JP2008066584A - Optical sensor - Google Patents

Abstract

An ultra-compact photosensor that accurately detects the absolute value of a minute long-wavelength infrared energy without being affected by the temperature of a portion that absorbs light.
A first mesa and a second mesa are provided on a substrate. The first mesa 21 includes the first conductivity type semiconductor layer 20 and has a portion where the electrode 61 of the first conductivity type semiconductor layer 20 is formed. The second mesa 41 is provided on the first mesa 21. The PIN photodiode includes an I layer 30 and a second conductivity type semiconductor layer 40, and the PN photodiode includes a second conductivity type semiconductor layer 40. A first conductive type semiconductor layer 20 is provided on the substrate 10, an I layer 30 is provided thereon, and a second conductive type semiconductor layer 40 is further provided thereon. When the light to be detected enters from the back surface of the semiconductor substrate 10 and enters the I layer 30, the light is absorbed by the I layer 30 to generate electrons and holes, and an output voltage of the photosensor is generated.
[Selection] Figure 1

Description

  The present invention relates to an optical sensor for converting light introduced from the back surface of a semiconductor substrate into a voltage signal, and more particularly to an optical sensor suitable for converting long-wavelength infrared light into a voltage signal.

  In recent years, development of high-sensitivity optical sensors corresponding to various wavelengths has been desired from the viewpoint of development of optical communication technology, energy saving and ensuring safety. In particular, optical sensors that detect mid- and far-infrared rays are capable of detecting infrared rays emitted from heat sources such as human bodies and automobiles at high speed, and are expected to be developed as energy-saving human sensors and sensors for collision prevention devices. .

  Examples of the optical sensor include an optical sensor having a PN or PIN junction structure using a semiconductor material. These optical sensors generate electrons and holes according to the bundle density of the light to be detected, and become current or voltage signals.

  In many cases, the optical sensor converts weak light into an electrical signal. Therefore, in addition to increasing the signal intensity by increasing the sensitivity, in order to reliably amplify and use the weak signal generated, noise caused by electromagnetic waves from inside and outside the sensor is reduced, and the S / N ratio is increased. It is desirable to increase.

  In order to increase the sensitivity of the optical sensor, it is necessary to efficiently use incident light and to suppress leakage current in order to efficiently extract generated electrons. Further, the sensitivity may be increased by using an amplification mechanism represented by avalanche doubling and reduction.

  For example, Patent Document 1 proposes an ultraviolet light sensor that uses this avalanche amplification phenomenon and is used at a high temperature. In order to use the avalanche effect, it is necessary to apply a high electric field to the sensor. With this technology, the leakage current when a high electric field is applied is suppressed by setting the angle formed with the substrate on the mesa side wall to an appropriate range. However, since light is incident from the surface side of the semiconductor layer, it is necessary to enter light from a narrow range, the light use efficiency is not high, and high sensitivity cannot be sufficiently achieved. there were.

  When the sensitivity wavelength provided by the optical sensor is mid- or far-infrared, the energy of light in this wavelength band is small, so the electrical signal obtained by photoelectric conversion is small, and the sensor internal and external noise is the S / N ratio of the sensor Adversely affects. Therefore, an optical sensor having a higher S / N ratio is required.

  In such a wavelength region, electrons and holes generated in the PN or PIN junction structure are attenuated inside the optical sensor through some leakage path before being output to the outside of the optical sensor, and the output signal of the sensor is lowered. As a result, the S / N ratio decreases. For example, in a PN or PIN structure having a mesa shape that is generally used, leakage current tends to occur on the side surface of the mesa structure. Therefore, generally, a method of cooling a portion that absorbs light is used in order to suppress the dark current and improve the S / N ratio.

  Further, for example, Patent Document 2 discloses a technique for providing an inversion layer in order to reduce the leakage current. According to this technology, leakage current is suppressed by the presence of the inversion layer formed in the mesa part, but the light utilization efficiency is low and the sensitivity is increased because the light absorption layer is formed in the mesa part. There was a problem that could not be fully achieved.

