JP2007314812A - Sputtering target and film-forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sputtering target which enables an oxide film superior in electroconductivity and transparency for visible light to be formed at a high deposition rate, and decreases transmissivity for visible light little even when a large number of such electroconductive films are stacked as are made from an oxide film, silver or the like, and to provide a film-forming method. <P>SOLUTION: This film-forming method includes: using a sputtering target containing 3-40 mol% TiO<SB>2</SB>and 60-97 mol% ZnO; and forming the oxide film with a sputtering technique on the surface of a substrate, or on the surface of a layered film which has been prepared by alternately layering the oxide film and the electroconductive film, on the surface of the substrate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a sputtering target and a film forming method using the same.

  As a transparent conductive laminate having high visible light transmittance and conductivity, a transparent conductive laminate in which transparent oxide films and silver films made of ZnO are alternately laminated on a transparent substrate has been proposed (patent) Reference 1). However, if the number of laminated transparent oxide films and silver films is increased in order to further increase the conductivity of the transparent conductive laminate, there is a problem that the visible light transmittance is lowered. Further, when a transparent oxide film is formed by a sputtering method using a sputtering target made of ZnO, there is a problem that the film forming speed is slow.

As a ZnO-based sputtering target having a low specific resistance, a ZnO sintered body having a sintering density of 5 g / cm ³ or more and a specific resistance of 1 Ω · cm or less containing an element having a positive trivalent or higher valence has been proposed ( Patent Document 2). Further, in Examples of Patent Document 2, 98 ZnO and TiO ₂ at a mass ratio (or molar ratio): 2 were mixed so that, molded, sintered bodies obtained by sintering is disclosed. However, even if the sintered body is used as a sputtering target, the visible light transmittance decreases if the number of laminated transparent oxide films and silver films is increased in order to further increase the conductivity of the transparent conductive laminate. .
Japanese Patent Publication No. 8-32436 JP-A-2-14959

  An object of the present invention is to form an oxide film excellent in conductivity and visible light transmittance at a high film formation rate, and visible light even when the number of stacked conductive films made of the oxide film and silver is increased. It is an object of the present invention to provide a sputtering target and a film forming method with little reduction in transmittance.

The sputtering target of the present invention is characterized by containing ₃ to 40 mol% of TiO ₂ and 60 to 97 mol% of ZnO.
The porosity of the sputtering target is preferably 3% or less.
The refractive index of the sputtering target at a measurement wavelength of 550 nm is preferably 2.00 to 2.25.

The film forming method of the present invention is characterized in that an oxide film is formed by a sputtering method using the sputtering target of the present invention.
The surface roughness of the oxide film is preferably Ra 0.6 nm or less.
The luminous transmittance of the laminate on which the oxide film is formed is preferably 70% or more.
The film formation rate is preferably 0.2 nm / second or more.

According to the sputtering target of the present invention, an oxide film excellent in conductivity and visible light transmission can be formed at a high film formation rate, and visible light transmission is possible even when the number of stacked oxide films and conductive films is increased. There is little decrease in rate.
According to the film formation method of the present invention, an oxide film excellent in conductivity and visible light transmittance can be formed at a high film formation speed, and even if the number of stacked oxide films and conductive films is increased, visible light can be formed. There is little decrease in transmittance.

<Sputtering target>
The sputtering target of the present invention (hereinafter abbreviated as “target”) contains ₃ to 40 mol% of TiO ₂ and 60 to 97 mol% of ZnO.

By setting the content of TiO ₂ to 3 mol% or more, when an oxide film formed using a sputtering target and a conductive film made of silver or the like are alternately stacked, a decrease in visible light transmittance is suppressed. Therefore, the number of stacked layers can be increased. The content of TiO ₂ is preferably 4 mol% or more, and more preferably 5 mol% or more from the viewpoint of the number of layers and specific resistance.
By setting the content of TiO ₂ to 40 mol% or less, the specific resistance of the sputtering target can be kept low.

