JP2010230453A - Infrared radiation element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an infrared radiation element that is compact and responds speedily, includes high radiation efficiency of infrared rays to applied energy, and saves energy. <P>SOLUTION: A heat insulating layer 2 and a heat generation layer 1 are provided on one surface of a single-crystal silicon substrate 3. The heat generation layer 1 radiates infrared rays by heat generation following energization. The heat insulating layer 2 is formed of porous silicon such that thermal conductivity may become smaller than that of the substrate 3, supports the heat generation layer 1, and radiates infrared rays toward the heat generation layer 1 by at least one of a partial temperature increase by heat transfer from the heat generation layer 1 following energization of the heat generation layer 1 and reflection of infrared rays entering from the heat generation layer 1. The heat generation layer 1 transmits infrared rays radiated from the heat insulating layer 2. Thus, by utilizing infrared rays radiated from the heat insulating layer 2 together with those radiated from the heat generation layer 1, radiation efficiency of infrared rays to applied power increases. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an infrared radiation element used mainly for applications requiring high-speed infrared modulation.

  2. Description of the Related Art Conventionally, there has been provided an apparatus that irradiates a detection target with infrared radiation emitted from an infrared radiation source and detects the detection target based on the amount of infrared absorption and reflection.

  For example, an infrared gas analyzer emits infrared light emitted from an infrared radiation source to the gas to be detected (methane, carbon monoxide, carbon dioxide, etc.) and outputs a light reception signal corresponding to the amount of received infrared light. The type and concentration of the gas are detected by detecting the infrared absorption wavelength and the amount of light absorption by the gas using the element.

  The infrared light receiving element receives infrared light through a filter that selectively passes only infrared light having a wavelength that is absorbed by the gas to be detected among infrared light emitted from an infrared radiation source. Therefore, the type of gas is specified by the wavelength that selectively passes through the filter, and the gas concentration is measured according to the amount of received infrared light (the output signal level of the infrared light receiving element).

  In this type of apparatus, in order to detect the amount of light absorption in the gas to be detected, it is necessary to intermittently irradiate the gas with infrared rays multiple times in one analysis. Therefore, in order to shorten the time for detecting the detection target, it is required to shorten the time interval for intermittently using an infrared radiation source capable of high-speed modulation. Moreover, since infrared rays are irradiated a plurality of times, it is necessary to increase the amount of infrared rays emitted relative to the input energy in order to reduce the energy (electric power) required for infrared rays emission. Furthermore, in order to improve detection accuracy, it is also necessary to make the amount of infrared radiation from the infrared radiation source constant.

  Typical infrared radiation sources include a filament formed in a coil shape with a material such as tungsten or platinum, or an infrared radiation efficiency by coating the surface of a coiled core material made of tungsten or platinum with a ceramic such as alumina. There is an infrared radiation source having a configuration in which a filament having an increased height is housed and the filament is housed in a light-transmitting airtight container.

  However, since this type of infrared radiation source cannot be used in a state where the filament is exposed to the outside air, the increase in size due to the hermetic container cannot be avoided, and it is difficult to reduce the size of the apparatus. In addition, since the heat capacity including the filament and the airtight container is increased, the rise time required for the infrared rays to rise to the required intensity after turning on the power, and the fall time required for the infrared rays to decrease to the specified intensity after the power is turned off are long. In addition, steep characteristics at the rise and fall cannot be obtained.

  Therefore, with this type of infrared radiation source, the frequency at which infrared rays are interrupted can only be set to about 0.1 to 10 Hz. In addition, since the rise and fall of the infrared radiation intensity are not steep, detection errors due to waveform rounding tend to occur.

  On the other hand, as an infrared radiation source that can be miniaturized, an infrared radiation element formed using MEMS (Micro Electro Mechanical Systems) technology has been proposed as schematically shown in FIG. 10 (for example, Patent Document 1). reference). The infrared radiation element shown in the figure has a rectangular frame-shaped support substrate 21 formed by etching a silicon substrate, and is formed of a polysilicon film doped on one surface of the support substrate 21 via a thermal insulating layer 22 of silicon nitride. An infrared radiation layer 23 is formed, and an incandescent filament 25 that is a metal layer is embedded in an insulating layer 24 of silicon nitride that covers the infrared radiation layer 23. An opening 26 is formed in a portion corresponding to the back side of the infrared radiation layer 23 in the support substrate 21.

  This infrared radiating element has an electrode 27 connected to an incandescent filament 25, heats the incandescent filament 25 by energizing between the electrodes 27, and heats the infrared radiating layer 23 by the incandescent filament 25. have. The incandescent filament 25 covers the surface of the infrared radiation layer 23, but has a very narrow width to prevent the infrared emissivity from the infrared radiation layer 23 from being significantly reduced.

  In this infrared radiating element, an opening 26 is formed in the support substrate 21 on the back side of the infrared radiating layer 23, and the surroundings of the incandescent filament 25 and the infrared radiating layer 23 that contribute to infrared radiation are surrounded by highly heat insulating air. Therefore, the heat capacity difference between the portion contributing to infrared radiation and the surroundings is large, and it is possible to respond to the intermittent current supplied to the incandescent filament 25 at a relatively high speed. That is, in the infrared radiation source including the above-described airtight container, the response is about 0.1 to 10 Hz, but when the configuration of FIG. 10 is adopted, it is possible to respond to 200 Hz. It is described in Document 1.

JP-A-9-184757

  As described above, the infrared radiation source is required to have high-speed response, radiation efficiency with respect to input energy, and stabilization of the amount of infrared radiation. If the configuration described in Patent Document 1 is adopted, high-speed response is achieved as described above.

  However, since the insulating layer 24 is interposed between the incandescent filament 25 and the infrared radiation layer 23, a part of the heat energy generated from the incandescent filament 25 is absorbed by the insulating layer 24 made of silicon nitride. As a result, the infrared radiation layer 23 does not contribute to heating, and as a result, there is a problem that the amount of infrared radiation relative to the input energy cannot be sufficiently increased.