JP 2004-193615 A Japanese Patent Publication No. 7-28049

  However, in the case of the optical sensor using the cooling mechanism of the part that absorbs light as described above, the power consumption necessary for the operation of the cooling mechanism is large, and the entire system is difficult to downsize. There is a problem of being narrowed.

  In addition, in the case of a mid- or far-infrared optical sensor, it is necessary to increase the element resistance in order to obtain high sensitivity, but there is a problem that it becomes susceptible to noise as the resistance increases.

  Further, by forming an inversion layer on the side surface of the mesa structure of the PN or PIN junction, there is a possibility that incident light may be absorbed by the inversion layer, and the maximum utilization efficiency of incident light cannot be obtained. .

  Furthermore, in the case of an optical sensor incident on the back surface of the substrate, when the detected light enters the optical sensor from the atmosphere, the light is partially reflected in the sensor window due to the difference between the refractive index of air and the optical sensor substrate There is a problem that the light is reflected on the surface, is not photoelectrically converted and is lost, and the S / N ratio of the optical sensor is lowered.

  The present invention has been made in view of such problems, and the object of the present invention is to use the detected light with maximum efficiency, with a very small size and low power consumption having a high S / N ratio, An object of the present invention is to provide an optical sensor suitable for converting long-wavelength infrared light into a voltage signal.

  The present invention has been made to achieve such an object, and the invention according to claim 1 includes a first mesa including at least a first conductivity type semiconductor layer on a surface of a semiconductor substrate, and at least a second mesa. It has a portion that absorbs light composed of a PN or PIN photodiode laminated with a second mesa including a conductive type semiconductor layer, and outputs a signal according to the luminance of light incident from the back surface of the semiconductor substrate as a voltage And a forward mesa shape in which an area of a surface of the second conductivity type semiconductor layer is smaller than an area of a contact surface between the semiconductor layer of the first conductivity type and the semiconductor substrate. It is characterized by that.

  According to a second aspect of the present invention, in the first aspect of the present invention, the area of the surface of the second mesa opposite to the first mesa side and the interface between the first mesa and the second mesa The area ratio of R is R, and 1.1 ≦ R ≦ 10000.

  The invention according to claim 3 is the invention according to claim 1, wherein the slope angle of the second mesa is 10 degrees or more and 60 degrees or less.

  According to a fourth aspect of the invention, in the first, second, or third aspect of the invention, the refractive index of the portion that absorbs light is larger than the refractive index of the semiconductor substrate.

  The invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein a protective layer having a refractive index smaller than that of the semiconductor layer is provided on the semiconductor layer.

  The invention according to claim 6 is the invention according to any one of claims 1 to 5, wherein the back surface of the semiconductor substrate is a rough surface.

  The invention according to claim 7 is the invention according to any one of claims 1 to 6, further comprising a reflective layer on the portion that absorbs the light.

The invention according to claim 8 is the invention according to any one of claims 1 to 7, wherein the portion that absorbs light includes InAs _X Sb _1-X (0 ≦ x ≦ 1). Features.

  The invention according to claim 9 is the invention according to any one of claims 1 to 8, characterized in that it has two or more portions that absorb the light and is connected in series.

  According to the present invention, the area of the surface of the second conductivity type semiconductor layer constituting the light absorbing portion is smaller than the area of the contact surface between the first conductivity type semiconductor layer and the semiconductor substrate. Since the mesa shape is provided, a light sensor with high light utilization efficiency, high output and high S / N ratio can be realized. In particular, when detecting infrared rays, it is possible to realize an optical sensor having a high output, a high S / N ratio, and low power consumption, which does not require a cooling mechanism.

Embodiments of the present invention will be described below with reference to the drawings.
The optical sensor of the present invention uses a photodiode composed of a PN or PIN junction composed of multiple semiconductor layers to perform so-called photoelectric conversion that absorbs light to be detected and converts it into an electrical signal. Light incident from the back surface where no layer is formed is absorbed by a multilayer semiconductor layer and a signal is output as a voltage. The semiconductor material used here is not particularly limited, and is appropriately selected depending on the intended sensitivity wavelength of the optical sensor.

