Cu—Ga alloy sputtering target, and method for producing same

Abstract

The present invention provides a sputtering target of a CuGa sintered body in which the oxygen content is further reduced and the abnormal discharging can be suppressed, and a method for producing the same. The sputtering target according to the present invention is a sintered body having: a texture in which Na compound phases are dispersed in a matrix with a phase and a phase of a CuGa alloy; and a component composition made of: 20 atomic % to 30 atomic % of Ga; 0.05 atomic % to 10 atomic % of Na; and the Cu balance and inevitable impurities including elements other than Na in the Na compound, wherein an average grain size of the phase is 30 m to 100 m, and an average grain size of the Na compound phases is equal to or less than 8.5 m.

Claims

1. A CuGa alloy sputtering target that is a sintered body having: a texture in which Na compound phases are dispersed in a matrix with a phase and a phase of a CuGa alloy; and a component composition made of: 20 atomic % to 30 atomic % of Ga; 1.3 atomic % to 10 atomic % of Na; and the Cu balance and inevitable impurities including elements other than Na in the Na compound, wherein an average grain size of the phase is 30 m to 100 m, and an average grain size of the Na compound phases is equal to or less than 8.5 m.

2. The CuGa alloy sputtering target according to claim 1, wherein a maximum grain size of the Na compound phases is equal to or less than 20 m.

3. The CuGa alloy sputtering target according to claim 1, wherein a main peak intensity in X-ray diffraction attributed to the phase is equal to or more than 5% of a main peak intensity in X-ray diffraction attributed to the phase.

4. The CuGa alloy sputtering target according to claim 1, wherein the Na compound phases are formed of at least one or more of NaF, Na.sub.2S, Na.sub.2Se, and Na.sub.3AlF.sub.6.

5. The CuGa alloy sputtering target according to claim 1, wherein an oxygen content of the sintered body is equal to or less than 200 mass ppm.

6. A method of producing the CuGa alloy sputtering target according to claim 1, the method comprising the step of performing pressureless sintering by heating a molded body formed from a powder mixture of a pure Cu powder, a CuGa alloy powder, and a Na compound, in a reducing atmosphere.

7. The CuGa alloy sputtering target according to claim 1, wherein the Na compound phases are formed of at least one or more of Na.sub.2S, Na.sub.2Se, and Na.sub.3AlF.sub.6.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a phase diagram of a CuGa alloy.

(2) FIG. 2 is a graph illustrating a diffraction peak measured by X-ray diffraction, regarding a sputtering target of Example 2 which contains 22.9 atomic % of Ga.

(3) FIG. 3 is a graph illustrating a diffraction peak measured by X-ray diffraction, regarding a sputtering target of Example 4 which contains 29.1 atomic % of Ga.

(4) FIG. 4 is a graph illustrating a diffraction peak measured by X-ray diffraction, regarding a sputtering target of Comparative Example 4 which contains 30.4 atomic % of Ga.

(5) FIG. 5 is a compositional image (COMPO image) obtained by an electron beam probe microanalyser (EPMA), regarding the sputtering target of Example 2 which contains 22.9 atomic % of Ga.

(6) FIG. 6 is an element distribution mapping image of Ga obtained by the EPMA, regarding the sputtering target of Example 2 which contains 22.9 atomic % of Ga.

(7) FIG. 7 is an element distribution mapping image of Na obtained by the EPMA, regarding the sputtering target of Example 2 which contains 22.9 atomic % of Ga.

DESCRIPTION OF EMBODIMENTS

(8) Hereinafter, embodiments of a CuGa alloy sputtering target and a method for producing the same according to the present invention are described.

(9) A sputtering target of the present embodiment is a sintered body having a component composition made of: 20 atomic % to 30 atomic % of Ga; 0.05 atomic % to 10 atomic % of Na; and the Cu balance and inevitable impurities including elements other than Na in the Na compound. In the metal matrix of the sintered body, a phase (Cu.sub.9Ga.sub.4 phase) and a phase (Cu.sub.3Ga phase) of a CuGa alloy co-exist, and has a texture in which Na compound phases are dispersed. The average grain size of the phase is 30 m to 100 m. In addition, the average grain size of the Na compound phases is equal to or less than 8.5 m. The maximum grain size of the Na compound phases is equal to or less than 20 m. In addition, the oxygen content of the sintered body is equal to or less than 200 mass ppm.

