Patent Publication Number: US-2023137163-A1

Title: Thermal radiation element, thermal radiation element module, and thermal radiation light source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-176702, filed on Oct. 28, 2021, and Japanese Patent Application No. 2022-132029, filed on Aug. 22, 2022, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a thermal radiation element. The present invention also relates to a thermal radiation element module and a thermal radiation light source including a thermal radiation element. 
     Related Art 
     In recent years, techniques for obtaining optical property independent of a material by forming a microstructure on the surface of the material have widely been studied. One of the microstructures is a plasmonic structure, and a plasmonic perfect absorber has been reported as one of the plasmonic structures. The plasmonic perfect absorber is one that exhibits a high absorptance in a specific wavelength band out of the plasmonic structures. The plasmonic perfect absorber is a resonator structure in which a conductor, an insulator, and a conductor are stacked, and is also called a metal-insulator-metal (MIM) structure. 
     According to Kirchhoff&#39;s law, in opaque, the emissivity is equal to the absorptance. Therefore, it has also been reported that the emissivity on the surface of a material can be controlled using the MIM structure. The emissivity is expressed by a ratio of emissive power of a real surface to that of a black body surface. The thermal radiation on the black body surface is defined by Planck&#39;s law and is multiplied by the emissivity to obtain the thermal radiation on the real surface. Note that thermal radiation is a phenomenon in which thermal energy of an object is emitted as electromagnetic waves in accordance with the temperature of the object such as a black body and an MIM structure. Hereinbelow, radiation means thermal radiation unless otherwise specified. 
     As a conventional art document relating to emissivity control, JP 2018-136576 A can be cited, for example. JP 2018-136576 A relates to a technique for emitting thermal radiation of narrow-band infrared rays by means of emissivity control at a specific wavelength using an MIM structure. 
     Also, as described in JP 2020-64820 A, a thermal radiation light source to which a technique of emissivity control using an MIM structure is applied is known. JP 2020-64820 A relates to a technique for suppressing oxidation of an MIM structure that may occur in a case where the MIM structure is operated in the atmosphere by using a layer that suppresses oxidation as a surface layer. 
     Meanwhile, as illustrated in (b) of FIG. 1 in JP 2018-136576 A and FIG. 1 in JP 2020-64820 A, the MIM structure is stacked on a substrate (base in JP 2018-136576 A). Hereinbelow, the substrate and the MIM structure stacked on the substrate are collectively referred to as a thermal radiation element. 
     In order to utilize thermal radiation using such a thermal radiation element, it is essential to heat the MIM structure to a predetermined operating temperature. As the operating temperature is higher, the intensity of the thermal radiation is higher, and radiation on the shorter wavelength is emitted. The temperature is a balance of thermal energy. The temperature increases as the input amount of thermal energy gets larger than the loss amount thereof. In a case where the same materials have the same energy amounts, the amounts of temperature rise depend on the volumes. The thermal energy required to raise the temperature of an object by 1° C. is defined as Equation (1), where the heat capacity is C[J/° C.], the specific heat is c[J/kg·° C.], the density is ρ [kg/m 3 ], and the volume is V[m 3 ]. 
         C=c×ρ×V   (1)
 
     SUMMARY OF THE INVENTION 
     As methods for heating the MIM structure in the thermal radiation light source, JP 2020-64820 A described above describes a method of self-heating the substrate by energizing the substrate and a method of externally heating the substrate and the MIM structure using an external heating unit (for example, a heater). In any of these methods, heat passes through the substrate in the heat transfer path in the case of heating the MIM structure. In this manner, the substrate functions as a heat source for the MIM structure. Therefore, in order to cause the temperature of the MIM structure to reach the above-described operating temperature, it is necessary to keep the temperature of the substrate, which is a heat source, equal to or higher than the operating temperature. 
     Since the substrate is thicker than the MIM structure, the volume of the substrate is inevitably larger. That is, the heat capacity C of the substrate is inevitably larger than the heat capacity C of the MIM structure. Therefore, the conventional method of heating the entire MIM structure using the substrate as a heat source (for example, the method described in JP 2020-64820 A) has room for improving energy efficiency. 
     The present invention has been made in view of the above-described problems, and an object of an aspect of the present invention is to enhance energy efficiency as compared with the thermal radiation element in JP 2020-64820 A, which is a conventional thermal radiation element. An object of another aspect of the present invention is to provide a thermal radiation element module and a thermal radiation light source including a thermal radiation element having higher energy efficiency than a conventional thermal radiation element. 
     In order to solve the above-described problems, a thermal radiation element according to an aspect of the present invention includes a substrate, made of a semiconductor, having a first principal plane and a second principal plane, a first conductor layer and a second conductor layer provided on the first principal plane and the second principal plane, respectively, and an electrode pair provided in an outer edge region of the first conductor layer. 
     Also, in order to solve the above-described problems, a thermal radiation element module according to an aspect of the present invention includes the thermal radiation element according to the aspect of the present invention, and a housing provided with a cavity housing the thermal radiation element and a power terminal supplying power to the electrode pair. In the thermal radiation element module, a configuration is employed in which, in the inside of the cavity, at least a part of the substrate is secured to the cavity using a bonding member. 
     Further, in order to solve the above-described problems, a thermal radiation light source according to an aspect of the present invention includes the thermal radiation element module according to the aspect of the present invention. 
