JP2023042644A - optical sensor device - Google Patents

Abstract

To provide an optical sensor device that has both weatherability and sensitivity.SOLUTION: An optical sensor device has: a substrate; a graphene layer provided on the substrate; a pair of electrodes connected to the graphene layer; and a resin layer which covers a light reception region of the graphene layer, and is transparent to light to be detected.SELECTED DRAWING: Figure 5

Description

The present disclosure relates to optical sensor devices.

Infrared sensors detect infrared rays emitted by objects with heat, and are applied to automatic doors, surveillance cameras, infrastructure inspections, and the like. Infrared has a wide wavelength range, and wideband, high-sensitivity sensors that operate at room temperature are in demand.

An electromagnetic wave detector using a graphene layer has been proposed (see Patent Document 1, for example). Graphene has high mobility and absorbs light in a wide wavelength band including visible and infrared. Due to these properties of graphene, research and development are proceeding toward the practical use of sensors that are smaller than before and that operate at room temperature.

WO2016/121408

A typical graphene sensor has a transistor structure in which electrodes are arranged at both ends of a graphene layer, and detects carriers generated when light is incident on the graphene as current. In graphene sensors with a transistor structure, when the graphene is exposed to the atmosphere, the drain current-gate voltage curve changes due to the effects of atmospheric moisture, contamination, and climate change. If the drain current-gate voltage curve changes, the change characteristics of the drain current depending on whether light is incident or not changes, and the amount of received light cannot be detected correctly. When a protective film is provided to cover the graphene surface, sufficient sensitivity cannot be obtained depending on the type of protective film.

An object of one aspect is to provide an optical sensor device that has both weather resistance and sensitivity.

In one embodiment, the optical sensor device comprises:
a substrate;
a graphene layer provided on the substrate;
a pair of electrodes connected to the graphene layer;
a resin layer that covers the light receiving region of the graphene layer and is transparent to the wavelength band of light to be detected;
have

An optical sensor device having both weather resistance and sensitivity is realized.

FIG. 2 illustrates environmental effects on graphene-based optical sensor devices. FIG. 3 is a diagram showing weather resistance of a protective film formed by an atomic layer deposition (ALD) method; It is a figure which shows the principle of the optical sensor device using graphene. 1 is a schematic plan view of an optical sensor device according to a first embodiment; FIG. FIG. 5 is a cross-sectional view taken along line IV-IV in FIG. 4; It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a manufacturing-process figure of the optical sensor device of 1st Embodiment. It is a figure which shows the effect of the resin layer of embodiment compared with an ALD alumina film. It is a plane schematic diagram of the optical sensor device of 2nd Embodiment. FIG. 9 is a cross-sectional view along the VV line in FIG. 8; It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. It is a manufacturing-process figure of the optical sensor device of 2nd Embodiment. FIG. 10 is a schematic plan view of an optical sensor device of another embodiment; FIG. 12 is a cross-sectional view along the VI-VI line in FIG. 11;

In the embodiment, weather resistance and sensitivity are improved by covering the light-receiving region of an optical sensor device using graphene with a resin layer transparent to the light to be detected. Weather resistance generally refers to a property that can withstand the outdoor natural environment and its changes. Includes the property that point fluctuations are suppressed.

Preferably, the resin layer has photosensitivity to light of a wavelength different from the light to be detected, or to electron beams. Such a resin layer is formed by a coating method such as a spin coating method, a spray coating method, a dip coating method, a die coating method, a bar coating method, etc., and is easy to form and pattern. The baking temperature is also 100° C. or lower, and has little effect on graphene.

By covering the light-receiving region of graphene with a resin layer, as will be described later, the p-doping effect of graphene is greater than that of an inorganic insulating film formed by ALD, and the sensitivity of the photosensor device is improved. . The term “p-doping effect” used herein refers to the effect of increasing the carrier (hole) concentration in graphene. Due to the effect of p-doping, the difference between the drain current when light is incident and when light is not incident is increased, and the photosensitivity can be improved.

