JP2021009916A - Optical sensor - Google Patents

Abstract

To improve the sensitivity of an optical sensor while suppressing an increase in the noise level, and improve the S/N ratio in the optical sensor.SOLUTION: An optical sensor 100 includes a semiconductor substrate 105 and a light-shielding member 150, and outputs the level of a current according to the light intensity. An opening 170 is formed in the light-shielding member 150, and is configured to partially block the light emitted to the semiconductor substrate 105. The semiconductor substrate 105 includes a first P-type layer 110 and a light receiving portion 115 formed on the first P-type layer 110 and including a first N-type layer 120. A first photodiode is formed by the first P-type layer 110 and the first N-type layer 120. When the optical sensor 100 is viewed in a plan view, the light receiving portion 115 is formed so as to be inside the opening 170, and the area of the opening 170 is more than twice the area of the light receiving portion 115.SELECTED DRAWING: Figure 4

Description

The present invention relates to an optical sensor, and more specifically, to a technique for improving the S / N ratio of the optical sensor.

Optical sensors that convert incident light into electric current are known. By integrating the current output from such an optical sensor, a voltage proportional to the intensity of the light incident on the optical sensor can be obtained. By using this voltage, it is possible to calculate the illuminance of the incident light or determine the type of light source.

According to Japanese Patent Application Laid-Open No. 2015-65357 (Patent Document 1), a light receiving unit in which a plurality of types of light receiving elements (photodiodes) are integrated in the same vertical structure is optically oriented by switching each light receiving element in a time-divided manner. An optical sensor device that improves the performance is disclosed.

JP-A-2015-65357

In an optical sensor device, it is required to improve the light detection sensitivity. In order to increase the detection sensitivity, it is generally effective to increase the area of the photodiode, which is the light receiving portion. However, on the other hand, if the area of the photodiode is increased, the parasitic capacitance of the photodiode increases. As a result, although the signal detection level in the photodiode rises, the noise level also rises at the same time, and the expected improvement in the S / N ratio may not be realized.

The present invention has been made to solve such a problem, and an object of the present invention is to improve the sensitivity of an optical sensor while suppressing an increase in noise level in the optical sensor, and to improve the sensitivity of the optical sensor. It is to improve the N ratio.

The optical sensor according to the present invention includes a semiconductor substrate and a light-shielding member, and outputs a current at a level corresponding to the light intensity. An opening is formed in the light-shielding member so as to partially block the light radiated to the semiconductor substrate. The semiconductor substrate includes a first P-type layer and a light receiving portion formed on the first P-type layer and including the first N-type layer. The first photodiode is formed by the first P-type layer and the first N-type layer. When the optical sensor is viewed in a plan view, the light receiving portion is formed so as to be inside the opening, and the area of the opening is more than twice the area of the light receiving portion.

Preferably, the light receiving unit further includes a second P-type layer formed on the first N-type layer and a second N-type layer formed on the second P-type layer. The second photodiode is formed by the first N-type layer and the second P-type layer. A third photodiode is formed by the second P-type layer and the second N-type layer.

Preferably, the first photodiode is used for infrared detection. The second photodiode and the third photodiode are used to detect visible light.

Preferably, the semiconductor substrate further includes a third N-type layer formed so as to be separated from the light receiving portion and surround the light receiving portion. The third N-type layer is connected to the ground potential and is formed so as to overlap the light-shielding member when the optical sensor is viewed in a plan view.

Preferably, the semiconductor substrate further includes a fourth N-type layer formed between the third N-type layer and the light-receiving portion so as to surround the light-receiving portion. The 4th N-type layer is thinner than the thickness of the light receiving portion and is connected to the ground potential.

According to the optical sensor according to the present invention, the area where light is incident on the semiconductor substrate by the opening is set to be at least twice the area of the light receiving portion (photodiode). As a result, in the semiconductor substrate, light is incident on the periphery of the region where the photodiode is formed, so that electrons and holes generated in the peripheral region are easily collected by the photodiode. That is, the collection efficiency of electrons and holes can be improved even if the area of the photodiode is the same as in the case where the area of the opening is narrow. As a result, the sensitivity can be increased without changing the parasitic capacitance of the photodiode, so that the S / N ratio can be improved.

