Infrared sensor and manufacturing method for infrared sensor

Abstract

To provide an infrared sensor that is improved in light absorption amount of graphene, and a manufacturing method therefor.SOLUTION: An infrared sensor comprises: a substrate; an insulation film provided on a surface of the substrate; a graphene layer arranged on the insulation film; a first electrode connected to one end of the graphene layer; a second electrode connected to the end on the opposite side of the graphene layer; a cavity provided above or below the graphene layer, and formed of resist or a gas under reduced pressure or non-reduced pressure; and a mirror layer facing the graphene layer across the cavity.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an infrared sensor and a method for manufacturing the infrared sensor.

Background Art

[0002] Infrared sensors that detect infrared light emitted by heat-bearing objects are widely applied to automatic doors, surveillance cameras, infrastructure inspections, etc. As materials that are sensitive to the wavelength of infrared light, two-dimensional layered materials such as graphene and transition metal dichalcogenides (TMDC) are used. Among them, graphene is a two-dimensional material in which carbon atoms are arranged in a two-dimensional honeycomb pattern, and due to its characteristic band structure, it absorbs light in a wide wavelength range from the ultraviolet region to the terahertz band. The light absorption efficiency of two-dimensional materials is about a few percent, and devices such as making the two-dimensional material into multiple layers or utilizing surface plasmon resonance have been devised to increase the light absorption rate. An optoelectronic device using a microcavity including a two-dimensional carbon lattice structure has been proposed (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] One object of the present disclosure is to provide an infrared sensor with improved light absorption of graphene.

Means for Solving the Problems

[0005] According to one embodiment, the infrared sensor includes a substrate, an insulating film provided on the surface of the substrate, a graphene layer disposed on the insulating film, A first electrode connected to one end of the graphene layer; A second electrode connected to the opposite end of the graphene layer; A cavity provided above or below the graphene layer and formed of a resist or a gas under reduced pressure or non-reduced pressure; A mirror layer facing the graphene layer with the cavity therebetween and comprising.

Advantages of the Invention

[0006] An infrared sensor with improved light absorption of graphene is realized.

Brief Description of the Drawings

[0007]

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Embodiments for Carrying Out the Invention

[0008] Hereinafter, embodiments for carrying out the invention will be described with reference to the drawings. The embodiments described below are examples for embodying the technical idea of the invention, and the present invention is not limited to the following configurations and numerical values. In the drawings, components having the same function may be denoted by the same reference numerals, and redundant descriptions may be omitted. Partial substitution or combination between different embodiments and configuration examples is possible. The sizes, positional relationships, etc. of the respective members shown in each drawing may be exaggerated for easy understanding of the invention. When referring to "above" or "below" in terms of positional relationship, it refers to the up and down in the stacking direction and is not an absolute direction.

[0009] (First Embodiment) FIG. 1 is a top view (a) and a cross-sectional view taken along line I-I (b) of the infrared sensor 10 according to the first embodiment. The infrared sensor 10 includes a substrate 11, an insulating film 12 provided on the surface 111 of the substrate 11, a graphene layer 15 disposed on the insulating film 12, a first electrode 17 connected to one end of the graphene layer 15, and a second electrode 18 connected to the opposite end of the graphene layer 15. In the coordinate system of FIG. 1, the plane parallel to the surface 111 of the substrate 11 is the X-Y plane, and the direction perpendicular to the X-Y plane is the Z direction. The stacking direction of the infrared sensor 10 is the Z direction.

[0010] The infrared sensor 10 includes a cavity 19 provided above the graphene layer 15 and a mirror layer 14 facing the graphene layer 15 with the cavity 19 interposed therebetween. In the configuration example of FIG. 1, the cavity 19 is formed of a resist layer 191, and the mirror layer 14 is provided on the surface of the resist layer 191. The back surface 112 of the substrate 11 is the light incident surface, and the graphene layer 15 functions as a light absorption layer that senses the incident infrared light.

[0011] The substrate 11 is at least a substrate transparent to infrared light, and a silicon (Si) substrate, a sapphire substrate, a silicon carbide (SiC) substrate, etc. are used. The incident light that enters from the back surface 112 of the substrate 11 and is transmitted through the graphene layer 15 without being completely absorbed is reflected by the mirror layer 14 and returns to the graphene layer 15. The mirror layer 14 is formed of a metal film having a high reflectivity to infrared light. The cavity 19 confines the light reflected by the mirror layer 14 and promotes light absorption in the graphene layer 15.

[0012] The thickness of the cavity 19 is set so as to satisfy the condition that the interference between the incident light to the mirror layer 14 and the reflected light from the mirror layer 14 reinforces at the position of the graphene layer 15. In other words, the distance from the graphene layer 15 to the mirror layer 14 is determined so that the intensity distribution of the optical electric field of the interference light becomes maximum at the graphene layer 15. Let the distance from the graphene layer 15 to the mirror layer 14 be d, the refractive index of the resist layer 191 in the cavity 19 be n, and the wavelength of the infrared light to be detected be λ. Then, the thickness of the resist layer 191 is set so that the distance d satisfies d = mλ / 4n (m is a positive odd number). Here, the refractive index of the protective layer 16 covering the surface of the graphene layer 15 is treated as negligible.

