JPS6120963B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a transparent conductive film, particularly a Cd-Sn oxide film,
The present invention also relates to a manufacturing method thereof and an electronic device used therefor. Metal oxide transparent conductive films used in photoelectric conversion devices, such as solar cells, solid-state imaging devices, or display devices, such as liquid crystal display boards, currently have
There are three types: SnO ₂ series, In ₂ O ₃ series, and Cd ₂ SnO ₄ series.
Among them, research on SnO ₂ -based and In ₂ O ₃ -based transparent conductive films has been conducted for a long time, but research on Cd ₂ SnO ₄ -based films still has a short history, and has recently been published in Applied Physics Letters (Appl. .Phys.Letters)
Vol.28, No.10, P622, 1976, Physical Review (Phys.Rev.) B, Vol.6, No.2, P453,
1972 and U.S. Patent No. 3,811,953, the manufacturing method and characteristics of this product were announced, and it has attracted attention. However, such a Cd ₂ SnO ₄ based transparent conductive film is
It is formed by the high frequency AC sputtering method using as a target a member obtained by press-molding and sintering a mixture of Cd ₂ SnO ₄ powder or SnO ₂ powder and CdO powder, so there is no need for target production or sputtering. It is difficult to operate and maintain the film manufacturing equipment used in this method, and it is not easy to produce a film having a predetermined composition.
Therefore, it is extremely difficult to produce a transparent conductive film with good reproducibility in which properties such as film thickness, specific resistance, light transmittance, temperature dependence of resistance, etc. are adjusted to predetermined values. On the other hand, when applied to the various electronic devices mentioned above, transparent conductive films must have low specific resistance, high light transmittance, low temperature dependence of resistance, and can be applied over a large area. There is a growing demand for extremely sophisticated properties such as uniform properties. The present invention was made to meet such demands, and one of its objectives is to provide a transparent conductive film with excellent electrical and/or physical properties. Another object of the invention is to provide a transparent conductive film with extremely low specific resistance. Another object of the present invention is to provide a method for manufacturing a transparent conductive film in which the target is easy to manufacture and the film manufacturing apparatus is easy to operate and maintain. Another object of the present invention is to provide a method for producing a transparent conductive film in which the electrical and/or physical properties of the produced film can be easily controlled. Another object of the present invention is to provide a novel industrial manufacturing method for a transparent conductive film with a simple manufacturing process. Hereinafter, first, the Cd--Sn oxide film production technique according to the present invention will be explained in detail. 1. Preparation of target: Since the DC bipolar sputtering apparatus described below is used, the material to be sputtered must be electrically conductive, and a cadmium-tin alloy target is prepared by the following procedure. (a) Surface-etch cadmium (Cd) with nitric acid and tin (Sn) with a mixture of nitric acid and hydrofluoric acid, and wash with distilled water. (b) To dry Cd and Sn and make the molar ratio of Cd and Sn 2:1, 68980 grams of
Prepare Cd and 36416 grams of Sn. (c) The prepared Cd and Sn were placed in a shear dish in the atmosphere, mixed thoroughly by vibration on a hot plate, heated to approximately 177°C to melt, and then rapidly cooled to form a circle with a diameter of 64 mm and a thickness of 3 mm. A disk-shaped Cd-Sn alloy target is created. The above melting temperature was set at 177°C because, as can be seen from the phase diagram of Cd-Sn shown in Figure 1, the eutectic point is 177°C when the ratio of Cd and Sn is 2:1. It is. 2. Sputtering apparatus: In the production of a transparent conductive film by the reactive cathode sputtering method in this invention, a device similar to a normal cathode sputtering apparatus using a DC power source is used. FIG. 2 shows the basic structure of such a sputtering device.