(10) The sputtering target has a crystal texture in which phases (Ga-rich region: Cu.sub.9Ga.sub.4 phase) are dispersed, and this phase contains more Ga than that in the sintered body. The Ga-rich region is, for example, an area observed to be white in a COMPO image obtained by an EPMA, as shown in FIG. 5.

(11) The average grain size of the phase is obtained in the following manner. A surface of a sample cut out from the sputtering target is polished so as to become a mirror surface, and is etched by using an etchant which is formed from nitric acid and pure water. Then, a microphotograph is captured by an optical microscope which can distinguish a crystal particle boundary at a magnification of a range from 50 times to 1000 times, and 10 straight lines for dividing one side of the obtained photograph into 11 equal parts are drawn. The number of crystal particles through which the 10 straight lines pass is calculated, and the average grain size of the phase is obtained by the following calculation expression.
Average grain size=(value obtained by correcting lengths of the 10 straight lines on the photograph to be actual lengths)/(the number of crystal particles through which the 10 straight lines pass)

(12) The average grain size of the Na compound phases is measured based on an element distribution mapping image of Na obtained by the EPMA, as shown in FIG. 7. In the image of FIG. 7, a white area indicates the presence of Na and represents the size of the Na compound phase. An occupied area S (m.sup.2) is measured and obtained by setting this white area as one Na compound particle. The grain size D of the Na compound phase is obtained from an expression of grain size D=(S/).sup.1/2. An average grain size (average value of D) and the maximum grain size (maximum value of D) are calculated from the number of Na compound particles observed in 10 square areas having one side of 100 m, and grain sizes D.

(13) The oxygen content is measured by an infrared absorbing method which is described in General rules for determination of oxygen in metallic materials of JIS Z 2613.

(14) The method for producing a sputtering target in the present embodiment has a step of performing pressureless sintering by heating a molded body in a reducing atmosphere. The molded body is formed from a powder mixture of a pure Cu powder, a CuGa alloy powder, and a Na compound.

(15) An example of the production method is described in detail below. First, a pure Cu powder in which D50 measured by Microtrac is 2 m to 3 m, a CuGa alloy atomized powder in which D50 is 20 m to 30 m, and a Na compound sieved to cause D50 to be 10 m to 20 m are weighed so as to be a target composition, and are mixed with each other in an Ar atmosphere by using a Henschel mixer to obtain a powder mixture. The CuGa alloy atomized powder is manufactured in such a manner that a CuGa alloy is dissolved in a gas-atomizing device so as to cause the concentration of Ga to be 50 atomic %, and is atomized by using an Ar gas.

(16) Then, a green compact (molded body) is obtained at a molding pressure of 500 kgf/cm.sup.2 to 2000 kgf/cm.sup.2 by using the obtained powder mixture. The green compact is arranged in a furnace. A reducing gas is flowed at 10 L/min to 100 L/min, and the green compact is heated up to a sintering temperature of 700 C. to 1000 C. at 10 C./min and is held for 5 hours. Then, the inside of the furnace is naturally cooled, and lathe work is performed on a surface portion and on an outer circumference portion of the obtained sintered body. Thereby, a sputtering target having a diameter of 50 mm and a thickness of 6 mm is manufactured.

(17) Then, the processed sputtering target is hot pressed by using a Cu backing plate and is used in sputtering.

(18) The CuGa alloy sputtering target manufactured in this manner is used in a direct-current (DC) magnetron sputtering device by using an Ar gas as a sputtering gas.

(19) In the CuGa alloy sputtering target of the present embodiment, the main peak intensity in X-ray diffraction attributed to the phase is equal to or more than 5% of the main peak intensity in X-ray diffraction attributed to the phase. The oxygen content is equal to or less than 200 mass ppm and the average grain size of the phases of the CuGa alloy is equal to or less than 100 m. Thus, the Na compound phases having an average grain size which is equal to or less than 8.5 m are dispersed and the maximum grain size thereof is suppressed so as to be equal to or less than 20 m. Accordingly, the abnormal discharging is significantly reduced.

(20) An increase of an amount of oxygen in a precursor film obtained through sputtering is suppressed by significantly reducing the oxygen content and it is possible to contribute to improvement of photoelectric conversion efficiency in a light-absorbing layer of a CIGS thin-film solar cell.

