JP2011133239A - Light intensity measuring device and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a conventional light intensity measuring device has a narrow measurement wavelength range and a narrow dynamic range. <P>SOLUTION: A light intensity measuring device is obtained by processing a nano-order irregular structure on the surface of a graphite substrate as a light absorbing material (step 101). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a light intensity measuring device and a manufacturing method thereof.

  The light intensity measuring device is used as, for example, a far-infrared light detector, a laser power meter, an illuminometer, a luminance meter, and the like.

  As a first conventional light intensity measuring device, there are a thermopile bolometer, a thermistor bolometer, and a pyroelectric bolometer having a wide measurement wavelength. This light intensity measuring apparatus includes a light absorber having a light receiving surface irradiated with light of a wavelength to be measured, a thermocouple, a thermistor, a pyroelectric sensor, etc. that converts a temperature rise of the light absorber caused by light absorption into an electric signal. The light intensity is measured by measuring this electric signal (refer to Patent Document 1). It is formed by coating fine particles of carbon black on the surface of a light absorber, for example, a thermal element.

  On the other hand, as a second conventional light intensity measuring device, there is a semiconductor photodetector called a photodiode. For example, use a Si semiconductor for 0.2-1.1 μm light and a HgCdTe semiconductor for 1-10 μm light, and measure the photovoltaic power generated by light irradiation with energy above the band gap. To measure the light intensity.

JP 59-79128

  However, in the above-described first conventional light intensity measuring device, the light absorption rate of carbon black is not so high, that is, the reflectance is not low. For example, the reflectance at a wavelength of 0.3-2 μm in the region including visible light is about 5%, and the reflectance at a wavelength of 10-20 μm in the far infrared region is as high as 20-30%. Therefore, there is a problem that the capability of the light intensity measuring device is low. In addition, since the heat conduction between the carbon black and the heat sensitive element is poor, the response speed of the light intensity measuring device is low, for example, by about 0.1 s, and the temperature of the heat sensitive element does not rise sufficiently. There is also a problem that it is difficult to measure. Furthermore, since the adhesion between the carbon black and the thermal element is poor, there is also a problem that when the high intensity light is irradiated, the temperature of the carbon black rises and the carbon black burns and disappears.

  On the other hand, the second conventional light intensity measuring device is superior in terms of sensitivity and response speed as compared with the first conventional light intensity measuring device described above. Semiconductor detector For example, Si semiconductor detector is required for 0.2-1.1μm light, and HgCdTe semiconductor detector is required for 1-10μm light. For example, it is difficult to measure the light intensity of broadband wavelength such as sunlight. There is a problem of being. In addition, a narrow gap semiconductor detector of, for example, an HgCdTe semiconductor that measures long-wavelength light in the far infrared region requires cooling of the Peltier device and the like, which causes a problem that the apparatus cost increases. Furthermore, there is also a problem that the semiconductor is destroyed when high intensity light is irradiated as compared with the first conventional light intensity measuring apparatus.

  In order to solve the above-described problems, a light intensity measurement device according to the present invention includes a carbon-based substrate having a nano-order uneven structure formed on a surface as a light absorber. As a result, the reflectance of the wavelength range of 0.3-2 μm in the region including visible light is lowered, and the reflectance of the far-infrared region of, for example, the wavelength of 2-50 μm is also lowered.

  Moreover, the manufacturing method of the light intensity measuring device according to the present invention includes a step of processing a nano-order uneven structure on the surface of a carbon-based substrate as a light absorber.

  According to the present invention, the reflectance of the region including the visible light and the far infrared region is low, so that an ideal absorption spectrum of the light absorber can be realized. Therefore, the measurement wavelength range and dynamic range of the light intensity measurement device can be widened.

It is a flowchart which shows the processing flow of the nano uneven structure of the graphite substrate as a light absorber which concerns on this invention. It is a scanning electron microscope (SEM) photograph of the surface of the graphite substrate before and behind the plasma etching of FIG. It is a graph which shows the reflectance of the wavelength 0.3-2 micrometer of the surface of the graphite substrate before and behind the plasma etching of FIG. It is a graph which shows the reflectance of the wavelength of 2-15 micrometers on the surface of the graphite substrate before and behind the plasma etching of FIG. It is a flowchart which shows the example of a change of the flow of FIG. It is a figure which shows embodiment of the light intensity measuring apparatus based on this invention using the carbon-type board | substrate formed with the flowchart of FIG. 1 or FIG.

  The principle of operation of the light absorber according to the present invention utilizes the light absorption action of the absorbed energy using the complete black body effect. That is, when the average reflectance of light is high, the light absorption efficiency is low. On the other hand, when the average reflectance of light is low, the light absorption efficiency is high. That is, when the reflectance R decreases, the emissivity I increases, and conversely, when the reflectance R increases, the emissivity I decreases. In this case, the emissivity is used as an index indicating the light radiation, that is, the light emission ability. When the light transmittance is almost zero, the emissivity is expressed as I≈1-R (reflectance).

