THERMOPILE SENSOR AND SENSOR ARRAY

A thermopile sensor according to the present disclosure includes a p-type portion and an n-type portion. The p-type portion has a first phononic crystal in which first holes are arranged in a plan view. The n-type portion has a second phononic crystal in which second holes are arranged in a plan view. The p-type portion and the n-type portion constitute a thermocouple. The boundary scattering frequency of phonons in the first phononic crystal is different from the boundary scattering frequency of phonons in the second phononic crystal. Alternatively, the ratio of the sum of the areas of the first holes to the area of the first phononic crystal in a plan view is different from the ratio of the sum of the areas of the second holes to the area of the second phononic crystal in a plan view.

BACKGROUND

1. Technical Field

The present disclosure relates to a thermopile sensor and a sensor array.

2. Description of the Related Art

To date, a thermopile sensor including a material having a phononic crystal is known.

For example, Japanese Unexamined Patent Application Publication No. 2017-223644 describes a thermopile infrared sensor including a beam that is a thin-film-like member having a two-dimensional phononic crystal. In the two-dimensional phononic crystal, through-holes having arbitrary diameters are arranged in a plane at an arbitrary period. The beam functions as a thermopile. According to Japanese Unexamined Patent Application Publication No. 2017-223644, it is possible to improve the sensitivity of an infrared sensor by introducing a phononic crystal into a beam of an infrared receiver.

In addition, U.S. Patent Application Publication No. 2015/0015930 and “Nomura et al., ‘Impeded thermal transport in Si multiscale hierarchical architectures with phononic crystal nanostructures’, Physical Review B 91, 205422 (2015)” each disclose a periodic structure constituted by through-holes that reduce the thermal conductivity of a thin film. In the periodic structure, the through-holes are regularly arranged with a period of a nanometer order in the range from 1 nanometer (nm) to 1000 nm in a plan view of the thin film. The periodic structure is a type of a phononic crystal structure.

SUMMARY

The technology described above has room for reexamination from the viewpoint of reducing the risk of breakage of a member in a thermopile sensor.

One non-limiting and exemplary embodiment provides a technology that is advantageous from the viewpoint of reducing the risk of breakage of a member in a thermopile sensor.

In one general aspect, the techniques disclosed here feature a thermopile sensor. The thermopile sensor includes: a p-type portion that includes a p-type material and has a first phononic crystal in which first holes are arranged in a plan view; and an n-type portion that includes an n-type material and has a second phononic crystal in which second holes are arranged in a plan view. The p-type portion and the n-type portion constitute a thermocouple. The thermopile sensor satisfies at least one condition selected from a group consisting of the following (I) and (II): (I) a boundary scattering frequency of phonons in the first phononic crystal is different from a boundary scattering frequency of phonons in the second phononic crystal; and (II) a ratio of a sum of areas of the first holes to an area of the first phononic crystal in a plan view of the first phononic crystal is different from a ratio of a sum of areas of the second holes to an area of the second phononic crystal in a plan view of the second phononic crystal.

The thermopile sensor according to the present disclosure is advantageous from the viewpoint of reducing the risk of breakage of a member.

DETAILED DESCRIPTIONS

A thermopile sensor includes a thermocouple including a p-type material and an n-type material. In the p-type material, holes behave as carriers for electrical conduction; and, in the n-type material, electrons behave as carriers for electrical conduction. As an example of the thermocouple in the thermopile sensor, a structure that has a hot junction in a sensing portion, which is spanned over a substrate by using a beam, and that has a cold junction on the substrate is conceivable. It is conceivable that, with this structure, the sensing portion of the thermocouple, having the hot junction, is not likely to be affected by heat of the substrate. It is conceivable that such a thermopile sensor can function as an infrared sensor when an infrared receiver that absorbs infrared rays is added to the spanned sensing portion in the thermopile sensor. In addition, it is conceivable that such a thermopile sensor can function as a gas sensor when a catalyst layer that reacts to a specific gas is added to the spanned sensing portion in the thermopile sensor. In such a thermopile sensor, the performance of the sensor, such as infrared ray detection sensitivity or gas detection sensitivity, tends to increase as the thermal insulation performance of the beam increases.

It is conceivable that the thermal conductivity of a member can be reduced by making the member porous. A member having a phononic crystal can have a thermal insulation performance that exceeds the classical effect of reduction of thermal conductance due to decrease in the volume of the member by making the member porous. For example, Japanese Unexamined Patent Application Publication No. 2017-223644 describes that the sensitivity of an infrared sensor can be improved by introducing a phononic crystal into a beam of an infrared receiver.

It is assumed that a p-type material and an n-type material of a thermocouple in a thermopile sensor have different thermo-physical properties. For example, Journal of Micromechanics and Microengineering 19, 125029 (2009) describes that the thermal conductivities of an n-type material and a p-type material whose base material is polysilicon are considerably different from each other. In a case where the thermal conductivities of a p-type material and an n-type material of a thermocouple of a thermopile sensor are different from each other, when the sensing portion detects infrared rays or a gas and the temperature of the sensing portion increases, the temperature distribution of the thermocouple may become nonuniform. As a result, a thermal stress is generated in the thermocouple. When the thermocouple is subjected to a sudden change in temperature or when the temperature gradient in the thermocouple is large, a large thermal stress may be generated in the thermocouple. If the generated thermal stress exceeds the yield stress of a member of the thermocouple, a crack or a permanent deformation occurs in the thermocouple. According to an examination by the inventors, among portions of a thermopile sensor into which a phononic crystal has been introduced in order to increase the thermal insulation performance, the temperature of a sensing portion tends to increase considerably when the thermopile sensor detects a detection target such as infrared rays or a gas. In this case, the risk of breakage of a member of the thermopile sensor due to a thermal stress tends to increase.

The inventors have intensely examined technologies expected to be advantageous from the viewpoint of reducing the risk of breakage of a member of the thermopile sensor. As a result, the inventors have found that it is possible to reduce a thermal stress by regulating a phononic crystal so that the boundary scattering frequencies of phonons in a p-type portion and an n-type portion of a thermocouple have a predetermined relationship. In addition, the inventors have found that it is possible to reduce a thermal stress by regulating the ratio of the sum of the areas of holes to the area of a phononic crystal in a plan view in a p-type portion and an n-type portion of a thermocouple. Based on these findings, the inventors have devised a thermopile sensor according to the present disclosure.

Summary of One Aspect according to Present Disclosure

The present disclosure provides the following thermopile sensor.

The thermopile sensor includes:a p-type portion that includes a p-type material and has a first phononic crystal in which first holes are arranged in a plan view; andan n-type portion that includes an n-type material and has a second phononic crystal in which second holes are arranged in a plan view.

The p-type portion and the n-type portion constitute a thermocouple.

The thermopile sensor satisfies at least one condition selected from a group consisting of the following (I) and (II):(I) a boundary scattering frequency of phonons in the first phononic crystal is different from a boundary scattering frequency of phonons in the second phononic crystal; and(II) a ratio of a sum of areas of the first holes to an area of the first phononic crystal in a plan view of the first phononic crystal is different from a ratio of a sum of areas of the second holes to an area of the second phononic crystal in a plan view of the second phononic crystal.

The thermopile sensor is advantageous from the viewpoint of reducing the risk of breakage of a member, because a thermal stress generated in the thermocouple tends to be small.

EMBODIMENTS OF PRESENT DISCLOSURE

Hereafter, embodiments of the present disclosure will be described with reference to the drawings. Embodiments described below each give a general or specific example. Numerical values, shapes, elements, the dispositions of the elements, the connections between the elements, steps, and the order of the steps described in the following embodiments are examples, and are not intended to limit the present disclosure. Elements of the following embodiments that are not described in an independent claim that represents the broadest concept will be described as optional elements. Each of the figures is a schematic view, and is not necessarily drawn strictly.

First Embodiment

FIGS.1A and1Billustrate a thermopile sensor1aaccording to a first embodiment. The thermopile sensor1aincludes a p-type portion11and an n-type portion12. The p-type portion11includes a p-type material. In addition, the p-type portion11has a first phononic crystal11cin which holes10hare arranged in a plan view. The n-type portion12includes an n-type material. In addition, the n-type portion12has a second phononic crystal12cin which holes10hare arranged in a plan view. In the thermopile sensor1a, the p-type portion11and the n-type portion12constitute a thermocouple10. The thermopile sensor1asatisfies at least one condition selected from a group consisting of the following (I) and (II).(I) The boundary scattering frequency of phonons in the first phononic crystal11cis different from the boundary scattering frequency of phonons in the second phononic crystal12c.(II) A ratio R1is different from a ratio R2. The ratio R1is the ratio of the sum of the areas of the holes10hto the area of the first phononic crystal11cin a plan view of the first phononic crystal11c. The ratio R2is the ratio of the sum of the areas of the holes10hto the area of the second phononic crystal12cin a plan view of the second phononic crystal12c.

