JPH0750431A - Infrared photodetector - Google Patents

Abstract

PURPOSE:To prevent a valley from being generated in the spectral sensitivity of an infrared photodetector by a method wherein the infrared incident surface of the photodetector is formed into a satin form to disperse the angle of incidence of infrared rays. CONSTITUTION:Infrared rays 11 enter through the rear of an infrared photodetector. The infrared rays 11 are properly scattered by a satin-shaped incident surface 9 of the photodetector. The entering infrared rays 11 reach a platinum silicide layer 2. A part of the infrared rays are absorbed. The residual infrared rays are reflected by a reflective film 8. After that, the reflection of the rays 11 is repeated in an SiO2 film 7. However, no interference is generated by the action of the satin-shaped incident surface 9. Electrons out of generated carries are taken out via gas rings 4. In such a way, a valley in the spectral sensitivity of the infrared photodetector is dissolved. In the rear incidence type photodetector, a lapping process, a polishing process and the formation of an anti-reflection film have been hitherto executed to smoothen the rear of the photodetector, but the lapping process has only to be executed, the manufacturing time of the infrared photodetector is significantly shortened and the manufacturing cost of the photodetector can be also reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared light receiving element, and more particularly to a Schottky type infrared light receiving element having excellent spectral sensitivity characteristics.

[0002]

2. Description of the Related Art In recent years, an infrared light receiving element has attracted attention. This element senses or visualizes infrared rays, which are invisible rays, and is expected in a wide range of fields such as industrial measurement, surveillance, medical care, space, and defense. Until now, Hg
Infrared light receiving elements such as a quantum type using CdTe or InSb, a pyroelectric type using a pyroelectric substance, and a Schottky type using a Schottky junction have been proposed. this house,
A device using a Schottky junction with silicon can use a highly complete silicon VLSI manufacturing technology, and
It is attracting attention as a material that can form a monolithic structure.

However, the infrared light receiving element using the Schottky junction with silicon has a sensitivity of about two digits lower than that of the quantum infrared light receiving element. Therefore, the sensitivity has been improved. An example is a Schottky type infrared light receiving element having an optical cavity structure. For example, SPIE
Volume 782, 1987 P114-P120, Television Society Technical Report 1992, P19-P24, and the like.

A conventional Schottky type infrared light receiving element having an optical cavity structure will be described with reference to FIG. 4 (cross-sectional view). The device in this figure is a back-illuminated type. Infrared 1
01 enters the element from the back surface of the element. In general,
The antireflection film 10 is installed on the incident surface. The incident infrared rays reach the silicide layer 2 which is the photoelectric conversion unit. Platinum silicide is generally used as the silicide. Part of the incident infrared rays is absorbed and generates holes and electrons that are carriers. Some of the holes go over the Schottky junction 3 and enter the silicon substrate 1. The same number of electrons is output from the guard ring 4 as an optical signal. The remaining carriers advance by the diffusion length of the carriers and disappear in the silicide layer 2. However, the optical cavity structure has a very thin silicide layer of 5 nm or less. Therefore, the probability that carriers will get over the Schottky junction is higher than that when the silicide layer is thick, and the sensitivity is improved. The remaining infrared rays that have reached the silicide layer are transmitted without being absorbed. The transmitted infrared rays are reflected by the reflective film 8 and enter the silicide layer 2 again. Further, reflection is repeated between the silicide layer 2 and the reflection film 8. As a result, infrared rays are attenuated while generating carriers. As the reflective film 8, a metal such as Al is generally used.

[0005]

However, the conventional Schottky type infrared light receiving element having an optical cavity structure has a problem in spectral sensitivity characteristics. FIG. 6 shows the spectral sensitivity characteristics of the device. The sensitivity preferably changes gently with a change in wavelength. However, the conventional element has a region in the spectral sensitivity where the sensitivity sharply decreases, which should be called a valley of the spectral sensitivity. Therefore, an improvement plan such as removing the reflection film 8 was also proposed. However, on the contrary, the sensitivity is lowered, and it has not been solved yet.

An object of the present invention is to provide a Schottky type infrared light receiving element having an optical cavity structure and having no valley in spectral sensitivity.

