JP7162647B2 - Cu-W-O sputtering target and oxide thin film - Google Patents

Description

The present invention relates to a Cu--WO sputtering target suitable for forming an oxide thin film with a high work function.

BACKGROUND ART ITO (indium tin oxide) is used as a transparent electrode (anode) in a light-emitting element such as an organic electroluminescence (organic EL) element. Holes injected by applying a voltage to the anode combine with electrons in the light emitting layer via the hole transport layer. In recent years, research has been conducted on using an oxide having a higher work function than ITO for the purpose of improving the efficiency of charge injection into the hole transport layer. For example, Non-Patent Document 1 describes oxide thin films in organic semiconductor devices such as TiO ₂ , MoO ₂ , CuO, NiO, WO ₃ , V ₂ O ₅ , CrO ₃ , Ta ₂ O ₅ and Co ₃ O ₄ . High work functions have been reported.

As shown in Non-Patent Document ₁ , WO3 has a relatively high work function. This WO ₃ film can be formed using a sputtering target made of a tungsten oxide sintered body (Patent Documents 1 and 2), but it is difficult to increase the density of the sintered body with a single WO ₃ phase. , DC sputtering was difficult due to high volume resistivity. Therefore, Patent Literature ₂ discloses that by adding WO2 to WO3 _, the density of the sintered body is increased and the electrical conductivity is increased to enable DC sputtering. Further, Patent Document 1 discloses that the density of a sintered body is increased by hot _- pressing WO3 powder in an oxygen-supplied atmosphere.

JP-A-3-150357 JP 2013-76163 A

Mark T Greiner and Zheng-Hong Lu, "Thin-Film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces", NPG Asia Materials (2013) 5,e55, 19 July 2013

As described above, an oxide film having a high work function is required as a film constituting an organic semiconductor device such as an organic EL. WO3 is an example of a material that exhibits a high work function, but when forming a film of WO3 _, DC sputtering _, which enables high-speed film formation, cannot be used because the sputtering target used for film formation has a high volume resistivity. There was a problem. Therefore, the present invention has been proposed to solve the above-described problems, and provides a sputtering target with low volume resistivity that can form a film with a high work function. The challenge is to

The present invention has been proposed to solve the above problems, and an aspect of the present invention that can solve the problems consists of tungsten (W), copper (Cu), oxygen (O) and unavoidable impurities The sputtering target is a Cu--WO sputtering target having a volume resistivity of 1.0×10 ³ Ω·cm or less.

According to the present invention, there is provided a sputtering target capable of forming a film having a high work function, and because of its low volume resistivity, DC sputtering is possible, thereby achieving an excellent effect of enabling high-speed film formation. have.

As described above _, WO3 has a _high work function, but it has been difficult to produce a sputtering target with a low volume resistivity that can be used for DC sputtering with a WO3 single phase. Also, when other oxide materials with a high work function (for example, single-phase CuO) are used, the volume resistivity is similarly high, making DC sputtering difficult. As a result of intensive research by the present inventors in response to such problems, sputtering with low volume resistivity that enables DC sputtering while maintaining a high work function by producing a mixed system of _CuO and WO The knowledge that the target can be obtained was obtained, leading to the present invention.

A sputtering target (referred to as a Cu—W—O sputtering target) according to an embodiment of the present invention is composed of tungsten (W), copper (Cu), oxygen (O), and unavoidable impurities, and has a volume resistivity of 1.0. ×10 ³ Ω·cm or less. If the sputtering target has a volume resistivity of 1.0×10 ³ Ω·cm or less, DC sputtering becomes possible, thereby enabling high-speed film formation. Preferably, the volume resistivity is 1.0×10 ² Ωcm or less. This enables high-speed film formation by more stable DC sputtering.