In an insulator and a semiconductor, heat is mainly carried by phonons, each of which is a quasi-particle that is quantization of lattice vibration. The thermal conductivity of a material including an insulator or a semiconductor is determined by the dispersion relation of phonons in the material. The dispersion relation of phonons may include the relation between frequency and wavenumber or a band structure. The frequency range of phonons that carry heat in an insulator and a semiconductor encompasses a wide range from 100 GHz to 10 THz. The thermal conductivity of a material including an insulator or a semiconductor is determined by the behavior of phonons in the frequency range. Phonons have, for each frequency, a finite mean free path that is determined by scattering caused by the phonons themselves and scattering caused by impurities. The mean free path corresponds to a distance that phonons can travel without being obstructed by scattering. Phonons have a mean free path in a wide range from several angstroms to several micrometers in accordance with frequency. Phonons having a long mean free path mainly carry heat in a material having a high thermal conductivity, and phonons having a short mean free path mainly carry heat in a material having a low thermal conductivity.

With a phononic crystal, the boundary scattering frequency of phonons can be regulated due to a structure in which holes are arranged, and the effective mean free path of phonons can be adjusted. As the representative length of the structure or the like becomes shorter, the boundary scattering frequency of phonons becomes higher. Therefore, for example, the boundary scattering frequency of phonons can be regulated by adjusting at least one condition selected from a group consisting of the following (i), (ii), and (iii). As the boundary scattering frequency of phonons increases, the thermal conductivity of the structure decreases. In addition, it is possible to regulate the effective thermal conductance of a phononic crystal by adjusting the following (ii).(i) the shortest distance between the nearest holes in a plan view of the phononic crystal(ii) the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view(iii) the specific surface area of the phononic crystal

For example, the thermal conductivity of the p-type material included in the p-type portion11and the thermal conductivity of the n-type material included in the n-type portion12are different from each other. On the other hand, the boundary scattering frequency of phonons in the first phononic crystal11cis different from the boundary scattering frequency of phonons in the second phononic crystal12c. Alternatively, the ratio R1is different from the ratio R2. Thus, the difference between the thermal conductivity of the p-type portion11and the thermal conductivity of the n-type portion12can be reduced, and the temperature distribution in the thermocouple10tends to be uniform. As a result, a thermal stress generated in the thermocouple10tends to be small, and the risk of breakage of a member, such as the thermocouple10, in the thermopile sensor1ais low. In the present specification, thermal conductivity means, for example, a value at 25° C.

As illustrated inFIGS.1A and1B, the thermopile sensor1aincludes a substrate20and a sensor layer15. The sensor layer15includes the thermocouple10. The sensor layer15has a connection portion15c, a beam15b, and a sensing portion15d. The connection portion15cconnects the sensor layer15to the substrate20. For example, the connection portion15cis in contact with the substrate20. The beam15bis connected to the sensing portion15d, and supports the sensing portion15din a state of being separated from the substrate20. The sensing portion15dand the beam15binclude the thermocouple10. The p-type portion11has, for example, a positive Seebeck coefficient. The n-type portion12has a negative Seebeck coefficient.

The thermopile sensor1ais configured as, for example, an infrared sensor, and the sensing portion15dincludes, for example, an infrared receiver15e.

As illustrated inFIGS.1A and1B, the thermopile sensor1afurther includes, for example, a signal-processing circuit30, wiring31, and an electrode pad33. When infrared rays are incident on the infrared receiver15e, the temperature of the infrared receiver15eincreases. At this time, the temperature of the infrared receiver15eincreases by a larger amount as the thermal insulation between the substrate20and a member on the substrate20, which are a heat bath, and the infrared receiver15ebecomes higher. As the temperature of the infrared receiver15eincreases, an electromotive force due to the Seebeck effect is generated in the thermocouple10. The signal-processing circuit30processes the generated electromotive force, and thus the thermopile sensor1adetects the infrared rays. Depending on the way the signal-processing circuit30processes the signal, the thermopile sensor1acan measure the intensity of infrared rays and/or the temperature of an object. An electric signal processed by the signal-processing circuit30can be read out through the electrode pad33.

The substrate20has a recessed portion25. The recessed portion25opens in one of main surfaces of the substrate20. As illustrated inFIG.1A, the sensing portion15dand the beam15boverlap the recessed portion25in a plan view the thermopile sensor1a. The sensing portion15dand the beam15bare spanned over the recessed portion25.

The substrate20is typically made of a semiconductor. The semiconductor is, for example, Si. However, the substrate20may be made of a semiconductor other than Si or a material other than a semiconductor.

The material of the wiring31is not limited to a specific material. The wiring31is made of, for example, an impurity semiconductor, a metal, or a metal compound. The metal and the metal compound may be, for example, materials used in a general semiconductor process, such as Al, Cu, TiN, and TaN.

The signal-processing circuit30has, for example, a known configuration that includes a transistor device and that can process an electric signal.

The sensor layer15may have a single-layer structure or a multilayer structure. As illustrated inFIG.1B, the sensor layer15is a single layer in which the thermocouple10faces the substrate20. In this case, the material of the sensor layer15may be a semiconductor material in which carriers for electrical conduction can be regulated to either of holes or electrons by doping. Examples of such a semiconductor material include Si, SiGe, SiC, GaAs, InAs, InSb, InP, GaN, ZnO, and BiTe. The base material of a semiconductor in the thermocouple10is not limited to these examples. The base material of a semiconductor in the thermocouple10may be a single crystal material, a polycrystal material, or an amorphous material. In a single crystal material, atoms are orderly arranged over a long distance. The thermocouple10includes a thin film having a thickness that is, for example, greater than or equal to 10 nm and less than or equal to 500 nm.

One end of the p-type portion11and one end of the n-type portion12are electrically connected to each other by, for example, a hot junction13. The hot junction13is made of, for example, a metal film or a metal compound film. Thus, a thermocouple element is constituted by the p-type portion11, the n-type portion12, and the hot junction13. The metal film or the metal compound film that forms the hot junction13is not limited to a specific film, and may be, for example, a film of a metal or a metal compound such as TiN, TaN, Al, Ti, or Cu, which is generally used in a semiconductor process. The beam15band the sensing portion15dof the sensor layer15may be constituted by the p-type portion11and the n-type portion12, or may include a non-doped portion that is not doped with impurities.

The hot junction13may be caused to function as an infrared absorbing layer by matching the sheet resistance of the metal film or the metal compound film that forms the hot junction13with the impedance of vacuum. For example, when the hot junction13includes TiN, it is possible to match the sheet resistance of the hot junction13with the impedance of vacuum by adjusting the thickness of the hot junction13to about 10 nm.

As illustrated inFIG.1A, the wiring31and the p-type portion11are electrically connected by, for example, the connection portion15c. The wiring31and the n-type portion12are electrically connected by, for example, the connection portion15c. The wiring31electrically connects the p-type portion11and the signal-processing circuit30, and the wiring31electrically connects the n-type portion12and the signal-processing circuit30.

The boundary scattering frequency of phonons in the first phononic crystal11cof the p-type portion11is higher than that in the other portion, and the first phononic crystal11chas a thermal conductivity that is lower than the thermal conductivity of the p-type material included in the p-type portion11. The boundary scattering frequency of phonons in the second phononic crystal12cof the n-type portion12is higher than that in the other portion, and the second phononic crystal12chas a thermal conductivity that is lower than the thermal conductivity of the n-type material included in the n-type portion12. The shape of the holes10hof the first phononic crystal11cand the holes10hof the second phononic crystal12cis not limited to a specific shape. The shape of the holes10hmay be a circle, or may be a polygon such as a triangle or a quadrangle, in a plan view of the first phononic crystal11cor the second phononic crystal12c. The holes10hmay be through-holes extending through the sensor layer15or may be non-through-holes. When the holes10hare through-holes, the thermal conductivity of the p-type portion11or the n-type portion12tends to be lower. When the holes10hare non-through-holes, the beam15btends to have high strength.

For example, in the first phononic crystal11cand the second phononic crystal12c, the arrangement of the holes10hhas periodicity. In other words, the holes10hare regularly arranged in a plan view of each of the first phononic crystal11cand the second phononic crystal12c. The period of the holes10his, for example, in the range from 1 nm to 5 μm. The wavelength of phonons that carry heat mainly encompasses a range from 1 nm to 5 μm, and thus the fact that the period of the holes10his in the range from 1 nm to 5 μm is advantageous in reducing the thermal conductivity of the first phononic crystal11cand the second phononic crystal12c.

FIGS.2A,2B,2C, and2Dillustrate examples of a unit cell10kof a phononic crystal. The unit cell of the first phononic crystal11cand the second phononic crystal12cis not limited to a specific unit cell. As illustrated inFIG.2A, the unit cell10kmay be a tetragonal lattice. As illustrated inFIG.2B, the unit cell10kmay be a triangular lattice. As illustrated inFIG.2C, the unit cell10kmay be a rectangular lattice. As illustrated inFIG.2D, the unit cell10kmay be a face-centered rectangular lattice.

Each of the first phononic crystal11cand the second phononic crystal12cmay include unit cells of different types.FIG.2Eillustrates an example of a phononic crystal. As illustrated inFIG.2E, for example, arrangement patterns of the holes10hhaving two different types of unit cells10kmay coexist in the first phononic crystal11cor the second phononic crystal12c.

As illustrated inFIG.1A, each of the first phononic crystal11cand the second phononic crystal12cis formed, for example, in the beam15b. Thus, it is possible to increase thermal insulation between the substrate20and the sensing portion15d, and the thermopile sensor1atends to have high sensitivity.