  Therefore, ideally, it is understood that the light absorption efficiency increases when, for example, a light absorber having a reflectance R of wavelength 0.3-50 μm is as close to 0 as possible.

  FIG. 1 is a flowchart showing a processing flow of a nano uneven structure of a graphite substrate as a light absorber according to the present invention.

In step 101 of FIG. 1, the graphite substrate having a mirror-like surface shown in FIG. 1A is etched by a plasma etching method using hydrogen gas, and the graphite substrate having a nano-order uneven structure shown in FIG. Get. The plasma etching conditions are, for example, as follows.
RF power: 100-1000W
Pressure: 133-13300Pa (1-100Torr)
Hydrogen flow rate: 5-500sccm
Etching time: 1-100 minutes

The plasma etching method in step 101 of FIG. 1 may be any of electron cyclotron resonance (ECR) etching method, reactive ion etching (RIE) method, atmospheric pressure plasma etching method, etc., and the processing gas is H Ar gas other than ₂ gas, N ₂ gas, O ₂ gas, CF ₄ gas, etc. may be used.

  Therefore, as shown in FIG. 3, the average reflectance of the region containing visible light at a wavelength of 0.3-2 μm is as low as 20-30% before plasma etching to 1% or less after plasma etching. As a result, the absorption of the region including visible light is the highest. Moreover, as shown in FIG. 4, the average reflectance in the far-infrared region, for example, at a wavelength of 2-15 μm, also decreases from 50% before plasma etching to 3% or less after plasma etching. As a result, the plasma-etched graphite substrate can be used as a light absorber as it is.

FIG. 5 shows a modified example of the flow of FIG. 1. In step 501, prior to the plasma etching step 101 of FIG. 1, irregular periodic micron (submicron) mechanical uneven structure processing by mechanical surface polishing such as sandblasting is performed. I do. Also, in step 502 after the plasma etching step 101 of FIG. 1, irregular-shaped micron (submicron) laser irradiation concavo-convex structure processing by surface polishing by high power laser irradiation such as CO ₂ laser, YAG laser, excimer laser or the like is performed. Do. Note that both steps 501 and 502 may be performed, but only one of them may be performed. In this case, it is preferable to perform step 501 because small nano-order irregularities are more fragile. As a result, a concavo-convex structure having an irregular period such as a micron order or a submicron order is formed. Accordingly, the surface area of the graphite substrate is increased and the light absorption rate is increased.

Further, in the irregular periodic micron (submicron) mechanical unevenness structure processing step 501 of FIG. 5, a large number of irregular periodic micron order or submicron order concave structures are formed on the surface of the graphite substrate to increase the surface area. It may be increased. For example, a resist layer is applied, and then a pattern of the resist layer is formed by photolithography using a photomask having an irregular periodic pattern, and the graphite substrate is formed with H ₂ gas and O ₂ using the resist layer pattern. Plasma etching using gas, for example, RIE is performed, and then the resist layer pattern is removed. Further, the surface area can be increased by forming an irregular periodic micron order or submicron order sword mountain type concavo-convex structure by a cutting method using a mechanical ruling engine or the like. This sword mountain-type uneven structure can also be formed by forming a reverse sword mountain mold by etching and pouring a liquid graphite material, such as carbon black, into it.

  Here, the irregular periodic structure of micron order or submicron order with a regular period causes a two-dimensional photonic crystal effect and increases the reflectivity in the far-infrared region. This is not preferable because an interference pattern is generated in the light and the flatness of the light absorption rate with respect to the wavelength is impaired.

3 is measured by a spectrophotometer having an integrating sphere whose inner surface is coated with BaSO ₄ particles or the like, while the reflectance of a wavelength of 2-15 μm in FIG. 4 is measured. Is performed by a Fourier Transform Infrared (FTIR) spectrometer having an integrating sphere coated with gold on the inner surface to collect all far-infrared reflected light.

  A dense graphite substrate can be obtained by mixing a metal with the above graphite substrate. Thereby, since the toughness of the dense graphite substrate is large, the workability as a light absorber and the adhesion to the heat sensitive element are improved, and the gap between the heat sensitive element and the light absorber is eliminated. When insulation is required between the thermal element and the light absorber, insulating graphite or a composite material of graphite and insulating ceramic is used as the light absorber.

  In the above-described embodiment, the graphite substrate is used. However, a carbon-based substrate other than the graphite substrate, for example, a substrate in which the surface of the diamond substrate is plasma etched to reduce the reflectance may be used.

