JPH04352370A - Infrared sensor - Google Patents

Abstract

PURPOSE:To manufacture an infrared sensor whose irregularity is small and which is provided with a stable characteristic by a method wherein a hard carbon film is used for a protective film by HgCdTe for the infrared sensor using HgCdTe. CONSTITUTION:Hard carbon films 2, as amorphous films, which use carbon as a main element are deposited on an n-type HgCdTe crystal substrate 1 by a plasma CVD method or the like. When the hard carbon films 2 are deposited by the plasma CVD method, the resistivity value and the hardness of the films are increased the smaller an RF output is, and the life of an active species is increased the lower a pressure is. As a result, a substrate temperature can be lowered, the uniformity of a large area can be realized, and there exists a tendency that the resistivity and the hardness are increased. Consequently, since the hard carbon films are used, protective films whose hardness is high and whose pinholes are in a very small amount can be manufactured at a low temperature. As a result, it is possible to manufacture an infrared sensor whose irregularity is small and which is provided with a stable characteristic.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to a surface protective film for an infrared sensor using a compound semiconductor HgCdTe.

0002

[Prior Art] Objects emit infrared rays with a wavelength corresponding to their temperature, and this can be detected with compound semiconductors such as HgCdTe and InSb. Infrared sensors using these can be used for weather observation, crime prevention, etc. It is used for disaster prevention, medical purposes, geology, resource surveys, etc.

HgCdTe is a mixed crystal of HgTe and CdTe, and is useful as a material for infrared sensors because the sensitivity wavelength of infrared rays can be arbitrarily selected over a range of 1 to 20 μm by changing the mixed crystal ratio. HgCdTe crystal has the following specificity when compared to other semiconductors.・Crystals are brittle and susceptible to mechanical damage. - Because the vapor pressure of Hg is high, the crystal temperature cannot be raised above 100°C. These specificities create various constraints on device processes. Furthermore, in infrared sensors, the behavior of carriers generated by incident photons determines the characteristics of the sensor, so efforts are being made to reduce carrier recombination. Carrier lifetime is determined by recombination processes in the bulk crystal and at crystal interfaces. When producing an infrared sensor using HgCdTe in this way, it is necessary to consider the properties of the interface as well as the bulk material properties of HgCdTe.

[0004] In order to solve the above problems, the following chemical inactivation (passivation);
A protective film is provided on the HgCdTe crystal surface for the purpose of preventing mechanical damage and reducing carrier recombination at the interface.

[0005] Infrared sensors using HgCdTe are classified into photoconductive type (hereinafter referred to as PC type), which utilizes resistance change upon irradiation with light, and photovoltaic type (hereinafter referred to as PV type), such as a photodiode. Ru. Among these, in PC type devices, as a method to extend the life of carriers, an anodic oxide film is formed as a protective film on the crystal surface in order to reduce interfacial recombination, and the fixed charges in the film cause the crystal surface to become in a state where majority carriers accumulate. I have to. However, the formation of an oxide film by anodic oxidation has poor controllability of film thickness, and it is difficult to obtain uniform film quality. On the other hand, a deposited ZnS film is widely used as a protective film for PV type elements. However, the deposited ZnS film has the following drawbacks. Since there are many interface states, the surface recombination current increases. Furthermore, since there are variations in the distribution of fixed charges, this causes non-uniform sensitivity. Furthermore, the properties are unstable due to poor moisture resistance. A SiO2 film has also been studied as a surface protective film, but although it has good interfacial properties, it has poor moisture resistance because the film is porous. Therefore, there has been a need for a protective film that is transparent to infrared rays and that does not have these drawbacks and can solve the above objectives. The present invention solves the above technical problems by using a hard carbon film as a protective film. The use of a hard carbon film as a protective film is disclosed in JP-A-1-1.
No. 16480, but this is applied to a thermistor and is completely different in purpose and configuration from the present invention.

[0006]

[Purpose] The purpose of the present invention is to provide an infrared sensor that can operate stably for a long time by solving problems such as variations in film quality and thickness and moisture resistance in the protective film of a conventional infrared sensor using HgCdTe. It is.

[0007]

[Structure] In order to achieve the above object, the present invention provides HgC
An infrared sensor using dTe is characterized by using a hard carbon film as a protective film of HgCdTe. Hard carbon film (i-Carbon) used in the present invention
(Also called diamond-like carbon) is an amorphous film whose main element is carbon, which is produced by decomposing a raw material gas such as methane gas using plasma or other means in the gas phase and depositing it on a substrate. It is characterized by being able to be formed at low temperatures, having high resistance and hardness, and having very few pinholes. In addition, the transmittance to infrared rays is extremely high. In this way, the hard carbon film is HgCdT
This material is very suitable for use as a protective film for infrared sensors using e-crystals.

