JP7457508B2 - Solid-state image sensor and its manufacturing method - Google Patents

Description

The present invention relates to a solid-state imaging device and a method for manufacturing the same, and more particularly to a junction-type solid-state imaging device in which a photoelectric conversion portion is a PN junction of crystalline selenium and gallium oxide, and a method for manufacturing the same.

In recent years, the number of pixels in solid-state imaging devices has increased, and the pixel area has decreased accordingly, resulting in a decrease in the sensitivity of solid-state imaging devices. Therefore, as shown in FIG. 9, a stacked image sensor is being considered that uses crystalline selenium, which has a high absorption coefficient in visible light, as a photoelectric conversion material instead of conventional silicon. By using a material with a high absorption coefficient in a stacked image sensor, it is possible to absorb sufficient light even in a thin film, and carrier multiplication by voltage application is possible, so higher sensitivity is expected (for example, non-patent (See literature 1, non-patent literature 2). On the other hand, in stacked image sensors, there is also concern that the sensitivity may be insufficient due to the fact that traveling carriers are electrons and that incident light is absorbed in the selenium film before reaching the depletion layer.

Therefore, a junction-type image sensor has been proposed that includes a signal readout circuit board, a pixel electrode, a crystalline selenium film, a gallium oxide film, a transparent electrode, and a transparent substrate in this order (see, for example, Patent Document 1). In the above-described stacked image sensor, since the photoelectric conversion section is directly stacked on the signal readout circuit board, the photoelectric conversion section cannot be heated above the heat resistance temperature (approximately 350 degrees) of the signal readout circuit board. On the other hand, as shown in FIG. 10, in a junction type image sensor, a first amorphous selenium film is formed on a signal readout circuit board, a gallium oxide film and a second amorphous selenium film are formed on a transparent substrate, and then each Since the amorphous selenium film is bonded, the photoelectric conversion section on the transparent substrate side can be heated above the heat resistance temperature of the signal readout circuit board. This is expected to improve the characteristics of the photoelectric conversion section due to crystallization, etc., and is expected to further increase the sensitivity of the junction type image sensor.

JP2017-208376A

S. Imura, K. Kikuchi, K. Miyakawa, H. Ohtake, M. Kubota, T. Okino, Y. Hi-rose, Y. Kato, N. Teranishi, IEEE Trans. Electron Devices 63, 86 (2016) S. Imura et al., "Low-voltage-operation avalanche photodiode based on n-gallium oxide/p-crystalline selenium heterojunction", Applied Physics Letters, Vol. 104, No. 24, pp. 242101-242101-4 (2014 )

However, when a high voltage is applied to a conventional junction-type image sensor in order to increase sensitivity through carrier multiplication, the signal readout circuit can be destroyed. As a result, studies have been conducted to achieve carrier multiplication at low voltages by adding impurities to the photoelectric conversion section, but there are concerns that adding impurities will increase dark current.

It is an object of the present invention, made in view of the above circumstances, to provide a solid-state imaging device and a method for manufacturing the same that can suppress an increase in dark current due to the addition of impurities while enabling carrier multiplication at low voltage. .

In order to solve the above-mentioned problems, the inventors of the present invention made extensive studies and came up with the idea of subjecting a gallium oxide film doped with impurities to high-temperature heat treatment to crystallize it in a junction-type imaging device. The present inventors have experimentally confirmed that this junction-type image sensor is capable of carrier multiplication at low voltage while suppressing an increase in dark current due to the addition of impurities. The present invention has been completed based on the above-mentioned findings, and its gist and structure are as follows.

A solid-state imaging device according to one embodiment includes a gallium oxide film containing impurities and crystallized, and a crystalline selenium film.

Furthermore, in the solid-state imaging device according to one embodiment, the signal readout circuit board, the first electrode, the bonding film, the crystalline selenium film, the gallium oxide film, the second electrode, and the transparent substrate are provided. It is characterized by being prepared in sequence.

A method for manufacturing a solid-state image sensor according to an embodiment includes a step of forming a first amorphous selenium film on a signal readout circuit board, a step of forming a gallium oxide film doped with impurities on a transparent substrate, and a step of forming a gallium oxide film doped with impurities on a transparent substrate. performing a first heat treatment on the gallium film to crystallize the gallium oxide film; forming a second amorphous selenium film on the crystallized gallium oxide film; The method is characterized in that it includes a step of bonding two amorphous selenium films, and a step of subjecting the bonded amorphous selenium film to a second heat treatment to crystallize the amorphous selenium film.

Furthermore, in the method for manufacturing a solid-state image sensor according to one embodiment, the step of crystallizing the gallium oxide film includes heating the gallium oxide film at a temperature of 700 degrees or more and 1200 degrees or less for 10 minutes or more. The method is characterized in that the step is a step of performing the first heat treatment for a period of time or less.

According to the present invention, it is possible to provide a solid-state imaging device and a method for manufacturing the same that can suppress an increase in dark current due to addition of impurities while enabling carrier multiplication at low voltage.