It is a block diagram of the optical sensor apparatus to which the optical sensor according to Embodiment 1 is applied. It is a figure for demonstrating the operation of the optical sensor device of FIG. It is a figure for demonstrating the relationship between the area of a photodiode and the S / N ratio. It is a figure for demonstrating the structure of the optical sensor according to Embodiment 1. FIG. It is a figure for demonstrating the structure of the optical sensor of the comparative example. It is a figure for demonstrating the structure of an optical sensor which follows a modification. It is a figure for demonstrating the light receiving part of the optical sensor of FIG. It is a figure for demonstrating the relationship between the wavelength of light and sensitivity in the light receiving part of FIG. It is a figure for demonstrating the light receiving part of the optical sensor according to Embodiment 2. It is a figure for demonstrating the relationship between the wavelength of light and sensitivity in the light receiving part of FIG.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

[Embodiment 1]
FIG. 1 is a block diagram of an optical sensor device 10 to which the optical sensor 100 according to the first embodiment is applied. With reference to FIG. 1, the optical sensor device 10 includes an operational amplifier OP, switches SWA and SWB, and a capacitor C1 in addition to the optical sensor 100. The optical sensor 100 includes a photodiode PD. The photodiode PD has a characteristic that when light is incident on the photodiode PD, a current of a level corresponding to the intensity of the light (hereinafter, also referred to as “photocurrent”) flows.

A predetermined bias voltage is connected to the non-inverting input terminal of the operational amplifier OP. A switch SWA and a capacitor C1 are connected in parallel between the inverting input terminal and the output terminal of the operational amplifier OP. That is, the operational amplifier OP functions as a full feedback type integrating amplifier by making the switch SWA non-conducting, and executes current-voltage conversion.

The switch SWB includes a switch SWB1 and a switch SWB2 that operate complementarily to each other. That is, when the switch SWB1 is in the conductive state, the switch SWB2 is in the non-conducting state, and when the switch SWB1 is in the non-conducting state, the SWB2 is in the non-conducting state. In the following, the operating state of the switch SWB1 is expressed as the operating state of the switch SWB.

One end of the switch SWB1 is connected to the inverting input terminal of the operational amplifier OP. The other end of the switch SWB1 is connected to the cathode of the photodiode PD, and the anode of the photodiode PD is connected to the ground potential. The switch SWB2 is connected between the cathode of the photodiode PD and the ground potential.

The operating states of the switch SWA and the switch SWB are switched by a control signal from a control device (not shown). The operational amplifier OP integrates the optical current input to the inverting input terminal while the switch SWA is in the non-conducting state, and outputs a voltage proportional to the intensity of the light applied to the optical sensor 100 as Aout. .. Although not shown in FIG. 2, the output terminal of the operational amplifier OP is connected to the control device via an A / D converter.

FIG. 2 is a diagram for explaining the operation of the optical sensor device 10 of FIG. The horizontal axis of FIG. 2 shows the time, and the vertical axis shows the states of the switches SWA and SWB, and the output Aout of the operational amplifier OP.

With reference to FIG. 2, both the switch SWA and the switch SWB are in a conductive state before the time t0. In this state, the optical current from the optical sensor 100 is input to the inverting input terminal of the operational amplifier OP, but since the switch SWA is in a conductive state, the output Aout of the operational amplifier OP remains the reference voltage (bias voltage). Yes (standby state).

When the switch SWA is set to the non-conducting state at time t0, the operational amplifier OP outputs a voltage obtained by time-integrating the optical current from the optical sensor 100 to the output Aout. Then, when the switch SWB is put into the non-conducting state at the time t1 when the predetermined period elapses, the voltage integrated in the predetermined period is output. Since the light current output from the optical sensor 100 has a magnitude corresponding to the intensity of the light applied to the optical sensor 100, the intensity of the light is determined by the output voltage Aout integrated by the operational amplifier OP in a predetermined period. Can be detected.

Although not shown in FIG. 2, by making the switches SWA and SWB conductive after the time t1, the electric charge stored in the capacitor C1 is discharged, and the state becomes a standby state as before the time t0. Be returned.

In the optical sensor device having such a configuration, in order to increase the detection sensitivity, it is necessary to increase the output current in the optical sensor 100 and / or reduce the capacitor C1.

