JP2014019934A - Sputtering target and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sputtering target of Cu-Ga sintered body capable of further reducing an oxygen content and suppressing an abnormal discharge, and a method for producing the same.SOLUTION: The sputtering target comprises a sintered body, which has a component composition containing 20 at% to less than 30 at% of Ga, and a remainder of Cu and inevitable impurities, in which a diffraction peak attributed to a γ phase and a diffraction peak attributed to a ζ phase of CuGa are observed via an X-ray diffraction. A main peak intensity of the diffraction peak attributed to the ζ phase is more than 10% of a main peak intensity of the diffraction peak attributed to the γ phase, an oxygen content is 100 ppm or less, and a mean particle diameter is 100 μm or less.

Description

  The present invention relates to a sputtering target used when forming a Cu—In—Ga—Se compound film (hereinafter sometimes abbreviated as CIGS film) for forming a light absorption layer of a CIGS thin film solar cell, and It relates to the manufacturing method.

  In recent years, thin film solar cells based on chalcopyrite compound semiconductors have come into practical use, and thin film solar cells based on this compound semiconductor form a Mo electrode layer serving as a positive electrode on a soda-lime glass substrate. A light absorption layer made of a CIGS film is formed on the Mo electrode layer, a buffer layer made of ZnS, CdS, or the like is formed on the light absorption layer, and a transparent electrode layer serving as a negative electrode is formed on the buffer layer. It has a basic structure formed.

  As a method for forming the light absorption layer, for example, a method of forming a film by multi-source deposition is known. Although the light absorption layer obtained by this method can obtain high energy conversion efficiency, the uniformity of the film thickness distribution tends to be lowered when the film is formed on a large-area substrate due to vapor deposition from a point source. Therefore, a method for forming a light absorption layer by a sputtering method has been proposed.

  As a method of forming the light absorption layer by sputtering, first, an In film is formed by sputtering using an In target. A Cu-Ga binary alloy film is formed on the In film by sputtering using a Cu-Ga binary alloy target, and the resulting In film and Cu-Ga binary alloy film are formed. A method (so-called selenization method) for forming a CIGS film by heat-treating a laminated precursor film in a Se atmosphere has been proposed.

  Furthermore, against the background of the above technology, the Cu-Ga alloy film and the In film laminated precursor film are formed from the metal back electrode layer side with a high Ga content Cu-Ga alloy layer, a low Ga content Cu-Ga alloy. A layer and an In layer are formed by a sputtering method in this order, and this is heat-treated in a selenium and / or sulfur atmosphere, so that the inside of the thin-film light absorption layer from the interface layer (buffer layer) side to the metal back electrode layer side There has been proposed a technique for realizing a thin film solar cell with a large open circuit voltage by gradually changing the Ga concentration gradient (stepwise) and preventing the thin film light absorption layer from peeling from other layers. In this case, the Ga content in the CuGa target has been proposed as 1 to 40 atomic% (see Patent Document 1).

As a CuGa target for forming such a CuGa alloy layer, for example, Patent Document 2 discloses a Cu-Ga alloy sintered body sputtering target obtained by sintering a Cu-Ga mixed fine powder produced by a water atomizer by hot pressing. Has been proposed. In this Cu-Ga alloy sintered compact sputtering target, it has a single composition, and the peak intensity other than the main peak (γ phase (Cu ₉ Ga ₄ phase)) by X-ray diffraction of the Cu—Ga alloy is higher than the main peak. The average crystal grain size is 5 to 30 μm. In addition, this target has an oxygen content of 350 to 400 ppm.

JP-A-10-135495 International Publication No. 2011/010529

The following problems remain in the conventional technology.
That is, in the technique described in Patent Document 2, production by hot pressing reduces the oxygen content and reduces abnormal discharge. From the solar cell manufacturer, a target with a lower oxygen content is obtained. This is the current situation. Moreover, in the target produced by the melting method, as described in Table 1 of Patent Document 2, the oxygen content can be greatly reduced to 40 to 50 ppm, but the average particle size becomes as large as 830 to 1100 μm. There is an inconvenience that abnormal discharge increases.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a sputtering target of a Cu-Ga sintered body capable of further reducing the oxygen content and suppressing abnormal discharge, and a method for manufacturing the same. And

  The present invention employs the following configuration in order to solve the above problems. That is, the sputtering target according to the first invention contains 20 at% or more and less than 30 at% of Ga, the remainder has a component composition consisting of Cu and inevitable impurities, and diffraction attributed to the γ phase of CuGa by X-ray diffraction A sintered body in which a peak and a diffraction peak attributed to the ζ phase are observed, and the main peak intensity of the diffraction peak attributed to the ζ phase is 10% or more of the main peak intensity of the diffraction peak attributed to the γ phase. The oxygen content is 100 ppm or less, and the average particle size is 100 μm or less.