     According to an aspect of the present invention, energy efficiency can be enhanced as compared with the thermal radiation element in JP 2020-64820 A, which is a conventional thermal radiation element. Also, according to another aspect of the present invention, it is possible to provide a thermal radiation element module and a thermal radiation light source including a thermal radiation element having higher energy efficiency than a conventional thermal radiation element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The upper diagram of  FIG.  1    is a plan view of a thermal radiation element module according to an embodiment of the present invention, and the lower diagram of  FIG.  1    is a cross-sectional view of the same thermal radiation element module; 
         FIG.  2    is a cross-sectional view of the thermal radiation element according to the embodiment of the present invention; 
         FIG.  3    is a plan view of the thermal radiation element illustrated in  FIG.  2    as viewed from the side provided with an electrode pair; 
         FIG.  4    is an enlarged perspective view in which a part of a plasmonic perfect absorber included in the thermal radiation element illustrated in  FIG.  2    is enlarged; 
         FIG.  5    is a cross-sectional view of a first modification example of the thermal radiation element illustrated in  FIG.  2   ; and 
         FIG.  6    is a cross-sectional view of a second modification example of the thermal radiation element illustrated in  FIG.  2   . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A thermal radiation element module M according to an embodiment of the present invention will be described with reference to  FIGS.  1  to  4   . The upper diagram of  FIG.  1    is a plan view of the thermal radiation element module M, and the lower diagram of  FIG.  1    is a cross-sectional view of the thermal radiation element module M. The plan view of the thermal radiation element module M is obtained in a case where an opening portion AP c  of a cavity C provided in a housing  20  is viewed in a planar view in the normal direction of the principal plane of an optical window  23 . The cross-sectional view of the thermal radiation element module M is obtained in a cross section along the normal direction of the principal plane of the optical window  23  and including a thermal radiation element  1 .  FIG.  2    is a cross-sectional view of the thermal radiation element  1 , and is an enlarged view of a portion of the thermal radiation element  1  in  FIG.  1   . In  FIG.  2   , the size of each component in the thickness direction is enlarged.  FIG.  3    is a plan view of the thermal radiation element  1  as viewed from the side provided with an electrode pair  16 .  FIG.  4    is an enlarged perspective view in which a part of a plasmonic perfect absorber  10  included in the thermal radiation element  1  is enlarged. 
     [Overview of Thermal Radiation Element Module] 
     As illustrated in the upper diagram and the lower diagram of  FIG.  1   , the thermal radiation element module M includes the thermal radiation element  1 , the housing  20 , electrode pads  21  and  22 , the optical window  23 , a bonding member  24 , and power terminals  25  and  26 . 
     Here, the thermal radiation element  1 , which is also an embodiment of the present invention, includes the plasmonic perfect absorber  10 , a substrate  14 , a conductor layer  15 , and the electrode pair  16 . Also, the plasmonic perfect absorber  10  includes a conductor layer  13 , an insulator layer  12 , and a conductor layer  11 . 
     In a case where the conductor layer  13 , the substrate  14 , and the conductor layer  15  constituting a part of the plasmonic perfect absorber  10  are energized using the power terminals  25  and  26 , the thermal radiation element module M emits electromagnetic waves (specifically, at least one of visible light, near-infrared light, mid-infrared light, and far-infrared light) caused by thermal radiation. In this manner, the thermal radiation element module M functions as a thermal radiation light source that emits electromagnetic waves of at least one of visible light, near-infrared light, mid-infrared light, and far-infrared light. That is, a thermal radiation light source using the thermal radiation element module M is also included in the scope of the present invention. Note that the thermal radiation light source may include not only the thermal radiation element module M but also a power supply module that supplies power to the thermal radiation element module M via the power terminals  25  and  26 . 
     The thermal radiation element module M uses the power terminals  25  and  26  to cause current to flow in the in-plane direction of the conductor layer  15  in a temperature region near the room temperature and to cause current to flow in the in-plane directions of the conductor layers  13  and  15  in a temperature region near the operating temperature. 
     The current flowing in the in-plane directions of the conductor layers  13  and  15  generates Joule heat. Therefore, the thermal radiation element module M emits the above-described electromagnetic waves by heating the thermal radiation element  1  to a predetermined operating temperature using the thermal energy. The operating temperature of the thermal radiation element  1  can appropriately be determined within a temperature range in which the eutectic reaction in the plasmonic perfect absorber  10  does not proceed. The higher the operating temperature, the higher the intensity of the light emitted by the plasmonic perfect absorber  10 . In the thermal radiation element  1  described in the present embodiment, the operating temperature is assumed to be 300° C. or more and 1200° C. or less. 
     &lt;Substrate&gt; 
     The substrate  14  is a plate-like member, made of a semiconductor, having a pair of principal planes  14   a  and  14   b . In the state illustrated in  FIG.  2   , the principal plane  14   a  is located on the upper side, and the principal plane  14   b  is located on the lower side. The shape of the substrate  14  can appropriately be determined, but is preferably a rectangular shape or a square shape. In the present embodiment, a square is employed as the shape of the substrate  14 . 
     In the present embodiment, silicon, which is an example of a semiconductor and has a resistivity of 1 nm, is employed as a material for the substrate  14 . However, the material for the substrate  14  is not limited to silicon as long as the material is a semiconductor whose resistivity decreases with an increase in temperature. Also, the resistivity of the semiconductor can appropriately be determined in accordance with the configuration of the thermal radiation element  1  (for example, thicknesses of the conductor layer  13 , the substrate  14 , and the conductor layer  15 ), an assumed operating temperature, and the like. In the present embodiment, the resistivity of the semiconductor constituting the substrate  14  is preferably 1×10 −2  Ωm or more and 2 Ωm or less. Also, the resistivity of the semiconductor constituting the substrate  14  is preferably measured using a resistance measurement method conforming to a standard (such as a standard defined by Japanese Industrial Standards or American Society for Testing and Materials). By using the substrate  14  made of a semiconductor whose resistivity is guaranteed in this manner, it is possible to suppress variations in temperature characteristics that may occur in the manufactured thermal radiation element  1 . The dopant doped in the semiconductor constituting the substrate  14  may be either n-type or p-type. 