By setting the ZnO content to 60 mol% or more, the specific resistance of the sputtering target can be kept low.
By making the content of ZnO 97 mol% or less, a reduction in visible light transmittance can be suppressed when an oxide film formed using a sputtering target and a conductive film made of silver or the like are alternately stacked. Therefore, the number of stacked layers can be increased. The content of ZnO is preferably 96 mol% or less, and more preferably 95 mol% or less from the viewpoint of the number of stacked layers and specific resistance.

The target may contain other elements other than Ti and Zn as long as the object of the present invention is not impaired. Examples of the element include Si, Al, Zr, Sn, In, Y, Nb, Mg, Ga, Ni, and Cu. The element may be contained as an oxide. The target is preferably substantially composed of TiO ₂ and ZnO from the viewpoint of the number of stacked layers and specific resistance. “Substantially” means that a trace amount component derived from impurities contained in the raw materials TiO ₂ and ZnO from the beginning is contained, but other components are not actively added.
The content of TiO ₂ and ZnO are determined by calculation from the TiO ₂ and ZnO charging ratio of the raw material.

  The porosity of the target is preferably 3% or less, more preferably 1% or less, and particularly preferably less than 0.5%. By setting the porosity to 3% or less, (i) the surface smoothness of the resulting oxide film is improved, (ii) the amount of moisture adhering to the surface of the sputtering target is reduced, and the production stability is increased during sputtering. (Iii) An effect of suppressing arcing is obtained to suppress the formation of protrusions (nodules) generated on the surface of the sputtering target during sputtering, and (iv) against thermal stress and mechanical stress applied to the sputtering target during sputtering. It becomes difficult to break and the durability of the sputtering target is improved. The porosity of the sputtering target is measured by the Archimedes method.

  The specific resistance of the target is preferably 0.03 Ω · cm or less, more preferably 0.02 Ω · cm or less, and particularly preferably 0.01 Ω · cm or less. By setting the specific resistance to 0.03 Ω · cm or less, (i) arcing during film formation can be suppressed and stable film formation can be achieved, (ii) film formation speed can be increased, and (iii) the resulting oxidation. The material film has excellent conductivity and is suitable as a base film for a conductive film made of silver. The specific resistance of the sputtering target is measured by the four probe method.

  The refractive index of the target at a measurement wavelength of 550 nm is preferably 2.00 to 2.25, more preferably 2.05 to 2.25. By setting the refractive index to 2.00 to 2.25, the refractive index of the obtained oxide film is increased, which is suitable as a sputtering target for forming an optical thin film. The refractive index of the sputtering target is measured by ellipsometry.

This target is produced, for example, by mixing raw material ZnO particles and TiO ₂ particles while pulverizing them with a ball mill or the like, forming a mixture, and sintering the mixture.

Examples of the mixing method include a dry method and a wet method. When the amount of TiO ₂ particles is small, sufficient mixing can be performed and uniformity is improved, so that the wet method is preferable.
Examples of the sintering method include a normal pressure sintering method and a pressure sintering method. As the pressure sintering method, a hot press method is preferable.

  When using the hot press method, the desired sputtering target is easily obtained, and therefore the maximum sintering temperature is preferably 1000 to 1200 ° C. The maximum temperature holding time is preferably 0.5 to 3 hours. By setting the maximum temperature holding time to 0.5 hours or longer, the sputtering target is sufficiently sintered. By setting the maximum temperature holding time to 3 hours or less, productivity can be improved without affecting the sinterability. The atmosphere during sintering is preferably a vacuum atmosphere or an inert gas atmosphere such as argon gas.

The raw material ZnO particles and TiO ₂ particles are preferably as high as possible and have a small average particle diameter. The average particle diameter of the ZnO particles and TiO ₂ particles is preferably 10 μm or less, more preferably 5 μm or less, and particularly preferably 2 μm or less.

