JP2023011110A - DUAL-WAVELENGTH INFRARED SENSOR AND IMAGING SYSTEM - Google Patents

Abstract

To maintain detection sensitivity to light of different wavelengths without switching a bias voltage for each pixel of a dual wavelength infrared sensor.SOLUTION: A dual wavelength infrared sensor includes a photodetector array having a plurality of pixels each of which includes a first contact layer, a first blocking layer, a second contact layer, a first light absorption layer sensitive to light of a first wavelength, a second blocking layer, a second light absorption layer sensitive to light of a second wavelength longer than the first wavelength, and a third contact layer in order from a light incident side. A unit block is formed by at least one first pixel for detecting light of the first wavelength and at least one second pixel for detecting light of the second wavelength, and this unit block is repeatedly arranged in the photodetector array. The first pixel is separated by a first isolation groove penetrating the third contact layer, the second light absorption layer, the second blocking layer, and the first light absorption layer, and the second pixel is separated by a second isolation groove extending from the third contact layer to the first contact layer.SELECTED DRAWING: Figure 7

Description

The present disclosure relates to dual wavelength infrared sensors and imaging systems.

2. Description of the Related Art There is known an infrared sensor in which one pixel is sensitive to different wavelength bands (see, for example, Patent Documents 1 and 2). In these two-wavelength infrared sensors, a laminated structure of absorption layers having sensitivities to different infrared wavelengths is used as one pixel, and infrared responses of different wavelengths are detected by switching the direction of the bias applied to each pixel.

An infrared detector using four pixels adjacent in the plane of a pixel array as one unit block, one pixel in the unit block as a pixel for long wavelength detection, and the other three pixels as pixels for short wavelength detection. known (see, for example, Patent Document 3).

FIG. 1 shows a cross-sectional configuration of a known infrared sensor. Section A is a schematic diagram of rows or columns containing only short wavelength (λ _S ) pixels, and section B contains alternating short wavelength (λ _S ) and long wavelength (λ _L ) pixels. FIG. 4 is a schematic diagram of a row or column; In each pixel, a lower contact layer (B-NCT), a λ _L absorption layer, an intermediate contact layer (M-CNT), a λ _S absorption layer, and an upper contact layer (T-CNT) are arranged in this order from the light incident side. is laminated with

A bias voltage applied between the intermediate contact layer (M-CNT) and the upper contact layer (T-CNT) reads out the charge accumulated in the λ _S absorption layer from the λ _S pixel. A bias voltage applied between the intermediate contact layer (M-CNT) and the lower contact layer (T-CNT) reads the charge accumulated in the λ _L absorption layer from the λ _L pixel. With this configuration, detection results can be obtained simultaneously from the λ _S pixel and the λ _L pixel without switching the direction of the voltage applied to each pixel in a time division manner. However, since only one response output of _λL or _λS can be extracted from each pixel, the missing wavelength information is supplemented by interpolation processing based on the output information of adjacent pixels.

In the configuration of FIG. 1, the λ _L absorption layer is arranged on the light incident side. The λ _L absorption layer and the λ _S absorption layer are formed of multiple quantum well layers and designed to have sharp absorption peaks at target wavelengths. Since the absorption peaks of the λ _L absorption layer and the λ _S absorption layer are independent of each other, the λ _S light is hardly absorbed by the λ _L absorption layer and reaches the λ _S absorption layer.

Japanese Unexamined Patent Application Publication No. 2013-21032 JP 2018-32663 A Japanese Patent Application Laid-Open No. 2020-21846

In recent years, in order to improve the sensitivity of dual-wavelength infrared sensors, a structure using T2SL (Type-II Super Lattice), which has high infrared absorption efficiency, as a light absorption layer has been adopted. Since the light absorption layer of T2SL has a very broad response characteristic, the response characteristics of the _λL absorption layer and the _λS absorption layer overlap, and the _λL absorption layer absorbs infrared light of _λS . Although it is conceivable to change the lamination order of the λ _L absorption layer and the λ _S absorption layer, it is not possible to read independent wavelength information from the absorption layers of each wavelength simply by changing the lamination order.

An object of the present disclosure is to provide a wavelength infrared sensor capable of maintaining detection accuracy for light of different wavelengths without switching the bias voltage for charge reading in each pixel.

In one embodiment, the two-wavelength infrared sensor comprises a first contact layer, a first blocking layer, a second contact layer, a first light absorption layer sensitive to light of the first wavelength, a second blocking layer, in order from the light incident side. a light-receiving element array in which a plurality of pixels having a second light absorption layer sensitive to light of a second wavelength longer than the first wavelength and a third contact layer are arranged;
At least one first pixel for detecting light of the first wavelength and at least one second pixel for detecting light of the second wavelength form a unit block, and the unit block is formed within the light receiving element array. placed repeatedly,
The first pixels are divided by a first separation groove penetrating the third contact layer, the second light absorption layer, the second blocking layer, and the first light absorption layer, and the second pixels are divided into the second pixels. 3 The contact layer is separated by a second separation groove reaching the first contact layer.

Detection accuracy for light of different wavelengths can be maintained without switching the bias voltage in each pixel of the dual-wavelength infrared sensor.

