JP3776266B2 - Infrared detector and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an infrared detector and a manufacturing method thereof, and more particularly to an infrared detector having a multi-quantum well structure having a focal plane array and a manufacturing method thereof.
[0002]
[Prior art]
Multiple quantum well infrared detectors (QWIP) having a focal plane array are used to detect infrared images in the wavelength range of 8-12 μm.
Such multiple quantum well (MQW) infrared detectors are described, for example, in B. F. Levine, J. Appl. Phys. 74 (8), 15 October 1993.
[0003]
The multiple quantum well infrared detector is composed of a GaAs substrate having an MQW structure in which AlGaAs and GaAs are alternately formed. The MQW layer is patterned by lithography to form a plurality of elements having a cross-sectional mesa shape or a plurality of pixels having a cross-sectional mesa shape. As an infrared detector having such an MQW layer, for example, there is a structure as shown in FIG.
[0004]
1A and 1B, a structure having an MQW layer 2 is formed on a GaAs substrate 1, and the MQW layer 2 is divided by a groove 3 formed by lithography, and a silicon substrate is formed in the divided portion. 9 is connected to a semiconductor circuit (not shown). The region delimited by the groove 3 is a pixel px.
Infrared rays are irradiated from the back side of the GaAs substrate 1. Irradiated electromagnetic waves such as infrared rays are components E parallel to the surface of the GaAs substrate._yAnd E_xHave Furthermore, the cross-sectional structure in one pixel region of the infrared detector is as shown in FIG.
[0005]
In FIG. 2, an n-type layer 1a made of GaAs, an MQW layer 2, and an n-type layer 3 made of GaAs are sequentially formed on an n-type GaAs substrate 1. A diffraction grating 4 is formed on the upper surface of the n-type layer 3. Furthermore, the diffraction grating 4 is connected to a silicon substrate 9 via bumps 5 made of indium. In FIG. 2, reference numeral 6 denotes a groove for dividing adjacent pixels.
[0006]
An electromagnetic wave such as an infrared ray entering the pixel through the GaAs substrate 1 is detected by photoinduced electron transition in the MQW layer 2. The light received by the one-pixel infrared detector is converted into an electric current and output to an electronic circuit (not shown) formed on the silicon substrate 9.
However, the MQW layer absorbs only electromagnetic waves having an electric field component perpendicular to the epitaxial surface. This usually means that the direction of the incident electromagnetic wave to be absorbed is required to change in that direction inside the detector pixel.
[0007]
In order to change the direction of the incident electromagnetic wave, a diffraction grating 4 formed by etching on the upper surface of the pixel is used as shown in FIG. The electromagnetic wave diffracted by the diffraction grating 4 is confined in the pixel by total internal reflection.
In QWIP, the configuration of optical coupling among several configurations for facilitating the absorption of electromagnetic waves has been reviewed in the aforementioned Levine literature.
[0008]
Research on optical coupling begins with a simple method of illuminating the MQW layer through a 45 ° inclined surface formed by polishing on the edge of the substrate.
The improvement in the coupling coefficient was made in early work by a metal elongated grating formed on the surface, which can diffract light at an angle close to 90 °. Thereafter, a one-dimensional or two-dimensional periodic grating is formed by etching inside the reflective layer on the substrate, so that the coupling coefficient is smaller in the one-dimensional periodic grating than in the case of the optical coupling shape having an inclined surface of 45 °. Is improved by 2 times, and in the two-dimensional periodic grating, the coupling coefficient is improved by 4 times.
[0009]
As a two-dimensional periodic grating, there is a periodic grating 4 shown in FIG. 3 (a), and one recess 4b as shown in FIGS. 3 (b) and 3 (c) is formed by etching at the center of one unit 4a. As shown in FIG. 3A, the unit 4a has a structure in which a plurality of units 4a are arranged vertically and horizontally.
Anderson et al. In J. Appl. Phys. 71 (7), 1 April 1992, pp. 3600-3610 or USP 5,229,614 detail theoretical analysis and perform experiments in the 8-10 μm wavelength range. It shows that the optical coupling coefficient is achieved 2 to 3 times as compared with the coupling shape with the inclination angle of 45 °. However, the periodic grating in the reflective layer (3) as shown in FIG. 3 only forms two light paths through the MQW layer 2 in the reflective film.
