Patent Application
20120298867
The present invention relates to an infrared flame detector.
Conventionally, there is studied and developed, in various organizations, an infrared flame detector that performs flame detection by detecting infrared radiation of a specific wavelength (4.3 μm or 4.4 μm) generated by resonance radiation (also referred to as CO2 resonance radiation) of carbon dioxide (CO2 gas) in a flame in a fire (e.g., Japanese Patent Application Laid-open No. H3-78899: Patent Document 1).
It is widely known that, as shown in
Non-Patent Document 1 mentioned above discloses a three-wavelength infrared flame detector having the structure shown in
However, the three-wavelength infrared flame detector disclosed in Non-Patent Document 1 described above includes, as individual components, the optical filters 2201, 2202, and 2203, and the infrared sensors 2401, 2402, and 2403. As a result, in the three-wavelength infrared flame detector, the size of a container (not shown) that has the optical filters 2201, 2202, and 2203 attached thereto and contains the infrared sensors 2401, 2402, and 2403 is much larger than that of a can package of the infrared flame detector described in Patent Document 1 described above.
On the other hand, as shown in
Further, as an infrared gas detector, as shown in
However, in the structure shown in
In contract to this, as an infrared radiation reception module that is accommodated and used in the package of the infrared gas detector, as shown in
Further, conventionally, there is proposed an infrared gas detector having the structure shown in
In each of the infrared optical filters 201, 202, 203, and 204 disclosed in Patent Document 3 described above, as shown in
Incidentally, in the infrared flame detector, it is necessary to set the center wavelength of the narrowband filter section that selectively transmits the infrared radiation of 4.3 μm generated by the resonance radiation of CO2 gas to 4.3 μm, set the transmission bandwidth thereof to about 0.2 μm, and the infrared flame detector is required to be able to detect a flame in the size of that of a lighter at a distance of 10 m or more.
Accordingly, in the field of the infrared flame detector, as the infrared radiation receiving element, a pyroelectric element or a thermopile capable of high-sensitivity measurement is used in many cases. Examples of a method for amplifying the output of the pyroelectric element include a current-voltage conversion circuit using an FET and a resister connected to the gate of the FET, and a current-voltage conversion circuit having a capacitor connected between an output terminal of an operational amplifier and an inverting input terminal thereof (Japanese Patent Application Laid-open No. H10-281866: Patent Document 4).
Incidentally, it can be considered that the infrared gas detector having the structure shown in
Additionally, in the infrared gas detector having the structure shown in
Furthermore, in the infrared gas detector having the structure shown in
Further, in the structure shown in
In contrast to this, in the infrared optical module having the structure shown in
However, in the infrared optical module having the structure shown in
In addition, in the amplification circuit constituted by the current-voltage conversion circuit described in Patent Document 4 described above, although it is necessary to separately amplify the outputs of the individual infrared radiation receiving elements, since the outputs of the individual infrared radiation receiving elements include a DC bias component resulting from ambient light such as sunlight, arc light, and infrared radiations from a fluorescent lamp and a heat source, when the intensity of the infrared radiation entering into the infrared radiation receiving element is extremely high, enhancement in the gain of the amplification circuit is limited due to the saturation of the output of the amplification circuit and an improvement in S/N ratio is also limited so that there is a possibility that there are cases where a fire cannot be detected in the infrared flame detector. Similarly, in the three-wavelength flame detector shown in
Further, since the pyroelectric element is what is called a differentiation type detecting element that absorbs infrared radiation as thermal energy and detects a change in electric charge amount resulting from the absorption (pyroelectric effect), the pyroelectric element can detect only the changed portion of the infrared radiation so that it is necessary to detect infrared radiation of a low frequency of about 0.1 to 10 Hz. However, the impedance of the above-described current-voltage conversion circuit is 100 GΩ to 1 TΩ that is extremely high so that the current-voltage conversion circuit is effective in realizing high S/N by the high impedance, but is susceptible to the influence of external radiant noises due to its high impedance.
The present invention has been achieved in view of the above-described problems, and an object thereof is to provide an infrared flame detector capable of achieving an increase in sensitivity and a reduction in cost.
The infrared flame detector in accordance with the present invention includes an infrared radiation receiving element accommodated in a package, and an infrared optical filter disposed forwardly of the infrared radiation receiving element in the package. The infrared radiation receiving element includes a set of two pyroelectric elements having mutually different polarities. The two pyroelectric elements are arranged side by side on a pyroelectric element forming substrate and are connected in anti-series or anti-parallel. The infrared optical filter includes a filter forming substrate, a set of two narrowband transmission filter sections, and a broadband blocking filter section. The filter forming substrate is made of an infrared radiation transmitting material. The two narrowband transmission filter sections are formed at positions respectively corresponding to the pyroelectric elements on a first surface of the filter forming substrate. The two narrowband transmission filter sections are configured to transmit infrared radiation of a first selective wavelength defined as a specific wavelength of infrared radiation generated by resonance radiation of CO2 gas resulting from a flame and infrared radiation of a second selective wavelength defined as a reference wavelength different from the specific wavelength, respectively. The broadband blocking filter section is formed on a second surface of the filter forming substrate. The broadband blocking filter is configured to absorb infrared radiation of a wavelength longer than an upper limit of an infrared reflection band defined by the narrowband transmission filter sections. Each of the narrowband transmission filter sections includes a first λ/4 multilayer film, a second λ/4 multilayer film, and a wavelength selection layer. The first λ/4 multilayer film is formed by stacking a plurality of types of thin films having different refractive indexes and the same optical film thicknesses on each other. The second λ/4 multilayer film is formed on an opposite side of the first λ/4 multilayer film from the filter forming substrate. The second λ/4 multilayer film is formed by stacking the plurality of types of the thin films on each other. The wavelength selection layer is interposed between the first λ/4 multilayer film and the second λ/4 multilayer film. The wavelength selection layer is designed to have an optical film thickness different from the optical film thicknesses of the thin film according to the selective wavelength.