(21) In the method for producing a sputtering target in the present embodiment, pressureless sintering is performed by heating the molded body formed from the powder mixture of the pure Cu powder, the CuGa alloy powder, and the Na compound, in the reducing atmosphere. Thus, mutual diffusion occurs from the raw material powders thereof during sintering, and the phase and the phase appear as a metal phase in the sintered body. As a result, the sintered body, in which an X-ray diffraction peak attributed to the phase of the CuGa alloy and an X-ray diffraction peak attributed to the phase are observed, with extremely low oxygen content is obtained.

(22) The reason for two phases of the phase and the phase being present together is as follows. That is, a Ga-rich liquid phase appears from the CuGa alloy powder, and so-called liquid phase sintering is performed in sintering. Thus, particles are easily rearranged and a high-density sintered body is obtained while a powder is sintered at normal pressure. In the process of cooling the sintered body, the sintered body is separated into the phase and the phase at the vicinity of 620 C. According to the CuGa phase diagram (source of reference, Desk Handbook: Phase Diagrams for Binary Alloys (ASM International)) shown in FIG. 1, it is expected that this phase separation necessarily occurs when an atom ratio of Ga is less than 30%. An advantage of coexistence of the two phases is that coarsening of a crystal particle in the phase is suppressed by the presence of the phase, the average grain size of a target structure becomes small, and the abnormal discharging during sputtering is less likely to occur.

EXAMPLES

(23) Next, results obtained by evaluating the sputtering target and the method for producing the same according to the present invention using examples manufactured based on the embodiments are described below.

(24) First, mixed powders of Examples 1 to 11 were prepared by blending the CuGa alloy atomized powder (CuGa powder in the table), Ga concentration of which was 50 atomic %; the Cu powder; and the Na compound (NaF, Na.sub.2S, Na.sub.2Se, and Na.sub.3AlF.sub.6), so as to have a weight ratio shown in Table 1. Next, green compacts (molded body) were formed at the molding pressure of 1500 kgf/cm.sup.2 by using each of the obtained mixed powders. As shown in Table 2, Examples 1 to 7 among the mixed powders were subjected to pressureless sintering in a hydrogen atmosphere. Examples 8 and 9 were subjected to pressureless sintering in a carbon monoxide atmosphere. Examples 10 and 11 were subjected to pressureless sintering in an atmosphere of an ammonia decomposition gas. The pressureless sintering was performed by holding the powder mixture at the sintering temperature of 840 C. for 5 hours with a reducing gas flowing at 50 L/min.

(25) As comparative examples, mixed powders of Comparative Examples 1 to 5 were prepared by blending the CuGa alloy atomized powder (CuGa powder in the table), Ga concentration of which was 50 atomic %; the Cu powder; and the Na compound (NaF, Na.sub.2S. Na.sub.2Se, and Na.sub.3AlF.sub.6), so as to have a weight ratio shown in Table 1. In Comparative Examples 3 and 4, the CuGa alloy powder was blended so as to cause the content of Ga to be out of range defined in the scope of the present invention. In Comparative Example 5, an excess of Na compound was blended. Then, a green compacts (molded body) were formed by using each of the obtained mixed powders, similarly to the above-explained Examples.

(26) As shown in Table 2, Comparative Example 1 among the mixed powders was subjected to pressureless sintering in an air atmosphere. Comparative Example 2 was sintered in a vacuum by using a hot pressing method. A condition of hot pressing was a holding time of 60 min at a holding temperature of 740 C. Comparative Examples 3 to 5 were subjected to pressureless sintering in a hydrogen atmosphere, similarly to the above-explained Examples.

(27) In Table 2, results of composition analysis relating to Ga, Na, and Cu in sputtering targets of Examples 1 to 11 and Comparative Examples 1 to 5 are shown in a target composition (atomic %) field. This composition of each of the targets was measured by using an ICP method thigh-frequency inductively-coupled plasma method).