  In the optical sensor of the present invention, after a PN or PIN structure is formed in a film shape on a semiconductor substrate, a portion that absorbs light is formed using a chemical or physical etching method, and the portion that absorbs light is removed from the substrate. The structure is such that the area of the surface of the second conductivity type semiconductor layer on the far side is a forward mesa shape that is smaller than the area of the contact surface of the first conductivity type semiconductor layer with the substrate. is required.

  That is, in the optical sensor of the present invention, the light is incident from the back surface of the semiconductor substrate where the semiconductor layer is not formed. Is efficiently introduced. Furthermore, since the optical sensor of the present invention outputs a signal as a voltage, a higher output can be obtained when the resistance of the PN or PIN photodiode portion is higher, but the resistance of the photodiode is determined by adopting a forward mesa structure. Since the area of the part (I layer in the case of a PIN photodiode, PN junction in the case of a PN photodiode) is reduced, the resistance of the photodiode is increased. Thus, by having both high photocurrent and high resistance, the output of the photosensor of the present invention becomes a high voltage.

1 and 2 are cross-sectional views for explaining an embodiment of the optical sensor of the present invention.
In the optical sensor 1 of the present invention, a first mesa 21 including at least a first conductivity type semiconductor layer 20 and a second mesa 41 including at least a second conductivity type semiconductor layer 40 are stacked on the surface of a semiconductor substrate 10. In addition, it has a portion 2 for absorbing light composed of a PN or PIN photodiode, and outputs a signal corresponding to the luminance of the light incident from the back surface of the semiconductor substrate 10 as a voltage.

  The surface area of the second conductivity type semiconductor layer 40 has a forward mesa shape that is smaller than the area of the contact surface between the first conductivity type semiconductor layer 20 and the semiconductor substrate 10.

  Reference numeral 30 denotes an I layer, 50 denotes a protective layer, 61 denotes an electrode of the first conductive type semiconductor layer, 62 denotes an electrode of the second conductive type semiconductor layer, 70 denotes an insulating layer, and 80 denotes a reflective layer. .

  First, the first mesa 21 and the second mesa 41 are formed on the semiconductor substrate 10. The first mesa 21 includes the first conductivity type semiconductor layer 20 and also includes a portion where the electrode 61 of the first conductivity type semiconductor layer 20 is formed. The second mesa 41 is formed on the first mesa 21 and includes at least an I layer 30 and a second conductivity type semiconductor layer 40 in the case of a PIN photodiode, but in the case of a PN photodiode, at least the second conductivity type. A semiconductor layer 40 is included. In both cases, the second mesa 41 may include a part of the semiconductor layer 20 of the first conductivity type.

  A semiconductor layer 20 of the first conductivity type is formed on the semiconductor substrate 10, and an I layer 30 that absorbs light is formed thereon. Furthermore, a second conductivity type semiconductor layer 40 is formed. When the light to be detected enters from the back surface of the semiconductor substrate 10 and enters the I layer 30, the light is absorbed by the I layer 30 to generate electrons and holes, and an output voltage of the photosensor is generated. Reference numeral 11 denotes a rough surface on the back surface of the semiconductor substrate 10.

  FIG. 1 shows the structure of a PIN photodiode. In the case of a photodiode having a PN structure, light incident on the substrate 10 is bonded to the first conductive type semiconductor layer 20 and the second conductive type semiconductor layer 40. The output voltage of the photosensor is generated by absorption of the generated depletion layer and generation of electrons and holes.

  Specifically, a first conductivity type semiconductor layer 20 to be an N layer doped with N dopant is formed on the substrate 10, and a second semiconductor layer 30 to be an I layer is formed thereon, and then P A semiconductor layer 40 of the second conductivity type that becomes a P layer doped with dopant may be formed. The case where the film was formed in the order of N → I → P from the interface with the substrate 10 has been described. Regardless of this order, the adhesion between the semiconductor layer and the substrate 10, the crystal mismatch buffering property, and the layer existing on the substrate side The light absorption characteristics are selected as appropriate. Such light absorption characteristics may be controlled by changing the material of the semiconductor layer, or may be controlled by doping concentration. As will be described later, when the optical sensor of the present invention is manufactured by InAsxSb1-x (0 ≦ x ≦ 1), it is preferable to stack in order of N → I → P because of easy control of light absorption characteristics. .