Each of the first phononic crystal11cand the second phononic crystal12cis, for example, a single crystal. Each of the first phononic crystal11cand the second phononic crystal12cmay be a polycrystal. In this case, each of the first phononic crystal11cand the second phononic crystal12chas domains, and a phononic crystal in each domain is a single crystal. In other words, a phononic crystal in a polycrystal state is a complex of phononic single crystals. In the domains, the holes10hare regularly arranged in different directions. In each domain, the orientations of unit cells are the same. In a plan view of each of the first phononic crystal11cand the second phononic crystal12c, the shapes of the domains may be the same or may be different. In a plan view of each of the first phononic crystal11cand the second phononic crystal12c, the sizes of the domains may be the same or may be different.

When the first phononic crystal11cor the second phononic crystal12cis a polycrystal, the shape of each domain in a plan view is not limited to a specific shape. The shape of each domain in a plan view is, for example, a polygon such as a triangle, square, or a rectangle, a circle, an ellipse, or a composite of these shapes. The shape of each domain in a plan view may be an indefinite shape. The number of domains included in the first phononic crystal11cor the second phononic crystal12cis not limited to a specific value.

For example, the thermal conductivity of the p-type material included in the p-type portion11is different from the thermal conductivity of the n-type material included in the n-type portion12. In other words, one of the thermal conductivities of the p-type material and the n-type material is higher than the thermal conductivity of the other of the p-type material and the n-type material. The boundary scattering frequency of phonons in the phononic crystal of one of the p-type portion11and the n-type portion12including a material having a higher thermal conductivity is higher than the boundary scattering frequency of phonons in the phononic crystal of the other of the p-type portion11and the n-type portion12. Alternatively, the ratio of the sum of the areas of the holes10hto the area of the phononic crystal in a plan view of one of the p-type portion11and the n-type portion12is greater than that of the other of the p-type portion11and the n-type portion12.

When the thermal conductivity of the p-type material of the p-type portion11is higher than the thermal conductivity of the n-type material of the n-type portion12, the thermopile sensor1asatisfies, for example, at least one condition selected from a group consisting of the following (Ia) and (Ha). In these conditions, the arrangement of the holes10hin the first phononic crystal11cand the second phononic crystal12cis not limited to a specific form.(Ia) The boundary scattering frequency of phonons in the first phononic crystal11cis higher than the boundary scattering frequency of phonons in the second phononic crystal12c.(IIa) The ratio R1is greater than the ratio R2.

The boundary scattering frequency of phonons in a phononic crystal can be regulated, for example, by adjusting at least one selected from a group consisting of (i), (ii), and (iii) described above. When the thermal conductivity of the p-type material of the p-type portion11is higher than the thermal conductivity of the n-type material of the n-type portion12, for example, at least one selected from a group consisting of the following (ia), (iia), and (iiia) is satisfied.(ia) The shortest distance between the nearest holes10hin a plan view of the first phononic crystal11cis shorter than the shortest distance between the nearest holes10hin a plan view of the second phononic crystal12c.(iia) The ratio R1is greater than the ratio R2.(iiia) The specific surface area SV1of the first phononic crystal11cis greater than the specific surface area SV2of the second phononic crystal12c. The specific surface area SV1is determined by dividing the surface area of the first phononic crystal11cby the volume of the first phononic crystal11c. The specific surface area SV2is determined by dividing the surface area of the second phononic crystal12cby the volume of the second phononic crystal12c.

When the thermal conductivity of the n-type material of the n-type portion12is higher than the thermal conductivity of the p-type material of the p-type portion11, the thermopile sensor1asatisfies, for example, at least one condition selected from a group consisting of the following (Ib) and (IIb). In these conditions, the arrangement of the holes10hin the first phononic crystal11cand the second phononic crystal12cis not limited to a specific form.(Ib) The boundary scattering frequency of phonons in the second phononic crystal12cis higher than the boundary scattering frequency of phonons in the first phononic crystal11c.(IIb) The ratio R2is greater than the ratio R1.

When the thermal conductivity of the n-type material of the n-type portion12is higher than the thermal conductivity of the p-type material of the p-type portion11, the thermopile sensor1asatisfies, for example, at least one selected from a group consisting of the following (ib), (iib), and (iiib).(ib) The shortest distance between the nearest holes10hin a plan view of the second phononic crystal12cis shorter than the shortest distance between the nearest holes10hin a plan view of the first phononic crystal11c.(iia) The ratio R2is greater than the ratio R1.(iiia) The specific surface area SV2of the second phononic crystal12cis greater than the specific surface area SV1of the first phononic crystal11c.

Regarding the conditions (ia) and (ib), it is assumed that the shortest distance between the nearest holes10his different from place to place in the first phononic crystal11cor the second phononic crystal12c. In this case, for example, the shortest distance between each hole10hand the nearest hole10his determined. Then, the sum of the shortest distances for the holes10hmay be divided by the number of the holes10hto determine the shortest distance between the nearest holes10hin a plan view of the first phononic crystal11cor the second phononic crystal12c.

Each ofFIGS.2F,2G,2H,2I,2J,2K,2L,2M,2N, and20illustrates an example of a phononic crystal that forms the first phononic crystal11cand the second phononic crystal12c.

One of the first phononic crystal11cand the second phononic crystal12cis, for example, a phononic crystal10aillustrated inFIG.2F. In addition, the other of the first phononic crystal11cand the second phononic crystal12cis, for example, a phononic crystal10billustrated inFIG.2G.

In a plan view of the phononic crystal10a, the diameter of each hole10his d1, and the shortest distance between the nearest holes10his c1. In a plan view of the phononic crystal10b, the diameter of each hole10his d2, and the shortest distance between the nearest holes10his c2. Although a relationship d1<d2is satisfied, the quotient of the diameter of each hole10hdivided by the period of the arrangement of the holes10his uniform in the phononic crystal10aand the phononic crystal10b. Therefore, the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view is uniform in the phononic crystal10aand the phononic crystal10b. On the other hand, because a relationship c1<c2is satisfied, the boundary scattering frequency of phonons in the phononic crystal10ais higher than the boundary scattering frequency of phonons in the phononic crystal10b.

The shortest distance between the nearest holes in a phononic crystal can be regulated by, for example, changing the period of regular arrangement of holes. For example, suppose a case where the base material of the phononic crystal is Si, the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view is 50%, and the holes are regularly arranged with a period that is less than or equal to 100 nm. In this case, when the period of the arrangement of the holes is changed by 10%, the thermal conductivity of the phononic crystal may change by greater than or equal to 15%. For example, suppose a case where the base material of the phononic crystal is Si, the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view is 50%, and the holes are regularly arranged with a period that is equal to or less than 50 nm. In this case, when the period of the arrangement of the holes is changed by 5%, the thermal conductivity of the phononic crystal may change by greater than or equal to 10%. Therefore, for example, it is conceivable to regulate the difference between the period of the arrangement of the holes of the phononic crystal of the p-type portion and the period of the arrangement of the holes of the phononic crystal of the n-type portion n-type portion to about 5%. In this case, the difference between the thermal conductivity of the phononic crystal of the p-type portion and the thermal conductivity of the phononic crystal of the n-type portion can be sufficiently reduced, and the risk of breakage of a member such as the thermocouple can be reduced in the thermopile sensor. As the ratio of the sum of the areas of holes to the area of a phononic crystal in a plan view becomes larger, the thermal conductivity of the phononic crystal may change by a larger amount due to a slight change in the period of the arrangement of the holes.

One of the first phononic crystal11cand the second phononic crystal12cmay be a phononic crystal10cillustrated inFIG.2H. In addition, the other of the first phononic crystal11cand the second phononic crystal12cmay be a phononic crystal10dillustrated inFIG.2I.

In a plan view of the phononic crystal10c, the diameter of each hole10his d3, and the shortest distance between the nearest holes10his c3. In a plan view of the phononic crystal10d, the diameter of each hole10his d4, and the shortest distance between the nearest holes10his c4. The period of the arrangement of the holes10his uniform in the phononic crystal10cand the phononic crystal10d. On the other hand, in the phononic crystal10cand the phononic crystal10d, relationships d3>d4and c3<c4are satisfied. The shortest distance between the nearest holes10h, the ratio of the sum of the areas of the holes10hto the area of the phononic crystal in a plan view, and the quotient of the sum of the circumferences of the holes10hin a plan view of the phononic crystal divided by the area of the phononic crystal are considered. Considering these matters, the boundary scattering frequency of phonons in the phononic crystal10cis higher than the boundary scattering frequency of phonons in the phononic crystal10d.

For example, suppose a case where the base material of a phononic crystal is Si, the holes are regularly arranged with a period of 300 nm, and the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view exceeds 19%. In this case, when the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view is changed by 2%, the thermal conductivity of the phononic crystal may change by greater than or equal to 10%. Therefore, for example, it is conceivable to regulate the difference between the ratio of the sum of the areas of holes to the area of the phononic crystal of the p-type portion in a plan view and the ratio of the sum of the areas of holes to the area of the phononic crystal of the n-type portion in a plan view to about 2%. In this case, the difference between the thermal conductivity of the p-type portion and the thermal conductivity of the n-type portion can be sufficiently reduced, and the risk of breakage of a member such as the thermocouple can be reduced in the thermopile sensor. As the period of the arrangement of holes in a phononic crystal becomes smaller, the thermal conductivity of the phononic crystal may change by a larger amount due to a slight change in the ratio of the sum of the areas of the holes to the area of the phononic crystal in a plan view.