The γ and ζ phases are described in “Binary Alloy Phase Diagrams (2nd edition)” (Copyright 1990 by ASM International®, ISBN: 0-87170-405-6), page 1410. R. Subramanian and D. E. It is defined in the item of Cu-Ga system by Laughlin, and each chemical formula and space group are as follows.
(Chemical formula) γ phase: Cu ₉ Ga ₄ , ζ phase: Cu ₃ Ga
(Space group) γ phase: P-43m, ζ phase: P63 / mmcE

In this sputtering target, since the oxygen content is 100 ppm or less and the average particle size is 100 μm or less, the low oxygen content and the small particle size make it possible to significantly reduce abnormal discharge.
Moreover, it contributes to the improvement of the photoelectric conversion efficiency in the light absorption layer of a CIGS thin film type solar cell by suppressing the increase in the oxygen amount in the precursor film | membrane obtained by sputtering by having reduced oxygen content significantly. be able to.
The reason why the Ga content is less than 30 at% is that when it is 30 at% or more, the diffraction peak attributed to the ζ phase almost disappears, and the peak intensity of the ζ phase with respect to the γ phase is less than 10%. This is because it becomes a single phase of γ phase.

A method for producing a sputtering target according to a second invention is a method for producing a sputtering target according to the first invention, wherein a compact comprising a mixed powder of pure Cu powder and Cu-Ga alloy powder is reduced. It has the process of heating in an atmosphere and carrying out atmospheric pressure sintering.
That is, in this method for producing a sputtering target, a compact made of a mixed powder of pure Cu powder and Cu—Ga alloy powder is heated in a reducing atmosphere and sintered at normal pressure. Γ phase and ζ phase appear due to mutual diffusion from the powder, and very few sintered bodies have observed diffraction peaks attributed to CuGa γ phase and diffraction peaks attributed to ζ phase by X-ray diffraction It can be obtained with an oxygen content.
In addition, shape maintenance becomes easy when it uses a pure Cu powder which is easy to carry out plastic deformation, when it uses a molded object. Moreover, although pure Cu powder will be oxidized also in the air | atmosphere at room temperature, since it is reduce | restored easily by the heating in a reducing atmosphere, it does not become a cause which increases oxygen content. Furthermore, by adding a Cu-Ga alloy powder of 50 at% Ga, liquid phase sintering is performed, and a high-density sintered body is obtained.

The present invention has the following effects.
That is, according to the sputtering target and the manufacturing method thereof according to the present invention, the main peak intensity of the diffraction peak attributed to the ζ phase is 10% or more of the main peak intensity of the diffraction peak attributed to the γ phase, and the oxygen content Is 100 ppm or less, and the average particle size is 100 μm or less, so the low oxygen content and the small particle size can greatly reduce abnormal discharge and increase the amount of oxygen in the precursor film obtained by sputtering. Can be suppressed.
Therefore, by forming the light absorption layer of the CIGS thin film solar cell by sputtering using the sputtering target of the present invention, it is possible to contribute to the improvement of the photoelectric conversion efficiency in the light absorption layer, and the power generation efficiency is high. A solar cell can be manufactured.

In the sputtering target which concerns on this invention, and its manufacturing method, it is a graph which shows the diffraction peak by X-ray diffraction about the Example containing Ga at 25at%. It is a graph which shows the diffraction peak by X-ray diffraction about the Example of this invention containing 28at% Ga. It is a graph which shows the diffraction peak by X-ray diffraction about the Example of this invention containing 29 at% of Ga. It is a graph which shows the diffraction peak by X-ray diffraction about the comparative example of this invention containing 30at% Ga. It is a composition image (COMPO image) by an electron beam microanalyzer (EPMA) about the Example of this invention containing 25at% of Ga. It is an element distribution mapping image by EPMA about the Example of this invention containing 25at% Ga.

  Hereinafter, one embodiment of the sputtering target and its manufacturing method concerning the present invention is described.