     The substrate  14  has a high resistivity at room temperature. In the case of intrinsic silicon as an example of silicon, the resistivity at room temperature is about 1×10 3  Ωm. Therefore, when the conductor layer  15  to be described below starts being energized, no current flows through the substrate  14 , and current flows only through the conductor layer  15 . 
     As described above, the resistivity of the substrate  14  decreases with an increase in temperature. In the case of intrinsic silicon, the resistivity at 300° C. is less than 1×10 −1  Ωm, the resistivity at 400° C. is less than 1×10 −2  Ωm, and the resistivity at 500° C. is about 1×10 −3  Ωm. Therefore, since the resistivity of the substrate  14  decreases as the temperature of the substrate  14  increases, the current flows not only through the conductor layer  15  but also through the conductor layer  13 . 
     In this manner, since parallel current paths of the conductor layer  13  and the conductor layer  15  are formed between an electrode  161  and an electrode  162  to be described below, the resistance value generated between the electrode  161  and the electrode  162  is obtained by combining the in-plane resistance value of the conductor layer  13 , the in-plane resistance value of the conductor layer  15 , and the perpendicular resistance value of the substrate  14  (resistance value between the conductor layer  13  and the conductor layer  15 ). In the thermal radiation element  1 , by appropriately adjusting the thicknesses of the conductor layer  13 , the substrate  14 , and the conductor layer  15 , it is possible to suppress a change in the resistance value generated between the electrode  161  and the electrode  162  at the operating temperature of the thermal radiation element  1 . Therefore, in the thermal radiation element  1 , the resistance value generated between the electrode  161  and the electrode  162  can be adjusted to any resistance value that can easily be monitored. 
     In addition, in the thermal radiation element  1 , the temperature of the plasmonic perfect absorber  10  can be found by monitoring a resistance value that can be generated between the electrode  161  and the electrode  162 . Since the spectrum of the electromagnetic waves emitted by the plasmonic perfect absorber  10  depends on the temperature of the plasmonic perfect absorber  10 , in the thermal radiation element  1 , a predetermined spectrum can be obtained by controlling the current supplied between the electrodes of the electrode pair  16  so that the resistance value that can be generated between the electrode  161  and the electrode  162  becomes a predetermined value. 
     As described above, in the thermal radiation element  1 , since the resistance value generated between the electrode  161  and the electrode  162  can accurately be monitored, the temperature of the plasmonic perfect absorber  10  can easily be controlled. In addition, in the thermal radiation element  1 , since it is not necessary to separately provide a thermometer for monitoring the temperature of the plasmonic perfect absorber  10 , it is possible to reduce the size and cost of the thermal radiation element  1 . 
     Note that a thickness t s  (refer to  FIG.  2   ) of the substrate  14  is preferably 100 μm or more and 1 mm or less. In the present embodiment, the thickness t s  is 200 μm. 
     The principal plane  14   a  is provided with the plasmonic perfect absorber  10  in which the conductor layer  13 , the insulator layer  12 , and the conductor layer  11  are stacked in this order. The plasmonic perfect absorber  10  will be described below. On the other hand, the principal plane  14   b  is provided with the conductor layer  15  and the electrode pair  16  stacked in this order. The conductor layer  15  and the electrode pair  16  will be described below. The principal plane  14   b  and the principal plane  14   a  are examples of a first principal plane and a second principal plane, respectively. 
     &lt;First Conductor Layer&gt; 
     As illustrated in  FIGS.  2  and  3   , the conductor layer  15  is provided so as to cover the entire region of the principal plane  14   b . In an aspect of the present invention, a first conductor layer refers to a conductor layer on which the electrode pair is stacked. Therefore, the conductor layer  15  is an example of the first conductor layer. The conductor layer  15  has current flow therethrough in the in-plane direction using the electrode pair  16  to be described below to function as a heater that heats the plasmonic perfect absorber  10  and the substrate  14 . Hence, the conductor constituting the conductor layer  15  preferably has a higher resistivity than copper, aluminum, gold, or the like. Also, a semiconductor has a characteristic of easily becoming a eutectic alloy with various metals. For example, tungsten, which has a melting point of 3422° C., exhibits a eutectic reaction with silicon at 650° C. or lower to cause the resistivity to be changed. For this reason, a preferable conductor constituting the conductor layer  15  is one whose temperature for a eutectic reaction with a semiconductor is high. Preferable examples of the conductor constituting the conductor layer  15  include hafnium nitride (HfN), titanium nitride (TiN), and molybdenum (Mo). 
     &lt;Electrode Pair&gt; 
     As illustrated in  FIGS.  2  and  3   , the electrodes  161  and  162  constituting the electrode pair  16  are provided on a principal plane  15   a , which is a principal plane (lower principal plane in  FIG.  2   ) on the opposite side of the substrate  14  out of the principal planes of the conductor layer  15 . 
     The electrodes  161  and  162  are provided in the outer edge region of the conductor layer  15  in order to cause current to flow throughout the conductor layer  15 . The outer edge region of the conductor layer  15  is an annular region along the four sides forming the conductor layer  15 . More specifically, the electrodes  161  and  162  are provided along a pair of opposite sides (a pair of opposite sides located on the left side and the right side in  FIG.  3   ) of the conductor layer  15  formed in a square shape similarly to the substrate  14 . Each of the electrodes  161  and  162  is a strip-shaped or rectangular conductor pattern. The electrode  161  is provided along a side located on the left side in  FIG.  3   , and the electrode  162  is provided along a side located on the right side in  FIG.  3   . 
     In the present embodiment, a three-layer film of Ti/Pt/Au in which titanium (Ti), platinum (Pt), and gold (Au) are stacked in this order on the principal plane  15   a  is used as each of the electrodes  161  and  162 . The thickness of each layer can appropriately be determined, but in the present embodiment, the thicknesses of Ti and Pt are 30 nm, and the thickness of Au is 500 nm. 
