JPH06275130A - Transparent conductive film - Google Patents

Abstract

PURPOSE:To reduce dispersion of carrier in a crystal magnetic field, to provide a transparent conductive film of low resistance by having a specific crystal surface preferentially orientated in relation to a substrate surface. CONSTITUTION:Magnesium oxide single crystal, the (100) crystal surface of which is optically polished, and a piece of Coning #7059 glass, the surface of which is optically polished, are fitted into an ion beam sputtering device, as a substrate, and are evacuated to the vacuum degree of 1X10<-6>Torr. An In2O3-10 wt.%SnO2 sintered body is fitted in the device as a target. After these substrates are maintained at 200 deg.C, an Ar/O2 mixed gas is introduced to the pressure of 2X10<-4>Torr, and an ITO film of thickness 100nm is formed at beam voltage of 1KV, and beam current of 80mA.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transparent conductive film which can be used as a transparent electrode for display devices such as liquid crystal displays and solar cells.

[0002]

2. Description of the Related Art A transparent conductive film is indispensable as a transparent electrode for a liquid crystal display or the like, but the upper limit of the film forming temperature and the required specific resistance differ depending on the liquid crystal driving system and the device manufacturing process. In the active matrix method that uses a thin film transistor (TFT), the TFT that was previously formed
The film formation temperature is 200 ℃ so as not to damage the device.
Limited to: On the other hand, STN (super twisted nema
In the tic method, the transparent conductive film also serves as a scanning signal electrode and a pixel electrode, so that a sheet resistance of 10 to 15 Ω / □ or less is required. Since the distance between the electrodes greatly affects the contrast, the film thickness of the conductive film is preferably 100 nm or less.
Therefore, a very small specific resistance is required. further,
In the color STN method, which has recently received attention, it is necessary to form a transparent conductive film on an organic color filter. Therefore, the thin film production temperature is limited to a low temperature of 200 ° C. or lower.
Moreover, the low specific resistance peculiar to the STN method is required. In the future, as the display area becomes larger and the display color becomes more and more, a technology capable of forming a low resistance film at an even lower temperature will be required.

A transparent conductive film which has been widely studied or put into practical use to date is Sn-doped In ₂ O ₃ (IT
O), F or Sb-doped SnO ₂ , In, Al,
There is ZnO or the like doped with Si or the like. It is known that each of these films is an n-type semiconductor having an energy gap of 3.5 eV or more, and that it is an oxygen deficiency due to an additive ion and a stoichiometric composition that acts as a donor. A vapor deposition method and a sputtering method are generally used as a method for producing the transparent conductive film. It is known that the specific resistance of the transparent conductive film largely depends on the film forming method and the film forming conditions, and the higher the substrate temperature at the time of forming the film, the smaller the specific resistance of the film. As the low resistance film, for example, Journal of Applied Physics 54 (1983) pp. 3497 to 3501 (J. Appl. P.
hys.54 (1983) p. 3497-3501) (known example 1), an ITO film having a resistivity of 2.1 × 10 ⁻⁴ Ωcm at a substrate temperature of 190 ° C. is obtained. .

[0004]

As described above, the transparent conductive film that is required in the future has a specific resistance of 1 to 1.5 × 10 ⁻⁴ Ωcm or less and must be formed at 200 ° C. or less. However, in the case of the above-mentioned known example 1, the characteristics are insufficient. By further optimizing the film formation conditions, some reduction in specific resistance can be expected, but 1.5 × 10
It is considered difficult to obtain a film having a specific resistance of ^-4 Ωcm or less at 200 ° C or less. This is because it is necessary to increase the number of conductive carriers or increase the mobility of carriers in order to reduce the specific resistance value. In order to increase the number of conduction carriers, it is necessary to optimize the doping amount and oxygen deficiency amount, which can be achieved to some extent by optimizing the film formation conditions, but there is a limit in terms of stabilizing the crystal structure. . Regarding the latter, for example, ITO
Membranes Journal of Applied Physics Vol. 72 (1992) 5288-5293 (J. Appl. P
hys. 72 (1992) p. 5288-5293) (Prior Art 2), the structure of the film is in a polycrystalline state, so that carrier scattering at the crystal grain boundaries is unavoidable. There is also a limit to mobility.

