The present invention relates to a sensor element and a sensor device for detecting electromagnetic waves.
In recent years, an electromagnetic wave sensor has been proposed which detects electromagnetic waves such as near-infrared light, terahertz waves, and microwaves by absorbing the electromagnetic waves using an absorber and by converting the generated heat into an electrical signal using a thermoelectric conversion element by the Seebeck effect (e.g., See Patent Literature 1).
The Seebeck effect generates an electromotive force in the same direction as a temperature difference direction, and thus there is a need to fabricate the sensor with a structure in which p-type modules and n-type modules are alternately arranged and stand in a direction perpendicular to a surface of a heat source. Therefore, the Seebeck effect-based electromagnetic wave sensors have a complicated three-dimensional structure, which leads to a rise in manufacturing cost and makes it difficult to achieve large-area and thin-film devices.
The invention has been made in view of the foregoing, and an object of the invention is to realize a simply structured and large-area sensor element and device for detecting electromagnetic waves using a thermoelectric conversion element.
A sensor element according to one embodiment of the invention is a sensor element for detecting electromagnetic waves and includes a substrate, an absorber configured to absorb the electromagnetic waves and generate heat, and at least one thermoelectric conversion element disposed between the substrate and the absorber to support a part of the absorber and configured to generate an electromotive force from the heat generated in the absorber by a transverse thermoelectric effect.
A sensor device according to one embodiment of the invention includes a substrate and a plurality of sensor structures arranged in a matrix on the substrate. Each of the plurality of sensor structures includes an absorber configured to absorb electromagnetic waves and generate heat, and at least one thermoelectric conversion element disposed between the substrate and the absorber to support a part of the absorber and configured to generate an electromotive force from the heat generated in the absorber by a transverse thermoelectric effect.
According to the invention, a sensor element for detecting electromagnetic waves includes a thermoelectric conversion element which exhibits a transverse thermoelectric effect. With this feature, it is possible to realize a simply structured and large-area sensor element and device.
Exemplary embodiments of the invention will be described below with reference to the accompanying drawings. The same reference signs are used to designate the same or similar elements throughout the drawings. The drawings are schematic, and a relationship between a planar dimension and a thickness and a thickness ratio between members are different from reality. Needless to say, there are portions having different dimensional relationships or ratios between the drawings.
First, a first embodiment of the invention will be described with reference to
The sensor structure 1004 includes a rectangular cuboid shaped thermoelectric conversion element 108, a pair of low thermal conductive insulating films 110 disposed on both sides of the thermoelectric conversion element 108 in a width direction (x-direction) to be bilaterally symmetrical, a pair of electrodes 112a and 112b disposed at both ends of the thermoelectric conversion element 108 in a longitudinal direction (y-direction), a high thermal conductive insulating film 106 disposed to cover the thermoelectric conversion element 108 and the low thermal conductive insulating films 110, and an absorber 104 disposed on the high thermal conductive insulating film 106.
The substrate 102 is made of a material such as AlN, MgO, or Al2O3.
The absorber 104 is a light receiving element that absorbs the electromagnetic waves and generates heat. The absorber 104 is a known absorber which is made of, for example, NiCr, a ferrite material (see Japanese Patent Application Laid-Open No. 2017-037999), a carbon nanotube thin film (see ACS Appl. Nano Mater. 2018, 1, 6, 2469-2475), or a metamaterial (see Phys. Rev. B 78, 241103 (R) (2008); Phys. Rev. B 82, 205117 (2010)). The electromagnetic waves absorbed by the absorber 104 are not restrictive as long as the electromagnetic waves are in a target wavelength band such as near-infrared light, terahertz waves, or microwaves, and generate heat.
The high thermal conductive insulating film 106 is made of an insulating material with a higher thermal conductivity than that of a material of the thermoelectric conversion elements 108. Examples of the material of the high thermal conductive insulating film 106 include AlN, SiC, SiN, or BN.
As shown in
The two low thermal conductive insulating films 110 are interlayer dielectric films having the same shape (rectangular cuboid shape) and the same size and disposed on both sides of the thermoelectric conversion element 108 in the width direction and between the substrate 102 and the high thermal conductive insulating film 106. The low thermal conductive insulating film 110 is made of an insulating material with a lower thermal conductivity than that of the material of the thermoelectric conversion elements 108. Examples of the material of the low thermal conductive insulating film 110 include SiO2.