  The present invention has been made in view of the above reasons, and an object of the present invention is to provide an infrared radiation element that is small in size and capable of high-speed response and has high infrared radiation efficiency with respect to input energy and can save energy. There is to do.

  The present invention relates to an infrared radiation element, which is formed on one surface of a substrate and radiates infrared rays by heat generated by energization, and the heat generation layer is laminated on one surface to support the heat generation layer. A heat insulating layer formed on one surface, the heat insulating layer is smaller in heat conductivity than the substrate, and a part of the temperature rise due to heat transfer from the heat generating layer due to energization of the heat generating layer, and from the heat generating layer The infrared ray is emitted in a direction toward the heat generation layer by at least one of the reflection of the incident infrared ray, and the heat generation layer has a function of passing the infrared ray radiated from the heat insulating layer.

  A configuration in which the other surface of the heat insulating layer is in contact with gas may be employed.

  Further, it is desirable that the thickness dimension of the heat insulating layer is set to a dimension in which the optical path length with respect to the infrared of the target wavelength is a natural number multiple of the half wavelength of the infrared.

  Alternatively, the other surface of the heat insulating layer is in contact with the substrate, and the boundary surface between the heat insulating layer and the substrate is a reflecting surface that reflects infrared light of a target wavelength from the heat insulating layer toward the substrate, and the heat insulating layer It is also possible to adopt a configuration in which the thickness dimension is set such that the optical path length with respect to the infrared ray of the target wavelength is an odd multiple of a quarter wavelength of the infrared ray.

  The thermal insulating layer may have a structure in which macropores that emit infrared rays by cavity emission during heating are formed in a bulk semiconductor, and nanopores are formed in the macropores.

  The thermal insulating layer can be formed of an electrical insulating film containing a semiconductor oxide.

  On the other hand, the heating layer is preferably formed of a material having a negative resistance temperature coefficient.

  The heat generating layer is preferably selected from TaN and TiN.

  According to the configuration of the present invention, since the heat generation layer is supported by the heat insulating layer, disconnection due to deformation of the heat generation layer is less likely to occur, and it is possible to reduce the thickness in order to reduce the heat capacity of the heat generation layer, As a result, high-speed response is possible. In other words, since the rise time from the start of energization to the heat generation layer to the emission of infrared rays is short, when the infrared radiation is repeated, the amount of infrared radiation is almost constant every time, like an infrared gas analyzer. When applied to an inspection device, high accuracy can be achieved. In addition, in the heat insulating layer, infrared rays are emitted in the direction toward the heat generating layer by at least one of a temperature rise due to heat transfer from the heat generating layer and reflection of infrared rays incident from the heat generating layer. Since the infrared radiation radiated from the heat insulation layer is passed, a part of the energy radiated from the heat generation layer toward the heat insulation layer is used as the energy of the infrared radiation, and the infrared radiation efficiency with respect to the input power is reduced. Can be increased. In other words, the input power required to emit a desired amount of infrared rays is reduced.

  In the configuration in which the heat generation layer is arranged on one surface of the heat insulation layer and the other surface is in contact with the gas, heat dissipation from the heat insulation layer is suppressed by the heat insulation of the gas, so that the heat is transmitted from the heat generation layer to the heat insulation layer. The temperature rise of the heat insulating layer due to heat is made uniform, and the amount of infrared radiation is increased.

  If the thickness of the thermal insulation layer is set so that it has an optical path length that is a natural number multiple of half the wavelength of the infrared ray of the target wavelength, the resonance condition is established in the thermal insulation layer for the infrared ray of the target wavelength. The relative radiant intensity can be increased for infrared wavelengths. In this embodiment, it is desirable to provide a reinforcing portion that reinforces the thermal insulating layer in order to suppress thermal deformation due to heat generation.

  Further, in the configuration in which the heat insulating layer is interposed between the heat generating layer and the substrate, the boundary surface between the insulating layer and the substrate becomes a reflecting surface, so that an odd number of a quarter wavelength with respect to the target wavelength infrared ray. By setting the thickness of the thermal insulation layer so that it has a double optical path length, resonance conditions are established in the thermal insulation layer for the infrared of the target wavelength, and the relative radiation intensity is increased for the infrared of the target wavelength. be able to.

  In a structure in which the thermal insulating layer has macropores that emit infrared rays by cavity radiation and nanopores are formed in the macropores, infrared radiation efficiency can be improved by utilizing the cavity radiation by the macropores. In addition, since the nanopores are formed in the macropores, it is possible to compensate for the decrease in strength of the thermal insulating layer that accompanies the formation of the macropores without inhibiting the cavity radiation by the macropores. Furthermore, by using a porous semiconductor having nanopores in the macropores as the thermal insulating layer, high thermal insulation performance can be obtained in the thermal insulating layer.

  In a configuration in which a thermal insulating layer is formed of an electrical insulating film containing a semiconductor oxide, the thermal insulating layer may be formed by oxidizing the semiconductor by a process such as thermal oxidation or performing CVD of a material containing the oxide. Therefore, the manufacturing process is relatively simple, and the mass productivity can be increased as compared with the case where the semiconductor is made porous.

  When the heat generating layer is formed using a material having a negative resistance temperature coefficient, the sheet resistance decreases as the temperature increases even if the driving voltage is the same, and the current flowing through the heat generating layer 1 increases. Therefore, the input power increases as the temperature rises, and the maximum temperature reached can be increased. In addition, when the power supply voltage is boosted by the booster circuit in order to obtain a voltage to be applied to the heat generating layer, an increase in the booster ratio of the booster circuit can be suppressed while increasing the maximum temperature reached. Can reduce power loss.

