OXIDE SINTERED BODY, SPUTTERING TARGET, AND OXIDE SEMICONDUCTOR THIN FILM OBTAINED USING SPUTTERING TARGET

Abstract

Provided are: a sintered oxide which achieves low carrier density and high carrier mobility when configured as an oxide semiconductor thin-film by using the sputtering method; and a sputtering target using the same. This sintered oxide contains indium, gallium and magnesium as oxides. It is preferable for the gallium content to be at least 0.08 and less than 0.20, inclusive, in terms of an atomic ratio (Ga/(In+Ga)), the magnesium content to be at least 0.0001 and less than 0.05 in terms of an atomic ratio (Mg/(In+Ga+Mg)), and the sintering to occur at 1,200-1,550 C., inclusive. An amorphous oxide semiconductor thin-film obtained by forming this sintered oxide as a sputtering target is capable of achieving a carrier density of less than 1.010.sup.18 cm.sup.3, and a carrier mobility of 10 cm.sup.2 V.sup.1 sec.sup.1 or higher.

Claims

1. An oxide sintered body comprising indium, gallium, and magnesium as oxides, wherein a gallium content is 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio, a magnesium content is 0.0001 or more and less than 0.05 in terms of Mg/(In+Ga+Mg) atomic ratio, and the oxide sintered body comprises; an In.sub.2O.sub.3 phase having a bixbyite-type structure; and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure as a formed phase other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase; and is substantially free of an In(GaMg)O.sub.4 phase, an MgGa.sub.2O.sub.4 phase, an In.sub.2MgO.sub.4 phase, and a Ga.sub.2O.sub.3 phase.

2. The oxide sintered body according to claim 1, wherein the magnesium content is 0.01 or more and 0.03 or less in terms of Mg/(In+Ga+Mg) atomic ratio.

3. The oxide sintered body according to claim 1, wherein the gallium content is 0.08 or more and 0.15 or less in terms of Ga/(In+Ga) atomic ratio.

4. The oxide sintered body according to claim 1, wherein the oxide sintered body is substantially free of positive divalent elements other than magnesium and positive trivalent to positive hexavalent elements other than indium and gallium.

5. The oxide sintered body according to claim 1, wherein an X-ray diffraction peak intensity ratio of the GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure defined by formula 1 below is in the range of 2% or more and 45% or less: 100I [GaInO.sub.3 phase (111)]/{I [In.sub.2O.sub.3 phase (400)]+I [GaInO.sub.3 phase (111)]}[%] . . . Formula 1 (in Formula 1, I [In.sub.2O.sub.3 phase (400)] represents a (400) peak intensity of the In.sub.2O.sub.3 phase having a bixbyite-type structure, and I [GaInO.sub.3 phase (111)] represents a (111) peak intensity of the -GaInO.sub.3 phase that is a composite oxide having a -Ga.sub.2O.sub.3-type structure.)

6. A sputtering target obtained by machining the oxide sintered body according to claim 1.

7. A crystalline oxide semiconductor thin film obtained by forming an amorphous film on a substrate by sputtering using the sputtering target according to claim 6 and crystallizing the amorphous film by heating in an oxidizing atmosphere.

8. The oxide semiconductor thin film according to claim 7, wherein the oxide semiconductor thin film has a carrier mobility of 10 cm.sup.2 V.sup.1 sec.sup.1 or more.

9. The oxide semiconductor thin film according to claim 7, wherein the oxide semiconductor thin film has a carrier density of less than 1.010.sup.18 cm.sup.3.

Description

PREFERRED MODE FOR CARRYING OUT THE INVENTION

[0031] An oxide sintered body, a sputtering target, and a semiconductor thin film obtained by using the target in the present invention will be described below in detail.

[0032] An oxide sintered body of the present invention contains indium, gallium, and magnesium as oxides. In the oxide sintered body, the gallium content is 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio, and the magnesium content is 0.0001 or more and less than 0.05 in terms of Mg/(In+Ga+Mg) atomic ratio.

[0033] The gallium content, in terms of Ga/(In+Ga) atomic ratio, is 0.08 or more and less than 0.20 and more preferably 0.08 or more and 0.15 or less. Gallium has an effect of reducing the amount of oxygen defects in the crystalline oxide semiconductor thin film of the present invention because gallium has high bonding strength to oxygen. When the gallium content is less than 0.08 in terms of Ga/(In+Ga) atomic ratio, this effect is not sufficiently obtained. On the other hand, when the gallium content is 0.20 or more, the crystallization temperature is too high and thus the oxide sintered body is required to be heated at a high temperature. Therefore, a carrier density required for an oxide semiconductor thin film is not achieved or it is impossible to enhance the crystallinity, and thus it is impossible to obtain sufficiently high carrier mobility.

[0034] The oxide sintered body of the present invention contains magnesium in addition to indium and gallium, which are in the composition ranges defined as described above. Magnesium concentration is 0.0001 or more and less than 0.05 and preferably 0.01 or more and 0.03 or less in terms of Mg/(In+Ga+Mg) atomic ratio.

[0035] Doping the oxide sintered body of the present invention with magnesium in this range reduces the carrier density because magnesium doping has an effect of neutralizing electrons generated mainly by oxygen defects. When the crystalline oxide semiconductor thin film of the present invention is used in TFTs, the on/off ratio of TFTs can be increased.