Here, as a method of increasing the output current in the optical sensor 100, it is conceivable to increase the area (light receiving area) of the photodiode PD formed in the optical sensor 100. However, as shown by the broken line in FIG. 1, in the optical sensor 100, a parasitic capacitance C2 is generated between the optical sensor 100 and the ground potential, and when the area of the photodiode PD is increased, the parasitic capacitance C2 is also increased accordingly.

Here, assuming that the capacitance of the capacitor C1 is constant, as the parasitic capacitance C2 of the photodiode PD increases, the feedback rate represented by C1 / C2 decreases. Since the closed loop gain of the integrating amplifier is almost inversely proportional to the feedback rate, the closed loop gain of the integrating amplifier increases as the feedback rate decreases. As a result, although the sensitivity of the operational amplifier OP increases, the noise component of the output voltage of the operational amplifier OP also increases, so that the S / N ratio of the photodiode PD may not be improved as desired.

FIG. 3 is a diagram for conceptually explaining the relationship between the area of the photodiode PD and the S / N ratio. In FIG. 3, the horizontal axis shows the area of the photodiode PD, and the vertical axis shows the S / N ratio. In FIG. 3, the broken line LN10 shows the relationship between the area of the photodiode PD and the S / N ratio in an ideal case, and the solid line LN11 shows the relationship between the area of the actual photodiode PD and the S / N ratio. Is shown.

With reference to FIG. 3, as the area of the photodiode PD increases, the output current of the optical sensor 100 with respect to the light intensity increases, so that the detection sensitivity of the optical sensor 100 increases. In the ideal case where the parasitic capacitance C2 of the photodiode PD does not change (broken line LN10), the S / N ratio is greatly improved as the area of the photodiode PD is increased.

However, as described above, in reality, as the area of the photodiode PD increases, the parasitic capacitance C2 of the photodiode PD also increases, so that the noise level at the output Aout of the operational amplifier OP also increases. As a result, the actual amount of improvement in the S / N ratio (solid line LN11) becomes smaller than in the ideal case (broken line LN10).

That is, in order to bring the improvement amount of the S / N ratio closer to the ideal state, it is necessary to increase the output current of the photodiode PD without increasing the parasitic capacitance C2 of the photodiode PD.

Therefore, in the first embodiment, in the optical sensor, a configuration is adopted in which the region where the light around the photodiode is irradiated is expanded on the semiconductor substrate on which the photodiode (light receiving portion) is formed. As a result, even if the area of the photodiode is the same, the electrons and holes generated by the irradiated light increase in the region of the semiconductor substrate around the photodiode, and the probability of being incorporated into the photodiode increases. As a result, the output current of the photodiode at the same light intensity increases without increasing the area of the photodiode (ie, without increasing the parasitic capacitance of the photodiode). Therefore, it is possible to improve the S / N ratio of the optical sensor device.

FIG. 4 is a diagram for explaining the structure of the optical sensor 100 according to the first embodiment. In FIG. 4, a plan view of the optical sensor 100 is shown in FIG. 4A in the upper row, and a cross-sectional view taken along line IV-IV in FIG. 4A is shown in FIG. 4B in the lower row. There is.

With reference to FIG. 4, the optical sensor 100 includes a semiconductor substrate 105 and a light-shielding member 150 arranged so as to cover the main surface of the semiconductor substrate 105.

The semiconductor substrate 105 is made of, for example, silicon, and the N-type layer 120 is formed on the surface of the substrate of the P-type layer 110. The N-type layer 120 has a substantially square shape when the optical sensor 100 is viewed in a plan view. The photodiode PD is formed by the PN junction surface between the P-type layer 110 and the N-type layer 120. That is, in the first embodiment, the portion of the N-type layer 120 corresponds to the light receiving portion 115.

On the surface of the substrate of the P-type layer 110, the N-type layer 130 is formed so as to be separated from the N-type layer 120 and surround the light receiving portion 115 (N-type layer 120). Although not shown, the N-type layer 130 is connected to a ground electrode. Further, a circuit layer 140 is formed so as to surround the N-type layer 130. Although the circuit layer 140 is not an essential configuration, for example, the switches SWA, SWB, the operational amplifier OP, and the like shown in FIG. 1 may be formed. The thickness of the N-type layer 130 is the same as that of the N-type layer 120. The N-type layer 130 also functions as a shielding wall for insulating the light receiving portion 115 and the circuit layer 140.