[0013] As shown in FIGS. 1(a) and 1(b), the cavity 19 covered by the mirror layer 14 is provided above the graphene layer 15 between the first electrode 17 and the second electrode 18 in the X direction on the substrate 11. The cavity 19 may be in contact with the air layer in the Y direction. The first electrode 17 and the second electrode 18 have a high reflectance to infrared light and confine the infrared light reflected by the mirror layer 14 in the X direction. The resist layer 191 has a transmittance of 80% or more, preferably 90% or more, more preferably 95% or more with respect to the infrared wavelength to be detected. Thereby, the infrared light transmitted through the graphene layer 15 is efficiently guided to the mirror layer 14, and the infrared light reflected by the mirror layer is efficiently re-incident on the graphene layer 15.

[0014] A protective layer 16 is inserted between the resist layer 191 and the graphene layer 15 in the cavity 19 to protect the surface of the graphene layer 15. The protective layer 16 is an insulating layer formed at the atomic layer level, such as alumina or boron nitride. By sandwiching the graphene layer 15 between the insulating film 12 and the protective layer 16, the external influence on the graphene layer 15 can be suppressed. When boron nitride is used for the protective layer 16, the carrier mobility of the graphene layer 15 can be maintained high, and the optical characteristics and electronic characteristics of the infrared sensor 10 can be kept good.

[0015] To enhance the light absorption of the graphene layer 15, surface plasmon resonance is utilized. Plasmons are collective oscillations of carriers in a substance. When light is incident on graphene with a fine periodic structure, plasmon resonance occurs at a predetermined wavelength determined by the periodic structure, and the free electrons in the graphene oscillate collectively. The generated plasmons are localized in the periodic structure of the graphene and confined, thereby promoting light absorption.

[0016] Figure 2 shows an example of the periodic pattern of the graphene layer 15 used in the infrared sensor 10. In Fig. 2(a), the graphene layer 15a has a pattern in which periodic holes (or antidots) 151 are formed. In Fig. 2(b), the graphene layer 15b has a stripe pattern of graphene ribbons 152 arranged at regular intervals. The extending direction of the graphene ribbons 152 is the X direction between the first electrode 17 and the second electrode 18. Near the edges of the holes 151 or the graphene ribbons 152, confinement of the oscillating electric field of light is strong, and light absorption can be enhanced. The diameter and spacing of the holes 151, and the width and spacing of the graphene ribbons 152 are designed according to the wavelength to be absorbed, that is, the wavelength at which plasmon resonance is to be generated, and are selected according to the wavelength, for example, in the range of 100 to 400 nm. The planar shape of the holes 151 is not limited to circular, and any planar shape can be used as long as it can excite plasmons, such as elliptical, polygonal, etc. Due to the periodicity of the graphene layer 15a or 15b, light in a desired wavelength range, for example, near-infrared light, can be selectively absorbed.

[0017] The graphene layers 15a and 15b may be two or more layers of graphene. By stacking multiple graphene layers, the light absorption rate is improved compared to single-layer graphene. When the potentials of the graphene layers 15 are the same, by stacking multiple layers of graphene, the areal density of carriers, that is, the two-dimensional electrical conductivity, increases compared to single-layer graphene. Even when using multilayer graphene, surface plasmon resonance is utilized to absorb light in a desired wavelength range.

[0018] Returning to FIG. 1, by combining the cavity 19 that confines the light reflected by the mirror layer 14 with the surface plasmon resonance of the graphene layer 15, the light absorption amount of the graphene layer 15 is increased, and the sensitivity of the infrared sensor 10 is enhanced. When light is incident on the graphene layer 15, electrons in the graphene absorb the light and obtain energy. In order to generate a potential gradient or a temperature gradient in the graphene layer 15 between the first electrode 17 and the second electrode 18, the first electrode 17 and the second electrode 18 may be formed asymmetrically. By forming the first electrode 17 and the second electrode 18 with different materials or different shapes, a potential gradient is generated in the X direction in the plane of the graphene layer 15, and the photovoltaic voltage can be detected. When the first electrode 17 and the second electrode 18 are formed of the same material and symmetrically, the energy diffusion between the electrons in the graphene layer 15 becomes isotropic. In this case, in order to generate a photovoltaic voltage, a gate voltage may be applied to a part of the graphene layer 15 to make the electron state asymmetric, or a DC bias may be applied to the drain electrode (for example, the first electrode 17) to generate a potential gradient between the first electrode 17 and the second electrode 18.

[0019] FIGS. 3A to 3I are manufacturing process diagrams of the infrared sensor 10 of the first embodiment. In FIG. 3A, a substrate 11 having an insulating film 12 formed on its surface is prepared, and a graphene layer 150 is formed on the insulating film 12. The insulating film 12 is, for example, an alumina film with a thickness of 20 nm formed by ALD. The graphene layer 150 may be obtained by transferring graphene synthesized by CVD or the like on another substrate onto the insulating film 12. In order to process the graphene layer 150 into a desired shape, a mask 161 with a predetermined shape is formed on the graphene layer 150 using photoresist.

[0020] In FIG. 3B, the graphene layer 150 in the region not covered by the mask 161 is removed by reactive ion etching (RIE) using oxygen plasma. As a result, a graphene layer 15 with a predetermined shape is obtained on the surface of the insulating film 12. In this RIE, a periodic pattern may be simultaneously formed on the graphene layer 15, or a periodic pattern may be formed by electron beam lithography after the RIE.