The electrodes 112a and 112b are made of a metal such as Cu, and detect the electromotive force generated in the thermoelectric conversion element 108 by the transverse thermoelectric effect. The electromotive force signal is amplified by a peripheral circuit (not shown) to detect the electromagnetic waves and to measure the intensity.
As shown in
As shown in
As shown in
As shown in
Next, as a measure indicating performance of the sensor elements of the first embodiment and the subsequent embodiments to be described later, sensitivity and noise equivalent power (hereinafter referred to as “NEP”) will be explained. When a voltage (V) generated in the sensor element is denoted by V and heat current density (W/m2) is denoted by q, V/q is defined as in Equation (1).
Here, SANE is a Nernst coefficient (μV/K) of the thermoelectric conversion element, κANE is a thermal conductivity (W/mK) of the thermoelectric conversion element, ptherm is a thermal collection rate, parea is a rate of the thermoelectric conversion element in a thermal conductive cross-sectional area of the sensor element, AANE is an area (m2) of the thermoelectric conversion element, and Arec is an area (m2) of the absorber. l denotes the length (m) of the thermoelectric conversion element in the longitudinal direction, as described above. κi (i=1, 2) denotes the thermal conductivities (W/mK) of the thermoelectric conversion element (i=1) and the low thermal conductive insulating film (i=2), and At (i=1, 2) denotes the areas (m2) of the thermoelectric conversion element (i=1) and the low thermal conductive insulating film (i=2). It is noted that, in this description, the terms “thermal conductive cross-sectional area” and “area” refer to the areas on the plane (x-y plane in
When the amount of heat (W) generated in the absorber is denoted by Q, the sensitivity V/Q of a single sensor element is given by Equation (2) from Equation (1).
It is noted that the sensitivity of the entire sensor element can be obtained by multiplying the sensitivity V/Q shown in Equation (2) by a light-to-heat conversion efficiency pr of the absorber. Equation (2) suggests that the higher the thermal resistance of the entire sensor element, the higher the sensitivity.
When thermal noise (V) caused by an element resistance is denoted by Vnoise, NEP is defined as Vnoise/(V/Q). Thermal noise Vnoise is defined as in Equation (3).
Here, kB=1.38×10−23 (J/K) is the Boltzmann constant, T is temperature (K) of the thermoelectric conversion element, R is an electrical resistance (Q) of the thermoelectric conversion element, p is an electrical resistivity (μΩcm) of the thermoelectric conversion element, and t and w are the thickness (m) and width (m) of the thermoelectric conversion element, respectively (see
Therefore, from Equations (2) and (3), the NEP of a single sensor element is given by Equation (4).
It is noted that the NEP of the entire sensor element can be obtained by dividing the NEP shown in Equation (4) by the conversion efficiency pr of the absorber.
Equations (2) and (4) suggest that the higher the thermal resistance of the entire sensor element and the larger the volume twl of the thermoelectric conversion element, the better the sensitivity of the sensor element and the lower the NEP.
Next, a method for fabricating the sensor element 10 of the first embodiment (hereinafter referred to as a “first fabrication method”) will be described with reference to
First, a resist is applied onto any substrate 102, and a resist pattern 127 with regions of the thermoelectric conversion element 108 opened is formed by a photolithography process. Then, as shown in
For example, an Fe3Ga thin film can be deposited on any substrate 102 at room temperature by DC magnetron sputtering. Here, the deposition can be performed with a base vacuum of 10−4 Pa or less under the deposition pressure range of 0.1 Pa to 1.5 Pa in a chamber. For example, the base vacuum may be 5×10−7 Pa, and the deposition pressure may be 0.5 Pa.
After formation of the thermoelectric conversion element 108, a resist is applied, and a resist pattern 131 with regions of the electrodes 112a and 112b opened is formed by the photolithography process. Then, as shown in
After formation of the electrodes 112a and 112b, a resist is applied, and a resist pattern 129 with regions of the low thermal conductive insulating films 110 opened is formed by the photolithography process. Then, as shown in
After formation of the low thermal conductive insulating films 110, a resist is applied, and a resist pattern 125 with regions of the absorber 104 and the high thermal conductive insulating film 106 opened is formed by the photolithography process. Then, as shown in
The width wa and length la of the absorber 104, the width whi and length lhi of the high thermal conductive insulating film 106, and the length 1 of the thermoelectric conversion element 108 are preferably 10 μm to 1000 μm, but they are not restrictive. The thickness ta of the absorber 104 is preferably 2 nm to 1000 nm, and the thickness thi of the high thermal conductive insulating film 106 is preferably 5 nm to 100 nm, but they are not restrictive. In addition, the width w of the thermoelectric conversion element 108 is preferably 0.1 μm to 200 μm, and the thickness t of the thermoelectric conversion element 108 is preferably 10 nm to 10 μm, but they are not restrictive. These sizes can be arbitrarily designed depending on the intended use or other factors.