     By connecting wires having different polarities to the electrodes  161  and  162 , respectively, and supplying power, current flows from one of the electrodes  161  and  162  to the other. That is, current flows through the conductor layer  15  in the in-plane direction of the principal plane  15   a . Therefore, the electrodes  161  and  162  provided on the principal plane  15   a  of the conductor layer  15  are an example of an electrode that allows current to flow in the in-plane direction of the principal plane of the conductor layer  15 . 
     Note that, in the present embodiment, a three-layer film of Ti/Pt/Au described above, which is an example of a multilayer film, is used as each of the electrodes  161  and  162 . Here, each of the Ti and Pt layers functions as an underlayer, and the Au layer functions as a main conductive layer. The Ti layer serving as the underlayer enhances adhesion of the electrodes  161  and  162  to the conductor layer  15  and reduces contact resistance that may be generated between the conductor layer  15  and the electrodes  161  and  162 . Also, the Pt layer serving as the underlayer prevents or suppresses diffusion that may be generated between the Au layer and the Ti layer and suppresses changes in the resistance of the electrodes. However, the constitution of the underlayer is not limited to Ti/Pt. The underlayer may be constituted by a single-layer film or a multilayer film of three or more layers. Also, a different metal can be used instead of Au as a layer that functions as the main conductor layer. For example, Ag or an alloy consisting primarily of Ag can be used. Further, in each of the electrodes  161  and  162 , the Ti/Pt underlayer may be omitted, and a single-layer film of Au can be employed. The configuration of each of the electrodes  161  and  162  is not limited to the above-described example, and can appropriately be determined in consideration of how high the conductivity is, how low the reactivity is, how high the melting point is, and the like. 
     &lt;Plasmonic Perfect Absorber&gt; 
     As illustrated in  FIG.  4   , the plasmonic perfect absorber  10  included in the thermal radiation element  1  includes the conductor layer  11 , the insulator layer  12 , and the conductor layer  13 . The conductor layer  11  is an example of a third conductor layer, and the conductor layer  13  is an example of a second conductor layer. On the principal plane  14   a  of the substrate  14 , the conductor layer  13 , the insulator layer  12 , and the conductor layer  11  are stacked in this order. 
     (Second Conductor Film) 
     The conductor layer  13  is a film, made of a conductor, formed on the principal plane  14   a , which is one principal plane (upper principal plane in  FIG.  2   ) of the substrate  14 , so as to cover the entire region of the principal plane  14   a . The conductor layer  13  is an example of the second conductor layer. 
     In the present embodiment, hafnium nitride (HfN) is employed as the conductor constituting the conductor layer  13 . However, the conductor constituting the conductor layer  13  is not limited to HfN as long as the conductor is a material having metallic conductive characteristics. In a case where the plasmonic perfect absorber  10  is formed on the surface of a base material which is assumed to have a high temperature at the time of use, the material for the conductor layer  13  is preferably a material having a high melting point such as HfN. A typical melting point of HfN is 3330° C. In addition, a preferable material for the conductor layer  13  is one whose temperature for a eutectic reaction with a semiconductor is high, such as HfN. HfN exhibits no eutectic reaction with silicon in a temperature range of 1200° C. or lower. 
     Note that the region of the principal plane  14   a  in which the conductor layer  13  is formed may be the entire principal plane  14   a  or a part of the principal plane  14   a , and can appropriately be determined. In the present embodiment, the conductor layer  13  is formed on the entire principal plane  14   a.    
     In the present embodiment, a thickness t 13  (refer to  FIG.  2   ) of the conductor layer  13  is 140 nm. However, the thickness t 13  is not limited to 140 nm, and can appropriately be determined within a range of, for example, 10 nm or more and 10 μm or less. Note that the thickness t 13  is an example of a thickness t1 described in the claims. 
     (Insulator Film) 
     The insulator layer  12  is a film, made of an insulator, formed on a principal plane  13   a , which is a principal plane (upper principal plane in  FIG.  2   ) on the opposite side of the substrate  14  out of the principal planes of the conductor layer  13 , so as to cover at least a part of the principal plane  13   a . In the present embodiment, as illustrated in  FIG.  2   , the insulator layer  12  is provided so as to cover the entire region of the principal plane  13   a . However, the region of the principal plane  13   a  in which the insulator layer  12  is provided may be the entire principal plane  13   a  or a part of the principal plane  13   a , and can appropriately be determined. 
     Note that, in the present embodiment, the insulator layer  12  which is a solid film having a uniform thickness is formed. However, the insulator layer  12  may be formed only in regions in which a plurality of conductor patterns  111  are formed. That is, similarly to the conductor layer  11 , the insulator layer  12  may include a plurality of conductor patterns regularly arranged, the plurality of conductor patterns each being in a circular shape or a regular polygonal shape. 
     In the present embodiment, SiO 2  is employed as a material for the insulator layer  12 . However, the material for the insulator layer  12  is not limited to SiO 2  as long as the material is an insulator. An example of such a material includes an insulating oxide. In a case where the plasmonic perfect absorber  10  is formed on the principal plane  14   a  of the substrate  14  which is assumed to have a high temperature at the time of use, the material for the insulator layer  12  is preferably any of SiO 2 , aluminum oxide (Al 2 O 3 ), aluminum nitride (AlN), and a mixture of SiO 2  and Al 2 O 3 . 
     In the present embodiment, a thickness t 12  (refer to  FIG.  2   ) of the insulator layer  12  is 180 nm. However, the thickness t 12  is not limited to 180 nm, and can appropriately be determined within a range of, for example, 10 nm or more and 10 μm or less. 