1 is a glass or metal container in which a low vacuum gas discharge is performed. 2 is a metal anode, which is kept at a positive DC potential of 1 KV or more. Reference numeral 3 denotes a metallic cathode, on which is placed a disc-shaped target 4 made of an alloy made of the component metals of the transparent conductive film, that is, a Cd-Sn alloy. This cathode 3, together with the target 4, is kept at a negative DC potential. Reference numeral 5 denotes a vacuum valve, which is connected to an exhaust system (not shown) comprising an oil diffusion pump, a trap, etc. for extracting the gas inside the container 1. 6 is a leak valve for steadily introducing an inert gas such as argon (Ar) into the container 1 after the air in the container 1 has been exhausted and maintained at a high vacuum; This is a leak valve for steadily introducing an active gas such as into the container 1. 8 is a substrate on which a sputtered film is to be formed; specifically, a glass plate on which a transparent conductive film is to be deposited;
Alternatively, the substrate is made of a silicon wafer such as a photoelectric device having a PN junction on its main surface, and is heated to a predetermined temperature by an appropriate heating means (not shown). Reference numeral 9 indicates a transparent conductive film deposited on this substrate 8. A shutter 10 is used to prevent the surface of the substrate 8 from being contaminated before a film is deposited on the substrate 8. 3. Production of transparent conductive film: The production of a transparent conductive film using the apparatus shown in FIG. 2 follows the steps below. In advance, the target 4 is fixed on the cathode 3, and the substrate 8 (for example, a slide glass) is fixed on the lower surface of the anode 2 using a suitable holder.
Heat to about 200℃. In operation, first, the exhaust device (not shown) is started, and the vacuum valve 5 is opened to remove the air or gas in the container 1, reducing the pressure in the container to 10 ^-5 Torr or less (preferably 5.0×
Maintain a vacuum of 10 ^-6 Torr or less. Next, the leak valve 6 is opened and argon (Ar) is introduced into the container 1 as an inert gas for sputtering the target material. as an inert gas
The reason why Ar is used is that the discharge state is relatively stable in DC sputtering targeting an alloy material such as the one of the present invention, and the deposition rate of the sputtered film is relatively high. Then further,
The leak valve 7 is opened to introduce oxygen as an active gas. Normally, the total pressure of the gas introduced into the container 1 is between 10 ^-3 and 10 ^-1 Torr (preferably 10 ^-2 Torr).
degree). The partial pressure of oxygen in the total pressure can be accurately controlled by controlling the leak valves 6 and 7, as will be described later. After being maintained in this state, a voltage of 1KV or more (preferably 2KV) is applied between the anode 2 and cathode 3.
Apply a DC voltage of 2.5KV to 2.5KV. Note that the distance between the anode and cathode is approximately several cm (actually 5.7 cm). Further, the surface of the substrate 8 is covered with a shutter 10 until a suitable time has elapsed after the start of discharge. Shutter 10 is then opened and an alloy target of a defined composition, i.e.
Cd and Sn metal atoms are sputtered from the Cd--Sn alloy target 4, react with oxygen gas, become oxides, and are deposited on the surface of the substrate 8 heated to about 200 DEG C. to form a film 9. The composition of the deposited film is controlled with good reproducibility by the alloy composition of the target 4 and the composition of the mixed gas in the container.When only inert gas is introduced, Cd-Sn When an alloy film is deposited and both an inert gas and an oxygen gas are introduced, an oxide semiconductor or insulator film having a composition dependent on the gas mixture ratio is deposited, as will be described later. Furthermore, the transmittance of the deposited film to visible light can be arbitrarily set within a range of opaque values with good reproducibility. In reality, if a so-called flowing method using a mixer is adopted instead of adjusting the partial pressure and total pressure using the leak valves 6 and 7 and the exhaust valve 5, the mixture ratio is always constant at a constant gas flow rate. This is more desirable because reactive sputtering can be performed under conditions such as . 4. Heat treatment of deposited film: In order to investigate the influence of heat treatment on the characteristics of the deposited film 9 formed on the substrate 8 as described above, the substrate (slide glass) 8 having the deposited film 9 was heated 4×
The characteristics of the deposited film 9 before and after the heat treatment were evaluated by heat treatment in the range of approximately 200°C to 400°C in a vacuum of 10 ^-6 Torr or less or in reduced pressure Ar gas of 5 × ¹⁰ -2 Torr or less. investigate. 5 Measurement of characteristics of deposited film Samples were prepared as shown in Figures 3a and 3b, and the specific resistance of the deposited film formed as described above was measured. In the figure, reference numerals 11 and 12 are gold electrodes formed on the substrate (slide glass plate) 8 and the deposited film 9 by vapor deposition technology;
3 and 14 are silver pastes for connecting measurement lead wires to gold electrodes. To measure the Hall effect of the deposited film 9, a measuring terminal (Ag paste) 1 is used as shown in FIG.
