5 μm, by setting the set wavelength of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 to 4 μm.
Preferably, the infrared transmitting material is Si.
In this case, costs are reduced compared with a case in which the infrared transmitting material is Ge or ZnS.
An infrared optical filter manufacturing method of the present invention comprises a basic step of alternately stacking two kinds of thin films having mutually different refractive indices but an identical optical film thickness, on one surface side of the substrate. Halfway the basic step, there is performed, at least once, a wavelength selection layer formation step. The wavelength selection layer formation step includes: a wavelength selection layer film-formation step of forming a wavelength selection layer formed of a material identical to that of the second top layer of the stacked film in said halfway of the basic step, on the stacked film; and a wavelength selection layer patterning step of etching an unwanted portion, in the wavelength selection layer formed in the wavelength selection layer film-formation step, by using an uppermost layer of the stacked film as an etching stopper layer, where the unwanted portion is a portion other than a portion corresponding to one arbitrary filter part. Optical film thickness of the wavelength selection layer is set in accordance with the selection wavelength of said one arbitrary filter part from among filter parts.
In this case, there can be provided an infrared optical filter that affords a high degree of freedom in the design of selection wavelengths, and that exhibits higher filter performance stability.
As illustrated in
As the infrared transmitting material of the substrate 1 there is used Si (i.e. a Si substrate is used as the substrate 1), but the infrared transmitting material is not limited to Si, and may be Ge, ZnS or the like. In the present embodiment, the plan-view shape of the filter part 21, 22 is a square of several millimeters, and the plan-view shape of the substrate 1 is a rectangular shape. However, the plan-view shapes and dimensions are not particularly limited to the foregoing.
In the infrared optical filter of the present embodiment, Al2O3, which is one kind of oxide, is used as the material (low refractive index material) of the thin film 21b that is a low refractive index layer in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22. Then, Ge, having a higher refractive index than Si, and being one kind of semiconductor material, is used as the material (high refractive index material) of the thin film 21a that is a high refractive index layer. The materials of the wavelength selection layers 231, 232 are respectively identical to the materials of the thin films 21b, 21a disposed second from the top of the first λ/4 multilayer film 21 that stands immediately below the wavelength selection layers 231, 232. In the second λ/4 multilayer film 22, those thin films 21b, 21b that are furthest from the substrate 1 are formed of the above-described low refractive index material. The low refractive index material is not limited to Al2O3, and there may be used SiO2, which is one kind of oxide. Herein, SiO2 has a lower refractive index than Al2O3, and hence there can be achieved a greater refractive index difference between the high refractive index material and the low refractive index material. In
In the present embodiment, the set wavelength λ0 of the first λ/4 multilayer film 21 and the second. λ/4 multilayer film 22 is set to 4 μm so that the above-described various gases and flame can be detected by appropriately setting the respective optical film thicknesses of the wavelength selection layers 231, 232. The physical film thickness of each thin film 21a, 21b is set to λ0/4nH and λ0/4nL, respectively, wherein nH is the refractive index of the high refractive index material and nL is the refractive index of the low refractive index material. In a concrete case where the high refractive index material is Ge and the low refractive index material is Al2O3, i.e. nH=4.0 and nL=1.7, the physical film thickness of the thin film 21a formed of the high refractive index material is set to 250 nm, and the physical film thickness of the thin film 21b formed of the low refractive index material is set to 588 nm.
Herein, assuming that there are 21 layers in a λ/4 multilayer film resulting from alternately stacking the thin film 21b formed of a low refractive index material and the thin film 21a formed of a high refractive index material, on one surface side of the substrate 1 formed of a Si substrate, as illustrated in
In
Accordingly, appropriately varying only the design of the optical film thickness of the wavelength selection layer 23, without modifying the design of the first λ/4 multilayer film 21 or the second λ/4 multilayer film 22, allows sensing various gases, for instance CH4 having a specific wavelength of 3.3 μm, SO3 having a specific wavelength of 4.0 μm, CO2 having a specific wavelength of 4.3 μm, CO having a specific wavelength of 4.7 μm, and NO having a specific wavelength of 5.3 μm, and allows sensing a flame having a specific wavelength of 4.3 μm. An optical film thickness “nd” ranging from 0 nm to 1600 nm corresponds to a physical film thickness “d” ranging from 0 nm to 941 nm. The reason why the transmission peak wavelength is 4000 nm in a case where the optical film thickness “nd” of the wavelength selection layer 23 is 0 nm, i.e. a case where the wavelength selection layer 23 is absent in
A method for manufacturing the infrared optical filter of the present embodiment will be explained next with reference to
Firstly, there is carried out a “first λ/4 multilayer film-formation step” of forming a first λ/4 multilayer film 21 by alternately stacking the thin film 21b having a predetermined physical film thickness (588 nm) formed of Al2O3 as a low refractive index material, and the thin film 21a having a predetermined physical film thickness (250 nm) formed of Ge as a high refractive index material, on the entire surface of one surface side of the substrate 1 formed of Si as an infrared transmitting material. Next, there is carried out a “wavelength selection layer film-formation step” of forming the wavelength selection layer 231 on the entire surface of said one surface side of the substrate 1 (herein, on the surface of the first λ/4 multilayer film 21), wherein the optical film thickness of the wavelength selection layer 231 is set in accordance with the selection wavelength of one filter part 21, and the wavelength selection layer 231 is formed of a material (herein, Al2O3 being the low refractive index material) identical to that of the thin film 21b positioned second from the top of the first λ/4 multilayer film 21. Thereby, the structure illustrated in
After the above-described wavelength selection layer film-formation step, there is carried out a “resist layer formation step” of forming a resist layer 31 that covers only the site corresponding to the filter part 21 by photolithography, to yield the structure illustrated in
Thereafter, there is carried out a “wavelength selection layer patterning step” of selectively etching an unwanted portion in the wavelength selection layer 231, using the resist layer 31 as a mask, and using the topmost thin film 21a of the first λ/4 multilayer film 21 as an etching stopper layer, to yield the structure illustrated in
After-the above-described wavelength selection layer patterning step, there is performed a “resist layer removal step” of removing the resist layer 31, to yield the structure illustrated in
After the above-described resist layer removal step, there is carried out a “second λ/4 multilayer film-formation step” of forming the second λ/4 multilayer film 22 by alternately stacking the thin film 21a having a predetermined physical film thickness (250 nm) formed of Ge as a high refractive index material, and the thin film 21b having a predetermined physical film thickness (588 nm) formed of Al2O3 as a low refractive index material, on the entire surface of the one surface side of the substrate 1, to yield the infrared optical filter having the structure illustrated in
In summary, the manufacturing method of the infrared optical filter of the present embodiment involves performing once a wavelength selection layer formation step in halfway during a basic step, where the basic step is composed of alternately stacking the two kinds of thin films 21b, 21a having mutually different refractive indices but an identical optical film thickness, on the one surface side of the substrate 1. Herein, the wavelength selection layer formation step is composed of: a wavelength selection layer film-formation step of forming, on the stacked film (herein, first λ/4 multilayer film 21) at the halfway of the basic step, a wavelength selection layer 23i (herein, i=1) formed of a material identical to that of the second layer from the top of the abovementioned stacked film, wherein the optical film thickness of the wavelength selection layer 23i is set in accordance with the selection wavelength of one arbitrary filter part 2i (herein, i=1) from among a plurality of filter parts 21, . . . , 2m (herein, m=2); and a wavelength selection layer patterning step of etching an unwanted portion in the wavelength selection layer 23 formed in the wavelength selection layer film-formation step, where the unwanted portion is a portion other than a portion corresponding to the abovementioned arbitrary one filter part 2i, by using an uppermost layer of the abovementioned stacked film as an etching stopper layer. Thereby, a plurality of filter parts 21, 22 is formed. Thus, an infrared optical filter having more selection wavelengths can be produced by performing more than once the wavelength selection layer formation step halfway during the above-described basic step. An infrared optical filter that senses all the above-described gases can thus be realized in one chip.