The light-shielding member 150 is formed by using a material such as aluminum. The light-shielding member 150 is formed with an opening 170 having a substantially square shape. The opening 170 is arranged so that the light receiving portion 115 becomes an internal region of the opening 170 and the N-shaped layer 130 and the circuit layer 140 are covered with the light shielding member 150 when the optical sensor 100 is viewed in a plan view. Be placed.

As shown in FIG. 4, on the surface of the semiconductor substrate 105, there is a portion where the P-shaped layer 110 of the semiconductor substrate 105 is exposed between the light receiving portion 115 and the light shielding member 150. Here, when the optical sensor 100 is viewed in a plan view, the area of the opening 170 is more than twice the area of the light receiving portion 115.

When the light sensor 100 is irradiated with light, the light shielding member 150 causes the light to enter only the region of the semiconductor substrate 105 inside the opening 170. At this time, electrons 160 and / or holes 165 are generated by the incident light within a range of a predetermined depth from the surface of the P-type layer 110. The generated electrons 160 and / or holes 165 are collected in the N-type layer 120 or the N-type layer 130, whereby a current flows from the P-type layer 110 to each N-type layer.

Here, in the P-type layer 110, the electrons 160 and / or holes 165 generated in the region AR1 below the opening 170 are mainly collected by the N-type layer 120 forming the photodiode PD. Further, the electrons 160 and / or holes 165 generated in the region AR2 below the light-shielding member 150 are mainly collected by the N-type layer 130 connected to the ground potential.

FIG. 5 is a diagram for explaining the structure of the optical sensor 100 # according to the comparative example. In FIG. 5, a plan view of the optical sensor 100 # is shown in FIG. 5 (a) in the upper row, and a sectional view taken along line VV of FIG. 5 (a) is shown in FIG. 5 (b) in the lower row. There is.

With reference to FIG. 5, the area of the light receiving portion 115 formed on the optical sensor 100 # of the comparative example is the same as that of the first embodiment. In the optical sensor 100 #, the separation distance between the light receiving portion 115 and the N-shaped layer 130 # formed around the light receiving portion 115 is shorter than that in the first embodiment of FIG. The opening width RCV2 of the opening 170 is also narrower than the opening width RCV1 in the first embodiment. Therefore, when the comparative example and the first embodiment are compared, the volume of the region AR1 that generates the electrons 160 and / or the holes 165 mainly collected by the N-type layer 120 forming the photodiode PD is larger than that of the comparative example. Also, the first embodiment is larger. That is, the amount of electrons 160 and / or holes 165 generated in the region AR1 is larger in the first embodiment than in the comparative example. Therefore, when the optical sensor 100 is irradiated with light of the same intensity, the current generated in the first embodiment is larger than that in the comparative example, so that the sensitivity of the optical sensor 100 is improved.

On the other hand, as described above, the area of the photodiode PD, which is the light receiving portion 115, is the same in both the first embodiment and the comparative example, so that the parasitic capacitance C2 of the photodiode PD is substantially the same. is there. As described above, in the optical sensor 100 of the first embodiment, as compared with the comparative example, the sensitivity of the photodiode PD can be increased without changing the parasitic capacitance C2 of the photodiode PD, so that the S / N ratio can be increased. Can be effectively improved.

(Modification example)
FIG. 6 is a diagram for explaining the structure of the optical sensor 100A according to the modified example. In FIG. 6, a plan view of the optical sensor 100A is shown in FIG. 6 (a) in the upper row, and a sectional view taken along line VI-VI of FIG. 6 (a) is shown in FIG. 6 (b) in the lower row. ..

In the modified example, the area of the opening 170 of the light-shielding member 150 is more than twice the area of the light-receiving portion 115, as in the optical sensor 100 of the first embodiment. However, in the modified example, when the optical sensor 100A is viewed in a plan view, the N-type layer 130A is arranged in a region other than the light receiving portion 115 inside the opening 170. The N-type layer 130A is connected to the N-type layer 130 located below the light-shielding member 150, and the thickness of the N-type layer 130A is thinner than that of the N-type layer 130.