1 is a schematic cross-sectional view showing an example of the configuration of a solid-state image sensor according to an embodiment of the present invention. FIG. 3 is a diagram showing the relationship between reverse bias voltage and dark current density according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing an example of a method for manufacturing a solid-state image sensor according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing an example of a method for manufacturing a solid-state image sensor according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing an example of a method for manufacturing a solid-state image sensor according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing an example of a method for manufacturing a solid-state image sensor according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing an example of a method for manufacturing a solid-state image sensor according to an embodiment of the present invention. 5A to 5C are schematic cross-sectional views showing an example of a method for manufacturing a solid-state imaging device according to an embodiment of the present invention. 1 is a flowchart illustrating an example of a method for manufacturing a solid-state image sensor according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of sample 1 according to an example. FIG. 2 is a schematic cross-sectional view of sample 2 according to the example. FIG. 3 is a schematic cross-sectional view of sample 3 according to an example. 3 is a diagram showing an example of voltage-current characteristics in Sample 1, Sample 2, and Sample 3 according to Experimental Example 1. FIG. 3 is a diagram showing an example of the surfaces of crystalline selenium films in Sample 1, Sample 2, and Sample 3 obtained by AFM measurement according to Experimental Example 2. FIG. 3 is a diagram showing an example of the relationship between diffraction angle and intensity in Sample 1, Sample 2, and Sample 3 obtained by XRD measurement according to Experimental Example 3. FIG. 3 is an enlarged view showing an example of the relationship between diffraction angle and intensity in Sample 1, Sample 2, and Sample 3 obtained by XRD measurement according to Experimental Example 3. FIG. FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a conventional stacked image sensor. 1 is a schematic cross-sectional view showing an example of the configuration of a conventional junction type image sensor.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. In addition, in principle, the same reference numerals are given to the same components, and redundant explanation will be omitted. In each figure, for convenience of explanation, the vertical and horizontal ratios of each structure are exaggerated from the actual ratios.

<Solid-state imaging device>
An example of the configuration of the solid-state image sensor 100 according to this embodiment will be described with reference to FIGS. 1 and 2.

The solid-state image sensor 100 includes a signal readout circuit board 60, a first electrode 40, a bonding film 30, a crystalline selenium film 20, a crystallized gallium oxide film 10 containing impurities, a second electrode 50, and a transparent substrate. 70 and are provided in this order. The gallium oxide film 10 containing impurities and crystallized, the crystalline selenium film 20, and the bonding film 30 function as a photoelectric conversion section 200. The first electrode 40 is connected to a negative electrode of a power source, and the second electrode 50 is connected to a positive electrode of a power source.

Hereinafter, in this specification, a case where tin is used as an impurity added to a gallium oxide film will be described as an example, but the impurity added to a gallium oxide film is not limited to tin. For example, silicon, germanium, or the like may be used as the impurity added to the gallium oxide film.

[Gallium oxide film]
The tin-containing crystallized gallium oxide film 10 is the photoelectric conversion section 200 in the solid-state imaging device 100, and functions as an n-type semiconductor. The tin-containing crystallized gallium oxide film 10 is formed by subjecting a tin-added gallium oxide film to high-temperature heat treatment and crystallizing it. Here, the crystallization of the tin-added gallium oxide film can be confirmed by, for example, observing diffraction peaks at diffraction angles of 38.4°, 59.2°, and 82.4° in the XRD pattern (see FIG. 8A).

The atomic percentage of tin added to the gallium oxide film is preferably 6.0 atom% or more and 9.0 atom% or less. The atomic percentage of tin added to the gallium oxide film is measured, for example, by Rutherford backscattering spectrometry (RBS), and the values measured by Rutherford backscattering spectrometry are used in the examples described below.

The heat treatment conditions for the tin-doped gallium oxide film are preferably 700 degrees or more and 1200 degrees or less and 10 minutes or more and 3 hours or less in an oxygen-containing atmosphere. Further, the conditions for the heat treatment applied to the tin-doped gallium oxide film are more preferably 700 degrees or more and 1100 degrees or less and 45 minutes or more and 2 hours or less in an oxygen-containing atmosphere. Further, the heat treatment conditions for the tin-doped gallium oxide film are more preferably 800 degrees or more and 1000 degrees or less and 1 hour or more and 2 hours or less in an oxygen-containing atmosphere. Among them, 800 degrees and 1 hour are most preferable.

The reasons why it is preferable to set the atomic percentage of tin added to the gallium oxide film within the above range are explained below.

The present inventors have experimentally confirmed that when tin is added to a gallium oxide film, the energy in the conduction band is reduced while the energy in the valence band is maintained, resulting in a narrowing of the band gap. Therefore, by optimizing the amount of tin added, the band offset of the conduction band between gallium oxide and crystalline selenium can be reduced. Therefore, if the atomic percentage of tin added to the gallium oxide film is set to 6.0 atom % or more, the operating voltage of the solid-state imaging device 100 can be reduced. Furthermore, when the atomic percentage of tin added to the gallium oxide film is 9.0 atom % or less, excessive carrier generation due to the addition of tin can be suppressed, and an increase in dark current of the solid-state imaging device 100 can be suppressed. Therefore, the atomic percentage of tin added to the gallium oxide film is preferably 6.0 atom % or more and 9.0 atom % or less.

The reason why it is preferable to set the conditions of the heat treatment applied to the tin-doped gallium oxide film to the above conditions will be explained.

Adding tin to the gallium oxide film enables carrier multiplication at low voltages, but it also roughens the surface of the crystalline selenium film, deteriorating the crystallinity of the photoelectric conversion part, and also reduces the quality of solid-state imaging devices. The present inventors experimentally confirmed that dark current increases. On the other hand, by subjecting a tin-doped gallium oxide film to high-temperature heat treatment and crystallizing it, it is possible to improve the crystallinity of the photoelectric conversion section, which has deteriorated due to the addition of tin, and to improve the dark current of solid-state imaging devices, which has increased due to the addition of tin. The present inventors have experimentally confirmed that it is possible to suppress the effects of oxidation, and in addition, that the treatment does not have an adverse effect on carrier multiplication at low voltage. Therefore, by optimizing the heat treatment conditions, it is possible to realize a solid-state imaging device that can suppress the increase in dark current due to the addition of tin while enabling carrier multiplication at low voltage.

