Oxide sintered compact and sputtering target formed from said oxide sintered compact

Abstract

An IGZO sintered compact composed of indium (In), gallium (Ga), zinc (Zn), oxygen (O) and unavoidable impurities, wherein the IGZO sintered compact has a flexural strength of 50 MPa or more, and a bulk resistance of 100 mcm or less. Provided is a sputtering target capable of suppressing the target cracks and reducing the generation of particles during deposition via DC sputtering, and forming favorable thin films.

Claims

1. An IGZO sintered compact composed of indium (In), gallium (Ga), zinc (Zn), oxygen (O) and unavoidable impurities, wherein the IGZO sintered compact has a flexural strength of 50 MPa or more, a bulk resistance of 23 mcm or more and 100 mcm or less, and a density of 6.26 g/cm.sup.3 or more, and an atomic ratio of In, Ga, and Zn satisfies the following formulae:
0.314In/(In+Ga+Zn)0.342;
0.314Ga/(In+Ga+Zn)0.342; and
0.325Zn/(In+Ga+Zn)0.364.

2. The IGZO sintered compact according to claim 1, wherein the IGZO sintered compact has an average crystal grain size of 6 to 22 m.

3. A plate-shaped or cylindrical sputtering target formed from the IGZO sintered compact according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The FIGURE is a diagram showing a relation of the flexural strength and the balk resistance of an IGZO sintered compact.

DETAILED DESCRIPTION

(2) The oxide sintered compact of the present invention is composed of indium (In), gallium (Ga), zinc (Zn), oxygen (O) and unavoidable impurities, and has a flexural strength (also known as modulus of rupture, bending strength, and transverse rupture strength) of 50 MPa or more and a bulk resistance of 100 mcm or less. When the flexural strength is less than 50 MPa, cracks may be generated in the target during sputtering. When the bulk resistance exceeds 100 mcm, even though DC sputtering may be possible, abnormal discharge may arise during sputtering that is performed for a long period. In some cases, a discharge may not occur in DC sputtering, and thus there is no choice but to apply RF sputtering, alternatively.

(3) Moreover, in the present invention, the atomic ratio of In, Ga and Zn of the oxide sintered compact preferably satisfies the following formulae:
0.314In/(In+Ga+Zn)0.342;
0.314Ga/(In+Ga+Zn)0.342; and
0.325Zn/(In+Ga+Zn)0.364.

(4) By making the composition of an IGZO sintered compact to be Zn-rich from a (111) composition, it is possible to realize a bulk resistance which enables high mechanical strength and stable DC sputtering.

(5) Note that the respective component amounts may vary during blending, mixing and sintering the raw material powders. For example, in cases where the intended composition is In:Ga:Zn=1:1:1, variation may arise in a range of In:Ga:Zn=10.02:10.02:10.02, and thus it may be out of a Zn-rich state, but such fact shall not be grounds for denying the present invention.

(6) The oxide sintered compact of the present invention preferably has an average crystal grain size of 6 to 22 m. The mechanical strength can be increased by causing the average grain size to fall within the foregoing numerical range. When the average grain size exceeds 22 m, the mechanical strength will decrease and, when excessive power is input during sputtering, there is a possibility that cracks may arise in the sintered compact due to the stress caused by the difference in thermal expansion between the sputtering target (sintered compact) and the backing plate that is bonded with the target.

(7) Meanwhile, when the average grain size is less than 6 m, there is a possibility that the sintering will not advance sufficiently, and, with such insufficient sintering, a sufficient reaction is not realized between the respective raw materials, and the composition may become uneven, or numerous pores may become generated in the sintered compact. Furthermore, such compositional unevenness and the existence of pores will cause deterioration in the flexural strength of the sintered compact, and an increase in the variation of the flexural strength. Furthermore, the pores will cause arcing or particle generation during sputtering, and have an adverse effect on the film characteristics.