  The angles between the first mesa 21 and the second mesa 41 of the optical sensor of the present invention and the surface of the substrate 10 are referred to as an angle θ1 of the first mesa 21 and an angle θ2 of the second mesa 41 as shown in FIG. The angle θ2 of the second mesa 41 is not particularly limited, but is preferably 10 to 60 degrees. A1 represents the interface between the first mesa 21 and the second mesa, and A2 represents the upper end surface of the second mesa 41.

  That is, when the angle θ2 of the second mesa 41 is larger than 60 degrees, not only the effect of combining the high photocurrent and the high resistance described above is reduced, but also when forming a metal wiring for electrical connection, which will be described later. Depending on the height of the semiconductor layer, the possibility of disconnection increases, and the metal wiring formation method may become complicated.

  On the other hand, when the angle θ2 of the second mesa 41 is less than 10 degrees, in order to make the junction (contact hole) with the metal wiring formed on the surface of the second conductivity type semiconductor layer 40 an appropriate size, In some cases, it is necessary to enlarge the portion that absorbs the heat, or the contact hole becomes too small, and the reliability of the electrical connection is lowered.

  In addition, when there are two or more light-absorbing portions made of PN or PIN photodiodes and are connected in series as described later, the unit area depends on the number of connections and the thickness of the semiconductor layer. In order to increase the number of photodiodes therein, a larger θ2 may be preferable.

FIG. 4 is a cross-sectional view for explaining another embodiment of the optical sensor of the present invention.
Even if the angle formed between the first conductive type semiconductor layer 20 and the inclined surfaces of the I layer 30 and the second conductive type semiconductor layer 40 in the second mesa 41 shown in FIGS. However, as shown in FIG. 4, the angles of the semiconductor layers 20a and 20b may be different. That is, as the angle θ1 formed by the semiconductor layer 20a of the first conductivity type and the substrate surface is closer to 90 degrees, more photodiodes can be formed in the same area, which may be preferable in design. On the other hand, in order to increase the light introduction efficiency into the light absorbing portion, it may be preferable that the area of the first conductive type semiconductor layer 20a and the substrate 10 is larger than the area of the light absorbing portion, In this case, it is desirable that θ1 is small. In the case of the mesa shape as shown in FIG. 4, the angle θ2 of the second mesa 41 connects the point M around the upper surface of the second mesa 41 and the point N around the interface between the first mesa 21 and the second mesa 41. This is the angle between the line and the substrate 10.

  Further, the smaller the angle θ2a formed between the I layer 30 and the surface of the substrate 10, the larger the output signal of the optical sensor. In the case of a PIN photodiode, the angle θ2b formed between the second conductivity type semiconductor layer 40 and the surface of the substrate 10 does not particularly affect the output of the sensor, but the electrode 62 of the second conductivity type semiconductor layer 40 (FIG. 1, FIG. 1). 2), when the manufacturing reliability is taken into consideration, a higher θ2b angle may be preferable. In the case of a PN photodiode, depending on the impurity concentration of each semiconductor layer, the depletion layer may spread to the first conductivity type semiconductor layer 20 or the second semiconductor type semiconductor layer 40 in some cases. In particular, when the depletion layer extends to the second conductivity type semiconductor layer 40, it may be preferable to reduce θ2b because the photodiode junction resistance increases and the output signal increases.

  3A to 3C are diagrams showing the relationship between the sensor resistance and the output signal and the angle θ2 of the second mesa in the present invention, and FIG. 3A shows the relationship between the angle θ2 of the second mesa and the resistance. (B) shows the relationship between the angle θ2 of the second mesa and the output voltage of the photodiode, and (c) shows the relationship between the output voltage of the photodiode and the area ratio R, respectively.

  Here, an optical sensor composed of a photodiode using GaAs as the substrate 10, an N-type InSb layer as the first conductivity type semiconductor layer 20, and a P-type InSb layer as the second conductivity type semiconductor layer 40 is used. The relationship between the output voltage when light is incident and the angle θ2 of the second mesa of the photodiode is shown. Here, a black body furnace having a temperature of 500 K was used as a light source.