  In the configuration using either TaN or TiN as the material of the heat generating layer, the heat generating layer has high oxidation resistance. Therefore, the heat generating layer can be exposed to the air and used. In other words, it is not necessary to use in a vacuum or inert gas, the package structure is simplified, no window material for sealing is required, there is no attenuation of infrared by the window material, Use efficiency can be increased. Furthermore, since these materials can adjust the sheet resistance by adjusting the nitrogen content, it is possible to adjust the thickness dimension necessary to obtain the desired sheet resistance to form a heat generating layer with a small heat capacity. As a result, high-speed response is possible.

1 is a cross-sectional view showing a first embodiment. It is a figure which shows a manufacturing process same as the above. It is a figure which shows the characteristic of the heat generating layer used same as the above. It is a figure which shows the temperature characteristic of the heat insulation layer used for the same as the above. It is a figure which shows the emissivity of the heat generating layer used for the same as the above. It is a top view of the heat generating layer used for the same as the above. It is sectional drawing which shows the structure of the heat insulation layer used for the same as the above. It is a figure which shows the emissivity of the heat-generating layer used for Embodiment 2. (A) and (b) are sectional views showing Embodiment 3. It is sectional drawing which shows a conventional structure.

(Embodiment 1)
As shown in FIG. 1, the infrared radiation element of the present embodiment includes a heat generation layer 1 that generates heat by energization and emits infrared light, and a heat insulating layer 2 that supports the back surface of the heat generation layer 1 on one surface of the substrate 3. It has a configuration. Here, the term “one surface” with respect to the substrate 3 means not a surface itself but a region in contact with the surface. Therefore, the heat generating layer 1 and the heat insulating layer 2 are formed on one surface of the substrate 3.

  The substrate 3 is a semiconductor substrate (for example, a single crystal silicon substrate) and is formed in a rectangular parallelepiped shape. The thermal insulating layer 2 is formed so that the thermal conductivity is sufficiently smaller than that of the substrate 3. The heat insulating layer 2 is formed in the depth direction on the one surface of the substrate 3, and one surface of the heat insulating layer 2 (upper surface in FIG. 1) is flush with one surface of the substrate 3 (upper surface in FIG. 1). Is formed.

  Specifically, the thermal insulating layer 2 is formed by making the substrate 3 porous by subjecting the region of the substrate 3 to the region where the peripheral portion of the one surface is left to be anodized. The conditions for anodic oxidation (current density, processing time, etc.) are appropriately set according to the conductivity type and conductivity of the substrate 3.

  Anodization is performed in an aqueous hydrogen fluoride solution, and is formed as a porous semiconductor layer (for example, a porous silicon layer) having a porosity of approximately 70%. The conductivity type of the substrate 3 may be either p-type or n-type. However, the p-type silicon substrate has a porosity higher than that of the n-type silicon substrate when anodized. Since it tends to be large, it is desirable to use a p-type silicon substrate for the substrate 3.

  Furthermore, in order to reduce the thermal conductivity in the heat insulating layer 2, a part or all of the heat insulating layer 2 may be oxidized or nitrided. If the oxidation or nitridation is performed, the electrical insulation is increased. The heat insulating layer 2 made porous by anodization has the characteristics that the heat capacity and the heat conductivity are small, the heat resistance is high, and the surface is smooth.

  On the other hand, a semiconductor oxide film may be formed by thermal oxidation instead of making it porous by anodic oxidation, and this semiconductor oxide film may be used for the thermal insulating layer 2. In the case where a semiconductor oxide film is used as the thermal insulating layer 2, if the thermal insulating layer 2 is formed by thermal oxidation or the thermal insulating layer 2 is formed by CVD using a material containing an oxide, it is compared with the porous structure. This simplifies the manufacturing process and increases the productivity. When forming the thermal insulation layer 2 by CVD, it is possible to use an oxide with high thermal insulation such as alumina, or a material containing this kind of oxide. Furthermore, a porous body of this kind of material can be formed as the heat insulating layer 2.

  The heat generating layer 1 is formed on one surface of the substrate 3 with the substrate 3 via the thermal insulating layer 2. The heat generating layer 1 is provided with a pair of electrodes 4 made of a highly conductive metal material (for example, aluminum). In FIG. 1, the electrodes 4 are provided so as to be separated from each other on the left and right. The shape of the heat generating layer 1 can be set as appropriate. For example, the heat generating layer 1 can be formed in a single rectangular shape, and the electrodes 4 can be formed on two opposite sides. Alternatively, the heating layer 1 having a plurality of strips that are long in the left-right direction in FIG. 1 and having one end in the longitudinal direction of each strip connected to each electrode 4 may be formed. In either case, by applying a voltage between the electrodes 4, the heat generating layer 1 can be energized to generate heat and emit infrared rays.

  The material of the heat generating layer 1 is preferably selected from TaN and TiN. These materials are excellent in heat resistance and oxidation resistance. Therefore, the heat generating layer 1 can be used in an air atmosphere, and the infrared radiation element can be mounted on the substrate as a bare chip without being housed in the package. In addition, even when housed in a package, it is not necessary to seal the window hole formed in the package in order to transmit infrared light, and there is no attenuation of infrared light by the window member attached to the window hole, so that infrared radiation efficiency is increased. Can do.

  Further, these materials have a physical property that the sheet resistance becomes a desired value (described later) in a thickness dimension (several tens of nm) suitable for forming the heat generating layer 1. In addition, the sheet resistance can be controlled by the partial pressure of nitrogen gas during film formation. However, materials other than TaN and TiN can be used as the material for forming the heat generating layer 1, and other metal nitrides or metal carbides may be used.

  In the infrared radiating element configured as described above, when a predetermined voltage is applied to both electrodes 4 and the heat generating layer 1 is energized, the heat generating layer 1 generates heat to emit infrared rays. Further, when the energization to the heat generating layer 1 is stopped, the temperature of the heat generating layer 1 is lowered and the infrared radiation is stopped. It is possible to increase the temperature during the voltage increase period and decrease the temperature during the voltage decrease period not only when the voltage applied to the heat generating layer 1 is intermittently applied but also when a voltage changing in a sine wave shape is applied. . That is, the intensity of infrared light can be modulated according to the voltage applied to the electrode 4.