[0005]

The present invention provides 1.5 × 10 ^-4
This shows one method of obtaining a transparent conductive film having a specific resistance of Ωcm or less. It is characterized by being formed on a substrate with crystallinity and has a transmittance of 85% or more with respect to light in the visible region. A thin film having a conductivity and conductivity is characterized in that a specific crystal plane is preferentially oriented with respect to the substrate surface. By having such characteristics, it is possible to reduce the scattering of carriers at the crystal grain boundaries as described above, and it is possible to obtain a transparent conductive film having a low resistance. In the present invention, the effect is extremely remarkable particularly when the film is in a single crystal state where no crystal grain boundary exists. As for the specific crystal plane that is preferentially oriented with respect to the substrate surface, the crystal plane having a low crystallographic index is more effective in lowering the resistance. This is (100), (110), (111) when the crystal has a cubic crystal or a symmetry close to it, such as ITO or doped SnO ₂ , Thus, when the crystal has hexagonal symmetry, it is (100), (001).

As a method for preferentially orienting a specific crystal plane with respect to the substrate surface, as shown in FIG.
There is a method of using a single crystal such as magnesium oxide or strontium titanate as a substrate. In that case, the crystal plane of the transparent conductive film that is preferentially oriented can be selected by appropriately selecting the crystal orientation of the surface of the single crystal substrate used and the thin film forming conditions. In addition to magnesium oxide and strontium titanate, a single crystal substrate to be used may be one having a corundum structure such as rock salt or other perovskite, garnet, or alumina, or one having a spinel structure.

In addition to such a method, there is a method of forming a thin metal layer on a substrate and then forming a transparent conductive film layer thereon. A metal film is easily crystallized even if formed at a low temperature, and it is easy to preferentially orient a specific crystal plane with respect to the substrate surface. By forming on the crystallized metal layer, the layer of the transparent conductive film can be easily crystallized and a specific crystal plane can be preferentially oriented. In that case, the thickness of the metal layer needs to be 15 nm or less in order to maintain the transmittance of light in the visible region. The layer made of this metal does not necessarily have to exist between the substrate and the transparent conductive film as shown in FIG. 2, and even when it exists only in a part thereof as shown in FIG. Effectively contributes to the crystal growth of. Further, the layer made of this metal is not directly formed on the substrate, but is effective even when formed on an insulating layer such as silicon oxide or silicon nitride as shown in FIG. is there. As the kind of this metal, gold, silver, platinum, copper, rhodium, palladium, aluminum, chromium, etc. are preferable.

As a method for obtaining a transparent conductive film having a low resistance, for example, as described in Japanese Patent Application No. 4-247612 (known example 3), a transparent conductive film is formed from a plurality of layers having different carrier concentrations. Then, there is a method of lowering the resistance value by increasing the carrier mobility of the high carrier concentration layer by utilizing the property of the two-dimensional electron gas. Even in such a transparent conductive film having a multi-layer structure, the crystal growth is performed so that a specific crystal plane is preferentially oriented with respect to the substrate surface in each layer, so that the scattering of electrons by grain boundaries in each layer is reduced. Therefore, the effect of increasing the carrier mobility of the high carrier concentration layer due to the effect of the two-dimensional electron gas is increased. In such a case, it is effective to use a single crystal such as magnesium oxide or strontium titanate as a substrate as shown in FIG.

The present invention can be easily realized by using an ordinary thin film forming apparatus. When a metal layer is formed between the substrate and the transparent conductive film layer, or when the transparent conductive film is composed of multiple layers with different carrier concentrations, two or more types of thin films are continuously formed without breaking the vacuum. It is effective to use a device that can. The vapor deposition method or the sputtering method may be used.
The sputtering method is a powerful method because it enables high-speed and uniform film formation on a large area, but on the other hand, since the film surface is exposed to plasma, it has the drawback of being easily damaged by plasma. is there. On the other hand, the vapor deposition method has no problem of plasma damage, but has a drawback that it is difficult to form a uniform film on a large area substrate. In that respect, the ion beam sputtering method in which accelerated noble gas ions are made to collide with a target and the repelled particles are deposited on the substrate does not expose the film surface to plasma. It is also possible to form a uniform film on a large area, which is an extremely effective method for forming a transparent conductive film.