FIG. 2 is a diagram showing an example of the relationship between the reverse bias voltage and the dark current density of the solid-state imaging device 100 when the heat treatment conditions are changed. The horizontal axis is the reverse bias voltage [V]. The vertical axis is the dark current density [nA/cm ² ].

Graph 201 shows voltage-current characteristics when a tin-doped gallium oxide film is heat-treated at 400 degrees for one hour in an oxygen-containing atmosphere. Graph 202 shows voltage-current characteristics when a tin-doped gallium oxide film is heat-treated at 600 degrees for 1 hour in an oxygen-containing atmosphere. Graph 203 shows voltage-current characteristics when a tin-doped gallium oxide film is heat-treated at 800 degrees for 10 minutes in an oxygen-containing atmosphere. Graph 204 shows voltage-current characteristics when a tin-doped gallium oxide film is heat-treated at 800 degrees for 30 minutes in an oxygen-containing atmosphere. Graph 205 shows voltage-current characteristics when a tin-doped gallium oxide film is heat-treated at 800 degrees for one hour in an oxygen-containing atmosphere.

Comparing graph 201, graph 202, and graph 205, it can be seen that graph 205 has the lowest dark current density. That is, when the heat treatment time is constant, it can be seen that the dark current reduction effect is large when the heat treatment temperature is 800 degrees, and the dark current reduction effect is small when the heat treatment temperature is 600 degrees. Therefore, it is suggested that the heat treatment temperature is preferably 700 degrees or higher, and more preferably 800 degrees or higher.

Comparing graphs 203, 204, and 205, it can be seen that graph 205 has the smallest dark current density. In other words, when the heat treatment temperature is constant, a heat treatment time of 1 hour has a large effect of reducing the dark current, and a heat treatment time of 30 minutes has a small effect of reducing the dark current. This suggests that the heat treatment temperature is preferably 45 minutes or more, and more preferably 1 hour or more.

Therefore, the heat treatment conditions for the tin-doped gallium oxide film are preferably 700 degrees or more and 1200 degrees or less and 10 minutes or more and 3 hours or less in an oxygen-containing atmosphere. Further, the conditions for the heat treatment applied to the tin-doped gallium oxide film are more preferably 700 degrees or more and 1100 degrees or less and 45 minutes or more and 2 hours or less in an oxygen-containing atmosphere. Further, the heat treatment conditions for the tin-doped gallium oxide film are more preferably 800 degrees or more and 1000 degrees or less and 1 hour or more and 2 hours or less in an oxygen-containing atmosphere. Among them, 800 degrees and 1 hour are most preferable.

The thickness of the crystallized gallium oxide film 10 containing tin is not particularly limited, but is preferably 1 nm or more and 100 nm or less. If the thickness of the crystallized gallium oxide film 10 containing tin is 1 nm or more, it is possible to efficiently block charge injection of holes from the electrodes. Furthermore, if the thickness of the crystallized gallium oxide film 10 containing tin is 100 nm or less, more preferably 50 nm or less, the externally applied voltage will be efficiently applied to the crystalline selenium film 20 side.

[Crystalline selenium film]
The crystalline selenium film 20 is the photoelectric conversion section 200 in the solid-state imaging device 100, and functions as a p-type semiconductor. The surface roughness of the crystalline selenium film 20 obtained by Atomic Force Microscope (AFM) measurement is 10 nm or more and 50 nm or less when the thickness of the crystalline selenium film 20 is 0.3 μm. The thickness is preferably 10 nm or more and 20 nm or less. The surface roughness of the crystalline selenium film 20 is one index for determining the crystallinity of the photoelectric conversion unit 200, and the amount of tin added to the gallium oxide film and the amount of tin added to the gallium oxide film are determined. It varies depending on the heat treatment conditions used. This is because the crystals of the crystalline selenium film 20 grow depending on the condition of the crystals of the tin-containing crystallized gallium oxide film 10 formed below (see FIG. 3D described later). . Therefore, by optimizing the amount of tin added to the gallium oxide film and the conditions of the heat treatment applied to the gallium oxide film to which tin has been added, as described above, the surface roughness of the crystalline selenium film 20 can be controlled within the range. It can be done. This makes it possible to smooth the surface of the crystalline selenium film 20 and improve the deterioration in crystallinity of the photoelectric conversion section 200 caused by the addition of tin. Further, it is considered that an increase in dark current of the solid-state imaging device 100 due to the addition of tin can be suppressed.

The crystalline selenium film 20 has a peak intensity of the diffraction peak of the Se (100) plane in an XRD pattern obtained by X-ray diffraction (XRD) measurement, and the film thickness of the crystalline selenium film 20 is 0.3 μm. Sometimes, it is preferably 5000 counts or more and 10000 counts or less, and more preferably 8000 counts or more and 10000 counts or less. The peak intensity of the diffraction peak of the Se (100) plane in the XRD pattern is one index for determining the crystallinity of the photoelectric conversion unit 200, and is an indicator of the amount of tin added to the gallium oxide film and the amount of tin added to the gallium oxide film. It changes depending on the conditions of the heat treatment applied to the gallium oxide film. Therefore, by optimizing the amount of tin added to the gallium oxide film and the conditions of the heat treatment applied to the gallium oxide film doped with tin as described above, the diffraction peak of the Se (100) plane in the XRD pattern can be The peak intensity of can be set within the range. Thereby, the peak intensity of the diffraction peak of the Se (100) plane in the XRD pattern can be increased, and the deterioration in crystallinity of the photoelectric conversion section 200 due to the addition of tin can be improved. Further, it is considered that an increase in dark current of the solid-state imaging device 100 due to the addition of tin can be suppressed.