In the above-described infrared optical filter of the present embodiment, the low refractive index material in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 is an oxide, while the high refractive index material is a semiconductor material of Ge. As a result, the refractive index difference between the high refractive index material and the low refractive index material can be increased as compared with a case in which both the high refractive index material and the low refractive index material are semiconductor materials. The reflectance bandwidth Δλ can be widened accordingly, and there can be expanded the range in which selection wavelengths can be set by selecting the film thickness of the wavelength selection layers 231, 232. The degree of freedom in the design of the selection wavelength can be increased as a result. In the infrared optical filter of the present embodiment, the materials of the wavelength selection layers 231, 232 are identical to the materials of the thin films 21b, 21a that are second from the top of the first λ/4 multilayer film 21. This allows, therefore, increasing the etching selectivity in a case where the wavelength selection layer 231 is patterned through etching, and preventing a decrease of the optical film thickness of the thin film 21a of the topmost layer (see
In the infrared optical filter of the present embodiment Ge is used as the high refractive index material, and Al2O3 or SiO2 is used as the low refractive index material. Therefore, a filter performance can be achieved such that the filter has a reflection band at an infrared region of about 3.1 μm to 5.5 μm, by setting to 4 μm the set wavelength of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22.
In the infrared optical filter of the present embodiment, the infrared transmitting material of the substrate 1 is Si. Therefore, costs can be reduced compared with a case in which the infrared transmitting material is Ge or ZnS.
A further invention is explained next.
Japanese Patent Application Publication No. 2006-39736 (referred to as Patent document 5) and Japanese Patent Application Publication No. 2003-227751 (referred to as Patent document 6) disclose the feature of using a combination of a narrow-band bandpass filter and a light blocking filter, wherein the bandpass filter is formed using a Si substrate or the like and configured to transmit infrared rays of a desired wavelength, and the light blocking filter is formed using a sapphire substrate and configured to block far infrared rays. In this constitution, far infrared rays in ambient light such as solar light or illumination light can be blocked by providing such a light blocking filter.
To block far infrared rays in an infrared optical filter having the configuration illustrated in
In order to use the optical filter 200 having the configuration illustrated in
Then, in a case where Ge is used as the high refractive index material and ZnS is used as the low refractive index material in order to use the optical filter 200 having the configuration illustrated in
In the light of the above, it is an object of the present invention to provide a low-cost infrared optical filter having infrared ray blocking performance over a wide band, from the near infrared to the far infrared, and that enables selective transmission of infrared rays of a desired selection wavelength, and to provide a method for manufacturing such a infrared optical filter.
An infrared optical filter of the present invention is an infrared optical filter that controls infrared rays in a wavelength range from 800 nm to 20000 nm, comprising: a substrate; and a filter part that is formed on one surface side of the substrate and is configured to selectively transmit infrared rays of a desired selection wavelength. The filter part comprises: a first λ/4 multilayer film in which a plurality of kinds of thin films having mutually different refractive indices but an identical optical film thickness are stacked; a second λ/4 multilayer film in which the plurality of kinds of thin films are stacked, said second λ/4 multilayer film being formed on the opposite side of the first λ/4 multilayer film from the substrate side; and a wavelength selection layer interposed between the first λ/4 multilayer film and the second λ/4 multilayer film, said wavelength selection layer having an optical film thickness different from the optical film thickness of each the thin film according to a desired selection wavelength. At least one kind of thin film from among the plurality of kinds of thin films is formed of a far infrared absorbing material that absorbs far infrared rays of a longer wavelength range than an infrared ray reflection band set by the first λ/4 multilayer film and the second λ/4 multilayer film.
In this case, infrared ray blocking performance over a wide band, from the near infrared to the far infrared, can be realized at a low cost, thanks to the light interference effect of the first λ/4 multilayer film and the second λ/4 multilayer film, and by virtue of the far infrared ray absorption effect of the thin film included in the first λ/4 multilayer film and the second λ/4 multilayer film. As a result, there can be realized a low-cost infrared optical filter which has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, and in which infrared rays of a desired selection wavelength can be selectively transmitted.
Preferably, the far infrared absorbing material is an oxide or a nitride.
In this case, it becomes possible to prevent changes in optical characteristics, caused by oxidation, of the thin film formed of the far infrared absorbing material, from among the plurality of kinds of thin films. Besides, it becomes possible to form the thin film of the far infrared absorbing material in accordance with an ordinary thin film formation method, such as vapor deposition or sputtering. Costs are thus lower.
Using Al2O3 as the far infrared absorbing material allows increasing the far infrared absorption ability as compared with a case in which SiOx or SiNx is used as the far infrared absorbing material.
Using Ta2O5 as the far infrared absorbing material allows increasing the far infrared absorption ability as compared with a case in which SiOx or SINx is used as the far infrared absorbing material.
Using SiNx as the far infrared absorbing material allows increasing the moisture resistance of the thin film that is formed of the far infrared absorbing material.
Using SiOx as the far infrared absorbing material allows increasing the refractive index difference in the first λ/4 multilayer film and the second λ/4 multilayer film, and reducing the number of stack layers in the first λ/4 multilayer film and the second λ/4 multilayer film.
The first λ/4 multilayer film and the second λ/4 multilayer film, preferably, are formed by alternately stacking the thin film formed of Ge, where Ge is a material having a higher refractive index than the far infrared absorbing material, and the thin film formed of the far infrared absorbing material.
In this case, the refractive index difference between the high refractive index material and the low refractive index material in the first λ/4 multilayer film and the second λ/4 multilayer film can be increased, and also the number of stack layers in the first λ/4 multilayer film and the second λ/4 multilayer film can be reduced, as compared with a case in which ZnS is used as the high refractive index material.
The first λ/4 multilayer film and the second λ/4 multilayer film, preferably, are formed by alternately stacking the thin film formed of Si, where Si is a material having a higher refractive index than the far infrared absorbing material, and the thin film formed of the far infrared absorbing material.
In this case, the refractive index difference between the high refractive index material and the low refractive index material in the first λ/4 multilayer film and the second λ/4 multilayer film can be increased, and also the number of stack layers in the first λ/4 multilayer film and the second λ/4 multilayer film can be reduced, as compared with a case in which ZnS is used as the high refractive index material.
Preferably, the substrate is a Si substrate.
In this case; costs can be lowered compared with a case in which there is used a Ge substrate, a ZnS substrate, a sapphire substrate or the like as the substrate.