     (Third Conductor Layer) 
     The conductor layer  11  is formed over the entire region of a principal plane  12   a , which is a principal plane (upper principal plane in  FIG.  2   ) on the opposite side of the insulator layer  12  out of the principal planes of the insulator layer  12 . However, the region of the principal plane  12   a  in which the conductor layer  11  is provided may be the entire principal plane  12   a  or a part of the principal plane  12   a , and can appropriately be determined. The conductor layer  11  is an example of a third conductor layer. In this manner, the insulator layer  12  and the conductor layer  11  described above are stacked on the principal plane  13   a  of the conductor layer  13 , which is an example of the second conductor layer, in this order. 
     The conductor layer  11  includes the plurality of (nine in  FIG.  4   ) conductor patterns  111  each formed in a circular shape. However, the shape of each of the conductor patterns  111  is not limited to the circular shape, and may be a regular polygonal shape. A preferable example of the regular polygonal shape includes a regular hexagonal shape. 
     Note that reference sign  111  is given to only one conductor pattern  111  out of the plurality of conductor patterns  111 . As illustrated in  FIG.  4   , the plurality of conductor patterns  111  are two-dimensionally and regularly arranged on the principal plane  12   a . In the present embodiment, as illustrated in  FIG.  4   , a square arrangement is employed as the two-dimensional and regular arrangement of the conductor patterns  111 . However, the two-dimensional and regular arrangement is not limited to the square arrangement, and may be, for example, a hexagonal arrangement. 
     In the present embodiment, hafnium nitride (HfN) is employed as the conductor constituting each conductor pattern  111  of the conductor layer  11 . However, the conductor constituting each conductor pattern  111  is not limited to HfN as long as the conductor is a material having metallic conductive characteristics. In this respect, the conductor constituting each conductor pattern  111  is the same as the conductor constituting the conductor layer  13 . 
     Also, in the present embodiment, a thickness t 11  (refer to  FIG.  2   , that is, the thickness of each of the conductor patterns  111 ) of the conductor layer  11  is 40 nm. However, the thickness t 11  is not limited to 40 nm, and can appropriately be determined within a range of, for example, 10 nm or more and 10 μm or less. Note that the thickness t 11  is an example of a thickness t 3  described in the claims. 
     Also, the thickness t 13  of the conductor layer  13  and the thickness t 11  of the conductor layer  11  preferably satisfy the relationship of t 13 &gt;1.5×t 11 . The thickness t 11  is an example of a thickness t3 of the third conductor layer, and the thickness t 13  is an example of a thickness t1 of the first conductor layer. 
     &lt;Housing&gt; 
     The housing  20  is a rectangular solid block. In the present embodiment, the material for the housing  20  is alumina, which is an example of ceramic. However, the ceramic constituting the housing  20  is not limited to alumina, and can appropriately be selected. In addition, the material for the housing  20  is not limited to ceramic, and may be metal, an alloy, or an organic compound such as resin. However, in a case where the operating temperature of the thermal radiation element  1  is set to 150° C. or higher, the material for the housing  20  is preferably any of metal, an alloy, and ceramic. 
     Out of the paired principal planes of the housing  20 , the principal plane located on the upper side in the state illustrated in  FIG.  1    is referred to as a principal plane  20   a , and the principal plane located on the lower side in the state illustrated in  FIG.  1    is referred to as a principal plane  20   b . The principal plane  20   a  is provided with the cavity C. The depth of the cavity C is smaller than the thickness of the housing  20 . Therefore, the cavity C does not penetrate the principal plane  20   b.    
     The cavity C includes two sub-cavities C 1  and C 2 . 
     The sub-cavity C 1  is formed in a region closer to the principal plane  20   a  (that is, a shallow region). The sub-cavity C 2  is formed in a region farther from the principal plane  20   a  than the sub-cavity C 1  (that is, a deep region). The size of the opening portion AP c  of the sub-cavity C 1  is determined so as to be able to include the thermal radiation element  1  in a planar view. Note that the reference sign AP c  is clearly illustrated in the upper diagram of  FIG.  1   , but illustration of AP c  is omitted in the lower diagram of  FIG.  1   . 
     On the other hand, the size of the opening portion of the sub-cavity C 2  formed on the bottom surface of the sub-cavity C 1  is determined so as to be included by the thermal radiation element  1  in a planar view. The cavity C configured in this manner is formed in a stepped shape. 
     In the sub-cavity C 1  of the cavity C, the thermal radiation element  1  is housed. The bottom wall of the sub-cavity C 1  is provided with the electrode pads  21  and  22  that are in partial contact with the electrodes  161  and  162 . In the thermal radiation element  1 , the electrodes  161  and  162  constituting a part of the substrate  14  are respectively secured to the electrode pads  21  and  22  provided on the bottom wall of the sub-cavity C 1  using conductive bonding members. That is, the electrodes  161  and  162  are electrically connected to the electrode pads  21  and  22 , respectively. 
     Note that the electrode pads  21  and  22  provide electric power to the electrodes  161  and  162 , and at the same time, transfer thermal energy generated in the thermal radiation element  1  to the housing  20 . To suppress heating of the housing  20  due to the thermal transfer, the contact area between the electrode pads  21  and  22  and the electrodes  161  and  162  is preferably small. On the other hand, if the contact area is too small, the contact resistance between the electrode pads  21  and  22  and the electrodes  161  and  162  becomes too large. This contact area can appropriately be set in view of the extent of the thermal transfer from the thermal radiation element  1  to the housing  20  and the level of the contact resistance between the electrode pads  21  and  22  and the electrodes  161  and  162 . 