[0021] In FIG. 3C, a resist pattern 163 for forming the first electrode 17 and the second electrode 18 by a lift-off method is formed. Next, a metal layer 165 serving as an electrode material is formed over the entire surface by electron beam evaporation. The metal layer 165 is formed of a good conductor such as Au, Pt, Ag, etc. In order to enhance the adhesion to the insulating film 12, a metal thin film with high adhesion to alumina may be inserted. In this example, the metal layer 165 is formed as a laminate of a 5-nm-thick Ti film and a 50-nm-thick Au film.

[0022] In FIG. 3D, the resist pattern 163 together with the upper metal layer 165 is removed with a stripping solution such as an organic solvent. The metal layer 165 in the portion not covered by the resist pattern 163 remains, and the first electrode 17 and the second electrode 18 are formed. The first electrode 17 is connected to one end in the X direction (see FIG. 1) of the graphene layer 15, and the second electrode 18 is connected to the other end of the graphene layer 15.

[0023] In FIG. 3E, a protective layer 160 is formed over the entire surface, and a resist pattern 164 for patterning is formed on the protective layer 160. As the protective layer 160, for example, a 5-nm-thick alumina layer is formed by ALD.

[0024] In FIG. 3F, using the resist pattern 164 as a mask, the unnecessary portion of the protective layer 160 is removed by etching. Thereby, a protective layer 16 covering the surface of the graphene layer 15 and the side surfaces of the first electrode 17 and the second electrode 18 is formed. The graphene layer 15 is sandwiched between the insulating film 12 and the protective layer 16, and the influence from the outside on the graphene layer 15 is suppressed.

[0025] In FIG. 3G, a resist layer 191 that forms a cavity 19 is formed on the graphene layer 15 via a protective layer 16. The resist layer 191 is formed by applying a liquid or paste-like resist and patterning it into a predetermined shape by exposure and development. The resist layer 191 is formed of a resist material that is not peeled off by the peeling liquid used in the subsequent lift-off process. For example, the resist layer 191 that forms the cavity 19 is formed of a negative resist such as SU-8 so that the resist layer 191 cured by exposure remains. The resist layer 191 has a high transmittance of 80% or more, preferably 90% or more, and more preferably 95% or more with respect to the infrared wavelength of the detection target. Thereby, the cavity 19 that efficiently confines infrared light can be formed without requiring a complicated film formation process.

[0026] In FIG. 3H, a resist pattern 166 for lift-off is formed. The resist pattern 166 is formed of, for example, a positive resist, and the region covering the resist layer 191 of the cavity 19 is exposed and removed with a developer. A metal layer 140 is formed to cover the surface of the exposed resist layer 191 of the cavity 19 and the surface of the resist pattern 166 that remains without being removed. The metal layer 140 is formed of a metal material having a high reflectivity with respect to infrared light. For example, an Al film with a thickness of 5 nm and an Au film with a thickness of 50 nm are formed by electron beam evaporation. Al and Au have a high reflectivity with respect to infrared light.

[0027] In FIG. 3I, the resist pattern 166 is peeled off together with the upper metal layer 140 using a solvent-based peeling liquid. By this lift-off, the metal layer 140 covering the resist layer 191 of the cavity 19 remains, and the unnecessary metal layer 140 is removed. The cured resist layer 191 is not peeled off by the peeling liquid for peeling the resist pattern 166 and remains on the protective layer 16. Thereby, the cavity 19 and the mirror layer 14 covering the surface of the cavity 19 are formed, and the infrared sensor 10 is obtained.

[0028] The graphene layer 15 having a periodic pattern faces the mirror layer 14 with the cavity 19 interposed therebetween in the stacking direction in the region between the first electrode 17 and the second electrode 18. Infrared light incident from the back surface 112 of the substrate 11 and incident on the mirror layer 14 without being completely absorbed by the graphene layer 15 is reflected by the mirror layer 14 and incident on the graphene layer 15 again. Due to surface plasmon resonance and the cavity configuration, the light absorption amount of the graphene layer 15 increases.

[0029] FIG. 4 is a top view (a) and a cross-sectional view taken along line II-II (b) of an infrared sensor array 100 using the infrared sensor 10 of FIG. 1. A plurality of infrared sensors 10 are arranged on the substrate 11. In the example of FIG. 4, the plurality of infrared sensors 10 are arranged in the X direction, and the infrared sensor array 100 is formed as a one-dimensional array. Each infrared sensor 10 constitutes a pixel. The infrared sensor array 100 may be configured as a one-dimensional array extending in the Y direction, or may be formed as a two-dimensional array extending in the X direction and the Y direction.

[0030] In each infrared sensor 10 mounted on the substrate 11, a periodic pattern that generates surface plasmon resonance in the graphene layer 15 is formed, and a mirror layer 14 facing the graphene layer 15 with the cavity 19 interposed therebetween is arranged. Absorption of infrared light having a target wavelength is promoted in each infrared sensor 10, and the sensitivity of the infrared sensor array 100 can be improved. In each infrared sensor 10, the area occupied by the graphene layer 15 may be made as large as possible.

[0031] (First Modified Example) FIG. 5 is a top view (a) and a cross-sectional view taken along line III-III (b) of an infrared sensor 10A which is a first modified example of the infrared sensor of FIG. 1. The infrared sensor 10A is entirely covered with a resist layer 192 except for the upper surfaces of the first electrode 17 and the second electrode 18. The resist layer 192 may be formed of the same material and in the same process as the resist layer 191. That is, after applying the resist material for forming the cavity 19 over the entire surface in FIG. 3G, when patterning by exposure and development, patterning is performed so that only the surfaces of the first electrode 17 and the second electrode 18 are exposed.