Preferably, the infrared optical filter comprises a plurality of the filter parts, and the optical film thickness of the wavelength selection layer of the plurality of the filter parts is different from each other.
In this case, infrared rays of a plurality of selection wavelengths can be selectively transmitted.
An infrared optical filter manufacturing method of the present invention is a method for manufacturing the infrared optical filter that is provided with a plurality of the filter parts such that the optical film thickness of the wavelength selection layer is different from each filter part, wherein the method comprises a step of forming, halfway a basic step of stacking the plurality of kinds of thin films on one surface side of a substrate, at least one pattern of the wavelength selection layer. Said pattern of the wavelength selection layer is formed by: forming a thin film which is a thin film formed of a material identical to that of the second top layer of the stacked film in said halfway of the basic step and having an optical film thickness set in accordance with a selection wavelength of the one arbitrary filter part from among filter parts, on the stacked film; and etching a portion, in the thin film formed on the stacked film, other than a portion corresponding to said one arbitrary filter part.
In this case, there can be provided a low-cost infrared optical filter which has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, and in which infrared rays of a plurality of desired selection wavelengths can be selectively transmitted.
An infrared optical filter manufacturing method of the present invention is a method for manufacturing the infrared optical filter that is provided with a plurality of the filter parts such that the optical film thickness of the wavelength selection layer is different from each filter part, wherein the wavelength selection layer having mutually different optical film thicknesses at respective sites corresponding to each filter part is formed through mask vapor deposition, between the first λ/4 multilayer film-formation step of forming the first λ/4 multilayer film on the one surface side of the substrate, and the second λ/4 multilayer film-formation step of forming the second λ/4 multilayer film on the opposite side of the first λ/4 multilayer film from the substrate side.
In this case, there can be provided a low-cost infrared optical filter which has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, and in which infrared rays of a plurality of desired selection wavelengths can be selectively transmitted.
An infrared optical filter of the present aspect is an infrared optical filter for controlling infrared rays in a wavelength range from 800 nm to 20000 nm. As illustrated in
As the material of the substrate 1 there is used Si, which is an infrared transmitting material (i.e. a Si substrate is used as the substrate 1), but the infrared transmitting material is not limited to Si, and may be, for instance, Ge, ZnS or the like. In the present aspect, the plan-view shape of the filter parts 21, 22 is a square of several millimeters, and the plan-view shape of the substrate 1 is a rectangular shape. However, the plan-view shapes and dimensions are not particularly limited to the foregoing.
In the infrared optical filter of the present aspect, Al2O3, which is one kind of far infrared absorbing material that absorbs far infrared rays, is used as the material (low refractive index material) of the thin film 21b that is the low refractive index layer in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, while Ge is used as the material (high refractive index material) of the thin film 21a that is the high refractive index layer. The materials of the wavelength selection layers 231, 232 are respectively identical to the materials of the thin films 21b, 21a disposed second from the top of the first λ/4 multilayer film 21 that stands immediately below the wavelength selection layers 231, 232. In the second λ/4 multilayer film 22, those thin films 21b, 21b that are furthest from the substrate 1 are formed of the above-described low refractive index material. The far infrared absorbing material is not limited to Al2O3, and may be, for instance, SiO2 or Ta2O5, which is an oxide other than Al2O3. Herein, SiO2 has a lower refractive index than Al2O3, and hence there can be achieved a greater refractive index difference between the high refractive index material and the low refractive index material.
Specific wavelengths for detecting (sensing) various gases and flame that can occur, for instance, in houses, include 3.3 μm for CH4 (methane), 4.0 μm for SO3 (sulfur trioxide), 4.3 μm for CO2 (carbon dioxide), 4.7 μm for CO (carbon monoxide), 5.3 μm for NO (nitrogen monoxide) and 4.3 μm for flame. A reflection band in the infrared region of about 3.1 μm to 5.5 μm is required, and thus the reflectance bandwidth Δλ must be 2.4 μm or greater, for selective detection of all the above-listed specific wavelengths.
In the present aspect, the set wavelength λ0 of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 is set to 4.0 μm, through appropriate setting of the respective optical film thicknesses of the wavelength selection layers 231, 232, in such a way so as to enable detection of the above-described various gases and flame. The physical film thickness of each thin film 21a, 21b is set to λ0/4nH and λ0/4nL, respectively, wherein nH is the refractive index of the high refractive index material, i.e. the material of the thin film 21a, and nL is the refractive index of the low refractive index material, i.e. the material of the thin film 21b. In a concrete case where the high refractive index material is Ge and the low refractive index material is Al2O3, i.e. nH=4.0 and nL=1.7, the physical film thickness of the thin film 21a formed of the high refractive index material is set to 250 nm, and the physical film thickness of the thin film 21b formed of the low refractive index material is set to 588 nm.
Here,
In
Accordingly, appropriately varying only the design of the optical film thickness of the wavelength selection layer 23, without modifying the design of the first λ/4 multilayer film 21 or the second λ/4 multilayer film 22, allows sensing various gases, for instance CH4 having a specific wavelength of 3.3 μm, SO3 having a specific wavelength of 4.0 μm, CO2 having a specific wavelength of 4.3 μm, CO having a specific wavelength of 4.7 μm, and NO having a specific wavelength of 5.3 μm, and allows sensing a flame having a specific wavelength of 4.3 μm. An optical film thickness “nd” ranging from 0 nm to 1600 nm corresponds to a physical film thickness “d” ranging from 0 nm to 941 nm. The reason why the transmission peak wavelength is 4000 nm in a case where the optical film thickness “nd” of the wavelength selection layer 23 is 0 nm, i.e. a case where the wavelength selection layer 23 is absent in
As the low refractive index material of the thin film 21b, there is used Al2O3, which is a far infrared absorbing material that absorbs infrared rays at a longer wavelength range than the infrared ray reflection band set by the first λ/4 multilayer film and the second λ/4 multilayer film. Herein, five kinds of far infrared absorbing materials, namely MgF2, Al2O3, SiOx, Ta2O5 and SiNx have been studied. Specifically, the film thickness of an MgF2 film, an Al2O3 film, an SiOx film, a Ta2O5 film and an SiNx film was set to 1 μm, and the film formation conditions on a Si substrate were set as given in Table 1 below. Results of the measurement of the transmission spectra of the MgF2 film, the Al2O3 film, the SiOx film, the Ta2O5 film and the SiNx film are illustrated in
In Table 1, “IB conditions” denote the conditions of ion beam assisting during film formation using the ion beam assisted deposition apparatus. “No IB” denotes no ion beam irradiation, “oxygen IB” denotes irradiation of an oxygen ion beam and “Ar IB” denotes irradiation of an argon ion beam. In
The above-described MgF2 film, Al2O3 film, SiOx film, Ta2O5 film and SiNx film were evaluated for “optical characteristic: absorption”, “refractive index” and “ease of film formation”. The results are given in Table 2 below.