The thickness of the crystalline selenium film 20 is not particularly limited, but is preferably 0.1 μm or more. If the thickness of the crystalline selenium film 20 is 0.1 μm or more, preferably 0.5 μm or more, the film thickness is sufficient, so that the solid-state image sensor 100 can obtain sufficient sensitivity in the entire visible light range. The upper limit of the thickness of the crystalline selenium film 20 is not particularly limited, but it is preferably 5 μm or less, preferably 2 μm or less, since the crystalline selenium film 20 can be formed efficiently.

[Joining film]
The bonding film 30 is the photoelectric conversion section 200 in the solid-state imaging device 100, and bonds the signal readout circuit board 60 provided with the first electrode 40 and the crystal selenium film 20. The material of the bonding film 30 is not particularly limited as long as it can bond the signal readout circuit board 60 and the crystalline selenium film 20; In order to reliably bond the signal readout circuit board 60 and the crystalline selenium film 20, it is more preferable to use tellurium.

The thickness of the bonding film 30 is not particularly limited, but can be 0.1 nm or more and 10 nm or less. If the thickness of the bonding film 30 is 0.1 nm or more, the adhesive strength between the signal readout circuit board 60 and the crystalline selenium film 20 can be effectively increased, which is preferable. In addition, if the thickness of the bonding film 30 is 10 nm or less, more preferably 3 nm or less, the bonding film 30 can prevent the formation of defects in the crystalline selenium film 20, which suppresses the increase in dark current, which is preferable (for example, see Non-Patent Document 1).

The bonding film 30 may be formed continuously over the entire area between the signal readout circuit board 60 and the crystalline selenium film 20. Alternatively, the bonding film 30 may have holes in a portion of the in-plane direction and be formed in a portion of the area between the signal readout circuit board 60 and the crystalline selenium film 20. In either case, it is sufficient that the bonding between the signal readout circuit board 60 and the crystalline selenium film 20 is ensured.

[First electrode]
The first electrode 40 is provided on the surface of the signal readout circuit board 60 in correspondence with each pixel, and is provided between the photoelectric conversion section 200 and the signal readout circuit board 60. A negative electrode of a power source is connected to the first electrode 40 . As the first electrode 40, for example, a metal film such as gold (Au), copper (Cu), aluminum (Al), tungsten (W), or molybdenum (Mo) can be used. Note that in FIG. 1, a part of the signal readout circuit board 60 is shown in the same layer as the first electrode 40 with different hatching than the first electrode 40.

[Second electrode]
The second electrode 50 is on the light incident side, is provided on the opposite side to the first electrode 40 with the photoelectric conversion section 200 interposed therebetween, and is provided between the photoelectric conversion section 200 and the transparent substrate 70 . The second electrode 50 is connected to the positive electrode of a power source. It is preferable that the second electrode 50 is formed of a material having translucency. As the second electrode 50, a transparent conductive oxide film such as ITO (indium tin oxide), IZO (zinc tin oxide), or AZO (aluminum-doped zinc) can be used. The second electrode 50 may be formed of the same material as the first electrode 40 or may be formed of a different material from the first electrode 40. Although the film thickness of the second electrode 50 is not particularly limited, it is preferably 5 nm or more and 30 nm or less.

[First board]
The signal readout circuit board 60 is, for example, a substrate in which a CMOS (Complementary Metal Oxide Semiconductor) structure is formed on a silicon substrate. The signal readout circuit board 60 has a first electrode 40 provided on its surface. The signal readout circuit board 60 is bonded to the crystal selenium film 20 in the photoelectric conversion section 200 via the bonding film 30.

[Second board]
The transparent substrate 70 is on the light incident side and is provided on the opposite side to the signal readout circuit board 60 with the photoelectric conversion unit 200 interposed therebetween. It is preferable that the transparent substrate 70 is formed of a material having translucency. As the transparent substrate 70, for example, a glass substrate, a sapphire substrate, a silicon substrate, etc. can be used. It is preferable that the material of the transparent substrate 70 is appropriately selected depending on the material of the second electrode 50.

In the solid-state imaging device 100 according to the present embodiment, the photoelectric conversion unit 200 includes at least a gallium oxide film 10 containing impurities and crystallized, and a crystalline selenium film 20. That is, the photoelectric conversion unit 200 has good crystallinity and has physical properties not found in the prior art. Thereby, it is possible to realize the solid-state imaging device 100 that can suppress an increase in dark current due to addition of impurities while enabling carrier multiplication at low voltage.

<Method for manufacturing solid-state image sensor>
An example of a method for manufacturing the solid-state imaging device 100 according to this embodiment will be described with reference to FIGS. 3A to 3F and FIG. 4.