By the way, assuming that a sinusoidal voltage is applied to the electrode 4 of the infrared radiation element having the above-mentioned configuration, the thermal conductivity of the thermal insulating layer 2 is αp [W / mK], and the volumetric heat capacity (specific heat capacity and If the product (density) is Cp [J / m ³ K] and the frequency at which the heat generating layer 1 can respond (twice the frequency of the applied voltage) is f [Hz], the heat diffusion length μ of the heat insulating layer 2 Is expressed by the following equation.
μ = (2αp / ωCp) ^1/2 (1)
However, ω = 2πf.

  It is necessary to set the thickness dimension Lp of the heat insulating layer 2 so that the temperature amplitude is attenuated and the heat is not radiated to the substrate 1 when heat that varies in an alternating manner is applied from the heat generating layer 1. That is, it is desirable to set the thickness dimension Lp of the thermal insulation layer 2 to a value at least larger than the thermal diffusion length μ (Lp> μ), but if it is too large, the temperature amplitude ratio decreases due to heat storage in the thermal insulation layer 2. Therefore, optimization is necessary.

Now, it is assumed that the heat insulating layer 2 is formed of porous silicon, and the frequency f at which the heat generating layer 1 can respond, the volumetric heat capacity Cp of the heat insulating layer 2, and the heat conductivity αp of the heat insulating layer 2 are f = 10 [ kHz], αp = 1.1 [W / mK], Cp = 1.05 × 106 [J / m3K] and substituting into the equation (1) results in Lp = 5.8 × 10 ⁻⁶ [m] . Therefore, if the thickness dimension Lp of the thermal insulating layer 2 is set to 10 [μm], it becomes possible to respond to a sinusoidal voltage of 10 [kHz] or more. Thus, by using the equation (1), the thickness dimension Lp of the heat insulating layer 2 can be set so as not to hinder the temperature rise of the heat generating layer 1.

The peak wavelength λ [μm] of infrared rays radiated from the heat generating layer 1 satisfies the Vienna displacement law, and the absolute temperature T [K] of the heat generating layer 1 has the following relationship.
λ = 2898 / T (2)
Therefore, by changing the temperature of the heat generating layer 1, the peak wavelength of infrared rays emitted from the heat generating layer 1 can be changed. In order to adjust the temperature of the heat generating layer 1, the amplitude and waveform of the voltage applied to the heat generating layer 1 are adjusted to change the Joule heat generated per unit time.

  For example, by applying a sinusoidal voltage of about 100 V between both electrodes 4, it is possible to design to emit infrared light having a peak wavelength of 3 to 4 [μm]. If the voltage is adjusted, It is also possible to emit infrared rays having a peak wavelength of 4 [μm] or more.

  By the way, when a voltage is applied to the heat generating layer 1 to generate heat, the temperature of the heat insulating layer 2 rises due to heat conduction. That is, when the temperature of the heat generating layer 1 rises, the heat insulating layer 2 is heated by infrared rays radiated to the back surface of the heat generating layer 1. Infrared radiation radiated to the back surface of the heat generating layer 1 is not used as infrared radiation radiated from the infrared radiation element, which causes a reduction in infrared radiation efficiency with respect to input power.

  Therefore, in the present embodiment, the heat generating layer 1 has a function of transmitting infrared rays. Specifically, the material described above is selected, and the thickness dimension of the heat generating layer 1 is set sufficiently smaller than the target infrared wavelength (by about two orders of magnitude) to provide infrared transparency. .

  In the configuration of the present embodiment, when the heat generating layer 1 is energized by applying a voltage to the electrode 4, infrared rays are emitted forward (upward in FIG. 1) from the heat generating layer 1 as indicated by E1 in FIG. At the same time, the heat insulating layer 2 is heated by heat transfer from the heat generating layer 1 to the heat insulating layer 2, and as indicated by E2 in FIG. Infrared rays are also emitted. Since the heat insulating layer 2 supports the heat generating layer 1, heat is directly transferred from the heat generating layer 1, and it can be said that the heat generating layer 1 constitutes a direct heat type infrared radiation source using the heat generating layer 1 as a heat source.

  Here, since the heat generating layer 1 has infrared transparency, infrared rays radiated from the heat insulating layer 2 toward the heat generating layer 1 are transmitted through the heat generating layer 1 and emitted to the front of the heat generating layer 1. . That is, the infrared radiation element radiates the infrared ray E1 emitted forward from the heat generating layer 1 and the infrared ray E2 emitted from the heat insulating layer 2 through the heat generating layer 1 and emitted forward of the heat generating layer 1. As a result, the infrared radiation efficiency with respect to the input power can be increased.

  Hereinafter, a process for manufacturing the infrared radiation element having the structure shown in FIG. 1 will be briefly described. First, as shown in FIG. 2A, a p-type silicon substrate 11 having a specific resistance of about 80 to 120 Ωcm is subjected to an oxidation process (pyro-oxidation) to thereby have a rectangular opening region made of a silicon oxide film. An anodic oxidation mask 12 is formed. Next, as shown in FIG. 2B, after the silicon oxide film on the back surface of the silicon substrate 11 is removed, an aluminum electrode 13 for back contact is formed by sputtering.

  Further, as shown in FIG. 2C, the opening region of the anodic oxidation mask 12 is anodized to form the thermal insulating layer 2 made of porous silicon. In the anodizing treatment, an electrolytic solution obtained by mixing a hydrogen fluoride aqueous solution and ethanol so that hydrogen fluoride is 30% is used, and only the surface of the silicon substrate 11 to be anodized is brought into contact with the electrolytic solution.

  Thereafter, a platinum electrode (not shown) is disposed on one surface of the silicon substrate 11 in the thickness direction on which the anodization mask 12 is formed, and set on a jig that can be energized on the other surface in the thickness direction. Then, anodization is performed by flowing a current having a predetermined current density for a predetermined time.