The evaluation item “optical characteristic: absorption” was evaluated on the basis of the absorption factor of far infrared rays of 6 μm or longer, calculated on the basis of the transmission spectra of
On the basis of Table 2, it was found that the evaluation item “ease of film formation” exhibited no significant differences between the five types, MgF2, Al2O3, SiOx, Ta2O5 and SiNx. Results of the evaluation items “optical characteristic: absorption” and “refractive index” led to the conclusion that any from among Al2O3, SiOx, Ta2O5 and SiNx is preferably used as the far infrared absorbing material. Far infrared absorption ability can be enhanced when Al2O3 or T2O5 is used as the far infrared absorbing material, as compared with when SiOx or SiNx is used as the far infrared absorbing material. Herein, Al2O3 is preferable to T2O5 in terms of increasing the refractive index difference with the high refractive index material. The moisture resistance of the thin film 21b formed of the far infrared absorbing material can be increased in a case where SiNx is used as the far infrared absorbing material. Using SiOx as the far infrared absorbing material allows increasing the refractive index difference with the high refractive index material, and there can be reduced the number of stack layers in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22.
A method for manufacturing the infrared optical filter of the present aspect will be explained next with reference to
Firstly, there is carried out a “first λ/4 multilayer film-formation step” of forming a first λ/4 multilayer film 21 by alternately stacking the thin film 21b having a predetermined physical film thickness (herein, 588 nm) formed of Al2O3 as a low refractive index material, and the thin film 21a having a predetermined physical film thickness (herein, 250 nm) formed of Ge as a high refractive index material, on the entire surface of one surface side of the substrate 1 formed of Si. Next, there is carried out a “wavelength selection layer film-formation step” of forming the wavelength selection layer 231 on the entire surface of said one surface side of the substrate 1 (herein, on the surface of the first λ/4 multilayer film 21), wherein the optical film thickness of the wavelength selection layer 231 is set in accordance with the selection wavelength of one filter part 21, and the wavelength selection layer 231 is formed of a material (herein, Al2O3 being a low refractive index material) identical to that of the thin film 21b positioned second from the top of the first λ/4 multilayer film 21, thereby to yield the structure illustrated in
After the above-described wavelength selection layer film-formation step, there is carried out a “resist layer formation step” of forming a resist layer 31 that covers only the site corresponding to the filter part 21, by photolithography, to yield the structure illustrated in
Thereafter, there is carried out a “wavelength selection layer patterning step” of selectively etching an unwanted portion in the wavelength selection layer 231, using the resist layer 31 as a mask, and using the topmost thin film 21a of the first λ/4 multilayer film 21 as an etching stopper layer, to yield the structure illustrated in
After the above-described wavelength selection layer patterning step, there is performed a “resist layer removal step” of removing the resist layer 31, to yield the structure illustrated in
After the above-described resist layer removal step, there is carried out a “second λ/4 multilayer film-formation step” of forming the second λ/4 multilayer film 22 by alternately stacking the thin film 21a having a predetermined physical film thickness (250 nm) formed of Ge as a high refractive index material, and the thin film 21b having a predetermined physical film thickness (588 nm) formed of Al2O3 as a low refractive index material, on the entire surface of the one surface side of the substrate 1, to yield the infrared optical filter having the structure illustrated in
In summary, the manufacturing method of the infrared optical filter of the present aspect involves performing once a wavelength selection layer formation step, in halfway during a basic step, where the basic step is composed of alternately stacking a plurality of kinds (herein, two kinds) of thin films 21b, 21a having different refractive indices but an identical optical film thickness, on the one surface side of the substrate 1. Herein, the wavelength selection layer formation step is composed of: a wavelength selection layer film-formation step of forming, on the stacked film (herein, first λ/4 multilayer film 21) at the halfway of the basic step, a wavelength selection layer 23i (herein, i=1) formed of a material identical to that of the second layer from the top of the abovementioned stacked film, wherein the optical film thickness of the wavelength selection layer 23i is set in accordance with the selection wavelength of one arbitrary filter part 2i (herein, i=1) from among a plurality of filter parts 21, . . . , 2m (herein, m=2); and a wavelength selection layer patterning step of etching an unwanted portion in the wavelength selection layer 23 formed in the wavelength selection layer film-formation step, where the unwanted portion is a portion other than a portion corresponding to the abovementioned arbitrary one filter part 2i, by using an uppermost layer of the abovementioned stacked film as an etching stopper layer. Thereby, a plurality of filter parts 21, 22 is formed. Thus, an infrared optical filter having more selection wavelengths can be produced by performing the wavelength selection layer formation step a plurality of times halfway during the above-described basic step. An infrared optical filter that senses all the above-described gases can thus be realized in one chip.
The above-described manufacturing method comprises the steps of, halfway during a basic step of stacking a plurality of kinds of thin films 21a, 21b on the one surface side of the substrate 1, forming a thin film formed of the material identical to that of the second layer from the top of the stacked film (herein, the first λ/4 multilayer film 21) at the halfway of the basic step, such that the optical film thickness of the thin film is set in accordance with the selection wavelength of one arbitrary filter part 2i (herein, i=1) from among filter parts 21, . . . , 2m (herein, m=2); and etching of a portion, in the thin film formed on the abovementioned stacked film, other than a portion corresponding to the abovementioned one arbitrary filter part 2i (herein, i=1), thereby at least one pattern of the wavelength selection layer 231 is formed. However, there may be patterned at least one wavelength selection layer 231. For instance, in a case where the material of a wavelength selection layer 232 is formed of the same material as the wavelength selection layer 231 and has an optical film thickness set to be smaller than that of the wavelength selection layer 231, the patterns of two wavelength selection layers 231, 232 may be formed by etching the thin film on the abovementioned stacked film up to halfway of the thin film.
The manufacturing method is not limited thereto the above-described one, and wavelength selection layers 231, . . . , 23m (herein, m=2) having mutually different optical film thicknesses at respective sites corresponding to each filter part 21, . . . , 2m (herein, m=2) may be formed through mask vapor deposition, between the first λ/4 multilayer film-formation step of forming the first λ/4 multilayer film 21 on the one surface side of the substrate 1, and the second λ/4 multilayer film-formation step of forming the second λ/4 multilayer film 22 over the first λ/4 multilayer film 21, on the reverse side of the side of the substrate 1.
In a case where in the above-described manufacturing method the far infrared absorbing material of the thin film 21b, which is one of the above-described two kinds of thin films 21a, 21b, is SiOx or SiNx, and the other thin film 21a is of Si, then, the two kinds of thin films 21a, 21b can share the same evaporation source by employing an ion beam assisted deposition apparatus having Si as the evaporation source, in such a manner of employing a vacuum atmosphere during formation of the thin film 21a of Si, irradiating an oxygen ion beam during formation of the thin film 21b of SiOx as an oxide, and irradiating a nitrogen ion beam during formation of the thin film 21b of SiNx as a nitride. Therefore, it becomes unnecessary to prepare an ion beam assisted deposition apparatus provided with a plurality of evaporation sources, and manufacturing costs can be cut accordingly. In a case where in the above-described manufacturing method the far infrared absorbing material of the thin film 21b, which is one of the above-described two kinds of thin films 21a, 21b, is SiOx or SiNx, and the other thin film 21a is of Si, then, the two kinds of thin films 21a, 21b can share a target by employing a sputtering apparatus that uses Si as the target, in such a manner of using, a vacuum atmosphere during formation of the thin film 21a of Si, an oxygen atmosphere during formation of the thin film 21b of SiOx, and a nitrogen atmosphere during formation of the thin film 21b of SiNx. Therefore, it becomes unnecessary to prepare a sputtering apparatus provided with a plurality of targets, and manufacturing costs can be cut accordingly.