The method for manufacturing the solid-state image sensor 100 includes a step of forming a first amorphous selenium film 20A on the signal readout circuit board 60 (S101), and a step of forming a tin-doped gallium oxide film 10A on the transparent substrate 70 (S101). S102), a step of crystallizing the tin-doped gallium oxide film 10A by performing a first heat treatment on the tin-doped gallium oxide film 10A (S103), and a step of crystallizing the tin-containing gallium oxide film 10. A step of forming a second amorphous selenium film 20B thereon (S104), a step of bonding the first amorphous selenium film 20A and a second amorphous selenium film 20B (S105), and a step of forming a second amorphous selenium film 20A, 20B on the bonded amorphous selenium film 20A, 20B. The method includes a step (S106) of performing two heat treatments to crystallize the bonded amorphous selenium films 20A and 20B.

The details of each step will be sequentially explained below. Note that the same reference numerals are given to the same constituent elements, and the description of the material, film thickness, etc. of each constituent element is the same as described above, and redundant explanation will be omitted. Furthermore, step S101 and steps S102 to S104 may be performed in parallel or in order.

[Formation process of first amorphous selenium film 20A: Step S101]
First, as shown in FIG. 3A, the bonding film 30 is formed on the signal readout circuit board 60 on which the first electrode 40 is provided by, for example, a vacuum evaporation method, a sputtering method, or the like. By forming the bonding film 30 on the signal readout circuit board 60, it is possible to improve the adhesive strength between the signal readout circuit board 60 and the crystalline selenium film 20 in a second heat treatment step described below. Thereby, peeling of the crystalline selenium film 20 can be prevented.

Next, the first amorphous selenium film 20A having a thickness of 10 nm or more and 500 nm or less is formed on the bonding film 30 by, for example, a vacuum evaporation method, a sputtering method, or the like. The thickness of the first amorphous selenium film 20A is preferably half the thickness of the crystalline selenium film 20 or less than half the thickness of the crystalline selenium film 20.

[Step of forming tin-added gallium oxide film 10A: step S102]
Next, as shown in FIG. 3B, the second electrode 50 is formed on the transparent substrate 70 by, for example, a vacuum deposition method or a sputtering method.

Next, the gallium oxide film 10A doped with tin is formed on the second electrode 50 by, for example, sputtering, pulsed laser deposition, vacuum deposition, or the like. For example, a single target made of metallic tin or gallium oxide containing tin oxide is sputtered using a sputtering device so that the atomic percentage of tin added to the gallium oxide film is 6.0 atom% or more and 9.0 atom% or less. A gallium oxide film 10A to which tin is added is formed on the second electrode 50 by sputtering using the same method. For example, a metal target of tin or a sintered target of tin oxide and a target of sintered tin oxide are sputtered so that the atomic percentage of tin added to the gallium oxide film is 6.0 atom % or more and 9.0 atom % or less. A tin-doped gallium oxide film 10A is formed on the second electrode 50 by co-sputtering using a target consisting of the following.

Note that the tin content in the tin-doped gallium oxide film 10A is approximately proportional to the tin content in the target, but is not linear. Therefore, the tin content in the tin-doped gallium oxide film 10A after film formation is determined depending on the output conditions (such as RF power), the atmospheric gas conditions (such as oxygen partial pressure), and the tin content of the target.

[First heat treatment step: Step S103]
Next, as shown in FIG. 3C, the tin-doped gallium oxide film 10A is subjected to a first heat treatment to crystallize the tin-doped gallium oxide film 10A.

The conditions of the first heat treatment are preferably 700 degrees or more and 1200 degrees or less, 10 minutes or more and 3 hours or less, in an oxygen-containing atmosphere. The conditions of the first heat treatment are more preferably 700 degrees or more and 1100 degrees or less, 45 minutes or more and 2 hours or less, in an oxygen-containing atmosphere. The conditions of the first heat treatment are more preferably 800 degrees or more and 1000 degrees or less, 1 hour or more and 2 hours or less, in an oxygen-containing atmosphere. By optimizing the conditions of the first heat treatment, the crystallinity of the photoelectric conversion section 200 to be formed later can be improved. In addition, by optimizing the conditions of the first heat treatment, an increase in the dark current of the solid-state imaging element 100 to be formed later can be suppressed. Note that the above-mentioned conditions of the first heat treatment are only an example, and are not particularly limited to these conditions.

[Step of forming the second amorphous selenium film 20B: Step S104]
3D, a second amorphous selenium film 20B having a thickness of 10 nm to 500 nm is formed on the tin-containing crystallized gallium oxide film 10 by, for example, vacuum deposition, sputtering, etc. The thickness of the second amorphous selenium film 20B is preferably half the thickness of the crystalline selenium film 20 or more than half the thickness of the crystalline selenium film 20.

[Joining process: step S105]
Next, as shown in FIG. 3E, the first amorphous selenium film 20A formed on the signal readout circuit substrate 60 and the second amorphous selenium film 20B formed on the transparent substrate 70 are bonded together so as to face each other.

For example, the first amorphous selenium film 20A and the second amorphous selenium film 20B are bonded by bringing the first amorphous selenium film 20A and the second amorphous selenium film 20B into contact and applying a pressure of 0.01 MPa to 100 MPa. . At this time, the first amorphous selenium film 20A and the second amorphous selenium film 20B may be heat-treated in advance in order to soften the first amorphous selenium film 20A and the second amorphous selenium film 20B. However, the temperature in the heat treatment is preferably 50 degrees or less.

[Second heat treatment step: Step S106]
Next, as shown in FIG. 3F, the bonded first amorphous selenium film 20A and second amorphous selenium film 20B are subjected to a second heat treatment to form the bonded first amorphous selenium film 20A and second amorphous selenium film 20B. crystallize. As a result, a photoelectric conversion section 200 is formed.