(28) TABLE-US-00001 TABLE 1 Raw material blending ratio (weight %) Cu CuGa NaF Na.sub.2S Na.sub.2Se Na.sub.23AlF.sub.6 Example 1 52.4 44.1 3.5 Example 2 46.0 50.5 3.5 Example 3 40.0 57.1 1.7 1.2 Example 4 39.9 59.4 0.7 Example 5 45.3 51.8 1.0 1.9 Example 6 31.7 62.9 2.0 3.4 Example 7 32.6 60.0 7.4 Example 8 42.7 56.8 0.5 Example 9 40.2 55.6 0.7 3.5 Example 10 46.4 48.6 5.0 Example 11 43.2 54.3 0.9 1.6 Comparative 45.0 52.6 2.4 Example 1 Comparative 43.4 55.0 1.6 Example 2 Comparative 56.3 40.6 3.1 Example 3 Comparative 31.1 65.5 3.4 Example 4 Comparative 38.7 51.6 5.8 3.9 Example 5

(29) TABLE-US-00002 TABLE 2 Sintering Target composition (atomic %) atmosphere Ga Na Cu Example 1 Hydrogen 20.3 5.1 Balance Example 2 Hydrogen 22.9 2.8 Balance Example 3 Hydrogen 26.7 4.4 Balance Example 4 hydrogen 29.1 1.0 Balance Example 5 Hydrogen 24.4 3.5 Balance Example 6 Hydrogen 28.0 5.9 Balance Example 7 Hydrogen 25.3 9.7 Balance Example 8 Carbon 27.5 0.5 Balance Monoxide Example 9 Carbon 24.7 4.2 Balance Monoxide Example 10 Ammonia 21.2 6.8 Balance Decomposition Gas Example 11 Ammonia 25.6 2.6 Balance Decomposition Gas Comparative Air 24.5 3.6 Balance Example 1 Comparative Vacuum 25.8 1.3 Balance Example 2 Comparative Hydrogen 18.3 4.6 Balance Example 3 Comparative Hydrogen 30.4 2.9 Balance Example 4 Comparative Hydrogen 21.2 10.5 Balance Example 5

(30) Regarding the sputtering targets of Examples of the present invention and the sputtering targets of Comparative Examples which were manufactured in this manner, results obtained by examining average grain sizes of the phases and the Na compound phases, the maximum grain size of the Na compound phases, analysis by X-ray diffraction, the oxygen content, and the number of occurrences of abnormal discharge are shown in Table 3.

(31) TABLE-US-00003 TABLE 3 Average grain Average grain Maximum grain size of size of size of X-ray Oxygen Abnormal phase Na compound Na compound diffrac- content discharge (m) (m) (m) tion (mass ppm) (times) Example 1 40 3.7 9.3 , 180 1 Example 2 50 4.4 9.7 , 130 0 Example 3 70 5.9 13.0 , 110 0 Example 4 80 5.1 12.2 , 80 0 Example 5 60 4.4 11.0 , 90 0 Example 6 90 8.3 19.1 , 70 1 Example 7 80 7.9 18.2 , 70 0 Example 8 80 4.8 10.1 , 100 0 Example 9 70 4.4 8.4 , 80 0 Example 10 50 7.7 16.2 , 160 0 Example 11 80 5.2 12.0 , 100 0 Comparative 20 15.0 36.0 , 400 13 Example 1 Comparative 20 18.3 36.6 , 350 5 Example 2 Comparative 10 4.0 7.6 , 290 6 Example 3 Comparative 130 8.9 22.3 60 3 Example 4 Comparative 70 10.1 24.2 , 100 4 Example 5

(32) In the analysis by X-ray diffraction, both the diffraction peaks attributed to the phase and the diffraction peak attributed to the phase were observed. When the intensity of the main peak in the diffraction peak attributed to the phase was equal to or 5% greater than the intensity of the main peak of the diffraction peak attributed to the phase, it is indicated by , in Table 3. When the intensity of the main peak in the diffraction peak attributed to the phase was 5% less than the intensity of the main peak of the diffraction peak attributed to the phase, it is indicated by in Table 3.

(33) Device and a measurement condition used in this analysis are as follows.

(34) Device: RINT-Ultima/PC manufactured by Rigaku Corporation.

(35) Tube: Cu

(36) Tube voltage: 40 kV

(37) Tube current: 40 mA

(38) Scanning range (2): 20 to 120

(39) Measurement step width: 0.02 at 2

(40) Scanning speed: 2 per minute

(41) Regarding abnormal discharge, sputtering was performed under the following film formation condition for 12 hours and the number of occurrences of abnormal discharge was measured.

(42) Power: Pulse DC500 W

(43) Total pressure: 0.4 Pa

(44) Sputtering gas: Ar=47.5 sccm, O.sub.2=2.5 sccm

(45) Target-Substrate (TS) distance: 70 mm

(46) Number of occurrences of abnormal discharge was measured by using the arc-counting function of the DC power source (type number: RPDG-50A) manufactured by MKS Instruments Inc.