The infrared optical filter of the present aspect explained above has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, by virtue of the light interference effect of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, and by virtue of the far infrared ray absorption effect of the far infrared absorbing material of one of the thin films included in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22. As a result, infrared ray blocking performance over a wide band, from the near infrared to the far infrared, can be realized, at a low cost, thanks to the light interference effect of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, and by virtue of the far infrared ray absorption effect of the thin film 21b included in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22. A low-cost infrared optical filter can be realized therefore that has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, and that enables selective transmission of infrared rays of a desired selection wavelength. In the infrared optical filter of the present aspect, an oxide or a nitride is used as the far infrared absorbing material that is the material of the thin film 21b. Therefore, it becomes possible to prevent changes in optical characteristics that result from oxidation of the thin film 21b that is of the far infrared absorbing material, from among the plurality of kinds of thin films 21a, 21b. Besides, it becomes possible to form the thin film 21b of the far infrared absorbing material in accordance with an ordinary thin film formation method, such as vapor deposition or sputtering. Costs are thus lower.
In the infrared optical filter of the present aspect, the thin film 21a formed of Ge, being a material having a higher refractive index than a far infrared absorbing material, and the thin film 21b formed of the far infrared absorbing material, are alternately stacked in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22. Therefore, the refractive index difference between the high refractive index material and the low refractive index material in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 can be made greater, and there can be reduced the number of stack layers in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, than in a case where the high refractive index material is Si or ZnS. In a case where Si is used as the high refractive index material, the refractive index difference between the high refractive index material and the low refractive index material in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 can be made greater, and the number of stack layers in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 can be reduced, as compared with a case where ZnS is used as the high refractive index material. In the present aspect, a Si substrate is used as the substrate 1, and hence costs can be lowered compared with a case in which the substrate 1 is a Ge substrate, a ZnS substrate, a sapphire substrate or the like.
The infrared optical filter of the present aspect comprises a plurality of filter parts 21, 22 such that the optical film thickness of the wavelength selection layers 231, 232 at the respective filter parts 21, 22 are dissimilar, as described above. Therefore, infrared rays of a plurality of selection wavelengths can be selectively transmitted.
In the above-described infrared optical filter of the present aspect, Al2O3 or SiOx is used as the low refractive index material in the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, and Ge is used as the high refractive index material. As a result, the refractive index difference between the high refractive index material and the low refractive index material can be increased as compared with a case in which both the high refractive index material and the low refractive index material are semiconductor materials. The reflectance bandwidth Δλ can be widened accordingly, and there can be expanded the range in which selection wavelengths can be set by selecting the film thickness of the wavelength selection layers 231, 232. The degree of freedom in the design of the selection wavelength can be increased as a result. In the infrared optical filter of the present aspect, the materials of the wavelength selection layers 231, 232 are identical to the materials of the thin films 21b, 21a that are second from the top of the first λ/4 multilayer film 21. This allows, therefore, increasing the etching selectivity in a case where the wavelength selection layer 231 is patterned through etching, and preventing a decrease in the optical film thickness of the thin film 21a of the topmost layer (see
In the infrared optical filter of the present aspect, Ge is used as the high refractive index material, and Al2O3 or SiO2 is used as the low refractive index material. Therefore, infrared rays of a wide band can be blocked even when using a Si substrate as the substrate 1. Also, an infrared optical filter having the above-described transmission peak wavelengths of 3.8 μm and a 4.3 μm can be realized in one chip, as illustrated in
The first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 are only necessary to have a periodic refractive index structure, and may be stacks of three or more types of thin film.
A yet further invention is explained next.
Patent documents 1, 3, 4 disclose infrared optical filters provided with a semiconductor substrate (for instance, a Si substrate, a Ge substrate or the like), and a narrow-band transmission filter part formed on one surface side of the semiconductor substrate, wherein the narrow-band transmission filter part includes a wavelength selection layer (spacer layer), for transmission of infrared rays of a desired selection wavelength, which is provided halfway a stacked structure of two kinds of thin films having mutually dissimilar refractive indices but an identical optical film thickness. Patent documents 1 to 3 disclose also infrared optical filters comprising a plurality of narrow-band transmission filter parts formed on the one surface side of the semiconductor substrate, in order to enable transmission of infrared rays of a plurality of selection wavelengths.
In the above-described narrow-band transmission filter parts, wavelength selection layers having dissimilar optical film thickness are provided in the periodic refractive index structure of the stacked structure of two kinds of thin films, to introduce thereby some local disarray in the periodic refractive index structure. According to the constitution, a transmission band of narrower spectral width than the reflectance bandwidth can be localized in the reflection band, where the reflectance bandwidth is determined in accordance with the refractive index difference of the two kinds of thin films that make up the periodic refractive index structure. The transmission peak wavelength of the transmission band can be modified by appropriately varying the optical film thickness of the wavelength selection layer.
The range over which the transmission peak wavelength can shift through modulation of the optical film thickness of the wavelength selection layer depends on the reflectance bandwidth of the periodic refractive index structure, such that the wider the reflectance bandwidth is, the wider becomes the range over which the transmission peak wavelength can shift. Herein, when nH denotes the refractive index of the above-described high refractive index material, nL denotes the refractive index of the low refractive index material, λ0 denotes a set wavelength equivalent to four times the optical film thickness, which is common to said thin films, and Δλ denotes the reflectance bandwidth, in order to widen the reflectance bandwidth Δλ, it is important to increase the value of the refractive index ratio nH/nL, i.e. it is important to increase the refractive index difference between the high refractive index material and the low refractive index material. As illustrated in a transmission spectrum diagram shown in
Regarding the two kinds of thin films in Patent document 1, PbTe is used as the high refractive index material, and ZnS is used as the low refractive index material, whereas Si is used as the high refractive index material in Patent documents 2, 3. Herein, Ge could conceivably be used as the high refractive index material and ZnS as the low refractive index material in order that increasing the refractive index difference.
Patent documents 5, 6 disclose the feature of using a combination of a narrow-band bandpass filter and a light blocking filter, wherein the bandpass filter is formed of a Si substrate or the like and configured to transmit infrared rays of a desired selection wavelength, and the light blocking filter is formed using a sapphire substrate and configured to block far infrared rays. In this configuration, far infrared rays in ambient light, for instance solar light, illumination light or the like can be blocked by providing such a light blocking filter.
Conceivable methods for forming each thin film and the wavelength selection layer in the narrow-band transmission filter parts of the above-described infrared optical filters include ordinary thin film formation methods such as vapor deposition, sputtering or the like.