Preferably, in this infrared flame detector, the broadband blocking filter section is constituted by a multilayer film formed by stacking multiple types of thin films having different refractive indexes on each other. At least one of the multiple types of the thin films is formed of an infrared absorbing material having a property of absorbing far-infrared radiation.
Preferably, in this infrared flame detector, the filter forming substrate is an Si substrate or a Ge substrate.
Preferably, in this infrared flame detector, the package is made of metal, and the filter forming substrate is electrically connected to the package.
Preferably, in this infrared flame detector, a component of an amplification circuit configured to amplify an output of the infrared radiation receiving element is accommodated in the package.
As shown in
The package 7 includes a metal stem 71 on which the circuit block 6 is mounted via a spacer 9 made of an insulating material and a metal cap 72 fixed to the stem 71 so as to cover the circuit block 6, and a plurality of (three in this embodiment) terminal pins 75 electrically connected to appropriate positions of the circuit block 6 are provided so as to extend through the stem 71. The stem 71 is formed into a disk shape, the cap 72 is formed into a bottomed cylindrical shape having an opened rear surface, and the rear surface is closed by the stem 71. Note that the spacer 9, the circuit block 6, and the stem 71 are fixed to one another using an adhesive.
In the above-described cap 72 constituting a part of the package 7, a rectangular (square in the present embodiment) window section 7a is formed in a front wall positioned forwardly of the infrared radiation receiving element 40, and an infrared optical filter 20 is disposed on the inside of the cap 72 so as to cover the window section 7a.
In addition, in the stem 71, a plurality of terminal holes 71b through which the above-described terminal pins 75 are inserted are provided so as to extend through the stem 71 in a thickness direction. The individual terminal pins 75 are inserted into the terminal holes 71b and sealed by sealing sections 74.
The above-described cap 72 and stem 71 are formed of steel plates. An outer brim section 72c provided to extend outward from the rear end edge of the cap 72 is welded to a flange section 71c formed in the periphery of the stem 71 by welding.
The circuit block 6 is constituted by a first circuit board 62, a resin layer 65, a shield plate 66, and a second circuit board 67. The first circuit board 62 is defined as a printed wiring board (e.g., a composite copper-clad laminate) having an IC 63 and chip-like electronic components 64 as constituent elements of the above-described signal processing circuit mounted on different surfaces thereof. The resin layer 65 is stacked on the surface of the first circuit board 62 on which the electronic components 64 are mounted. The shield plate 66 includes an insulating base and a metal layer (hereinafter referred to as a shield layer) formed on a surface of the insulating base. For example, the insulating base is made of glass epoxy resin, and the metal layer is made of a metal material (e.g., copper or the like). The shield layer 66 is stacked on the resin layer 65. The second circuit board 67 is defined as a printed wiring board (e.g., the composite copper-clad laminate) which has the infrared radiation receiving element 40 mounted thereon and is stacked on the shield plate 66. Note that the shield layer formed only of a copper foil or a metal plate may be used as an alternative to the shield plate 66.
In the first circuit board 62, the IC 63 is flip-chip mounted on the lower surface in
The above-described infrared radiation receiving element 40 is a dual element in which a set of two pyroelectric elements 41 and 42 having mutually different polarities are arranged side by side on a pyroelectric element forming substrate 41 made of a pyroelectric material (e.g., lithium tantalate or the like) and are connected in anti-series such that a differential output of the two pyroelectric elements 41 and 42 is obtained (see
In the second circuit board 67, since a thermal insulation hole 67a for thermally insulating the pyroelectric elements 41 and 42 of the infrared radiation receiving element 40 from the second circuit board 67 is provided to extend therethrough in the thickness direction, a gap is formed between the pyroelectric elements 41 and 42 of the infrared radiation receiving element 40 and the shield plate 66, and sensitivity is thereby enhanced. Note that, instead of providing the thermal insulation hole 67a, a support section for supporting the infrared radiation receiving element 40 may also be provided to protrude from the second circuit board 67 such that a gap is formed between the pyroelectric elements 41 and 42 of the infrared radiation receiving element 40 and the second circuit board 67.
In the circuit block 6, through holes 62b, 65b, 66b, and 67b into which the above-described terminal pins 75 are inserted are provided to extend through each of the first circuit board 62, the resin layer 65, the shield plate 66, and the second circuit board 67 in the thickness direction, and the infrared radiation receiving element 40 and the above-described signal processing circuit are electrically connected via the terminal pins 75. Note that, when a component-embedded substrate formation method is adopted in which the first circuit board 62, the resin layer 65, the shield plate 66, and the second circuit board 67 are stacked on one another and the through holes 62b, 65b, 66b, and 67b are formed by performing a one-time perforation process of forming through holes extending through the circuit block 6 in the thickness direction, production steps can be simplified and electrical connections in the circuit block 6 are facilitated.