The conditions for the second heat treatment are preferably 100 degrees or more and 220 degrees or less and 30 seconds or more and 1 hour or less. Note that the conditions for the second heat treatment are merely an example, and are not particularly limited to these conditions.

The solid-state imaging device 100 according to this embodiment can be manufactured by going through each process including the above-mentioned optional steps. In the solid-state imaging device 100 according to the present embodiment, the photoelectric conversion unit 200 includes at least a gallium oxide film 10 containing impurities and crystallized, and a crystalline selenium film 20. That is, the photoelectric conversion unit 200 has good crystallinity and has physical properties not found in the prior art. Thereby, it is possible to realize the solid-state imaging device 100 that can suppress an increase in dark current due to addition of impurities while enabling carrier multiplication at low voltage.

<Example>
Hereinafter, the present invention will be described in further detail using Examples with reference to FIGS. 5A to 8B, but the present invention is not limited to the following Examples. For convenience of explanation, the reference numerals in FIGS. 5A to 5C refer to the components used in FIGS. 1 and 3A to 3F.

[Preparation of sample]
Sample 1 according to the comparative example, Sample 2 according to the comparative example, and Sample 3 according to the example were prepared as evaluation samples by the method shown below. Sample 1, Sample 2, and Sample 3 imitate elements formed on the transparent substrate side before bonding (see FIG. 3D) in a bonded image sensor. Therefore, it may be considered that the evaluation results obtained using Sample 1, Sample 2, and Sample 3 substantially match the evaluation results obtained using an actual solid-state image sensor (junction type image sensor).

《Sample 1》
As shown in FIG. 5A, a first transparent electrode 202 made of an ITO film with a thickness of 10 nm was formed on a sapphire substrate 201 by a sputtering method. Next, a 20 nm thick gallium oxide film 10B was formed on the first transparent electrode 202 by sputtering. As a target, a target made of gallium oxide to which tin was not added was used. Next, a bonding film 30 made of a tellurium film having a thickness of 1 nm was formed on the gallium oxide film 10B by vacuum evaporation. Next, an amorphous selenium film having a thickness of 0.5 μm was formed on the bonding film 30 by vacuum evaporation. Next, the sapphire substrate 201 on which the first transparent electrode 202, the gallium oxide film 10B, the bonding film 30, and the amorphous selenium film are formed is subjected to heat treatment at 200 degrees for 1 minute to reduce the film thickness to 0.5 μm. A crystalline selenium film 20 was formed. Finally, a second transparent electrode 203 made of an ITO film with a thickness of 30 nm was formed on the crystalline selenium film 20 by sputtering to obtain a sample 1.

《Sample 2》
As shown in FIG. 5B, a first transparent electrode 202 made of an ITO film with a thickness of 10 nm was formed on a sapphire substrate 201 by a sputtering method. Next, a tin-doped gallium oxide film 10A having a thickness of 20 nm was formed on the first transparent electrode 202 by sputtering. As a target, a target made of tin-doped gallium oxide to which 6.0 atom % of tin was added was used. Next, a bonding film 30 made of a tellurium film having a thickness of 1 nm was formed on the tin-doped gallium oxide film 10A by vacuum evaporation. Next, an amorphous selenium film having a thickness of 0.5 μm was formed on the bonding film 30 by vacuum evaporation. Next, the first transparent electrode 202, the tin-doped gallium oxide film 10A, the bonding film 30, and the sapphire substrate 201 on which the amorphous selenium film is formed are heat-treated at 200 degrees for 1 minute to form a film. A crystalline selenium film 20 with a thickness of 0.5 μm was formed. Finally, a second transparent electrode 203 made of an ITO film with a thickness of 30 nm was formed on the crystalline selenium film 20 by sputtering to obtain a sample 2.

《Sample 3》
As shown in FIG. 5C, a first transparent electrode 202 made of an ITO film with a thickness of 10 nm was formed on a sapphire substrate 201 by a sputtering method. Next, a tin-doped gallium oxide film 10A having a thickness of 20 nm was formed on the first transparent electrode 202 by sputtering. As a target, a target made of tin-doped gallium oxide to which 6.0 atom % of tin was added was used. Next, the sapphire substrate 201 on which the first transparent electrode 202 and the tin-doped gallium oxide film 10A are formed is heat-treated at 800 degrees for one hour in an oxygen-containing atmosphere to reduce the film thickness to 20 nm. A crystallized gallium oxide film 10 containing tin was formed. Next, on the crystallized gallium oxide film 10 containing tin, a bonding film 30 made of a tellurium film having a thickness of 1 nm was formed by vacuum evaporation. Next, an amorphous selenium film having a thickness of 0.5 μm was formed on the bonding film 30 by vacuum evaporation. Next, the sapphire substrate 201 on which the first transparent electrode 202, the tin-containing crystallized gallium oxide film 10, the bonding film 30, and the amorphous selenium film are formed is subjected to heat treatment at 200 degrees for 1 minute. A crystalline selenium film 20 having a thickness of 0.5 μm was formed. Finally, a second transparent electrode 203 made of an ITO film with a thickness of 30 nm was formed on the crystalline selenium film 20 by sputtering to obtain a sample 3. Note that the tin-doped gallium oxide film is not completely crystallized at the time of the first heat treatment.