[0010]
Sarusi et al., In Appl. Phys. Lett. 64 (1994), pp. 960-962, increased the number of paths through the MQW layer by adopting a pseudo-random lattice, and further angled by a pseudo-random lattice. It shows that the coupling efficiency is about 14 times that of the optical coupling of the 45 ° inclined surface. It has been experimentally verified that when the peak response wavelength is 16.4 μm, a coupling coefficient that is eight times the coupling coefficient of the coupling by the inclined surface having an angle of 45 ° can be obtained. The light confinement to the pixels in these experiments is achieved by a thin GaAs substrate.
[0011]
For example, as shown in FIG. 4, the pseudo random lattice has a structure having three-step surfaces 4c, 4d, and 4e having different heights, and is formed by two etching processes. The maximum unit cell size is about 5.7 μm, and the minimum shape is 1.25 μm.
Furthermore, according to the document of IEEE Trans. Electron Devices, 44 (1997), pp. 45-50 published by Gunpala et al., A pseudorandom grating is used for QWIP for a wavelength of 15 μm. Further, IEEE Trans. Electron Devices, 44 (1997), pp. 51-57, a pseudorandom grating is used for a QWIP element for a wavelength of 8.5 μm. However, these devices have a very low response, with a response of 0.3 A / W at a wavelength of 8.5 μm and a response of 0.4 A / W at a wavelength of 14 μm. This response is slightly smaller than twice the optical coupling response of the 45 ° inclined surface. At an 8.5 μm wavelength, an optimal pseudorandom grating has a unit cell with a width of 2.9 μm, and for that size, a minimum width of 0.4 μm, which is practically difficult to produce, is required.
[0012]
Therefore, it has been considered that at a wavelength of 8.5 μm, a periodic grating having a minimum line width larger than the above pseudorandom must be used. This results in a further lowering of the diffraction capability.
As described above, the optical coupling structure having very high optical coupling capability in QWIP uses a pseudo random grating. However, this optical coupling structure is difficult to achieve at short wavelengths. Usually, a periodic grating arranged in a cross shape having a low coupling ability compared to a pseudorandom grating was considered suitable for a peak wavelength of about 8.5 μm. As an initial structure, there is a structure in which the upper surface of a pixel combined with a reflection grating is angled, a planar metal grating, a sawtooth wave grating, or the like.
[0013]
[Problems to be solved by the invention]
By the way, when a two-dimensional periodic diffraction grating as shown in FIG. 3 is used, light is reflected by 90 ° with respect to the substrate surface by the periodic grating at the second diffraction (not shown). 2. A large optical coupling ability like the pseudo random diffraction grating as shown in FIG. 4 cannot be obtained.
[0014]
However, when a pseudorandom grating used at a wavelength of 8.5 μm is constructed from a structure as shown in FIG. 4, the number of steps is increased as described above, and the number of steps is increased. The width of the low step region is as small as about 0.4 μm, and it is difficult to pattern the width with high accuracy, and it is difficult to align the pattern of the lithography twice.
[0015]
If the patterning accuracy is lowered, the optical coupling in the multiple quantum well infrared detector is weakened.
An object of the present invention is to provide an infrared detector having a new grating pattern that can detect infrared rays having a short wavelength by reducing the number of times of patterning, and a method of manufacturing the infrared detector.
[0016]
[Means for Solving the Problems]
  As illustrated in FIGS. 5 to 7, the above-described problems are formed on the substrate 11 and formed on the light absorption layer 13 having the quantum well structure and the light absorption layer 13.A light transmission layer 14 formed on the upper surface of the light transmission layer 14, and a first elliptic curve and a second elliptic curve surrounding the first elliptic curve and having the same center of the ellipse and the major axis orthogonal to each other. Including a shape divided into four along the major axis and minor axis of the second elliptic curve,Further, the invention is solved by an infrared detector characterized by having a diffraction pattern 14b having a step-shaped groove 14a having a one-step cross section.
[0017]
In the above-described infrared detector, the groove 14a has a one-step step shape with respect to the uppermost surface of the diffraction pattern 14b.
In the above infrared detector, the diffraction pattern 14 b is covered with a reflective film 16.