Among the above-described three terminal pins 75, one of them is an electric power supply terminal pin 75 (75a), another one thereof is a signal output terminal pin 75 (75b), and the remaining one thereof is a ground terminal pin 75 (75c), and the shield layer in the shield plate 66 is electrically connected to the ground terminal pin 75c. The sealing sections 74 (74a, 74b) for sealing the terminal pins 75a and 75b are formed of sealing glass having insulating properties, and the sealing section 74 (74c) for sealing the terminal pin 75c is formed of a metal material. That is, the terminal pins 75a and 75b are electrically insulated from the stem 71, while the ground terminal pin 75c has the same potential as the stem 71. Accordingly, although the potential of the shield plate 66 is set to the ground potential, the potential of the shield plate 66 may be set to a specific potential other than the ground potential as long as the specific potential allows the shield plate 66 to exert its shielding function.
In the production of the infrared flame detector of the present embodiment, after the circuit block 6 on which the infrared radiation receiving element 40 is mounted is attached to the stem 71 via the spacer 9, the inside of the metal package 7 constituted by the cap 72 and the stem 71 may be sealed by welding the outer brim section 72c of the cap 72 to which the infrared optical filter 20 is fixed so as to close the window section 7a and the flange section 71c of the stem 71 to each other. In order to prevent characteristics of the infrared radiation receiving element 40 from being changed by the influence of humidity or the like, dry nitrogen is sealed in the package 7. Note that, as described above, the package 7 in the present embodiment is a can package, and it is possible to achieve an improvement in weather resistance resulting from an improvement in airtightness as well as enhancing a shielding effect to external noises. However, the package 7 may also be constituted by a ceramic package having the shielding effect.
The above-described infrared optical filter 20 has a filter main body section 20a formed with narrowband filter sections 21 and 22 and a broadband blocking filter section 3 described later, and a flange section 20b provided to extend outward from the periphery of the filter main body section 20a and fixed to the periphery of the window section 7a in the cap 72. In the infrared optical filter 20, the plan-view shape of the filter section 20a is rectangular (square in the present embodiment), and the shape of the outer periphery of the flange section 20b is formed into a rectangular (square in the present embodiment) shape. Note that, in the present embodiment, although the plan-view shape of the filter main body section 20a is a square shape of several mm square, the plan-view shape or the dimensions of the filter main body section 20a are not particularly limited.
As shown in
In the above-described infrared optical filter 20, the set of the two narrowband transmission filter sections 21 and 22 are arranged side by side on the first surface of the filter forming substrate 1. The narrowband transmission filter sections 21 and 22 include first λ/4 multilayer films 21, second λ/4 multilayer films 22, and wavelength selection layers 231 and 232, respectively. The first λ/4 multilayer film 21 is formed by stacking plural types (two types in this embodiment) of thin films 21b and 21a having different refractive indexes and the same optical film thicknesses on each other. The second λ/4 multilayer film 22 is formed on the opposite surface of the first λ/4 multilayer film 21 from the filter forming substrate 1, and formed by stacking plural types of the thin films 21a and 21b described above. Each of the wavelength selection layers 231 and 232 is interposed between the first and second λ/4 multilayer films 21 and 22 and is designed to have an optical film thickness different from the optical thickness of the thin films 21a and 21b according to a corresponding selective wavelength. Note that the tolerance of the variation in the optical film thickness of the two types of the thin films 21a and 21b is about ±1% and the tolerance of the variation in physical film thickness is determined according to the variation in the optical film thickness.
In addition, in the infrared optical filter 20, Al2O3 which is a far-infrared radiation absorbing material having a property of absorbing far-infrared radiation is adopted as a material (a low refractive index material) of the thin film 21b defining a low refractive index layer in each of the first and second λ/4 multilayer films 21 and 22, and Ge is adopted as a material (a high refractive index material) of the thin film 21a defining a high refractive index layer. Further, in the infrared optical filter 20, as materials of the wavelength selection layers 231 and 232, there are used the same materials as those of the thin films 21b and 21a that are the second films from the top of the first λ/4 multilayer film 21 immediately below the wavelength selection layers 231 and 232, and the thin films 21b and 21b of the second λ/4 multilayer film 22 that are farthest from the filter forming substrate 1 are formed of the above-described low refractive index material. The far-infrared radiation absorbing material is not limited to Al2O3, there may be adopted SiO2 and Ta2O5 as oxides other than Al2O3 and, since the refractive index of SiO2 is lower than that of Al2O3, a difference in refractive index between the high refractive index material and the low refractive index material can be made larger by SiO2 than the difference by Al2O3.