It is a cross-sectional schematic diagram of the conventional dual wavelength infrared sensor. It is a figure explaining the subject when applying T2SL to the dual wavelength infrared sensor of a conventional structure. FIG. 2 is a schematic plan view showing a configuration when the λ _L absorption layer and the λ _S absorption layer are interchanged in the configuration of FIG. 1; FIG. 2 is a schematic cross-sectional view showing a configuration when the λ _L absorption layer and the λ _S absorption layer are interchanged in the configuration of FIG. 1; It is a cross-sectional schematic diagram of a light receiving element array that can be considered in the process of reaching the embodiment. It is a plane schematic diagram of the light receiving element array of the two-wavelength infrared sensor of the embodiment. 7A and 7B are schematic diagrams of a cross section A and a cross section B of the light receiving element array of FIG. 6; FIG. 3 is an energy potential diagram of a single pixel configuration; It is a figure explaining operation|movement of the short wavelength pixel of a 2 wavelength infrared sensor. It is a figure explaining operation|movement of the long wavelength pixel of a 2 wavelength infrared sensor. It is a figure which shows the crystal growth structure of the light receiving element array of embodiment. FIG. 12 is a view showing a processed cross section of the crystal growth structure of FIG. 11; It is a cross-sectional schematic diagram of a two-wavelength infrared sensor. It is a cross-sectional schematic diagram of the modification of a light receiving element array. 1 is a schematic block diagram of an imaging system using a dual-wavelength infrared sensor; FIG. It is a figure explaining the interpolation process of the 2 wavelength signal of embodiment. It is a figure explaining the interpolation process of the 2 wavelength signal of embodiment.

<Structures considered in the process leading to the embodiment and their problems>
FIG. 2 is a diagram for explaining problems that arise when applying T2SL to the two-wavelength infrared sensor of FIG. The dashed line in the figure indicates the infrared response characteristics of the short wavelength (λ _S ) absorption layer formed of T2SL, and the solid line indicates the infrared response characteristics of the long wavelength (λ _L ) absorption layer formed of T2SL. Unlike the light absorption layer using multiple quantum well layers, the infrared response characteristic of the light absorption layer of T2SL is broad. Most of the response characteristics of the λ _S absorber layer overlap with the response characteristics of the λ _L absorber layer. Therefore, when the λ _L absorption layer is positioned on the light incident side as shown in FIG. 1, most of the λ _S infrared light is absorbed by the λ _L absorption layer. The λ _S infrared light is attenuated in the λ _L absorption layer before reaching the λ _S absorption layer, and the detection sensitivity on the λ _S side decreases.

The first thing that can be considered is to replace the positions of the λ _L absorption layer and the λ _S absorption layer in the configuration of FIG. That is, the λ _S absorption layer is provided on the light incident side, and the λ _L absorption layer is arranged on the far side from the light incident surface. However, by simply exchanging the positions of the λ _L absorption layer and the λ _S absorption layer, it is not possible to appropriately extract the photodetection signal of each wavelength.

3 and 4 show configurations in which the T2SL configuration is employed and the λ _L and λ _S absorption layers are interchanged. 3 is a schematic plan view of the light receiving element array, and FIG. 4 is a schematic cross-sectional view of the light receiving element array. In FIG. 3, a plurality of pixels including the _λL pixel and the _λS pixel are arranged in the XY plane. A unit block is formed by four pixels adjacent to each other in the X direction and the Y direction of the light receiving element array. Of the four pixels included in the unit block, one pixel is the _λL pixel and the remaining three pixels are the _λS pixels.

4A is a schematic diagram of the cross section A in FIG. 3, and FIG. 4B is a schematic diagram of the cross section B of FIG. Unlike FIG. 1, a λ _S absorption layer is arranged on the light incident side. Since the λ _S pixel does not have a λ _L absorption layer, the electrodes EL _{λS for short wavelengths are made higher than the electrodes EL λL for} long wavelengths to align the electrode surfaces.

The _λL pixels are surrounded by _λS pixels. The intermediate contact layer (M-CNT) is divided for each pixel and cannot be used as a common contact layer. Therefore, the lower contact layer (B-CNT) is used as a common contact layer. In the λ _S pixel, the charges accumulated in the λ _S absorption layer can be read out by selecting individual electrodes EL _λS and applying a bias voltage. However, in the λ _L pixel, the output of the λ _L absorption layer is mixed with the output of the λ _S absorption layer, causing crosstalk within one pixel. With this configuration, the incident light quantity of λ _L cannot be detected correctly.

FIG. 5 is yet another configuration example that can be considered in the process of reaching the embodiment. In the structure using the T2SL light absorption layer, a block layer (BLC) is inserted between the λ _S absorption layer and the λ _L absorption layer so that the output current of the λ _L absorption layer does not mix with the output of the λ _S absorption layer. A lower contact layer (B-CNT) is used as a common contact layer and a common bias voltage (eg 0V) is applied. By applying bias voltages of opposite polarities to the λ _S pixel and the λ _L pixel, the λ _S pixel and the λ _L pixel can be driven simultaneously without switching the bias in each pixel.

When a positive bias is applied to the λ _S pixel and a negative bias is applied to the λ _L pixel, electrons of photoexcited carriers generated in the λ _S absorption layer of the λ _S pixel are drawn from the electrode EL _λS , Holes are drawn to the bottom electrode (B-CNT). At this time, holes, which are photo-excited carriers in the λ _L absorption layer of the λ _S pixel, are blocked by the block layer BLC. However, holes drawn from the λ _S absorption layer to the lower electrode (B-CNT) enter the adjacent λ _L pixel, and a current path in the direction of the arrow can occur. Signals of unintended wavelengths interfere between adjacent pixels.