(8) Moreover, the oxide sintered compact of the present invention preferably has a density of 6.10 g/cm.sup.3 or more. When the oxide sintered compact of the present invention is used as a sputtering target, superior effects are yielded in that high densification of the sintered compact will increase the uniformity of the sputtered film, and particle generation during sputtering can be significantly reduced.

(9) A representative example of the production method of the oxide sintered compact of the present invention is as follows.

(10) As raw materials, indium oxide (In.sub.2O.sub.3), gallium oxide (Ga.sub.2O.sub.3), and zinc oxide (ZnO) are prepared. In order to avoid the adverse effects on the electrical properties caused by impurities, it is desirable to use raw materials having a purity of 4N or higher. The respective raw materials are weighed to achieve a predetermined compositional ratio. Note that these raw materials contain unavoidable impurities.

(11) Next, the respective raw materials are mixed so that the oxide sintered compact is to have a predetermined compositional ratio. If the mixing is insufficient, the respective components in the target may become segregated and cause an abnormal discharge such as arcing, or cause particle generation during the sputtering. Thus, the mixing is preferably performed sufficiently. Furthermore, by subjecting the mixed powder to pulverization and granulation, it is possible to improve the moldability and sinterability of the mixed powder, and thereby obtain a high-density sintered compact. As the means for performing the mixing and pulverization processes, for instance, a commercially available mixer, ball mill, beads mill or the like may be used, and as the means for the granulation process, for instance, a commercially available spray drier may be used.

(12) Next, the mixed powder is filled in a mold, and subject to uniaxial pressing under the following conditions; specifically, a surface pressure of 400 to 1000 kgf/cm.sup.2, and a holding time of 1 to 3 minutes, to obtain a green compact. When the surface pressure is less than 400 kgf/cm.sup.2, it is not possible to obtain a green compact having a sufficient density. Moreover, even if excessive surface pressure is applied, the density of the green compact hardly increases beyond a certain level, and a density distribution tends to become generated in the green compact in principle when being subject to uniaxial pressing, and causes deformation and cracks during sintering. Thus, a surface pressure of 1000 kgf/cm.sup.2 or more is not particularly required for production.

(13) Next, this green compact is subject to double vacuum packing in vinyl, and subject to CIP (cold isostatic pressing) under the following conditions; specifically, a pressure of 1500 to 4000 kgf/cm.sup.2, and a holding time of 1 to 3 minutes. When the pressure is less than 1500 kgf/cm.sup.2, it is not possible to obtain a sufficient effect of CIP. Meanwhile, even if pressure of 4000 kgf/cm.sup.2 or more is applied, the density of the green compact hardly increases beyond a certain level, and therefore, a surface pressure of 4000 kgf/cm.sup.2 or more is not particularly required for production.

(14) Next, the green compact is subject to sintering at a temperature of 1300 to 1430 C. and a holding time of 10 to 24 hours in an air atmosphere or an oxygen atmosphere to obtain a sintered compact. The sintering temperature of lower than 1300 C. is undesirable since oxygen becomes less eliminated from the sintered compact, the oxygen defect concentration will deteriorate, and the carrier concentration will deteriorate (that is, the bulk resistance will increase). Meanwhile, when the sintering temperature is 1430 C. or higher, the size of the crystal grains in the sintered compact becomes too large, and the mechanical strength of the sintered compact may deteriorate. Moreover, when the holding time is less than 10 hours, it is not possible to obtain a sintered compact having a sufficient density, and, when the holding time is longer than 24 hours, this is undesirable from the perspective of production cost.

(15) Moreover, in the molding/sintering processes, HP (hot pressing) and HIP (hot isostatic pressing) may be used other than the foregoing methods. The sintered compact obtained as described above is processed into a target shape via machining such as grinding and/or polishing to obtain a sputtering target. Note that, upon preparing an oxide semiconductor film, the sputtering target obtained as described above is sputtered under predetermined conditions to deposit a film, and, as needed, the deposited film is subject to annealing at a predetermined temperature to obtain an oxide semiconductor film.