One of the first phononic crystal11cand the second phononic crystal12cmay be a phononic crystal10eillustrated inFIG.2J. In addition, the other of the first phononic crystal11cand the second phononic crystal12cmay be a phononic crystal10fillustrated inFIG.2K.

In a plan view the phononic crystal10e, the diameter of each hole10his d5, and the shortest distance between the nearest holes10his c5. In a plan view of the phononic crystal10f, the diameter of each hole10his d5, and the shortest distance between the nearest holes10his c6. The diameter of each hole10his uniform in a plan view of the phononic crystal10eand the phononic crystal10f. On the other hand, a relationship c5<c6is satisfied. The shortest distance between the nearest holes10h, the ratio of the sum of the areas of the holes10hto the area of the phononic crystal in a plan view, and the quotient of the sum of the circumference of the holes10hin a plan view of the phononic crystal divided by the area of the phononic crystal are considered. Considering these matters, the boundary scattering frequency of phonons in the phononic crystal10eis higher than the boundary scattering frequency of phonons in the phononic crystal10f.

One of the first phononic crystal11cand the second phononic crystal12cmay have a phononic crystal10gillustrated inFIG.2L. In addition, the other of the first phononic crystal11cand the second phononic crystal12cmay have a phononic crystal10millustrated inFIG.2M.

In a plan view of the phononic crystal10gand the phononic crystal10m, the diameter of each hole10his d7, and the shortest distance between the nearest holes10his c7. The unit cell of the arrangement of the holes10his a triangular lattice in a plan view of the phononic crystal10g, and the unit cell of the arrangement of the holes10hin a plan view the phononic crystal10mis a tetragonal lattice. The filling factor of a triangular lattice is higher than the filling factor of a tetragonal lattice. The ratio of the sum of the areas of the holes10hto the area of the phononic crystal in a plan view, and the quotient of the sum of the circumferences of the holes10hin a plan view of the phononic crystal divided by the area of the first phononic crystal are considered. Considering these matters, the boundary scattering frequency of phonons in the phononic crystal10gis higher than the boundary scattering frequency of phonons in the phononic crystal10m.

One of the first phononic crystal11cand the second phononic crystal12cmay have a phononic crystal10iillustrated inFIG.2N. In addition, the other of the first phononic crystal11cand the second phononic crystal12cmay have a phononic crystal10jillustrated inFIG.2O.

Each of the phononic crystal10iand the phononic crystal10jhas multiple types of arrangement patterns, regarding the arrangement of the holes10h. In a plan view, the phononic crystal10ihas an arrangement pattern of the holes10hsuch that the diameter of each hole10his d8and the shortest distance between the nearest holes10his c8. In addition, in a plan view, the phononic crystal10ihas an arrangement pattern of the holes10hsuch that the diameter of each hole10his d9and the shortest distance between the nearest holes10his c9. In a plan view, the phononic crystal10jhas an arrangement pattern of the holes10hsuch that the diameter of each hole10his d8and the shortest distance between the nearest holes10his c8. In addition, in a plan view, the phononic crystal10jhas an arrangement pattern of the holes10hsuch that the diameter of each hole10his d10and the shortest distance between the nearest holes10his c10. A relationship d9>d10is satisfied. Considering the ratio of the sum of the areas of the holes10hto the area of the phononic crystal in a plan view, the boundary scattering frequency of phonons in the phononic crystal10iis higher than the boundary scattering frequency of phonons in the phononic crystal10j.

The difference between the thermal conductivity of the first phononic crystal11cand the thermal conductivity of the second phononic crystal12cis not limited to a specific value. The difference is, for example, less than or equal to 10% of the lower one of the thermal conductivity of the first phononic crystal11cand the thermal conductivity of the second phononic crystal12c. Thus, the temperature of the thermocouple10tends to be maintained uniform, and the risk of breakage of a member due to a thermal stress in the thermopile sensor1acan be reduced. The difference between the thermal conductivity of the first phononic crystal11cand the thermal conductivity of the second phononic crystal12cmay be greater than or equal to 10% of the lower one of these thermal conductivities. It can be understood that it is effective that the difference between the thermal conductivity of the first phononic crystal11cand the thermal conductivity of the second phononic crystal12cis less than the difference between the thermal conductivity of the p-type material included in the p-type portion11and the thermal conductivity of the n-type material included in the n-type portion12.

The difference between the thermal conductivity of the first phononic crystal11cand the thermal conductivity of the second phononic crystal12cis, for example, less than or equal to 5 W/(m·K), may be less than or equal to 1 W/(m·K), or may be less than or equal to 0.5 W/(m·K).

The thermopile sensor1acan be modified from various viewpoints. Each ofFIGS.3A,3B,3C,3D,3E, and3Fillustrates a modification of the thermopile sensor1a. Except for portions that will be specifically described, these modifications are configured in the same way as the thermopile sensor1a. Elements that are the same as or in correspondence with those of the thermopile sensor1awill be denoted by the same numerals, and descriptions of such elements will be omitted. Descriptions of the thermopile sensor1aalso apply to these modifications, unless technologically contradictory.

As illustrated inFIG.3A, a thermopile sensor1bincludes, for example, an infrared reflection layer40. The infrared reflection layer40is disposed on the bottom surface of the recessed portion25. Thus, the thermopile sensor1btends to have high sensitivity to infrared rays. The material of the infrared reflection layer40is not limited to a specific material. The material may be a metal such as Al, Cu, W, or Ti, may be a metal compound such as TiN or TaN, or may be highly-conductive Si.

As illustrated inFIG.3B, a thermopile sensor1cincludes, for example, an infrared absorbing layer14. The infrared absorbing layer14is disposed, for example, on the hot junction13. Thus, the thermopile sensor1ctends to have higher sensitivity to infrared rays. The configuration of the infrared absorbing layer14is not limited to a specific configuration. The infrared absorbing layer14may be a film made of a material such as TaN, Cr, or Ti, a porous metal film, or may be a dielectric film made of SiO2or the like.

As illustrated inFIG.3C, in a thermopile sensor1d, each of the first phononic crystal11cand the second phononic crystal12cis also formed in the sensing portion15din addition to the beam15b. In this case, thermal insulation between the substrate20and the sensing portion15dcan be further increased.

In the thermopile sensor1e, the thermocouples10are arranged in parallel. In this case, even if one or more of the thermocouples10malfunction, sensing can be performed by using the other thermocouples10.

In the thermopile sensor1f, the thermocouples10are arranged in series. In this case, the thermopile sensor1ftends to have high sensitivity, because output power corresponding to the sum of thermoelectromotive forces generated by the thermocouples10can be obtained. As illustrated inFIG.3E, the thermopile sensor1fincludes a cold junction16. The cold junction16is connected to the connection portion15cand electrically connects the thermocouples10. The cold junction16includes, for example, a metal film. As illustrated inFIG.3E, in the thermopile sensor1f, the p-type portion11and the n-type portion12are formed in the same beam15b. Therefore, the first phononic crystal11cand the second phononic crystal12care formed in the same beam15b.

As illustrated inFIG.3F, a thermopile sensor1gincludes an insulating film18. Thus, multilayered wiring layers can be formed, and an electromotive force generated in the thermocouple10can be efficiently transmitted to the signal-processing circuit30.

An example of a method of manufacturing the thermopile sensor1awill be described. A method of manufacturing the thermopile sensor1ais not limited to the following method.

As illustrated inFIG.4A, the recessed portion25, having a depth of about 1 μm, is formed in one of main surfaces of the substrate20by photolithography and etching. The substrate20is, for example, a Si substrate. Next, as illustrated inFIG.4B, a sacrificial layer51including a material, such as SiO2, different from the material of the substrate20is formed in such a way as to cover the recessed portion25. Next, as illustrated inFIG.4C, the sacrificial layer51outside of the recessed portion25is removed by using a method such as chemical mechanical polishing (CMP). Next, as illustrated inFIG.4D, the signal-processing circuit30, including a transistor device, is formed in a region of the substrate20from which the sacrificial layer51has been removed. Subsequently, as illustrated inFIG.4E, the sensor layer15, including a semiconductor such as polycrystal Si, is formed by using a method such as chemical vapor deposition (CVD), and doping is performed on a predetermined region of the sensor layer15, thereby forming the p-type portion11and the n-type portion12. The doping is performed, for example, by using a method such as ion implantation.