[0010]

When the method according to the present invention is used, it is possible to obtain a sufficient mobility even at a temperature of 200 ° C. or lower, which is normally not sufficient.
A transparent conductive film having a low resistance can be obtained due to the effect of reducing carrier scattering at the crystal grain boundaries. By using such a low-resistance transparent conductive film as an electrode, the thickness or width of the electrode can be reduced, so that a high-definition display device can be manufactured.

[0011]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited to this.

(Example 1) 1) A magnesium oxide single crystal whose surface showing the crystal plane of (100) was optically polished and Corning # 7059 glass whose surface was optically polished were mounted as a substrate in an ion beam sputtering apparatus. Evacuate to a vacuum of 1 × 10 ^-6 Torr. An In ₂ O ₃ -10 wt% SnO ₂ sintered body is mounted as a target in this apparatus. After holding these substrates at 200 ° C., 2 × 10 6 Ar / O ₂ mixed gas was added.
A pressure of ^-4 Torr is introduced, and an ITO film having a thickness of 100 nm is formed under the conditions of a beam voltage of 1 KV and a beam current of 80 mA. The carrier concentration of these ITO films obtained from the measurement of the Hall coefficient is 1.3 × 10 ²¹ cm ^{−3 for} the magnesium oxide single crystal substrate and 1.2 × 10 ^{21 for} the glass substrate.
^{It is 21} cm ^-3 .

2) Similar to 1), an ITO film is formed by ion beam sputtering using a magnesium oxide single crystal and Corning # 7059 glass as a substrate. At this time, the substrate temperature is 400 ° C., and the other conditions are the same as in 1). These IT obtained from the measurement of Hall coefficient
The carrier concentration of the O film is 1.3 × 10 ²¹ cm ^-3 in the case of a magnesium oxide single crystal substrate, and 1. 1 in the case of a glass substrate.
It is 4 × 10 ²¹ cm ^-3 .

3) I formed by the steps 1) and 2)
The crystalline state of the TO film was examined by the X-ray diffraction method. The results are summarized in Table 1.

[0015]

[Table 1]

The ITO film on the glass substrate was non-oriented, but when a magnesium oxide single crystal was used as the substrate, an (111) or (100) oriented ITO film was obtained. I using magnesium oxide single crystal as a substrate
The X-ray diffraction pattern of the TO film is shown in FIG.

4) I formed by the steps 1) and 2)
The specific resistance of the TO film is summarized in Table 2.

[0018]

[Table 2]

The specific resistance of the ITO film on the oriented magnesium oxide single crystal substrate is smaller than that of the ITO film on the non-oriented glass substrate. Since the value of the carrier concentration hardly changes depending on the substrate, it is considered that this difference is due to the increase of the carrier mobility due to the orientation.

(Example 2) 1) Using a strontium titanate single crystal whose surface showing the crystal planes of (100) and (110) was optically polished and Corning # 7059 glass whose surface was optically polished as a substrate, In ₂ O ₃ -10 wt% SnO ₂ sintered body as the target was equipped in DC sputtering apparatus, 1 × 10
After evacuating to ^-6 Torr vacuum, these substrates are exposed to 200
Hold at ° C. An Ar / O ₂ mixed gas is introduced up to a pressure of 4 × 10 ⁻³ Torr, and sputtering is performed under a sputtering voltage condition of −100 V to form an ITO film having a thickness of 100 nm. The carrier concentration of these ITO films obtained from the measurement of the Hall coefficient is 1.3 × 10 ²¹ c in the case of a glass substrate.
m ^-3 , in the case of strontium titanate single crystal, (10
Both the (0) plane and the (110) plane are 1.2 × 10 ²¹ cm ⁻³ .