In the infrared detector described above, the diffraction pattern 14b diffracts light that has passed through the light absorption layer 13 from the substrate 11 side, is strong at an angle close to parallel to the surface of the light absorption layer 13, and absorbs the light. A light intensity distribution that weakens at an angle close to 18 degrees from a line perpendicular to the surface of the layer 13 is provided.
[0018]
  In the infrared detector described above, the groove 14a constituting the diffraction pattern 14b has a depth α that is smaller than half of the absorption wavelength of the light absorption layer 13.
[0019]
In the above-described infrared detector, the area of the diffraction pattern that is not the area of the region where the groove is formed is equal.
In the infrared detector described above, the light absorption layer 13 has a structure of any one of a multiple quantum well layer, a quantum wire, or a quantum box. In this case, the multiple quantum well layer is made of a compound semiconductor. The multiple quantum well layer may employ a structure in which a plurality of GaAs and AlGaAs layers are alternately stacked.
[0020]
  The above-described problems are, as illustrated in FIGS. 5 to 7, the step of forming the light absorption layer 13 having a quantum well structure on the substrate 11, and the formation of the light transmission layer 14 on the light absorption layer 13. Process,A shape sandwiched between a first elliptic curve and a second elliptic curve surrounding the first elliptic curve and having the same center of the ellipse and the major axis orthogonal to each other is defined as a major axis and a minor axis of the second elliptic curve. Shape divided into four alongA step of forming a mask 15 having a pattern having a pattern on the light transmissive layer 14; and etching the light transmissive layer 14 in a region not covered by the mask 15, thereby diffracting the light transmissive layer 14. The problem is solved by a method of manufacturing an infrared detector, comprising the step of forming a pattern 14b and the step of removing the mask 15.
[0021]
In the above-described infrared detector, the diffraction pattern 14b is formed by etching a cross section into a single groove 14a.
In addition, the above-mentioned figure number and code | symbol were quoted in order to make an understanding of this invention easy, and are not limited to this.
Next, the operation of the present invention will be described.
[0022]
According to the present invention, a diffraction pattern having a curved surface (for example, an elliptic curve) in a planar shape is formed on a light absorption layer having a multiple quantum well structure.
Such a diffraction pattern was confirmed to have a high optical coupling factor with respect to infrared rays having a wavelength of 8.5 μm. Further, since the grooves constituting the diffraction pattern have a single step shape, they can be formed by one lithography, and the manufacturing process can be shortened. Moreover, the minimum pattern width of the diffraction pattern is about 0.6 μm, and pattern formation is easy.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
The optical coupling grating design process of the multiple quantum well infrared detector (QWIP) is based on the calculation of diffracted light inside the pixel using diffraction theory. The inventor of the present application has found a diffraction pattern (diffraction grating) that can obtain a theoretically high coupling coefficient and can be used at a wavelength as short as 8.5 μm.
[0024]
Below, the formation process of the QWIP diffraction pattern and the structure of the QWIP according to the present invention will be described.
First, as shown in FIG. 5A, a semiconductor layer constituting a multiple quantum well infrared detector (QWIP) is formed. That is, a first n-type GaAs layer 12 having a thickness of 1 μm, an MQW layer 13 and a second n-type GaAs layer 14 having a thickness of 2 μm are formed on an n-type GaAs substrate 11. The MQW layer 13 is formed by alternately forming a plurality of layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs), or gallium arsenide and indium gallium arsenide (InGaAs), or a combination thereof. is there.
[0025]
The growth of the layers from the first n-type GaAs layer 12 to the second n-type GaAs layer 14 is formed by, for example, the MOVPE method, the MBE method, or the like.
Subsequently, as shown in FIG. 5 (b), a resist is applied on the second n-type GaAs layer 14, and this is exposed and developed to form a resist pattern 15.
The resist pattern 15 is a diffraction pattern including a curve. For example, as shown in FIG. 6A, a pattern having a first ellipse curve smaller than that in the second ellipse (ellipse) curve is formed. It has an opening plane shape divided into two. The arrangement of the first ellipse and the second ellipse is shifted by 90 ° from each other. As a diffraction pattern in which the basic shapes shown in FIG. 6 (a) are integrated, for example, there are flat patterns as shown in FIGS. 6 (b) and 7 (a).