(16) In the present invention, the flexural strength is measured based on a three-point bending test in accordance with JISR1601:2008 (titled Testing Method for Flexural Strength (Modulus of Rupture) of Fine Ceramics at Room Temperature). Specifically, the measurement is performed using ten samples having dimensions, in which the total length is 40 mm L 0.1 mm, the width is 4 mm0.1 mm, and the thickness is 3 mm0.1 mm, at a span between supports of 30 mm f 0.1 mm, and a crosshead speed of 0.5 mm/min, and the measurement results are averaged.

(17) For obtaining the average grain size, samples are extracted from a total of five locations; specifically, a part near the center and the four corners of a rectangular plate-shaped target. For each sample, a 300SEM image is produced by scanning an arbitrary cross-section surface of the target, five straight lines are drawn on the image to obtain code lengths by measuring the length in which each straight line intersects with the crystal grains, these code lengths are averaged, and a value obtained by multiplying the average value by a coefficient of 1.78 is used as the crystal grain size.

(18) Moreover, the sintered compact density and the bulk resistance are obtained, for each of the samples extracted from a total of five locations; specifically, a part near the center and the four corners of a rectangular plate-shaped target, by measuring the sintered compact density by the Archimedean method and the bulk resistance by the four probe method, and respectively calculating the average values by dividing the measurement results by the number of measurement locations.

EXAMPLES

(19) The present invention is now explained based on Examples and Comparative Examples. Note that the following Examples are merely illustrative, and the present invention is not limited to such Examples. In other words, the present invention is limited only based on the scope of claims, and the present invention also covers the other modes and modifications included therein.

Example 1

(20) An In.sub.2O.sub.3 powder, a Ga.sub.2O.sub.3 powder, and a ZnO powder were weighed so that the compositional ratio of the sintered compact becomes 1.00:1.00:1.01 in terms of an atomic ratio of In, Ga and Zn, and these powders were mixed and pulverized based on a wet process, and subsequently dried and granulated with a spray drier to obtain a mixed powder. Next, the mixed powder was subject to uniaxial pressing at a surface pressure of 400 to 1000 kgf/cm.sup.2 to obtain a green compact. Next, the obtained green compact was subject to double vacuum packing in vinyl, and CIP (cold isostatic pressing) at 1500 to 4000 kgf/cm.sup.2, and thereafter sintered in an oxygen atmosphere at a temperature of 1430 C. for 20 hours.

(21) The flexural strength of the thus obtained IGZO sintered compact was 55 MPa and the bulk resistance was 36.0 mcm, and it was possible to obtain an IGZO sintered compact having a high mechanical strength and a low resistance. Moreover, the average grain size of the sintered compact was 20.8 m and the density was 6.3 g/cm.sup.3, and a high-density IGZO sintered compact was obtained. The foregoing results are shown in Table 1.

(22) TABLE-US-00001 TABLE 1 Sintering Sintering Sintered compact Crystal Flexural Bulk temperature time composition (atomic ratio) grain size Density Strength density ( C.) (h) In Ga Zn (m) (g/cm.sup.3) (MPa) (mcm) Comparative Example 1 1430 20 1.00 1.00 1.00 35.9 6.33 33 35 Example 1 1430 20 1.00 1.00 1.01 20.8 6.32 55 36 Example 2 1430 20 1.00 1.00 1.02 18.3 6.33 57 36 Example 3 1430 20 1.00 1.00 1.05 14.0 6.26 63 27 Example 4 1430 20 1.00 1.00 1.10 9.9 6.33 87 23 Comparative Example 2 1430 5 1.00 1.00 1.02 5.5 6.01 38 40 Example 5 1350 10 1.00 1.00 1.00 14.4 6.34 60 82 Example 6 1350 10 1.00 1.00 1.02 10.7 6.34 77 60 Comparative Example 3 1300 20 1.00 1.00 1.00 18.4 6.34 54 118 Example 7 1300 20 1.00 1.00 1.01 11.1 6.34 80 92 Example 8 1300 20 1.00 1.00 1.02 10.9 6.35 86 83 Example 9 1300 20 1.00 1.00 1.05 8.5 6.35 152 58 Example 10 1300 20 1.00 1.00 1.10 6.2 6.32 167 47 Comparative Example 4 1250 20 1.00 1.00 1.00 11.6 6.32 85 203 Comparative Example 5 1250 20 1.00 1.00 1.01 8.5 6.32 162 123 Comparative Example 6 1250 20 1.00 1.00 1.02 7.6 6.30 144 139 Comparative Example 7 1250 20 1.00 1.00 1.05 5.9 6.20 140 131 Comparative Example 8 1250 20 1.00 1.00 1.10 5.3 6.09 110 143