[0032] Similar to the infrared sensor 10 in FIG. 1, the graphene layer 15 has a periodic pattern, and its surface is covered with a protective layer 16. The graphene layer 15 faces the mirror layer 14A with the cavity 19A formed in the resist layer 191 interposed therebetween. Infrared light incident from the back surface 112 of the substrate 11 and not completely absorbed by the graphene layer 15 is reflected by the mirror layer 14A and re-incident on the graphene layer 15. Due to this cavity structure and the surface plasmon resonance of the graphene layer 15, the amount of infrared absorption in the graphene layer 15 increases. The resist layer 192 provided around the infrared sensor 10A protects the side surfaces of each infrared sensor 10A and stabilizes the structure. The configuration of FIG. 5 may be applied to the infrared sensor array 100 of FIG. 4.

[0033] (Second Modified Example) FIG. 6 is a schematic cross-sectional view of an infrared sensor 10B, which is a second modified example of the infrared sensor in FIG. 1. The top view of the infrared sensor 10B is the same as that in FIG. 5(a), and the cross-sectional configuration along the line III-III is the configuration in FIG. 6. The infrared sensor 10B has a gate electrode 13B between the substrate 11 and the insulating film 12. The gate electrode 13B applies a gate voltage to the graphene layer 15. The gate electrode 13B may be formed of a second graphene layer or a layered TMDC. Alternatively, it may be formed of a metal oxide such as ITO with a high transmittance to infrared light. By applying a gate voltage to the graphene layer 15, the electronic state of the graphene layer 15 can be modulated to optimize the light absorption efficiency.

[0034] When the gate electrode 13B is formed of a second graphene layer or TMDC, these two-dimensional materials are about monolayer or a few layers thick, and no periodic pattern is formed. Therefore, the light absorption rate of the incident light on the gate electrode 13B from the back surface 112 of the substrate 11 is low, and it does not inhibit the light absorption in the graphene layer 15 that functions as a sensing layer. In the configuration of FIG. 6, the wiring to the gate electrode 13B may be formed in the front (-Y direction) or the depth direction (+Y direction) of the paper surface.

[0035] (Third Modified Example) FIG. 7 is a schematic cross-sectional view of an infrared sensor 10C, which is a third modification of the infrared sensor shown in FIG. 1. The top view of the infrared sensor 10C is the same as that in FIG. 5(a), and FIG. 7 shows another cross-sectional configuration along the line III-III. The infrared sensor 10C has a half-gate electrode 13C between a substrate 11 and an insulating film 12. The half-gate electrode 13C applies a gate voltage to a partial region of the graphene layer 15. Thereby, the electronic state of the graphene layer 15 becomes asymmetric in the X direction, and an electromotive force can be detected between the first electrode 17 and the second electrode 18. The material of the half-gate electrode 13C is the same as that of the gate electrode 13B in FIG. 6, and it is formed to a thickness that does not inhibit light absorption in the graphene layer 15.

[0036] The wiring to the half-gate electrode 13C can be drawn out in front of the paper surface or in the depth direction, similar to the gate electrode 13B in FIG. 6. In this case, the wiring to the first electrode 17 and the second electrode 18 can be realized by bonding wirings 170 and 180, or flip-chip wiring. Needless to say, the infrared sensor 10B in FIG. 6 can also adopt the bonding wirings 170 and 180 and the flip-chip wiring structure in FIG. 7.

[0037] (Second Embodiment) FIG. 8 is a top view of an infrared sensor array 200 including infrared sensors 20a, 20b, and 20c according to the second embodiment. FIG. 9A is a cross-sectional view taken along the line IV-IV in FIG. 8, and FIG. 9B is a cross-sectional view taken along the line V-V in FIG. 8. In the second embodiment, a gas cavity under reduced pressure or non-reduced pressure is provided above the graphene layer. The gas is air or a gas of homonuclear diatomic molecules such as nitrogen gas or hydrogen gas. The reduced-pressure gas includes a vacuum or a state close to a vacuum, and may be, for example, a thin air layer.

[0038] The infrared sensors 20a, 20b, and 20c included in the infrared sensor array 200 are arranged in a predetermined direction and separated from each other by the partition wall 193. In the example of FIG. 8, the infrared sensors 20a, 20b, and 20c are arranged in the Y direction, and the mirror layer 24 extends in the Y direction and is commonly used for the infrared sensors 20a, 20b, and 20c. The infrared sensors 20a, 20b, and 20c have the same configuration and may be collectively referred to as the "infrared sensor 20" hereinafter. Instead of the mirror layer 24 extending in the arrangement direction of the infrared sensors 20, individual mirror layers 24 may be provided for each infrared sensor 20. On one side of the mirror layer 24, the first electrodes 27a, 27b, and 27c (which may be collectively referred to as the "first electrode 27") are arranged, and on the other side of the mirror layer 24, the second electrodes 28a, 28b, and 28c (which may be collectively referred to as the "second electrode 28") are arranged.)