Incidentally, the first selective wavelength defined as the specific wavelength of infrared radiation generated by the resonance radiation of CO2 gas in the flame in a fire is 4.3 μm (or 4.4 μm). As for the absorption wavelength of infrared radiation in various gases that can be generated in a house or the like, the absorption wavelength of CH4 (methane) is 3.3 μm, that of CO (carbon monoxide) is 4.7 μm, and that of NO (nitrogen monoxide) is 5.3 μm. Consequently, in the infrared optical filter 20 of the present embodiment, the second selective wavelength defined as the reference wavelength is 3.9 μm that is a value relatively close to the first selective wavelength. In order to selectively detect the infrared radiations of the first selective wavelength and the second selective wavelength, the narrowband transmission filter sections 21 and 22 need to have a reflection band in an infrared region of about 3.1 μm to 5.5 μm, and a reflection bandwidth Δλ of 2.4 μm or more is essential. Note that, if it is assumed that a designed center wavelength corresponding to four times of the optical film thickness common to the thin films 21a and 21b is ±0, as shown in
In the present embodiment, the designed center wavelength λ0 of each of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 is set to 4.0 μm such that it becomes possible to detect the infrared radiation of the above-described first selective wavelength by appropriately setting the optical film thickness of each of the wavelength selection layers 231 and 232. In addition, if it is assumed that the refractive index of the high refractive index material as the material of the thin film 21a is nH and the refractive index of the low refractive index material as the material of the thin film 21b is nL, the physical film thicknesses of the thin films 21a and 21b are set to λ0/4nH and λ0/4nL. Specifically, in a case where the high refractive index material is Ge and the low refractive index material is Al2O3, nH and nL are set to satisfy 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.
In
In addition,
From
Next,
From
Consequently, by appropriately changing only the design of the optical film thickness of the wavelength selection layer 23 without changing the design of each of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22, it becomes possible to perform the sensing of not only a flame corresponding to the specific wavelength of 4.3 μm but also various gases such as CH4 corresponding to the specific wavelength of 3.3 μm, CO corresponding to the specific wavelength of 4.7 μm, and NO corresponding to the specific wavelength of 5.3 μm. Note that the range of the optical film thickness “nd” of 0 nm to 1600 nm corresponds to the range of the physical film thickness “d” of 0 nm to 941 nm. In addition, the reason why the transmission peak wavelength becomes 4000 nm in a case where the optical film thickness “nd” of the wavelength selection layer 23 is 0 nm, i.e., the wavelength selection layer 23 does not exist in
Although, as the low refractive index material of the thin film 21b, there is adopted Al2O3 as the far-infrared radiation absorbing material with a property of absorbing infrared radiation in a range of a wavelength longer than an upper limit of the infrared reflection band defined by the first and second λ/4 multilayer films 21 and 22 (i.e., the infrared reflection band set by the narrowband transmission filter sections 21 and 22), five types of MgF2, Al2O3, SiOx, Ta2O5, and SiNx were examined as the far-infrared radiation absorbing material. Specifically,
The “IB conditions” in Table 1 denote conditions of ion beam assist when the film deposition is performed by the ion beam assisted deposition apparatus, the “No IB” denotes that no ion beam irradiation is performed. The “Oxygen IB” denotes that oxygen ion beam irradiation is performed. The “Ar IB” denotes that argon ion beam irradiation is performed. In addition, in
Furthermore, Table 2 shown below shows the result of examination of “Optical Characteristic: Absorption”, “Refractive Index”, and “Easiness in Film Deposition” of each of the MgF2 film, the Al2O3 film, the SiOx film, the Ta2O5 film, and the SiNx film as evaluation items.
As for the evaluation item of the “Optical Characteristic: Absorption”, the evaluation was made on the basis of the absorptivity of the far-infrared radiation of 6 μm or more calculated from the transmission spectrum of
From Table 2, as for the five types of MgF2, Al2O3, SiOx, Ta2O5, and SiNx, there was no significant difference in the evaluation item of the “Easiness in Film Deposition” among them and, as a result of focusing attention on the “Optical Characteristic: Absorption” and the “Refractive Index”, a conclusion was reached that it was preferable to adopt any of Al2O3, SiOx, Ta2O5, and SiNx as the far-infrared radiation absorbing material. In a case where Al2O3 or T2O5 is adopted as the far-infrared radiation absorbing material, it is possible to improve the absorptivity of the far-infrared radiation as compared with a case where the far-infrared radiation absorbing material is SiOx or SiNx. However, from the viewpoint of increasing the refractive index difference with respect to the high refractive index material, Al2O3 is more preferable than T2O5. In addition, in a case where SiNx is adopted as the far-infrared radiation absorbing material, it is possible to enhance the moisture resistance of the thin film 21b formed of the far-infrared radiation absorbing material. Further, when SiOx is adopted as the far-infrared radiation absorbing material, it is possible to increase the refractive index difference with respect to the high refractive index material, and achieve a reduction in the number of stacked layers (layer number) of each of the first and second λ/4 multilayer films 21 and 22.