2) The crystalline state of these ITO films was examined by the X-ray diffraction method. The results are summarized in Table 3.

[0022]

[Table 3]

When a strontium titanate single crystal is used as a substrate, (110) depending on the crystal plane of the substrate surface.
Alternatively, an (111) oriented ITO film was obtained.

3) Table 4 collectively shows the specific resistance of the ITO film formed by the process of 1).

[0025]

[Table 4]

As in the case of Example 1, the specific resistance of the ITO film on the oriented strontium titanate single crystal substrate was
It becomes smaller than that of the ITO film on the glass substrate in the non-oriented state.

(Embodiment 3) 1) Corning # 7059 glass whose surface is optically polished is used as a substrate in the same ion beam sputtering apparatus as in Embodiment 1, and the chamber is evacuated to a vacuum degree of 1 × 10 ^-6 Torr. This equipment is equipped with platinum and In ₂ O ₃ -10 wt% SnO.
_{2 A} sintered body is mounted as a target, and a platinum film and an ITO film can be formed without breaking the vacuum state. After holding the glass substrate at 200 ° C., platinum was used as a target, Ar / O ₂ mixed gas was introduced up to a pressure of 2 × 10 ⁻⁴ Torr, and a platinum film of 5 nm was formed under the conditions of a beam voltage of 1 KV and a beam current of 80 mA. Up to the thickness of.

2) Following 1), the target is In
₂ O ₃ -5 wt% SnO ₂ sintered body, beam voltage 1K
An ITO film having a thickness of 100 nm is formed on the platinum film under the conditions of V and beam current of 80 mA.

3) A transparent conductive film as shown in FIG. 7 was formed by the above steps 1) to 2). X of this film
The line diffraction pattern is shown in FIG. The ITO film on the platinum film is (1
Oriented to 11). The specific resistance measured on the surface of this laminated film was 1.1 × 10 ⁻⁴ Ωcm. The spectral characteristics of the transmittance of this laminated film are shown in FIG. It can be seen that there is a transmittance of 85% or more in the visible light region of 350 nm to 800 nm. The platinum layer is thin enough that it does not adversely affect the transmittance.

Example 4 1) Using the laminated transparent conductive film shown in Example 3 as a pixel electrode, a display device having the pixel structure shown in FIG. 10 is manufactured as a prototype. The difference from the conventional type is that the pixel electrode is thin platinum layer and IT.
Since it is only composed of an O layer, there is no increase or decrease in the number of photomasks in the process as compared with the conventional type.

2) In the pixel structure of FIG. 10, the specific resistance of the pixel electrode 11 is reduced by about 40% as compared with the conventional material, so that the width of the pixel electrode can be narrowed accordingly. As a result, the pixel scale is 10% compared to the conventional type.
It has become possible to make it smaller. This brings about an effect of improving the pixel density with respect to a conventional display device of the same size.

3) Further, in the conventional pixel scale, the distance between the pixel electrode and the drain wiring 17 can be increased. This contributed to the reduction of the short circuit between the source electrode 14 and the drain wiring via the pixel electrode.

[0033]

EFFECTS OF THE INVENTION By orienting a transparent conductive film on a specific crystal plane with respect to a substrate, carrier scattering at crystal grain boundaries is reduced, and a transparent conductive film having a specific resistance of 1.5 × 10 ⁻⁴ Ωcm or less is obtained. It was found that a film was obtained and the transmittance in the visible light region was not impaired.

[Brief description of drawings]

FIG. 1 is an explanatory view of a case where a transparent conductive film having a specific crystal orientation with respect to a substrate is formed on a single crystal substrate.

[FIG. 2] Thickness 1 made of metal between the substrate and the transparent conductive film
Explanatory drawing at the time of forming the transparent conductive film which has specific crystal orientation by providing a layer of 5 nm or less.

FIG. 3 is an explanatory view of a case where a transparent conductive film having a specific crystal orientation is formed by providing a layer made of a metal and having a thickness of 15 nm or less on a part between the substrate and the transparent conductive film.