[0036] It is preferred that the oxide sintered body of the present invention be substantially free of M, which are positive divalent elements other than magnesium and positive trivalent to positive hexavalent elements other than indium and gallium. The term substantially free of M as used herein means that the content of each M, in terms of M/(In+Ga+M) atomic ratio, is 500 ppm or less, preferably 200 ppm or less, and more preferably 100 ppm or less. Specific examples of the element M include positive divalent elements, such as Cu, Ni, Co, Zn, Ca, Sr, and Pb; positive trivalent elements, such as Al, Y, Sc, B, and lanthanoids; positive tetravalent elements, such as Sn, Ge, Ti, Si, Zr, Hf, C, and Ce; positive pentavalent elements, such as Nb and Ta; and positive hexavalent elements, such as W and Mo.

1. Structure of Oxide Sintered Body

[0037] The oxide sintered body of the present invention is composed of an In.sub.2O.sub.3 phase having a bixbyite-type structure and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure as a formed phase other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase. Nodules are generated when the oxide sintered body is composed only of an In.sub.2O.sub.3 phase, for example, in the same manner as in Comparative Example 11 of Patent Document 4 (PCT International Publication No. WO2003/014409) regardless of the inclusion of Mg. On the other hand, an In(GaMg)O.sub.4 phase, an MgGa.sub.2O.sub.4 phase, and an In.sub.2MgO.sub.4 phase are all phases having a high resistance so as to be a cause of arcing or nodules. The In.sub.2MgO.sub.4 phase has a specific resistance of about 10.sup.2 .Math.cm (Non-Patent Document 1) so as to have an electrical resistance higher by one to two orders than that of the In.sub.2O.sub.3 phase or the GaInO.sub.3 phase, and thus nodules are likely to be generated as the In.sub.2MgO.sub.4 phase is likely to remain by sputter deposition. The In(GaMg)O.sub.4 phase has a higher specific resistance of about 10.sup.0 .Math.cm (Non-Patent Document 2) so as to be a cause of the nodule generation. The MgGa.sub.2O.sub.4 phase has an even higher specific resistance since it does not contain In so as to be a cause of arcing. In addition, an oxide semiconductor thin film obtained by sputter deposition using an oxide sintered body in which these phases are formed includes an In.sub.2O.sub.3 phase exhibiting low crystallinity and the carrier mobility thereof tends to decrease.

[0038] Gallium and magnesium are dissolved in the In.sub.2O.sub.3 phase. In addition, gallium makes up the GaInO.sub.3 phase or the (Ga, In).sub.2O.sub.3 phase. When gallium and magnesium are dissolved in the In.sub.2O.sub.3 phase, gallium and magnesium substitute for indium, which is a trivalent cation, at the lattice positions. It is not preferred that gallium be less dissolved in the In.sub.2O.sub.3 phase because of unsuccessful sintering or the like, and as a result, a Ga.sub.2O.sub.3 phase having a -Ga.sub.2O.sub.3-type structure is formed. Since the Ga.sub.2O.sub.3 phase has low conductivity, abnormal discharge arises.

[0039] The oxide sintered body of the present invention preferably includes only a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure other than the In.sub.2O.sub.3 phase having a bixbyite-type structure, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase other than the In.sub.2O.sub.3 phase having a bixbyite-type structure in the range in which the X-ray diffraction peak intensity ratio thereof defined by formula 1 below is 2% or more and 45% or less.

100I[GaInO.sub.3 phase(111)]/{I[In.sub.2O.sub.3 phase(400)]+I [GaInO.sub.3 phase(111)]}[%]Formula 1

(in Formula 1, I [In.sub.2O.sub.3 phase (400)] represents a (400) peak intensity of the In.sub.2O.sub.3 phase having a bixbyite-type structure, and I [GaInO.sub.3 phase (111)] represents a (111) peak intensity of the -GaInO.sub.3 phase that is a composite oxide having a -Ga.sub.2O.sub.3-type structure.)

2. Method for Producing Oxide Sintered Body

[0040] To produce the oxide sintered body of the present invention, a magnesium oxide powder and an oxide powder composed of an indium oxide powder and a gallium oxide powder are used as raw material powders.

[0041] In the process for producing the oxide sintered body of the present invention, these raw material powders are mixed and then compacted, and the compact is sintered by ordinary-pressure sintering. The formed phases in the structure of the oxide sintered body of the present invention strongly depend on the conditions in each step for producing the oxide sintered body, for example, the particle size of the raw material powders, the mixing conditions, and the sintering conditions.

[0042] The structure of the oxide sintered body of the present invention is preferably composed of; an In.sub.2O.sub.3 phase having a bixbyite-type structure; and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure as a formed phase other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase in a desired ratio, and for this, the mean particle size of each raw material powder above is preferably 3 m or less and more preferably 1.5 m or less. As described above, the oxide sintered body includes, in addition to the In.sub.2O.sub.3 phase, the GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure or the GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and the (Ga, In).sub.2O.sub.3 phase, and thus the mean particle size of each raw material powder is preferably 1.5 m or less in order to suppress excessive formation of these phases.

