Classifications

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  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/32—Gas-filled discharge tubes
  • H01J37/34—Gas-filled discharge tubes operating with cathodic sputtering
  • H01J37/3411—Constructional aspects of the reactor
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  • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
  • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
  • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
  • H01L29/76—Unipolar devices, e.g. field effect transistors
  • H01L29/772—Field effect transistors
  • H01L29/78—Field effect transistors with field effect produced by an insulated gate
  • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
  • H01L29/7869—Thin film transistors, i.e. transistors with a channel being at least partly a thin film having a semiconductor body comprising an oxide semiconductor material, e.g. zinc oxide, copper aluminium oxide, cadmium stannate
  • H01L29/78693—Thin film transistors, i.e. transistors with a channel being at least partly a thin film having a semiconductor body comprising an oxide semiconductor material, e.g. zinc oxide, copper aluminium oxide, cadmium stannate the semiconducting oxide being amorphous
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
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  • C04B35/03—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on magnesium oxide, calcium oxide or oxide mixtures derived from dolomite
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/626—Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B
  • C04B35/62605—Treating the starting powders individually or as mixtures
  • C04B35/6261—Milling
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  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
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  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
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  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
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  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
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  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
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  • H01L21/02587—Structure
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/74—Physical characteristics
  • C04B2235/77—Density
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/80—Phases present in the sintered or melt-cast ceramic products other than the main phase
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/32—Processing objects by plasma generation
  • H01J2237/33—Processing objects by plasma generation characterised by the type of processing
  • H01J2237/332—Coating