  As is clear from FIGS. 3A and 3B, the resistance (FIG. 3A) and the output voltage (FIG. 3B) of the photodiode improve as the angle θ2 of the second mesa decreases. I understand that

  FIG. 3C shows the relationship between the output voltage of the photodiode and the area ratio R. The photodiode used here has an I layer, θ2 = θ2a = θ2b, and θ1 is about 45 °.

  Hereinafter, the relationship between the shape of the mesa structure of the optical sensor of the present invention and the area of each semiconductor layer will be described.

  In the case of the photodiode of FIG. 1, the substrate-side surface of the second conductivity type semiconductor layer 40 of the present invention is an interface between the second conductivity type semiconductor layer 40 and the I layer 30. In the case of a PN photodiode, the surface of the second conductivity type semiconductor layer 40 on the substrate side is an interface between the second conductivity type semiconductor layer 40 and the first conductivity type semiconductor layer 20. The area ratio of the surface on the substrate side of the second conductivity type semiconductor layer 40 and the area ratio of the interface between the first conductivity type semiconductor layer 20 and the substrate 10 is R, and R may be 1 or more. 1.1 ≦ R ≦ 10000 is more preferable. That is, as R is larger, the absorption efficiency of incident light is improved and the output of the optical sensor is increased. As shown in FIG. 3C, it can be seen that the output voltage increases as the area ratio R increases. The relationship between R and the output voltage varies depending on the values of θ1 and θ2, but the tendency of the output voltage to increase as R increases does not change.

Next, the substrate, the semiconductor layer, and the protective layer formed on the semiconductor layer used in the present invention will be described.
In the optical sensor of the present invention, light is incident from the surface (back surface) where the semiconductor layer is not formed. Therefore, the band gap energy of the portion that absorbs light of the semiconductor layer (I layer in the case of a PIN photodiode) It is necessary to be smaller than the band gap energy of the substrate. The difference between the band gap energies is not particularly limited, but is preferably 0.4 eV or more, more preferably 0.8 eV or more in order to increase the photocurrent.

Next, the refractive indexes of the substrate and the semiconductor layer will be described.
The optical sensor of the present invention may preferably have a refractive index of the substrate 10 smaller than that of the semiconductor layers 20 and 40 in order to make maximum use of light incident from the back surface of the substrate 10. At least, the refractive index of the substrate 10 is preferably smaller than the refractive index of the portion that absorbs light (I layer in the case of a PIN photodiode).

  That is, incident light is efficiently absorbed by the absorption layer (PIN layer in the case of a PIN photodiode) without being totally reflected at the interface between the substrate 10 and the semiconductor layer. In the case of the first conductivity type semiconductor layer 20, the second conductivity type semiconductor layer 40, and the PIN photodiode, the material of the I layer may be the same, but the material of each layer may be different.

FIG. 5 is a sectional view for explaining still another embodiment of the optical sensor of the present invention.
As described above, the material of the substrate 10 may include only one material, but the substrate 10 may also have a multilayer structure as in the embodiment shown in FIG. In this embodiment, the substrate 10 is composed of a layer 10A on the back side of the substrate 10, an intermediate layer 10B on the substrate, and a layer 10C on the semiconductor layer side of the substrate. In this case, the band gap energy of each layer is higher than that of the semiconductor absorption layer so that each layer has a high transmittance with respect to the detection light or does not absorb the detection light more than the absorption layer. Is preferred. The difference is preferably larger than 0.4 eV as described above. An example of a substrate having a multilayer structure is a substrate having a single-layer or multilayer optical filter formed on the back surface. By using such a substrate, for example, the wavelength of the light to be detected can efficiently enter the photodiode, and light absorption can be further enhanced.