Next, as illustrated inFIG.4F, a phononic crystal is formed in each of the p-type portion11and the n-type portion12of the sensor layer15. In accordance with the shape of the holes, multiple lithographic technologies are used to form the phononic crystal. For example, photolithography is used to form a phononic crystal having a period that is greater than or equal to 300 nm. Electron-beam lithography is used to form a phononic crystal having a period that is in the range from 100 nm to 300 nm. Block copolymer lithography is used to form a phononic crystal having a period that is in the range from 1 nm to 100 nm. A method of forming the phononic crystal is not limited to these methods. The phononic crystal may be formed by using another lithographic technology such as nanoimprinting lithography. Whichever of the lithographic technologies is used, the phononic crystal can be formed in an arbitrary region of the sensor layer15. A phononic crystal including multiple types unit cells10killustrated inFIGS.2E,2N, and2O can be formed by forming beforehand a drawing pattern, corresponding to the unit cells, by photolithography or electron-beam lithography. The phononic crystal including multiple types of unit cells may be formed by using a combination of multiple types of lithographic technologies. For example, unit cells having a small period are formed on a desirable region by block copolymer lithography or electron-beam lithography. Subsequently, unit cells having a large period are overlappingly formed on the same region by photolithography.

When a phononic crystal is to be formed by photolithography, a photomask in which holes having different diameters, different periods, or different unit cells is prepared. A pattern for forming the first phononic crystal11cmay be formed on a photomask that is the same as a photomask for forming the second phononic crystal12cor may be formed on another photomask. Through exposure and development processes, patterns of the first phononic crystal11cand the second phononic crystal12cdrawn on the photomask are transferred to a resist film applied onto the sensor layer15. Subsequently, by etching the sensor layer15from the upper surface of the resist film, the holes10hare formed in the first phononic crystal11cand the second phononic crystal12c. Lastly, the holes10hin the first phononic crystal11cand the second phononic crystal12care obtained by removing the resist film.

A case where a phononic crystal is to be formed by electron-beam lithography will be described. A pattern of holes having different diameters, different periods, or different unit cells in a region corresponding to the first phononic crystal11cand a region corresponding to the second phononic crystal12cis input to an electron-beam irradiation apparatus. The sensor layer15is irradiated with an electron beam that is scanned in accordance with the input data. Thus, patterns of the first phononic crystal11cand the second phononic crystal12care directly drawn on a resist film applied onto the sensor layer15. After the drawn patterns have been developed, the sensor layer15is etched from the upper surface of the resist film to which the pattern has been transferred. Thus, the holes10hin the first phononic crystal11cand the second phononic crystal12care formed. Lastly, the holes10hin the first phononic crystal11cand the second phononic crystal12care obtained by removing the resist film.

When a phononic crystal is to be formed by block copolymer lithography, for example, block copolymers having different compositions are used to form the first phononic crystal11cand the second phononic crystal12c. The period of and the arrangement pattern of the self-organized structure in a block copolymer changes in accordance with the type of the block copolymer or the composition ratio of polymers in the block copolymer. Therefore, by using two types of block copolymers having different compositions, it is possible to form two types of phononic crystals whose diameters, periods, or unit cells are different. First, by using a first block copolymer, the first phononic crystal11cis formed by block copolymer lithography. Subsequently, by using a second block copolymer, the second phononic crystal12cis formed by block copolymer lithography. Known process conditions can be used for the block copolymer lithography.

As illustrated inFIG.4G, after the first phononic crystal11cand the second phononic crystal12chave been formed, the sensing portion15dand the beam15bare formed in the sensor layer15by photolithography and etching. At this time, a contact hole52is also formed. Next, as illustrated inFIG.4H, a film including a material such as TiN, TaN, Al, Cr, Ti, or Cu is formed on the sensor layer15. By etching the film, the hot junction13, the wiring31, and the electrode pad33are formed.

In order to reduce the electrical resistance of the wiring31, the material of the hot junction13and the material of the wiring31may be made different from each other. The thickness of a film for the wiring31may be made greater than the thickness of a film for the hot junction13. The material of the wiring31may be a metal having a low electrical resistance, such as Al or Cu. In this case, the thickness of the wiring31is, for example, in the range from 100 nm to 500 nm. The thermocouple10is constituted by the p-type portion11, the n-type portion12, the hot junction13, and the wiring31formed in the sensor layer15.

Lastly, by removing the sacrificial layer51by selective etching, the recessed portion25is formed in the substrate20. Thus, the beam15band the sensing portion15din the sensor layer15are spanned in a state of being separated from the substrate20. Alternatively, the sensing portion15dmay be spanned in a state of being separated from the substrate20by removing a part of the substrate20by anisotropic etching. In this case, the step of forming the recessed portion25in the substrate20can be omitted.

In a case where the thermopile sensor1gis to be manufactured, the insulating film18is formed on the uppermost layer in a state illustrated inFIG.4G. In this case, it is possible to expose the beam15band the sensing portion15dof the sensor layer15by removing a region of the insulating film18that overlaps the beam15band the sensing portion15dof the sensor layer15by photolithography and etching.

Second Embodiment

FIGS.5A and5Billustrate a thermopile sensor1haccording to a second embodiment. Except for portions that will be specifically described, the thermopile sensor1his configured in the same way as the thermopile sensor1a. Elements of the thermopile sensor1hthat are the same as or in correspondence with those of the thermopile sensor1awill be denoted by the same numerals, and descriptions of such elements will be omitted. Descriptions of the thermopile sensor1aalso apply to the thermopile sensor1h, unless technologically contradictory.

As illustrated inFIGS.5A and5B, the sensor layer15of the thermopile sensor1hhas a support layer15s. The thermocouple10is disposed on the support layer15s. The sensor layer15includes, for example, a thermocouple layer151including the p-type portion11and the n-type portion12. The thermocouple layer151is disposed on the support layer15s. The strength of a structure in which the beam15band the sensing portion15dare spanned tends to be high due to the support layer15s. In addition, a stress generated in the thermocouple10can be regulated due to the support layer15s.

The thickness of the support layer15sis not limited to a specific value. The thickness is, for example, greater than or equal to 10 nm and less than or equal to 500 nm. The material of the support layer15smay be the same as or may be different from the material of the thermocouple layer151. The material of the support layer15sis not limited to a specific material. The material may be a semiconductor material such as Si, SiGe, SiC, GaAs, InAs, InSb, InP, GaN, or ZnO, or may be an insulator material such as SiO2, SiN, or Al2O3. The material of the support layer15smay be a single crystal material, may be a polycrystal material, or may be an amorphous material.

In the thermopile sensor1h, the base material of the p-type portion11and the base material of the n-type portion12may be the same material, or may be different materials. For example, the base material of the p-type portion11may be Si, and the base material of the n-type portion12may be SrTiO3. Other examples of the base materials of the p-type portion11and the base material of the n-type portion12include BiTe, Bi, Sb, Constantan, Chromel, and Alumel. Chromel is a registered trademark of Concept Alloys, Inc. The base material of the thermocouple layer151may be another material.

In the thermopile sensor1h, holes may be formed in the support layer15s. In this case, the holes may be formed in the support layer15sin such a way as to form a phononic crystal. The holes in the support layer15smay be formed in correspondence with the holes10hin the first phononic crystal11cor the second phononic crystal12c. The holes in the support layer15smay be formed in an arrangement pattern that is different from the arrangement pattern of the holes10h.

The thermal conductance Gb of the beam15bis represented as Gb=Gt+Gs by using the thermal conductance Gt of the thermocouple layer15tand the thermal conductance Gs of the support layer15s. For example, if the thermal conductivity of the base material of the thermocouple layer15tis greater than or equal to 5 times the thermal conductivity of the base material of the support layer15s, the absolute value of the thermal conductance Gb of the beam15bis close to the absolute value of the thermal conductance Gt of the thermocouple layer15t. In this case, the thermal conductance of the support layer15sdoes not greatly affect the thermal conductance of the beam15b. For example, suppose a case where a semiconductor or a semimetal, such as Si, SrTiO3, or Bi, is used for the thermocouple layer151and an amorphous insulator, such as SiO2or SiN, is used for the support layer15s. In this case, the thermal conductivity of the base material of the thermocouple layer151may be greater than or equal to 5 times the thermal conductivity of the base material of the support layer15s. In this case, the thermal conductance Gt of the thermocouple layer151is dominant in the thermal conductance Gb of the beam15b. Therefore, an effect obtained by regulating the boundary scattering frequency of phonons in the first phononic crystal11cand the boundary scattering frequency of phonons in the second phononic crystal12cis not greatly affected by the presence or absence of holes in the support layer15s. In a case where the support layer15shas holes, each hole may be a through-hole or may be a non-through-hole.

FIG.5Cillustrates a modification of the thermopile sensor1h. As illustrated inFIG.5C, in a thermopile sensor1i, the support layer15shas multiple support layers. The support layer15shas, for example, a first support layer15saand a second support layer15sb. The first support layer15sais disposed between the second support layer15sband the thermocouple layer151in the thickness direction of the first support layer15sa. With such a configuration, the strength of a structure in which the beam15band the sensing portion15dare spanned tends to be higher, and a stress generated in the thermocouple10can be regulated to be in a more desirable range.

An example of a method of manufacturing the thermopile sensor1hwill be described. A method of manufacturing the thermopile sensor1his not limited to the following method.