The sputtering target according to the present embodiment is composed of W, Cu, O and unavoidable impurities, and the content ratio of W and Cu is preferably W/(Cu+W)≧0.5 in terms of atomic ratio.
When W/(Cu+W)<0.5, the volume resistivity increases and a desired high work function may not be obtained. Preferably, W/(Cu+W)≧0.7, more preferably W/(Cu+W)≧0.8, still more preferably W/(Cu+W)≧0.9. Further _, when the WO3 single phase is used, the volume resistivity of the sputtering target is high as described above, so that W/(Cu+W)<1. In addition, the unavoidable impurities are impurities mixed in raw materials, manufacturing processes, etc., and may contain an amount that does not particularly affect characteristics such as work function. It can be said that there is no problem.

The sputtering target according to this embodiment preferably has a relative density of 95% or more. Preferably, the relative density is 98% or more. Such a high-density sputtering target can prevent cracks and fractures during sputtering, and can reduce particles during film formation. The relative density of the sputtering target is also related to volume resistivity, and the lower the relative density value, the higher the volume resistivity. Therefore, in order to lower the volume resistivity, it is necessary to increase the relative density by strictly adjusting the content ratio of W and Cu in the sputtering target as well as the manufacturing method and manufacturing conditions of the sputtering target.

A sputtering target according to an embodiment of the present invention has a work function of 4.5 eV or more. A film having a high work function can be produced by using a sputtering target having such a high work function. And such a film with a high work function can improve the efficiency of charge injection into the hole transport layer in organic semiconductor devices such as organic EL and organic solar cells, and improve luminous efficiency or conversion efficiency. can be expected.

A method for manufacturing a sputtering target according to this embodiment will be described below. However, it is clear that the following manufacturing conditions and the like are not limited to the disclosed range, and that some omissions and changes may be made.

Tungsten oxide (WO ₃ ) powder and copper oxide (CuO) powder are prepared as raw material powders, and these raw material powders are weighed so as to have a desired composition ratio. As the copper oxide, Cu ₂ O or the like can be used in addition to CuO. Next, wet pulverization is performed using zirconia beads having a ball diameter of 0.5 to 3.0 mm. Then, pulverization is performed until the median particle size reaches 0.1 to 5.0 μm, and then granulation is performed. Next, the obtained granulated powder is press-molded. The pressing pressure is preferably 300-400 kgf/cm ² . Cold isostatic pressing (CIP) is then performed. CIP pressure is preferably 1000 to 2000 kgf/cm ² . Next, the obtained compact is sintered at normal pressure for 10 to 20 hours in oxygen flow. At this time, the sintering temperature is preferably 900°C or higher and lower than 950°C. If the temperature is less than 900°C, a high _- density _sintered body cannot be obtained. Moreover, it is not preferable because it melts. After that, the obtained sintered body is cut into a target shape, polished, or the like, so that a sputtering target can be produced. When hot press sintering is used, CuO may be reduced to Cu by the carbon sintering member, resulting in rapid wear of the member.

In the specification of the present application, various physical properties of sputtering targets and the like were analyzed using the following measurement methods.
(Component Composition of Sputtering Target and Film)
Device: SPS3500DD manufactured by SII
Method: ICP-OES (Inductively Coupled Plasma Emission Spectrometry)
(Component composition of film)
Device: JXA-8500F made by JEOL
Method: EPMA (electron probe microanalyzer)
Acceleration voltage: 5-10keV
Irradiation current: 2.0×10 ⁻⁷ to 2.0 to 10 ⁻⁸ A
Probe diameter: 10 μm
Five smooth film-forming portions where no dust or the like adhered and the substrate surface was not visible were selected, point analysis was performed, and the average composition was calculated.

(Volume resistivity of sputtering target)
The volume resistivity of the sputtering target was obtained by measuring the surface of the sputtering target at 5 points (1 point at the center and 4 points near the outer periphery) and taking the average value thereof. The following equipment was used for the measurement.
Apparatus: Resistivity measuring instrument Σ-5+ manufactured by NPS
Method: Constant current application method Method: DC 4-probe method

(Regarding the relative density of the sputtering target)
Relative density (%) = Archimedes density / true density x 100
Archimedes Density: A small piece is cut from the sputtering target and the density is calculated from the piece using the Archimedes method.
True density: _Calculate the atomic ratio of _Cu and W from elemental analysis. Assuming that the theoretical densities are d _CuO and d _WO3 , the true density (g/cm ³ )=100/(a/d _CuO +b/d _WO3 ) is calculated. The theoretical density of CuO is d _CuO =6.31 g/cm ³ , and the theoretical density of WO ₃ is d _WO3 =7.16 g/cm ³ .