Next, a case where the refractive index of each layer of the substrate 10 is different from the refractive index of each layer of the semiconductor will be described.
The refractive index of the layer 10A on the back side of the substrate 10 is n1, the refractive index of the intermediate layer 10B of the substrate 10 is n2, the refractive index of the layer 10C on the semiconductor side of the substrate 10 is n3, and the refractive index of the semiconductor layer 20 of the first conductivity type. In the case where the refractive index is n4, the refractive index of the I layer in the case of a PIN photodiode is n5, and the refractive index of the second conductivity type semiconductor layer 40 is n6, the relationship between the respective refractive indexes is as shown in the equation (1). Is preferred.
n1 <n2 <n3 <n4 ≦ n5 ≧ n6 (1)
However, the refractive index of each layer of n1-6 shows the refractive index with respect to to-be-detected light.
That is, it is preferable that the refractive index of the layer that absorbs light (I layer in the case of a PIN photodiode) is larger than the refractive indexes of the other semiconductor layers and the semiconductor layers. If the relationship of n1 <n2 <n3 <n4 <n5 is maintained, the light incident on the substrate 10 is not totally reflected on each interface of each layer, and the relationship of n5> n6 is maintained, so that the light is It becomes easy to totally reflect at the interface with the two-conductivity-type semiconductor layer 40 and is reflected by the light absorbing layer (I layer in the case of a PIN photodiode), thereby increasing the photocurrent.

Next, the protective layer of the photosensor of the present invention will be described.
The protective layer 50 is formed so as to cover at least the semiconductor layer. The role of the protective layer 50 is aligned with the role of protecting the mesa structure, and has the role of increasing the photocurrent of the photosensor by total reflection of light at the interface between the semiconductor layer and the protective layer 50. In order to increase the photocurrent using the total reflection of light, it is necessary to make the refractive index of the protective layer 50 smaller than the refractive index of the semiconductor layer. That is, when the refractive index of the protective layer 50 is set to n7, it is preferable that the relationship of the refractive index of each layer has a relationship as shown by the following formula (2).
n1 <n2 <n3 <n4 ≦ n5> n7 (2)
However, the refractive index of each layer of n1-5 and n7 shows the refractive index with respect to the to-be-detected light.
By doing so, the reflection efficiency by total reflection can be improved by covering with a material having a refractive index lower than that of the semiconductor layer having a mesa structure. As a specific material, a silicon oxide insulating layer having a refractive index of about 1.47 or a silicon nitride insulating layer having a refractive index of about 1.9 is the most typical example. In many semiconductor materials, silicon nitride or silicon oxide can be used because it has a lower refractive index.

Next, the back surface of the optical sensor substrate of the present invention will be described.
Examples of the semiconductor substrate used in the optical sensor of the present invention include a substrate using GaAs, Si, or other compound materials. Using these substrates, a semiconductor layer is laminated by LPE (Liquid Phase Epitaxy), MBE (Molecular Beam Epitaxy), and CVD (Chemical Vapor Deposition), and is stacked through physical and chemical etching. A mesa structure is created. As described above, a semiconductor layer is formed using LPE, MBE, or CVD. From the viewpoint of film formation stability and process uniformity, a double-side polished substrate having both surfaces mirror-finished is often used.

  In the optical sensor of the present invention, it is preferable that the back surface on which light is incident is a rough surface 11 when used. That is, the light incident on the substrate is usually reflected and lost by the reflectance determined by the refractive index of the substrate and the refractive index of the atmosphere. However, by making the back surface of the substrate rough, the amount of scattered light incident on the substrate (forward scattering) can be increased, and light loss is suppressed. As shown in FIGS. 1, 2, 4, and 5, the incident surface of the detected light is the back surface of the substrate 10, and is generated at the interface between the atmosphere and the substrate 10 by controlling the roughness of the back surface. The reflection of the reflected light can be suppressed, and the light intensity entering the PN or PIN structure can be increased.

  Furthermore, when light is incident on the back surface obliquely, if the back surface is mirror-finished, most of the light is reflected to the outside world and is not subjected to photoelectric conversion. However, since it proceeds to the inside of the substrate, it is possible to enter the PN or PIN mesa portion, and light absorption is enhanced. Another effect of the rough surface will be described with reference to FIG. When the vertical light P shown in FIG. 6 is incident on the back surface of the substrate 10, when the light is incident on the back surface of the substrate 10, part of the light travels straight, but part of the light rays are scattered and travels in the other direction. . Light that travels straight ahead is not absorbed by the photodiode, but part of the scattered light enters the photodiode, increasing the light absorption efficiency. This improves the sensitivity of the sensor. Although the vertical light Q is also scattered on the back surface of the substrate, the scattered light is reflected at the interface between the semiconductor layer and the protective layer, and can proceed to a portion that absorbs the light. Such an effect becomes more prominent when the above-described refractive index relationship is satisfied.