FIG. 6 is a diagram showing an embodiment of a light intensity measuring apparatus according to the present invention using a carbon-based substrate processed according to the flowchart of FIG. 1 or FIG. 5 as a light absorber. The light intensity measuring device of FIG. 6 includes a light absorber 601 whose light absorption surface is integrally formed with a carbon-based substrate, and a heat resistant adhesive (not shown) on the surface opposite to the light absorption surface of the light absorber 601. It is composed of a pyroelectric element such as BaTiO ₃ and LiNbO _{3 that} are combined, and a thermal element 602 that generates a photovoltaic power Ev. In this case, heat conduction between the light absorbing surface of the light absorber 601 and the heat sensitive element 602 is good, and the response speed is as fast as 0.01 s or less, so that temperature transfer from the light absorber 601 to the heat sensitive element 602 can be sufficiently performed. Therefore, it is possible to pick up light of low to high intensity from the nW region to the GW region at the pulse peak value. Further, the light absorber 601 does not burn or disappear even by high intensity light irradiation.

Next, the thermal process of the light intensity measuring device in FIG. 6 will be considered. 6, when the energy W _in per unit time is incident on the light absorber 601, the following relationship occurs.
W _in = E ₁ + E ₂ + E ₃ (1)
Where E ₁ is the convective energy lost per unit time by convection in the air,
E ₁ = ηA (TT ₀ )
E ₂ is the radiation energy lost per unit time due to radiation,
E ₂ = σA (T ⁴ -T ₀ ⁴ )
E ₃ is the transmitted energy per unit time transmitted to the thermal element 602, and
E ₃ = κA∂T / ∂x
It is represented by here,
T ₀ is 300K at room temperature
κ is the thermal conductivity of the light absorber 601 and is 230 W / mK
A is the area of the light absorber 601 1 × 10 ⁻⁴ m ²
η is the heat transfer coefficient due to convection, for example about 1 W / m ² K
σ is the Stefan-Boltzmann constant, 5.67 × 10 ^-8 W / K ⁴ m ²
d is the thickness of the light absorber 601 1 × 10 ⁻³ m
T is the surface temperature of the light absorber 601. Therefore, equation (1) becomes equation (2).
-κA∂T / ∂x = ηA (TT ₀ ) + σA (T ⁴ -T ₀ ⁴ )-W _in (2)
The thickness d of the light absorber 601 is as small as 1 mm, also, if the temperature T _D at room temperature T ₀ of the thermal element 602, (2) becomes (3) of the following four equations.
-κA (TT ₀ ) / d = ηA (TT ₀ ) + σA (T ⁴ -T ₀ ⁴ )-W _in (3)
In equation (3), when evaluating the heat loss of each term,
κA (TT ₀ ) / d ≫ ηA (TT ₀ ) + σA (T ⁴ -T ₀ ⁴ )
Therefore, Equation (3) can be expressed by Equation (4).
κA (TT ₀ ) / d = W _in (4)
That is, most of the incident energy W _in would be carried into the thermal element 602. By the way, the sensitivity of the thermosensitive element 602 is about 10 ⁴ V / W. If the voltage value readable from a general S / N ratio is 1 μV, the corresponding light intensity is 100 pW (10 ⁻¹⁰ W). It becomes very low strength. Therefore, it becomes possible to measure very low intensity light.

  In the present invention, the nano-order is assumed to be about 10 to 500 nm, and the sub-micron order is assumed to be about 0.2 to 1 μm.

101: Nano uneven structure processing step 501: Irregular period micron (submicron) mechanical uneven structure processing step 502: Irregular period micron (submicron) laser irradiation uneven structure processing step 601: Light absorber (carbon) System board)
602: Thermal element

Claims (11)

  A light intensity measuring apparatus comprising a carbon-based substrate having a nano-order first uneven structure formed on a surface as a light absorber material.   2. The black body radiation source according to claim 1, wherein the first uneven structure is formed on a second uneven structure having an irregular period larger than the size of the first uneven structure.   The light intensity measurement apparatus according to claim 2, wherein the size of the second concavo-convex structure is not less than a submicron order.   The light intensity measuring device according to claim 2, wherein the second uneven structure is a sword mountain structure provided on a surface of the carbon-based substrate. further,
The light intensity measuring apparatus according to claim 1, further comprising a thermosensitive element coupled to an opposite surface of the first concavo-convex structure of the carbon-based substrate.   A manufacturing method of a light intensity measuring device comprising a step of processing a surface of a carbon-based substrate as a light absorber material into a nano-order first uneven structure.   Furthermore, the manufacturing method of the light intensity measuring apparatus of Claim 6 which comprises the process of processing the 2nd uneven structure of the irregular period larger than the size of the said 1st uneven structure on the surface of the said carbon-type board | substrate.   The method of manufacturing a light intensity measuring apparatus according to claim 7, wherein a size of the second concavo-convex structure is not less than a submicron order.   The method for manufacturing a light intensity measuring apparatus according to claim 6, wherein the first uneven structure processing step is a plasma etching step. The second concavo-convex structure processing step includes:
A photolithography process for forming a photoresist layer having the irregular periodic pattern;
An etching step of forming the recess on the surface of the carbon-based substrate using the photoresist layer;
The method for manufacturing a light intensity measuring device according to claim 7, further comprising: removing the photoresist layer after forming the recess. The method for manufacturing a light intensity measuring device according to claim 7, wherein the second uneven structure is a sword mountain structure provided on a surface of the carbon-based substrate.

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