In the above-mentioned samples 2 and 3, the atomic percentage of tin added to the gallium oxide film was 6.0 atom %, as measured by Rutherford backscattering spectrometry.

[Experiment example 1]
Voltage at the time of light irradiation when a voltage is applied using Sample 1, Sample 2, and Sample 3 shown in FIGS. 5A to 5C so that the first transparent electrode 202 becomes a positive electrode and the second transparent electrode 203 becomes a negative electrode. - Measured photocurrent characteristics. Furthermore, using Sample 1, Sample 2, and Sample 3 shown in FIGS. 5A to 5C, when a voltage was applied such that the first transparent electrode 202 became the positive electrode and the second transparent electrode 203 became the negative electrode, the Voltage-dark current characteristics were measured. In measuring the voltage-photocurrent characteristics, a light source with a light intensity of 2.5 μW/cm ² and a wavelength of 550 nm was used.

[Evaluation in Experimental Example 1]
FIG. 6 is a diagram showing an example of voltage-photocurrent characteristics during light irradiation and voltage-dark current characteristics during darkness. The solid line shows the voltage-photocurrent characteristics during light irradiation. The dotted line indicates voltage-dark current characteristics in the dark. The horizontal axis indicates voltage [V]. The vertical axis indicates current density [pA/cm ² ].

From the solid line graph in FIG. 6, it can be seen that in Samples 2 and 3, the photocurrent rises around a voltage of 15V, whereas in Sample 1, the photocurrent does not rise around a voltage of 15V. In other words, in Sample 1, the photocurrent rises slowly and requires a very high voltage for carrier multiplication, whereas in Samples 2 and 3, the photocurrent rises quickly and requires a very high voltage for carrier multiplication. It can be seen that the voltage can be reduced.

This is because the carrier concentration of non-doped gallium oxide is lower than that of crystalline selenium, and the depletion layer does not spread efficiently to the crystalline selenium side. However, by adding tin, the carrier concentration of gallium oxide increases and the depletion layer does not expand efficiently. This is thought to be due to the depletion layer expanding toward the crystalline selenium side. Therefore, it is suggested that carrier multiplication at low voltage is possible by adding tin to the gallium oxide film.

Moreover, from the solid line graph in FIG. 6, it can be seen that sample 2 and sample 3 have substantially the same photocurrent. Therefore, even if a gallium oxide film doped with tin is subjected to heat treatment, the photocurrent is not significantly affected, suggesting that the effect of carrier multiplication at low voltage is not lost.

From the dotted line graph in FIG. 6, it can be seen that among Sample 1, Sample 2, and Sample 3, Sample 1 has the smallest overall dark current. Furthermore, it can be seen that among Sample 1, Sample 2, and Sample 3, Sample 2 has the largest overall dark current. Therefore, by adding tin to a gallium oxide film, the dark current increases, but by further heat-treating the gallium oxide film to which tin has been added, it is possible to suppress the increase in dark current due to the addition of tin. This suggests something.

From Experimental Example 1, by optimizing the amount of tin added to the gallium oxide film and the conditions of heat treatment applied to the gallium oxide film to which tin has been added, carrier multiplication at low voltage was made possible. This suggests that it is possible to realize a solid-state imaging device 100 that can suppress an increase in dark current due to the addition of tin.

[Experimental Example 2]
Using Sample 1, Sample 2, and Sample 3 shown in FIGS. 5A to 5C, the surface roughness Ra of the crystalline selenium film 20 was measured by an atomic force microscope.

[Evaluation in Experimental Example 2]
FIG. 7 is a diagram showing an example of the surfaces of crystalline selenium films in Sample 1, Sample 2, and Sample 3 obtained by AFM measurement.

The surface roughness of the crystalline selenium film 20 in Sample 1 was 20.8 [nm]. The surface roughness of the crystalline selenium film 20 in Sample 2 was 41.1 [nm]. The surface roughness of the crystalline selenium film 20 in Sample 3 was 17.7 [nm].

It can be seen from FIG. 7 that the surface of the crystalline selenium film 20 in Samples 1 and 3 has few irregularities and is smooth to some extent. On the other hand, it can be seen that the surface of the crystalline selenium film 20 in Sample 2 has many irregularities and is rough.

The reason why differences appear on the surface of the crystalline selenium film 20 is that the crystals of the crystalline selenium film 20 grow depending on the condition of the crystals of the gallium oxide film formed below. . The rougher the surface of the crystalline selenium film 20, the worse the crystallinity of the entire photoelectric conversion section can be evaluated, and it is thought that the dark current in the evaluation sample becomes larger. On the other hand, it is considered that the smoother the surface of the crystalline selenium film 20, the better the crystallinity of the entire photoelectric conversion section, and the smaller the dark current in the evaluation sample.

In other words, since the surface of the crystalline selenium film 20 in sample 1 is smooth, the crystallinity of the crystalline selenium film 20 formed on the gallium oxide film 10B to which no tin is added is good, and the photoelectric conversion section It can be evaluated as having good crystallinity. In addition, since the surface of the crystalline selenium film 20 in sample 2 is rough, the crystallinity of the crystalline selenium film 20 formed on the tin-doped gallium oxide film 10A is not good, and the crystallinity of the photoelectric conversion section is poor. It can be evaluated as bad. In addition, since the surface of the crystalline selenium film 20 in sample 3 is smooth, the crystallinity of the crystalline selenium film 20 formed on the crystallized gallium oxide film 10 containing tin is good, and the photoelectric conversion section is It can be evaluated as having good crystallinity.