Examples 2 to 4, Comparative Example 1

(23) An In.sub.2O.sub.3 powder, a Ga.sub.2O.sub.3 powder, and a ZnO powder were weighed so that the compositional ratio of the sintered compact becomes an atomic ratio of In, Ga and Zn indicated in Table 1, and these powders were mixed and pulverized based on a wet process, and subsequently dried and granulated with a spray drier to obtain a mixed powder. Next, the mixed powder was subject to uniaxial pressing at a surface pressure of 400 to 1000 kgf/cm.sup.2 to obtain a green compact. Next, the obtained green compact was subject to double vacuum packing in vinyl, and CIP at 1500 to 4000 kgf/cm.sup.2, and thereafter sintered in an oxygen atmosphere at a temperature of 1430 C. for 20 hours.

(24) The IGZO sintered compacts obtained under the conditions of Examples 2 to 4 all had a flexural strength of 50 MPa or more and a bulk resistance of 100 mcm or less, and it was possible to obtain IGZO sintered compacts having a high mechanical strength and a low resistance. Moreover, the average grain size of the sintered compacts was 22 m or less and the density was 6.10 g/cm.sup.3 or more, and high-density IGZO sintered compacts were obtained. Meanwhile, the IGZO sintered compact obtained under the conditions of Comparative Example 1 had a low bulk resistance, but exhibited a low flexural strength value of 33 MPa.

Examples 5 and 6

(25) An In.sub.2O.sub.3 powder, a Ga.sub.2O.sub.3 powder, and a ZnO powder were weighed so that the compositional ratio of the sintered compact becomes an atomic ratio of In, Ga and Zn indicated in Table 1, and these powders were mixed and pulverized based on a wet process, and subsequently dried and granulated with a spray drier to obtain a mixed powder. Next, the mixed powder was subject to uniaxial pressing at a surface pressure of 400 to 1000 kgf/cm.sup.2 to obtain a green compact. Next, the obtained green compact was subject to double vacuum packing in vinyl, and CIP at 1500 to 4000 kgf/cm.sup.2, and thereafter sintered in an oxygen atmosphere at a temperature of 1350 C. for 10 hours.

(26) The IGZO sintered compacts obtained under the conditions of Examples 5 and 6 all had a flexural strength of 50 MPa or more and a bulk resistance of 100 mcm or less, and it was possible to obtain IGZO sintered compacts having a high mechanical strength and a low resistance. Moreover, the average grain size of the sintered compacts was 22 m or less and the density was 6.10 g/cm.sup.3 or more, and high-density IGZO sintered compacts were obtained.