Samples provided with 5 and 16 were prepared and the Hall effect was measured. 6 Various process conditions and characteristic measurement results (1) Sputtering time and film thickness: Figure 4 shows that the total pressure inside container 1 is 6.0×
Sputtering time and thickness of deposited film 9 when sputtering a Cd-Sn alloy target under the conditions of 10 ^-3 Torr, a mixing ratio of Ar/O ₂ = 4/1, and a DC applied voltage between anode and cathode of 2 KV. FIG. 3 is a characteristic measurement diagram showing the relationship between As can be seen from the figure, the relationship between sputtering time and film thickness is proportional, but it is thought that the reason for some variation in film thickness is due to the stability of the discharge state. (2) Film thickness and specific resistance: Figure 5 shows that the total pressure inside container 1 is 3.0×
The thickness of the deposited film 9 formed by sputtering a Cd-Sn alloy target under the conditions of 10 ^-2 Torr, a mixing ratio of Ar/O ₂ = 4/1, and a DC applied voltage of 2 KV between the anode and cathode. The graph shows the relationship between the deposited film before and after heat treatment, and the solid line 17 is the characteristic curve before heat treatment, and the dotted line 18 is the characteristic curve after heat treatment at about 300°C in Ar or vacuum. The characteristic curve after addition is shown. As you can see from the figure, the film thickness is 200
When the resistivity is extremely thin, such as less than 500 angstroms, the resistivity largely depends on the film thickness, but when the film thickness exceeds about 500 angstroms, the degree of dependence is small, and the tendency does not change drastically between before and after heat treatment. do not have. (3) Dependence of specific resistance on oxygen partial pressure The properties of the Cd-Sn oxide film produced by reactive sputtering using the Cd-Sn alloy target according to the present invention are influenced by the magnitude of the oxygen partial pressure in the container 1. box office. In other words, once the composition of the target material is fixed, Cd-Sn with desired characteristics such as resistivity can be produced with good reproducibility by simply controlling the amount or partial pressure ratio of active gas in the atmosphere during discharge and sputtering. A transparent conductive film made of an oxide film is obtained. Figure 6 shows that the total pressure of the atmosphere inside container 1 is 0.6.
A DC voltage of 2KV is applied between the anode and cathode at a constant ×10 ^-2 Torr, and a Cd-Sn oxide film with a thickness of approximately 500 to 900 angstroms is formed on the slide glass plate 8 by sputtering from the Cd-Sn alloy target. FIG. 3 is a characteristic diagram showing the relationship between the specific resistance of the deposited film and various oxygen partial pressure ratios (ie, O ₂ /(Ar+O ₂ )×100%) when In addition, the total pressure inside the container should be 10 ^-1 to 10 ^-3 Torr.