[0043] Indium oxide powder is a raw material for ITO (indium-tin oxide), and fine indium oxide powder having good sintering properties has been developed along with improvements in ITO. Since indium oxide powder has been continuously used in large quantities as a raw material for ITO, raw material powder having a mean particle size of 0.8 m or less is available these days.

[0044] However, since the amount of gallium oxide powder or magnesium oxide powder used is still smaller than that of indium oxide powder used, it is difficult to obtain raw material powder having a mean particle size of 1.5 m or less for gallium oxide powder or magnesium oxide powder. Therefore, when only coarse gallium oxide powder is available, the powder needs to be pulverized into particles having a mean particle size of 1.5 m or less.

[0045] In the process for sintering the oxide sintered body of the present invention, ordinary-pressure sintering is preferably employed. Ordinary-pressure sintering is a simple and industrially advantageous method, and is also an economically preferable means.

[0046] When ordinary-pressure sintering is used, a compact is first produced as described above. Raw material powders are placed in a resin pot and mixed with a binder (for example, PVA) and the like by wet ball milling or the like. In the production of the oxide sintered body of the present invention, the ball mill mixing is preferably performed for 18 hours or longer in order to suppress excessive formation of the GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure, or the GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and the (Ga, In).sub.2O.sub.3 phase other than the In.sub.2O.sub.3 phase or to avoid formation of the Ga.sub.2O.sub.3 phase having a -Ga.sub.2O.sub.3-type structure. In this case, hard ZrO.sub.2 balls are used as mixing balls. After mixing, the slurry is taken out, filtered, dried, and granulated. Subsequently, the resultant granulated material is compacted under a pressure of about 9.8 MPa (0.1 ton/cm.sup.2) to 294 MPa (3 ton/cm.sup.2) by cold isostatic pressing to form a compact.

[0047] The sintering process by ordinary-pressure sintering is preferably preformed in an atmosphere containing oxygen. The volume fraction of oxygen in the atmosphere is preferably over 20%. In particular, when the volume fraction of oxygen is over 20%, the oxide sintered body is further densified. An excessive amount of oxygen in the atmosphere causes the surface of the compact to undergo sintering in advance during the early stage of sintering. Subsequently, sintering proceeds while the inside of the compact is reduced, and a highly dense oxide sintered body is finally obtained.

[0048] In an atmosphere free of oxygen, the surface of the compact does not undergo sintering and as a result, densification of the sintered body does not proceed. If oxygen is free, indium oxide decomposes particularly at about 900 C. to 1000 C. to form metal indium, which makes it difficult to obtain a desired oxide sintered body.

[0049] The temperature range of ordinary-pressure sintering is preferably 1200 C. or higher and 1550 C. or lower, and more preferably sintering is performed at 1350 C. or higher and 1450 C. or lower in an atmosphere obtained by introducing oxygen gas into air in a sintering furnace. The sintering time is preferably 10 to 30 hours, and more preferably 15 to 25 hours.

[0050] When the sintering temperature is in the above range, and the oxide powder composed of the indium oxide powder and gallium oxide powder, and the magnesium oxide powder that are controlled to have a mean particle size of 1.5 m or less are used as raw material powders, the oxide sintered body that is composed of; an In.sub.2O.sub.3 phase having a bixbyite-type structure; and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase is obtained.

[0051] At a sintering temperature lower than 1200 C., the sintering reaction does not sufficiently proceed and a disadvantage is caused that the density of the oxide sintered body is less than 6.4 g/cm.sup.3. On the other hand, the formation of the (Ga, In).sub.2O.sub.3 phase is remarkable when the sintering temperature is higher than 1550 C. The (Ga, In).sub.2O.sub.3 phase has a higher electrical resistance than the GaInO.sub.3 phase so as to cause a decrease in deposition rate. There is no problem when the sintering temperature is 1550 C. or lower, namely, the (Ga, In).sub.2O.sub.3 phase is formed in a small amount. From this viewpoint, the sintering temperature is preferably 1200 C. or higher and 1550 C. or lower and more preferably 1350 C. or higher and 1450 C. or lower.

[0052] The temperature elevation rate until the sintering temperature is reached is preferably in the range of 0.2 to 5 C./min in order to cause debinding without forming cracks in the sintered body. As long as the temperature elevation rate is this range, the temperature may be increased to the sintering temperature in a combination of different temperature elevation rates as desired. During the temperature elevation process, a particular temperature may be maintained for a certain time in order for debinding and sintering to proceed. In particular, it is effective to retain a temperature of 1100 C. or lower for a certain time in order to promote the dissolution of magnesium into the In.sub.2O.sub.3 phase. The retention time is not particularly limited, but it is preferably 1 hour or longer and 10 hours or shorter. After sintering, oxygen introduction is stopped before cooling. The temperature is preferably decreased to 1000 C. at a temperature drop rate in the range of 0.2 to 5 C./min, and particularly 0.2 C./min or more and less than 1 C./min.

3. Target

[0053] The target of the present invention is obtained by cutting the above oxide sintered body into a predetermined size, grinding the surface thereof, and bonding the oxide sintered body to a backing plate. The target preferably has a flat shape, but may have a cylindrical shape. When a cylindrical target is used, it is preferred to suppress particle generation due to target rotation.