The sputtering target of this embodiment contains Ga of 20 at% or more and less than 30 at%, and the remainder has a component composition composed of Cu and inevitable impurities, and is converted into a CuGa γ phase (Cu ₉ Ga ₄ phase) by X-ray diffraction. It consists of a sintered body in which the diffraction peak attributed to the diffraction peak attributed to the ζ phase (Cu ₃ Ga phase) is observed, and the main peak intensity of the diffraction peak attributed to the ζ phase is the diffraction peak attributed to the γ phase. It is 10% or more of the main peak intensity, the oxygen content is 100 ppm or less, and the average particle size is 100 μm or less.

This sputtering target has a crystal structure in which a phase (Ga-rich region) containing a relatively large amount of Ga is dispersed. The Ga-rich region is, for example, a region observed in white in a COMPO image by EPMA as shown in FIG.
The main peak intensity of the diffraction peak is the intensity of the strongest of a plurality of diffraction peaks belonging to a specific metal phase.

The average grain size is a magnification that can discriminate the crystal grain boundary after polishing the sample surface cut out from the target to a mirror surface and etching with an etching solution composed of nitric acid and pure water. Take a photomicrograph with the optical microscope inside, draw 10 straight lines that divide one side of the obtained photo into 11 equal parts, count the crystal grains that pass through these 10 straight lines, and use the following formula Looking for.
Average crystal grain size = (value obtained by correcting the length of 10 line segments in the photograph) / (number of crystal grains through which 10 line segments pass)

  The oxygen content is measured by the infrared absorption method described in JIS Z 2613 “General Rules for Determination of Oxygen of Metallic Materials”.

The method for producing the sputtering target of the present embodiment includes a step of heating a molded body made of a mixed powder of pure Cu powder and Cu-Ga alloy powder in a reducing atmosphere to perform atmospheric pressure sintering. Yes.
An example of this production method will be described in detail. First, pure Cu powder having an average particle diameter of 2 to 3 μm and Cu—Ga alloy atomized powder having an average particle diameter of 20 to 30 μm are weighed to obtain a target composition, and a Henschel mixer is used. Is used for 1 minute at 2800 rpm in an Ar atmosphere to obtain a mixed powder.

Next, the obtained mixed powder is made into a green compact (molded body) with a molding pressure of 500 to 2000 kgf / cm ² . The green compact is placed in a furnace and heated at 10 ° C./min to a firing temperature of 700 to 1000 ° C. while flowing a reducing gas at 10 to 100 L / min, and held for 5 hours. Thereafter, the inside of the furnace is naturally cooled, and the surface portion and the outer peripheral portion of the obtained sintered body are turned to produce a sputtering target having a diameter of 50 mm and a thickness of 6 mm. The Cu—Ga alloy atomized powder is prepared by filling a carbon crucible with Cu and Ga raw materials at a specified composition ratio and by gas atomizing with Ar gas.

Next, the processed sputtering target is bonded to a backing plate made of Cu or SUS (stainless steel) or other metal (for example, Mo) using In as solder, and is subjected to sputtering.
When storing the processed target, it is preferable to apply a vacuum pack or a pack obtained by replacing the entire target with a vacuum in order to prevent oxidation and moisture absorption.

  The sputtering target thus produced is subjected to DC magnetron sputtering using Ar gas as a sputtering gas. In this case, direct current (DC) sputtering may be performed using a pulsed DC power supply to which a pulse voltage is applied or a DC power supply without a pulse.

In the sputtering target of this embodiment, the main peak intensity of the diffraction peak attributed to the ζ phase is 10% or more of the main peak intensity of the diffraction peak attributed to the γ phase, the oxygen content is 100 ppm or less, and the average Since the particle size is 100 μm or less, abnormal discharge can be significantly reduced because the oxygen content is low and the particle size is small.
Moreover, it contributes to the improvement of the photoelectric conversion efficiency in the light absorption layer of a CIGS thin film type solar cell by suppressing the increase in the oxygen amount in the precursor film | membrane obtained by sputtering by having reduced oxygen content significantly. be able to.

  Moreover, in the manufacturing method of the sputtering target of this embodiment, since the molded object consisting of the mixed powder of pure Cu powder and Cu-Ga alloy powder is heated in a reducing atmosphere and sintered at normal pressure, A sintered body in which a γ phase and a ζ phase appear due to mutual diffusion from each raw material powder, and a diffraction peak attributed to the γ phase of CuGa and a diffraction peak attributed to the ζ phase are observed by X-ray diffraction. Can be obtained with a very low oxygen content.