In the modified example optical sensor 100A, the electrons 160 and / or holes 165 generated in the surface layer portion of the P-type layer 110 below the opening 170 are N-type layers rather than the N-type layer 120 forming the photodiode PD. 130A makes it easier to collect. Therefore, in the N-type layer 120, electrons 160 and / or holes 165 generated in the region AR1A slightly inside the surface of the semiconductor substrate 105A are collected.

Here, it is known that the position (depth) from the substrate surface where electrons 160 and / or holes 165 are generated in the P-type layer 110 differs depending on the wavelength of the irradiated light. Specifically, the shorter the wavelength of the irradiated light, the more the excitation of electrons occurs at the shallow position of the P-type layer 110, and the longer the wavelength of light, the more the excitation of electrons occurs at the deeper position. Therefore, as in the modified example, by forming the N-type layer 130A connected to the ground potential in the portion other than the light receiving portion 115 of the light irradiation region (inside the opening 170), it is relatively like infrared rays. While increasing the sensitivity of light having a long wavelength, it is possible to suppress the sensitivity of light having a relatively short wavelength such as visible light. The thickness of the N-type layer 130A can be appropriately adjusted according to the wavelength of light for which sensitivity is desired to be lowered.

[Embodiment 2]
In the first embodiment, an example in which one photodiode is formed in the light receiving portion has been described. In the second embodiment, an example in which a plurality of photodiodes are formed in the light receiving portion and the sensitivities of light of a plurality of wavelengths included in the incident light are detected will be described.

FIG. 7 is a diagram for explaining the light receiving unit 115 of the optical sensor 100 according to the first embodiment. As described above, the optical sensor 100 has a configuration in which one N-type layer 120 is formed on the P-type layer 110, and the N-type layer 130 corresponds to the light receiving portion 115. In this case, the photodiode PD is formed at the junction surface between the P-type layer 110 and the N-type layer 130.

FIG. 8 is a diagram for explaining the relationship between the wavelength of light and the sensitivity of the light receiving unit 115 of FIG. 7. As described above, the position (depth) at which electrons are excited in the P-type layer 110 differs depending on the wavelength of light. In the light receiving unit 115 of FIG. 7, one photodiode is formed, and electrons and / or holes generated at a shallow position of the P-type layer 110, and electrons and / or holes generated at a deep position of the P-type layer 110. / Or holes are collected. Therefore, as shown by the solid line L20 in FIG. 8, both the light having a wavelength in the visible light region and the light having a wavelength in the infrared region are detected. With such an optical sensor, it is not possible to detect the intensities of a plurality of lights contained in the incident light.

FIG. 9 is a diagram for explaining a light receiving unit 115B of the optical sensor 100B according to the second embodiment. In the semiconductor substrate 105B of the optical sensor 100B, the N-type layer 121 is formed on the P-type layer 110, the P-type layer 122 is formed on the N-type layer 121, and the N-type layer 123 is further formed on the P-type layer 122. Is formed. The configuration other than the portion shown in FIG. 9 is the same as that in FIG. 4 of the first embodiment, and the description of the overlapping elements will not be repeated.

With such a configuration, a photodiode PDA is formed on the junction surface between the P-type layer 110 and the N-type layer 121, and a photodiode PDB is formed on the junction surface between the N-type layer 121 and the P-type layer 122, and the P-type is formed. A photodiode PDC is formed at the junction surface between the layer 122 and the N-type layer 123. In the second embodiment, the N-type layer 121, the P-type layer 122, and the N-type layer 123 correspond to the light receiving portion 115B.

In the light receiving portion 115B of the second embodiment, the P-type layer and the N-type layer are formed at different depths of the semiconductor substrate 105B. Therefore, the electrons and / or holes collected by each photodiode correspond to the intensity of light that excites the electrons of the corresponding depth.

FIG. 10 is a diagram for explaining the relationship between the wavelength of light detected by the light receiving unit 115B of FIG. 9 and the sensitivity. With reference to FIG. 10, since the photodiode PDA formed by the P-type layer 110 and the N-type layer 121 is formed at the deepest position, the sensitivity to light in the infrared region having a relatively long wavelength is high (solid line). L30).