Thus, in order to operate a two-wavelength infrared sensor using T2SL correctly without switching the bias of each pixel, it is necessary to devise a pixel structure. A specific configuration of the embodiment will be described below. The same reference numerals may be given to the same components to omit overlapping descriptions.

<Configuration of Embodiment>
6 is a schematic plan view of the light receiving element array 20 of the two-wavelength infrared sensor of the embodiment, and FIG. 7 is a schematic view of cross sections A and B of FIG. In the embodiment, in a two-wavelength infrared sensor using T2SL, mixing of signals of unintended wavelengths is prevented within a pixel and between adjacent pixels, and detection sensitivity to light of each wavelength is maintained without bias switching. .

As shown in FIG. 6, λ _S pixels 21 having sensitivity to λ _S and λ _L pixels 22 having sensitivity to λ _L are arranged in the XY plane in a periodic arrangement pattern. The center-to-center distance between adjacent pixels, that is, the pixel pitch, is less than or equal to the optical resolution of _λL among the response wavelengths of _λS and _λL . This is because the interpolation process, which will be described later, utilizes the information of the _λL light that cannot be fully focused on the _λL pixel 22 and leaks and spreads to surrounding pixels.

In the light receiving element array 20, a plurality of pixels including at least one _λS pixel 21 and at least one _λL pixel 22 form a unit block 25. FIG. In FIG. 6, the unit block 25 is formed of 2×2=4 pixels, but it is not limited to this example. As will be described later, the unit block 25 may be formed of two adjacent pixels, or the unit block 25 may be formed of 3×3=9 pixels.

The arrangement of the unit blocks 25 is repeated within the XY plane of the light receiving element array 20 . All pixels have a λ _S absorption layer and a λ _L absorption layer. The λ _S pixels 21 included in the unit block 25 detect λ _S light, and the λ _L pixels 22 _detect Detect light. In each pixel, the amount of received infrared rays of undetectable wavelengths is interpolated from the output values of the peripheral pixels to obtain output signals of two wavelengths for each pixel. A specific example of interpolation processing will be described later.

7A shows the section A in FIG. 6, and FIG. 7B shows the section B in FIG. The light receiving element array 20 includes, in order from the light incident side, a first contact layer 201, a first block layer 202, a second contact layer 203, a λ _S absorption layer 205, a second block layer 206, a λ _L absorption layer 207, and A stack of third contact layers 209 is included. The conductivity type of the light-receiving layer and the contact layer is designed depending on which of photoexcited carriers, electrons or holes, is taken out as a signal. In the following, a configuration in which electrons are extracted as a signal, that is, a configuration in which a p-type conductor is applied to the lamination will be described as an example. In this case, the first blocking layer 202 and the second blocking layer 206 may be formed of p-type or i-type T2SL.

Cross-section A includes only λ _S pixels 21 . Each λ _S pixel 21 is separated from each other by a shallow isolation trench 27 . The isolation trench 27 reaches the second contact layer 203 from the surface of the third contact layer 209 . For the λ _S pixels 21, the first contact layer 201 and the second contact layer 203 can be used as common contact layers.

Section B includes alternating λ _S pixels 21 and λ _L pixels 22 . The _λL pixels 22 are separated by deep isolation trenches 28 . The isolation trench 28 reaches the first contact layer 201 from the surface of the third contact layer 209 . For the _λL pixels 22, the first contact layer 201 can be used as a common contact layer.

The third contact layer 209 is electrically connected to the individual protruding electrodes 41 and 42 via the ohmic electrode 212 and base electrode 213 . Although the detailed configuration is omitted for simplification of the illustration, the layered structure of each pixel may be covered with a protective film except for the electrode portion.

For the sake of convenience, the individual electrode provided in the _λS pixel 21 is referred to as the protruding electrode 41, and the individual electrode provided in the _λL pixel 22 is referred to as the protruding electrode 42. The protruding electrodes 41 and 42 are formed in the same process and have the same configuration. electrode. The light receiving element array 20 is flip-chip bonded to the readout circuit by protruding electrodes 41 and 42 .

FIG. 8 is an energy potential diagram of a one-pixel configuration. The energy bandgap of the λ _L absorbing layer 207 is narrower than that of the λ _S absorbing layer 205 . When electrons are extracted as signals, the first blocking layer 202 and the second blocking layer 206 have a large potential barrier on the valence band side and a small potential barrier on the conduction band side with respect to the layers in contact above and below the lamination. As for the barrier on the conduction band side, the potential difference B1 of the first blocking layer 202 with respect to the layers in contact above and below the stack is larger than the potential difference B2 of the second blocking layer 206 with respect to the layers contacting above and below with the stack (B1> B2).

When a hole is taken out as a signal, the energy potential diagram is upside down from that shown in FIG. That is, the contact layer of each pixel is made of an n-type conductor. The first blocking layer 202 and the second blocking layer 206 have a large potential barrier on the conduction band side and a small potential barrier on the valence band side with respect to layers in contact with each other above and below the stack. On the valence band side where the potential barrier is small, the potential difference of the first blocking layer 202 with respect to the first contact layer 201 and the second contact layer 203 is greater than that of the second blocking layer 206 with respect to the λ _S absorption layer 205 and the λ _L absorption layer 207. be larger than the potential difference of

9 and 10 are diagrams for explaining the operation of the two-wavelength infrared sensor using the light receiving element array 20 of FIG. 9 shows the operation of the _λS pixel 21, and FIG. 10 shows the operation of the _λL pixel 22. FIG. All of them are premised on a configuration in which electrons are taken out as signals.