<Film formation method>
The film forming method of the present invention uses this target to form an oxide film on the surface of a base material by sputtering or on the surface of a laminated film in which oxide films and conductive films are alternately laminated on the base material. Is the method.
Examples of the base material include glass, ceramics, plastic, and metal.
Examples of the conductive film include a film made of silver or a film made of a silver alloy based on silver such as an alloy of silver and platinum.

The atmosphere gas at the time of sputtering is preferably a gas containing 80 to 100% by volume of argon gas, and is a mixture in which argon gas is 80 to 95% by volume and the remainder is oxygen gas in that an oxide film is easily formed. Gas is more preferred.
The pressure during sputtering is preferably 1.00 × 10 ^{−1 to} 7.00 × 10 ⁻¹ Pa in terms of stable discharge when the atmospheric gas is argon gas or a mixed gas of argon gas and oxygen gas. .

The input power during sputtering is preferably 0.1 to 10 W / cm ^{2 in} terms of power density. The power density is (input power) / (sputtering target area). By setting the input power to 0.1 W / cm ² or more, the film formation rate becomes sufficiently high, and the specific resistance of the obtained oxide film becomes low. By setting the input power to 10 W / cm ² or less, overheating of the sputtering target can be suppressed, and damage to the sputtering target can be prevented.

  The substrate temperature is preferably room temperature to 400 ° C. In the film forming method of the present invention, the substrate temperature may be 200 ° C. or lower because the resulting oxide film has high crystallinity and is thermally stable. Therefore, an oxide film can be easily formed even on a plastic substrate that cannot be formed when the substrate temperature is high.

  The deposition rate is preferably 0.2 nm / second or more, and more preferably 0.3 nm / second or more. By increasing the deposition rate to 0.2 nm / second or more, productivity is improved and the cost is reduced.

  The surface roughness of the oxide film thus formed is preferably Ra 0.6 nm or less, and more preferably Ra 0.45 nm or less. By setting the surface roughness to Ra 0.6 nm or less, even if the number of stacked oxide films and conductive films is increased, the decrease in visible light transmittance is reduced. The surface roughness of the oxide film is measured as root mean square roughness RMS (unit: nm).

  The luminous transmittance of the laminate on which the oxide film is formed is preferably 70% or more, and more preferably 75% or more. By setting the luminous transmittance to 70% or more, a clear image can be obtained through such a laminate. The luminous transmittance is measured as a stimulus value Y defined in JIS Z8701.

Since the target described above contains ₃ to 40 mol% of TiO ₂ and 60 to 97 mol% of ZnO, the number of stacked oxide films and conductive films formed using the sputtering target is increased. There is little decrease in visible light transmittance. The reason why the visible light transmittance is hardly lowered even when the number of stacked oxide films and conductive films is increased is considered to be the surface properties of the obtained oxide film, particularly smoothness. That is, the smoothness of the oxide film is excellent. Specifically, the surface roughness is 0.6 nm or less, so even if the number of stacked oxide films and conductive films is increased, the light between each film can be reduced. It is considered that the decrease in visible light transmittance is reduced because of less scattering.
In addition, since the surface of the oxide film is smooth, the characteristics of the dissimilar material film are easily exhibited when a dissimilar material film such as a conductive film is stacked over the oxide film. When the dissimilar material film is a conductive film, the film thickness can be reduced, which is suitable for multilayering. Therefore, it is suitable as a sputtering target for forming a laminated film, particularly a transparent conductive laminated film.

Since this target has low specific resistance and high conductivity, it can form a film over a large area. Further, it can sufficiently cope with direct current sputtering with a high film formation rate, and can also cope with any sputtering method such as high-frequency sputtering.
Further, by using this target, the film forming speed is increased, so that the productivity of the transparent conductive laminate is increased, which is advantageous in terms of cost and the like.
An oxide film formed using this target is suitable as a transparent film because of its high visible light transmittance. The oxide film is suitable as a transparent conductive film because of its low specific resistance.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.
Examples 2 to 6 are examples, and examples 1 and 7 are comparative examples.
Each characteristic of the sputtering target, oxide film, and transparent conductive laminate in the examples was measured as follows.