[0007]

As a result of earnest research, the present inventor found for the first time that a valley of spectral sensitivity is caused by interference between incident light and reflected light. That is, in the conventional optical cavity structure, between the incident light that has entered the silicide layer and the reflected light that has been transmitted through the silicide layer and reflected by the reflective film, or further, the multiple reflected light that has been reflected. , There was interference. Then, the interference causes a valley in the spectral sensitivity in a specific wavelength region.

As a result of research conducted by the inventor of the present invention, it is possible to slightly vary the incident angle of infrared rays by making the surface on which infrared rays are conventionally smooth to have a satin finish, thereby solving the above problem. It has been found that the above is effective, and has completed the present invention. That is, the present invention is a Schottky type infrared light receiving element having an optical cavity structure, in which a satin-shaped infrared light incident surface is arranged.

[0009]

The present invention is based on the opposite idea. If the surface on which the infrared rays are incident is made satin-finished, the incident light is appropriately scattered and interference can be reduced.
The reason why the satin-finished incident surface is effective for eliminating the valley of the spectral sensitivity will be described with reference to the drawings and the formula. FIG. 5 is an enlarged sectional view of a photoelectric conversion part of an infrared light receiving element (an example) having a conventional optical cavity structure. The platinum silicide layer 2 is arranged in the photoelectric conversion part. The film thickness is 1-5
nm. Al is used for the metal film 8 for reflection, and its film thickness is 1 μm. Platinum silicide layer 2 and SiO
_The interface between the SiO ₂ film 7 and the Al ₂ film is defined as the interface A21.
The interface composed of the film 8 is referred to as an interface B22.

First, the behavior of incident infrared rays will be described.
Infrared rays 1 incident on the platinum silicide layer 2 at an angle of _PtSi
Part of 01 is absorbed by the platinum silicide layer 2,
The rest reaches the interface A21. The infrared light that has reached the interface A21 is divided into a reflection component 102 and a transmission component 103.
The transmission component 103 is incident on the SiO ₂ film 7 at an incident angle θ _SiO2 determined by the ratio of the refractive indices of platinum silicide and SiO ₂ according to Snell's law. The transmitted infrared ray 103 reaches the interface B22 and is reflected to become an infrared ray 104. If the Al film 8 is 0.1 μm or more, almost all of the infrared rays 103 are reflected, and the component absorbed by the Al film 8 is very small compared to the reflected component. The infrared ray 104 has an interface A21.
To reach. Then, it is divided into a component 105 that transmits to the platinum silicide layer 2 and a component 106 that reflects toward the interface B22. To be precise, the infrared ray component 105 is also reflected at the interface 3 between the silicon substrate 1 and the platinum silicide. However, the film thickness of platinum silicide is 1 to 5 nm.
This value is approximately 10 ⁻³ times the wavelength of infrared rays. Therefore, the reflection at the interface 3 can be considered to be included in the reflection at the interface A21. Hereinafter, similarly, the interface A21 and the interface B2
The reflection between 2 is repeated. Each time, a part of the platinum silicide layer penetrates and infrared rays are attenuated.

Next, consider the influence of interference in the vicinity of the interface A21. Regarding the interference of light that occurs between two interfaces,
It has been analyzed in the field of anti-reflection film, for example, 19
1979 Published by Science Co., Ltd. Kazumi Murata "Optics" (P4
9, P70, etc.). Infrared 102,10
5, 108 ... Combined amplitude a _r and its energy intensity I
_r is represented by the following formula. The initial amplitude is 1.

[0012]

[Equation 1]

[0013]

[Equation 2]

Δ represents a phase difference caused by one round trip of the SiO ₂ film 7, and is represented by the following equation.

[0015]

[Equation 3]

N _SiO2 and d _SiO2 are the refractive index and the film thickness of the SiO ₂ film 7. This product is called the optical thickness.
r _A is the interface A21 of the infrared rays 104, 107, 110, ...
The amplitude reflectance at is shown. r _B is the infrared rays 103, 10
6 shows the amplitude reflectance at the interface B22. The amplitude reflectances r _{A and} r _B are determined by the Fresnel's formula and the refractive indices of the two materials forming the interface. Where r _A
Is platinum silicide and SiO ₂ , and r _B is SiO ₂ and Al. Although the refractive index varies depending on the wavelength, the refractive index n _A of platinum silicide is about 3.5 and the refractive index n _{B of} Al is about 6. Here, it is assumed that r _A = −r _B. And when substituting into the formula of energy intensity,
The following formula is obtained.