  The reason why the two phases of γ phase and ζ phase coexist is that the liquid phase of Ga-rich appears from the Cu-Ga alloy powder during firing, so that the rearrangement of particles is easy by so-called liquid phase sintering. This is because a high-density sintered body is obtained in spite of normal-pressure powder sintering and the sintered body is separated into a γ phase and a ζ phase at around 620 ° C. in the process of cooling. According to the Cu—Ga phase diagram described in “Binary Alloy Phase Diagrams (2nd edition)”, this phase separation is expected to occur whenever the atomic ratio of Ga is less than 30%. The advantage of two-phase coexistence is that the presence of the ζ phase suppresses the enlargement of crystal grains in the γ layer, the average grain size of the target structure is reduced, and abnormal discharge during sputtering is less likely to occur.

  Next, the evaluation results of the sputtering target and the manufacturing method thereof according to the present invention will be described with reference to the examples manufactured based on the above embodiment.

First, Cu-Ga alloy powder (CuGa powder in the table) having the component composition and particle size shown in Table 1 and Cu powder were blended so as to have the amounts shown in Table 1, and Examples 1 to 5 mixed powder. Next, the obtained mixed powder was made into a green compact (molded body) with a molding pressure of 1500 kgf / cm ² . Further, among these mixed powders, Examples 1 to 3 were subjected to normal pressure sintering in a hydrogen atmosphere, Example 4 was subjected to normal pressure sintering in a carbon monoxide atmosphere, and Example 5 was an ammonia cracking gas atmosphere. Sintered at normal pressure. In addition, these atmospheric pressure sintering was performed by flowing reducing gas at 50 L / min, and hold | maintaining at the calcination temperature of 840 degreeC for 5 hours.

  On the other hand, as a comparative example, a Cu-Ga alloy powder (CuGa powder in the table) having the component composition and particle size shown in Table 1 and Cu powder are blended so as to have the amount shown in Table 1, It was set as the mixed powder of Comparative Examples 1-4. In Comparative Examples 2 and 3, the Ga content deviated from the scope of the present invention. Next, the obtained mixed powder was used as a green compact (molded body) in the same manner as in the above example. In Comparative Examples 5 and 8, only the Cu—Ga alloy powder was used as the raw material powder.

Further, among these powders, Comparative Example 1 was subjected to normal pressure sintering in an air atmosphere, and Comparative Examples 2, 3 and 8 were subjected to normal pressure sintering in a hydrogen atmosphere in the same manner as in the Examples. No. 5 was sintered in a vacuum by a hot press method. The hot press conditions at this time are a holding temperature of 740 ° C. and a holding time of 60 minutes.
Further, Comparative Examples 6 and 7 were produced by a casting method so as to have the composition shown in Table 1.

Table 1 shows the results of examining the average particle diameter, the analysis by X-ray diffraction, the oxygen content, the density, and the number of abnormal discharges for the examples and comparative examples of the present invention thus prepared. However, in Comparative Example 8, the shape of the disk could not be maintained and collapsed, so that the green compact could not be produced and the target could not be produced, so these measurements were not performed. For Comparative Example 8, a portion of the lump that remained unbroken during compacting was fired and the Ga composition was measured.
The composition of the target was measured using an ICP method (high frequency inductively coupled plasma method).

  In the X-ray diffraction analysis (XRD), both a diffraction peak attributed to the γ phase and a diffraction peak attributed to the ζ phase are observed, and the main peak intensity of the diffraction peak attributed to the ζ phase is A case where it is 10% or more of the main peak intensity of the assigned diffraction peak is expressed as “γ, ζ” in Table 1, and the main peak intensity of the diffraction peak attributed to the ζ phase is the diffraction peak attributed to the γ phase. The case where it is less than 10% of the main peak intensity is represented as “γ” in Table 1.

In this XRD, a target sample was wet-polished with SiC-Paper (grit 180) and dried, and then used as a measurement sample. The apparatus and measurement conditions used for this analysis are shown below.
Equipment: Rigaku Electric (RINT-Ultima / PC)
Tube: Cu
Tube voltage: 40 kV
Tube current: 40 mA
Scanning range (2θ): 20 ° to 120 °
Slit size: Divergence (DS) 2/3 degree, Scattering (SS) 2/3 degree, Light reception (RS) 0.8mm
Measurement step width: 0.02 degrees at 2θ Scan speed: 2 degrees per minute Sample stage rotation speed: 30 rpm

For abnormal discharge, sputtering was performed for 12 hours under the following film forming conditions, and the number of abnormal discharges was measured.
・ Power supply: Pulse DC500W
・ Total pressure: 0.4Pa
Sputtering gas: Ar = 47.5 sccm, O ₂ : 2.5 sccm
-Target-substrate (TS) distance: 70 mm
The number of abnormal discharges was measured by the arc count function of a DC power supply (model number: RPDG-50A) manufactured by MKS Instruments.