FIG. 1 shows the effect of environment on graphene-based optical sensor devices. As a sample, a graphene optical device with a transistor structure is fabricated in which a pair of electrodes is provided on a graphene layer on a substrate and a gate electrode is provided on the back surface of the substrate. Using the fabricated sample, the gate voltage-drain current characteristics are measured in vacuum without incident light. The sample is vacuum annealed at 150° C. for 1 hour to simulate a low moisture content on the device surface. A sample without vacuum annealing simulates a high moisture content on the device surface.

The gate voltage-drain current characteristics differ greatly between the state where the moisture content on the device surface is small (line A) and the state where the moisture content is large (line B). The Dirac point, ie, the point at which the drain current is minimum with respect to the gate voltage, is near 0 V when the device surface has a small amount of moisture. On the other hand, when the moisture content on the device surface is large, the Dirac point shifts significantly to the positive side of the gate voltage, and the drain current-gate voltage characteristic itself changes, narrowing the dynamic range.

In order to improve the weather resistance of an optical sensor device using graphene, it is conceivable to cover the entire device with a protective film. For example, it is conceivable to prevent the effects of moisture and contamination by forming a transparent insulating protective film by the ALD method capable of forming a dense film.

FIG. 2 shows weather resistance of the protective film formed by the ALD method. An Al ₂ O ₃ film with a thickness of 20 nm is formed as a protective film on the same sample as the sample used in FIG. Line C shows the electrical properties of the sample annealed after forming the overcoat by ALD. Line D shows the electrical properties measured in vacuum without annealing after forming the protective film by ALD. Line E shows the electrical properties measured in the atmosphere without annealing after forming the protective film by ALD.

Regardless of the presence or absence of annealing, the Dirac point is almost constant and near the gate voltage of 0V. It can be seen that formation of a protective layer by the ALD method suppresses fluctuations in the Dirac point regardless of changes in the amount of water in the environment, and improves weather resistance.

The inventors have found that the protective film formed by the ALD method has weather resistance, but the effect of p-doping is insufficient. If the effects of weather resistance and p-doping can be combined, a photosensor device with stable operation and high sensitivity can be realized. In the embodiment, a resin layer transparent to the light to be detected is used as the protective layer.

FIG. 3 shows the principle of an optical sensor device. The horizontal axis is the gate voltage, and the vertical axis is the drain current. The Dirac point at which the drain current is minimum when there is no incident light is at DP1. When light enters the graphene, carriers are generated in the graphene, and the Dirac point shifts to DP2 on the low voltage side. In other words, with the same gate voltage, more drain current flows. The amount of incident light can be determined by measuring the change in drain current. Instead of measuring drain current, change in electrical resistance may be measured.

In the vicinity of the Dirac point, the current change (inclination) is large when light is incident. By applying a bias voltage between the source and the drain, it is possible to read the current change or electrical resistance change at the Dirac point. If the effect of p-doping is large, the change in drain current or the change in electrical resistance will be large with the same amount of incident light, and the sensitivity will be high. The inventors have found that the p-doping effect can be sufficiently obtained by using a resin layer as the protective layer. The effect of p-doping by the resin layer will be described later with reference to FIG.

<First embodiment>
4 is a schematic plan view of the optical sensor device 10A of the first embodiment, and FIG. 5 is a cross-sectional view taken along line IV-IV of FIG. The optical sensor device 10A includes a substrate 11, a graphene layer 15 provided on the substrate 11 with an insulating film 13 interposed therebetween, electrodes 18 and 19 connected to the graphene layer 15, and at least a light receiving region 155 of the graphene layer 15. and a covering resin layer 25 . The light receiving area 155 of the graphene layer 15 is the area extending between the electrodes 18 and 19 . The resin layer 25 is transparent to light detected by the optical sensor device 10A, and is transparent to visible light and infrared light in this example.

The resin layer 25 is a resin coat formed by a spin coating method, a spray coating method, a dip coating method, a die coating method, a bar coating method, or the like, and has good adhesion to the base. The thickness of the resin layer 25 is from 1 μm to several μm. When the resin layer 25 is used as a protective layer, a large-scale film forming apparatus equipped with a vacuum chamber is not required.