[0054] In order to use it as a sputtering target, the density of the oxide sintered body of the present invention is preferably 6.4 g/cm.sup.3 or more, and it is preferably 6.8 g/cm.sup.2 or more when the gallium content is 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio. It is not preferred that the density be less than 6.4 g/cm.sup.3 since nodules are generated during use in mass production.

4. Oxide Semiconductor Thin Film and Method for Depositing Oxide Semiconductor Thin Film

[0055] The crystalline oxide semiconductor thin film of the present invention is obtained as follows: once forming an amorphous thin film on a substrate by sputtering using the sputtering target; and heating the amorphous oxide thin film.

[0056] The sputtering target is formed from the oxide sintered body. The structure of the oxide sintered body, namely, the structure composed basically of an In.sub.2O.sub.3 phase having a bixbyite-type structure and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure, is important. In order to obtain the crystalline oxide semiconductor thin film of the present invention, it is important that the crystallization temperature of the amorphous oxide thin film that is once formed is sufficiently high, which is related to the structure of the oxide sintered body. That is, when the oxide sintered body includes not only an In.sub.2O.sub.3 phase having a bixbyite-type structure but also a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure as in the oxide sintered body used in the present invention, the oxide thin film obtained from this oxide sintered body after film deposition has a high crystallization temperature, namely a crystallization temperature of preferably 250 C. or higher, more preferably 300 C. or higher, and even more preferably 350 C. or higher. That is, the oxide thin film is a stable amorphous film. In contrast, when the oxide sintered body is composed only of an In.sub.2O.sub.3 phase having a bixbyite-type structure, for example, as disclosed in Patent Document 2 (PCT International Publication No. WO2010/032422), the oxide thin film obtained after film deposition has a low crystallization temperature of about from 190 to 230 C. and a complete amorphous film is not formed in some cases. This is because microcrystals are already generated after film deposition and thus patterning by wet etching is difficult due to the generation of residues in this case.

[0057] Ordinary sputtering is used in the process for forming the amorphous thin film. In particular, direct current (DC) sputtering is industrially advantageous because the thermal effects are minimized during film deposition and high-rate deposition is achieved. To form the oxide semiconductor thin film of the present invention by direct current sputtering, a gas mixture of an inert gas and oxygen, particularly a gas mixture of argon and oxygen, is preferably used as a sputtering gas. Sputtering is preferably performed in a chamber of a sputtering apparatus at an internal pressure of 0.1 to 1 Pa, particularly 0.2 to 0.8 Pa.

[0058] The substrate is typically a glass substrate and is preferably an alkali-free glass substrate. In addition, any resin sheet and resin film that withstands the temperature for the above process can be used.

[0059] In the process for forming the amorphous thin film, presputtering can be performed as follows: for example, after evacuation to 210.sup.4 Pa or less, introducing a gas mixture of argon and oxygen until the gas pressure reaches 0.2 to 0.5 Pa; and generating a direct current plasma by applying direct current power so that the direct current power with respect to the area of the target, namely, the direct current power density, is in the range of about 1 to 7 W/cm.sup.2. It is preferred that, after this presputtering for 5 to 30 minutes, the substrate position be corrected as desired and then sputter deposition be performed. In sputter deposition, the direct current power to be applied in an acceptable range is increased in order to increase the deposition rate.

[0060] The crystalline oxide semiconductor thin film of the present invention is obtained by forming the amorphous thin film and crystalizing the amorphous thin film by heating. The conditions for heat treatment are a temperature of the crystallization temperature or higher and an oxidizing atmosphere. An atmosphere containing oxygen, ozone, water vapor, nitrogen oxides, or the like is preferable as the oxidizing atmosphere. The temperature for heat treatment is preferably 250 to 600 C., more preferably 300 to 550 C., and even more preferably 350 to 500 C. The time for heat treatment, namely, the time during which the temperature for heat treatment is maintained, is preferably 1 to 120 minutes and more preferably 5 to 60 minutes. The method until the oxide sintered body is crystallized involves: for example, once forming an amorphous film at a low temperature, for example, near room temperature or at a substrate temperature of from 100 to 300 C., and then crystalizing the oxide thin film by performing heating at the crystallization temperature or higher, or heating a substrate to the crystallization temperature of the oxide thin film or higher to form a crystalline oxide thin film. The heating temperature in these two methods is only about 600 C. or lower, and there is no significant difference in treatment temperature as compared to a known semiconductor process described, for example, in Patent Document 5 (Japanese Unexamined Patent Application, Publication No. 2012-253372).

[0061] The composition of indium, gallium, and magnesium in the amorphous thin film and the crystalline oxide semiconductor thin film substantially corresponds to the composition of the oxide sintered body of the present invention. That is, the crystalline oxide semiconductor thin film contains indium and gallium as oxides and further contains magnesium. The gallium content is 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio. The magnesium content is 0.0001 or more and less than 0.05 in terms of Mg/(In+Ga+Mg) atomic ratio. The gallium content is more preferably 0.08 or more and 0.15 or less in terms of Ga/(In+Ga) atomic ratio. In addition, the magnesium content is more preferably 0.01 or more and 0.03 or less in terms of Mg/(In+Ga+Mg) atomic ratio.