An explanation is given hereinbelow of a method of producing the narrowband transmission filter sections 21 and 22 in the infrared optical filter 20 with reference to
First, a first λ/4 multilayer film formation step of forming the first λ/4 multilayer film 21 by alternately stacking the thin film 21b that is made of Al2O3 as the low refractive index material and has a predetermined physical film thickness (588 nm in this embodiment) and the thin film 21a that is made of Ge as the high refractive index material and has a predetermined physical film thickness (250 nm in this embodiment) on the entire first surface of the filter forming substrate 1 constituted of the Si substrate is performed. Subsequently, by performing a wavelength selection layer deposition step of depositing, on the entire surface on the one surface side (the surface of the first λ/4 multilayer film 21 in this embodiment) of the filter forming substrate 1, the wavelength selection layer 231 that is made of the same material (Al2O3 in this embodiment) as that of the thin film 21b positioned at the second position from the top of the first λ/4 multilayer film 21 and has an optical film thickness set according to the selective wavelength of one narrowband transmission filter section 21, the structure shown in
After the above-described wavelength selection layer deposition step, by performing a resist layer formation step of forming a resist layer 31 that covers only the portion corresponding to the narrowband transmission filter section 21 by using a photolithography technique, the structure shown in
Thereafter, by performing a wavelength selection layer patterning step of selectively etching an unnecessary portion of the wavelength selection layer 231 by using the resist layer 31 as a mask and the thin film 21a as the uppermost film of the first λ/4 multilayer film 21 as an etching stopper layer, the structure shown in
After the above-described wavelength selection layer patterning step, by performing a resist layer removal step of removing the resist layer 31, the structure shown in
After the above-described resist layer removal step, by performing a second λ/4 multilayer film formation step of forming the second λ/4 multilayer film 22 by alternately stacking the thin film 21a that is made of Ge as the high refractive index material and has a predetermined physical film thickness (250 nm) and the thin film 21b that is made of Al2O3 as the low refractive index material and has a predetermined physical film thickness (588 nm) on the entire first surface of the filter forming substrate 1, the structure shown in
That is, in the production of the narrowband transmission filter sections 21 and 22 of the infrared optical filter 20, in the middle of the basic step of stacking a plurality of types (two types in this case) of the thin films 21b and 21a having different refractive indexes and the same optical film thicknesses on the first surface of the filter forming substrate 1, a wavelength selection layer formation step including the wavelength selection layer deposition step of depositing a wavelength selection layer 23i (i=1 in this embodiment) that is made of the same material as that of the second layer from the top of the multilayer film (the first λ/4 multilayer film 21 in this embodiment) in the middle of the basic step and has an optical film thickness set according to the selective wavelength of an arbitrary narrowband transmission filter section 2i (i=1 in this case) out of a plurality of the narrowband transmission filter sections 21, . . . , 2m (m=2 in this embodiment) on the above-described multilayer film, and the wavelength selection layer patterning step of etching an unnecessary portion of the wavelength selection layer 23i deposited in the wavelength selection layer deposition step other than a portion corresponding to the arbitrary narrowband transmission filter section 2i by using the uppermost layer of the multilayer film as the etching stopper layer is performed once, and a plurality of the narrowband transmission filter sections 21 and 22 are formed. If the wavelength selection layer formation step is performed plural times in the middle of the basic step described above, it is possible to produce the infrared optical filter 20 having more selective wavelengths with one chip.
In addition, in the above-described production method, in the middle of the basic step of stacking the plurality of types of the thin films 21a and 21b on the first surface of the filter forming substrate 1, by depositing the thin film that is made of the same material as that of the second layer from the top of the multilayer film (the first λ/4 multilayer film 21 in this embodiment) in the middle of the basic step and has an optical film thickness set according to the selective wavelength of the arbitrary filter section 2i (i=1 in this case) out of the filter sections 21, . . . , 2m (m=2 in this embodiment) on the above-described multilayer film, and etching the portion of the thin film deposited on the multilayer film other than the portion corresponding to the arbitrary filter section 2i (i=1 in this case), the pattern of at least one wavelength selection layer 231 is formed. However, the present embodiment is not limited thereto. It is only necessary to form the pattern of at least one wavelength selection layer 231 in the middle of the basic step and, e.g., in a case where the wavelength selection layer 232 is made of the same material as that of the wavelength selection layer 231 and the optical film thickness thereof is set to be smaller than that of the wavelength selection layer 231, the patterns of the two wavelength selection layers 231 and 232 may also be formed by etching the thin film on the multilayer film halfway.
Further, the present invention is not limited to the above-described production method, and the wavelength selection layers 231, . . . , 23m (m=2 in this embodiment) having mutually different optical film thicknesses may also be formed at positions corresponding to positions of the filter sections 21, . . . , 2m (m=2 in this embodiment) by mask vapor deposition between the first λ/4 multilayer film formation step of forming the first λ/4 multilayer film 21 on the first surface of the filter forming substrate 1 and the second λ/4 multilayer film formation step of forming the second λ/4 multilayer film 22 on the opposite side of the first λ/4 multilayer film from the filter forming substrate 1.
Furthermore, in the above-described production method, of the two types of the thin films 21a and 21b described above, in a case where the far-infrared radiation absorbing material of the thin film 21b is SiOx or SiNx and the material of the thin film 21a is Si, it is preferable that the ion beam assisted deposition apparatus having Si as an evaporation source is used, a vacuum atmosphere is used when the thin film 21a made of Si is deposited, the oxygen ion beam is irradiated when the thin film 21b made of SiOx as an oxide is deposited, and a nitrogen ion beam is irradiated when the thin film 21b made of SiNx as a nitride is deposited. With this, in the above-described production method, it is possible to achieve commonality of the evaporation source between the two types of the thin films 21a and 21b, and hence it is not necessary to prepare an ion beam assisted deposition apparatus having a plurality of evaporation sources, and it is possible to achieve a reduction in production cost. Similarly, in the above-described production method, of the two types of the thin films 21a and 21b described above, in the case where the far-infrared radiation absorbing material of the thin film 21b is SiOx or SiNx and the material of the thin film 21a is Si, it is preferable that a sputtering apparatus having Si as a target is used, the vacuum atmosphere is used when the thin film 21a made of Si is deposited, an oxygen atmosphere is used when the thin film 21b made of SiOx is deposited, and a nitrogen atmosphere is used when the thin film 21b made of SiNx is deposited. With this, in the above-described production method, it is possible to achieve commonality of the target between the two types of the thin films 21a and 21b, and hence it is not necessary to prepare a sputtering apparatus having a plurality of targets, and it is possible to achieve a reduction in production cost.