[0039] As shown in the cross-sectional view taken along line IV-IV of FIG. 9A, the infrared sensor 20 includes a substrate 11, an insulating film 22 provided on the surface 111 of the substrate 11, a graphene layer 25 disposed on the insulating film 22, a first electrode 27a connected to one end of the graphene layer 25, and a second electrode 28a connected to the opposite end of the graphene layer 25. The surface of the graphene layer 25 is covered with a protective layer 26, and a cavity 29 is provided above the graphene layer 25. The cavity 29 is formed by an air layer 195. By removing air to form a thin air layer (or vacuum layer), it is also possible to reduce unnecessary scattering sources. Instead of air, a gas of homonuclear diatomic molecules such as nitrogen, oxygen, and hydrogen that does not absorb the infrared light to be detected may be used. Thereby, the transmittance of the infrared light to be detected can be improved, and the infrared light can be efficiently confined between the mirror layer 24 and the graphene layer 25.)

[0040] The thickness of the cavity 29 is set so as to satisfy the condition that the interference between the incident light to the mirror layer 24 and the reflected light from the mirror layer 24 is enhanced at the position of the graphene layer 25. When the distance from the graphene layer 25 to the mirror layer 24 is d, the wavelength of the light to be detected is λ, and the refractive index of the air layer 195 is 1, the thickness of the cavity 29 is set so that the distance d satisfies d = mλ / 4 (m is a positive odd number). Here, the refractive index of the protective layer 26 is treated as negligible.

[0041] As shown in the V-V cross-sectional view of FIG. 9B, the mirror layer 24 is bridged between the separation walls 193 that partition the respective infrared sensors 20, and the cavity 29 of the air layer 195 is formed. On the bottom surface of the cavity 29, a graphene layer 25 covered with a protective layer 26 is located. The graphene layer 25 has a periodic pattern similar to the graphene layer 15 of the first embodiment, and by combining the surface plasmon resonance with the above-described cavity configuration, the light absorption efficiency is improved.

[0042] FIGS. 10A to 10D are manufacturing process diagrams of the infrared sensor 20. The manufacturing process is shown in both the IV-IV cross-section and the V-V cross-section of FIG. 8. Up to the step of forming the graphene layer 25, the first electrode 27, and the second electrode 28 via an insulating film on the substrate 11 and forming a protective layer on the entire surface, the steps of FIGS. 3A to 3E of the first embodiment are the same. In the second embodiment, the formation of the protective layer 26 covering the graphene layer 25 will be described. Also in the second embodiment, a pattern having a predetermined period is formed in the graphene layer 25.

[0043] In FIG. 10A, unnecessary portions of the protective layer covering the entire surface on the substrate 11 are removed by photolithography and etching. By this etching, a protective layer 26 covering the surface of the graphene layer 25 is formed, and at the same time, the surfaces of the first electrode 27a and the second electrode 28a are exposed.

[0044] In FIG. 10B, a resist is applied to the entire surface, and the separation walls 193 that partition the respective infrared sensors 20 are formed by exposure and development. The separation walls 193 are formed of, for example, a negative resist, and the exposed and cured portions remain as the separation walls 193.

[0045] In FIG. 10C, a resist 251 for lift-off of the mirror layer 24 is applied over the entire surface, patterned by exposure and development, and the unnecessary portion of the resist 251 is removed. Next, a metal layer 240 is formed over the entire surface by electron beam evaporation. The metal layer 240 is formed of a good conductor such as Au, Pt, Ag, etc. As an example, the metal layer 240 is formed as a stack of an Al film with a thickness of 5 nm and an Au film with a thickness of 50 nm.

[0046] In FIG. 10D, the resist 251 is peeled off together with the upper metal layer 240 with a stripping solution such as an organic solvent. The previously formed separation wall 193 remains without being peeled off by this organic solvent. The metal layer 240 formed on the upper surface of the separation wall 193 and extending in the sensor array direction (Y direction in FIG. 8) is supported and bridged by the separation wall 193 and remains as the mirror layer 24 even when the lower resist 251 is dissolved. Thereby, a cavity 29 of the air layer 195 is formed above the graphene layer 25 covered with the protective layer 26.

[0047] The infrared sensor 20 and the infrared sensor array 200 of the second embodiment have an improved light absorption efficiency and sensitivity due to the cavity structure of the air layer 195 and the surface plasmon resonance by the periodic pattern of the graphene layer 25 of each infrared sensor 20.

[0048] (Third Embodiment) FIG. 11 is a cross-sectional schematic view of an infrared sensor 30 according to the third embodiment. The infrared sensor 30 of the third embodiment has a cavity structure below the graphene layer. The infrared sensor 30 includes a substrate 11, an insulating film 32 provided on the surface 111 of the substrate 11, a graphene layer 35 disposed on the insulating film 32, a first electrode 37 connected to one end of the graphene layer 35, and a second electrode 38 connected to the opposite end of the graphene layer 35. The surface of the graphene layer 35 is covered with a protective layer 36, and the protective layer 36 serves as the light incident surface.

[0049] The protective layer 36 is formed of alumina, boron nitride, or the like. The graphene layer 35 is sandwiched between the insulating film 32 and the protective layer 36, and the influence from the outside is suppressed. When the protective layer 36 is formed of boron nitride, the carrier mobility of the graphene layer 35 can be maintained high, and the optical characteristics and electronic characteristics of the infrared sensor 10 can be kept good.

[0050] Below the graphene layer 35, a cavity 39 reaching a predetermined depth from the surface of the substrate 11 is provided. The cavity 39 is formed of a resist layer 391. A mirror layer 34 is disposed on the bottom surface of the cavity 39. Infrared light that is incident on the graphene layer 35 from the side of the protective layer 36 and that cannot be completely absorbed is reflected by the mirror layer 34 and re-incident on the graphene layer 35. A periodic pattern is formed on the graphene layer 35, and the amount of light absorption increases due to surface plasmon resonance and re-incidence by the mirror layer 34.