[0062] The crystalline oxide semiconductor thin film of the present invention is preferably composed only of an In.sub.2O.sub.3 phase having a bixbyite structure. In the In.sub.2O.sub.3 phase, gallium is dissolved to substitute for indium, which is a trivalent cation, at the lattice positions, and magnesium is dissolved to substitute, as in the oxide sintered body. In the oxide semiconductor thin film of the present invention, the carrier density decreases to less than 1.010.sup.18 cm.sup.3 because magnesium doping has an effect of neutralizing carrier electrons generated mainly by oxygen defects, and more preferably a carrier density of 3.010.sup.17 cm.sup.3 or less is obtained. It is known that an oxide semiconductor thin film containing indium in a great amount is in a degenerate state at the carrier density of 4.010.sup.18 cm.sup.3 or more, and a TFT including such an amorphous oxide semiconductor thin film as a channel layer thus does not exhibit normally-off characteristics. Therefore, the crystalline oxide semiconductor thin film according to the present invention is advantageous in that the carrier density is controlled so that the TFT exhibits normally-off characteristics. On the other hand, the carrier mobility tends to decrease together with a decrease in carrier density, but the carrier mobility is preferably 10 cm.sup.2 V.sup.1 sec.sup.1 or more and more preferably 15 cm.sup.2 V.sup.1 sec.sup.1 or more.

[0063] The crystalline oxide semiconductor thin film of the present invention is subjected to micromachining, which is required in applications such as TFTs, by wet etching or dry etching. When an amorphous film is once formed at a low temperature, and then the oxide thin film is crystallized by heating at a temperature of the crystallization temperature or higher, micromachining can be performed by wet etching using a weak acid after the amorphous film formation. Most weak acids can be used, but a weak acid composed mainly of oxalic acid is preferably used. For example, ITO-06N available from Kanto Chemical Co., Inc., or the like can be used. When the crystalline oxide thin film is formed by heating the substrate at the crystallization temperature of the oxide thin film or higher, for example, wet etching or dry etching with a strong acid such as aqueous ferric chloride can be employed, and dry etching is preferably employed from the standpoint of damage to the surrounding area of a TFT.

[0064] Although the thickness of the crystalline oxide semiconductor thin film of the present invention is not limited, the thickness is 10 to 500 nm, preferably 20 to 300 nm, and more preferably 30 to 100 nm. When the thickness is less than 10 nm, unfavorable crystallinity is obtained, and as a result, high carrier mobility is not achieved. Meanwhile, when the thickness is more than 500 nm, it is disadvantageous in that a problem associated with productivity arises.

[0065] The mean transmittance of the crystalline oxide semiconductor thin film of the present invention in the visible range (400 to 800 nm) is preferably 80% or more, more preferably 85% or more, and still more preferably 90% or more. When the crystalline oxide semiconductor thin film of the present invention is used in a transparent TFT and the mean transmittance is less than 80%, the light extraction efficiency of, for example, liquid crystal elements and organic EL elements as transparent display devices decreases.

EXAMPLES

[0066] A more detailed description is provided below by way of Examples of the present invention, but the present invention is not limited by these Examples.

<Evaluation of Oxide Sintered Body>

[0067] The composition of metal elements in the obtained oxide sintered body was determined by ICP emission spectrometry. The formed phases were identified by a powder method with an X-ray diffractometer (available from Philips) using rejects of the obtained oxide sintered body.

<Evaluation of Basic Properties of Oxide Thin Film>

[0068] The composition of the obtained oxide thin film was determined by ICP emission spectrometry. The thickness of the oxide thin film was determined with a surface profilometer (available from KLA-Tencor Corporation). The deposition rate was calculated from the film thickness and the film deposition time. The carrier density and carrier mobility of the oxide thin film were determined with a Hall-effect measurement apparatus (available from TOYO Corporation). The formed phases in the film were identified by X-ray diffraction measurement.

(Production and Evaluation of Sintered Body)

[0069] An indium oxide powder, a gallium oxide powder, and a magnesium oxide powder were prepared as raw material powders so that each powder has a mean particle size of 1.5 m or less. These raw material powders were prepared so as to obtain the Ga/(In+Ga) atomic ratio and the Mg/(In+Ga+Mg) atomic ratio of Examples and Comparative Examples shown in Table 1 and Table 2. The raw material powders were placed in a resin pot together with water and mixed by wet ball milling. At this time hard ZrO.sub.2 balls were used, and the mixing time was 18 hours. After mixing, the slurry was taken out, filtered, dried, and granulated. The granulated material was compacted by cold isostatic pressing under a pressure of 3 ton/cm.sup.2.

[0070] Next, the compact was sintered as described below. The compact was sintered at a sintering temperature of between 1000 and 1550 C. for 20 hours in an atmosphere obtained by introducing oxygen into air in a sintering furnace at a rate of 5 L/min per 0.1 m.sup.3 furnace volume. At this time, the temperature was increased by 1 C./min, oxygen introduction was stopped during cooling after sintering, and the temperature was decreased to 1000 C. by 10 C./min.

[0071] The composition of the obtained oxide sintered body was analyzed by ICP emission spectrometry. As a result, it was confirmed that the composition of the metal elements substantially corresponds to the composition prepared at the time of mixing raw material powders in all Examples.