(About work function)
As for the bulk body (sputtering target), a sample having a length of 20 mm, a width of 10 mm, and a thickness of 5 to 10 mm was produced. The surface to be measured was polished using abrasive paper of No. 2000 count. As for the sputtered film, a sample of 20×20 mm formed on a Si substrate was prepared and measured under the following conditions. It should be noted that the measurement result of the work function does not depend on the size of the sample. In addition, if the surface to be measured is not polished or is polished with a low-grade abrasive paper and the surface is insufficiently polished, the work function cannot be measured accurately, and its value may be measured high. .
Method: Atmospheric photoelectron spectroscopy Apparatus: Riken Keiki AC-5 apparatus Conditions: Measurable work function range: 3.4 eV to 6.2 eV
Light source power: 2000W

Hereinafter, description will be made based on examples and comparative examples. It should be noted that this embodiment is merely an example, and the present invention is not limited by this example. That is, the present invention is limited only by the scope of the claims, and includes various modifications other than the examples included in the present invention.

(Example 1)
_CuO powder and WO3 powder were prepared, and these powders were weighed at _CuO :WO3 = 50:50 (mol%). Next, wet ball mill mixing pulverization was performed for 24 hours using zirconia beads of 3.0 mm to obtain a mixed powder having a median diameter of 0.8 μm or less. Next, this mixed powder was pressurized at a surface pressure of 400 kgf/cm ² and then subjected to CIP at a pressure of 1800 kgf/cm ² to produce a compact.
Next, normal pressure sintering was performed at a sintering temperature of 940° C. for 10 hours in an oxygen flow to produce a sintered body. After that, this sintered body was machined and finished into a sputtering target shape.
As a result of evaluating the sputtering target obtained in Example 1, the relative density was 103.3% and the volume resistivity was 1.0×10 ³ Ω·cm. Moreover, as a result of measuring the work function of the sputtering target, a high work function of 4.5 eV was obtained. Table 1 shows the above results. As a result of component analysis of the sputtering target, it was confirmed that there was almost no change from the ratio of the raw materials when they were charged.

(Examples 2-5)
CuO powder and WO ₃ powder were prepared, and these powders were weighed so as to have the molar ratios shown in Table 1. Next, wet ball mill mixing pulverization was performed for 24 hours using zirconia beads of 3.0 mm to obtain a mixed powder having a median diameter of 0.8 μm or less. Next, after pressurizing this mixed powder under the condition of a surface pressure of 400 kgf/cm ² , CIP was performed under the condition of a pressure of 1800 kgf/cm ² to produce a compact.
Next, normal pressure sintering was performed at a sintering temperature of 940° C. for 10 hours in an oxygen flow to produce a sintered body. After that, each sintered body was machined and finished into a sputtering target shape.
The sputtering targets of Examples 2 to 5 all had relative densities of 99% or more and volume resistivities of 1.0×10 ³ Ω·cm or less. Moreover, as a result of measuring the work function of the sputtering targets, all of them had a high work function of 4.5 eV. As a result of component analysis of the sputtering target, it was confirmed that there was almost no change from the ratio of the raw materials when they were charged.

(Comparative example 1)
In Comparative Example 1, only _CuO powder was used, and WO3 powder was not used. The Cu powder was mixed and pulverized by a wet ball mill for 24 hours using zirconia beads of 3.0 mm to obtain a mixed powder having a median diameter of 0.8 μm or less. Next, after pressurizing this mixed powder under the condition of a surface pressure of 400 kgf/cm ² , CIP was performed under the condition of a pressure of 1800 kgf/cm ² to produce a compact.
Next, a sintered body was produced by normal pressure sintering at a sintering temperature of 950° C. for 10 hours in an oxygen flow. After that, this sintered body was machined and finished into a sputtering target shape.
As a result of evaluating the sputtering target obtained in Comparative Example 1, the relative density was 98.3% and the volume resistivity was 3.3×10 ⁵ Ω·cm. Moreover, the work function of the sputtering target was measured and found to be 4.2 eV. As a result of component analysis of the sputtering target, it was confirmed that there was almost no change from the ratio of the raw materials when they were charged.