In FIG. 9, a positive bias such as +0.5 V is applied to the projection electrode 41 of the λ _S pixel 21, and a negative bias such as −0 V is applied to the first contact layer 201 and the second contact layer 203. , λ _S absorber layer 205 to read out the photocurrent. The first contact layer 201 and the second contact layer 203 are set at the same potential, and are connected to a common electrode at the periphery of the light receiving element array 20, as will be described later with reference to FIG.

By applying −0 V to the first contact layer 201 and the second contact layer 203 and +0.5 V to the third contact layer, between the conduction band of the λ _S absorption layer 205 and the λ _L absorption layer 207, A potential difference occurs. On the conduction band side, the potential difference between the second blocking layer 206 and the λ _S absorption layer 205 and λ _L absorption layer 207 is set small as shown in FIG.

Among the photoexcited carriers generated in the λ _S absorption layer 205 by the incident infrared rays, electrons overcome the barrier of the second blocking layer 206 and move to the third contact layer 209 along the bottom of the conduction band. Holes generated in the λ _S absorption layer 205 move to the second contact layer 203 along the top of the valence band. At this time, holes generated in the λ _L absorption layer 207 are blocked by the second blocking layer 206 and do not flow to the second contact layer 203 .

Since the first contact layer 201 and the second contact layer 203 are set to the same potential, extra carriers (electrons) from the first contact layer 201 are blocked by the first blocking layer 202 . The first blocking layer 202 is designed so that potential barriers exist in both the conduction band and the valence band even if a slight potential difference occurs between the first contact layer 201 and the second contact layer 203. Excess carrier flow between the first contact layer 201 and the second contact layer 203 is blocked. A photocurrent responsive to λ _S is thereby drawn from the second contact layer 203 .

In FIG. 10, a negative bias, for example, −0.5 V is applied to the projecting electrode 42 of the λ _L pixel 22, and −0 V is applied to the first contact layer 201 so that light from the λ _L absorption layer 207 Read the current. In the _λL pixel 22, the second contact layer 203 is separated by the deep separation groove 28, and no external voltage is applied to the second contact layer 203 (in a floating state). On the other hand, in the λ _S pixel 21 , the second contact layer 203 is continuous with the adjacent λ _S pixel 21 in the direction perpendicular to the plane of the drawing, and is used as a common contact layer for the λ _S pixel 21 .

In the λ _L pixel 22, the first contact layer 201 is −0 _V , the second contact layer 203 is in a floating state, and the third contact layer is −0.5 _V. Also, a potential difference is generated between the first contact layer 201 and the second contact layer 203 . Minority carrier electrons generated in the λ _L absorption layer 207 of the λ _L pixel 22 by the incidence of infrared rays overcome the barrier between the second block layer 206 and the first block layer 202 and enter the first contact layer 201 . Moving. Holes generated in the λ _L absorption layer 207 move to the third contact layer 209 .

At this time, holes generated in the λ _S absorption layer 205 are blocked by the valence band of the second blocking layer 206 and cannot move toward the third contact layer 209 . Therefore, a photocurrent responsive to λ _L is drawn from the projecting electrode 42 . In this way, it is possible to suppress both interference of different wavelength information within the pixel and interference between the _λS pixel 21 and the _λL pixel.

In FIGS. 9 and 10, a positive bias is applied to the _λS protruding electrode 41, a negative bias is applied to the _λL protruding electrode 42, and a negative bias is applied to the first contact layer 201 and the second contact layer 203. Application of the common potential is performed simultaneously. Light of the target wavelength can be correctly detected by each of the λ _S pixel 21 and the λ _L pixel 22 without switching the bias application direction in each pixel.

Since the λ _S absorption layer 205 is arranged on the side closer to the first contact layer 201 , which is the light incident surface, and the λ _L absorption layer 207 is arranged on the far side, the λ _L absorption layer 207 in the λ _S pixel 21 Not affected by light attenuation. Therefore, the problem of reduced λ _S sensitivity is also resolved.

FIG. 11 is a diagram showing the crystal growth structure of the light receiving element array 20 of the embodiment. A layer structure of the light receiving element array 20 of the two-wavelength infrared sensor is crystal-grown on the substrate 221 by a crystal growth method such as MBE (Molecular Beam Epitaxy). Specifically, on a GaSb (100) wafer, each layer constituting the laminate of FIG. 11 is epitaxially grown in sequence. If electrons are to be detected as signals, the laminate of FIG. 11 may be made of a p-type conductor.

A GaSb buffer layer 222 with a thickness of 100 nm is grown on a GaSb substrate 221 . The buffer layer 222 contains, for example, a p-type impurity such as boron at a concentration of 1E18 cm^-3.

On the buffer layer 222, the first contact layer 201, the first blocking layer 202, the second contact layer 203, the _λS absorbing layer 205, the second blocking layer 206, the _λL absorbing layer 207, the third contact layer 209, and the A cap layer 210 is grown in this order.

The first contact layer 201 is formed of a superlattice in which InAs (3 nm)/GaSb (1 nm) are repeated 100 times, and contains p-type impurities at a concentration of 1E18 cm^-3.