Even when the ratio is changed within the range of , the relationship between the oxygen partial pressure ratio and the specific resistance shows almost the same tendency as above. As can be understood from Figure 6 above, when the oxygen partial pressure ratio is approximately 10%, the specific resistance becomes minimum, 4.78×
We were able to form a transparent conductor film made of Cd-Sn oxide with a thickness of 10 ^-4 Ωcm. It has also been found that by setting the oxygen partial pressure ratio in the range of about 5% to 15%, a transparent thin film with good conductivity and a nearly constant resistivity can be obtained despite variations in the oxygen partial pressure ratio. Furthermore, considering the application to the actual electronic equipment mentioned above, 10 ^-3 Ωcm
Although it is necessary to form a transparent conductive film with a specific resistance as shown below, it has been found that by setting the oxygen partial pressure ratio in the range of about 3 to 20%, this requirement can be achieved even with relatively small variations in specific resistance. (4) Temperature characteristics of resistance and conductivity in low temperature region Figures 7 and 8 show that Ar/O ₂ =
The temperature characteristics of resistance and conductivity of a transparent conductive film formed in a 4/1 atmosphere from room temperature (288 K) to liquid nitrogen temperature (77 K) in the air are shown. As can be seen from these figures, there is almost no dependence on temperature regardless of film thickness. In addition, after lowering the ambient temperature from 288K to 77K, I raised it again to 288K, but there was almost no history due to temperature. (5) Heat treatment effect and spectral transmittance Total gas pressure 3.0×10 ^-2 Torr, Ar/O ₂ = 4/1
5.0× for the spatter film formed by
10 ^- ₂ Torr in Ar or 0.5 x 10 ^-5 ~ 3.0 x
Figure 9 shows the results of measuring the specific resistance of the sputtered film after heat treatment at 300°C in a vacuum of 10 ^-5 Torr. In the figure, the solid curve shows the latter relationship with the heat treatment time in vacuum, and the dotted curve shows the former change in resistivity due to the heat treatment in Ar gas. As can be seen from the figure, when heat treated at 300°C, there are cases where the resistance decreases rapidly and other cases where the resistance is almost constant and stable. In Ar gas, _O2
In the heat treatment in gas, the difference fields were hardly different from each other. In addition, samples were heat-treated in Ar gas at 300℃ for 3 hours so that the resistivity approached a constant value.
It was found that when heat treated again at 400°C, the resistance increased as shown in Figure 10. Furthermore, FIG. 11 shows the results of comparative measurement of light transmittance before and after such heat treatment. The sample in the figure is a sputtered film formed under the conditions of a total gas pressure of 3.0×10 ^-2 Torr and Ar/O ₂ =4/1 in an inert gas atmosphere such as Ar gas (5.0×10 ^-2 Torr). ) 300
It was subjected to heat treatment (annealing) at ℃ for 6 hours. In the figure, a solid line 19 shows the characteristics before heat treatment, and a dotted line 20 shows the characteristics after heat treatment. As can be seen from the figure, when the heat treatment is applied, the transmittance of the rising edge of light with wavelengths around 300 to 500 nm increases significantly. It was confirmed that this phenomenon occurs not only in Ar gas but also when heat treatment is performed in high vacuum (3×10 ⁻⁵ Torr). (6) Oxygen partial pressure ratio and absorption coefficient The light absorption coefficient α was calculated from the spectral transmittance measured in this way based on the following equation, and the results are shown in FIG. T=I/I ₀ = exp(-αd) α=(ln1/T)/d Here, T is the transmittance, I ₀ is the intensity of the incident light,
I indicates the intensity of transmitted light, and d indicates the film thickness. In FIG. 12, curves 21, 22, 2
3 and 24 are the oxygen partial pressure ratio (O ₂ /(Ar+
This is a characteristic curve showing the relationship between wavelength and absorption coefficient of sputtered films formed in an atmosphere containing 100%, 50%, 20%, and 2% O ₂ (total gas pressure 6.0×10 ⁻³ Torr). It can be seen from the figure that the higher the oxygen partial pressure ratio, the better the permeability. (7) Hall effect of Cd-Sn oxide film Carrier concentration, carrier mobility, and Hall coefficient were measured for Cd-Sn oxide films with different oxygen partial pressures in the form shown in Figure 3c.