The thermopile sensor1hcan be manufactured, for example, by applying the manufacturing method of the first embodiment. As illustrated inFIG.6A, in the same way as in the method of manufacturing the thermopile sensor1a, the signal-processing circuit30is formed by filling the recessed portion25formed in the substrate20, which is a Si substrate, with the sacrificial layer51made of a dielectric such as SiO2. The support layer15sis formed from a material, such as SiN, different from the material of the sacrificial layer51on the substrate20on which the sacrificial layer51has been formed. Next, the thermocouple layer151including a semiconductor such as Si is formed on the support layer15s. The p-type portion11and the n-type portion12are formed in the thermocouple layer151by doping. The doping can be performed by using a method such as ion implantation.

Next, as illustrated inFIG.6B, by using a method the same as that in the first embodiment, phononic crystals whose boundary scattering frequencies of phonons are different from each other are formed in the p-type portion11and the n-type portion12of the thermocouple layer151. In this case, lithography is performed on the thermocouple layer151. A phononic crystal similar to that in the thermocouple layer151may be formed in the support layer15sby adjusting the etching time when etching the thermocouple layer15t.

After the phononic crystals have been formed, the shape of the thermocouple layer151is regulated in accordance with the shapes of the p-type portion11and the n-type portion12by photolithography and etching. Subsequently, the support layer15sis shaped in accordance with the shapes of the sensing portion15dand the beam15bby photolithography and etching. As illustrated inFIG.6C, a contact hole52is formed by etching the thermocouple layer151and the support layer15s. Next, a film including a material such as TiN, TaN, Al, Cr, Ti, or Cu is formed on the sensor layer15. By etching the film, as illustrated inFIG.6D, the hot junction13, the wiring31, and the electrode pad33are formed.

Lastly, by removing the sacrificial layer51by selective etching, the recessed portion25is formed in the substrate20. Thus, the beam15band the sensing portion15din the sensor layer15are spanned in a state of being separated from the substrate20. Alternatively, the sensing portion15dmay be spanned in a state of being separated from the substrate20by removing a part of the substrate20by anisotropic etching. In this case, the step of forming the recessed portion25in the substrate20can be omitted.

Third Embodiment

FIGS.7A and7Billustrate a thermopile sensor1jaccording to a third embodiment. Except for portions that will be specifically described, the thermopile sensor1jis configured in the same way as the thermopile sensor1a. Elements of the thermopile sensor1jthat are the same as or in correspondence with those of the thermopile sensor1awill be denoted by the same numerals, and descriptions of such elements will be omitted. Descriptions of the thermopile sensor1aalso apply to the thermopile sensor1j, unless technologically contradictory.

As illustrated inFIGS.7A and7B, in the thermopile sensor1j, the sensor layer15has a protective layer15pthat covers the p-type portion11and the n-type portion12. The sensor layer15includes, for example, the thermocouple layer151including the p-type portion11and the n-type portion12. A contact hole is formed at the center of the protective layer15p, and the hot junction13is formed inside of the contact hole and on a region of the main surface of the protective layer15paround the contact hole. The strength of a structure in which the beam15band the sensing portion15dare spanned tends to be high due to the protective layer15p. In addition, a stress generated in the thermocouple10can be regulated due to the protective layer15p. The thermocouple10can be protected from oxidation environments and chemical solutions due to the protective layer15p.

The thickness of the protective layer15pis not limited to a specific value. The thickness is, for example, greater than or equal to 10 nm and less than or equal to 500 nm. The material of the protective layer15pmay be the same as or may be different from the material of the thermocouple layer151. The material of the protective layer15pis not limited to a specific material. The material may be a semiconductor material such as Si, SiGe, SiC, GaAs, InAs, InSb, InP, GaN, or ZnO, or may be an insulator material such as SiO2, SiN, or Al2O3. The material of the protective layer15pmay be a single crystal material, may be a polycrystal material, or may be an amorphous material.

In the thermopile sensor1j, the base materials of the p-type portion11and the base material of the n-type portion12may be the same material, or may be different materials. For example, the base material of the p-type portion11may be Si, and the base material of the n-type portion12may be SrTiO3. Other examples of the base materials of the p-type portion11and the base material of the n-type portion12include BiTe, Bi, Sb, Constantan, Chromel, and Alumel. Chromel is a registered trademark of Concept Alloys, Inc. The base material of the thermocouple layer15tmay be another material.

In the thermopile sensor1j, holes may be formed in the support layer15s. In this case, holes may be formed in the protective layer15pin such a way as to form a phononic crystal. The holes in the protective layer15pmay be formed in correspondence with the holes10hin the first phononic crystal11cor the second phononic crystal12c, or may be formed in an arrangement pattern that is different from the arrangement pattern of the holes10h.

The thermal conductance Gb of the beam15bis represented as Gb=Gt+Gp by using the thermal conductance Gt of the thermocouple layer15tand the thermal conductance Gp of the protective layer15p. For example, if the thermal conductivity of the base material of the thermocouple layer15tis greater than or equal to 5 times the thermal conductivity of the base material of the protective layer15p, the absolute value of the thermal conductance Gb of the beam15bis close to the absolute value of the thermal conductance Gt of the thermocouple layer15t. In this case, the thermal conductance of the protective layer15pdoes not greatly affect the thermal conductance of the beam15b. For example, suppose a case where a semiconductor or a semimetal, such as Si, SrTiO3, or Bi, is used for the thermocouple layer151and an amorphous insulator, such as SiO2or SiN, is used for the protective layer15p. In this case, the thermal conductivity of the base material of the thermocouple layer151may be greater than or equal to 5 times the thermal conductivity of the base material of the protective layer15p. In this case, the thermal conductance Gt of the thermocouple layer151is dominant in the thermal conductance Gb of the beam15b. Therefore, an effect obtained by regulating the boundary scattering frequency of phonons in the first phononic crystal11cand the boundary scattering frequency of phonons in the second phononic crystal12cis not greatly affected by the presence or absence of holes in the protective layer15p. In a case where the protective layer15phas holes, each hole may be a through-hole or may be a non-through-hole.

FIGS.7C and7Dillustrate modifications of the thermopile sensor1j. As illustrated inFIG.7C, in a thermopile sensor1k, the protective layer15phas multiple protective layers. The protective layer15phas, for example, a first protective layer15paand a second protective layer15pb. The first protective layer15pais disposed between the second protective layer15pband the thermocouple layer15tin the thickness direction of the first protective layer15pa. With such a configuration, the strength of a structure in which the beam15band the sensing portion15dare spanned tends to be higher, and a stress generated in the thermocouple10can be regulated to be in a more desirable range. In addition, the thermocouple10can be more reliably protected from oxidation environments and chemical solutions.

As illustrated inFIG.7D, in a thermopile sensor1l, the hot junction13may be covered by the protective layer15p. Thus, the hot junction13can be protected by the protective layer15p.

An example of a method of manufacturing the thermopile sensor1jwill be described. A method of manufacturing the thermopile sensor1jis not limited to the following method.

The thermopile sensor1jcan be manufactured, for example, by applying the manufacturing method of the first embodiment. As illustrated inFIG.8A, in the same way as in the method of manufacturing the thermopile sensor1a, the signal-processing circuit30is formed by filling the recessed portion25formed in the substrate20, which is a Si substrate, with the sacrificial layer51made of a dielectric such as SiO2. The thermocouple layer151including a semiconductor, such as Si, is formed on the substrate20on which the sacrificial layer51has been formed. The p-type portion11and the n-type portion12are formed in the thermocouple layer151by doping. The doping can be performed by using a method such as ion implantation.

Next, by using a method the same as that in the first embodiment, phononic crystals whose boundary scattering frequencies of phonons are different from each other are formed in the p-type portion11and the n-type portion12of the thermocouple layer151. After the phononic crystals have been formed, the shape of the thermocouple layer151is regulated in accordance with the shapes of the p-type portion11and the n-type portion12by photolithography and etching. In this case, as illustrated inFIG.8A, the contact hole52is formed.

Next, as illustrated inFIG.8B, the protective layer15p, including a material such as SiN, is formed on the thermocouple layer15t. Subsequently, the protective layer15pis shaped in accordance with the shapes of the sensing portion15dand the beam15bby photolithography and etching. As illustrated inFIG.8C, a contact hole54and a contact hole56are formed by etching the protective layer15p. Next, a film including a material such as TiN, TaN, Al, Cr, Ti, or Cu is formed on the sensor layer15. By etching the film, as illustrated inFIG.8D, the hot junction13, the wiring31, and the electrode pad33are formed.

Lastly, by removing the sacrificial layer51by selective etching, the recessed portion25is formed in the substrate20. Thus, the beam15band the sensing portion15din the sensor layer15are spanned in a state of being separated from the substrate20. Alternatively, the sensing portion15dmay be spanned in a state of being separated from the substrate20by removing a part of the substrate20by anisotropic etching. In this case, the step of forming the recessed portion25in the substrate20can be omitted.

Fourth Embodiment

FIGS.9A and9Billustrate a thermopile sensor1maccording to a fourth embodiment. Except for portions that will be specifically described, the thermopile sensor1mis configured in the same way as the thermopile sensor1a. Elements of the thermopile sensor1mthat are the same as or in correspondence with those of the thermopile sensor1awill be denoted by the same numerals, and descriptions of such elements will be omitted. Descriptions of the thermopile sensor1aalso apply to the thermopile sensor1m, unless technologically contradictory.