In the substrate 11, the surface on which the graphene layer 15 and the electrodes 18 and 19 are formed is the main surface, and the side opposite to the main surface is the back surface. For the sake of convenience, the lamination direction of the main surface is referred to as the "upper" side, but such an expression representing the direction does not indicate an absolute positional relationship. Substrate 11 is, for example, a silicon substrate, and gate electrode 14 is provided on the back surface of substrate 11 . The gate electrode 14 and the electrodes 18 and 19 form a transistor structure.

Electrode pads 16 and 17 are provided for bonding wires with a bonder. Electrode pads 16 and 17 are formed of, for example, a laminate of titanium (Ti) and gold (Au). Chromium (Cr) may be used as a base of Au instead of Ti.

Electrodes 18 and 19 are made of Au, Ti, Cr, palladium (Pd), platinum (Pt), combinations thereof, or the like. If the electrodes 18 and 19 have sufficient adhesion to the insulating film 13, the electrode pads 16 and 17 may be omitted.

Contact holes 22 and 23 reaching the electrodes 18 and 19 are formed in the resin layer 25, respectively. By filling the contact holes 22 and 23 with a conductive material and providing wiring or terminals, the current flowing through the channel of the graphene layer 15 can be extracted to the outside.

Resin layer 25 may be, for example, a photosensitive resin having sensitivity to ultraviolet rays. In this case, the contact holes 22 and 23 can be easily formed only by exposing and developing the resin layer 35 . When the resin layer 25 is formed of a photoresist, the photoresist may be of either a negative type or a positive type from the viewpoint of patterning, but the negative type may be superior to the positive type in terms of weather resistance.

By protecting the optical sensor device 10A with the resin layer 25, penetration of moisture and contamination is prevented and weather resistance is improved. In addition, as will be described later, the p-doping effect of the graphene layer 15 can be improved to increase the photosensitivity.

6A to 6G are manufacturing process diagrams of the optical sensor device 10A of the first embodiment. In FIG. 6A, for example, a silicon substrate with a thermal oxide film is provided. A silicon substrate is used as the substrate 11 , and a thermally oxidized film on one surface (main surface) is used as the insulating film 13 . A gate electrode 14 is formed by removing the thermal oxide film on the surface (back surface) of the substrate 11 opposite to the insulating film 13 . A two-layer electrode of Ti and Au may be formed on the back surface of the substrate 11 by electron beam (EB) vapor deposition to serve as the gate electrode. The gate electrode 14 becomes a back gate.

In FIG. 6B, electrode pads 16 and 17 are formed on the insulating film 13 . The electrode pads 16 and 17 are formed as a two-layer metal film of Ti and Au by, for example, a lift-off method. In order to keep the electrical conductivity as high as possible, the thickness of the underlying Ti film that serves as the adhesion layer may be made as thin as possible.

In FIG. 6C, a graphene layer 15 is provided over the entire surface. The graphene layer 15 is formed by using a resin or the like from graphene synthesized on a catalyst such as a copper foil by a mechanical peeling method in which HOPG (Highly oriented pyrolytic graphite) is peeled off with a tape or the like, or by a chemical vapor deposition method. Alternatively, the insulating film 13 and the electrode pads 16 and 17 may be coated with a transfer method such as a transfer method in which the film is peeled off and transferred. The graphene layer 15 may be a monolayer graphene or a layer with a thickness of several atomic layers. Since the amount of light absorption per layer of graphene is small, multiple layers of graphene may be stacked to increase the amount of light absorption. After providing the graphene layer 15, annealing may be performed at a temperature of 150 to 200° C. in order to increase the adhesion between the insulating film 13 and the graphene.

In FIG. 6D, the graphene layer 15 is patterned into a predetermined shape. Graphene can be processed by oxygen ashing, oxygen ion beam, or the like.

