Patent Application
20210243858
The present invention relates to a radiation light source which has a simple structure with multi-layered, or laminated conductors and insulator materials and enables low-cost and large-area production and has wavelength controllability such as band narrowing effect and is preferable for, for example, infrared processing. In addition, when a high-temperature resistant material is employed as at least part of the conductors or the insulator materials as needed, the radiation light source can be made to stably operate at high temperatures for a long period of time.
Each substance has a particular absorption spectrum. When a substance is irradiated with light having a specific wavelength at which the substance shows high absorption, it is possible, for example, to dry, anneal, or form the substance with high efficiency. Furthermore, when gases are irradiated with a narrowband light corresponding to an absorption wavelength particular to a gas molecule, it is possible to monitor the abundance of the gas molecule based on variation of absorption which depends on the gases present in a light path.
The former example may be applied to applications to roll-to-roll printing and coating or resin drying. For example, when a solvent is irradiated with infrared light having a wavelength corresponding to an absorption wavelength of the solvent, it is possible to save energy and dry the solvent at high speed while preventing unwanted temperature rise. What is more, the former example prevents overheat of a product or the interior of a processing device, which enables molding, reactions, and processing with high accuracy while preventing deterioration of the product and the device.
The latter example may be applied to applications to, for example, non-dispersive infrared absorption (NDIR). When a target sample gas is irradiated with infrared light having an adequately narrow band according to an infrared absorption wavelength particular to the gas, it is possible to detect the gas of interest with high selectivity. The narrower the wavelength width of infrared light to be emitted, the more accurately and selectively the absorption of gas molecules can be measured, which enhances the identification accuracy of molecular species and the measurement sensitivity.
As a wavelength-selective infrared radiation light source, there is disclosed a structure that emits infrared light having a specific wavelength by heating a three-dimensional uneven structure (Non-Patent Literature 1, Patent Literatures 1 and 2) or a two-dimensional microfabricated metal-insulator-metal structure (MIM structure) (Patent Literature 3). A diffraction grating device having the three-dimensional uneven structure emits light with a narrow band but has a complicated structure and is not suitable for large-area production. Such a device also has problems that a direction of radiation is not always perpendicular to a heater surface and that its wavelength fluctuates widely depending on radiation angles. In regard to a device having the two-dimensional patterned MIM structure, a half width is about 10% of a radiation wavelength at its narrowest, which is unsuitable for applications that require high selectivity of wavelength. Especially for use as a gas sensor, such a device has too wide a radiation wavelength width compared to absorption bandwidths of gas molecules. Therefore, it is difficult to separate a signal of a target gas molecule from signals of other gas molecules.
On the other hand, there is disclosed a narrowband radiation light source having a resonant structure between a multi-layered distributed Bragg reflector and plasmonic reflector layer (Non-Patent Literature 2). There are other similar structures disclosed. However, to put narrowband radiation light sources into practical applications, the present inventors have specifically studied radiation light sources as one disclosed in Non-Patent Literature 2 and have found that laminated Si on Au or Ag causes exfoliation at about 300° C. due to interlayer adhesiveness and thermal expansion, which hinders practical applications of narrowband radiation light sources. The inventors have also found that using Ti or Cr as an adhesive layer enhances adhesiveness but deteriorates plasmonic properties, leading to deterioration of radiation properties. Furthermore, the inventors have also found that disposing a metal in the outermost layer on the side close to the air (Non-Patent Literature 3) or disposing an easily-oxidizable semiconductor layer such as Si or Ge in the outermost layer provides no assurance of prolonged stable operation because high-temperature operation in the air causes a change in physical properties of the material and a radiation wavelength changes depending on operating temperatures. Still further, as a result of further study, the inventors have found that the use of a doped semiconductor material such as Si, Ge, and ZnO or the use of a narrow-gap semiconductor as a constituent material changes optical properties in the mid- and far-infrared regions due to carrier generation by thermal excitation, which may change an infrared radiation spectrum along with temperature rise.
An object of the present invention is to provide a radiation light source device which can adjust a bandwidth and, if desired, set to have a half width about single digit or double digits narrower than a radiation wavelength or an even narrower half width and to achieve the radiation light source device as a simple and large-area radiator with a simple multi-layered structure without three- or two-dimensional nano/micro patterning. Another object of the present invention is to enable stable and long-life operation of the device at high temperatures by appropriating selecting materials.