[0051] The depth of the cavity 39 is set so as to satisfy the condition that the interference between the light incident on the mirror layer 34 and the light reflected from the mirror layer 34 strengthens at the position of the graphene layer 35. Assuming the distance from the graphene layer 35 to the mirror layer 34 is d, the wavelength of the light to be detected is λ, and the refractive index of the resist layer 391 in the cavity 39 is n, the depth of the cavity 39 is set so that the distance d from the graphene layer 35 to the mirror layer 34 satisfies d = mλ / 4 (m is a positive odd number). Here, the refractive index of the insulating film 32 is treated as negligible.

[0052] Figs. 12A to 12E are manufacturing process diagrams of the infrared sensor 30. In Fig. 12A, a recess 116 reaching a predetermined depth from the surface 111 of the substrate 11 is formed at a predetermined location on the substrate 11, and a mirror layer 34 is formed on the bottom surface of the recess 116. The mirror layer 34 is formed, for example, by a lift-off method.

[0053] In FIG. 12B, a resist is applied over the entire surface, and a resist layer 391 is formed inside the recess 116 by photolithography and etching to obtain a cavity 39. An insulating film 32 is formed over the entire surface including the surface of the resist layer 391 by the ALD method or the like. As the insulating film 32, an alumina film having a thickness of about 20 nm is formed.

[0054] In FIG. 12C, separately synthesized graphene is transferred onto the insulating film 32 and patterned into a desired shape to form a graphene layer 35. The graphene layer 35 is patterned into a desired shape by the photolithography method and RIE. In this patterning process, periodic patterns such as antidots and stripes may be formed in the graphene layer 35. Thereby, a graphene layer 35 facing the mirror layer 34 with the cavity 39 in between is obtained.

[0055] In FIG. 12D, a first electrode 37 is connected to one end of the graphene layer 35, and a second electrode 38 is connected to the other end of the graphene layer 35. In FIG. 12E, for example, an alumina layer is formed over the entire surface by the ALD method, and a protective layer 36 covering the surface of the graphene layer 35 is formed by photolithography and etching. Thereby, an infrared sensor 30 is obtained.

[0056] (Modification example) FIG. 13 is a schematic cross-sectional view of an infrared sensor 30A as a modification example of the third embodiment. The infrared sensor 30A has a cavity 39A of an air layer 392 below the graphene layer 35. A gate electrode 33 for applying a gate voltage to the graphene layer 35 is provided between the graphene layer 35 and the cavity 39A. The gate electrode 33 may be formed of a second graphene layer, or may be formed of a transparent electrode such as ITO that transmits infrared light or TMDC.

[0057] The gate electrode 33 applies a gate voltage to the graphene layer 35 to modulate the electronic state of the graphene layer 35. The gate electrode 33 is drawn out to the outside of the first electrode 37 and the second electrode 38 and connected to the metal pad 31. The gate electrode 33 may have the half-gate structure shown in FIG. 7. The gate voltage is applied to the graphene layer 35 through the insulating film 322. Bonding wirings shown in FIG. 7 may be connected to each of the first electrode 37, the second electrode 38, and the metal pad 31.

[0058] FIGS. 14A to 14P are manufacturing process diagrams of the infrared sensor 30A in FIG. 13. In FIG. 14A, an insulating film 321 is formed on the surface 111 of the substrate 11. A silicon substrate may be used as the substrate 11. An alumina film with a thickness of about 20 nm is formed on the surface 111 of the silicon substrate by the ALD method or the like.

[0059] In FIG. 14B, a recess 320 reaching a predetermined depth from the surface of the insulating film 321 is formed at a predetermined position on the substrate 11.

[0060] In FIG. 14C, a resist is applied over the entire surface, the resist is patterned to form a lift-off resist 254 in a region excluding the recess 320, and then a metal layer 340 is formed on the bottom surface of the recess 320 and the upper surface of the resist 254. The metal layer 340 is formed by electron beam evaporation of an Au(50 nm) / Al(5 nm) layer having a high reflectivity for infrared light.

[0061] In FIG. 14D, the resist 254 is peeled off together with the upper metal layer 340 by lift-off. A mirror layer 34 is formed on the bottom surface of the recess 320.

[0062] In FIG. 14E, a separately synthesized graphene layer 330 by CVD or the like is transferred onto the insulating film 321. This graphene layer 330 is a graphene layer 330 for a gate electrode and has a thickness of several layers. A resist mask 255 having a predetermined shape is formed on the graphene layer 330.

[0063] In FIG. 14F, by using oxygen plasma for RIE, the graphene layer 330 is processed into a predetermined shape, and the resist mask 255 is removed to form the gate electrode 33. An air layer cavity 39A is formed in the recess 320 below the gate electrode 33. By evacuating the air to form a thin air layer (vacuum layer), it is also possible to reduce unnecessary scattering sources. Further, a gas of homonuclear diatomic molecules other than air may be used under reduced pressure or non-reduced pressure.

[0064] In FIG. 14G, an insulating film 322 is formed over the entire surface by ALD. The insulating film 322 is an alumina film with a thickness of about 20 nm.

