JPH11330504A - Msm photoconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To contrive to realize a MSM(metal semiconductor metal) photoconductor element with a small distortion and thermal stress due to a protection film and no dark current. SOLUTION: A MSM photoconductor element is provided with a first electrode 2 and a second electrode 3 formed on a substrate 1 composed of a photoconductive material and in proximity each other in an irradiation part of lights 5. Here, the element comprises a protection film 6 of Al2 O3 formed in an irradiation part of the lights 5 on each upper face of the first and second electrodes 2, 3 and an upper face of the substrate 1 surrounding the first and second electrodes 2, 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a broadband optical semiconductor for converting an optical signal into an electric signal, which has a structure in which an electrode is formed close to the surface of an optical semiconductor substrate and is used in an optical communication system or the like. The present invention relates to an MSM photoconductor element used as a switch.

[0002]

2. Description of the Related Art FIG. 1 shows an MSM (Metal-Semiconductor-Met).
al) It is a perspective view of a photoconductor element. The MSM photoconductor element is a metal-semiconductor-metal element formed on a substrate 1 made of a photoconductive material and provided with a first electrode 2 and a second electrode 3 which are close to each other at a portion irradiated with light 5. Has a broadband optical response characteristic, and the resistance between metal electrodes changes when light 5 is irradiated. When a bias 4 is applied between these metal electrodes and light 5 is applied between the electrode gaps, conduction occurs. The MSM photoconductor element is used as a semiconductor optical switch by utilizing this characteristic.

As a conventional example, a substrate 1 made of a photoconductive material includes a semi-insulating GaAs substrate, a semi-insulating InP substrate, a low-temperature grown GaAs epitaxial substrate, and the like. As the electrode 3, Ti
/ Pt / Au, Ti / Au and the like. In such an MSM photoconductor device, a first electrode 2 and a second electrode 3 made of a photoconductive material are protected to protect the fine first electrode 2 and second electrode 3 having a gap of about several μm between the electrodes. A protective film 6 is coated on the second and third electrodes 3. This protective film 6 also functions as an anti-reflection film,
It also serves to improve the incidence efficiency of the light 5. Many of the conventional MSM photoconductor elements have no protective film or use SiO ₂ or Si ₃ N ₄ for the protective film 6 in many cases.

[0004]

However, the thermal expansion coefficient and the thermal conductivity of the above-mentioned conventional protective film 6 made of SiO ₂ or Si ₃ N ₄ are limited to the semi-insulating GaAs substrate, semi-insulating InP substrate, Since the thermal expansion coefficient and the thermal conductivity of the substrate 1 made of a photoconductive material such as a grown GaAs epitaxial substrate do not match, there is a problem that thermal stress is generated and the surface of the substrate 1 made of the photoconductive material is distorted. . In addition, there is a problem that a dark current, which is one of the electrical characteristics, occurs due to the thermal stress.

In view of the above problems, an object of the present invention is to provide an MSM photoconductor element in which no distortion or thermal stress is generated on the surface of a substrate 1 made of a photoconductive material, so that a dark current does not leak. It is in.

[0006]

In order to achieve the above object, in the present invention, a protective film of an MSM photoconductor element is formed of Al ₂ O ₃ . That is, in the MSM photoconductor device of the present invention, the first electrode and the second electrode are formed on the substrate made of the photoconductive material, and the first electrode and the second electrode are formed on the substrate made of the photoconductive material. A protective film made of Al ₂ O _{3 is} deposited by electron beam evaporation so as to cover a portion of the electrode irradiated with light.

According to the present invention, Al ₂ O ₃ having the same thermal expansion coefficient and thermal conductivity as that of a substrate made of a photoconductive material is used as a protective film, so that the surface of the substrate made of a photoconductive material can be prevented from being distorted. No thermal stress occurs, and therefore no dark current leakage occurs.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An MSM photoconductor device according to an embodiment of the present invention has the same appearance as the conventional example shown in FIG. A comb-shaped first electrode 2 and a second electrode 3 are formed on a surface of a substrate 1 made of a photoconductive material such as a semi-insulating GaAs substrate, a semi-insulating InP substrate, and a low-temperature grown GaAs epitaxial substrate. What is different from the conventional example is that the first electrode 2 and the second electrode 3 in the form of fine combs having a gap of about several μm between the substrate 1 made of a photoconductive material and the first electrode 2 are protected. The protective film 6 coated on the second and third electrodes 3 is made of Al ₂ O ₃ .
The protective film 6 made of Al ₂ O ₃ substantially matches the thermal expansion coefficient and the thermal conductivity of the substrate 1 made of a photoconductive material, and the protective film 6 made of Al ₂ O ₃ improves the light incidence efficiency. It also serves the purpose of making it work.