  Table 1 shows the relationship between the roughness of the back surface and the photocurrent. As is clear from Table 1, the photocurrent is improved by the roughness of the back surface.

Next, the reflective layer in the optical sensor of the present invention will be described with reference to FIG.
As described above, it has been described that the portion of the incident light that has not been absorbed is reflected to the portion that absorbs light at the interface between the semiconductor layer and the protective layer 50. Depending on the relationship between the refractive indexes of the layers, the protective layer 50 may not be reflected. When the protective layer 50 is transparent to the light to be detected, light that is not reflected at the interface between the protective layer 50 and the semiconductor layer passes through the protective layer 50. A part of the light is reflected by the metal wiring 61 to the semiconductor layer that absorbs light. However, since the electrode 61 of the first conductivity type semiconductor layer 20 and the electrode 62 of the second conductivity type semiconductor layer 40 are electrically insulated, a metal-free gap is generated. When part of the light that has passed through the protective layer 50 (the amount that has not been reflected) passes through the gap, it is emitted to the atmosphere. The reflective layer 80 is preferably formed so as to reflect the light to the light absorption site. Such an effect of the reflective layer 80 depends on the relationship between the refractive index of the semiconductor material and the protective layer 50, but becomes more prominent when the angle of inclination of the second mesa is reduced and the reflective effect of the protective layer 50 is insufficient. become.

  The material of the reflective layer 80 is not limited as long as it can reflect the above-described leakage light, but a material having a high reflectance with respect to the light to be detected may be used. Specific examples include Al and Au.

  Such a reflective layer 80 cuts off noise generated by electromagnetic waves entering the sensor from the outside, although it depends on the structure of the substrate and photodiode, especially when formed of a conductive metal. There is also an effect. In addition, an insulating layer 70 is formed for electrical insulation between the reflective layer 80 and the electrodes 61 and 62. The insulating layer 70 may be made of the same material as the protective layer 50, but may be different. Specifically, materials such as silicon nitride and silicon oxide can be exemplified.

  The semiconductor material used in the optical sensor of the present invention and the material for absorbing light must be selected in consideration of the wavelength of the light to be detected. At the same time, as described above, it is preferable that the refractive indexes of the substrate 10 and the protective layer 50 also satisfy the expressions (1) and (2). For example, when light in the vicinity of 10 μm is to be detected, InAsxSb1-x (0 ≦ x ≦ 1) is preferably used as the semiconductor material for the portion that absorbs light. Such a material can form a multilayer film on a commonly used GaAs substrate by using the film formation method described above, so let's detect light with a wavelength of around 10 μm. In such a case, it is preferable to use such a material.

Further, since the refractive index of InAs _X Sb _1-X is larger than that of GaAs, the total reflection of light at the interface between the substrate 10 and the first conductivity type semiconductor layer 20 hardly occurs as described above. Also preferred from a rate perspective.

Next, it will be described that there are two or more light-absorbing portions made of PN or PIN photodiodes and connected in series.
FIG. 7 is a cross-sectional view for explaining still another embodiment of the optical sensor of the present invention.
When detecting a weak optical signal, it may be difficult to obtain a high output signal with a single photodiode, particularly for infrared rays having a long wavelength near 10 μm. In this case, as shown in FIG. 7, two or more PN or PIN structures having a forward mesa shape are formed, and by connecting PN or PIN junctions formed by etching processing in series, the output signal of the sensor is obtained. Can be multiplied. Such a structure is preferably used in the optical sensor of the present invention in which light is incident from the back surface of the semiconductor substrate because the amount of light is not lost due to reflection of light by the connection wiring.

  For example, the photosensor can be operated like a solar cell, and the photocurrent generated by the incident light can be output as an open voltage in proportion to the resistance. In this case, when n PN or PIN junctions are connected in series, an open circuit voltage proportional to the number n of photodiodes is obtained between the connection termination A and the connection termination B as shown in FIG.