     In the present embodiment, a silver (Ag) paste, which is a sintered bonding material, is employed as the bonding member. By heating the sintered silver paste to about 200° C., objects can be bonded to each other in a non-pressurized state. The silver paste used in the present embodiment has heat resistance to withstand the operating temperature of the thermal radiation element  1  (for example, any temperature of 300° C. or more and 900° C. or less) when sintered, and is thus preferable as a bonding member. Note that, in the lower diagram of  FIG.  1   , illustration of the bonding member is omitted. 
     In this manner, since the thermal radiation element  1  has a high operating temperature, in order to enhance energy efficiency, it is preferable to suppress thermal energy dissipated by thermal transfer from the thermal radiation element  1  to the housing  20 . In the thermal radiation element module M, since the sub-cavity C 2  as well as the sub-cavity C 1  is formed in the housing  20 , it is possible to limit the path for thermal transfer that can be formed between the thermal radiation element  1  and the housing  20 . 
     Note that, in the present embodiment, the electrode pads  21  and  22  are in contact with parts of the electrodes  161  and  162 , respectively. By limiting the contact area between the electrodes  161  and  162  and the electrode pads  21  and  22 , the path for thermal transfer can be limited between the electrode pads  21  and  22  and the electrodes  161  and  162 . 
     As illustrated in the lower diagram of  FIG.  1   , the housing  20  is provided with the power terminals  25  and  26 . The power terminal  25  is inserted from the outside into the inside of the housing  20 . In addition, the tip of the power terminal  25  is electrically connected to the electrode pad  21 . Similarly, the power terminal  26  is inserted from the outside into the inside of the housing  20 . In addition, the tip of the power terminal  26  is electrically connected to the electrode pad  22 . Note that the portions at which the power terminals  25  and  26  are respectively inserted into the housing  20  are sealed so as to maintain sealing performance. 
     As illustrated in the upper view and the lower view of  FIG.  1   , the opening portion AP c  is covered with the optical window  23 . The material for the optical window  23  preferably has translucency and heat resistance to withstand the temperature that the housing  20  is to reach (for example, any temperature of 270° C. or less), and in the present embodiment, a plate-like member made of borosilicate glass is used. 
     The optical window  23  is bonded to the principal plane  20   a  of the housing  20  using the bonding member  24 . In the present embodiment, gold (Au) tin (Sn) solder is used as the bonding member  24 . That is, the opening portion AP c  is sealed by the optical window  23 . 
     Also, the thermal radiation element module M is configured so that the pressure inside the cavity C is lower than the pressure outside the cavity C (for example, the atmospheric pressure). This configuration can be achieved, for example, by sealing the cavity C under a reduced pressure environment in which the pressure is lower than the atmospheric pressure. The pressure inside the cavity C is not limited, but is preferably 1×10 3  Pa or less, and more preferably 1×10 1  Pa or less. As the pressure inside the cavity C is lower, the heat insulating property of the cavity C can be enhanced. 
     [Sum-Up] 
     The thermal radiation element  1  according to an aspect of the present invention includes the substrate  14 , made of a semiconductor (made of silicon in the present embodiment), having the principal plane  14   b , which is an example of a first principal plane, and the principal plane  14   a , which is an example of a second principal plane, a first conductor layer (conductor layer  15 ) and a second conductor layer (conductor layer  13 ) provided on the first principal plane (principal plane  14   b ) and the second principal plane (principal plane  14   a ), respectively, and the electrode pair  16  provided in an outer edge region of the first conductor layer (conductor layer  15 ). 
     According to this configuration, since the resistivity of the substrate  14  decreases in a state where the plasmonic perfect absorber  10  has reached the operating temperature, the conductor layer  13  as well as the conductor layer  15  is used as a heat source. In this manner, since the conductor layer  13  constituting a part of the plasmonic perfect absorber  10  can directly be used as a heat source, the thermal radiation element  1  can enhance energy efficiency as compared with the thermal radiation element in Patent Document 2, which is a conventional thermal radiation element. 
     Also, in the thermal radiation element  1 , the resistance value that can be generated between the electrode  161  and the electrode  162  is obtained by combining the in-plane resistance values of the conductor layer  13  and the conductor layer  15 , and the perpendicular resistance value of the substrate  14  (resistance value between the conductor layer  13  and the conductor layer  15 ). Therefore, the resistance value that can be generated between the electrode  161  and the electrode  162  can be set to any resistance value that can easily be monitored regardless of the in-plane resistance values of the conductor layer  13  and the conductor layer  15 . 
     Accordingly, in the thermal radiation element  1 , since the resistance value generated between the electrode  161  and the electrode  162  can accurately be monitored, the temperature of the plasmonic perfect absorber  10  can easily be controlled. 
     In addition, in the thermal radiation element  1 , since it is not necessary to separately provide a thermometer for monitoring the temperature of the plasmonic perfect absorber  10 , it is possible to reduce the size and cost of the thermal radiation element  1 . 
     The thermal radiation element  1  further includes the insulator layer  12  and a third conductor layer (conductor layer  11 ) stacked in order on a surface (principal plane  13   a  of the conductor layer  13  in the present embodiment) of the second conductor layer (conductor layer  13 ), the insulator layer  12  and the third conductor layer (conductor layer  11 ) constituting the plasmonic perfect absorber  10  together with the second conductor layer (conductor layer  13 ) (together with the conductor layer  13  in the present embodiment). 
     According to this configuration, the spectrum of the electromagnetic waves emitted by the thermal radiation element  1  can be controlled by appropriately setting each parameter of the plasmonic perfect absorber. 
     Note that, as described below with reference to  FIG.  5   , a modification example of the thermal radiation element  1  may further include an insulator layer  12 A and the third conductor layer (conductor layer  11 A) stacked in order on the principal plane  15   a , which is a surface of the first conductor layer (conductor layer  15 ), the insulator layer  12 A and the third conductor layer (conductor layer  11 A) constituting a plasmonic perfect absorber  10 A together with the first conductor layer (conductor layer  15 ). That is, the plasmonic perfect absorber may be provided on the second conductor layer (conductor layer  13 ) or on the first conductor layer (conductor layer  15 ). 