In FIG. 6E, electrodes 18 and 19 are formed to overlap both ends of the processed graphene layer 15 . The electrodes 18 and 19 respectively cover the electrode pads 16 and 17 on the substrate 11, but may be integrally formed of the same material. A region of the graphene layer 15 extending between the electrodes 18 and 19 is a light receiving region 155 . Graphene layer 15 forms a channel between electrodes 18 and 19 .

In FIG. 6F, a resin layer 25 is formed on the entire surface. As the resin layer 25, a resin that is transparent in the wavelength region of the light to be detected and has a p-doping effect on the graphene layer 15, such as an epoxy resin, is used. In this example, SU-8 resist from MicroChem is used. SU-8 resist is coated on the entire surface with a spinner to a thickness of 1 μm to several μm (1 μm in this example) and baked at 95° C. with a hot plate.

In FIG. 6G, contact holes 22 and 23 reaching the electrodes 18 and 19 are formed at predetermined positions of the resin layer 25 by exposure and development. The exposure may be UV irradiation using a mask aligner exposure device. Electron beam exposure may be used depending on the type of resin used. Wiring or terminals may be formed in the contact holes 22 and 23 .

In the above description, the manufacturing process was described focusing on individual photosensor devices 10A, but a large number of photosensor devices 10A may be simultaneously formed in each chip area on a silicon wafer to form a photosensor device array.

By applying a predetermined bias voltage between the electrodes 18 and 19, an electric field is applied to the graphene layer 15, and electron-hole pairs generated by light (for example, infrared rays) incident on the light receiving region 155 of the graphene layer 15 are generated. flows through the graphene layer 15 . Drain current can be controlled by gate voltage. By applying a Dirac point gate voltage that minimizes the drain current when the light is off, the light incidence increases the drain current. Light incidence can be detected by detecting this increase in drain current.

FIG. 7 shows the p-doping effect of resin layer 25 . In order to confirm the p-doping effect of the resin layer 25, the drain current-gate voltage characteristics are measured in the absence of incident light on the samples fabricated in the steps of FIGS. 6A to 6G. For comparison, a sample having an alumina film formed by the ALD method as a protective layer is prepared, and the drain current-gate voltage characteristics are similarly measured without incident light.

FIG. 7(a) shows the measurement results in the atmosphere of seven types of samples a to g using the resin layer 25 as a protective layer. FIG. 7(b) shows the measurement results in the air of five types of samples h to l using an ALD alumina film with a thickness of 20 nm as a protective layer.

In samples a to g having the resin layer 25, single-layer graphene was used, the length of the graphene layer 15 (the length between the electrodes 18 and 19) was constant at 30 mm, and the width was varied from 1 mm to 50 mm. there is The resin layer 25 is SU-8 resist. In samples h to l of the comparative examples, single-layer graphene is used, and the length of the graphene layer is constant at 30 mm, and the width is varied between 5 mm and 300 mm.

In the samples of the embodiment, the Dirac point is stable around 15 V regardless of the change in the width of the graphene layer 15, except for sample a with a width of 1 mm. It can be seen that the effect of p-doping appears.

On the other hand, in samples h to l of the comparative example, the Dirac point shifts as the width of the graphene layer 15 changes, so the setting of the gate voltage must be changed according to the design of the device. Also, the Dirac point is 5 V or less except for sample h, and the effect of p-doping is small.

From this measurement result, it can be seen that the optical sensor device 10A having both weather resistance and sensitivity can be realized by providing the resin layer 25 transparent to the light to be detected.

<Second embodiment>
FIG. 8 is a schematic plan view of the optical sensor device 10B of the second embodiment, and FIG. 9 is a cross-sectional view taken along line VV of FIG. In the second embodiment, a plurality of holes 151 are provided in the light receiving region 155 of the graphene layer 15 in order to enhance the light absorption of the graphene layer 15 .

The optical sensor device 10B includes a substrate 11, a graphene layer 15 provided on the substrate 11 with an insulating film 13 interposed therebetween, electrodes 18 and 19 connected to the graphene layer 15, and at least a light receiving region 155 of the graphene layer 15. and a covering resin layer 25 . The resin layer 25 is transparent to light detected by the optical sensor device 10B, and is transparent to visible light and infrared light in this example.