Next, the hard carbon film used in the present invention will be explained in detail. The hard carbon film used in the present invention is a hard carbon film (i-C film,
Diamond-like carbon film, amorphous diamond film,
Also called diamond thin film. ). One of the outstanding features of this hard carbon film is that, since it is a vapor-phase grown film, its various physical properties can be controlled over a wide range by changing the film-forming conditions.

[0009] In order to form such a hard carbon film, an organic compound gas, particularly a hydrocarbon gas, is used. The phase state of these raw materials does not necessarily have to be a gas phase at room temperature and normal pressure, but can be melted, evaporated, or evaporated by heating or reduced pressure.
As long as it can be vaporized through sublimation or the like, either liquid phase or solid phase can be used. Regarding hydrocarbon gas as raw material gas, for example, CH4, C2H6, C3H8, C4H1
Gases containing at least all hydrocarbons such as paraffinic hydrocarbons such as C2H4, acetylenic hydrocarbons such as C2H4, olefinic hydrocarbons, diolefinic hydrocarbons, and even aromatic hydrocarbons can be used. Furthermore, other than hydrocarbons, compounds containing at least the carbon element can be used, such as alcohols, ketones, ethers, esters, CO, and CO2.

In the method of forming a hard carbon film from a raw material gas in the present invention, active species for film formation are formed through a plasma state generated by a plasma method using direct current, low frequency, high frequency, microwave, etc. However, since the deposition is performed under low pressure for the purpose of increasing the area, improving uniformity, and forming a film at a low temperature, a method using a magnetic field effect is more preferable. This active species can also be formed by high temperature pyrolysis. In addition, it may be formed through an ionic state generated by an ionization vapor deposition method, an ion beam vapor deposition method, etc., or it may be formed from neutral particles generated by a vacuum vapor deposition method, a sputtering method, etc. However, it may also be formed by a combination of these.

An example of the deposition conditions for the hard carbon film produced in this manner is as follows in the case of the plasma CVD method. RF output: 0.1 to 50 W/cm2 Pressure: 10-3 to 10 Torr Deposition temperature: Room temperature to 950°C Due to this plasma state, the raw material gas is decomposed into radicals and ions and reacts, thereby forming carbon atoms C on the substrate.
A hard carbon film containing at least one of amorphous (amorphous) and microcrystalline (crystal size is several tens of angstroms to several μm) is deposited. Further, Table 1 shows various properties of the hard carbon film.

[Table 1] Note) Measurement method; Specific resistance (ρ): Determined from IV characteristics using a coplanar cell. Optical bandgap (Egopt): Obtain the absorption coefficient (α) from the spectral characteristics and determine from the relationship shown in Equation 1.

[Equation 1] Amount of hydrogen in the film [C(H)]: 29 from the infrared absorption spectrum
It is determined by integrating the peak around 00/cm and multiplying it by the absorption cross section A. That is, [C(H)]=A・∫α(v)/v・dvSP3/SP
2 ratio: The infrared absorption spectrum is decomposed into Gaussian functions assigned to SP3 and SP2, respectively, and determined from the area ratio. Vickers hardness (H): Based on micro Vickers meter. Refractive index (n): By ellipsometer. Defect density: Based on ESR.

[0012] As a result of analysis by Raman spectroscopy and IR absorption method, the hard carbon film thus formed shows that the carbon atoms have SP3 hybrid orbitals and SP2 hybrid orbitals, as shown in FIGS. 3 and 4, respectively.
It has become clear that there are interatomic bonds that form a hybrid orbital. The ratio of SP3 bonds to SP2 bonds can be approximately estimated by peak-separating the IR spectrum. 2800-3150/cm for IR spectrum
Although the spectra of many modes overlap and are measured,
The attribution of peaks corresponding to each wave number is clear, and if we perform peak separation using a Gaussian distribution as shown in Figure 5, calculate the area of each peak, and find the ratio, we can obtain SP.
3/SP2 can be known. Moreover, according to X-ray and electron diffraction analysis, it has been found that it is in an amorphous state (a-C:H) and/or an amorphous state containing microcrystalline grains of about 50 Å to several μm.

In the case of the plasma CVD method, which is generally suitable for mass production, the lower the RF output, the higher the resistivity and hardness of the film, and the lower the pressure, the longer the life of the active species. It is possible to achieve uniformity over a large area, and the specific resistance and hardness tend to increase. Furthermore, since the plasma density decreases at low pressures, methods using magnetic field confinement effects are particularly effective in increasing resistivity.