With reference to FIGS. 2, 3, and 4, a method of manufacturing an MSM photoconductor device in this embodiment will be described. FIG. 2 shows a lift-off process in which a first electrode 2 and a second electrode 3 are formed on a substrate 1 made of a photoconductive material such as an insulating GaAs substrate, a semi-insulating InP substrate, and a low-temperature grown GaAs epitaxial substrate. And FIG.
Shows an example of the shape of the first electrode 2 and the second electrode 3 of the MSM photoconductor element, and FIG. 4 shows the same as each of the electrodes after forming the first electrode 2 and the second electrode 3. 5 shows a lift-off process as a step of forming the protective film 6.

First, a pattern is formed in the shape of an electrode by a lift-off process. In this embodiment, first, a photoresist film 7 is formed on a substrate 1 made of a photoconductive material at a position other than a desired electrode portion by using photolithography. FIG. 2A shows this state. In this figure, the thickness of the substrate 1 made of a photoconductive material is about 600 μm. Subsequently, a metal film 8 is deposited as an electrode material on the entire surface of the substrate 1 made of a photoconductive material having the coated photoresist film 7 by an electron beam evaporation apparatus. Fig. 2 (2)
Shown in Then, along with the removal of the photoresist film 7, the metal film other than the desired electrode portion is removed, and a desired pattern of the first electrode 2 and the second electrode 3 is formed. This state is shown in FIG.
See (3). In this figure, the thickness of the first electrode 2 and the second electrode 3 is about 400 nm.

In this embodiment, the comb-shaped structure shown in FIG. 1 is used as the first electrode 2 and the second electrode 3 which are close to each other on the substrate 1 made of a photoconductive material. Various shapes as shown in FIG. FIG. 3A shows a case where a first electrode 2 and a second electrode 3 on a substrate 1 made of a photoconductive material are arranged close to each other in a rectangular shape, and FIG. FIG. 3C shows a case where the first electrode 2 and the second electrode 3 are disposed close to each other on the substrate 1 made of a photoconductive material so as to have a projection.
Denotes a first electrode 2 and a second electrode 2 on a substrate 1 made of a photoconductive material.
This is a case where the electrodes 3 are arranged in a T-shape such that the area of each electrode facing each other is larger than that in the case of FIG. When the irradiation light intensity is the same, the resistance value changes depending on the shapes of the first electrode 2 and the second electrode 3 formed on the substrate 1 made of a low photoconductive material.

Next, the substrate 1 made of a photoconductive material and the
A first electrode 2 formed on a substrate 1 made of an electrically conductive material;
A protective film 6 is coated on the second electrode 3. Security
As the material of the protective film 6, the substrate 1 made of a photoconductive material and the heat
It is important that the materials have the same expansion coefficient and thermal conductivity
It is. Therefore, the material of the protective film 6 has its thermal expansion coefficient and
The thermal conductivity of the substrate 1 made of a photoconductive material has a coefficient of thermal expansion or heat.
Al with almost the same conductivity _TwoO_ThreeChoose Al_TwoO_ThreeOr
The protective film 6 made of the same material as the first electrode 2 and the second electrode 3
Pattern in the shape of protective film by lift-off process
I do.

In this embodiment, first, a desired protective film is formed on a substrate 1 made of a photoconductive material and on a first electrode 2 and a second electrode 3 formed on the substrate 1 made of the photoconductive material. A photoresist film 7 is formed at a location other than 6 using photolithography. FIG. 4A shows this state. Next, electrons are applied to the entire surface of the substrate 1 made of a photoconductive material having the coated photoresist film 7 and the first electrode 2 and the second electrode 3 formed on the substrate 1 made of the photoconductive material. Al ₂ O ₃ is deposited as a protective film material by a beam deposition apparatus. This state is shown in FIG. Subsequently, along with the removal of the photoresist film 7, Al ₂ O in a portion other than the desired protective film 6 is formed.
_{3 The} film 9 is removed, and a desired pattern of the protective film 6 is formed. This state is shown in FIG. The thickness of the protective film 6 satisfies the condition of the antireflection film, that is, d = λ / 4n.
It is about 133 nm at a wavelength of 0 nm. Here, d is the thickness of the protective film 6, λ is the wavelength of the light 5, and n is the refractive index of the protective film 6.