In the above-described narrowband transmission filter sections 21 and 22 of the infrared optical filter 20, by appropriately setting the optical film thickness “nd” of each of the wavelength selection layers 231 and 232, as shown in
Note that each of the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 may appropriately have the refractive index periodic structure, and may be formed by stacking three or more types of thin films on one another.
Next, an explanation is given of the broadband blocking filter section 3 of the infrared optical filter 20.
The broadband blocking filter section 3 is constituted by a multilayer film formed by stacking multiple types (two types in this embodiment) of thin films 3a and 3b having different refractive indexes on each other. In the broadband blocking filter section 3, Al2O3 as one type of the far-infrared radiation absorbing material having a property of absorbing far-infrared radiation is adopted as a material of the thin film 3a as a low refractive index layer having a relatively low refractive index. Ge is adopted as a material of the thin film 3b as a high refractive index layer having a relatively high refractive index. The thin film 3a and the thin film 3b are alternately stacked on each other. The number of stacked layers is 11 though the number of stacked layers is not particularly limited. However, in the broadband blocking filter section 3, it is preferable to constitute the uppermost layer that is farthest from the filter forming substrate 1 by the thin film 3a as the low refractive index layer in terms of the stability of optical characteristics. The far-infrared radiation absorbing material is not limited to Al2O3, and there may also be adopted SiO2 and Ta2O5 as oxides other than Al2O3 as the far-infrared radiation absorbing material. The refractive index of SiO2 is lower than that of Al2O3 so that the difference in refractive index between the high refractive index material and the low refractive index material can be made larger by using SiO2. In addition, as the far-infrared radiation absorbing material, SiNx as a nitride may also be adopted.
As described above, in the broadband blocking filter section 3, of the two types of the thin films 3a and 3b, although the thin film 3a is formed of Al2O3 as the far-infrared radiation absorbing material that absorbs the far-infrared radiation, it is only necessary to form at least one of the plurality of the types by using the far-infrared radiation absorbing material. For example, the broadband blocking filter section 3 may be a multilayer film in which a Ge film, an Al2O3 film, and an SiOx film as three types of thin films are stacked in the order of Ge film-Al2O3 film-Ge film-SiOx film-Ge film-Al2O3 film-Ge film . . . from the side closer to a semiconductor substrate 1 constituted by an Si substrate and, in this case, two types of the thin films out of the three types of the thin films are formed of the far-infrared radiation absorbing material.
The above-described broadband blocking filter section 3 absorbs far-infrared radiation in a range of a wavelength longer than an upper limit of the infrared reflection band set by the narrowband transmission filter sections 21 and 22. In the broadband blocking filter section 3, although Al2O3 is adopted as the far-infrared radiation absorbing material having a property of absorbing the infrared radiation, similarly to the above-described narrowband transmission filter sections 21 and 22, as the far-infrared radiation absorbing material, five types of MgF2, Al2O3, SiOx, Ta2O5, and SiNx were examined.
In order to verify the effect of the ion beam assist, the present inventors prepared samples obtained by variously changing the amount of ion beam irradiation in the deposition of the Al2O3 film on the Si substrate, and analyzed the difference in film quality among the Al2O3 films of the individual samples by FT-IR (Fourier-transform infrared spectroscopy).
In addition, as described above, in the case where Al2O3 or T2O5 is adopted as the far-infrared radiation absorbing material, it is possible to improve absorption of the far-infrared radiation as compared with the case where the far-infrared radiation absorbing material is SiOx or SiNx.
Further, when the present inventors measured a transmission spectrum of a reference example in which the Al2O3 film of 1 μm was deposited on the Si substrate, the present inventors obtained measured values as shown in “A1” of
“A1” of
In
In the production of the infrared optical filter 20 of the present embodiment, first, a broadband blocking filter section formation step of forming the broadband blocking filter section 3 by alternately stacking the thin film 3a made of, e.g., the Al2O3 film and the thin film 3b made of, e.g., the Ge film on the second surface of the filter forming substrate 1 constituted by the Si substrate is performed. Thereafter, the narrowband transmission filter sections 21 and 22 may appropriately be formed on the first surface of the filter forming substrate 1 in the manner described above.