The first block layer 202 is formed of a superlattice in which InAs (3 nm)/AlSb (1 nm) are repeated 20 times, and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a wide energy bandgap that is deep on the valence band side.

The second contact layer 203 is formed of a superlattice in which InAs (3 nm)/GaSb (1 nm) are repeated 100 times, and contains p-type impurities at a concentration of 1E18 cm^-3.

The λ _S absorption layer 205 is formed of a superlattice of 200 cycles of InAs (3 nm)/GaSb (1 nm), and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a response sensitivity over a band of 1-7 μm and a cut-off wavelength of response at 5 μm.

The second block layer 206 is formed of a superlattice in which InAs (5 nm)/AlSb (1 nm) are repeated 20 times, and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a deep, wide energy bandgap on the valence band side. is smaller than the potential difference between

The λ _L absorption layer 207 is formed of a superlattice in which InAs (4 nm)/GaSb (2 nm) are repeated 200 periods, and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a response sensitivity over a band of 1-11 μm and a cut-off wavelength of response at 9 μm. A desired infrared spectral response can be obtained by adjusting the film thicknesses of InAs and GaSb in each superlattice of the λ _S absorption layer 205 and the λ _L absorption layer 207 .

The third contact layer 209 is formed of a superlattice in which InAs (4 nm)/GaSb (2 nm) are repeated 80 times, and contains p-type impurities at a concentration of 1E18 cm^-3. The cap layer 210 is an undoped InAs layer with a thickness of 30 nm.

The layered structure of FIG. 11 is subjected to the element forming process described below to fabricate the light receiving element array 20 having the pixel structure of FIG.

(1) The cap layer 210 on the surface is etched to form a marker for pattern matching, and dry etching is performed from the surface to form the separation groove 27 reaching the surface of the second contact layer 203 . At this time, most of the λ _S absorption layer 205 is also removed by etching from the surface of the position where the common electrode is to be provided on the periphery of the pixel array. As a result, shallow isolation trenches are formed throughout the stack. This separation groove becomes a separation groove 27 that separates the _λS pixels 21 .

(2) The second contact layer 203 and the first block layer 202 are dry-etched to reach the surface of the first contact layer 201 by patterning to form a deep separation groove 28 around the _λL pixel 22 . A separation groove 28 is formed. At this time, the second contact layer 203 and the first block layer 202 are dry-etched at the same time at the position where the common electrode is to be provided on the outer periphery of the pixel array. As a result, as shown in FIG. 12, a pixel separation structure is obtained in which the section A and the section B are separated by separation grooves having different depths.

(3) CVD (Chemical Vapor Deposition) method is used to cover the entire pixel isolation structure obtained in FIG. Electrodes 212 are formed. For example, a resist pattern is formed on the SiO2 protective film, and openings are formed at required portions of the SiO2 protective film to expose the third contact layer 209 . An AuGe film is formed by vacuum deposition, and the AuGe film on the resist mask is removed together with the resist pattern by lift-off.

(4) A SiO2 film is formed again on the entire surface, and dry etching is performed to remove the SiO2 film in the regions where the base electrodes 213 of the protruding electrodes 41 and 42 are to be formed and in the region where the wiring on the periphery of the array is to be formed. A metal laminated film of Ti/Pt is formed on the entire surface by a sputtering method. After that, the Ti/Pt metal laminated film at unnecessary portions is removed by ion milling. A base electrode 213 (see FIG. 7) connected to the ohmic electrode 212 is formed in the pixel region including the λ _S pixel 21 and the λ _L pixel 22 . Wirings connected to the first contact layer 201 and the second contact layer 203 are formed in the outer periphery of the pixel array.

(5) The entire pixel array is covered with a SiO2 protective film, and openings are formed in portions where the projecting electrodes 41 and 42 and the common electrode are to be formed. Protruding electrodes 41 and 42 of indium or the like and common electrodes 431 and 432 (see FIG. 13) are formed in the openings by a lift-off method. Thereby, the configuration of FIG. 7 is obtained.

FIG. 13 is a schematic cross-sectional view of a two-wavelength infrared sensor 50 using the light receiving element array 20. As shown in FIG. The light receiving element array 20 includes a pixel region 230 including the λ _S pixels 21 and the λ _L pixels 22 and a peripheral region 240 outside the pixel region 230 . The light receiving element array 20 is flip-chip bonded to the readout circuit 30 . The readout circuit 30 has a detection circuit for each pixel formed of, for example, a CMOS circuit.

In the pixel region 230 of the light-receiving element array 20, the _λS pixel 21 and the _λL pixel 22 are connected to corresponding detection circuits of the readout circuit 30 by protruding electrodes 41 and 42, respectively. In the peripheral region 240 , the first contact layer 201 and the common electrode 431 are connected by the ohmic electrode 214 and the wiring 215 . The second contact layer 203 and the common electrode 432 are connected by the ohmic electrode 216 and the wiring 217 .

A positive bias V+ is applied from the readout circuit 30 to the λ _S pixel 21 . A negative bias V− is applied from the readout circuit 30 to the _λL pixel 22 . A common bias potential V0 is applied to the common electrodes 431 and 432 from the readout circuit 30 and set to the same potential. These bias potentials are simultaneously applied to the light-receiving element array 20, and the _λS pixel 21 extracts the _λS signal, and the _λL pixel 22 extracts the _λL signal. When holes are extracted as signals, a negative bias V− is applied to the λ _S pixel 21 and a positive bias V+ is applied to the λ _L pixel 22 .