A plurality of holes 151 are formed in the light receiving region 155 of the graphene layer 15 . By providing the holes 151 in the graphene layer 15, absorption of light can be enhanced during light reception. The incidence of light excites plasmons on the surface of the graphene layer 15, that is, collective oscillation of free electrons caused by the incident light. In the vicinity of the edge of the hole 151, confinement of the oscillating electric field of light is strong, and light absorption can be enhanced compared to a graphene layer in which the hole 151 is not formed. The hole 151 does not necessarily have to completely penetrate the graphene layer 15 and reach the insulating film 13 .

Electrodes 18 and 19 overlapping the graphene layer 15 are arranged at both ends of the graphene layer 15 . A gate electrode 19 is provided on the back surface of the substrate 11 to form a back gate type transistor configuration.

The configuration of the optical sensor device 10B is the same as the configuration of the optical sensor device 10A of the first embodiment, except that the graphene layer 16 is provided with the holes 151 . The same components as those of the optical sensor device 10A are denoted by the same reference numerals, and overlapping descriptions are omitted. The effects of weather resistance and p-doping by the resin layer 25 are the same as in the first embodiment, but the amount of light absorbed by the graphene layer 15 is increased, so the sensitivity is further improved.

10A to 10K are manufacturing process diagrams of the optical sensor device 10B. Figures 10A to 10D are the same as the process of Figures 6A to 6D. That is, the thermally oxidized film on one side of the silicon substrate with the thermally oxidized film is used as the insulating film 13, and the thermally oxidized film on the other side is removed to form the gate electrode 14. Next, as shown in FIG. After forming the electrode pads 16 and 17 on the insulating film 13, the graphene layer 16 is provided on the entire surface, and the graphene layer 15 is patterned into a predetermined shape.

In FIG. 10E, a resist 31 for forming holes is applied to the entire surface. The resist 31 is a resin that can be easily patterned and easily removed with a solvent. As such a resin, an acrylic resin, polymethyl methacrylate (PMMA), hydrogen silsesquioxane (HSQ), or the like can be used. Alternatively, a plurality of types of these may be stacked.

In FIG. 10F, an opening 32 having a predetermined shape is formed in the resist 31 by electron beam drawing, and a part of the graphene layer 15 is exposed in the opening 32 . The diameter of the opening 32 is about 100-400 nm.

In FIG. 10G, periodic holes 151 are formed in the graphene layer 15 with a directional oxygen ion beam using the resist 31 with the openings 32 formed therein as a mask. The diameter of the holes 151 formed in the graphene layer 15 may be, for example, in the range of 100 to 400 nm, and the diameter, arrangement, etc. of the holes 151 can be adjusted according to the wavelength of the light to be absorbed. The planar shape of the hole is not limited to a circular shape, and may be any planar shape as long as it can excite plasmons.

In FIG. 10H, resist 31 is removed. This resist 31 is for patterning the holes 151 and is easily removed with a solvent or the like.

The steps of FIGS. 10I-10K are the same as those of FIGS. 6E-G. That is, the electrodes 18 and 19 are formed to cover the electrode pads 16 and 17 and connected to both ends of the graphene layer 15 in which the holes 151 are formed, and the resin layer 25 is formed on the entire surface. As the resin layer 25, a resin that is transparent in the wavelength region of the light to be detected, has a low dielectric constant, and has good adhesion to the underlying layer is used. For example, SU-8 resist manufactured by MicroChem is spin-coated to a thickness of 1 to several μm. After baking at a low temperature of 100° C. or less, contact holes 22 and 23 reaching the electrodes 18 and 19 are formed by exposure and development. Wiring or terminals may be formed in the contact holes 22 and 23 .

Also in the optical sensor device 10B, the resin layer 25 improves the weather resistance, and the p-doping effect improves the sensitivity. In the optical sensor device 10B, by providing the periodic holes 151 in the graphene layer 25, light absorption can be enhanced and the sensitivity can be further improved.