The infrared optical filters disclosed in Patent documents 1, 3, 4 could conceivably be used for controlling infrared rays in a wavelength range from 800 nm to 20000 nm. However, blocking of far infrared rays requires providing separately the light blocking filter formed of a sapphire substrate for blocking the far infrared rays, in the same way as in Patent documents 5, 6. However, sapphire substrates are more expensive than semiconductor substrates, and entail higher costs. As a result, this limits the scope for cost reduction in, for instance, gas sensors and flame detection sensors that utilize the infrared optical filters. Meanwhile, infrared optical filters that use Ge as the high refractive index material and ZnS as the low refractive index material in narrow-band transmission filter parts require 70 or more stacked layers of two kinds of thin films in order to realize far infrared ray blocking performance without having to provide separately a light blocking filter of a sapphire substrate. This drives up manufacturing costs and may give rise to cracking of the narrow-band transmission filter parts.
In the light of the above-described issues, it is an object of the present invention to provide a low-cost infrared optical filter having infrared ray blocking performance over a wide band, from the near infrared to the far infrared.
An infrared optical filter of the present invention is an infrared optical filter that controls infrared rays in a wavelength range from 800 nm to 20000 nm. The infrared optical filter of the present invention comprises a semiconductor substrate and a wide-band blocking filter part formed on one surface side of the semiconductor substrate. The wide-band blocking filter part is formed of a multilayer film in which a plurality of kinds of thin films having different refractive indices is stacked. At least one kind of thin film from among the plurality of kinds of thin films is formed of a far infrared absorbing material that absorbs far infrared rays.
In this case, the light interference effect elicited by the multilayer film, and the far infrared ray absorption effect elicited by said thin film included in the multilayer film, allow realizing infrared ray blocking performance over a wide band from the near infrared to the far infrared, without employing a sapphire substrate. Thus, it can be realized a low-cost infrared optical filter having infrared ray blocking performance over a wide band, from the near infrared to the far infrared.
Preferably, the far infrared absorbing material is an oxide or a nitride.
In this case, it becomes possible to prevent changes in optical characteristics, which might be caused by oxidation, of the thin film formed of the far infrared absorbing material.
Using Al2O3 as the far infrared absorbing material allows increasing the far infrared absorption ability as compared with a case in which SiOx or SiNx is used as the far infrared absorbing material.
Using Ta2O5 as the far infrared absorbing material allows increasing the far infrared absorption ability as compared with a case in which SiOx or SiNx is used as the far infrared absorbing material.
Using SiNx as the far infrared absorbing material allows increasing the moisture resistance of the thin film that is formed of the far infrared absorbing material.
Using SiOx as the far infrared absorbing material allows increasing the refractive index difference in the multilayer film, and reducing the number of stack layers in the multilayer film.
The multilayer film, preferably, is formed by alternately stacking the thin film formed of Ge, where Ge is a material having a higher refractive index than the far infrared absorbing material, and the thin film formed of the far infrared absorbing material.
In this case, the refractive index difference between the high refractive index material and the low refractive index material in the multilayer film can be increased, and also the number of stack layers in the multilayer film can be reduced, as compared with a case in which Si, PbTe or ZnS is used as the high refractive index material.
The multilayer film, preferably, is formed by alternately stacking the thin film formed of Si, where Si is a material having a higher refractive index than the far infrared absorbing material, and the thin film formed of the far infrared absorbing material.
In this case, the refractive index difference between the high refractive index material and the low refractive index material in the multilayer film can be increased, and also the number of stack layers in the multilayer film can be reduced, as compared with a case in which ZnS is used as the high refractive index material.
Preferably, the semiconductor substrate is a Si substrate.
This allows reducing costs compared with a case where the semiconductor substrate is a Ge substrate or a ZnS substrate.
An infrared optical filter of the present aspect is an infrared optical filter for controlling infrared rays in a wavelength range from 800 nm to 20000 nm. The infrared optical filter of the present aspect is provided with, as illustrated in
As the semiconductor substrate 1 there is used a Si substrate, but the semiconductor substrate 1 is not limited to a Si substrate, and there may be used a Ge substrate, a ZnS substrate or the like. In the present aspect, the plan-view shape of the semiconductor substrate 1 is a square plan-view shape of several mm2, but the plan-view shape and dimensions of the semiconductor substrate 1 are not particularly limited.
The wide-band blocking filter part 2 is made up of a multilayer film in which there are stacked a plurality of kinds (herein, two kinds) of thin films 2a, 2b having dissimilar refractive indices. In the wide band transmission filter part 2, Al2O3, which is one kind of far infrared absorbing material having far infrared ray absorption property, is used as the material of the thin film 2a that is a low refractive index layer having a relatively low refractive index, while Ge is used as the material of the thin film 2b that is a high refractive index layer having a relatively high refractive index. Herein, the thin film 2a and the thin film 2b are alternately stacked in such a manner that there are 29 stacked layers, but the number of stack layers is not particularly limited thereto. In the wide-band blocking filter part 2, from the viewpoint of the stability of optical characteristic, the topmost layer that is furthest from the semiconductor substrate 1 is preferably made up of a thin film 2a that is the low refractive index layer. The far infrared absorbing material is not limited to Al2O3, and may be an oxide other than Al2O3, for instance SiO2 or Ta2O5. Herein, SiO2 has a lower refractive index than Al2O3, and hence there can be achieved a greater refractive index difference between the high refractive index material and the low refractive index material. SiNx, which is a nitride, can be used as the far infrared absorbing material.
In the wide-band blocking filter part 2, as described above, the thin film 2a, being one of the two kinds of thin films 2a, 2b, is formed of Al2O3, which is a far infrared absorbing material that absorbs far infrared rays. Herein, the wide-band blocking filter part 2 may be formed of plurality of kinds of materials at least one of which is a far infrared absorbing material. The multilayer film may be formed by stacking, for instance, three kinds of thin films of a Ge film, an Al2O3 film and a SiOx film so that stacking a Ge film-Al2O3 film-Ge film-SiOx film-Ge film-Al2O3 film-Ge film . . . over the semiconductor substrate 1 in this order from the side nearest to the semiconductor substrate 1 that is of a Si substrate. In this case, two kinds of thin films from among the three kinds of thin films are formed of far infrared absorbing materials.
The narrow-band transmission filter part 3 is provided with: a periodic refractive index structure having a stacked structure of a plurality of kinds (herein, two kinds) of thin films 3a, 3b having dissimilar refractive indices but an identical optical film thickness; and a wavelength selection layer 3c provided halfway the periodic refractive index structure and having an optical film thickness dissimilar from that of the thin films 3a, 3b. In the narrow-band transmission filter part 3, a stacked film of the thin films 3a, 3b on the side closer to the semiconductor substrate 1 than the wavelength selection layer 3c constitutes a first λ/4 multilayer film 31, while a stacked film of the thin films 3a, 3b on a side further from the semiconductor substrate 1 than the wavelength selection layer 3c constitutes a second λ/4 multilayer film 32 (that is, the wavelength selection layer 3c is interposed between the first λ/4 multilayer film 31 and the second λ/4 multilayer film 32).