[0017]

[Equation 4]

The energy intensity I _r changes with the phase difference δ. Then, −δ = (2m−1) π (where m
= 1, 2, 3 ...), I _r has a maximum value, and −δ
= 2 mπ, I _r has a minimum value of zero. Where m
= 1, the energy intensity I _r becomes maximum
From Equation 3, it is when the _{n SiO2 d SiO2 COS θ SiO2 =} λ 1/4. Further, the I _r is the minimum value is when the _{n SiO2 d SiO2 COS θ SiO2 =} λ 2/2.

In the conventional device, the left sides are both constant,
_{λ 1/4 = λ 2/} 2, therefore, becomes λ ₂ = λ _1/2.
That is, the element having the conventional optical cavity structure having the maximum value at a certain wavelength always has the minimum value at the half wavelength. The infrared light receiving element having the spectral sensitivity characteristic shown in FIG. 2 is designed with an optical film thickness n _SiO2 d _SiO2 of 1 μm. Vertical incidence of infrared rays, that is, incident angle θ
_{If SiO2} = 0 ° is fixed, the maximum value of the energy intensity is obtained at the wavelength of 4 μm, and the minimum value is obtained at the wavelength of 2 μm. Figure 6 shows exactly this result. The energy intensity mentioned here is the light energy intensity due to the influence of interference. In the light receiving element,
There is an inherent sensitivity characteristic, and the maximum value of the energy intensity does not always have the maximum sensitivity. This is why the maximum does not occur at 4 μm in FIG.

Infrared rays incident from a certain direction are not scattered by the conventional structure. Therefore, the incident angle θ _SiO2 is always constant. However, in the present invention, the incident angle θ _SiO2 is not uniform because it is appropriately scattered by the satin-shaped incident surface. Therefore, n _SiO2 d _SiO2 COS θ _SiO2 is not constant and is moderately averaged around the incident angle θ _SiO2 . Therefore, interference is less likely to occur, and the maximum and minimum values of energy intensity are moderated appropriately.

As described above, the satin-shaped incident surface is effective for eliminating the valley of the spectral sensitivity. The satin-like shape referred to in the present invention means a state of fine irregularities (preferably about 10 to 100 μm). In the conventional element, the incident surface is a smooth surface so that infrared rays are not scattered. This is a process for the incident infrared rays to efficiently reach the photoelectric conversion unit. The element of the present invention causes appropriate scattering due to the opposite idea. However, this does not reduce the light receiving efficiency. Although this intentionally scatters, the extent is
This is because the light receiving efficiency does not deteriorate. It is also within the scope of the present invention to dispose an antireflection film on the device of the present invention or to make the surface of the antireflection film satin like the conventional device. In the latter case, the surface of the antireflection film becomes the incident surface.

In the present invention, as the photoelectric conversion portion, in addition to platinum silicide, for example, palladium silicide, iridium silicide, PtSi and SiGe, P ⁺⁺ Si.
For example, one using a Schottky junction of Ge and Si can be cited. The photoelectric conversion part has to be very thin (generally 5 nm or less) because it has an optical cavity structure. A reflective film is provided on the photoelectric conversion unit via an insulating film such as SiO ₂ or SiN. As the reflective film, A
In addition to l, for example, a metal such as W or Cu, or a reflective film formed of a dielectric multilayer film based on the optical interference theory is used.

There may be one element, or a line sensor in which a plurality of elements are arranged in a line or an image pickup element in which a plurality of elements are arranged in a grid. The satin-shaped incident surface is obtained by rough polishing (lapping) or etching. Etching
Dry etching using plasma or the like and wet etching using a solution can be used.