According to an aspect of the present invention, there is provided a multi-layered radiation light source including: a plasmonic reflector layer; a resonator layer consisting of an insulator layer, said resonator layer being adjacent to the plasmonic reflector layer; and a distributed reflector layer having a structure in which a plurality of types of insulator layers having different refractive indexes are alternately laminated, said distributed reflector layer being arranged on the resonator layer on the opposite side of the plasmonic reflector layer, in which the multi-layered radiation light source emits infrared light from the distributed reflector layer to the outside by heating the plasmonic reflector layer.
At least one of the plurality of types of insulator layers which constitutes the distributed reflector layer may have high-temperature resistance.
According to another aspect of the present invention, there is provided a multi-layered radiation light source including: a metallic total reflecting layer; a resonator layer consisting of an insulator layer, said resonator layer is adjacent to the metallic total reflecting layer; and a partially reflecting layer being configured to reflect part of incident light, said partially reflecting layer being arranged on the resonator layer on the opposite side of the metallic total reflecting layer, in which a metal in the metallic total reflecting layer is an optical metallic material having a complex permittivity with a negative real part at a wavelength to be used, and the multi-layered radiation light source emits infrared light from the partially reflecting layer to the outside by heating the metallic total reflecting layer.
The partially reflecting layer may be an interface between the resonator layer and an external space formed by a surface of the resonator layer on the opposite side of the total reflecting layer.
The partially reflecting layer may be a metallic layer configured to reflect part of incident light.
The metallic layer that reflects part of the incident light may have high-temperature resistance.
The partially reflecting layer may be a distributed reflector layer having a structure in which a plurality of types of insulator layers having different refractive indexes are alternately laminated.
At least one of the plurality of types of insulator layers may have high-temperature resistance.
The insulator layer which constitutes the resonator layer and the insulator layer having a low refractive index in the distributed reflector layer may be composed of the same material.
Alternatively, the insulator layer which constitutes the resonator layer and the insulator layer having a low refractive index in the distributed reflector layer may be composed of different materials.
In the plurality of types of insulator layers which constitute the distributed reflector layer, the insulator layer having a high refractive index may have a refractive index 1.3 times or more a refractive index of the insulator layer having a low refractive index.
In the plurality of types of insulator layers which constitute the distributed reflector layer, at least the insulator layer in contact with the air may be composed of an oxide or SiC.
In the distributed reflector layer, the insulator layer having a low refractive index may be a material selected from the group consisting of SiO2, Al2O3, and Si3N4, and the insulator layer having the high refractive index may be a material selected from the group consisting of Si, Ge, SiC, Ta2O5, Nb2O5, and HfO2.
The plasmonic reflector layer or the metallic total reflecting layer may have high-temperature resistance.
The plasmonic reflector layer or the metallic total reflecting layer may be selected from the group consisting of LaB6, Au, W, Mo, Cu alloy, Al alloy, and Ni alloy, having a complex permittivity with a negative real part, and from the group consisting of metallic nitride, metallic carbide, conductive metallic oxide, silicon carbide, silicon oxide, aluminum oxide, and metallic boride, having a complex permittivity with a negative real part in the infrared region.
The metallic carbide may be selected from the group consisting of TiC and TaC.
The plasmonic reflector layer or the metallic total reflecting layer may be selected from the group consisting of TiN and TaN, having a complex permittivity with a negative real part.
The plasmonic reflector layer or the metallic total reflecting layer may be a transparent conductive oxide having a complex permittivity with a negative real part.
The plasmonic reflector layer or the metallic total reflecting layer may be composed of a material having a FOM of 1 or more.
A substrate may be arranged on the plasmonic reflector layer or the metallic total reflecting layer on the opposite side of the resonator layer, and the plasmonic reflector layer or the metallic total reflecting layer may be heated though the substrate.
The substrate or a surface of the substrate may be composed of a conductor having a resistance, and the substrate or the surface of the substrate may be heated by energizing the substrate.
The substrate may contain N-type doped SiC.
The plasmonic reflector layer or the metallic total reflecting layer may be energized to be heated.