[0026]
After forming the resist pattern 15 as described above, as shown in FIG. 5C, the portion of the second n-type GaAs layer 14 that is not covered with the resist pattern 15 is etched to form the second n-type GaAs. A groove 14a is formed on the upper surface of the layer 14 so that the planar shape includes a part of an elliptic curve and the cross-sectional shape has one step. As a result of the pattern shown in FIG. 7A being transferred to the second n-type GaAs layer 14, a diffraction grating 14a having a cross section as shown in FIG. The etching depth α of the second n-type GaAs layer 14 is, for example, an amount equivalent to or less than ½ of the wavelength λ′p showing the strongest absorption among the light absorbed by the MQW layer 13. .
[0027]
The portion where the 1/4 elliptical pattern shown in FIG. 7 (a) is connected is an etched portion, and the total area of the etched region is made substantially equal to the total area of the non-etched region. Yes.
Thus, after forming the groove 14a having an elliptic curve on the upper surface of the second n-type GaAs layer 14, the resist pattern 15 is removed with a solvent. As a result, a new diffraction pattern having a groove 14 a is formed on the upper surface of the second n-type GaAs layer 14.
[0028]
Subsequently, as shown in FIG. 5 (d), a reflective film 16 made of a material having a high reflectivity such as gold, silver, or aluminum is sputtered or deposited on the inside of the recess 14a and the second n-type GaAs layer 14. Formed by vacuum evaporation. The reflective film 16 made of a conductive material is used as a cathode.
Thereafter, the reflective film 16 is patterned by lithography, bumps 17 made of indium (In) are formed on the reflective film 16, and the bumps are connected to a semiconductor integrated circuit formed on a silicon substrate (not shown). . This completes the basic structure of the infrared detector.
[0029]
In the manufacturing process of the infrared detector as described above, since a diffraction pattern is formed by one photolithography using one resist pattern 15, two or more photolithography processes shown in FIG. 4 are required. Compared with the formation of the conventional pseudorandom lattice, the process becomes shorter.
Moreover, since the narrowest width of the patterns constituting the diffraction pattern in this embodiment is about 0.6 μm, the pattern accuracy can be increased.
[0030]
Next, based on the basic pattern shape shown in FIG. 6 constituting the diffraction pattern of the top surface of one pixel of the light receiving element, a region to be etched and a region not to be etched are shown by mathematical expressions.
First, the coordinate of the point P in the region to be etched is (xy) with the longitudinal direction of the 1/4 elliptical pattern shown in FIG. 6 as the x axis, the short direction as the y axis, and the etching direction as the z axis. , Z). And the coordinates of the reference point (x₀, Y₀, Z₀).
[0031]
X at point P is x₀<X ≦ x₀Is in the range of + C and y at point P is y₀<Y ≦ y₀Within the range of + C. C is a constant.
The following (1) to (8) show etching regions due to the difference in the elliptical shape divided into four. In (1) to (8), the symbol d represents the etching depth of the second n-type GaAs layer 14, and z = z₀-D indicates etched recess 14a, z = z₀Indicates a region that is not etched. a₁And b₁Are half the short axis and half the long axis of the first ellipse shown in FIG.₂And b₂These show half of the major axis and half of the minor axis of the second ellipse shown in FIG. 6 (a), respectively.