The obtained optical response outputs are sequentially scanned and read by the readout circuit 30 and output from the two-wavelength infrared sensor 50 as a time series signal.

<Modified example of light receiving element array>
FIG. 14 shows a cross-sectional configuration of a light receiving element array 20A as a modified example of the light receiving element array 20. As shown in FIG. 14A is a schematic diagram of the cross section A in FIG. 6, and FIG. 14B is a schematic diagram of the cross section B in FIG.

14, an n-type first block layer 232 is used instead of the wide-gap first block layer 202 used in the configuration of FIG. The n-type first block layer 232 includes an n-type region 232 a used in the λ _S pixel 21 and a p-type region 232 b used in the λ _L pixel 22 . The p-type region 232b may be formed by forming the n-type first block layer 232 as a whole and then partially implanting p-type impurity ions to invert it to p-type. When adopting the configuration of FIG. 14, it is desirable to provide an electrode terminal for applying a bias voltage to the n-type first block layer 232 in the peripheral region 240 of the light receiving element array.

In the _λS pixel 21, a pnp-type connection 220 is electrically formed by the first contact layer 201, the second contact layer 203, and the n-type region 232a of the first block layer 232 sandwiched between these contact layers. ing. On the other hand, in the _λL pixel 22, the first contact layer 201, the first blocking layer 232, and the second contact layer 203 are formed as p-type conductors, as in FIG.

A positive bias potential is applied to the _λS pixel 21 and a negative bias potential is applied to the _λL pixel 22 . The same common bias voltage is applied to the first contact layer 201 and the second contact layer 203 to set them to the same potential.

A voltage of +0.5 V, for example, is applied to the projecting electrode 41 of the λ _S pixel 21 , and −0 V is applied to the first contact layer 201 and the second contact layer 203 , thereby reading photocurrent from the λ _S absorption layer 205 . At this time, a positive potential is applied from the electrode terminal of the outer peripheral portion 240 to the n-type first block layer 232 . Of the photoexcited carriers generated in the λ _S absorption layer 205 , electrons overcome a slight barrier in the conduction band of the second blocking layer 206 and move to the third contact layer 209 , and holes move to the second contact layer 203 . move to Holes generated in the λ _L absorption layer 207 are blocked by the wide-gap second blocking layer 206 and do not flow to the second contact layer 203 .

Between the first contact layer 201 and the second contact layer 203, when a positive potential is applied to the n-type first blocking layer, a reverse-biased state of the pn junction diode is generated, and the first contact layer 201 and the second contact layer 201 are connected to the second contact layer. Current flow between layers 203 is interrupted. Therefore, excess carriers (electrons) from the first contact layer 201 can be prevented from flowing into the second contact layer 203 . Thereby, only the photocurrent responsive to λ _S is drawn from the second contact layer 203 .

A voltage of −0.5 _V , for example, is applied to the projecting electrode 42 of the λ _L pixel 22 in FIG. In the _λL pixel 22, the second contact layer 203 is in a floating state. Minority carrier electrons generated in the λ _L absorption layer 207 of the λ _L pixel 22 overcome the conduction band barrier of the second block layer 206 and pass through the first block layer 232 formed as a p-type region 232b. and move to the first contact layer 201 . Holes generated in the λ _L absorption layer 207 move to the third contact layer 209 .

Holes generated in the λ _S absorption layer 205 are blocked in the valence band of the second blocking layer 206 . Therefore, only the photocurrent responsive to λ _L is drawn from the protruding electrode 42 . In this manner, in the configuration of FIG. 14 as well, as in FIG. 7, it is possible to suppress both interference of two wavelengths within the pixel and interference between the _λS pixel 21 and the _λL pixel 22. . As with the light receiving element array 20, the light receiving element array 20A can also realize a fine-pixel two-wavelength infrared sensor 50 using T2SL with high sensitivity.

<Imaging system>
FIG. 15 is a schematic diagram of an imaging system 100 using a two-wavelength infrared sensor 50. As shown in FIG. Imaging system 100 includes dual-wavelength infrared sensor 50 and signal processing circuitry 60 . The signal processing circuit 60 is realized by a DSP (Digital Signal Processor) or the like.

The charge amounts sequentially read from the _λS pixel 21 and the _λL pixel 22 of the two-wavelength infrared sensor 50 are input to the signal processing circuit 60 as analog electric signals. The analog electrical signal may be a signal that has undergone processing such as noise cancellation and amplification by the readout circuit 30 .

The signal processing circuit 60 has an AD converter 61 , an interpolation processing circuit 62 , a correction coefficient memory 63 , a frame memory 64 and a sensitivity correction processing circuit 65 . The AD converter 61 samples the input analog electric signal at a predetermined rate and converts it into a digital signal. The frame memory 64 sequentially stores digitally converted pixel outputs and holds digital data for one frame.

The interpolation processing circuit 62 reads from the frame memory 64 the output value of the pixel and the output values of the pixels surrounding the pixel at different wavelengths for each pixel. Interpolate the amount of light received for wavelengths that do not exist. By this interpolation processing, the interpolation processing circuit 62 outputs a λ _S digital signal and a λ _L digital signal for each pixel.

The sensitivity correction processing circuit 65 corrects sensitivity variations in each pixel. The sensitivity of the two-wavelength infrared sensor 50 is affected by variations in the response characteristics of the _λS pixel 21 and the _λL pixel 22, variations in transistor characteristics of the readout circuit 30, and the like. The sensitivity correction processing circuit 65 uses the correction coefficients stored in the correction coefficient memory 63 to correct the sensitivity of the signals output in time series from the two-wavelength infrared sensor 50 and converted to digital.