The current density and the time during which the current flows are adjusted according to the target porosity and thickness dimension of the heat insulating layer 2. For example, in order to form a thermal insulation layer having a porosity of about 70%, the current density is set to 100 [mA / cm ² ].

After the thermal insulating layer 2 is formed, the heat generating layer 1 is formed in the opening region surrounded by the anodic oxidation mask 12 as shown in FIG. The central portion of the heat generating layer 1 is formed in the opening region, and the end of the heat generating layer 1 is formed on the anodizing mask 12 in the peripheral portion of the opening region. When the heat generating layer 1 is formed of TaN, a reactive sputtering method is employed, a Ta target is used, and TaN is formed in a mixed atmosphere of Ar gas and N ₂ gas.

If the heat generating layer 1 is formed by the reactive sputtering method, the composition of TaN can be controlled by the input power and the partial pressure of N ₂ gas, so that the specific resistance of the heat generating layer 1 can be controlled. Become. That is, the sheet resistance value of the heat generating layer 1 can be adjusted to a desired value depending on the film forming conditions. The thickness dimension of the heat generating layer 1 is, for example, 40 [nm], and the sheet resistance is, for example, 300 [Ωsq] at the highest temperature achieved during driving.

  After the heat generation layer 1 is formed, the electrodes 4 are formed on both ends of the heat generation layer 1 as shown in FIG. For the formation of the electrode 4, for example, a vapor deposition method using a metal mask can be employed.

  By the way, as shown in FIG. 3, the reflectance (curve A), the transmittance (curve B), and the absorptance (curve C) of the heat generating layer 1 with respect to infrared rays of the wavelength of interest depend on the sheet resistance of the heat generating layer 1. It has been theoretically shown to change. On the other hand, according to Kirchhoff's law, it is known that the absorptivity and emissivity are equal when in a thermal equilibrium state.

  Further, in the configuration of the present embodiment, as indicated by a curve A in FIG. 4, the thermal insulating layer 2 has a temperature distribution in the thickness direction (the depth direction from the heat generating layer 1), and the back side of the thermal insulating layer 2. Since the substrate 3 is provided, the temperature difference between the portion of the thermal insulating layer 2 close to the heat generating layer 1 (left end portion in FIG. 4) and the portion close to the substrate 3 (right end portion in FIG. 4) becomes large.

That is, it is expected that the temperature of the heat insulating layer 2 rises as the heat generating layer 1 generates heat, and infrared rays are also emitted from the heat insulating layer 2. Therefore, assuming that the transmittance of the heat generating layer 1 with respect to infrared rays of the wavelength of interest is Th, the emissivity is Eh, and the emissivity of the heat insulating layer 2 is Ei, the apparent emissivity E of the heat generating layer 1 is expressed by the following equation. Can do.
E = Eh + Th · Ei (3)
Here, the emissivity Ei of the heat insulating layer 2 is a conversion value with respect to the radiant power when the temperature is the same as that of the heat generating layer 1. When Ei = 0.5, the apparent emissivity E of the heat generating layer 1 with respect to the sheet resistance of the heat generating layer 1 is calculated as shown in FIG. As can be seen from a comparison between FIG. 3 and FIG. 5, the maximum value of the emissivity is a combination of the heat generating layer 1 and the heat insulating layer 2 as in this embodiment, rather than the case where only the heat generating layer 1 is used alone. This is much better when used for infrared radiation.

  In the illustrated example, the maximum value of the emissivity when the heat generating layer 1 is used alone is about 0.5, and the sheet resistance at this time is 187 Ωsq. Further, when the heat generating layer 1 and the heat insulating layer 2 are used in combination with infrared radiation, the maximum value of the emissivity is 0.714, and the sheet resistance at this time is 471 Ωsq.

  By the way, when the infrared radiation intensity is controlled by the driving voltage applied to the heat generating layer 1, if the power supplied to the heat generating layer 1 is the same, the driving voltage can be reduced as the heat generating layer 1 has a lower sheet resistance. . If the driving voltage is low, loss due to boosting can be reduced and the electric field strength in the infrared radiation element can be reduced to reduce the possibility of breakage. Therefore, it is desirable that the sheet resistance is small. Here, since the sheet resistance range in which the emissivity exceeds 0.7 is about 300 to 800 Ωsq, the sheet resistance is set to 300 Ωsq which is the lower limit of this range. In this case, the apparent emissivity (emissivity when the heat-generating layer 1 and the heat insulating layer 2 are used in combination) is improved by 40% compared to when the heat-generating layer 1 is used alone.

  In order to be usable as an infrared radiation element, in the range where the emissivity E is 0.25 or more, the condition that the heat generation layer 1 does not break due to heat generated by energization (the sheet resistance does not become too large) is set. In order to satisfy this condition, it is desirable to set the sheet resistance of the heat generating layer 1 during infrared radiation to 30 to 1000 Ωsq.

  Furthermore, the heat generating layer 1 has a negative resistance temperature coefficient in which the sheet resistance decreases as the temperature increases. Therefore, even if the drive voltage is the same, the sheet resistance decreases as the temperature rises, and the current flowing through the heat generating layer 1 increases. That is, the input power increases with the temperature rise, and the ultimate temperature can be increased.

By the way, TaN is used for the heat generating layer 1 and the resistance temperature coefficient is set to −0.001 [° C. ⁻¹ ], the maximum temperature reached during driving is set to 500 [° C.], and the sheet resistance at that temperature is 300 [Ωsq ], The sheet resistance at room temperature is 571 [Ωsq].

  As described above, when the heat generating layer 1 has a negative resistance temperature coefficient, the power supply voltage is boosted by a booster circuit in order to obtain a voltage to be applied to the heat generating layer 1, while increasing the maximum temperature reached. An increase in the boost ratio of the booster circuit can be suppressed, and power loss in the booster circuit can be suppressed.

  Further, as shown in FIG. 6, without changing the sheet resistance at room temperature, it is possible to reduce the driving voltage with respect to the input power by changing the shape of the heat generating layer 1.