(Bending strength)
The bending strength of the sputtering target was measured by preparing a 3 × 4 × 50 mm test piece and bending at room temperature using a bending tester at a crosshead speed of 0.5 mm / min and a span of 30 mm.
(Bulk density, porosity)
The bulk density and porosity of the sputtering target were measured by the Archimedes method.

(Resistivity)
The specific resistance of the sputtering target was measured with a resistance meter (trade name: Loresta, manufactured by Mitsubishi Yuka Co., Ltd.).
(Refractive index)
The refractive index of the oxide film obtained by sputtering was measured with an ellipsometer (trade name: SE700, manufactured by SENTTECH).

(Deposition rate)
The film formation rate of the oxide film was measured by using a stylus type surface shape measuring instrument (manufactured by Veeco, trade name: DEKTAK).
(Surface roughness)
The surface roughness of the oxide film was measured using an atomic force microscope AFM (trade name: Nanoscope 3a, manufactured by Digital Instruments).

(Visibility transmittance)
The luminous transmittance (stimulus value Y defined in JIS Z 8701) of the transparent conductive laminate was measured with a color analyzer (trade name: TC1800, manufactured by Tokyo Denshoku Co., Ltd.).
(Surface resistance)
The surface resistance of the oxide film was measured using an eddy current resistance measuring device (trade name: SRM12, manufactured by Nagy).

(Manufacturing conditions of sputtering target)
[Example 1]
ZnO powder (average particle size 1 μm) was dried with an evaporator, the dried product was crushed with a juicer mixer, and passed through a 150 mesh sieve to obtain a particle size-adjusted raw material powder. The obtained raw material powder is filled into a graphite mold, and pressure-sintered at a heating rate of 400 ° C./hour, a maximum temperature of 900 ° C., a maximum temperature holding time of 2 hours, and a pressure of 98 MPa using a resistance heating hot press apparatus did. Argon gas was used as the atmospheric gas. The size of the obtained sputtering target was 101 mm in diameter × about 5 mm in thickness. The characteristics of the obtained sputtering target are shown in Example 1 column of Table 1.

[Example 2]
ZnO powder (average particle diameter 1 μm) and TiO ₂ powder (average particle diameter 0.8 μm) were weighed so that the molar ratio was 95: 5, wet-mixed for 24 hours with a ball mill mixer, and then dried with an evaporator. The dried product was crushed with a juicer mixer and passed through a 150-mesh sieve to obtain a raw material powder whose particle size was adjusted. The obtained raw material powder is filled into a graphite mold, and pressure-sintered at a heating rate of 400 ° C./hour, a maximum temperature of 1100 ° C., a maximum temperature holding time of 2 hours, and a pressure of 98 MPa using a resistance heating type hot press apparatus. did. Argon gas was used as the atmospheric gas. The characteristics of the obtained sputtering target are shown in Example 2 column of Table 1.

[Examples 3 to 6]
A sputtering target was produced in the same manner as in Example 2 except that the molar ratio of the raw material ZnO powder and TiO ₂ powder was changed to the molar ratio shown in Examples 3 to 6 in Table 1. The characteristics of the obtained sputtering target are shown in Example 3 to Example 6 column of Table 1.

[Example 7 (comparative example)]
A sputtering target was obtained in the same manner as in Example 1 except that the raw material ZnO powder was changed to TiO ₂ powder. The characteristics of the sputtering target are shown in Example 7 column of Table 1.