After the above-described manufacturing process is completed, the wafer is cleaved to the size of the device, and becomes an individual MSM photoconductor device. The chipped MSM photoconductor element is modularized by bonding. Here, an important manufacturing process is a step of forming the protective film 6. In general, electron beam evaporation, sputtering, P-CVD or the like can be considered as a method for forming a film. Here, the protective film 6 must be formed in such a manner that pressure is not applied to the substrate 1 made of the photoconductive material as much as possible. Therefore, electron beam evaporation is used in the step of forming the protective film 6. Sputtering and P-CVD cannot be used because the strain increases.

Electron beam evaporation directly heats the vaporized material, so that the vaporized material is less contaminated and is suitable for vapor deposition of high melting point metals, oxides and other inorganic compounds. Deposition by sputtering irradiates the target material with heavy charged particles,
This is a method in which particles coming out of the target material due to the impact are attached to the facing sample. The sputtered film has a stronger adhesion to the substrate and a lower epitaxial temperature than a normal vapor deposition method. P-CVD is a method in which a reaction gas is turned into plasma to make it in a non-equilibrium state, and a film is deposited on a substrate by a chemical reaction in a plasma phase.

Table 1 summarizes the dark current of each protective film 6 formed by electron beam evaporation, sputtering, and P-CVD measured by applying a bias 4 of 1 V. M
The SM photoconductor element has the same structure as that shown in FIG. 1 and has a comb-shaped first electrode 2 and a second electrode 3.
Is 6 μm, and the substrate 1 made of the used photoconductive material is a semi-insulating InP substrate.

[0017]

[Table 1]

As can be seen from the above Table 1, the dark current value of the MSM photoconductor element in which Al ₂ O ₃ is deposited by electron beam as the protective film 6 has the smallest value. This is considered to be due to a small thermal stress during film formation. That is, the substrate 1 made of a photoconductive material
It is expected that the dark current value will be determined by the magnitude of the difference between the coefficient of thermal expansion and the thermal conductivity of the material of the protective film 6 and the material of the protective film 6.

Further, the MSM photoconductor element in which Al ₂ O ₃ was vapor-deposited by electron beam as the protective film 6 had excellent withstand voltage characteristics, and the dark current value did not increase sharply even when the bias 4 voltage was 10 V. Further, the characteristics as a protective film are excellent, and a high-temperature and high-humidity test (voltage of bias 4 of 10 V, 6
(Under conditions of 0 ° C, relative humidity of 85% and 1000 hours)
I found that I could endure.

[0020]

As described above, according to the present invention,
By depositing Al ₂ O ₃ as a protective film 6 on the surface of an optical semiconductor device such as an MSM photoconductor by electron beam evaporation,
A protective film having a lower dark current value than iO ₂ or Si ₃ N ₄ can be formed.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an appearance of an MSM photoconductor element using an optical film material in a conventional example and an example of the present invention.

FIG. 2 is a view showing a lift-off process, which is a step of forming a first electrode 2 and a second electrode 3 on a substrate made of a photoconductive material according to the present invention.

FIG. 3 is a diagram showing an example of the shape of each electrode of the MSM photoconductor element.

FIG. 4 is a step of forming a protective film 6 on a substrate 1 made of a photoconductive material and on a first electrode 2 and a second electrode 3 formed on the substrate 1 made of the photoconductive material in the present invention. It is a figure showing about lift-off process which is.

[Explanation of symbols]

1 ... made of photoconductive material substrate 2 ... first electrode 3 ... second electrode 5 ... light 6 ... protective film 7 ... photoresist film 8 ... metal film 9 ... Al ₂ O ₃ film

Claims (3)

[Claims] An MSM comprising a metal-semiconductor-metal comprising a substrate made of a photoconductive material, and a first electrode and a second electrode formed on the substrate and adjacent to each other in a light-irradiated portion. (Metal-Semiconductor-Metal)
A photoconductor element, wherein protection of Al ₂ O ₃ formed on each of the upper surfaces of the first and second electrodes and on the light-irradiated portion of the upper surface of the substrate around the first and second electrodes. An MSM photoconductor element comprising a film. 2. The MSM photoconductor device according to claim 1, wherein d is a thickness of the protective film, λ is a wavelength of light, and n is a refractive index of the protective film. Has a thickness d of λ / 4n. 3. The MSM photoconductor device according to claim 1, wherein said protective film is formed by electron beam evaporation.