  Now, as shown in FIG. 6A, the heat generating layer 1 whose planar shape is a square shape is a basic shape. With respect to this basic shape, as shown in FIG. 6B, the resistance value of the heat generating layer 1 can be reduced by making the width dimension of the heat generating layer 1 in the direction connecting the two electrodes 4 smaller than the basic shape. Here, the sheet resistance is not changed. In this configuration, the applied voltage can be reduced while keeping the same input power per unit area of the heat generating layer 1 correlated with the heat generation state.

  Further, as shown in FIG. 6C, three electrodes 4 may be provided to apply a voltage between the central electrode 4 and the electrodes 4 at both ends. In this configuration, the heat generating layers 1 shown in FIG. 6B are connected in parallel, and the resistance value is further reduced as compared with the configuration of FIG. 6B with the same sheet resistance as the configuration of FIG. 6B. Can be made. That is, it is possible to reduce the applied voltage and suppress an increase in the boost ratio of the booster circuit, thereby suppressing power loss. In addition, in the shape of FIG. 6C, the shape of the region that emits infrared rays is square, and the light emission pattern is square, so that optical handling is easy.

  By the way, in the present embodiment, as described above, the porous semiconductor obtained by making the substrate 3 porous by anodic oxidation is used as the heat insulating layer 2, and in addition to generating infrared rays by the heat generated by the heat generating layer 1, the heat generating layer 1 Although the infrared emissivity is increased by generating infrared rays by heating the thermal insulating layer 2, the infrared emissivity can be further increased by adopting the following structure as the thermal insulating layer 2. It becomes possible.

That is, as shown in FIG. 7, the thermal insulation layer 2 has several μm macropores (not shown, but the surface of the bulk silicon is microscopically undulated). It has a structure in which nanopores of nm are formed. This configuration is formed by adjusting the conductivity type, specific resistance, and anodic oxidation conditions (electrolyte composition, current density, processing time) of the semiconductor substrate 11 during anodic oxidation. For example, a p-type silicon substrate having a high resistance of about 100 Ωcm is used as the semiconductor substrate 11, and a hydrofluoric acid solution having a high concentration of hydrofluoric acid of about 25% is used as an anodic oxidation condition, and a comparison of about 100 mA / cm ² is performed. What is necessary is just to process with a large current density.

  When the structure shown in FIG. 7 is adopted as the heat insulating layer 2, when the heat insulating layer 2 is heated by the heat from the heat generating layer 1, cavity radiation is generated by macropores, and the infrared emissivity can be further increased. In addition, since the nanopores are formed in the macropores, only nano-sized fine irregularities are generated on the surface of the heat insulating layer 2, and the smoothness of the surface of the heat insulating layer 2 can be maintained. . That is, when the heat generating layer 1 is formed, the surface state of the heat insulating layer 2 hardly affects the heat generating layer 1, so that the heat generating layer 1 can be formed to a thickness of about several tens of nm.

  In order for the thermal insulating layer 2 to contribute to infrared radiation, it is necessary to set the thickness dimension of the thermal insulating layer 2 to 0.5 μm or more. This thickness dimension is set by the relationship with the frequency of the voltage applied to the heat generating layer 1 using the equation (1).

  By adopting the configuration of the present embodiment, the heat capacity of the heat generating layer 1 can be reduced, and the thermal insulation of the heat insulating layer 2 is increased, so that a rise in voltage application to the heat generating layer 1 is achieved. Response is better. Moreover, not only infrared rays radiated by the heat generated by the heat generating layer 1 but also infrared rays radiated from the heat insulating layer 2 by heating the heat insulating layer 2 with heat to the back of the heat generating layer 1 can be used. Since the emissivity is increased, the infrared radiation intensity can be increased even with single pulse energization. Further, in the configuration in which the macropore for performing infrared cavity emission of the wavelength of interest is provided in the heat insulating layer 2, it is possible to emit infrared rays with higher intensity.

(Embodiment 2)
In the present embodiment, in order to further increase the utilization efficiency of infrared rays radiated to the back of the heat generating layer 1, the infrared rays are reflected at the boundary surface between the heat insulating layer 2 and the substrate 3, and the reflected infrared rays are reflected on the heat generating layer 1. A configuration that radiates forward is adopted. In order to reflect infrared rays at the interface between the thermal insulating layer 2 and the substrate 3, a material that is transparent to the infrared rays and has a refractive index smaller than that of the substrate 3 is selected as the material of the thermal insulating layer 2. .

Now, assuming that the transmittance of the heat generating layer 1 is Th, the emissivity of the heat generating layer 1 is Eh, and the reflectance at the interface between the heat insulating layer 2 and the substrate 3 is Rs, the apparent emissivity E of the heat generating layer 1 is Is expressed by the following equation.
E = Eh + Th · Eh · Rs (3)
Now, assuming that the reflectance Rs = 0.8, the apparent emissivity E of the heat generating layer 1 with respect to the sheet resistance of the heat generating layer 1 is calculated as shown in FIG. As can be seen from a comparison between FIG. 3 and FIG. 8, the maximum emissivity is higher when the heat generating layer 1 and the heat insulating layer 2 are used in combination than when the heat generating layer 1 is used alone. When the reflection at the boundary surface between the substrate 3 and the substrate 3 is used, the number is significantly increased.

  In the example shown in FIG. 8, the maximum value of the emissivity when the heat generation layer 1 and the heat insulating layer 2 are used in combination with infrared radiation and the reflection at the boundary surface between the heat insulating layer 2 and the substrate 3 is used. The sheet resistance at this time is 270 Ωsq. That is, the emissivity is improved by 21% compared to when the heat generating layer 1 is used alone (maximum emissivity is 0.5).

  As in the first embodiment, in the range where the emissivity E is 0.25 or more, if the condition that the heat generating layer 1 is not disconnected due to the heat generated by energization (the sheet resistance does not become too large) is set, heat generated during infrared radiation It is desirable that the sheet resistance of the layer 1 is 30 to 1000 Ωsq.