We determined whether the semiconductor is P or N type. The results are shown in the table below. The strength of the magnetic field used here was 6K to 8KG. Furthermore, these samples were all after sputtering and were not heat-treated.

[Table] (8) Analysis of Cd-Sn oxide film The crystal structure of the film was analyzed using X-ray diffraction and electron diffraction. It was found by X-ray diffraction that most of the thin film on the glass substrate was deposited in an amorphous form. It remains amorphous even after heat treatment at 300°C. 13th
As shown in Fig. 14 and the X-ray diffraction pattern of the thin film when the oxygen partial pressure is 29%,
Cd ₂ SnO ₄ (130) peak and CdSnO ₃
A peak of (200) was found. On the other hand, as a result of forming a Cd-Sn oxide film with a thickness of less than 1000 angstroms on a NaCl (001) substrate and analyzing it by electron diffraction, amorphous Cd ₂ SnO ₄ and polycrystalline CdSnO ₃ were observed. Served. Here, in Fig. 13, a weak peak appears at (130), and in Fig. 14,
A relatively large peak appears at (200).
From this, it is understood that Cd ₂ SnO ₄ is amorphous and CdSnO ₃ is polycrystalline. From the above, it is clear that the Cd according to the present invention described above.
−The transparent conductive film made of Sn oxide is Cd ₂ SnO ₄ and
It is thought to be composed of a mixture with _CdSnO3 . The specific embodiments of the present invention and various characteristic measurement results have been described above.
As shown in the figure, which shows the relationship between the thickness of the transparent conductive film and the figure of merit, from the practical standpoint of the electronic device mentioned above,
It is desirable that the film thickness be on the order of microns.
As shown in the figure below 1000 Å, as the film thickness increases, this FM also increases. That is, it is preferable that the transparent conductive film has a large FM value. Furthermore, Figure 16 shows the relationship between oxygen partial pressure and deposition rate, and this tendency appears to be due to the fact that the probability of positive ionization during discharge changes depending on the oxygen ratio. . It is also believed that as the proportion of oxygen increases, a reaction occurs between spatter atoms and oxygen, which slows down the deposition rate. As shown in FIG. 6, the dependence of resistivity on oxygen partial pressure can be divided into two regions with oxygen partial pressure ranging from 3 to 20%. i.e. 2-10%
In the region of , the resistivity is high due to metal richness and gradually decreases as oxygenation increases. On the other hand, in the 10% to 100% range, the specific resistance increases as the oxygen deficiency is satisfied.
When the oxygen partial pressure approaches 100%, the composition becomes stoichiometric, that is, Cd ₂ SnO ₄
Although it has a resistivity close to that of an insulator, it is thought that the composition of the thin film is actually close to that of Cd ₂ SnO ₄ . The above region of oxygen partial pressure of 3 to 20% exists between these two regions, and corresponds to the so-called transition region where these two modes coexist. It is considered that the specific resistance of the film has a relatively small displacement and the value of the specific resistance also falls within a minimum range. This way of thinking about the transition region can also be used when sputtering metal oxides other than Cd-Sn oxide films, as well as using target materials with compositions other than Cd/Sn=2/1.