In the narrow-band transmission filter part 3, Al2O3, which is one kind of far infrared absorbing material that absorbs far infrared rays, is used as the material of the thin film 3a that is a low refractive index layer having a relatively low refractive index, while Ge is used as the material of the thin film 3b that is a high refractive index layer having a relatively high refractive index. The thin film 3a, the thin film 3b and the wavelength selection layer 3c yield a total of 11 stacked layers, but the number of stack layers is not particularly limited thereto. The far infrared absorbing material used in the narrow-band transmission filter part 3 is not limited to Al2O3, and may be an oxide other than Al2O3, for instance SiO2 or Ta2O5. Herein, SiO2 has a lower refractive index than Al2O3, and hence there can be achieved a greater refractive index difference between the high refractive index material and the low refractive index material. SiNx, which is a nitride, can be used as the far infrared absorbing material. Herein, Si, PbTe or the like may be used instead of Ge as the high refractive index material of the narrow-band transmission filter part 3. The low refractive index material of the narrow-band transmission filter part 3 is not limited to a far infrared absorbing material, and may be an infrared transmitting material such as ZnS. Preferably, herein, the topmost layer of the narrow-band transmission filter part 3 furthest from the semiconductor substrate 1 is made up of the thin film 3a, which is a low refractive index layer. More preferably, the thin film 3a of the furthest layer is made up of an oxide or a nitride, from the viewpoint of the stability of optical characteristic.
The wavelength selection layer 3c of the narrow-band transmission filter part 3 has an optical film thickness dissimilar from the optical film thickness of the thin films 21a, 21b, in accordance with the desired selection wavelength. Specific wavelengths for detecting (sensing) various gases and flame that can occur, for instance, in houses, include 3.3 μm for CH4 (methane), 4.0 μm for SO3 (sulfur trioxide), 4.3 μm for CO2 (carbon dioxide), 4.7 μm for CO (carbon monoxide), 5.3 μm for NO (nitrogen monoxide) and 4.3 μm for flame. In order to sense any one of the foregoing, a set wavelength λ0 of the first λ/4 multilayer film 31 and the second λ/4 multilayer film 32 may be set in such a manner that there is formed a reflection band that encompasses the selection wavelengths consisting of the specific wavelengths of the sensing target, and the optical film thickness of the wavelength selection layer 3c may be appropriately set in such a way so as to localize a narrow transmission band that encompasses the selection wavelengths.
The above-described wide-band blocking filter part 2 absorbs far infrared rays of a wavelength range that is longer than the infrared reflection band set by the narrow-band transmission filter part 3. Herein, for the wide-band blocking filter part 2, Al2O3 is used as the far infrared absorbing material that absorbs infrared rays, but five kinds of far infrared absorbing materials, namely MgF2, Al2O3, SiOx, Ta2O5 and SiNx have been studied.
Specifically, the film thickness of an MgF2 film, an Al2O3 film, an SiOx film, a Ta2O5 film and an SiNx film was set to 1 μm, and the film formation conditions on a Si substrate were set as given in Table 3 below. Results of the measurement of the transmission spectra of the MgF2 film, the Al2O3 film, the SiOx film, the Ta2O5 film and the SiNx film are illustrated in
An ion beam assisted deposition apparatus was used as the film formation apparatus for the MgF2 film, the Al2O3 film, the SiOx film, the Ta2O5 film and the SiNx film. As illustrated in
In Table 3, “IB conditions” denote the conditions of ion beam assisting during film formation using the ion beam assisted deposition apparatus, such that “no IB” denotes no ion beam irradiation, “oxygen IB” denotes irradiation of an oxygen ion beam and “Ar IB” denotes irradiation of an argon ion beam.
In order to check the effect of ion beam assisting, the inventors of the present application prepared samples with various ion beam irradiation doses upon formation of the Al2O3 film on the Si substrate. The difference in film quality of the Al2O3 film of the samples was analyzed by FT-IR (Fourier transform infrared spectroscopy).
The above-described MgF2 film, Al2O3 film, SiOx film, Ta2O5 film and SiNx film were evaluated for “optical characteristic: absorption”, “refractive index” and “ease of film formation”. The results are given in Table 4 below.
The evaluation item “optical characteristic: absorption” was evaluated on the basis of the absorption factor of far infrared rays of 6 μm or longer, calculated on the basis of the transmission spectra of
On the basis of Table 4, it was found that the evaluation item “ease of film formation” exhibited no significant differences between the five types, MgF2, Al2O3, SiOx, Ta2O5 and SiNx. Results of the evaluation items “optical characteristic: absorption” and “refractive index” led to the conclusion that any from among Al2O3, SiOx, Ta2O5 and SiNx is preferably used as the far infrared absorbing material. Far infrared absorption ability can be enhanced when Al2O3 or T2O5 is used as the far infrared absorbing material, as compared with when SiOx or SiNx is used as the far infrared absorbing material. Herein, Al2O3 is preferable to T2O5 in terms of increasing the refractive index difference with the high refractive index material. The moisture resistance of the thin film 2a formed of the far infrared absorbing material can be increased in a case where SiNx is used as the far infrared absorbing material. Using SiOx as the far infrared absorbing material allows increasing the refractive index difference with the high refractive index material, and allows reducing the number of stack layers of the thin film 2a and the thin film 2b.
The inventors of the present application performed measurements of the transmission spectra of a reference example which a 1 μm Al2O3 film was formed on a Si substrate. The actually measured values were such as those illustrated by “A1” in
Tables 5 and 6 illustrate a design example of the film thickness (physical film thickness) of the multilayer film of the wide-band blocking filter part 3 and that of the narrow-band transmission filter part 2, of a working example of the infrared optical filter in which the wavelength of a narrow-band bandpass filter was designed to a 4.4 μm, using the above-described novel optical parameters of the Al2O3 film. A simulation result of the transmission spectrum of this working example is illustrated as “A1” in
In
To manufacture the infrared optical filter of the present aspect, there may be carried out: firstly, a “wide-band blocking filter part formation step” of forming the wide-band blocking filter part 2 by alternately stacking the thin film 2a formed of such as an Al2O3 film, and the thin film 2b formed of such as a Ge film, on the one surface side of the semiconductor substrate 1 formed of a Si substrate; then, a “first λ/4 multilayer film-formation step” of forming the first λ/4 multilayer film 31 by alternately stacking the thin film 3a formed of such as an Al2O3 film and the thin film 3b formed of such as a Ge film, on the other surface side of the semiconductor substrate 1; next, a “wavelength selection layer formation step” of forming the wavelength selection layer 3c on the other surface side of the semiconductor substrate 1 (herein, on the first λ/4 multilayer film 31); and a “second λ/4 multilayer film-formation step” of farming thereafter the second λ/4 multilayer film 32.
For instance, the thin films 2a, 2b of the wide-band blocking filter part 2 can be formed continuously, and also the first λ/4 multilayer film 31, the wavelength selection layer 3c and the second λ/4 multilayer film 32 of the narrow-band transmission filter part 3 can be formed continuously, if vapor deposition or sputtering are adopted as the method for forming the thin films 2a, 2b, the thin films 3a, 3b and the wavelength selection layer 3c. However, ion beam assisted deposition is preferably used to irradiate an oxygen ion beam during formation of the thin films 2a, 3a in a case where the low refractive index material is Al2O3, as described above, in order that the thin films 2a, 3a can be made more compact by lowering moisture content. SiOx, T2O5 or SiNx, which are a far infrared absorbing material other than Al2O3, may also be used as the low refractive index material. In any case, the thin films 2a, 3a made of a far infrared absorbing material are preferably formed by ion beam assisted deposition. Doing so allows controlling precisely the chemical composition of the thin films 2a, 3a made of a low refractive index material, while enhancing the compactness of the thin films 2a, 3a, and allows preventing fluctuation of the optical parameters.