[0024]

EXAMPLES The present invention will be described in more detail with reference to the following examples with reference to the drawings. However, the present invention is not limited to these examples. FIG. 1 is a sectional view of an element according to this example. This element is a back-illuminated type. 1
Is a P-type silicon substrate and has a thickness of about 350 μm.
2 is a platinum silicide layer which is a photoelectric conversion part and has a film thickness of 1 to
It is 5 nm. 3 is a Schottky junction in which a Schottky barrier is formed at the interface between the P-type silicon 1 and the platinum silicide layer 2. Reference numeral 4 denotes an n-type region called a guard ring, which prevents an increase in dark current and also takes out generated electrons, which are carriers, to the outside. 5
Is to apply a potential to the substrate in the high concentration P-type region.
Reference numeral 6 is a thermal oxide film of silicon. An insulating layer 7 made of SiO ₂ has a film thickness of 0.77 μm. The film thickness was set such that the refractive index was 1.3 and the infrared energy intensity was maximized at a wavelength of 4 μm. Reference numeral 8 is an infrared reflecting film made of Al, and has a film thickness of 1 μm. The infrared incident surface 9 was finished in a satin finish by lapping. As the lapping treatment conditions, water and an abrasive having an average particle size of 20 μm were used for treatment at a pressure of 400 g / cm ² for 10 minutes.

The infrared ray 11 enters from the back surface of the device.
The light is appropriately scattered by the satin-shaped incident surface 9. The incident infrared rays reach the platinum silicide layer 2. And a part of infrared rays is absorbed. The remaining infrared rays are reflected by the reflective film 8. After that, the reflection is repeated in the SiO ₂ film 7.
However, due to the action of the satin-shaped incident surface, no interference occurs. The electrons of the generated carriers are taken out through the guard ring 4. FIG. 2 shows the spectral sensitivity characteristics of the device according to this example.

FIG. 3 is a sectional view of a device according to another embodiment of the present invention. The same reference numerals indicate the same elements as those of the previous embodiment. The element is a front-illuminated type,
The matte surface is on the surface. The infrared ray 11a enters from the surface of the element. Then, it is appropriately scattered by the satin-shaped incident surface 9a. The incident infrared rays are platinum silicide layer 2
To reach. And a part of infrared rays is absorbed. The remaining infrared rays are reflected by the reflective film 8a. After that, the reflection is repeated in the silicon substrate 1. However, interference does not occur due to the action of the satin-finished incident surface. The electrons of the generated carriers are taken out through the guard ring 4.

[0027]

As described above, according to the present invention, the valley of the spectral sensitivity is eliminated. Further, in the back-illuminated element, conventionally, the back surface is smoothed by lapping, polishing, and forming an antireflection film. This is a process for the incident infrared rays to efficiently reach the photoelectric conversion unit. However, in the present invention, only the lapping process is required. Therefore, there is also an effect that the manufacturing time can be greatly shortened and the manufacturing cost can be reduced accordingly.

[Brief description of drawings]

FIG. 1 is a sectional view of an infrared light receiving element according to an embodiment of the present invention.

FIG. 2 is a spectral sensitivity characteristic diagram of the infrared light receiving element of FIG.

FIG. 3 is a sectional view of an infrared light receiving element according to another embodiment of the present invention.

FIG. 4 is a sectional view of a conventional infrared light receiving element.

FIG. 5 is an enlarged cross-sectional view of the light receiving area of FIG.

FIG. 6 is a spectral sensitivity characteristic diagram of a conventional infrared light receiving element.

[Explanation of symbols]

1 P-type silicon substrate 2 Platinum silicide layer 3 Schottky junction 4 Guard ring (n type region) 5 High concentration P Mold region 6 ... Thermal oxide film 7 ... SiO ₂ film 7a ... SiO ₂ film 8 ... Reflective film (Al) 8a ... Reflective film (Al) 9・ ・ ・ Satin-shaped incident surface (back surface) 9a ・ ・ ・ Satin-shaped incident surface (front surface) 10 ・ ・ ・ Anti-reflection film 11 ・ ・ ・ Infrared ray 11a ・ ・ ・ Infrared ray 110 / infrared or more

Claims (1)

[Claims] 1. A Schottky type infrared light receiving element having an optical cavity structure, in which a reflecting metal film and a very thin photoelectric conversion part are arranged, wherein the infrared light receiving surface has a satin finish.