     Further, in the thermal radiation element  1 , a configuration is employed in which the third conductor layer (conductor layer  11 ) includes the plurality of conductor patterns  111  two-dimensionally and regularly arranged, the plurality of conductor patterns  111  each being in a circular shape or a regular polygonal shape. 
     According to this configuration, the wavelength range of the light emitted from the plasmonic perfect absorber  10  can be adjusted by adjusting the size and the regular arrangement of the plurality of conductor patterns  111 . 
     Also, in the thermal radiation element  1 , a configuration is employed in which the thickness t1 (t 13 ) of the conductor layer (conductor layer  13 ), out of the first conductor layer (conductor layer  15 ) and the second conductor layer (conductor layer  13 ), constituting the plasmonic perfect absorber  10  together with the insulator layer  12  and the third conductor layer (conductor layer  11 ), and the thickness t3 (t 11 ) of the third conductor layer (conductor layer  11 ) satisfy the relationship of t1&gt;1.5×t3 (t 13 &gt;1.5×t 11 ). 
     According to this configuration, since the resistance value of the conductor layer  13  can appropriately be lowered, large current easily flows through the conductor layer  13 . Therefore, the thermal energy generated in the conductor layer  13  can be increased. 
     Also, in the thermal radiation element  1 , a configuration is employed in which the conductor layer (conductor layer  13  in the present embodiment), out of the first conductor layer (conductor layer  15 ) and the second conductor layer (conductor layer  13 ), constituting the plasmonic perfect absorber  10  together with the insulator layer  12  and the third conductor layer (conductor layer  11 ), and the third conductor layer (conductor layer  11 ) are made of HfN. 
     HfN is known to have a high melting point and to be less likely to exhibit a eutectic reaction with a semiconductor. Therefore, according to this configuration, it is possible to suppress the eutectic reaction that can proceed with the semiconductor constituting the substrate  14 . Therefore, the operating temperature of the thermal radiation element  1  can be increased. 
     In the thermal radiation element  1 , a configuration is employed in which the insulator layer  12  is made of at least one of SiO 2 , Al 2 O 3 , and AlN. 
     According to this configuration, the insulator layer  12  having a high insulating property can easily be formed. 
     Also, in the thermal radiation element  1 , a configuration is employed in which the thickness t s  of the substrate  14  is 100 μm or more and 1 mm or less. 
     According to this configuration, the resistance value that can be generated between the electrode  161  and the electrode  162  can be adjusted to any resistance value that can easily be monitored. 
     Also, in the thermal radiation element  1 , a configuration is employed in which the substrate  14  is made of silicon. 
     According to this configuration, the cost of the substrate  14  can be suppressed. Also, by appropriately setting the doping amount of the dopant and the thickness of the substrate, it is possible to easily adjust the perpendicular resistance value of the substrate  14 . 
     The thermal radiation element module M according to an aspect of the present invention includes the thermal radiation element  1 , and the housing  20  provided with the cavity C housing the thermal radiation element  1  and the power terminals  25  and  26  supplying power to the electrodes  161  and  162  serving as an electrode pair. In the thermal radiation element module M, a configuration is employed in which, in the inside of the cavity C, at least a part of the substrate  14  is secured to the cavity C using a bonding member. Specifically, the electrodes  161  and  162  are secured to the electrode pads  21  and  22  using a bonding member (sintered silver paste), respectively. 
     The thermal radiation element module M configured as described above has a similar effect to that of the thermal radiation element  1 . Also, the thermal radiation element included in the thermal radiation element module M is not limited to the thermal radiation element  1 , and may be a thermal radiation element  1 A illustrated in  FIG.  5    or a thermal radiation element  1 B illustrated in  FIG.  6   . The thermal radiation elements  1 A and  1 B will be described below. 
     In the thermal radiation element module M, a configuration is employed in which, in a case where the opening portion AP c  of the cavity C is viewed in a planar view (refer to the upper diagram of  FIG.  1   ), the opening portion AP c  includes the thermal radiation element  1 , the opening portion AP c  is sealed by the optical window  23  having translucency, and the pressure inside the cavity C is lower than the pressure outside the cavity C. 
     According to this configuration, since the heat insulating property of the cavity C can be enhanced as compared with a case where the pressure inside the cavity C is equal to or higher than the pressure outside the cavity C, the thermal energy generated by the conductor layer  13  can be restricted from being dissipated to the outside of the cavity C. Accordingly, the energy efficiency of the entire thermal radiation element module M can be enhanced. 
     Also, in the thermal radiation element module M, the thermal radiation element  1  is heated to a predetermined operating temperature using thermal energy generated by the conductor layer  13  and the conductor layer  15  to cause electromagnetic waves (specifically, at least one of visible light, near-infrared light, mid-infrared light, and far-infrared light) caused by thermal radiation to be emitted. In this manner, the thermal radiation element module M functions as a thermal radiation light source that emits electromagnetic waves of at least one of visible light, near-infrared light, mid-infrared light, and far-infrared light. That is, a thermal radiation light source including the thermal radiation element module M is also included in the scope of the present invention. 
     Here, the effect achieved by an aspect of the present invention has been described using the thermal radiation element  1  illustrated in  FIGS.  1  and  2   . However, the effect of the thermal radiation element  1  described above can similarly be obtained in the thermal radiation elements  1 A and  1 B described below. 
     First Modification Example 
     The thermal radiation element  1 A according to a first modification example of the thermal radiation element  1  will be described with reference to  FIG.  5   .  FIG.  5    is a cross-sectional view of the thermal radiation element  1 A, which is a cross-sectional view corresponding to  FIG.  2    illustrating the thermal radiation element  1 . 