Next, the performance of the sensor element 10 will be examined when the sensor element 10 whose size and materials are defined in Table 1 is fabricated by the first fabrication method.
Table 2 shows the thermal conductivity κANE, area AANE, Nernst coefficient SANE, and electrical resistivity ρ of the Fe3Ga thin film as the thermoelectric conversion element 108.
In this case, from Equations (2) and (4), the V/Q and NEP of a single sensor element 10 are calculated as follows, and can be said to be practical numerical values depending on how the sensor element 10 is used.
The method for fabricating the sensor element 10 is not limited to the first fabrication method described above. For example, a fabrication method including annealing and etching processes (hereinafter referred to as a “second fabrication method”) may be employed.
The second fabrication method for fabricating the sensor element 10 will be described with reference to
First, the substrate 102 is annealed in a high vacuum chamber. Then, as shown in
Next, as shown in
The process after formation of the thermoelectric conversion element 108 is the same as that shown in
For example, to deposit the Fe3Ga thin film by the second fabrication method, first, the substrate 102 made of MgO (001) is annealed at 800° ° C. for 10 minutes in a high vacuum (<10−5 Pa) chamber. Next, the Fe3Ga thin film is formed at room temperature by DC magnetron sputtering. Here, the deposition can be performed with a base vacuum of 10−4 Pa or less under the deposition pressure range of 0.1 Pa to 1.5 Pa in the chamber. For example, the base vacuum may be 5×10−7 Pa, and the deposition pressure may be 0.5 Pa. Next, post annealing is performed at 500° ° C. for 30 minutes in the same chamber. Accordingly, the Fe3Ga thin film with a D03 structure can be obtained.
Next, the performance of the sensor element 10 having the Fe3Ga thin film with the D03 structure as the thermoelectric conversion element 108 will be examined.
Table 3 shows the thermal conductivity κANE, area AANE, Nernst coefficient SANE, and electrical resistivity ρ of the Fe3Ga thin film with the DOS structure. Here, the materials and size of the sensor element 10 are the same as those defined in Table 1 except that the material of the substrate 102 is MgO.
As shown in Table 3, the Fe3Ga thin film with the D03 structure has the higher Nernst coefficient SANE and the lower electrical resistivity ρ than the Fe3Ga thin film shown in Table 2. In this case, from Equations (2) and (4), the V/Q and NEP of a single sensor element 10 are calculated as follows, indicating that the sensitivity is higher than the sensor element 10 fabricated by the first fabrication method which does not include an annealing process, and that the noise immunity is improved.
For example, to deposit a Co2MnGa thin film by the second fabrication method, the substrate 102 made of MgO (001) is annealed at 800° ° C. for 10 minutes in a high vacuum (<10−5 Pa) chamber. Next, the Co2MnGa thin film is formed at room temperature by DC magnetron sputtering. Also in this case, the deposition can be performed with a base vacuum of 10−4 Pa or less under the deposition pressure range of 0.1 Pa to 1.5 Pa in the chamber. For example, the base vacuum may be 1×10−6 Pa, and the deposition pressure may be 1.2 Pa. Next, post annealing is performed at 550° C. for 60 minutes in the same chamber. Accordingly, the Co2MnGa thin film with an L21 structure can be obtained.
Next, the performance of the sensor element 10 having the Co2MnGa thin film with the L21 structure as the thermoelectric conversion element 108 will be examined.
Table 4 shows the thermal conductivity κANE, area AANE, Nernst coefficient SANE, and electrical resistivity ρ of the Co2MnGa thin film with the L21 structure. Here, the materials and size of the sensor element 10 are the same as those defined in Table 1 except for the material of the substrate 102 (MgO) and the material of the thermoelectric conversion element 108.
In this case, from Equations (2) and (4), the V/Q and NEP of a single sensor element 10 are calculated as follows, indicating that the sensitivity and noise immunity are almost equivalent to those of the sensor element 10 including the Fe3Ga thin film with the above-mentioned D03 structure.
Thus, by employing the second fabrication method including the post annealing process and enhancing crystallizability of the thermoelectric conversion element 108, it is possible to increase the sensitivity of the sensor element 10 and improve the noise immunity.