In a case where in the above-described manufacturing method the far infrared absorbing material of one thin film 2a (3a) from among the above-described two kinds of thin films 2a (3a), 2b (3b) is SiOx (SiO2) or SiNx (Si3N4) and the material of the other thin film 2b (3b) is Si, then the evaporation source 64 used for forming the two kinds of thin films 2a (3a), 2b (3b) can be shared, by using Si as the evaporation source 64b in the ion beam assisted deposition apparatus of
The infrared optical filter of the present aspect explained above comprises the semiconductor substrate 1 and the wide-band blocking filter part 2 formed on the side of the one surface of the semiconductor substrate 1, wherein the wide-band blocking filter part 2 is formed of a multilayer film in which a plurality of kinds of thin films 2a, 2b having dissimilar refractive indices are stacked. Herein, one kind of thin film 2a from among the plurality of kinds of thin films 2a, 2b, is formed of a far infrared absorbing material that absorbs far infrared rays. Therefore, the light interference effect elicited by the multilayer film, and the far infrared ray absorption effect elicited by the thin film 2a included in the multilayer film, allow realizing infrared ray blocking performance over a wide band from the near infrared to the far infrared, without employing a sapphire substrate. Thus, it allows realizing a low-cost infrared optical filter having infrared ray blocking performance over a wide band, from the near infrared to the far infrared. In brief, the wide-band blocking filter part 2 is composed of a multilayer film in which a plurality of kinds of thin films 2a, 2b having dissimilar refractive indices are stacked, and has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, by virtue of the light interference effect elicited by the multilayer film, and by virtue of the far infrared ray absorption effect elicited by the far infrared absorbing material of the thin film 2a in the multilayer film.
In the infrared optical filter of the present aspect, the narrow-band transmission filter part 3 is formed on the side of the other surface of the semiconductor substrate 1. Thus, the infrared optical filter of the present aspect has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, by virtue of the light interference effect elicited by the first λ/4 multilayer film 31 and the second λ/4 multilayer film 32, and by virtue of the far infrared ray absorption effect elicited by the far infrared absorbing material of the thin film 3a in the multilayer film, where the multilayer film is made up of the first λ/4 multilayer film 31, the wavelength selection layer 3c and the second λ/4 multilayer film 32. As a result, there can be realized a low-cost infrared optical filter, which has infrared ray blocking performance over a wide band, from the near infrared to the far infrared, and which selectively transmits infrared rays of a desired selection wavelength.
An oxide or a nitride is used as the far infrared absorbing material in the infrared optical filter of the present aspect. Changes in optical characteristics, arising from oxidation, of the thin films 2a, 3a formed of the far infrared absorbing material, can be prevented thereby. In both the wide-band blocking filter part 2 and the narrow-band transmission filter part 3 of the infrared optical filter of the present aspect, the topmost layers that are furthest from the semiconductor substrate 1 are formed of the above-described oxide or nitride, respectively. This allows preventing changes in the properties of the thin films 2a, 3a at the topmost layer on account of reactions with moisture, oxygen or the like in air, or adsorption and/or adhesion of impurities, thereby enhancing the stability of filter performance, and allows reducing reflection at the surface of the wide-band blocking filter part 2 and the narrow-band transmission filter part 3, so that filter performance is enhanced.
In the infrared optical filter of the present aspect, a multilayer film of the wide-band blocking filter part 2 is configured through alternate stacking of the thin film 2a formed of far infrared absorbing material, and the thin film 2b formed of Ge, which is a material having a higher refractive index than that of the far infrared absorbing material. Therefore, the refractive index difference between the high refractive index material and the low refractive index material can be made greater, and there can be reduced the number of stack layers of the multilayer film, as compared with a case where Si, PbTe or ZnS is used as the high refractive index material. Then, when using Si as the high refractive index material, the refractive index difference between the high refractive index material and the low refractive index material can be increased, and the number of stack layers of the multilayer film can be reduced to a greater extent than when using ZnS as the high refractive index material. With respect to the narrow-band transmission filter part 3, the number of stack layers can also be reduced, for the same reasons as set forth above. In the present aspect, a Si substrate is used as the semiconductor substrate 1. This allows reducing costs compared with a case where the semiconductor substrate 1 is a Ge substrate or a ZnS substrate.
In the above-described infrared optical filler, one narrow-band transmission filter part 3 is provided on the other surface side of the semiconductor substrate 1. However, the infrared optical filter may also be configured in such a manner that a plurality of narrow-band transmission filter parts 3 having mutually different selection wavelengths is arranged side by side on the other surface side of the semiconductor substrate 1 so that infrared rays of a plurality of selection wavelengths can be selectively transmitted in one chip. In this case, each the optical film thickness of the wavelength selection layer 23 may be appropriately set according to the corresponding selection wavelength of the narrow-band transmission filter part 3. In this case as well, using Al2O3 or SiOx as the low refractive index material and. Ge as the high refractive index material, in the first λ/4 multilayer film 31 and the second λ/4 multilayer film 32, allows increasing the refractive index difference between the high refractive index material and the low refractive index material, whereby there can be widened the reflectance bandwidth Δλ and the selection wavelength range at which selection is enabled through setting of the film thickness of each wavelength selection layer 23, as compared with a case where both the high refractive index material and the low refractive index material are semiconductor materials. The degree of freedom in the design of the selection wavelengths is increased as a result.
The patterning method of the wavelength selection layer 23 is not particularly limited, and may be a method that combines thin film formation techniques with photolithography techniques and etching techniques, or may be a lift-off method, or a mask vapor deposition method.
In a method that combines thin film formation techniques with photolithography techniques and etching techniques, and in a case where the low refractive index material is an oxide (Al2O3) and the high refractive index material is a semiconductor material (Ge), as described above, etching can be performed with high etching selectivity, as compared with dry etching, by adopting wet etching using a hydrofluoric acid solution as the etching solution. That is because an oxide such as Al2O3 and SiO2 is readily soluble in a hydrofluoric acid solution, while Ge is very difficult to dissolve in hydrofluoric acid solution. As an example, etching can be performed with high etching selectivity, with an etching-rate ratio between Al2O3 and Ge of about 500:1, at an etching rate of Al2O3 of about 300 nm/min, by performing wet etching using a hydrofluoric acid solution in the form of dilute hydrofluoric acid composed of a mixed liquid of hydrofluoric acid (HF) and pure water (H2O) (for instance, dilute hydrofluoric acid having a hydrofluoric acid concentration of 2%).
The first λ/4 multilayer film 31 and the second λ/4 multilayer film 32 may be a stack of three or more kinds of thin films, so long as the stack has a periodic refractive index structure.
The infrared optical filter explained with reference to
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-031634 | Feb 2009 | JP | national |
| 2009-031637 | Feb 2009 | JP | national |
| 2009-031638 | Feb 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/051620 | 2/4/2010 | WO | 00 | 8/11/2011 |