[0065] In FIG. 14H, a resist is applied over the entire surface and patterned into a predetermined shape to form a resist mask 256 having an opening 257. The opening 257 is an opening for a via hole to access the gate electrode 33. An insulating film 32A is formed by the insulating films 321 and 322.

[0066] In FIG. 14I, a wet etching is used to form a via hole 258 that reaches the gate electrode 33 in the insulating film 322. A part of the gate electrode 33 is exposed in the via hole 258. Thereafter, the resist mask 256 is removed.

[0067] In FIG. 14J, a separately synthesized graphene layer 350 by CVD or the like is transferred onto the insulating film 32A. In FIG. 14K, a resist mask 259 having a predetermined shape is formed on the graphene layer 350.

[0068] In FIG. 14L, the graphene layer 350 is patterned into a predetermined shape by RIE using the resist mask 259. In this patterning, a periodic pattern for surface plasmon resonance may be formed in the graphene layer 35. In the patterning of the graphene layer 350, the via hole 258 reaching the underlying gate electrode 33 appears. Thereafter, the resist mask 259 is removed to obtain a graphene layer 35 having a predetermined shape.

[0069] In FIG. 14M, a resist mask 261 for electrode formation is formed by photolithography, and a metal layer 370 for electrodes is formed by electron beam evaporation.

[0070] In FIG. 14N, the resist mask 261 is removed together with the upper metal layer 370 by the lift-off method. Thereby, a first electrode 37 connected to the graphene layer 35, a second electrode 38, and a metal pad 31 connected to the gate electrode 33 are obtained.

[0071] In FIG. 14O, a protective layer 360 is formed over the entire surface by the ALD method. The protective layer 360 may be alumina or boron nitride. In this example, alumina with a thickness of 5 nm is formed as the protective layer 360.

[0072] In FIG. 14P, a resist is applied over the protective layer 360 and patterned into a predetermined shape. Using this resist pattern as a mask, etching is performed to remove the portions of the protective layer 360 that covered the first electrode 37, the second electrode 38, and the metal pad 31. The surfaces of the first electrode 37, the second electrode 38, and the metal pad 31 are exposed, while the protective layer 36 covering the surface of the graphene layer 35 remains. Thereby, an infrared sensor 30A is obtained.

[0073] The infrared sensor 30A has an air cavity 39A below the graphene layer 35. Among the infrared light incident from the side of the protective layer 36, the infrared light that could not be completely absorbed by the graphene layer 35 is reflected by the mirror layer 34 on the bottom surface of the cavity 39A and re-incident on the graphene layer 35. The gate electrode 33 is formed of several layers of graphene layers, and its light absorption amount is extremely small, so it hardly affects the absorption of infrared light by the graphene layer 35. The depth of the cavity 39A is set such that the interference between the incident light on the mirror layer 34 and the reflected light from the mirror layer 34 becomes constructive interference at the position of the graphene layer 35. This cavity configuration and the surface plasmon resonance of the graphene layer 35 increase the light absorption amount of the graphene layer.

[0074] The present disclosure has been described based on specific examples above, but the present disclosure is not limited to the above-described examples. Each configuration shown in the first to third embodiments, including examples and modifications, may be combined with each other. For example, a gate electrode may be provided on the infrared sensor 20 of the second example with a second graphene layer. Also in this case, the back surface 112 of the substrate serves as the light incident surface. The infrared sensors 30 of the third embodiment may be stacked in the vertical direction to form a configuration that absorbs infrared light in different wavelength ranges. In this case, the period of the pattern of the lower graphene layer and the thickness of the cavity may be designed to respond to light of the first wavelength, and the period of the pattern of the upper graphene layer and the thickness of the cavity may be designed to respond to light of a second wavelength different from the first wavelength. For light of any wavelength, the absorption amount can be improved by the cavity structure and surface plasmon resonance. The infrared sensor of the embodiment can detect thermal radiation light of a substance and is applicable to night vision thermography and the like.