Various sensor devices can be provided that include the sensor structure 1004 shown in
Next, a second embodiment of the invention will be described with reference to
The sensor structure 2004 includes a first thermoelectric conversion element 208 and a second thermoelectric conversion element 209 that have a rectangular cuboid shape and are located separately from each other on the substrate 202, a low thermal conductive insulating film 210 as an interlayer dielectric film, a pair of electrodes 212a and 212b disposed at both ends of the first thermoelectric conversion element 208 in the longitudinal direction (y-direction), a pair of electrodes 213a and 213b disposed at both ends of the second thermoelectric conversion element 209 in the longitudinal direction (y-direction), a high thermal conductive insulating film 206 disposed to cover the first thermoelectric conversion element 208, the second thermoelectric conversion element 209 and the low thermal conductive insulating film 210, and an absorber 204 disposed on the high thermal conductive insulating film 206.
As shown in
The low thermal conductive insulating film 210 is disposed between the substrate 202 and the high thermal conductive insulating film 206 in a region where the first thermoelectric conversion element 208 and the second thermoelectric conversion element 209 are not located. In
As shown in
Since the material and function of each component of the sensor element 20 are the same as those of the corresponding components of the sensor element 10 of the first embodiment, and since the shape and sizes of the absorber 204, the high thermal conductive insulating film 206, the first thermoelectric conversion element 208, and the second thermoelectric conversion element 209 are also the same as those of the corresponding components of the sensor element 10 of the first embodiment, the description thereof will be omitted.
Next, a method for fabricating the sensor element 20 of the second embodiment (hereinafter referred to as a “third fabrication method”) will be described with reference to
First, the substrate 202 is annealed in a high vacuum chamber. Then, as shown in
Next, as shown in
After formation of the first thermoelectric conversion element 208 and the second thermoelectric conversion element 209, a resist is applied, and a resist pattern 241 with regions of the electrodes 212a, 212b, 213a, and 213b opened is formed by the photolithography process. Then, as shown in
Next, as shown in
After formation of the electrodes 212a, 212b, 213a, and 213b, a resist is applied, and a resist pattern 239 with regions of the low thermal conductive insulating film 210 opened is formed by the photolithography process. Then, as shown in
Next, as shown in
After formation of the low thermal conductive insulating film 210, a resist is applied, and a resist pattern 235 with regions of the absorber 204 and the high thermal conductive insulating film 206 opened is formed by the photolithography process. Then, as shown in
Finally, as shown in
Next, the performance of the sensor element 20 will be examined when the sensor element 20 whose size and materials are defined in Table 5 is fabricated by the third fabrication method.
The thermal conductivity κANE, area AANE, Nernst coefficient SANE, and electrical resistivity ρ of each of the first thermoelectric conversion element 208 and the second thermoelectric conversion element 209 are the same as those defined in Table 4.
In this case, from Equations (2) and (4), the V/Q and NEP of a single sensor element 20 are calculated as follows, indicating that the sensitivity is improved as compared to the sensor element 10 of the first embodiment including the Co2MnGa thin film. Here, Equations (2) and (4) include contributions from the two thermoelectric conversion elements.
Various sensor devices including the sensor structure 2004 shown in
Next, a third embodiment of the invention will be described with reference to
The sensor element 30 has substantially the same configuration as the sensor element 20 of the second embodiment except that the low thermal conductive insulating film 210 is removed. Specifically, the sensor structure 3004 of the sensor element 30 includes a high thermal conductive insulating film 306, an absorber 304 disposed on the high thermal conductive insulating film 306, a first thermoelectric conversion element 308 and a second thermoelectric conversion element 309 that have a rectangular cuboid shape and support both ends of the absorber 304 and the high thermal conductive insulating film 306, a pair of electrodes 312a and 312b disposed at both ends of the first thermoelectric conversion element 308 in the longitudinal direction (y-direction), and a pair of electrodes 313a and 313b disposed at both ends of the second thermoelectric conversion element 309 in the longitudinal direction (y-direction). A hollow region 310 is provided between the first thermoelectric conversion element 308 and the second thermoelectric conversion element 309 and between the high thermal conductive insulating film 306 and the substrate 302.
As shown in
Since the shape, size, material, and function of each component of the sensor element 30 are the same as those of the corresponding components of the sensor element 20 of the second embodiment, the description thereof will be omitted.