  Therefore, the output voltage of the photosensor can be improved. As a result, the weak output signal of a single photodiode is multiplied, and amplification and comparison processing by an external circuit is facilitated. Therefore, this method is particularly effective for detecting long-wavelength infrared rays.

  When other numbers of photodiodes are connected in series, a gap is generated between the photodiodes. However, even with respect to the vertical ray P shown in FIG. 6, the incident light completely passes through the substrate and does not return to the outside world. The incident light is scattered on the back surface of the substrate 10 and most of the photodiode. Proceed to where it was formed and be absorbed. Therefore, a high-sensitivity optical sensor can be realized by setting the optimum roughness to the series connection and the back surface of the substrate.

  The optical sensor of the present invention relates to an optical sensor suitable for converting long-wavelength infrared light into a voltage signal, and can realize an optical sensor with high light utilization efficiency, high output, and high S / N ratio. In particular, when detecting infrared rays, it is possible to realize an optical sensor having a high output, a high S / N ratio, and low power consumption, which does not require a cooling mechanism. In addition, since it is ultra-compact and has low power consumption, high sensitivity, and a high S / N ratio, it can be applied to portable electronic devices and the like.

It is sectional drawing (the 1) for demonstrating one Example of the optical sensor of this invention. It is sectional drawing (the 2) for demonstrating one Example of the optical sensor of this invention. FIG. 4 is a diagram illustrating a relationship between a sensor resistance and an output signal and an angle θ2 of the second mesa in the present invention, where (a) is a relationship between the angle θ2 of the second mesa and R, and (b) is an angle of the second mesa. The relationship between θ2 and the output voltage of the photodiode, (c) shows the relationship between the output voltage of the photodiode and the area ratio R, respectively. It is sectional drawing for demonstrating the other Example of the optical sensor of this invention. It is sectional drawing for demonstrating other Example of the optical sensor of this invention. It is a figure which shows the angle of the mesa structure side surface and board | substrate in this invention. It is sectional drawing for demonstrating other Example of the optical sensor of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical sensor 2 Light absorption part 10 Semiconductor substrate 10A Substrate side layer 10B Substrate intermediate layer 10C Substrate semiconductor side layer 11 Rough surface 12 Substrate part 20 where no light absorption part is formed ( 20a, 20b) First conductivity type semiconductor layer 21 First mesa 30 I layer 40 Second conductivity type semiconductor layer 41 Second mesa 50 Protective layer 60 Metal wiring 61 Electrode 62 of first conductivity type semiconductor layer Second Conductive type semiconductor layer electrode 70 Insulating layer 80 Reflective layer A, B Connection termination P, Q Normal incident light

Claims (9)

A portion that absorbs light comprising a PN or PIN photodiode in which a first mesa including at least a first conductivity type semiconductor layer and a second mesa including at least a second conductivity type semiconductor layer are stacked on a surface of a semiconductor substrate. An optical sensor that outputs a voltage according to the luminance of light incident from the back surface of the semiconductor substrate,
A light having a forward mesa shape in which an area of a surface of the semiconductor layer of the second conductivity type is smaller than an area of a contact surface between the semiconductor layer of the first conductivity type and the semiconductor substrate. Sensor.   The area ratio of the surface of the second mesa opposite to the first mesa side and the area ratio of the interface between the first mesa and the second mesa is R, and 1.1 ≦ R ≦ 10000. The optical sensor according to claim 1.   2. The optical sensor according to claim 1, wherein the slope angle of the second mesa is 10 degrees or more and 60 degrees or less.   4. The optical sensor according to claim 1, wherein a refractive index of the portion that absorbs light is larger than a refractive index of the semiconductor substrate.   The optical sensor according to claim 1, further comprising a protective layer having a refractive index smaller than that of the semiconductor layer on the semiconductor layer.   6. The optical sensor according to claim 1, wherein the back surface of the semiconductor substrate is a rough surface.   The optical sensor according to claim 1, further comprising a reflective layer on a portion that absorbs the light. The optical sensor according to claim 1, wherein the portion that absorbs light includes InAs _X Sb _1-X (0 ≦ x ≦ 1).   9. The optical sensor according to claim 1, wherein the optical sensor has two or more portions that absorb light and are connected in series.

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