On the other hand, the photodiode PDB formed by the N-type layer 121 and the P-type layer 122 and the photodiode PDC formed by the P-type layer 122 and the N-type layer 123 are both visible light having a relatively short wavelength. The sensitivity of the region to light is increased (single-point chain line L31, broken line L32). Among them, the sensitivity of the photodiode PDC becomes higher with respect to light on the shorter wavelength side (broken line L32).

In the optical sensor device of the second embodiment, an integration circuit as shown in FIG. 1 is formed for each photodiode, and the intensity of light detected by each photodiode is individually detected.

As described above, in the optical sensor of the second embodiment, a plurality of photodiodes are formed by forming a plurality of P-type layers and N-type layers in the light receiving portion, and the output of each photodiode is detected individually. This makes it possible to detect the intensity of light having different wavelengths. Further, also in the second embodiment, the S / N ratio can be improved by making the area of the opening of the light-shielding member more than twice the area of the light-receiving portion, as in the first embodiment.

The "P-type layer 110" and "P-type layer 122" in the above-described embodiment correspond to the "first P-type layer" and "second P-type layer" in the present invention, respectively. Further, the "N-type layer 120" and the "N-type layer 121" in the embodiment both correspond to the "first N-type layer" in the present invention. Further, the "N-type layer 123", "N-type layer 130", and "N-type layer 130A" in the embodiment are the "second N-type layer", "third N-type layer", and "fourth N-type layer" in the present invention. Corresponds to each. Further, the "photodiode PD" and the "photodiode PDA" in the embodiment both correspond to the "first photodiode" of the present invention. Further, the "photodiode PDB" and the "photodiode PDC" in the embodiment correspond to the "second photodiode" and the "third photodiode" in the present invention, respectively.

The embodiments disclosed this time should be considered as exemplary in all respects and not restrictive. The scope of the present disclosure is shown by the claims rather than the description of the embodiments described above, and is intended to include all modifications within the meaning and scope equivalent to the claims.

10 Optical sensor device, 100, 100A, 100B Optical sensor, 105, 105A, 105B, 105 # Semiconductor substrate, 110, 122 P-type layer, 115, 115B Receiver, 120, 121, 123, 130, 130A, 130 # N Mold layer, 140 circuit layer, 150 shading member, 160 electrons, 165 holes, 170 openings, AR1, AR1A, AR2, AR2A region, C1 capacitor, C2 parasitic capacitance, OP operational amplifier, PD, PDA to PDC photodiode, SWA , SWB, SWB1, SWB2 switches.

Claims (5)

An optical sensor that outputs a level of current according to the light intensity.
With a semiconductor substrate
An opening is formed, and the semiconductor substrate is provided with a light-shielding member configured to partially block light emitted from the semiconductor substrate.
The semiconductor substrate is
1st P type layer and
A light receiving portion formed on the first P-type layer and including the first N-type layer is included.
A first photodiode is formed by the first P-type layer and the first N-type layer.
When the optical sensor is viewed in a plan view, the light receiving portion is formed so as to be inside the opening, and the area of the opening is at least twice the area of the light receiving portion. .. The light receiving part is
The second P-type layer formed on the first N-type layer and
Further including a second N-type layer formed on the second P-type layer,
A second photodiode is formed by the first N-type layer and the second P-type layer.
The optical sensor according to claim 1, wherein a third photodiode is formed by the second P-type layer and the second N-type layer. The first photodiode is used for infrared detection and
The optical sensor according to claim 2, wherein the second photodiode and the third photodiode are used for detecting visible light. The semiconductor substrate further includes a third N-type layer formed so as to be separated from the light receiving portion and surround the light receiving portion.
The third N-type layer is connected to a ground potential and is formed so as to overlap the light-shielding member when the optical sensor is viewed in a plan view, according to any one of claims 1 to 3. Optical sensor. The semiconductor substrate further includes a fourth N-type layer formed between the third N-type layer and the light-receiving portion so as to surround the light-receiving portion.
The optical sensor according to claim 4, wherein the 4N type layer is thinner than the thickness of the light receiving portion and is connected to the ground potential.

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