[0075] In view of the above disclosure, the following appendices are presented. (Appendix 1) A substrate, An insulating film provided on the surface of the substrate, A graphene layer disposed on the insulating film, A first electrode connected to one end of the graphene layer, A second electrode connected to the opposite end of the graphene layer, A cavity provided above or below the graphene layer and formed of a resist or a gas under reduced pressure or non-reduced pressure, A mirror layer facing the graphene layer with the cavity therebetween, Comprising An infrared sensor. (Appendix 2) The cavity is formed by a resist layer above the graphene layer, The mirror layer is provided on the surface of the resist layer, The infrared sensor according to Appendix 1. (Appendix 3) The cavity is formed by an air layer above the graphene layer, The mirror layer covers the air layer, The infrared sensor according to Appendix 1. (Appendix 4) A protective layer disposed between the graphene layer and the cavity and covering the surface of the graphene layer, The infrared sensor according to any one of Appendices 1 to 3 having the same. (Appendix 5) A gate electrode disposed between the graphene layer and the substrate and applying a gate voltage to the graphene layer, The infrared sensor according to any one of Appendices 1 to 4 having the same. (Appendix 6) The gate electrode is formed of a second graphene layer, a transition metal dichalcogenide, or a metal oxide having a high transmittance to infrared light, The infrared sensor according to Appendix 5. (Appendix 7) A second insulating film covering the gate electrode, Having the same, and the graphene layer is disposed on the second insulating film, The infrared sensor according to Appendix 6. (Appendix 8) The cavity reaches a predetermined depth from the surface of the substrate below the graphene layer, The mirror layer is disposed on the bottom surface of the cavity and faces the graphene layer with the cavity interposed therebetween, The infrared sensor according to Appendix 1. (Appendix 9) A resist layer is disposed in the cavity, and the insulating film is disposed between the resist layer and the graphene layer, The infrared sensor according to Appendix 8. (Appendix 10) The inside of the cavity is an air layer, and has a second graphene layer covering the air layer and a third electrode connected to the second graphene layer, The infrared sensor according to Appendix 8. (Appendix 11) The graphene layer has a periodic pattern. The infrared sensor according to any one of Appendices 1 to 10. (Appendix 12) Prepare a substrate with an insulating film formed on its surface. Form a graphene layer with a predetermined shape on the insulating film. Connect a first electrode to one end of the graphene layer and a second electrode to the opposite end of the graphene layer. Form a resist layer or a gas cavity under reduced pressure or non-reduced pressure above the graphene layer. Provide a mirror layer facing the graphene layer with the cavity in between. A method for manufacturing an infrared sensor. (Appendix 13) Form the cavity with the resist layer above the graphene layer between the first electrode and the second electrode. Form the mirror layer on the surface of the resist layer. A method for manufacturing the infrared sensor according to Appendix 12. (Appendix 14) Form a separation wall surrounding the periphery of the first electrode and the second electrode except for the region between the first electrode and the second electrode. Form a mirror layer bridged by the separation wall above the graphene layer. Obtain the cavity of the air layer between the mirror layer and the graphene layer. A method for manufacturing the infrared sensor according to Appendix 12. (Appendix 15) Form a recess with a predetermined depth from the surface of the substrate. Form a mirror layer on the bottom surface of the recess. Form a resist or a gas cavity under reduced pressure or non-reduced pressure inside the recess where the mirror layer is formed. Form a graphene layer at a position facing the mirror layer with the cavity in between. Connect a first electrode to one end of the graphene layer and a second electrode to the other end of the graphene layer. A method for manufacturing an infrared sensor.

Description of Symbols

[0076] 10, 10A, 10B, 10C, 20, 30, 30A Infrared Sensors 11 Substrate 111 Surface 112 Back Surface 12, 22, 32, 32A, 321, 322 Insulating Films 13B, 13C, 33 Gate Electrodes 13C Half Gate Electrode 14, 14A, 24, 34 Mirror Layers 15, 15a, 15b, 25, 35 Graphene Layers 16, 26, 36 Protective Layers 17, 27, 37 First Electrodes 18, 28, 38 Second Electrodes 19, 19A, 29, 39, 39A Cavities 31 Metal Pads 100, 200 Infrared Sensor Arrays 116, 320 Recesses 151 Anti-Dots 152 Graphene Ribbons 191, 391 Resist Layers 195, 392 Air Layers

Claims

1. A substrate, an insulating film provided on the surface of the substrate, a graphene layer disposed on the insulating film, a first electrode connected to one end of the graphene layer, a second electrode connected to the opposite end of the graphene layer, a cavity provided above or below the graphene layer and formed of a resist or a gas under reduced pressure or non-reduced pressure, a mirror layer facing the graphene layer with the cavity therebetween, comprising an infrared sensor.

2. The cavity is formed by a resist layer above the graphene layer, and the mirror layer is provided on the surface of the resist layer, The infrared sensor according to Claim 1.

3. The cavity is formed by an air layer above the graphene layer, and the mirror layer covers the air layer, The infrared sensor according to Claim 1.

4. The cavity reaches a predetermined depth from the surface of the substrate below the graphene layer, and the mirror layer is disposed on the bottom surface of the cavity and faces the graphene layer with the cavity therebetween, The infrared sensor according to Claim 1.

5. A resist layer is disposed in the cavity, and the insulating film is disposed between the resist layer and the graphene layer, The infrared sensor according to Claim 4.

6. The inside of the cavity is an air layer, and has a second graphene layer covering the air layer and a third electrode connected to the second graphene layer, The infrared sensor according to Claim 4.

7. Prepare a substrate with an insulating film formed on its surface, Form a graphene layer of a predetermined shape on the insulating film, Connect a first electrode to one end of the graphene layer and connect a second electrode to the opposite end of the graphene layer, Form a resist layer or a gas cavity under reduced pressure or non-reduced pressure above the graphene layer, Provide a mirror layer facing the graphene layer with the cavity therebetween, A method for manufacturing an infrared sensor.

8. Form the cavity with the resist layer above the graphene layer between the first electrode and the second electrode, Form the mirror layer on the surface of the resist layer, The method for manufacturing an infrared sensor according to Claim 7.

9. Form a separation wall surrounding the periphery of the first electrode and the second electrode except for the region between the first electrode and the second electrode, Form a mirror layer bridged by the separation wall above the graphene layer, Obtain the cavity of the air layer between the mirror layer and the graphene layer, The method for manufacturing an infrared sensor according to claim 7.

10. Form a recess with a predetermined depth from the surface of the substrate, Form a mirror layer on the bottom surface of the recess, Inside the recess on which the mirror layer is formed, form a cavity of a resist or a gas under reduced pressure or non-reduced pressure, Form a graphene layer at a position facing the mirror layer with the cavity interposed therebetween, Connect a first electrode to one end of the graphene layer and connect a second electrode to the other end of the graphene layer. The method for manufacturing an infrared sensor.

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