[0072] Next, the phases of the oxide sintered body were identified by X-ray diffraction measurement, and as a result, only the diffraction peak attributed to an In.sub.2O.sub.3 phase having a bixbyite-type structure or only the diffraction peaks attributed to an In.sub.2O.sub.3 phase having a bixbyite-type structure, a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure, and a (Ga, In).sub.2O.sub.3 phase were confirmed as shown in Table 1 and Table 2.

[0073] Incidentally, in the case of including a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure, the X-ray diffraction peak intensity ratio of the GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure defined by Formula 1 below is shown in Table 1 and Table 2.

100I[GaInO.sub.3 phase(111)]/{I[In.sub.2O.sub.3 phase(400)]+I [GaInO.sub.3 phase(111)]}[%]Formula 1

TABLE-US-00001 TABLE 1 Density Ga/ Mg/(In + Sintering of GaInO.sub.3 (In + Ga) Ga + Mg) temper- sintered (111) Peak Atomic Atomic ature body intensity Structure of ratio ratio ( C.) (g/cm.sup.3) ratio sintered body Comparative 0.05 0.01 1400 7.05 Only In.sub.2O.sub.3 Example 1 Comparative 0.08 0 1450 7.00 Only In.sub.2O.sub.3 Example 2 Comparative 0.08 0.00001 1450 7.01 Only In.sub.2O.sub.3 Example 3 Example 1 0.08 0.0001 1450 7.00 2 In.sub.2O.sub.3/GaInO.sub.3 Example 2 0.08 0.01 1400 7.01 5 In.sub.2O.sub.3/GaInO.sub.3 Example 3 0.08 0.02 1350 6.88 5 In.sub.2O.sub.3/GaInO.sub.3 Example 4 0.08 0.03 1400 6.92 5 In.sub.2O.sub.3/GaInO.sub.3 Example 5 0.08 0.04 1400 6.83 6 In.sub.2O.sub.3/GaInO.sub.3 Comparative 0.08 0.05 1400 6.44 In.sub.2O.sub.3/In(GaMg)O.sub.4 Example 4 Example 6 0.10 0.0001 1400 7.01 9 In.sub.2O.sub.3/GaInO.sub.3 Example 7 0.10 0.03 1400 6.89 12 In.sub.2O.sub.3/GaInO.sub.3 Example 8 0.12 0.0001 1400 6.98 12 In.sub.2O.sub.3/GaInO.sub.3 Example 9 0.12 0.01 1400 7.00 16 In.sub.2O.sub.3/GaInO.sub.3 Example 10 0.15 0.0001 1400 6.94 21 In.sub.2O.sub.3/GaInO.sub.3 Example 11 0.15 0.02 1400 6.90 26 In.sub.2O.sub.3/GaInO.sub.3

TABLE-US-00002 TABLE 2 Density Ga/ Mg/(In + Sintering of GaInO.sub.3 (In + Ga) Ga + Mg) temper- sintered (111) Peak Atomic Atomic ature body intensity Structure of ratio ratio ( C.) (g/cm.sup.3) ratio sintered body Example 12 0.15 0.03 1400 6.83 29 In.sub.2O.sub.3/GaInO.sub.3 Comparative 0.15 0.05 1400 6.39 In.sub.2O.sub.3/In(GaMg)O.sub.4 Example 5 Example 13 0.19 0.0001 1400 6.89 31 In.sub.2O.sub.3/GaInO.sub.3 Example 14 0.19 0.01 1400 6.84 36 In.sub.2O.sub.3/GaInO.sub.3 Example 15 0.19 0.01 1200 6.74 31 In.sub.2O.sub.3/GaInO.sub.3 Example 16 0.19 0.03 1400 6.80 45 In.sub.2O.sub.3/GaInO.sub.3 Example 17 0.19 0.03 1550 6.77 29 In.sub.2O.sub.3/GaInO.sub.3 Comparative 0.30 0.05 1400 6.37 In.sub.2O.sub.3/In(GaMg)O.sub.4/ Example 6 MgGa.sub.2O.sub.4

[0074] The oxide sintered body was machined to a size of 152 mm in diameter and 5 mm in thickness. The sputtering surface was ground with a cup grinding wheel so that the maximum height Rz was 3.0 m or less. The machined oxide sintered body was bonded to an oxygen-free copper backing plate by using metal indium to provide a sputtering target.

(Evaluation on Sputter Deposition)

[0075] Film deposition by direct current sputtering was performed at room temperature without heating the substrate by using the sputtering targets of Examples and Comparative Examples and an alkali-free glass substrate (Eagle XG manufactured by Corning Incorporated). The sputtering target was attached to a cathode of a direct current magnetron sputtering apparatus (available from Tokki Corporation) having a direct current power supply with no arcing control function. At this time, the target-substrate (holder) distance was fixed at 60 mm. After evacuation to 110.sup.4 Pa or less, a gas mixture of argon and oxygen was introduced at an appropriate oxygen ratio, which depends on the gallium content in each target. The gas pressure was controlled to 0.6 Pa. A direct current plasma was generated by applying a direct current power of 300 W (1.64 W/cm.sup.2). After presputtering for 10 minutes, the substrate was placed directly above the sputtering target, namely, in the stationary opposing position, and an oxide thin film having a thickness of 50 nm was deposited. The composition of the obtained oxide thin film was confirmed to be substantially the same as that of the target. As a result of X-ray diffraction measurement, the oxide thin film was confirmed to be amorphous. The obtained amorphous oxide thin film was subjected to a heat treatment at 300 to 700 C. within 30 minutes in air by using a RTA (Rapid Thermal Annealing) apparatus. It was confirmed that the oxide thin film after the heat treatment was crystallized from the result of X-ray diffraction measurement, and the main peak was In.sub.2O.sub.3 (111). The Hall-effect measurement was performed on the obtained crystalline oxide semiconductor thin films to obtain the carrier density and the carrier mobility. The obtained evaluation results are summarized in Table 3 and Table 4.