The correction coefficient memory 63 stores correction coefficients (including offset values and gain values) for each pixel. The sensitivity correction processing circuit 65 may perform sensitivity correction by multiplying the output of the interpolation processing circuit 62 by a correction coefficient read from the correction coefficient memory 63 . A sensitivity correction processing circuit 65 applies sensitivity correction to each of the λ _S signal and the λ _L signal for each pixel, and outputs the result as an image signal.

Since the imaging system 100 uses a compact, high-resolution two-wavelength infrared sensor 50, the entire apparatus can be made compact. From the correlation of the two wavelength outputs for each pixel, the absolute value of the temperature of the observed object can be accurately measured. In addition, the reflected light component of natural light (sunlight) and the temperature radiation component from the object itself can be distinguished from the infrared information received from the target object.

<Interpolation processing>
16 and 17 are diagrams for explaining interpolation processing for two-wavelength signals according to the embodiment. This interpolation processing is performed by the interpolation processing circuit 62 of the signal processing circuit 60 . As shown in FIG. 16, one unit block 25 is made up of four adjacent pixels. The unit block 25 has one _λL pixel 22 and three _λS pixels 21 . Inside the unit block 25A, the position of the _λL pixel 22 is (3), and the position of the _λS pixel 21 adjacent to the _λL pixel 22 in the horizontal direction (X direction) or the vertical direction (Y direction) is ( 1), and let (2) be the position of the _λS pixel 21 that is diagonal to the _λL pixel 22 . The current value output at each of the pixel positions (1) to (3) is the detection value of the corresponding single wavelength. , interpolation processing is performed.

FIG. 17 shows the interpolation processing performed for each pixel at pixel positions (1) to (3). At pixel position (1) in FIG. 17A, the output value from this _λS pixel 21 is used as it is as the _λS output. This λ _S pixel 21 does not output the λ _L detection value. Therefore, as the _λL output of the _λS pixel 21, the average value of the pixels A and B of the adjacent _λL pixels 22 is used. Thereby, detection values corresponding to two wavelengths can be obtained from one pixel.

The pixel size or pixel pitch is set to be larger than the infrared resolution on the short wavelength (λ _S ) side and below the infrared resolution on the long wavelength (λ _L ) side. In this case, the λ _S light is condensed in the λ _S pixel 21 at the pixel position (1) and reflected in the detection output at the pixel position (1), and light leakage to adjacent pixels is small. On the other hand, the incident light of λ _L has a smaller pixel pitch (size) than the optical resolution of this wavelength, so the incident light cannot be condensed to the size of one pixel, and the incident light is imaged across a plurality of pixels. be. λ _L incident light to pixel position (1) leaks out and enters adjacent λ _L pixels 22 (pixels A and B in the drawing). Therefore, the detected outputs of the two pixels A and B adjacent to pixel location (1) each partially reflect the λ _L incident light component on pixel location (1). By using this, even if the detection output of _λL is not directly obtained from the _λS pixel 21 at the pixel position (1), by averaging the outputs of the adjacent pixels A and B, _λS It is possible to roughly estimate the incident amount of λ _L at the pixel 21 for the light beam.

At the pixel position (2) in FIG. 17B, the output value from the _λS pixel 21 is used as it is as the _λS output. This λ _S pixel 21 does not output the λ _L detection value. Therefore, as the λ _L output of this pixel, the average value of four λ _L pixels 22 (pixel A to pixel D) adjacent in the diagonal direction is used. The _λL infrared light incident on the _λS pixel 21 at the pixel position (2) is not condensed into this pixel size and is also incident on the surrounding pixels A to D. From the output, we can infer the amount of λ _L incident on pixel location (2). Thereby, detection values corresponding to two wavelengths can be obtained from one pixel.

At pixel position (3) in FIG. 17C, the output value from this _λL pixel 22 is used as it is as the _λL output. This _λL pixel 22 does not output the detection value of the short wavelength _λS . Therefore, the average value of four _λS pixels 21 (pixel A to pixel D) adjacent in the horizontal and vertical directions is used as the short wavelength output. The λ _S infrared light incident on the λ _L pixel 22 at pixel position (3) is considered to have an intensity distribution that is continuous with the λ _S incident light on the surrounding pixels AD. Therefore, by using the detection values of the surrounding pixels, detection values corresponding to two wavelengths can be obtained from one pixel.

By supplementing the missing wavelength output at each pixel position with the interpolation process described above, output signals for two wavelengths can be generated. As a result, the detection sensitivity of two wavelengths can be maintained without switching the bias application direction in each pixel using T2SL. The two-wavelength infrared sensor 50 and imaging system 100 of the embodiment can be applied to security systems, unmanned exploration systems, nighttime surveillance systems, and the like.

Although the invention has been described with reference to specific embodiments, the invention is not limited to the configurations illustrated in the embodiments. The configuration of T2SL used in each pixel is not limited to the example described above. By adjusting the composition and thickness of the thin film that constitutes the T2SL, it is possible to impart sensitivity to the target wavelength. Both the _λS pixel 21 and the _λL pixel 2 need not necessarily be formed of T2SL. If one of the light absorption layers is T2SL, broad response characteristics will cause attenuation of one wavelength and problems of crosstalk, but this problem is solved by the configuration of the embodiment. The conductivity types of the contact layer and the light absorption layer are appropriately determined according to the carrier to be detected. The bias voltage value applied to each pixel can be adjusted as appropriate. When an n-type conductor is used as a whole in the configuration of FIG. 14, the first contact layer 201, the first blocking layer 232, and the second contact layer 203 of the _λS pixel 21 may form an npn junction.