Next, a method for fabricating the sensor element 30 of the third embodiment (hereinafter referred to as a “fourth fabrication method”) will be described with reference to
The steps of forming the first thermoelectric conversion element 308, the second thermoelectric conversion element 309, and the electrodes 312a, 312b, 313a, and 313b on the substrate 302 are the same as those shown in
After formation of the first thermoelectric conversion element 308, the second thermoelectric conversion element 309, and the electrodes 312a, 312b, 313a, and 313b, a resist is applied, and a resist pattern 339 with areas of the hollow region 310 opened is formed by the photolithography process. Then, as shown in
Next, as shown in
After formation of the interpolating layer 320, a resist is applied, and a resist pattern 335 with regions of the absorber 304 and the high thermal conductive insulating film 306 opened is formed by the photolithography process. Then, as shown in
Next, as shown in
Finally, as shown in
Next, the performance of the sensor element 30 will be examined when the sensor element 30 whose size and materials are defined in Table 6 is fabricated by the fourth fabrication method.
The thermal conductivity κANE, area AANE, Nernst coefficient SANE, and electrical resistivity ρ of each of the first thermoelectric conversion element 308 and the second thermoelectric conversion element 309 are the same as those defined in Table 4.
In this case, the V/Q and NEP of a single sensor element 30 are calculated as follows based on Equations (2) and (4). Here, Equations (2) and (4) include contributions from the two thermoelectric conversion elements.
Thus, the sensor element 30 of the third embodiment has significantly improved sensitivity and noise immunity as compared to the sensor element 10 of the first embodiment and the sensor element 20 of the second embodiment. This indicates that the heat current from the absorber 304 is concentrated on the first thermoelectric conversion element 308 and the second thermoelectric conversion element 309 by providing the vacuum hollow region 310 between the first thermoelectric conversion element 308 and the second thermoelectric conversion element 309.
Various sensor devices including the sensor structure 3004 shown in
According to the sensor elements and sensor devices of the first to third embodiments, it is possible to realize a simply structured and large-area device. Moreover, a very inexpensive device can be fabricated without using a microfabrication apparatus. Furthermore, the thinner thermoelectric conversion elements allow a design with a small heat capacity and enhance a response speed. Such excellent properties allow, for example, the sensor element to be integrated into a baggage inspection device or other such devices to inspect contents of baggage at a high speed. In addition, background noise can be suppressed, and an S/N ratio can be increased. Furthermore, depending on structure and arrangement of the sensor elements, the resolution in the order of μW can be realized with a temperature width of 100 K or more.
Next, a fourth embodiment of the invention will be described with reference to
While the thermoelectric conversion elements of the first to third embodiments are thin films, a thermoelectric conversion element of the fourth embodiment has a bulk structure.
As shown in
The sensor element 40 is obtained by arranging on the substrate 402 the first thermoelectric conversion element 408 and the second thermoelectric conversion element 409 that have a rectangular cuboid shape in parallel to be separated from each other in the width direction, and placing a structure including the thin-film absorber 404 and the thin-film high thermal conductive insulating film 406 on the first thermoelectric conversion element 408 and the second thermoelectric conversion element 409 which are served as support stands.
Next, the performance of the sensor element 40 of the fourth embodiment will be examined below on the assumption that the sensor element 40 has size and materials defined in Table 7.
Table 8 shows the thermal conductivity κANE, area AANE, Nernst coefficient SANE, and electrical resistivity ρ of each of the first thermoelectric conversion element 408 and the second thermoelectric conversion element 409.
In this case, the V/Q and NEP of a single sensor element 40 are calculated as follows based on Equations (2) and (4). Here, Equations (2) and (4) include contributions from the two thermoelectric conversion elements.
Thus, the bulk-structured sensor element 40 has lower sensitivity and noise immunity than those of the sensor element including the thin-film thermoelectric conversion element shown in the first to third embodiments, but can obtain a signal equivalent to that of the Seebeck effect-based electromagnetic wave sensor. In addition, the thicknesses of the first thermoelectric conversion element 408 and the second thermoelectric conversion element 409 are larger than those of the thermoelectric conversion elements of the first to third embodiments, which leads to an advantage that an electric resistance value is small.
As shown in
The sensor elements and sensor devices of the embodiments do not show phase transition like conventional bolometers, which makes it possible to detect electromagnetic waves with high signal linearity in a wide range. In addition, unlike the Seebeck effect-based electromagnetic wave sensors, the area of the absorber can be designed to be large, which allows the electromagnetic waves to be efficiently converted into heat.
Number | Date | Country | Kind |
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2021-070428 | Apr 2021 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2022/017602 | 4/12/2022 | WO |