Therefore, by adding tin to the gallium oxide film, the surface of the crystalline selenium film becomes rough and the crystallinity of the photoelectric conversion part deteriorates, but if the gallium oxide film to which tin is added is further heat-treated, , the surface of the crystalline selenium film becomes smooth, suggesting that it is possible to improve the deterioration of crystallinity of the photoelectric conversion section due to the addition of tin.

From Experimental Example 2, by optimizing the amount of tin added to the gallium oxide film and the conditions of the heat treatment applied to the gallium oxide film to which tin has been added, the crystallinity of the photoelectric conversion part can be improved and the photoelectric conversion can be improved. It is suggested that the properties of the parts can be improved. This suggests that it is possible to realize the solid-state imaging device 100 that can suppress the increase in dark current due to the addition of tin.

[Experiment example 3]
X-ray diffraction measurements were performed using Sample 1, Sample 2, and Sample 3 shown in FIGS. 5A to 5C. Out-of-plane measurements were applied to evaluate the lattice planes parallel to the surfaces of Sample 1, Sample 2, and Sample 3.

The measurement conditions are shown below.
X-ray source: Cu-Kα line Output setting: 45kV, 200mA
Angle measurement range: 2θ=10°~90°
Scan speed: 2° (2θ/min), continuous scan Divergence slit: 5°
Scattering slit: 5°
Light receiving slit: 1mm

[Evaluation in Experimental Example 3]
8A and 8B are diagrams showing an example of the relationship between diffraction angle and intensity in Sample 1, Sample 2, and Sample 3 obtained by XRD measurement. FIG. 8B is an enlarged view of FIG. 8A. The horizontal axis is the diffraction angle 2θ [deg. ] is shown. The vertical axis is the strength [a. u. ] is shown.

The peak intensity of the diffraction peak of the Se (100) plane in the XRD pattern of Sample 1 was 7975 counts. Further, the half width of the diffraction peak was 0.2433 degrees. The peak intensity of the diffraction peak of the Se (100) plane in the XRD pattern of sample 2 was 4971 counts. Further, the half width of the diffraction peak was 0.2499 degrees. The peak intensity of the diffraction peak of the Se (100) plane in the XRD pattern of sample 3 was 6060 counts. Further, the half width of the diffraction peak was 0.2546 degrees.

Figure 8A shows that sample 3 has multiple prominent peaks in the XRD pattern. Figure 8B also shows that sample 1 has the largest peak intensity, sample 2 has the smallest peak intensity, and sample 3 has a peak intensity between the peak intensities of sample 1 and sample 2.

Therefore, by adding tin to the gallium oxide film, the crystallinity of the photoelectric conversion part deteriorates, but by further heat-treating the gallium oxide film to which tin has been added, the photoelectric conversion part due to the addition of tin can be improved. It is suggested that deterioration of crystallinity can be improved.

From Experimental Example 3, by optimizing the amount of tin added to the gallium oxide film and the conditions of the heat treatment applied to the gallium oxide film to which tin has been added, the crystallinity of the photoelectric conversion part can be improved and the photoelectric conversion can be improved. It is suggested that the properties of the parts can be improved. This suggests that it is possible to realize the solid-state imaging device 100 that can suppress the increase in dark current due to the addition of tin.

The above examples suggest that by subjecting the gallium oxide film to which tin has been added to high-temperature heat treatment to crystallize it, it is possible to improve the deterioration in crystallinity of the photoelectric conversion section caused by the addition of tin. This suggests that it is possible to realize the solid-state imaging device 100 that can suppress the increase in dark current due to the addition of tin while enabling carrier multiplication at low voltage.

Note that in the above embodiment, tin is used as an impurity to be added to the gallium oxide film, but silicon, germanium, etc. may be selected as the impurity to be added to the gallium oxide film. Of course, the same effect can be obtained even if

Although the embodiments described above have been described as representative examples, it will be apparent to those skilled in the art that many modifications and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited to the above-described embodiments, and various modifications and changes can be made without departing from the scope of the claims. For example, the order of each process described in the flowchart of the embodiment is not limited to the above and can be changed as appropriate. Furthermore, it is possible to combine a plurality of steps into one, or to divide one step.

1 Sample 2 Sample 3 Sample 10 Crystallized gallium oxide film containing tin 10A Gallium oxide film added with tin 10B Gallium oxide film 20 Crystalline selenium film 20A First amorphous selenium film 20B Second amorphous selenium film 30 Bonding film 40 1 electrode 50 2nd electrode 60 signal readout circuit board 70 transparent substrate 100 solid-state image sensor 200 photoelectric conversion section 201 sapphire substrate 202 first transparent electrode 203 second transparent electrode

Claims (2)

forming a first amorphous selenium film on the signal readout circuit board;
forming a gallium oxide film doped with impurities on a transparent substrate;
performing a first heat treatment on the gallium oxide film to crystallize the gallium oxide film;
forming a second amorphous selenium film on the crystallized gallium oxide film;
a step of joining the first amorphous selenium film and the second amorphous selenium film;
performing a second heat treatment on the bonded amorphous selenium film to crystallize the amorphous selenium film;
A method for manufacturing a solid-state image sensor, comprising: The step of crystallizing the gallium oxide film includes:
a step of subjecting the gallium oxide film to the first heat treatment at 700 degrees or more and 1200 degrees or less for 10 minutes or more and 3 hours or less in an atmosphere containing oxygen;
2. The method of manufacturing a solid-state image sensor according to claim 1 .

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