Next, an explanation is given of an infrared flame detection apparatus that uses the infrared flame detector of the present embodiment with reference to
The infrared flame detection apparatus shown in
As a comparative example of the infrared flame detection apparatus having the structure shown in
The infrared flame detection apparatus having the structure shown in
In contrast to this, since the infrared flame detection apparatus having the structure shown in
In addition, since the infrared flame detection apparatus having the structure shown in
When a fire occurs, the resonance radiation of CO2 gas is caused by the heat of a flame, and infrared radiation having a peak wavelength of 4.3 μm is emitted. The emission spectrum of the infrared radiation generated by the resonance radiation of CO2 gas is a narrowband emission spectrum having a peak wavelength of 4.3 μm. On the other hand, in ambient light such as sunlight, a heat source, arc, and illumination, the emission of a spectrum having a specific wavelength is rare in general, and the ambient light usually provides a broadband broad emission spectrum. Consequently, in the present embodiment, as described above, the first selective wavelength is set to 4.3 μm as the peak wavelength of the resonance radiation of CO2 gas, and the second selective wavelength is set to 3.9 μm in the vicinity of 4.3 μm. The second selective wavelength is preferably set to a wavelength with which the infrared intensity of the second selective wavelength of the ambient light is as close to the infrared intensity of 4.3 μm of the ambient light as possible.
In the infrared flame detection apparatus having the structure shown in
I1=(Is1+Id1)×G1
I2=(Is2+Id2)×G2
are satisfied. Accordingly, the output signal of the subtracter 164 is given by I1−I2=(Is1+Id1)×G1−(Is2+Id2)×G2. However, in a case where the values of the DC bias components Id1 and Id2 resulting from the ambient light are extremely large, the saturation of each of the amplification sections 1631 and 1632 is caused so that the S/N ratio is lowered.
In contrast to this, in the infrared radiation receiving element 40 in the infrared flame detection apparatus having the structure shown in
I=(Is1+Id1)−(Is2+Id2)
is satisfied. Since the emission spectrum of the ambient light is broad in general, when the first and second selective wavelengths are selected so as to be included in the wavelength band in which the radiant intensities of the ambient light become substantially identical,
Id1=Id2
can be considered to hold so that
I=Is1−Is2
is satisfied, it is possible to cancel the influence of the DC bias component by the ambient light such as sunlight or the like (i.e., in a case where a fire does not occur and the infrared radiation resulting from the resonance radiation of CO2 gas is not generated, the output of the infrared radiation receiving element 40 becomes substantially zero), it is possible to increase the gain of the amplification section 63a configured to amplify the output of the infrared radiation receiving element 40, and an improvement in S/N ratio is allowed.
In addition, in the infrared flame detector of the present embodiment, as described above, the infrared optical filter 20 includes the filter forming substrate 1, the set of the two narrowband transmission filter sections 21 and 22, and the broadband blocking filter section 3. The filter forming substrate 1 is made of the infrared radiation transmitting material. The set of the two narrowband transmission filter sections 21 and 22 are formed at positions corresponding to positions of the pyroelectric elements 41 and 42 on the first surface of the filter forming substrate 1. The set of the two narrowband transmission filter sections 21 and 22 are configured to transmit the infrared radiation of the first selective wavelength as the specific wavelength (4.3 μm) generated by the resonance radiation of CO2 gas resulting from a flame and the infrared radiation of the second selective wavelength as the reference wavelength (e.g., 3.9 μm) different from the specific wavelength, respectively. The broadband blocking filter section 3 is formed on the second surface of the filter forming substrate 1. The broadband blocking filter 3 is configured to absorb the infrared radiation of the wavelength longer than the upper limit of the infrared reflection band defined by the narrowband transmission filter sections 21 and 22. In the infrared flame detector, the narrowband transmission filter sections 21 and 22 include the first λ/4 multilayer films 21 and 21, the second λ/4 multilayer films 22 and 22, and the wavelength selection layers 231 and 232, respectively. The first λ/4 multilayer film 21 is formed by means of stacking plural types of the thin films 21a and 21b having different refractive indexes and the same optical film thickness on each other. The second λ/4 multilayer film 22 is disposed on the opposite side of the first λ/4 multilayer film 21 from the filter forming substrate 1. The second λ/4 multilayer is formed by means of stacking plural types of the thin films 21a and 21b on each other. Each of the wavelength selection layers 231 and 232 is interposed between the first λ/4 multilayer film 21 and the second λ/4 multilayer film 22 and has optical film thickness which is different from the common optical thickness of the thin films 21a and 21b and is selected on the basis of the corresponding selective wavelength. Thus, in the infrared flame detector of the present embodiment, it is possible to achieve a reduction in cost by reducing the size of the infrared radiation receiving filter 20 having a plurality of the narrowband transmission filter sections 21 and 22, and reduce the distance between the centers of the plurality of the narrowband transmission filter sections 21 and 22 to reduce the difference in optical path length between the infrared radiation of the specific wavelength and the infrared radiation of the reference wavelength, and achieve an improvement in the light receiving efficiency of each of the pyroelectric elements 41 and 42 of the infrared radiation receiving element 40.
Further, in the infrared flame detector of the present embodiment, the broadband blocking filter section 3 of the infrared optical filter 20 is constituted by the multilayer film formed by means of stacking multiple types of the thin films 3a and 3b having different refractive indexes on each other. At least one type of the thin film 3a out of the multiple types of the thin films 3a and 3b is formed of the far-infrared radiation absorbing material having a property of absorbing the far-infrared radiation. Thus, according to the infrared flame detector of the present embodiment, it is possible to realize the infrared radiation blocking function in a broad band from near infrared radiation to far infrared radiation without using the sapphire substrate by making use of a light interference effect by the multilayer film constituting the broadband blocking filter section 3 and a far-infrared radiation absorption effect by the thin film 3a constituting the multilayer film while reducing the number of layers in the multilayer film, and thereby achieve a reduction in cost.