<Other embodiments>
FIG. 11 is a schematic plan view of an optical sensor device 10C that is another embodiment, and FIG. 12 is a cross-sectional view taken along line VI-VI of FIG. The photosensor device 10C has an embedded gate electrode 24 in place of the back gate.

A gate electrode 24 is provided on the insulating film 13 on the substrate 11 . The substrate 11 is not limited to a silicon substrate, and any insulating substrate such as a sapphire substrate, MgO substrate, and resin substrate may be used. When a silicon substrate is used, the insulating film 13 may be a thermal oxide film, or may be a new insulating film formed by removing the thermal oxide film. When an insulating substrate is used, the insulating film 13 may be formed on the surface of the substrate 11 by sputtering or the like. The gate electrode 24 is formed on the insulating film 13 by EB vapor deposition or the like.

Another insulating film 27 is provided to cover the gate electrode 24 . The graphene layer 15 is arranged on the insulating film 27 . The insulating film 27 immediately below the graphene layer 15 becomes a gate insulating film. By applying a gate voltage to the graphene layer 15 through the insulating film 27, carrier density of electrons, holes, etc. in the graphene layer 15 is controlled.

In the photosensor device 10C, since all the electrodes are provided on the main surface side of the substrate 11, fabrication of an integrated circuit is possible. Also in the optical sensor device 10C, the resin layer 25 is provided to cover the graphene layer 15, thereby improving the weather resistance and improving the sensitivity due to the p-doping effect.

As another configuration, instead of forming the electrode pads 16 and 17 on the insulating film 13, after forming the contact holes 18 and 19 in the resin layer 25, the electrode pads may be formed in the contact holes by the lift-off method. .

Although the disclosure has been described above based on specific examples, the disclosure is not limited to the examples described above. Each embodiment may be combined with each other. For example, the buried gate electrode configuration of FIGS. 11 and 12 may be combined with the perforated graphene layer 15 of the second embodiment. The shape (including width and length) and the number of layers of the graphene layer 15 can be designed appropriately. When forming the hole 151 in the graphene layer 15, the hole 151 is not limited to a circular hole, and may be a polygonal hole, an elliptical hole, or the like. The arrangement of the holes 151 is not limited to the matrix arrangement of FIG. 8, and may be a staggered arrangement or a close arrangement.

The resin layer 25 may be made of epoxy resin as a main component, with several to 10% by weight of an ultraviolet (UV) curable resin and approximately 1 to 5% by weight of an adhesion promoter added thereto. By protecting at least the light receiving region 155 of the graphene layer 15 with the resin layer 25, an improvement in weather resistance and an improvement in sensitivity due to the effect of p-doping are expected.

10A, 10B, 10C Optical sensor device 11 Substrate 13 Insulating films 14, 24 Gate electrodes 16, 17 Electrode pads 18, 19 Electrode 15 Graphene layer 25 Resin layer 151 Hole

Claims (8)

a substrate;
a graphene layer provided on the substrate;
a pair of electrodes connected to the graphene layer;
a resin layer that covers the light receiving region of the graphene layer and is transparent to the light to be detected;
An optical sensor device having The resin layer has photosensitivity to light of a wavelength different from that of the light to be detected, or to an electron beam.
An optical sensor device according to claim 1 . The resin layer has photosensitivity to ultraviolet light,
3. The optical sensor device of claim 2. The resin layer is a negative or positive photoresist,
Optical sensor device according to any one of claims 1 to 3. The resin layer is SU-8 photoresist,
Optical sensor device according to any one of claims 1 to 4. The resin layer is a layer formed by a coating method,
Optical sensor device according to any one of claims 1 to 5. The graphene layer has a periodic array of a plurality of holes formed in the light receiving region,
The resin layer covers the light receiving region in which the hole is formed.
Optical sensor device according to any one of claims 1 to 6. a gate electrode provided on the back surface of the substrate or between the substrate and the graphene layer;
8. The optical sensor device of any one of claims 1 to 7, further comprising:

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