The radiation light source according to the present invention may flexibly change wavelength widths of a thermal radiation spectrum by appropriately selecting the type of multi-layered structure and the thickness of each laminated film. Accordingly, it is possible to obtain an optimal radiation spectrum for heating according to the intended use or according to an absorption spectrum of an object to be heated. The radiation spectrum is narrower than radiation spectra of blackbody and graybody heaters in the related art. Accordingly, it is possible to decrease the temperature of an object to be processed during drying or heat processing and to reduce product damage caused by high temperature and to prevent ignition of vaporized solvents. In addition, the radiation light source according to the present invention has a simple multi-layered structure and does not require microfabrication by lithography and can be produced simply by film formation. Accordingly, it is possible to increase an area of a heater and to reduce cost.
When an oxide insulator or a high-temperature resistant insulator material such as SiC is employed in the outermost layer of the multi-layered distributed Bragg reflector, it is possible to prevent changes in refractive index and in structure due to oxidation during operation in the air at high temperatures up to 550° C. or about 600° C., preferably about 800° C., more preferably about 1000° C., and still more preferably even higher temperatures, which enables stable operation for a long period of time while reducing temperature dependence. The high-temperature resistance is a property which prevents changes in refractive index and in structure due to oxidation and does not affect repetitive operation of the multi-layered radiation light source according to the present invention in the air at the aforementioned temperature range, that is, 550° C. or about 600° C., preferably about 800° C., more preferably about 1000° C., and still more preferably even higher temperatures. Furthermore, employing an insulator as a material of the thin-film resonator prevents changes in optical conductivity due to thermal excitation and to prevent a resonant wavelength (radiation wavelength) from changing with temperature and with time. Still further, employing conductive ceramics such as high-melting-point plasmonic metals, alloys, metallic carbides, or metallic borides as a plasmonic material which constitutes a surface of the plasmonic reflector layer enables long-life operation at high temperatures. For applications in which directivity is required, such as in heating furnaces and sensors, a light emitting device can be implemented by setting the lamination cycle number for the distributed Bragg reflector to be three or more selecting a combination of materials of small and similar coefficients of thermal expansion and good adhesiveness from among metallic oxides, carbides, borides, and the like.
The left side of
A radiant structure employed in an embodiment of the present invention consists of three parts: a multi-layered distributed reflector layer (distributed Bragg reflector) having a structure in which insulator materials (or intrinsic semiconductors) having high-contrast refractive indexes are alternately laminated; a plasmonic reflector layer (plasmonic reflector (using, for example, Tamm plasmon)); and a resonator layer (thin-film resonator) consisting of an insulator (or an intrinsic semiconductor) sandwiched between the distributed reflector layer and plasmonic reflector layer. A heater (heat source) is in contact with the plasmonic reflector side, and the distributed Bragg reflector on the opposite side irradiates an object in the air (or in a vacuum). Hereinafter described are two typical device structures proposed in the present invention. Herein, a sharp or narrowband radiation peak is generated in what is called a photonic band gap. In order to make the photonic band gap wide to the possible extent, a difference in refractive index is increased between the alternately laminated materials in the distributed Bragg reflector.
For example, as shown in
The insulator on the uppermost layer (the side farthest from the heat source, that is, a surface of the distributed Bragg reflector facing an object to be irradiated) with the high-temperature resistance prevents changes in refractive index and in structure due to oxidation during high-temperature operation in the air and enables stable operation for a long period of time while reducing temperature dependence. For example, exposing LaB6 to the air causes surface oxidation and changes in property at around 800° C. SiC shows no change of property up to a higher temperature of 1600° C. But when LaB6 is buried under an alumina layer or the like, the inlying LaB6 becomes resistant to high temperatures of 1000° C. and higher. Therefore, when a material having a particularly good high-temperature resistance is disposed in the uppermost layer or in several layers on the side close to the uppermost layer, a material with less high-temperature resistance can be used as inner layers. The same applies to the following other device structures.