Furthermore, in the infrared flame detector of the present embodiment, the narrowband transmission filter sections 21 and 22 of the infrared optical filter 20 have the infrared radiation blocking function in the broad band from near infrared radiation to far-infrared radiation caused by the light interference effect by the first and second λ/4 multilayer films 21 and 22 and the far-infrared radiation absorption effect by the thin film 21b in the multilayer film constituted by the first λ/4 multilayer film 21, the wavelength selection layers 231 and 232, and the second λ/4 multilayer film 22. Therefore, it is possible to realize the low-cost infrared optical filter 20 having the infrared radiation blocking function in the broad band from near infrared radiation to far-infrared radiation, and capable of selectively transmitting infrared radiation of a desired selective wavelength.
Moreover, in the above-described infrared optical filter 20, since the oxide or nitride is adopted as the far-infrared radiation absorbing material, it is possible to prevent the oxidation of the thin films 3a and 21b made of the far-infrared radiation absorbing material and the resultant change of optical characteristics thereof. Additionally, in the above-described infrared optical filter 20, since the uppermost layer of each of the broadband blocking filter section 3 and the narrowband transmission filter sections 21 and 22 that is farthest from the filter forming substrate 1 is formed of the above-mentioned oxide or nitride, it is possible to prevent the change of the physical properties of the thin film 3a or 21b as the uppermost layer resulting from the reaction with moisture or oxygen in air or absorption or adhesion of impurities to thereby enhance the performance of the filter, and also reduce reflection on the surface of each of the broadband blocking filter section 3 and the narrowband transmission filter sections 21 and 22 to thereby achieve an improvement in the performance of the filter.
In addition, in the above-described infrared optical filter 20, since the multilayer film of the broadband blocking filter section 3 is constituted by alternately stacking the thin film 3a formed of the far-infrared radiation absorbing material and the thin film 3b formed of Ge as the high refractive index material having a refractive index higher than that of the far-infrared radiation absorbing material on each other, it is possible to increase the difference in refractive index between the high refractive index material and the low refractive index material to be larger than that in the case where the high refractive index material is Si, PbTe, or ZnS, and reduce the number of stacked layers of the multilayer film. Further, in the case where Si is adopted as the high refractive index material, it is possible to increase the difference in refractive index between the high refractive index material and the low refractive index material in the multilayer film to be larger than that in a case where the high refractive index material is ZnS, and reduce the number of stacked layers (layer number) of the multilayer film. Furthermore, as for the narrowband transmission filter sections 21 and 22, it is possible to reduce the number of stacked layers because of the same reason.
In the present embodiment, although the Si substrate is used as the filter forming substrate 1 of the infrared optical filter 20, the filter forming substrate 1 is not limited to the Si substrate, and a Ge substrate may also be used.
In the infrared flame detector of the present embodiment, as described above, by using the Si substrate or the Ge substrate as the filter forming substrate 1, it is possible to achieve a reduction in cost as compared with a case where the filter forming substrate 1 is a sapphire substrate, an MgO substrate, or a ZnS substrate.
Further, in the infrared flame detector of the present embodiment, the package 7 is made of metal, and the filter forming substrate 1 is bonded to and electrically connected to the cap 72 of the package 7 by using a bonding section 58 made of a conductive bonding material (e.g., a silver paste, a solder, or the like). With this, in the infrared flame detector of the present embodiment, it is possible to perform electromagnetic shielding with the filter forming substrate 1 and the package 7, prevent the influence of external radiant noises (electromagnetic noises) on the infrared radiation receiving element 40, and achieve an increase in sensitivity due to an improvement in S/N ratio.
Furthermore, in the infrared flame detector of the present embodiment, the window section 7a of the cap 72 is opened in the rectangular shape, a stepped section 20c that is positioned on the inner peripheral surface and the peripheral portion of the window section 7a in the cap 72 is formed in the infrared optical filter 20, and the stepped section 20c in the infrared optical filter 20 is fixed to the cap 72 via the bonding section 58 made of the above-described bonding material. Consequently, it is possible to enhance parallelism between the infrared optical filter 20 and the infrared radiation receiving element 40, enhance the precision in the distance between each of the narrowband transmission filter sections 21 and 22 and each of the pyroelectric elements 41 and 42 of the infrared radiation receiving element 40 in the optical axis direction of each of the narrowband transmission filter sections 21 and 22 of the infrared optical filter 20, and also enhance the precision in the alignment between the optical axis of each of the narrowband transmission filter sections 21 and 22 and the optical axis of each of the light receiving surfaces of the pyroelectric elements 41 and 42.
Moreover, in the infrared flame detector of the present embodiment, since the components of the amplification section (the amplification circuit) 63a configured to amplify the output of the infrared radiation receiving element 40 are accommodated in the package 7, it is possible to reduce an electric path between the infrared radiation receiving element 40 and the amplification section 63a and, since the amplification section 63a is also electromagnetically shielded, it is possible to achieve an increase in sensitivity by a further improvement in S/N ratio.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-279693 | Dec 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/071813 | 12/6/2010 | WO | 00 | 7/30/2012 |