It is thought that this method can also be applied when using Cd-Sn alloy materials. The following is understood from the various experimental results described above. (1) We were able to obtain a good transparent conductive film using a DC bipolar sputtering method using an alloy target with Cd/Sn=2/1. (2) Inert gas Ar and active gas in the spatsuta container
The specific resistance of the sputtered film produced can be controlled to a predetermined value by adjusting the O ₂ mixing ratio. Especially when the oxygen partial pressure is set between 3 and 20%, 10 ^-4
Has a small resistivity on the order of Ωcm and a wavelength of 500nm
We were able to form a transparent conductive film that has a transmittance of approximately 90% for light with a wavelength of . (3) The resistance and permeability of the film can be improved by heat-treating the sputtered film formed by the above-mentioned reactive sputtering method in Ar or vacuum. (4) We were able to obtain a transparent conductive film whose conductivity has almost no temperature dependence at low temperatures below room temperature. (5) Cd made by the above reactive sputtering method
−As a result of analyzing the Sn oxide film, Cd ₂ SnO ₄ (130)
A peak of CdSnO _{3 and CdSnO 3} (200) was found in X-ray diffraction, and when a film deposited on a NaCl (001) substrate was analyzed using electron diffraction, it was found that amorphous Cd ₂ SnO ₄
and polycrystalline CdSnO ₃ . (6) According to the present invention, a transparent conductive film made of a mixture of Cd ₂ SnO ₄ and CdSnO ₃ could be obtained.

[Brief explanation of the drawing]

Fig. 1 is a Cd-Sn state diagram, Fig. 2 is a schematic diagram of the principle of a sputtering device, and Figs. 3a, b, and c are a plan view, a cross-sectional view, and a top view of a substrate having a sputtering film on its surface, respectively. , Figure 4 is a characteristic diagram showing the relationship between sputtering time and sputter film thickness, Figure 5 is a characteristic diagram showing the relationship between sputter film thickness and specific resistance, and Figure 6 is a characteristic diagram showing the relationship between sputter film thickness and specific resistance.
The figure is a characteristic diagram showing the relationship between oxygen partial pressure and specific resistance, Figures 7 and 8 are characteristic diagrams for explaining the temperature dependence of the resistance and conductivity of sputtered films, respectively, and Figures 9 and 10 are The figure is a characteristic diagram for explaining the relationship between heat treatment time and specific resistance after sputter film formation.
The figure is a characteristic diagram for explaining the change in spectral transmittance before and after heat treatment for the sputtered film, Figure 12 is a characteristic diagram for explaining the absorption coefficient with respect to the wavelength of irradiated light, and Figures 13 and 14 are An X-ray diffraction pattern diagram of a sputtered film. Figure 15 is a characteristic diagram for explaining the figure of merit of a transparent conductive film with respect to film thickness. Figure 16 is a diagram showing the relationship between oxygen partial pressure and deposition rate of a sputtered film. It is a characteristic diagram for explaining the relationship. 1... Spatuta container, 2... Anode, 3...
Cathode, 4... Target, 5, 6, 7... Valve, 8... Substrate, 9... Sputtered film, 10...
Shyatsuta.

Claims (1)

[Claims] 1. A transparent conductive film made of a mixture of Cd ₂ SnO ₄ and CdSnO ₃ . The transparent conductive film according to claim 1, having a specific resistance of 2 10 ^-3 Ωcm or less. 3. The transparent conductive film according to claim 1, which has a transmittance of about 90% to visible light. 4. A substrate and a substrate formed on the surface of the substrate.
1. An electronic device comprising: a transparent conductive film made of a mixture of Cd ₂ SnO ₄ and CdSnO ₃ ; and an electrode connected to the transparent conductive film. 5. The electronic device according to claim 4, wherein the transparent conductive film has a specific resistance of 10 ^-3 Ωcm or less. 6 A target made of an alloy of cadmium and tin is placed in a mixed gas of oxygen gas with an oxygen partial pressure of 3 to 20% and an inert gas, and the mixed gas is discharged under a high electric field to generate cadmium-tin. A method for producing a transparent conductive film characterized by generating an oxide film. 7. The method for manufacturing a transparent conductive film according to claim 6, characterized in that argon gas is used as the inert gas. 8. The method for producing a transparent conductive film according to claim 6, wherein the target is made of an alloy of cadmium and tin in a molar ratio of 2:1. 9. The method for producing a transparent conductive film according to claim 6, wherein the high electric field is generated by applying a DC voltage.