In the aforementioned device structure described with reference to
In addition to the above structures, there is a third type of device structure. Instead of forming a film on a metal serving as a substrate, an infrared transparent insulator supporting substrate is disposed on the side close to a distributed Bragg reflector. In this case, the order of film formation is reversed. Examples of the infrared transparent substrate include sapphire (Al2O3) that transmits light with wavelengths from 0.3 to 6 μm, fused quartz substrate that transmits light with wavelengths from 0.2 to 3 μm, and ultralow-doped Si substrate that transmits light with wavelengths from 1.1 to 10 μm (for example, ultralow-doped Si wafer (50,000 Ωcm or more) grown by FZ method which is used for low-temperature operation at 200° C. or lower). On this transparent substrate, opposite to the aforementioned process, two types of insulator films are formed alternately, and after formation of a resonator layer, a plasmonic reflector layer is formed. This process may be followed by forming a film and other structures that perform various functions such as protection of the plasmonic reflector layer from chemical and physical influences during various processes after the film formation of the plasmonic reflector layer or during the actual use.
In any type of structure, the lowermost plasmonic reflector layer used herein has a high melting point of 1600° C. or higher, more preferably 2000° C. or higher, and has a permittivity with a negative real part in a wavelength band to be used and an imaginary part equal to or less than the modulus of the real part of the permittivity. Furthermore, it is more preferable that a material used herein should have a small coefficient of thermal expansion.
To describe further about the high-melting-point material (high-temperature resistant material, or heat resistant material), the radiation light source according to the present invention is usually desired to have resistance to high temperatures, but in practice, it is often the case that resistance to about 800° C. is enough. However, as a result of experiments (to be described), the inventors have found that even when a melting point of a material used in the radiation light source is raised above the upper limit of the operating temperature of the radiation light source, it is not enough to satisfy this condition. Through actual use, the inventors have found that the radiation light source is broken at a temperature far below the melting point. From this result and the known fact that a surface or an interface generally starts to melt at about two thirds of a melting point of a bulk, the inventors have derived a specific condition on the melting point of the aforementioned high-temperature resistant metal. Note that this material may be not only a single element metal but also a heat-resistant alloy or the like which has resistance to the aforementioned high temperatures and does not cause breakage of the radiation light source structure.
Furthermore, as shown in
In
In regard to the insulator layers that compose the distributed reflector layer and the resonator layer, it is preferable to select materials having a large real part of a permittivity and a small imaginary part of the permittivity as can be seen from
As a result of further study, the inventors of the present application have found that the aforementioned plasmonic reflector layer may not be associated with plasmon polaritons and will do as long as the plasmonic reflector layer generally shows metallic properties. Therefore, the inventors have found that the above description of the invention of the present application is valid even when the term “plasmonic” is replaced with the term “metallic”. The following description uses the term “metallic” which is a broader concept.
Note that the term “metallic” or “plasmonic” materials herein not only refers to what is called metals, that is, metals and alloys of single elements but also refers to materials having optical metallic properties, or materials having a complex permittivity with a negative real part. For example, aforementioned LaB6 is usually regarded as ceramics, but LaB6 is one of the typical examples of metal since it has a complex permittivity with a negative real part in a wide wavelength range. Furthermore, the metal herein does not have to show a negative real part of the complex permittivity at all wavelengths but at least at a wavelength of interest, specifically, a peak when the device operates as a narrowband radiation light source. In other words, herein, “plasmonic material”≡“optical metal”≡“material having free carriers and a negative permittivity”. Still further, non-plasmonic materials are also employable since those materials perform as optical metals. For example, as will be described, SiO2 has a region where the real part of the permittivity becomes negative in a narrow wavelength range of 8 to 9 μm which is close to an absorption wavelength of an optical phonon. Other types of materials also show a similar phenomenon in a particular wavelength range. This indicates that a dielectric exhibits the same physical behavior (produces resonant polarization) as the plasmonic material or metallic material herein at around a frequency of an optical phonon of a polar material. The present invention may also employ such a material in a broad sense as a substitute material for metal at a specific wavelength. The principle of the present invention will now be generally described below.
A typical embodiment of the present invention includes three types of narrowband radiation light source devices. All of these examples have a mechanism of wavelength control by the same physical origin called Gires-Tournois interferometer. As can be seen from
Furthermore, when an intensity distribution of an electric field as a result of the multiple reflections is close to the total reflecting layer, electric charges in the metal vibrate violently, which increases the degree of Joule heat due to the loss. However, when the center of distribution of the electric field is far from the metallic section, the loss is reduced, causing a narrow band. All layers may have a thickness changed from the Gires-Tournois structures shown in
In some cases, window glass or the like requires a material that is transparent to visible light and has a high heat shield property. Adjusting various parameters of the configuration of the radiation light source according to the present invention makes it possible to increase absorption in a wavelength region, which greatly contributes to temperature rise in a room or the like in the infrared region. Accordingly, the structure of the present invention can be directly used for this type of heat shield.