  As can be seen from these results, all of the examples of the present invention have an average particle size as small as 68 to 84 μm, and two phases of γ phase and ζ phase are observed in X-ray diffraction. In these examples, the oxygen content is very low, 55 to 75 ppm, and the number of abnormal discharges is greatly reduced to 1 or less.

  On the other hand, in the comparative example 1 which carried out the atmospheric pressure sintering in air | atmosphere, while oxygen content is as high as 300 ppm, the frequency | count of abnormal discharge has increased significantly with 13 times. Further, in Comparative Example 2 where Ga is less than the composition range of the present invention, the oxygen content is increased to 105 ppm and the number of abnormal discharges is increased to 3 times. Furthermore, in Comparative Example 3 in which Ga is out of the composition range of the present invention, it becomes a single γ phase, and the number of abnormal discharges has increased to three.

  In Comparative Examples 4 and 5 baked by the hot press method, the oxygen content greatly increases to 300 ppm or more, and the number of abnormal discharges also increases. Further, in Comparative Examples 6 and 7 produced by the casting method, the average particle size is as large as 500 nm or more, and the number of abnormal discharges is increased to 8 times and 6 times, respectively.

  Next, the Examples and Comparative Examples of the present invention in which the Ga content was 25 at%, 28 at%, 29 at%, and 30 at%, respectively, were allowed to flow at 50 L / min for 5 hours at a firing temperature of 840 ° C. FIGS. 1 to 4 show the results of measurement by holding the atmospheric pressure sintering and measuring the diffraction peak by XRD.

As can be seen from these results, when the Ga content is 25 at%, 28 at%, and 29 at%, the diffraction peak attributed to the γ phase (Cu ₉ Ga ₄ phase) of CuGa and the ζ phase (Cu ₃ Ga phase) are attributed. The diffraction peak attributed to the ζ phase is 10% or more of the main peak intensity of the diffraction peak attributed to the γ phase. It can be seen that the phase is clearly formed in the tissue. However, it can be seen that when the Ga content is 30 at%, the diffraction peak attributed to the ζ phase almost disappears and the structure is substantially a single phase of γ phase.

Next, an embodiment of the present invention containing 25 at% Ga was produced by atmospheric pressure sintering performed at a firing temperature of 840 ° C. for 5 hours while flowing hydrogen gas at 50 L / min. A composition image (COMPO image) observed by EPMA and element mapping images of Cu, Ga, O (oxygen), and C (carbon) are shown in FIGS. In these element mapping images of EPMA, the original image is a color image, but is converted into a black and white image in gray scale, and the Cu mapping image tends to have a high content in a portion with high brightness. is there. Moreover, about Ga mapping image, it exists in the tendency for content to be high in the part where the brightness is dark. Further, in the COMPO image, the whitest portion indicates a region having a relatively high Ga content.
As can be seen from these images, the examples of the present invention have a crystal structure in which a phase (Ga-rich region) containing a relatively large amount of Ga is dispersed.

In order to use the present invention as a sputtering target, surface roughness: 1.5 μm or less, electrical resistance: 1 × 10 ⁻⁴ Ω · cm or less, metal impurity concentration: 0.1 atomic% or less, resistance Folding strength: It is preferably 150 MPa or more. Each of the above-described embodiments satisfies these conditions.
The technical scope of the present invention is not limited to the above-described embodiment and examples, and various modifications can be made without departing from the spirit of the present invention.
For example, although the sputtering target of the said embodiment and the said Example is a flat thing, it is good also as a cylindrical sputtering target.

Claims (2)

Containing 20 at% or more and less than 30 at% Ga, with the balance being composed of Cu and inevitable impurities,
It consists of a sintered body in which a diffraction peak attributed to the γ phase of CuGa and a diffraction peak attributed to the ζ phase are observed by X-ray diffraction,
The main peak intensity of the diffraction peak attributed to the ζ phase is 10% or more of the main peak intensity of the diffraction peak attributed to the γ phase;
The oxygen content is 100 ppm or less,
A sputtering target having an average particle size of 100 μm or less. A method for producing the sputtering target according to claim 1,
A method for producing a sputtering target, comprising: a step of heating a molded body composed of a mixed powder of pure Cu powder and Cu-Ga alloy powder in a reducing atmosphere to perform normal pressure sintering.