(Single layer film formation experiment)
A glass substrate having a plate thickness of 2 mm was prepared as the substrate.
Using a sputtering target of Example 1 while introducing a mixed gas of 85% by volume of argon gas and 15% by volume of oxygen gas into a DC sputtering apparatus, DC sputtering was performed under the conditions of a pressure of 0.42 Pa and an input power of 3.75 W / cm ^2. Then, an oxide film having a thickness of 40 nm was formed as a single layer film on the substrate. The properties of the obtained oxide film are shown in Example 1 column of Table 2. Similarly, the results of film formation using the sputtering targets of Examples 2 to 6 are shown in the columns of Example 2 to Example 6 in Table 2, respectively.

(Laminated film formation experiment)
Four laminated films in which the oxide film A, the conductive film, and the oxide film B were laminated in this order were formed on the base material, and the oxide film A was formed as the uppermost layer. A total of 13 laminated films were formed, and the luminous transmittance characteristics were compared. The results are shown in Example 2 to Example 6 column of Table 3, respectively. Note that a glass substrate having a plate thickness of 2 mm was used as the substrate.

  That is, the structure of the laminated film is, in order from the base material, oxide film A1 (film thickness 40 nm), conductive film 1 (film thickness 10 nm), oxide film B1 (film thickness 2 nm), oxide film A2 (film thickness). 80 nm), conductive film 2 (thickness 10 nm), oxide film B2 (thickness 2 nm), oxide film A3 (thickness 80 nm), conductive film 3 (thickness 10 nm), oxide film B3 (thickness 2 nm) The oxide film A4 (film thickness 80 nm), the conductive film 4 (film thickness 10 nm), the oxide film B4 (film thickness 2 nm), and the oxide film A5 (film thickness 40 nm) were 13 films (total film thickness 368 nm). .

Among these, the oxide film A was formed by DC sputtering using the sputtering targets of Examples 2 to 6. The film forming conditions were a pressure of 0.42 Pa and a constant input power of 3.75 W / cm ² while introducing a mixed gas of 85 vol% argon gas and 15 vol% oxygen gas into the DC sputtering apparatus. The film formation time was set to 40 nm for A1 and A5, and to 80 nm for A2, A3, and A4.

The conductive film is a film of 99 atomic% silver to 1 atomic% gold. The conductive film was formed by DC sputtering using a silver and gold sputtering target having a desired composition. The film forming conditions were a constant pressure of 0.42 Pa and an input power of 2.5 W / cm ² while introducing argon gas into the DC sputtering apparatus. The film formation time was such that the film thickness was 10 nm.

The oxide film B was formed by DC sputtering using a sputtering target of 5 mol% Al ₂ O ₃ -95 mol% ZnO. The film forming conditions were a constant pressure of 0.25 Pa and an input power of 2.5 W / cm ² while introducing argon gas into the DC sputtering apparatus. The film formation time was such that the film thickness was 2 nm.

The transparent conductive laminate in which the oxide film formed using the sputtering target of the present invention and the conductive film made of silver or the like are alternately laminated on the base material is conductive (electromagnetic shielding), visible light Excellent transparency and near-infrared shielding, and wide transmission and reflection bands. Transparent electrodes for plasma display electromagnetic wave shielding films, plasma display protection plates, liquid crystal display elements, automotive windshields, heat mirrors, etc. It is useful as an electromagnetic shielding window glass.

Claims (7)

A sputtering target containing ₃ to 40 mol% of TiO ₂ and 60 to 97 mol% of ZnO.   The sputtering target according to claim 1, wherein the porosity is 3% or less.   The sputtering target of Claim 1 or 2 whose refractive index in measurement wavelength 550nm is 2.00-2.25.   The film-forming method which forms an oxide film by sputtering method using the sputtering target in any one of Claims 1-3.   The film-forming method of Claim 4 whose surface roughness of an oxide film is Ra0.6nm or less.   The film-forming method of Claim 4 or 5 whose luminous transmittance of the laminated body in which the oxide film was formed is 70% or more. The film-forming method in any one of Claims 4-6 whose film-forming speed | rate is 0.2 nm / sec or more.