  In the configuration of the present embodiment, a porous semiconductor that is made porous without forming macropores is used as the thermal insulating layer 2. That is, there is no effect of cavity radiation by the heat insulating layer 2, and when silicon having a small infrared absorption rate is used as the semiconductor, the infrared light transmittance of the heat insulating layer 2 is that of the porous semiconductor forming the heat insulating layer 2. The infrared absorption characteristics of gas molecules existing in the pores (gap formed between nano-sized fine particles = nanopores) will dominate. Therefore, if the gas contained in the heat insulating layer 2 is air or nitrogen that hardly absorbs infrared rays, the heat insulating layer 2 becomes substantially transparent to infrared rays.

By the way, in view of the fact that infrared rays are reflected at the boundary surface between the thermal insulating layer 2 and the substrate 3, when the infrared rays are resonated by the thermal insulating layer 2, the intensity of infrared rays emitted from the thermal insulating layer 2 can be increased. It is considered possible. If the thickness dimension of the thermal insulation layer 2 is Lp [m] and the refractive index of the thermal insulation layer 2 is n, the optical path length is n · Lp, so the wavelength of the target infrared ray in vacuum is λ [m]. If so, the resonance condition is expressed by the following equation.
n · Lp = (2m−1) λ / 4 (4)
Here, m is a positive integer. For example, if the porosity of the heat insulating layer 2 is 70%, the refractive index is 1.35 when the target infrared wavelength is 4 μm. Therefore, if m = 5, the heat insulating layer 2 The thickness dimension Lp is 6.7 μm. This thickness dimension Lp satisfies the required thickness dimension Lp of the thermal insulation layer 2 of 5.8 μm or more in the driving conditions and the thermal properties of the thermal insulation layer 2 described in the first embodiment.

  As described in the first embodiment, the microstructure formed by anodic oxidation is adjusted by the conductivity type, specific resistance, and anodic oxidation conditions (electrolyte composition, current density, time) of the semiconductor substrate 11. be able to. For example, by using a low-resistance p-type silicon substrate as the semiconductor substrate 11 and increasing the concentration of hydrofluoric acid as the composition of the electrolytic solution, the thermal insulating layer 2 having only nanopores can be formed. Other configurations and operations are the same as those of the first embodiment.

(Embodiment 3)
In each of the above-described embodiments, an example in which the thermal insulating layer 2 is formed on one surface of the substrate 3 has been described. However, in this embodiment, a configuration in which a gas contacts the back surface of the thermal insulating layer 2 is employed. Specifically, the gas layer 5 is formed between the thermal insulating layer 2 and the substrate 3 as shown in FIG. 9A, or the back side of the thermal insulating layer 2 in the substrate 3 as shown in FIG. 9B. The opening 6 penetrating the front and back is formed in the part to be.

  In the structure of FIG. 9A, after the thermal insulating layer 2 made of porous silicon is formed by anodic oxidation, the gas layer 5 is formed on the back side of the thermal insulating layer 2 by anodic oxidation. The thermal insulating layer 2 and the gas layer 5 are formed by changing conditions such as current density during anodic oxidation. For example, when forming the thermal insulating layer 2, the current density is set to be relatively small under the porous condition, and when forming the gas layer 5, the current density is set to be large so as to be the condition for electrolytic polishing. After the thermal insulation layer 2 is formed, since the thermal insulation layer 2 has a higher resistance than the substrate 1 due to the porous structure, the gas layer 5 is formed while leaving the thermal insulation layer 2 under electrolytic polishing conditions. Can be formed.

  Further, in order to form the structure of FIG. 9B, a trench is formed from the back surface of the substrate 1 by anisotropic etching using an alkaline solution such as KOH, leaving several μm to the heat insulating layer 2. After the anisotropic etching is completed, reactive ion etching is performed to open the opening 6 to the thermal insulating layer 2.

  By the way, in Embodiment 1, although the thickness dimension of the heat insulation layer 2 is set larger than the dimension Lp calculated | required by (1) Formula, in this embodiment, the thickness dimension of the heat insulation layer 2 is set to (1) Formula. Is set to be smaller than the dimension Lp required in the above.

  That is, in Embodiment 1, it is necessary to set the thickness dimension of the thermal insulating layer 2 to be larger than the dimension Lp so that infrared rays radiated from the heat generating layer 1 toward the thermal insulating layer 2 do not reach the substrate 3. On the other hand, in the present embodiment, the thickness dimension of the heat insulating layer 2 is determined from the dimension Lp so that the infrared rays emitted from the heat generating layer 1 toward the heat insulating layer 2 are in contact with the gas on the back surface of the heat insulating layer 2. Is set too small.

  Considering the same conditions as in the first embodiment, it is assumed that the heat insulating layer 2 is formed of porous silicon and a sinusoidal voltage is applied to the heat generating layer 1. 2 and the thermal conductivity αp of the thermal insulating layer 2 are f = 10 [kHz], αp = 1.1 [W / mK], and Cp = 1.05 × 106 [J / m3K], respectively. (1), Lp = 5.8 [μm], and in this embodiment, it is necessary to make the thickness dimension of the thermal insulating layer 2 smaller than this dimension Lp (= 5.8 [μm]). is there.

Furthermore, in order to increase the infrared radiation efficiency, it is necessary to satisfy the resonance condition represented by the following equation, where λ [m] is the wavelength of the desired infrared ray in vacuum and n is the refractive index of the thermal insulating layer 2. desirable.
n · Lp = mλ / 2 (5)
However, m is a natural number.

  The thickness Lp of the thermal insulating layer 2 formed of porous silicon in the present embodiment is 1 if the desired infrared wavelength is 4 [μm], and n = 1.35 and m = 1 in the equation (5). .5 [μm]. That is, since 1.5 [μm] <5.8 [μm], the conditions required in the present embodiment can be satisfied.