Furthermore, since the structure and physical properties of the hard carbon film can be controlled over a wide range as shown in Table 1, there is an advantage that device characteristics can be designed freely.

[0015] In the hard carbon film as described above, in order to further control the physical properties as necessary, impurities may be added such as Group III elements, Group IV elements, Group V elements, and alkali metal elements of the periodic table. , an alkaline earth metal element, a nitrogen atom, an oxygen atom, a chalcogen element, or a halogen atom can be doped therein. This impurity doping further increases the stability of the element and the degree of freedom in device design. The amount of these impurities is usually 5 at % or less of the total constituent atoms for Group III elements of the periodic table, 35 at % or less for Group IV elements, and 5 at % or less for Group V elements. Below, the amount of alkali metal elements is 5 at% or less, the amount of alkaline earth metal elements is 5 at% or less, and the amount of nitrogen atoms is 5 at% or less.
The amount of oxygen atoms is 5 atomic % or less, the amount of chalcogen elements is 35 atomic % or less, and the amount of halogen elements is 35 atomic % or less. Note that the amounts of these elements or atoms can be measured by a conventional method of elemental analysis, for example, Auger analysis. Further, this amount can be adjusted by adjusting the amount of other compounds contained in the source gas and the film forming conditions.

The hard carbon coating described above can be applied to both PC type and PV type infrared sensors. First, an example of application to a PC-type infrared sensor will be explained based on FIG. 1. Figure 1 shows a P
FIG. 2 is a cross-sectional view of a C-type infrared sensor. Protective films 2 made of hard carbon films are provided on both sides of the n-type HgCdTe crystal 1 . Changes in conductivity caused by carriers generated by incident photons are detected as voltage changes between the electrodes 3. FIG. 2 is a cross-sectional view of a photodiode using HgCdTe crystal 4. As shown in FIG. A hard carbon protective film 5 is provided on the surface of the HgCdTe crystal 4 having a pn junction. Electrodes 6 and 7 are arranged on the n-type and p-type sides, respectively.

[0017]

EXAMPLES The present invention will be explained in more detail based on examples. Example 1 An example in which a PC-type infrared sensor is manufactured will be described with reference to FIG. After polishing the n-type Hg0.8Cd0.2Te crystal, it was etched with Br2-methanol. Next, a hard carbon film 2 as a protective film was formed by plasma CVD. In this embodiment, a parallel plate type plasma CVD apparatus is used. A raw material gas containing a mixture of CH4 and hydrogen is introduced into the device, and a high-frequency electric field of about 13.56 MHz is applied between the parallel plate electrodes to decompose the raw material gas into radicals and ions and cause a reaction, resulting in a thickness of 800 Å. deposited. During the deposition of the hard carbon film, the substrate temperature is below 100°C. Further, a part of the hard carbon film was removed by etching for contact use, and In metal was deposited to form an electrode 3. Example 2 An example in which a photodiode is manufactured will be described with reference to FIG. After polishing p-type Hg0.8Cd0.2Te4,
Etched with Br2-methanol. The pn junction was fabricated by implanting B(+) ions into a part of the p-type crystal. Next, a hard carbon film 5 as a protective film was provided in the same manner as in Example 1 to a thickness of 2000 Å. Further, a part of the hard carbon film was removed by etching for contact use, and In metal was deposited to form an electrode 6. Au was vapor-deposited on the back surface of the crystal, and an electrode 7 was provided.

[0018]

[Effect] In an infrared sensor using HgCdTe,
Because a hard carbon film is used as a protective film for HgCdTe,
A protective film with high hardness and very few pinholes can be produced at low temperatures. Therefore, it is possible to manufacture an infrared sensor having stable characteristics with little variation.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an embodiment of a PC-type infrared sensor according to the present invention.

FIG. 2 is a cross-sectional view showing an embodiment of a photodiode according to the present invention.

FIG. 3 is a spectrum diagram showing the results of an IR absorption analysis of the hard carbon film used in the protective film of the present invention.

FIG. 4 is a spectrum diagram showing the results of Raman spectroscopy analysis of the hard carbon film used as the insulating layer of the thin film two-terminal device of the present invention.

FIG. 5 is a diagram showing a Gaussian distribution of the IR spectrum of the hard carbon film.

[Explanation of symbols]

1 n-type HgCdTe crystal substrate 2 hard carbon film 3 In electrode 4 HgCdTe crystal substrate with pn junction 5 A
u electrode

Claims (1)

[Claims] 1. An infrared sensor using HgCdTe, characterized in that a hard carbon film is used as a protective film for the HgCdTe.