The sensor layer15of the thermopile sensor1mincludes the support layer15sand the protective layer15p. The thermocouple10is disposed on the support layer15s. The protective layer15pcovers the p-type portion11and the n-type portion12. The sensor layer15includes, for example, the thermocouple layer151including the p-type portion11and the n-type portion12. The thermocouple layer151is disposed between the support layer15sand the protective layer15pin the thickness direction thereof. A contact hole is formed at the center of the protective layer15p, and the hot junction13is formed inside of the contact hole and on a region of the main surface of the protective layer15paround the contact hole. With such a configuration, the strength of a structure in which the beam15band the sensing portion15dare spanned tends to be higher, and a stress generated in the thermocouple10can be regulated to be in a more desirable range. In addition, the thermocouple10can be more reliably protected from oxidation environments and chemical solutions.

In the thermopile sensor1m, the materials of the support layer15s, the protective layer15p, and the thermocouple layer15tmay be, for example, the materials described in the second and third embodiments.

In the thermopile sensor1m, holes may be formed in the support layer15sand the protective layer15p. In this case, the holes may be formed in the support layer15sand the protective layer15pin such a way as to form a phononic crystal. The holes in the support layer15sand the protective layer15pmay be formed in correspondence with the holes10hin the first phononic crystal11cor the second phononic crystal12c. The holes in the support layer15sand the protective layer15pmay be formed in an arrangement pattern that is different from the arrangement pattern of the holes10h. As described in the second and third embodiments, an effect obtained by regulating the boundary scattering frequencies of phonons in the first phononic crystal11cand the second phononic crystal12cis not greatly affected by the presence or absence of holes in the support layer15sand the protective layer15p. In a case where the support layer15sand the protective layer15phave holes, each hole may be a through-hole or may be a non-through-hole.

FIGS.9C and9Dillustrate modifications of the thermopile sensor1m. As illustrated inFIG.9C, in a thermopile sensor1n, the support layer15shas multiple support layers. In addition, the protective layer15phas multiple protective layers. The support layer15shas, for example, the first support layer15saand the second support layer15sb. The first support layer15sais disposed between the second support layer15sband the thermocouple layer15tin the thickness direction of the first support layer15sa. The protective layer15phas, for example, the first protective layer15paand the second protective layer15pb. The first protective layer15pais disposed between the second protective layer15pband the thermocouple layer15tin the thickness direction of the first protective layer15pa. With such a configuration, the strength of a structure in which the beam15band the sensing portion15dare spanned tends to be higher, and a stress generated in the thermocouple10can be regulated to be in a more desirable range. In addition, the thermocouple layer15tcan be more reliably protected from oxidation environments and chemical solutions.

As illustrated inFIG.9D, in a thermopile sensor1o, the hot junction13may be covered by the protective layer15p. Thus, the hot junction13can be protected by the protective layer15p.

The thermopile sensor1mcan be manufactured by applying the manufacturing methods described in the second and third embodiments.

Fifth Embodiment

FIG.10Aillustrates a thermopile sensor1paccording to a fifth embodiment. Except for portions that will be specifically described, the thermopile sensor1pis configured in the same way as the thermopile sensor1a. Elements of the thermopile sensor1pthat are the same as or in correspondence with those of the thermopile sensor1awill be denoted by the same numerals, and descriptions of such elements will be omitted. Descriptions of the thermopile sensor1aalso apply to the thermopile sensor1p, unless technologically contradictory.

As illustrated inFIG.10A, in the thermopile sensor1p, the substrate20includes a first substrate21and an interlayer film22. The interlayer film22is disposed between the first substrate21and the sensor layer15in the thickness of direction of the interlayer film22. The recessed portion25is formed in the interlayer film22. The material of the interlayer film22is an insulator or a semiconductor, such as SiO2, SiN, or Si.

As illustrated inFIG.10A, the sensor layer15includes the support layer15s, the thermocouple layer15t, and the protective layer15p. The sensor layer15may have a single layer structure including the thermocouple layer15t, or may have another multilayer structure.

FIGS.10B,10C, and10Dillustrate modifications of the thermopile sensor1p. As illustrated inFIGS.10B,10C, and10D, each of thermopile sensors1q,1r, and is includes the infrared reflection layer40. In the thermopile sensor1q, the infrared reflection layer40is disposed on the bottom surface of the recessed portion25. In the thermopile sensor1r, the infrared reflection layer40is disposed on the main surface of the first substrate21. In the thermopile sensor1s, the infrared reflection layer40is formed in such a way as to form a part of the main surface of the first substrate21. For example, when the first substrate21is a Si substrate, the infrared reflection layer40is obtained by doping a region of the main surface of the first substrate21corresponding to the infrared reflection layer40with a high-concentration dopant.

The material of the infrared reflection layer40is not limited to a specific material. The material may be a metal such as Al, Cu, W, or Ti, may be a metal compound such as TiN or TaN, or may be highly-conductive Si.

An example of a method of manufacturing the thermopile sensor1pwill be described. The thermopile sensor1pcan be manufactured, for example, by applying the method of manufacturing the thermopile sensor1aaccording to the first embodiment. As illustrated inFIG.11A, the signal-processing circuit30including a transistor device is formed on the first substrate21, which is a Si substrate. When the infrared reflection layer40is to be formed in such a way as to form a part of the main surface of the first substrate21as in the thermopile sensor1s, the infrared reflection layer40may be formed by doping a part of the main surface of the first substrate21with a high-concentration dopant.

Next, as illustrated inFIG.11B, the interlayer film22, which is made of a dielectric such as SiO2, is formed on the surface of the first substrate21. At this time, protrusions and recesses may be generated on the surface of the interlayer film22in accordance with the height of the signal-processing circuit30. As illustrated inFIG.11C, the protrusions and recesses may be removed by CMP to flatten the surface of the interlayer film22. Thus, in the following lithography step, a photoresist and a film of block copolymer tend to be formed in a desirable state.

Next, as illustrated inFIG.11D, the recessed portion25is formed in the interlayer film22by photolithography and etching. In forming the recessed portion25, the main surface of the first substrate21may become exposed. In this case, as illustrated inFIG.11E, a dielectric protective film53is formed in the recessed portion25. The dielectric protective film53is, for example, a dielectric film having a thickness of about 100 nm and made of SiN or the like. Thus, the main surface of the first substrate21is protected.

Next, as illustrated inFIG.11F, a sacrificial layer55is formed in such a way as to cover the recessed portion25. The material of the sacrificial layer55is different from the material of the interlayer film22. The material of the sacrificial layer55is, for example, Si. Next, as illustrated inFIG.11G, the sacrificial layer55outside of the recessed portion25is removed by using a method such as CMP. The thermopile sensor1pcan be manufactured by subsequently applying the manufacturing methods described in the first embodiment, the second embodiment, and the third embodiment.

Sixth Embodiment

FIGS.12A and12Billustrate sensor arrays2aand2baccording to a sixth embodiment. The sensor arrays2aand2binclude thermopile sensors1aforming a one-dimensional array or a two-dimensional array. The thermopile sensors1aforming a one-dimensional array or a two-dimensional array are connected to each other by the signal-processing circuit30and the wiring31. In the sensor arrays2aand2b, the thermopile sensors forming a one-dimensional array or a two-dimensional array may include any one of the thermopile sensors1bto1s, instead of the thermopile sensor1a. In the sensor arrays2aand2b, the thermopile sensors forming a one-dimensional array or a two-dimensional array may include multiple types of sensors selected from the thermopile sensors1bto1s.

EXAMPLES

Hereafter, referring to Examples, the present embodiment will be described in more detail. However, a thermopile sensor according to the present embodiment is not limited to any of the forms described in the following Examples.

A p-type Si film into which boron ions had been injected as an impurity with a dose of 1×1016cm−2and an n-type Si film into which phosphorous ions had been injected as an impurity with a dose of 4×1015cm−2were prepared. The thickness of each of the p-type Si film and the n-type Si film was 150 nm.

In accordance with a thermoreflectance method, the thermal conductivities of the p-type Si film and the n-type Si film were measured. Before a phononic crystal was formed, the thermal conductivity of the p-type Si film at 25° C. was 28 W/(m·K). Before a phononic crystal was formed, the thermal conductivity of the n-type Si film at 25° C. was 39 W/(m·K). It can be understood that the thermal conductivity of the n-type Si film was higher than the thermal conductivity of the p-type Si film by about 40%.

A phononic crystal was formed in each of the p-type Si film and the n-type Si film. In forming the phononic crystal, holes were arranged in a tetragonal lattice pattern. In addition, the period P of the arrangement of the holes and the diameter D of the holes were regulated so that the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view of the phononic crystal became 50%. Due to forming of the phononic crystal, the boundary scattering frequency of phonons increased, and the thermal conductivities of the p-type Si film and the n-type Si film decreased. A phononic crystal having a period P of 1000 nm and a diameter D of 800 nm was formed in the p-type Si film. On the other hand, a phononic crystal having a period P of 400 nm and a diameter D of 320 nm was formed in the n-type Si film. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 7.6 W/(m·K), and the thermal conductivity of the n-type Si film having the phononic crystal was 7.7 W/(m·K). The thermal conductivity of the n-type Si film having the phononic crystal was higher than the thermal conductivity of the p-type Si film having the phononic crystal by only about 1%. It is conceivable that, when a thermocouple is formed by using such p-type Si film and n-type Si film, a thermal stress generated in the thermocouple is low.