TABLE-US-00003 TABLE 3 Heat Ga/ treat- (In + Mg/(In + Sintering ment Film Crystal Ga) Ga + Mg) temper- temper- thick- structure Carrier Carrier Atomic Atomic ature ature ness of density mobility ratio ratio ( C.) ( C.) (nm) thin film (10.sup.17 cm.sup.3) (cm.sup.2/V .Math. s) Comparative 0.05 0.01 1400 300 50 Only In.sub.2O.sub.3 12 22.9 Example 1 Comparative 0.08 0 1450 300 50 Only In.sub.2O.sub.3 14 24.5 Example 2 Comparative 0.08 0.00001 1450 300 50 Only In.sub.2O.sub.3 12 24.4 Example 3 Example 1 0.08 0.0001 1450 300 50 Only In.sub.2O.sub.3 6.8 23.8 Example 2 0.08 0.01 1400 325 50 Only In.sub.2O.sub.3 3.0 22.2 Example 3 0.08 0.02 1350 325 50 Only In.sub.2O.sub.3 0.94 20.7 Example 4 0.08 0.03 1400 325 50 Only In.sub.2O.sub.3 0.42 17.4 Example 5 0.08 0.04 1400 325 50 Only In.sub.2O.sub.3 0.087 14.1 Comparative 0.08 0.05 1400 325 50 Only In.sub.2O.sub.3 0.023 9.8 Example 4 Example 6 0.10 0.0001 1400 325 50 Only In.sub.2O.sub.3 3.1 22.4 Example 7 0.10 0.03 1400 325 50 Only In.sub.2O.sub.3 0.32 17.6 Example 8 0.12 0.0001 1400 350 50 Only In.sub.2O.sub.3 4.1 21.3 Example 9 0.12 0.01 1400 350 50 Only In.sub.2O.sub.3 1.2 18.6 Example 10 0.15 0.0001 1400 375 50 Only In.sub.2O.sub.3 3.6 19.3 Example 11 0.15 0.02 1400 375 50 Only In.sub.2O.sub.3 0.74 16.7

TABLE-US-00004 TABLE 4 Heat Ga/ treat- (In + Mg/(In + Sintering ment Film Crystal Ga) Ga + Mg) temper- temper- thick- structure Carrier Carrier Atomic Atomic ature ature ness of density mobility ratio ratio ( C.) ( C.) (nm) thin film (10.sup.17 cm.sup.3) (cm.sup.2/V .Math. s) Example 12 0.15 0.03 1400 375 50 Only In.sub.2O.sub.3 0.24 15.1 Comparative 0.15 0.05 1400 375 50 Only In.sub.2O.sub.3 0.011 8.2 Example 5 Example 13 0.19 0.0001 1400 450 50 Only In.sub.2O.sub.3 1.7 14.3 Example 14 0.19 0.01 1400 450 50 Only In.sub.2O.sub.3 0.64 12.9 Example 15 0.19 0.01 1200 450 50 Only In.sub.2O.sub.3 0.74 11.7 Example 16 0.19 0.03 1400 450 50 Only In.sub.2O.sub.3 0.25 11.5 Example 17 0.19 0.03 1550 450 50 Only In.sub.2O.sub.3 0.3 10.8 Comparative 0.30 0.03 1400 700 50 Only In.sub.2O.sub.3 0.0048 5.7 Example 6

(Evaluation on Nodule Generation)

[0076] The evaluation on the nodule generation due to the sputter deposition simulating mass production was carried out for the sputtering targets of Example 2 and Comparative Examples 1, 2, 4, and 6. A load-lock-type pass-type magnetron sputtering apparatus equipped with a direct current power supply with no arcing suppression function (available from ULVAC Technologies, Inc.) was used as the sputtering apparatus. A square-shaped target having 5 inches in height and 15 inches in width was used as the target. After evacuation of the sputtering chamber for the evaluation on sputter deposition to 710.sup.5 Pa or less, a gas mixture of argon and oxygen was introduced into the sputtering chamber at an appropriate oxygen ratio, which depended on the gallium content in each target, and the gas pressure was controlled to 0.6 Pa. The reason for selecting the sputtering gas under such conditions is because it is impossible to perform an appropriate evaluation when the degree of vacuum in the sputtering chamber is higher than 110.sup.4 Pa so that the moisture pressure in the chamber is high or a hydrogen gas is added. As it is well known in the ITO or the like, when H.sup.+ derived from moisture or the hydrogen gas is incorporated into the film, the crystallization temperature of the film is increased and the film which adheres to the target non-erosion portion is likely to be amorphous. As a result, the film stress decreases and thus the target non-erosion portion is less likely to peel off and the nodules are less likely to be generated. The direct current power was set to 2500 W (direct current power density: 5.17 W/cm.sup.2) in consideration of that the direct current power density employed in mass production is generally about 3 to 6 W/cm.sup.2.