Examples 7 to 10, Comparative Example 3

(27) An In.sub.2O.sub.3 powder, a Ga.sub.2O.sub.3 powder, and a ZnO powder were weighed so that the compositional ratio of the sintered compact becomes an atomic ratio of In, Ga and Zn indicated in Table 1, and these powders were mixed and pulverized based on a wet process, and subsequently dried and granulated with a spray drier to obtain a mixed powder. Next, the mixed powder was subject to uniaxial pressing at a surface pressure of 400 to 1000 kgf/cm.sup.2 to obtain a green compact. Next, the obtained green compact was subject to double vacuum packing in vinyl, and CIP at 1500 to 4000 kgf/cm.sup.2, and thereafter sintered in an oxygen atmosphere at a temperature of 1300 C. for 20 hours.

(28) The IGZO sintered compacts obtained under the conditions of Examples 7 to 10 all had a flexural strength of 50 MPa or more and a bulk resistance of 100 mcm or less, and it was possible to obtain IGZO sintered compacts having a high mechanical strength and a low resistance. Moreover, the average grain size of the sintered compacts was 22 m or less and the density was 6.10 g/cm.sup.3 or more, and high-density IGZO sintered compacts were obtained. Meanwhile, the IGZO sintered compact obtained under the conditions of Comparative Example 3 had a high transverse intensity, but exhibited a high bulk resistance value in excess of 100 mcm.

Comparative Examples 4 to 8

(29) An In.sub.2O.sub.3 powder, a Ga.sub.2O.sub.3 powder, and a ZnO powder were weighed so that the compositional ratio of the sintered compact becomes an atomic ratio of In, Ga and Zn indicated in Table 1, and these powders were mixed and pulverized based on a wet process, and subsequently dried and granulated with a spray drier to obtain a mixed powder. Next, the mixed powder was subject to uniaxial pressing at a surface pressure of 400 to 1000 kgf/cm.sup.2 to obtain a green compact. Next, the obtained green compact was subject to double vacuum packing in vinyl, and CIP at 1500 to 4000 kgf/cm.sup.2, and thereafter sintered in an oxygen atmosphere at a temperature of 1250 C. for 20 hours. The IGZO sintered compacts obtained under the conditions of Comparative Examples 4 to 8 all had a high flexural strength, but exhibited a high bulk resistance value in excess of 100 mcm. Moreover, while the sintered compacts of Comparative Examples 7 and 8 had a small crystal grain size, numerous pores were observed in the sintered compacts.

Comparative Example 2

(30) An In.sub.2O.sub.3 powder, a Ga.sub.2O.sub.3 powder, and a ZnO powder were weighed so that the compositional ratio of the sintered compact becomes an atomic ratio of In, Ga and Zn indicated in Table 1, and these powders were mixed and pulverized based on a wet process, and subsequently dried and granulated with a spray drier to obtain a mixed powder. Next, the mixed powder was subject to uniaxial pressing at a surface pressure of 400 to 1000 kgf/cm.sup.2 to obtain a green compact. Next, the obtained green compact was subject to double vacuum packing in vinyl, and CIP at 1500 to 4000 kgf/cm.sup.2, and thereafter sintered in an oxygen atmosphere at a temperature of 1430 C. for 5 hours. While the obtained IGZO sintered compact had a small crystal grain size, there were numerous pores in the sintered compact, and, when used as a target, there was concern of the generation of arcing or particles during sputtering.

(31) The relation of flexural strength and bulk resistance of the IGZO sintered compacts in the foregoing Examples and Comparative Examples is shown in the FIGURE. By appropriately adjusting the sintered compact composition and the sintering temperature, it was possible to prepare sintered compacts having a flexural strength of 50 MPa or more and a balk resistance of 100 mcm or less as shown in the FIGURE.

(32) The oxide sintered compact of the present invention can be used to form a sputtering target capable of realizing both a high flexural strength and a low bulk resistance. This target is free from cracks and has limited generation of particles when subject to DC sputtering, and thus it is possible to form high quality thin films. By using this kind of sputtering target, it is possible to yield a superior effect of being able to stably mass-produce oxide semiconductor films and the like. The oxide semiconductor film of the present invention is particularly useful as an active layer of TFT in a backplane of a flat-panel display or a flexible panel display.