(1) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
  y₁= Y₀+ [A₁ ²b₁ ²−b₁ ²(x −x₀)²]^1/2/ A₁,
y₂= Y₀+ [A₂ ²b₂ ²−b₂ ²(x −x₀)²]^1/2/ A₂  Then,
The area to be etched is x₀<X ≤ x₀+ A₁And y₁<Y ≤ y₂An area that satisfies the condition or x₀+ A₁<X ≤ x₀+ A₂And y₀<Y ≤ y₂An area that satisfies the condition₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
[0032]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
(2) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
    y₁= (Y₀+ C) + [a₁ ²b₁ ²−a₁ ²(x −x₀)²]^1/2/ B₁,
y₂= (Y₀+ C) + [a₂ ²b₂ ²−a₂ ²(x −x₀)²]^1/2/ B₂  Then,
The area to be etched is x₀<X ≤ x₀+ B₁And y₂<Y ≤ y₁An area that satisfies the condition or x₀+ B₁<X ≤ x₀+ B₂And y₂<Y ≤ y₀An area that satisfies the condition + C, and y₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
[0033]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
(3) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
    y₁= (Y₀+ C) + [a₁ ²b₁ ²−b₁ ²(x −C −x₀)²]^1/2/ A₁,
y₂= (Y₀+ C) + [a₂ ²b₂ ²−b₂ ²(x −C −x₀)²]^1/2/ A₂  Then,
The region to be etched is (x₀+ C -a₂) <x ≤ (x₀+ C -a₁) And y₂<Y ≤ y₀An area that satisfies the condition of + C or (x₀+ C -a₁) <x ≤ (x₀+ C) and y₂<Y ≤ y₁An area that satisfies the condition₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
[0034]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
(4) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
    y₁= Y₀+ [A₁ ²b₁ ²−a₁ ²(x −C −x₀)²]^1/2/ B₁,
y₂= Y₀+ [A₂ ²b₂ ²−a₂ ²(x −C −x₀)²]^1/2/ B₂  Then,
The region to be etched is (x₀+ C -b₂) <x ≤ (x₀+ C -b₁) And y₀<Y ≤ y₂An area that satisfies the condition₀+ C -b₁) <x ≦ (x₀+ C) and y₀<Y ≤ y₁An area that satisfies the condition₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
[0035]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
(5) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
    y₁= Y₀+ [A₁ ²b₁ ²−b₁ ²(x −C −x₀)²]^1/2/ A₁,
y₂= Y₀+ [A₂ ²b₂ ²−b₂ ²(x −C −x₀)²]^1/2/ A₂  Then,
The region to be etched is (x₀+ C -a₂) <x ≦ (x₀+ C -a₁) And y₀<Y ≤ y₂Or (x₀+ C -a₁) <x ≤ (x₀+ C) and y₁<Y ≤ y₂An area that satisfies the condition₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
[0036]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
(6) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
y₁= Y₀+ [A₁ ²b₁ ²−a₁ ²(x −x₀)²]^1/2/ B₁,
y₂= Y₀+ [A₂ ²b₂ ²−a₂ ²(x −x₀)²]^1/2/ B₂  Then,
The area to be etched is x₀<X ≤ (x₀+ B₁) And y₁<Y ≤ y₂An area that satisfies the condition₀+ B₁) <X ≤ (x₀+ B₂) And y₀<Y ≤ y₂An area that satisfies the condition₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀This is a region that satisfies + C.
[0037]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
(7) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
    y₁= (Y₀+ C) + [a₁ ²b₁ ²−b₁ ²(x −x₀)²]^1/2/ A₁,
y₂= (Y₀+ C) + [a₂ ²b₂ ²−b₂ ²(x −x₀)²]^1/2/ A₂  Then,
The area to be etched is x₀<X ≤ x₀+ A₁And y₂<Y ≤ y₁An area that satisfies the condition or x₀+ A₁<X ≤ x₀+ A₂And y₂<Y ≤ y₀An area that satisfies the condition + C, and y₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
(8) Conditions for the etching pattern shape shown in FIG.
▲ 1 ▼ z = z₀Area etched to a depth of −d:
  y₁= (Y₀+ C) + [a₁ ²b₁ ²−a₁ ²(x −C −x₀)²]^1/2/ B₁,
y₂= (Y₀+ C) + [a₂ ²b₂ ²−a₂ ²(x −C −x₀)²]^1/2/ B₂Then,
The region to be etched is (x₀+ C -b₂) <x ≤ (x₀+ C -b₁) And y₂<Y ≤ y₀An area that satisfies the condition₀+ C -b₁) <x ≤ (x₀+ C) and y₂<Y ≤ y₁An area that satisfies the condition₀≦ y₂≦ y₀+ C and y₀≦ y₁≦ y₀It is an area that satisfies the condition of + C.
[0038]
(2) Area not etched: Areas other than (1) are not etched and z = z₀It becomes.
In (1) to (8), x₀And y₀Is x₀= MC, y₀Increase in steps such as nC. However, m and n are both integers.
When the response wavelength is 8.5 μm, the maximum value of C is 3.5 μm, and a₁, B₁, A₂And b₂Are 0.4 μm, 0.9 μm, 3.0 μm, and 2.4 μm, respectively.
[0039]
The pattern shown in FIG. 7A is formed using the equations shown in (1) to (8).