FIG. 4 shows a case where a transparent conductive film having a specific crystal orientation is formed by providing a layer made of a metal and having a thickness of 15 nm or less between an insulating film layer and a transparent conductive film formed on a substrate. Explanatory drawing of.

FIG. 5 is an explanatory diagram of a case where a transparent conductive film including a plurality of layers each having a specific crystal orientation with respect to the substrate and having different conduction carrier concentrations is formed on a single crystal substrate.

FIG. 6 shows (111) or (10) in the first embodiment.
The characteristic view of the X-ray diffraction pattern of the transparent conductive film oriented in (0).

FIG. 7 is a sectional view of a transparent conductive film according to Example 3.

FIG. 8 is a characteristic diagram of an X-ray diffraction pattern of the transparent conductive film according to Example 3.

FIG. 9 is a spectral characteristic diagram of the transmittance of the transparent conductive film according to Example 3.

FIG. 10 is a cross-sectional view of a pixel of a display device using the transparent conductive film according to the third embodiment as a pixel electrode.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Transparent conductive film having a specific crystal orientation, 2 ... Single crystal substrate, 3 ... Layer with a thickness of 15 nm or less, 4 ... Substrate, 5 ... Insulating film layer, 6 ... Transparent conductive film having high carrier concentration Layer of film, 7 ... Layer of transparent conductive film having low carrier concentration, 8 ... ITO film, 9 ... Platinum layer, 10 ... Glass substrate,
11 ... Pixel electrode, 12 ... ITO layer, 13 ... Platinum layer, 14
... silicon film, 15 ... source electrode, 16 ... silicon nitride film, 17 ... drain wiring, 18 ... alumina film, 19 ... gate electrode, 20 ... substrate.

Claims (11)

[Claims] 1. A method comprising one or more kinds of cations and oxygen,
In a thin film formed on the substrate with crystallinity and having a transmittance of 80% or more for light in the visible range and conductivity, a specific crystal plane is preferentially oriented with respect to the surface of the substrate. A transparent conductive film characterized by the above. 2. Consisting of one or more cations and oxygen,
In a thin film formed on the substrate with crystallinity and having a transmittance of 80% or more for light in the visible range and conductivity, (1
A transparent conductive film characterized in that the crystal plane of (00) is preferentially oriented with respect to the substrate surface. 3. Consists of one or more cations and oxygen,
In a thin film formed on the substrate with crystallinity and having a transmittance of 80% or more for light in the visible range and conductivity, (1
A transparent conductive film, wherein the crystal plane of 10) is preferentially oriented with respect to the substrate surface. 4. Consisting of one or more cations and oxygen,
In a thin film formed on the substrate with crystallinity and having a transmittance of 80% or more for light in the visible range and conductivity, (1
A transparent conductive film, wherein the crystal plane of 11) is preferentially oriented with respect to the substrate surface. 5. The transparent conductive film according to claim 1, 2, 3 or 4, wherein a magnesium oxide (MgO) single crystal is used as a substrate. 6. The transparent conductive film according to claim 1, 2, 3 or 4, wherein a strontium titanate (SrTiO ₃ ) single crystal is used as the substrate. 7. The transparent conductive film according to claim 1, wherein a layer made of a metal having a thickness of 15 nm or less is present in all or a part between the substrate and the transparent conductive film. 8. A transparent conductive film comprising a plurality of layers having different conduction carrier concentrations, wherein a specific crystal plane in each layer is preferentially oriented with respect to the substrate surface. 9. The accelerated inert gas ion according to claim 1, 2, 3, 4, 5, 6, 7 or 8, which collides with a target made of the same kind of element as the element constituting the transparent conductive film. A transparent conductive film formed by depositing the repelled particles on a substrate. 10. Claims 1, 2, 3, 4, 5, 6, 7, 8
Alternatively, in item 9, the display device uses the transparent conductive film as a transparent electrode. 11. Claims 1, 2, 3, 4, 5, 6, 7, 8
Alternatively, in 9, the device for converting sunlight into electric power using the transparent conductive film as a transparent electrode.