  That is, the heat insulating layer 2 does not hinder the temperature rise of the heat generating layer 1 and can reduce the volumetric heat capacity of the heat generating layer 1 and the heat insulating layer 2 as a whole. Therefore, the heat generating layer 1 can respond to a change in the applied voltage at a high speed, and can increase the modulation frequency of the applied voltage.

  In addition, by forming the thermal insulating layer 2 from porous silicon, the volumetric heat capacity is reduced and the thermal response time is shortened, so that the heating efficiency of the heat generating layer 1 is further increased. In addition, in this embodiment, the back surface of the thermal insulation layer 2 is in contact with the gas, and since the gas generally has a lower thermal conductivity than the thermal insulation layer 2, the heat insulation is performed by insulating the back surface of the thermal insulation layer 2. The heat conduction path from the insulating layer 2 to the periphery of the heat generating layer 1 is reduced, and heat dissipation to the peripheral portion of the heat generating layer 1 is suppressed. Therefore, as shown by curve B in FIG. 3, the temperature of the heat insulating layer 2 supporting the heat generating layer 1 rises so that a large temperature difference does not occur in the depth direction when the heat generating layer 1 generates heat. .

By the way, when the gas layer 5 is formed like the structure of FIG. 9A, it is desirable to set the thickness dimension Lg of the gas layer 5 under the following conditions. That is, the voltage applied to the heat generating layer 1 is sinusoidal, the frequency of the applied voltage is f [Hz], the thermal conductivity of the gas layer 5 is αg [W / mK], and the volumetric heat capacity of the gas layer 5 is Cg [J / m ³ K], the thickness dimension Lg of the gas layer 5 is set to a range represented by the following formula.
0.05Lg '<Lg <3Lg' ... (6)
However, Lg ′ = (2αg / ωCg) ^1/2 and ω = 2πf.

For example, the voltage applied to the heat generating layer 1 is a sine wave with a frequency f = 10 [kHz], and the thermal conductivity αg and volumetric heat capacity Cg of the gas layer 5 are αg = 0.0254 [W / mK] and Cg = Assuming that 1.21 × 10 ³ [J / m ³ K], 1.3 (μm) <Lg <77.5 [μm] from the formula (5), the thickness dimension Lg of the gas layer 5 is For example, by setting to 25 [μm], the condition of the expression (5) can be satisfied. Desirably, the thickness is set so that the temperature amplitude ratio is maximized within this range.

  The gas layer 5 having the configuration shown in FIG. 9A has either a heat insulating property or a heat radiating function depending on the temperature and the thickness Lg of the heat insulating layer 2 if the temperature of the substrate 3 is constant. Therefore, by appropriately adjusting the thickness dimension Lg of the gas layer 5 in the condition range of the formula (5), the gas layer 1 is provided with heat insulation during the period when the voltage applied to the heat generating layer 1 increases, During the period in which the voltage applied to the heat generating layer 1 falls, the gas layer 1 can be provided with heat dissipation.

  That is, the timing of using the heat insulation and heat dissipation of the gas layer 5 can be made to substantially coincide with the timing of increase / decrease of the voltage applied to the heat generating layer 1, and the voltage applied to the heat generating layer 1 is modulated at a high frequency. Even in the case where the temperature is set, the temperature of the heat generating layer 1 can be changed so as to be substantially synchronized with the frequency of the voltage. That is, it becomes possible to improve responsiveness compared with the case where the gas layer 2 is not provided.

  Moreover, when the heat generating layer 1 heats the heat insulating layer 2, at least a part of infrared rays radiated to the back side of the heat generating layer 1 is radiated forward of the heat generating layer 1, as in the first embodiment. Thus, the infrared radiation efficiency with respect to the input power can be increased. Furthermore, by setting the thickness dimension Lp of the thermal insulating layer 2 so as to satisfy the resonance condition, it is possible to further increase the infrared radiation intensity using the resonance. Other configurations and operations are the same as those in the first and second embodiments.

1 Heat generation layer 2 Thermal insulation layer 3 Substrate 4 Electrode 5 Gas layer

Claims (8)

  A heat generating layer that is formed on one surface of the substrate and emits infrared rays due to heat generated by energization, and a heat insulating layer formed on one surface of the substrate to support the heat generating layer by laminating the heat generating layer on one surface. The thermal insulation layer has a thermal conductivity smaller than that of the substrate, and due to at least one of a rise in temperature due to heat transfer from the heat generation layer accompanying energization of the heat generation layer and reflection of infrared rays incident from the heat generation layer, An infrared radiation element characterized in that infrared radiation is emitted in a direction toward the heat generation layer, and the heat generation layer has a function of allowing infrared radiation radiated from the heat insulating layer to pass therethrough.   The infrared radiation element according to claim 1, wherein the other surface of the thermal insulating layer is in contact with a gas.   2. The infrared radiation element according to claim 1, wherein a thickness dimension of the heat insulating layer is set such that an optical path length with respect to an infrared ray having a target wavelength is a natural number multiple of a half wavelength of the infrared ray.   The other surface of the thermal insulation layer is in contact with the substrate, and a boundary surface between the thermal insulation layer and the substrate is a reflection surface that reflects infrared rays of a target wavelength from the thermal insulation layer toward the substrate, and the thermal insulation layer The infrared radiation element according to claim 1, wherein the thickness dimension of the infrared radiation element is set such that an optical path length with respect to an infrared ray having a target wavelength is an odd multiple of a quarter wavelength of the infrared ray.   5. The heat insulating layer has a structure in which macropores that emit infrared rays by cavity emission during heating are formed in a bulk semiconductor, and nanopores are formed in the macropores. 6. The infrared radiation element described in 1.   The infrared radiation element according to claim 1, wherein the thermal insulation layer is formed of an electrical insulation film containing a semiconductor oxide.   The infrared radiation element according to claim 1, wherein the heat generating layer is made of a material having a negative resistance temperature coefficient.   The infrared radiation element according to claim 1, wherein the heat generation layer is selected from TaN and TiN.

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