The smallest figure of the diffraction pattern appears when two patterns as shown in FIG. 6 (a) are combined as shown in FIG. 6 (b). In this case, the width of the smallest shape is 2a₁It becomes. That a₁The value is 0.4 μm when the wavelength is 8.5 μm under the conditions (1) to (8). The smallest shape in FIG. 6B is an ellipse, a rectangle and not h, but the average width is not 0.8 μm but 0.6 μm.
[0040]
For example, as shown in FIG. 7B, the light diffracted by the diffraction pattern 14b has a strong component close to the plane of the MQW layer 13 and a weak component at an angle close to 18 degrees from a line perpendicular to the plane. Has an intensity distribution of
While the deviation (increase / decrease) in the etching region from the design value of the conventional pseudorandom grating shown in FIG. 4 and the correlation response are shown by the broken line in FIG. The deviation (increase / decrease) in the area and the correlation response are shown by the solid line in FIG. When these are compared, it has been clarified that the manufacturing margin is large because there is little decrease in the response to the deviation from the design value in the present invention.
[0041]
FIG. 16 shows a logical evaluation of the influence of the QWIP coupling system number on manufacturing errors. In the case of a conventional pseudorandom grating, even if the first etching is configured without error, the response becomes low due to an error in the etching region caused by the second etching.
The planar shape of the grating grooves having the elliptical component shown in FIG. 7 is a pattern that does not include a linear component. However, as shown in FIGS. 17 and 18, a groove having a pattern including a linear portion may be adopted.
[0042]
FIG. 17A shows another example of a diffraction pattern including an elliptic curve for a square pixel with a side of 28 μm. In this case, the length C in the above (1) to (8) is 3.5 μm, and a₂Is 3.0 μm, b₂Is 2.6 μm, a₁Is 0.4 μm, b₁Is 0.9 μm. The cross section taken along the line II-II in FIG. 17 (a) is as shown in FIG. 17 (b).
[0043]
FIG. 18A shows still another example of a lattice including an elliptic curve for a square pixel with a side of 28 μm. In this case, the length C in the above (1) to (8) is 3.5 μm, and a₂Is 3.0 μm, b₂Is 2.8 μm, a₁Is 0.5 μm, b₁Is 0.7 μm. The cross section taken along the line III-III in FIG. 18 (a) is as shown in FIG. 18 (b).
[0044]
When light with a wavelength of 8.5 μm is incident on an infrared detector, the grating shown in FIG. 17 (a) has an optical coupling factor that is greater than that of the grating shown in FIG. About 18% higher.
In the above infrared detector, the light absorption layer is composed of an MQW layer, but may be composed of a quantum wire or a quantum box (dot).
[0045]
The quantum well infrared detector described above is used, for example, in an infrared focal plane array (IRFPA).
[0046]
【The invention's effect】
As described above, according to the present invention, a diffraction grating having a curved surface (for example, an elliptic curve) in a planar shape and having a step-shaped groove having a single cross section is formed on a light absorption layer having a quantum well structure. As a result, the amount of light absorbed by the light absorption layer can be increased, and the optical coupling factor for infrared rays having a wavelength of 8.5 μm can be increased.
[0047]
Since the grooves constituting the diffraction grating have a single step shape, they can be formed by one lithography, and the manufacturing process can be shortened. In addition, the minimum pattern width of the diffraction grating is about 0.6 μm, and a reduction in pattern accuracy can be prevented. Furthermore, the manufacturing margin is large as shown in FIG.
[Brief description of the drawings]
FIG. 1 (a) is a plan view of a conventional infrared detector, and FIG. 1 (b) is a cross-sectional view of the infrared photodetector.
FIG. 2 is a cross-sectional view of a photodetector having a conventional pseudorandom grating.
3A is a plan view of a conventional two-dimensional periodic diffraction grating, FIG. 3B is a plan view of one pixel of the diffraction grating, and FIG. 3C is a plan view of the diffraction grating. It is sectional drawing of 1 pixel.
4A is a plan view of a conventional pseudorandom lattice, FIG. 4B is a plan view of one pixel of the lattice, and FIG. 4C is a cross section of one pixel of the lattice. FIG.
FIGS. 5 (a) to 5 (d) are cross-sectional views showing a manufacturing process of an infrared detection element according to an embodiment of the present invention.