[0077] As the evaluation on the nodule generation, the presence or absence of the nodule generation was evaluated by observing the target surface after continuous sputtering discharge at 50 kWh under the above conditions.

[Evaluation]

[0078] The results in Table 1 and Table 2 indicate that, when the gallium content is 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio and the magnesium content is 0.0001 or more and less than 0.05 in terms of Mg/(In+Ga+Mg) atomic ratio in Examples 1 to 17, the oxide sintered bodies are composed of; an In.sub.2O.sub.3 phase having a bixbyite-type structure; and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure as a formed phase other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase.

[0079] In contrast, the oxide sintered body of Comparative Example 1 in which the gallium content is below 0.08 in terms of Ga/(In+Ga) atomic ratio and the oxide sintered bodies of Comparative Examples 2 and 3 in which the magnesium content is below 0.0001 in terms of Mg/(In+Ga+Mg) atomic ratio are an oxide sintered body composed only of an In.sub.2O.sub.3 phase having a bixbyite-type structure. That is, the oxide sintered body of the present invention that is composed of; an In.sub.2O.sub.3 phase having a bixbyite-type structure; and a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure as a formed phase other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a -Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase is not obtained. In addition, the magnesium content is 0.05 or more in terms of Mg/(In+Ga+Mg) atomic ratio in the oxide sintered bodies of Comparative Examples 4 to 6, thus the formed phase other than the In.sub.2O.sub.3 phase having a bixbyite-type structure includes an In(GaMg)O.sub.4 phase and an MgGa.sub.2O.sub.4 phase, and the desired oxide sintered body of the present invention is not obtained.

[0080] In addition, in the evaluation on the nodule generation of Example 2 and Comparative Examples 1, 2, 4, and 6, the nodules generated were not observed in the target of Example 2 that was an oxide sintered body of the present invention. On the other hand, a great number of generated nodules were observed in the targets of Comparative Examples 1, 2, 4, and 6. In Comparative Examples 1 and 2, it is believed that this is because the structure of the sintered body is composed only of an In.sub.2O.sub.3 phase having a bixbyite-type structure although the density of the sintered body is high. In Comparative Examples 4 and 6, the fact that the density of the sintered body is low and the In(GaMg)O.sub.4 phase is included in the oxide sintered body, which phase has a high electrical resistance and is likely to remain by sputtering, is believed as the cause. In addition, arcing more frequently occurred in the oxide sintered body of Comparative Example 6 as compared to Example 2 or other Comparative Examples 1, 2, and 4 since the oxide sintered body of Comparative Example 6 includes an MgGa.sub.2O.sub.4 phase.

[0081] In addition, the properties of the oxide semiconductor thin film which is a crystalline oxide semiconductor thin film containing indium, gallium, and magnesium as oxides and in which the gallium content is controlled to 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio and the magnesium content is controlled to 0.0001 or more and less than 0.05 in terms of Mg/(In+Ga+Mg) atomic ratio are shown in Table 3 and Table 4.

[0082] The oxide semiconductor thin films of Examples are all found to be composed only of an In.sub.2O.sub.3 phase having a bixbyite-type structure. In addition, the oxide semiconductor thin films of Examples are found to have a carrier density of less than 1.010.sup.18 cm.sup.3 and a carrier mobility of 10 cm.sup.2 V.sup.1 sec.sup.1 or more.

[0083] Among them, the oxide semiconductor thin films of Examples 2 to 4, 7, 9, 11, and 12 in which the gallium content is 0.08 or more and 0.15 or less in terms of Ga/(In+Ga) atomic ratio and the magnesium content is 0.01 or more and 0.03 or less in terms of Mg/(In+Ga+Mg) atomic ratio exhibit good properties, a carrier density of 3.010.sup.17 cm.sup.3 or less and a carrier mobility of 15 cm.sup.2 V.sup.1 sec.sup.1 or more.

[0084] In contrast, the oxide semiconductor thin films of Comparative Examples 1 to 3 are not suitable for the active layer in TFTs because the carrier density is above 1.010.sup.18 cm.sup.3 although they are an oxide semiconductor thin film composed only of an In.sub.2O.sub.3 phase having a bixbyite-type structure. In contrast, in the oxide semiconductor thin films of Comparative Examples 4 and 5, the magnesium content is 0.05 or more in terms of Mg/(In+Ga+Mg) atomic ratio and the carrier mobility is below 10 cm.sup.2 V.sup.1 sec.sup.1, and thus the desired oxide semiconductor thin film of the present invention is not obtained. In addition, in the oxide semiconductor thin film of Comparative Example 6, the gallium content is 0.20 or more in terms of Ga/(In+Ga) atomic ratio and the carrier mobility is below 10 cm.sup.2 V.sup.1 sec.sup.1, and thus the desired oxide semiconductor thin film of the present invention is not obtained.