Comparative Example 1

A p-type Si film having a phononic crystal and an n-type Si film having a phononic crystal were obtained in the same way as in Example 1 except for the following. A phononic crystal having a period P of 1000 nm and a diameter D of 800 nm was formed in each of the p-type Si film and the n-type Si film. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 7.6 W/(m·K), and the thermal conductivity of the n-type Si film having the phononic crystal was 10.5 W/(m·K). The thermal conductivity of the n-type Si film having the phononic crystal was higher than the thermal conductivity of the p-type Si film having the phononic crystal by as large as about 39%.

A p-type Si film having a phononic crystal and an n-type Si film having a phononic crystal were obtained in the same way as in Example 1 except for the following. A phononic crystal having a period P of 150 nm and a diameter D of 120 nm was formed in the p-type Si film. On the other hand, a phononic crystal having a period P of 100 nm and a diameter D of 80 nm was formed in the n-type Si film. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 1.89 W/(m·K) and the thermal conductivity of the n-type Si film having the phononic crystal was 1.91 W/(m·K). In this case, the thermal conductivity of the n-type Si film having the phononic crystal was higher than the thermal conductivity of the p-type Si film having the phononic crystal by only about 1%. It is conceivable that, when a thermocouple is formed by using such p-type Si film and n-type Si film, a thermal stress generated in the thermocouple is low.

Comparative Example 2

A p-type Si film having a phononic crystal and an n-type Si film having a phononic crystal were obtained in the same way as in Example 2 except for the following. A phononic crystal having a period P of 150 nm and a diameter D of 120 nm was formed in each of the p-type Si film and the n-type Si film. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 1.89 W/(m·K), and the thermal conductivity of the n-type Si film having the phononic crystal was 2.63 W/(m·K). The thermal conductivity of the n-type Si film having the phononic crystal was higher than the thermal conductivity of the p-type Si film having the phononic crystal by as large as about 39%.

By comparing Example 1 with Comparative Example 1 and comparing Example 2 with Comparative Example 2, it can be understood that the boundary scattering frequency of phonons in a phononic crystal tends to become higher as the period P in the phononic crystal becomes smaller. Therefore, the thermal conductivity of a p-type Si film and an n-type Si film can be reduced further as the period P of the phononic crystal becomes smaller. It can be understood that, by differentiating the period P of the phononic crystal in a p-type portion from that in an n-type portion, the difference in thermal conductivity between the p-type portion and the n-type portion of the thermocouple can be reduced. In the phononic crystal in the p-type Si film of Example 1, the distance between the nearest holes was 200 nm. On the other hand, in the phononic crystal in the n-type Si film of Example 1, the distance between the nearest holes was 80 nm. In the phononic crystal in the p-type Si film of Example 2, the distance between the nearest holes was 30 nm. On the other hand, in the phononic crystal in the n-type Si film of Example 2, the distance between the nearest holes was 20 nm. It can be understood that, by differentiating the distance between the nearest holes in the phononic crystal in the p-type portion from that in the n-type portion, the difference in thermal conductivity between the p-type portion and the n-type portion of the thermocouple can be reduced.

A p-type Si film into which boron ions had been injected as an impurity with a dose of 4×1015cm−2and an n-type Si film into which phosphorous ions had been injected as an impurity with a dose of 1×1016cm−2were prepared. The thickness of each of the p-type Si film and the n-type Si film was 150 nm.

In accordance with a thermoreflectance method, the thermal conductivities of the p-type Si film and the n-type Si film were measured. Before a phononic crystal was formed, the thermal conductivity of the p-type Si film at 25° C. was 38 W/(m·K). Before a phononic crystal was formed, the thermal conductivity of the n-type Si film at 25° C. was 30 W/(m·K). It can be understood that the thermal conductivity of the p-type Si film was higher than the thermal conductivity of the n-type Si film by about 27%.

A phononic crystal was formed in each of the p-type Si film and the n-type Si film. In forming the phononic crystal, holes were arranged in a tetragonal lattice pattern, and the period P of the arrangement of the holes of the phononic crystal was regulated to 300 nm. A phononic crystal having a period P of 300 nm and a diameter D of 180 nm was formed in the p-type Si film. The ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view of the phononic crystal of the p-type Si film was 28%. On the other hand, a phononic crystal having a period P of 300 nm and a diameter D of 150 nm was formed in the n-type Si film. The ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view of the phononic crystal of the n-type Si film was 20%. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 11.5 W/(m·K), and the thermal conductivity of the n-type Si film having the phononic crystal was 12.1 W/(m·K). The thermal conductivity of the n-type Si film having the phononic crystal was higher than the thermal conductivity of the p-type Si film having the phononic crystal by only about 5%. It is conceivable that, when a thermocouple is formed by using such p-type Si film and n-type Si film, a thermal stress generated in the thermocouple is low.

Comparative Example 3

A p-type Si film having a phononic crystal and an n-type Si film having a phononic crystal were obtained in the same way as in Example 3 except for the following. A phononic crystal having a period P of 300 nm and a diameter D of 150 nm was formed in each of the p-type Si film and the n-type Si film. The ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view of the p-type Si film and the n-type Si film was 20%. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 15.3 W/(m·K), and the thermal conductivity of the n-type Si film having the phononic crystal was 12.1 W/(m·K). The thermal conductivity of the p-type Si film having the phononic crystal was higher than the thermal conductivity of the n-type Si film having the phononic crystal by as large as about 27%.

By comparing Example 3 with Comparative Example 3, it can be understood that the boundary scattering frequency of phonons in a phononic crystal tends to become higher as the diameter of holes in the phonic crystal becomes larger. Therefore, the thermal conductivity of a p-type Si film and an n-type Si film can be reduced further as the diameter of holes in a phononic crystal becomes larger. It can be understood that, by differentiating the diameter of holes in a phononic crystal in a p-type portion from that in an n-type portion, the difference in thermal conductivity between the p-type portion and the n-type portion of the thermocouple can be reduced. In the phononic crystal in the p-type Si film of Example 3, the distance between the nearest holes was 120 nm. On the other hand, in the phononic crystal in the n-type Si film of Example 3, the distance between the nearest holes was 150 nm. It can be understood that, by differentiating the distance between the nearest holes in the phononic crystal in the p-type portion from that in the n-type portion, the difference in thermal conductivity between the p-type portion and the n-type portion of the thermocouple can be reduced.

A p-type Si film and an n-type Bi film into each of which boron ions had been injected as an impurity with a dose of 4×1015cm−2were prepared. The thickness of each of the p-type Si film and the n-type Bi film was 150 nm.

In accordance with a thermoreflectance method, the thermal conductivities of the p-type Si film and the n-type Bi film were measured. Before a phononic crystal was formed, the thermal conductivity of the p-type Si film at 25° C. was 38 W/(m·K). On the other hand, before a phononic crystal was formed, the thermal conductivity of the n-type Bi film at 25° C. was 8 W/(m·K). It can be understood that the thermal conductivity of the p-type Si film was higher than the thermal conductivity of the n-type Bi film by about 375%.

A phononic crystal was formed in each of the p-type Si film and the n-type Bi film. In forming the phononic crystal, holes were arranged in a tetragonal lattice pattern, and the period P of the arrangement of the holes and the diameter D of the holes were regulated so that the ratio of the sum of the areas of holes to the area of the phononic crystal in a plan view of the phononic crystal became 50%. A phononic crystal having a period P of 150 nm and a diameter D of 120 nm was formed in the p-type Si film. On the other hand, a phononic crystal having a period P of 500 nm and a diameter D of 400 nm was formed in the n-type Bi film. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 2.6 W/(m·K), and the thermal conductivity of the n-type Bi film having the phononic crystal was 2.7 W/(m·K). The thermal conductivity of the n-type Bi film having the phononic crystal was higher than the thermal conductivity of the p-type Si film having the phononic crystal by only about 3%. It is conceivable that, when a thermocouple is formed by using such p-type Si film and n-type Bi film, a thermal stress generated in the thermocouple is low.

Comparative Example 4

A p-type Si film having a phononic crystal and an n-type Bi film having a phononic crystal were obtained in the same way as in Example 4 except for the following. A phononic crystal having a period P of 500 nm and a diameter D of 400 nm was formed in each of the p-type Si film and the n-type Bi film. In this case, the thermal conductivity of the p-type Si film having the phononic crystal was 9.0 W/(m·K), and the thermal conductivity of the n-type Bi film having the phononic crystal was 2.7 W/(m·K). The thermal conductivity of the p-type Si film having the phononic crystal was higher than the thermal conductivity of the n-type Bi film having the phononic crystal by as large as about 237%.

According to the Examples described above, it can be understood that, by regulating the boundary scattering frequency of phonons in a phononic crystal, even when there is a difference between the thermal conductivities of base materials, the difference in thermal conductivity between a p-type portion and an n-type portion of a thermocouple can be reduced.

INDUSTRIAL APPLICABILITY

A thermopile sensor according to the present disclosure can be used for various uses including uses as an infrared sensor.