FIG. 6 (a) is a plan view showing a basic pattern constituting the diffraction pattern of the embodiment of the present invention, and FIG. 6 (b) is a plane of the diffraction pattern obtained by combining the two basic patterns. FIG.
7 (a) is a plan view showing a first example of a diffraction pattern according to an embodiment of the present invention, and FIG. 7 (b) is a cross-sectional view taken along the line II of FIG. 7 (a).
FIG. 8 is a diagram showing an etched region and a non-etched region of the first element of the pattern constituting FIG. 7 (a).
FIG. 9 is a diagram showing an etched region and a non-etched region of the second element of the pattern constituting FIG. 7 (a).
FIG. 10 is a diagram showing an etched region and a non-etched region of the third element of the pattern constituting FIG. 7 (a).
FIG. 11 is a diagram showing an etched region and a non-etched region of the fourth element of the pattern constituting FIG. 7 (a).
FIG. 12 is a diagram showing an etched region and a non-etched region of the fifth element of the pattern constituting FIG. 7 (a).
FIG. 13 is a diagram showing an etching region and a non-etching region of the sixth element of the pattern constituting FIG. 7 (a).
FIG. 14 is a diagram showing an etching region and a non-etching region of a seventh element of the pattern constituting FIG. 7 (a).
FIG. 15 is a diagram showing an etched region and a non-etched region of the eighth element of the pattern constituting FIG. 7 (a).
FIG. 16 is a diagram showing the relationship between the coupling efficiency and the shift (increase / decrease) of the etching region from the design value in the embodiment of the present invention and the conventional QWIP.
17 (a) is a plan view showing a second example of the diffraction pattern according to the embodiment of the present invention, and FIG. 17 (b) is a cross-sectional view taken along the line II-II of FIG. 17 (a). .
18 (a) is a plan view showing a third example of the diffraction pattern according to the embodiment of the present invention, and FIG. 18 (b) is a cross-sectional view taken along line III-III in FIG. 18 (a). .
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... GaAs substrate, 12 ... 1st n-type GaAs layer, 13 ... MQW layer (light absorption layer), 14 ... 2nd n-type GaAs layer, 14a ... Groove (step), 14b ... Diffraction pattern, 16 ... Reflection film.

Claims (9)

A light absorption layer formed on the substrate and having a quantum well structure;
A light transmission layer formed on the light absorption layer ;
A shape formed on the upper surface of the light transmission layer and sandwiched between a first elliptic curve and a second elliptic curve that surrounds the first elliptic curve and has the same center of the ellipse and whose major axes are orthogonal to each other. An infrared detector comprising: a diffraction pattern including a step-shaped groove having a cross section of one step including a shape divided into four along the major axis and minor axis of the elliptic curve . The infrared detector according to claim 1, wherein the groove has a step shape with respect to the uppermost surface of the diffraction pattern. The infrared detector according to claim 1, wherein the diffraction pattern is covered with a reflective film. The diffraction pattern diffracts light that has passed through the light absorption layer from the substrate side, is strong at an angle close to parallel to the surface of the light absorption layer, and is close to 18 degrees from a line perpendicular to the surface of the light absorption layer The infrared detector according to claim 1, wherein a light intensity distribution that weakens with an angle is imparted. The infrared detector according to claim 1, wherein the groove constituting the diffraction pattern has a depth smaller than half of the absorption wavelength of the light absorption layer. 2. The infrared detector according to claim 1, wherein an area of the diffraction pattern in which the groove is formed is equal to an area that is not defined. The infrared detector according to claim 1, wherein the light absorption layer has a structure of any one of a multiple quantum well layer, a quantum wire, or a quantum box. The infrared detector according to claim 7, wherein the multiple quantum well layer has a structure in which a plurality of GaAs and AlGaAs are alternately stacked. Forming a light absorption layer having a quantum well structure on a substrate;
Forming a light transmission layer on the light absorption layer;
A shape sandwiched between a first elliptic curve and a second elliptic curve surrounding the first elliptic curve and having the same center of the ellipse and the major axis orthogonal to each other is defined as a major axis and a minor axis of the second elliptic curve. Forming a mask formed with a pattern having a shape divided into four along the light-transmitting layer;
Forming a diffraction pattern in the light transmission layer by etching the light transmission layer in a region not covered by the mask; and
And a step of removing the mask.

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