(47) As can be understood from these results, in any one of the sputtering targets of Examples according to the present invention, the average grain size of the phase was as small as 40 m to 90 m, and the two phases of the phase and the phase in X-ray diffraction were observed. In the sputtering targets of these Examples, the oxygen content was significantly low at 70 mass ppm to 180 mass ppm, and the average grain size and the maximum grain size of the Na compound phase were small. Thus, the number of occurrences of abnormal discharge was significantly reduced so as to be equal to or less than one time.

(48) On the contrary, in the sputtering target of Comparative Example 1 subjected to pressureless sintering in air, the oxygen content was high as 400 mass ppm, and the number of occurrences of abnormal discharge was significantly increased to 13 times. In the sputtering target of Comparative Example 3 having a small amount of Ga which is out of the composition range defined in the scope of the present invention, the oxygen content was increased to 290 mass ppm and the number of occurrences of abnormal discharge was also significantly increased to 6 times. In the sputtering target of Comparative Example 4 having a large amount of Ga which was out of the composition range defined in the scope of the present invention, it turned into a single-phase texture of the phase mostly and the number of occurrences of abnormal discharge was also significantly increased to 3 times.

(49) In the sputtering target of Comparative Example 2 sintered by using the hot pressing method, the oxygen content was increased to 350 mass ppm and the number of occurrences of abnormal discharge was also significantly increased to 5 times.

(50) Next, Examples according to the present invention and Comparative Examples in which the content of Ga was set to 22.9 atomic %, 29.1 atomic %, and 30.4 atomic % were manufactured by performing pressureless sintering in which the sintered body was held at the sintering temperature of 840 C. for 5 hours while flowing hydrogen gas at 50 L/min. Results obtained by measuring diffraction peaks of Examples and Comparative Examples through X-ray diffraction (XRD) are shown in FIGS. 2 to 4.

(51) As can be understood from these results, in the sputtering target in which the content of Ga was 22.9 atomic % and 29.1 atomic %, both of the diffraction peak attributed to the phase (Cu.sub.9Ga.sub.4 phase) of CuGa and the diffraction peak attributed to the phase (Cu.sub.3Ga phase) were observed. Intensity of the main peak in the diffraction peak attributed to the phase was equal to or 5% greater than the intensity of the main peak in the diffraction peak attributed to the phase. Thus, it can be understood that the two phases of the phase and the phase were clearly formed in the texture. However, when the content of Ga was 30.4 atomic %, the intensity of the diffraction peak attributed to the phase was less than 5%. Thus, it can be understood that the texture was formed of the single-phase of the phase mostly.

(52) Next, the sputtering target of Example 2 which contains 22.9 atomic % of Ga was manufactured by performing pressureless sintering in which the sintered body was held at the sintering temperature of 840 C. for 5 hours while flowing hydrogen gas at 50 L/min. A compositional image (COMPO image) and an element-mapping image of Ga which were obtained by observing the obtained structure using the EPMA are shown in FIGS. 5 and 6. In the COMPO image, the whitest portion indicates an area in which the content of Ga was high relatively to the sintered body. Although the original image of the element-mapping image of Ga is a color image, an image obtained by converting the original image into a monochrome image using a gray scale is shown. In the image, a portion having low brightness has a high Ga content. As can be understood from these images, the examples according to the present invention have a crystal texture in which phases (Ga-rich region) having more Ga than the sintered body were dispersed.

(53) In order to use the examples of the present invention as a sputtering target, it is preferable that surface roughness be equal to or less than 1.5 m, an electrical resistance be equal to or less than 110.sup.4 .Math.cm, the amount of metal impurities be equal to or less than 0.1 atomic %, and a transverse strength be equal to or greater than 150 MPa. All of the examples satisfy these conditions.

(54) The technological range of the present invention is not limited to the descriptions of the embodiments and the examples, and various changes in a range without departing from the scope of the present invention may be added.

(55) For example, the sputtering targets of the embodiments and the examples have a plate-like shape. However, the sputtering target may have a cylindrical shape. More specifically, the sputtering target may have a disc plate-like shape, a short-side plate-like shape, a polygonal shape, and an elliptical plate-like shape, or a cylindrical shape.

INDUSTRIAL APPLICABILITY

(56) It is possible to stably form a film by performing sputtering using an AgIn alloy sputtering target according to the present invention.