     In the thermal radiation element  1  illustrated in  FIG.  2   , the plasmonic perfect absorber  10  is provided on the conductor layer  13 , which is a conductor layer on the side on which the electrode pair  16  is not provided, out of the conductor layer  13  and the conductor layer  15 . That is, the conductor layer  13  constitutes the plasmonic perfect absorber  10  together with the insulator layer  12  and the conductor layer  11 . 
     On the other hand, in the thermal radiation element  1 A according to the present modification example, a plasmonic perfect absorber  10 A is provided on the conductor layer  15 , which is a conductor layer on the side on which an electrode pair  16 A is provided, out of the conductor layer  13  and the conductor layer  15 . That is, the conductor layer  15  constitutes the plasmonic perfect absorber  10 A together with the insulator layer  12 A and the conductor layer  11 A. Note that the insulator layer  12 A and the conductor layer  11 A constituting the plasmonic perfect absorber  10 A are configured in the same manner as the insulator layer  12  and the conductor layer  11  constituting the plasmonic perfect absorber  10 . Therefore, in the present modification example, the description thereof is omitted. 
     In this manner, in an aspect of the present invention, of the conductor layers provided on the principal plane  14   a  and the principal plane  14   b  of the substrate  14 , the conductor layer provided with the electrode pair  16  is defined as the first conductor layer, and the conductor layer not provided with the electrode pair  16  is defined as the second conductor layer, and the plasmonic perfect absorber may be provided on the first conductor layer (on the side provided with the principal plane  14   b ) or on the second conductor layer (on the side provided with the principal plane  14   a ). 
     Second Modification Example 
     The thermal radiation element  1 B according to a second modification example of the thermal radiation element  1  will be described with reference to  FIG.  6   .  FIG.  6    is a cross-sectional view of the thermal radiation element  1 B, which is a cross-sectional view corresponding to  FIG.  2    illustrating the thermal radiation element  1 . 
     In the thermal radiation element  1  illustrated in  FIG.  2   , a plasmonic perfect absorber is employed as an example of a meta-surface structure provided on one principal plane (principal plane  14   a  in  FIG.  2   ) side of the substrate  14 . 
     On the other hand, in the thermal radiation element  1 B according to the present modification example, a nanoscale rough structure is employed as an example of a meta-surface structure provided on one principal plane (principal plane  14 Ba in  FIG.  6   ) side of a substrate  14 B. By employing the nanoscale random structure as the meta-surface structure, the band of the emitted electromagnetic waves can be widened as compared with the case of employing the plasmonic perfect absorber. 
     Note that, in the thermal radiation element  1 B, the nanoscale random structure may be formed on a principal plane  14 Bb. 
     Example 
     The thermal radiation element  1  as an example of the present invention will be described below. In the present example, the thermal radiation element  1  configured as illustrated in  FIG.  2    is employed as a thermal radiation element, and each of the components is designed as follows. 
     As the substrate  14 , a plate-shaped member made of silicon, having a thickness t s  of 200 μm, and formed in a square shape, 5 mm on each side, was used. The resistivity of this silicon at room temperature was 1 Ωm. 
     As the conductor layer  15 , an HfN film having a thickness t 15  of 140 nm was used. 
     As the conductor layer  13 , an HfN film having the thickness t 13  of 140 nm was used. 
     As the insulator layer  12 , a SiO 2  film having the thickness t 12  of 180 nm was used. 
     As the conductor layer  11 , an HfN film having the thickness t 11  of 40 nm was used. Also, as the shape of each of the plurality of conductor patterns  111  constituting the conductor layer  11 , a circular shape having a diameter of 400 nm was employed. In addition, as the regular arrangement of the plurality of conductor patterns  111 , a square arrangement having a regular space of 650 nm was employed. 
     As each of the electrodes  161  and  162 , an Au film having a thickness of 500 nm was employed. In addition, as the shape of each of the electrodes  161  and  162  in a planar view, a rectangle having a width of 500 μm and a length of 5 mm was employed. 
     The electric resistivity of HfN employed in the present example was about 1×10 −3  (Ω·mm), and the electric resistivity of SiO 2  employed in the present example was about 1×10 15  (Ω·mm). Also, in the thermal radiation element  1 A in the present example, the plasmonic perfect absorber  10 A had a thickness of 340 nm in total. 
     In the present example, an operating temperature in the range of 350° C. or higher and 500° C. or lower was employed as an example of the operating temperature of the thermal radiation element  1 , and the resistance value generated between the electrode  161  and the electrode  162  was measured. Here, the voltage applied between the electrode  161  and the electrode  162  was fixed to 5 V, and the current flowing between the electrode  161  and the electrode  162  was measured. As a result, the resistance values generated between the electrode  161  and the electrode  162  were 6.3 Ω, 5.3 Ω, 4.6Ω, and 4.3Ω at 350° C., 400° C., 450° C., and 500° C., respectively. 
     In this manner, in the thermal radiation element  1 , as the temperature rises, the resistivity of silicon constituting the substrate  14  decreases while the resistivity of HfN constituting the conductor layer  13  and the conductor layer  15  slightly increases. Therefore, as the temperature increased, the resistance value generated between the electrode  161  and the electrode  162  decreased. 
     In the present example, the spectrum of the electromagnetic waves emitted by the thermal radiation element  1  was measured, and it was confirmed that near-infrared light of 1.0 μm or more and 2.0 μm or less was stably emitted. That is, it has been found that the thermal radiation element  1  in the present example can suitably be used as a thermal radiation light source that stably emits near-infrared light of 1.0 μm or more and 2.0 μm or less in a case where the operating temperature is set to 350° C. or more and 500° C. or less. 
     [Additional Remarks] 
     The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope indicated in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments are also included in the technical scope of the present invention.