(Comparative Examples 2 and 3)
In Comparative Examples 2 and ₃ , only WO3 powder was used, and CuO powder was not used. The WO ₃ powder was subjected to wet ball mill mixing pulverization for 24 hours using zirconia beads of 3.0 mm to obtain mixed powder having a median diameter of 0.8 μm or less. Next, after pressurizing this mixed powder under the condition of a surface pressure of 400 kgf/cm ² , CIP was performed under the condition of a pressure of 1800 kgf/cm ² to produce a compact.
Next, the sintering temperature was set to 1100° C. (Comparative Example 2) and 940° C. (Comparative Example 3) in an oxygen flow for 10 hours at normal pressure to produce sintered bodies. After that, this sintered body was machined and finished into a sputtering target shape.
As a result of evaluating the sputtering targets obtained in Comparative Examples 2 and 3, the relative density was less than 95% and the volume resistivity was more than 1.0×10 ³ Ω·cm. Moreover, the work function of the sputtering target was measured and found to be 4.4 eV. As a result of component analysis of the sputtering target, it was confirmed that there was almost no change from the ratio of the raw materials when they were charged.

(Comparative Example 4)
_CuO powder and WO3 powder were prepared, and these powders were weighed at _CuO :WO3 = 30:70 (mol%). Next, wet ball mill mixing pulverization was performed for 24 hours using zirconia beads of 3.0 mm to obtain a mixed powder having a median diameter of 0.8 μm or less. After pressurizing this mixed powder at a surface pressure of 400 kgf/cm ² , CIP was performed at a pressure of 1800 kgf/cm ² to produce a compact.
Next, a sintered body was produced by normal pressure sintering at a sintering temperature of 850° C. for 10 hours in an oxygen flow. After that, this sintered body was machined and finished into a sputtering target shape.
As a result of evaluating the sputtering target obtained in Comparative Example 4, the volume resistivity was 3.1×10 ⁴ Ω·cm.

Next, using the sputtering target of Example 3, sputtering film formation was performed. The film formation conditions were as follows. As a result of measuring the work function of the obtained sputtered film, it was 4.6 eV under Ar gas and 4.8 eV under Ar gas + 6% O ₂ , which were the desired high work functions. As a result of component analysis of the sputtered film, it was confirmed that there was almost no change from the ratio of the raw materials when they were charged.
(Deposition conditions)
Apparatus: SPL-500 sputtering apparatus manufactured by Canon ANELVA Substrate: Silicon substrate Deposition power density: 1.0 W/cm ²
Deposition atmosphere: Ar or Ar+6% O ₂
Gas pressure: 0.5 Pa
Film thickness: 50 nm

The Cu—W—O sputtering target according to the embodiment of the present invention has a low volume resistivity, is capable of DC sputtering, has a high relative density, and does not cause cracks or cracks in the target during film formation. It can be used on a practical, commercial level. INDUSTRIAL APPLICABILITY The present invention is particularly useful for forming transparent electrodes in light-emitting devices such as organic electroluminescence devices.

Claims (4)

A sputtering target comprising tungsten (W), copper (Cu), oxygen (O) and unavoidable impurities, having a volume resistivity of 1.0×10 ³ Ω·cm or less and a work function of 4.5 eV A Cu—W—O sputtering target that satisfies the above . The Cu-WO sputtering target according to claim 1, having a relative density of 95% or more. 3. The Cu-WO sputtering target according to claim 1, wherein the content ratio of W and Cu satisfies the atomic ratio of 0.5≦W/(Cu+W)<1. A thin film composed of tungsten (W), copper (Cu), oxygen (O) and unavoidable impurities, wherein the content ratio of W and Cu satisfies 0.5 ≤ W / (Cu + W) < 1 in atomic ratio, An oxide thin film having a work function of 4.5 eV or more .