Hereinafter described are typical values of refractive indexes, real parts of complex permittivity (the larger this value is on the negative side, the better the performance), melting points, and coefficients of thermal expansion in regard to typical materials to be used in the present invention. Note that there are still other materials included in the present invention other than the following materials. When values are not known or are informed but significantly different between several results, the values are considered to be unreliable. In such cases, the values are described as “(unknown)”.
(In addition to TiN, metallic nitrides such as TaN are also employable.)
(In regard to TiAl and SUS430, the exact real parts of complex permittivities in the infrared region are unknown. However, TiAl and SUS430 are metals and have metallic properties. Accordingly, both materials are employable for the metallic total reflecting layer of the narrowband multi-layered radiation light source according an embodiment of the present invention. In regard to the values of indium tin oxide, indium oxide accounts for 90% by weight, and tin oxide accounts for 10% by weight. In addition to ITO, conductive metallic oxides such as tungsten oxide and molybdenum oxide are also employable.)
Alternatively, a simplified structure may be used in which the distributed Bragg reflector (distributed reflector layer) and the thin-film resonator (resonator layer) are combined into one section to form a structure of about two or three layers laminated by combining materials with high refractive index contrast. Furthermore, as already described, instead of the distributed Bragg reflector, the air (more precisely, a dielectric-air interface that simply causes Fresnel reflection; and of course, an interface between the dielectric and other gases or vacuum) or a metallic partially reflecting layer may be disposed.
Still further, the radiant structure according to the present invention may be formed on a surface of a high heat-resistant semiconductor material such as N-type doped SiC, and the SiC may be electrically energized and heated to a high temperature. Alternatively, the radiant structure according to the present invention may be formed on a heat-resistant insulating substrate such as alumina or Si3N4, and an electric current may be flowed through the metallic total reflecting layer of the radiant structure to heat the substrate.
The radiation light source according to the present invention emits light by heating but can suppress light having a wavelength unnecessary for heating a product. Accordingly, it is possible to offer the prospect of saving energy used for the entire radiation. In addition, when the light source is heated with the same input power as a blackbody light source that emits broadband light, the total amount of radiation energy is smaller than that of the blackbody light source. Accordingly, it is possible to hold the temperature of the light source element high and to emit light with higher intensity than the blackbody light source at a resonant wavelength of the light source. The greatest feature of the radiation light source according to the present invention is that the radiation light source enables large-area, inexpensive, and stable operation even at high temperatures, and the radiation light source is of vital use to a practical large-area and high-intensity light source. Furthermore, the radiation light source irradiates a product of interest to the necessary extent, which can prevent unwanted temperature rise and deterioration by heat. This enables highly accurate molding or drying and opens the way to a new highly accurate production process. Still further, the radiation light source has a sharp radiation wavelength band. Accordingly, it is possible to selectively excite or selectively avoid specific molecular vibrations with high accuracy, leading to a new production process in which products are produced while being accurately processed and synthesized according to desired chemical bonds, molecular structures, or reactions. Similarly, it is possible to produce a light source that emits infrared light in a narrowband corresponding an absorption band of a chemical bond of a specific gas or vibration of a molecular species. Taking advantage of this potential, it is possible to achieve, for example, a compact and high-performance infrared light source that requires no filter and has a simple structure, and it is highly probable that such an infrared light source is applied to a small and highly accurate NDIR sensor component.
Note that the radiation light source according to the present invention is not only operable in high-temperature regions such as 550° C. or higher but also sufficiently effective in lower temperature regions. For example, the radiation light source according to the present invention inherently emits light in a plane, which is convenient for heating a large-area object. Surface emission is feasible even with the structure in the related art shown in
As a metallic total reflecting layer, LaB6, that is, a material having a small thermal expansion and having a permittivity with a largely negative real part and a small imaginary part in the infrared wavelength band is laminated with a thickness of 100 nm or more, and on the layer of LaB6, a resonator layer consisting of Al2O3 (Al2O3 cavity) is formed with a thickness of 1205 nm. On the resonator layer, a SiC layer with a thickness of 323 nm and an Al2O3 layer with a thickness of 625 nm are repetitively laminated. The lower left side of
Hereinafter described is a method for producing a radiation light source for carrying out the present invention.