The number of pixels forming the unit block 25 is not limited to 2×2=4 pixels, and the unit block 25 may be formed by 2×1, 3×3, 4×4, 5×5, or the like. A 2×1 unit block is formed by one _λS pixel 21 and one _λL pixel 22 . In this case, the planar pixel arrangement of the light receiving element array 20 becomes a checkerboard pattern. The interpolation processing is the same as in FIG. 17(A). A 3×3 unit block includes three _λL pixels 22 and six _λS pixels 21 arranged diagonally in the matrix. The interpolation processing is similar to that of FIG. 17, but when the distance from the _λS pixel 21 of interest to the surrounding _λL pixels 22 for interpolation differs depending on the positions of the _λL pixels 22, the calculation of the average value is weighted. You may In the case of a 4×4 unit block, interpolation processing for a 2×2 unit block is repeated.

Even when these replacements and modifications are performed, the _λS pixel 21 and the _λL pixel 22 of the dual-wavelength infrared sensor 50 operate simultaneously, and interpolation processing is performed for each pixel, enabling dual-wavelength imaging. This is a function that cannot be achieved with a configuration that operates as a two-wavelength image sensor by bias switching operation for each pixel. As in the embodiment, when performing correlation processing of two-wavelength information, etc., it is important that the data are acquired at the same time. Focusing on one wavelength, it seems that pixels are spatially missing, but with regard to the λ _L pixels 22, which are arranged in a small number, the λ _L infrared light condensed and imaged by the lens optical system is one pixel. An image is formed across a plurality of pixels because the light cannot be fully collected inside. Therefore, the interpolation process can sufficiently estimate the λ _L response at the missing pixel position.

20 light receiving element array 21 λ _S pixel (first pixel)
22 λ _L pixel (second pixel)
27 first separation groove 28 second separation groove 201 first contact layers 202, 232 first block layer 232a n-type region 232b p-type region 203 second contact layer 205 λ _S absorption layer (first light absorption layer)
206 second block layer 207 λ _L absorption layer (second light absorption layer)
209 third contact layer 30 readout circuits 41, 42 projecting electrodes 50 two-wavelength infrared sensor 60 signal processing circuit 100 imaging system

Claims (10)

In order from the light incident side, a first contact layer, a first blocking layer, a second contact layer, a first light absorption layer sensitive to light of a first wavelength, a second blocking layer (206), and a second layer longer than the first wavelength. a light-receiving element array in which a plurality of pixels having a second light absorption layer sensitive to light of two wavelengths and a third contact layer are arranged;
has
At least one first pixel for detecting light of the first wavelength and at least one second pixel for detecting light of the second wavelength form a unit block, and the unit block is formed within the light receiving element array. placed repeatedly,
the first pixel is divided by a first separation groove that penetrates the third contact layer, the second light absorption layer, the second block layer, and the first light absorption layer;
The second pixels are divided by a second separation groove reaching the first contact layer from the third contact layer,
Dual wavelength infrared sensor. The first blocking layer and the second blocking layer have a first potential barrier in either a conduction band or a valence band with respect to the first light absorbing layer and the second light absorbing layer, and having a second potential barrier lower than the first potential barrier in the other of the electron band and the conduction band;
The potential difference between the first blocking layer and the first contact layer and the second contact layer on the side of the second potential barrier is equal to the second blocking layer, the first light absorbing layer and the second contact layer. greater than the potential difference between the two light absorbing layers,
The two-wavelength infrared sensor according to claim 1. the first contact layer, the second contact layer, the first light absorption layer, the second light absorption layer, and the third contact layer are of the same conductivity type;
The two-wavelength infrared sensor according to claim 1 or 2. the first contact layer and the second contact layer have the same conductivity type, and the first blocking layer has a conductivity type opposite to that of the first contact layer and the second contact layer;
The two-wavelength infrared sensor according to claim 1. The first contact layer and the second contact layer are each connected to a common electrode, and the common electrode sets the first contact layer and the second contact layer to the same potential.
The two-wavelength infrared sensor according to any one of claims 1 to 4. a reverse bias is applied to the first blocking layer;
The two-wavelength infrared sensor according to claim 4. Bias voltages with different polarities are simultaneously applied to the first pixel and the second pixel,
The two-wavelength infrared sensor according to any one of claims 1 to 6. At least one of the first light absorption layer and the second light absorption layer is formed of a type 2 superlattice,
The two-wavelength infrared sensor according to any one of claims 1 to 7. 9. The dual-wavelength infrared sensor according to claim 1, wherein the pixel pitch of said light receiving element array is equal to or smaller than the optical resolution of said second pixels. A two-wavelength infrared sensor according to any one of claims 1 to 9;
a signal processing circuit connected to the output of the two-wavelength infrared sensor;
has
The signal processing circuit interpolates the received amount of light of a second wavelength at the first pixel using the output of the second pixels surrounding the first pixel with respect to the output of the first pixel; generating first wavelength information and second wavelength information at the first pixel;
imaging system.

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