First, materials having a small coefficient of thermal expansion such as glass, quartz, alumina, Si, W, Mo, Ta, AlN, Si3N4, and Fernico alloys were used as a substrate material in contact with a heat source, and then, on the substrate, metallic conductive materials having high melting points and small coefficients of thermal expansion such as W, Mo, LaB6, TiC, and TiN were formed into mirror-like films as a metallic total reflecting layer. W and Mo were deposited by DC sputtering with an electron beam evaporation device manufactured by ULVAC or i-Miller (CFS-4EP-LL) manufactured by Shibaura Mechatronics Corporation.
Film formation of LaB6 was performed with an electron beam evaporation device manufactured by Eiko Corporation (revised EB350), under a base pressure in the range of 10−8 Pa (that is, 1×10−8 Pa or more and less than 1×10−7 Pa) and under a pressure during vapor deposition in the range of 10−6 Pa or less (that is, a pressure of 10−6 Pa or lower pressures) so as to set a deposition rate to about 3.5 nm/sec. A target of vapor deposition was changed to a single crystal prepared by FZ from a sintered compact prepared by vacuum hot pressing, which yielded a higher performance LaB6 film (ε′=−250 at a wavelength of 3 μm). When the hot-pressed sintered compact was used as the target, good metallic properties were obtained by setting the base temperature during the film formation to about 740° C. to 800° C. (see
On the other hand, in pulsed laser deposition (PLD), a real part of a permittivity was positive, and a film with metallic properties could not be formed (a pressure during vapor deposition was 5×10−5 Pa or less, a film formation temperature was 800° C., and a deposition rate was 0.004 nm/sec). Since a LaB6 film known in the related art did not show such metallic properties or, if not at all, showed quite poor metallic properties, LaB6 could not be used in reality as the metallic total reflecting layer of the narrowband multi-layered radiation light source according to an embodiment of the present invention. However, the inventors of the present application have found that a LaB6 film prepared by the aforementioned method shows a FOM almost equivalent to that of Au in the infrared region, as already described with reference to
More specifically, in a case where a LaB6 film is used as an example, a preferable FOM is 1 or more as described above. In addition, a LaB6 film formed by the novel film formation method found by the inventors has a higher FOM and shows better properties than many other materials having metallic properties. Accordingly, the LaB6 film can have a more preferable FOM of 2 or more. As also found by the inventors, in the novel film formation method, it is possible to achieve a higher FOM with single crystal LaB6 instead of using a hot-pressed sintered compact as a LaB6 target. In this case, an even more preferable FOM is 5 or more.
Film formation of TiN was performed by PLD at a pressure of 5×10−6 Pa or less during vapor deposition and a deposition rate of 0.01 nm/sec or more, whereby obtaining a good metallic film.
Plasmon materials such as W and Mo have high adhesion to ceramics at high temperatures. These materials also have a small coefficient of thermal expansion. Accordingly, these materials are preferable in that a difference in coefficient of thermal expansion is small when a film of the materials is formed on a material with a small coefficient of thermal expansion such as AlN, Si3N4, and Fernico alloys, which causes small thermal stress at an interface. Alternatively, a plate material of Mo or W having a mirror-finished surface on one side may double as the substrate and the metallic total reflecting layer.
On a surface of the metallic total reflecting layer, an insulator with good adhesion and a relatively low refractive index such as Al2O3 or SiO2, or an insulator with a high refractive index such as SiC or high-purity Si is formed as a resonator layer. On this resonator layer, a distributed reflector layer having alternately changed refractive indexes is periodically arranged to form the structure shown in
Hereinafter described are Examples of the radiation light source described with reference to
Furthermore, breakage experiments were conducted on radiation light source structures when the radiation light source according to the present invention was used at high temperatures. Specifically, radiation light sources having the structures shown in
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| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-100713 | May 2018 | JP | national |
| 2019-072426 | Apr 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/020572 | 5/24/2019 | WO | 00 |