Definitions

• the present invention relates to an oxide sintered body, a target, and an oxide semiconductor thin film obtained by using the target, and more particularly to a crystalline oxide semiconductor thin film which has low carrier density and high carrier mobility and contains indium, gallium, and magnesium, a sputtering target that is suitable for the formation of the crystalline oxide semiconductor thin film and contains indium, gallium, and magnesium, and an oxide sintered body that is suitable for obtaining the sputtering target and contains indium, gallium, and magnesium.
• TFTs Thin film transistors
• FETs field effect transistors
• TFTs are three-terminal elements having a gate terminal, a source terminal, and a drain terminal in the basic structure.
• TFTs are active elements having a function of switching the current between the source terminal and the drain terminal so that a semiconductor thin film deposited on a substrate is used as a channel layer in which electrons or holes move and a voltage is applied to the gate terminal to control the current flowing in the channel layer.
• TFTs are electronic devices that are most widely used these days in practical application. Typical applications of TFTs include liquid-crystal driving elements.
• MIS-FETs metal-insulator-semiconductor-FETs
• a polycrystalline silicon film or an amorphous silicon film is used as a channel layer material.
• MIS-FETs including silicon are opaque to visible light and thus fail to form transparent circuits. Therefore, when MIS-FETs are used as switching elements for driving liquid crystals in liquid crystal displays, the aperture ratio of a display pixel in the devices is small.
• Patent Document 1 proposes a transparent semi-insulating amorphous oxide thin film which is a transparent amorphous oxide thin film deposited by vapor deposition and containing elements of In, Ga, Zn, and O.
• the composition of the oxide is InGaO 3 (ZnO) m (m is a natural number less than 6) when the oxide is crystallized.
• the transparent semi-insulating amorphous oxide thin film is a semi-insulating thin film having a carrier mobility (also referred to as carrier electron mobility) of more than 1 cm 2 V ⁇ 1 sec ⁇ 1 and a carrier density (also referred to as carrier electron density) of 10 16 cm ⁇ 3 or less without doping with an impurity ion.
• Patent Document 1 also proposes a thin film transistor in which the transparent semi-insulating amorphous oxide thin film is used as a channel layer.
• the transparent amorphous oxide thin film (a-IGZO film) containing elements of In, Ga, Zn, and O and deposited by any method of vapor deposition selected from sputtering and pulsed laser deposition has an electron carrier mobility in the range of about from 1 to 10 cm 2 V ⁇ 1 sec ⁇ 1 , which is relatively high.
• oxygen defects are likely to be generated in the amorphous oxide thin film, and electron carriers do not always stably behave in response to external factors such as heat. It has been pointed out that these facts often have adverse effects and cause problems associated with instability of devices, such as TFTs, when the devices are formed.
• Patent Document 2 proposes a thin film transistor including an oxide thin film in which gallium is dissolved in indium oxide.
• the oxide thin film In the oxide thin film, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more.
• the oxide thin film has an In 2 O 3 bixbyite structure.
• An oxide sintered body is proposed as the material of the oxide thin film in which gallium is dissolved in indium oxide.
• the Ga/(Ga+In) atomic ratio In the oxide sintered body, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more.
• the oxide sintered body has an In 2 O 3 bixbyite structure.
• Patent Document 3 a sputtering target containing a sintered body which contains In, Ga, and Mg and one or more kinds of compounds selected from a compound represented by In 2 O 3 , a compound represented by In(GaMg)O 4 , a compound represented by MgGa 2 O 4 , and a compound represented by In 2 MgO 4 has been proposed.
• the target disclosed in Patent Document 3 includes a phase such as Ga 2 MgO 4 , which has low conductivity so as to cause arcing, and thus there is a problem that abnormal discharge is caused.
• An object of the present invention is to provide a sputtering target that allows a crystalline oxide semiconductor thin film to have low carrier density, an oxide sintered body most suitable for obtaining the sputtering target, and an oxide semiconductor thin film that is obtained by using the sputtering target and has low carrier density and high carrier mobility.
• an oxide semiconductor thin film formed as described below has a carrier mobility of 10 cm 2 V ⁇ 1 sec ⁇ 1 or more.
• the oxide semiconductor thin film is formed by using an oxide sintered body obtained as follows: adding a small amount of magnesium to an oxide sintered body containing gallium as an oxide so that the indium and gallium Ga/(In+Ga) ratio is 0.08 or more and less than 0.20; and sintering the oxide sintered body.
• the amount of magnesium added is 0.0001 or more and less than 0.05 in terms of Mg/(In+Ga+Mg) ratio.
• the oxide sintered body thus sintered is substantially composed of an In 2 O 3 phase having a bixbyite-type structure and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase.
• an oxide sintered body includes indium, gallium, and magnesium as oxides.
• 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 oxide sintered body is composed of; an In 2 O 3 phase having a bixbyite-type structure; and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase; and is substantially free of an In(GaMg)O 4 phase, an MgGa 2 O 4 phase, an In 2 MgO 4 phase, and a Ga 2 O 3 phase.
• the magnesium content is 0.01 or more and 0.03 or less in terms of Mg/(In+Ga+Mg) atomic ratio in the oxide sintered body according to the first embodiment.
• the gallium content is 0.08 or more and 0.15 or less in terms of Ga/(In+Ga) atomic ratio in the oxide sintered body according to the first or second embodiment.
• the oxide sintered body according to any one of the first to third embodiments is substantially free of positive divalent elements other than magnesium and positive trivalent to positive hexavalent elements other than indium and gallium.
• an X-ray diffraction peak intensity ratio of the GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure defined by formula 1 below is in the range of 2% or more and 45% or less in the oxide sintered body according to any one of the first to fourth embodiments:
• a sputtering target is obtained by machining the oxide sintered body according to any one of the first to fifth embodiments.
• a crystalline oxide semiconductor thin film is obtained by forming a film on a substrate by sputtering using the sputtering target according to the sixth embodiment, followed by crystallization by heating in an oxidizing atmosphere.
• the oxide semiconductor thin film according to the seventh embodiment has a carrier mobility of 10 cm 2 V ⁇ 1 sec ⁇ 1 or more.
• the oxide semiconductor thin film according to the seventh or eighth embodiment has a carrier density of less than 1.0 ⁇ 10 18 cm ⁇ 3 .
• An oxide sintered body of the present invention that contains indium and gallium as oxides and further contains magnesium so that the Mg/(In+Ga+Mg) atomic ratio is 0.0001 or more and less than 0.05 can provide a crystalline oxide semiconductor thin film of the present invention by sputter deposition and subsequent heating, for example, when the oxide sintered body is used as a sputtering target.
• the crystalline oxide semiconductor thin film has a bixbyite structure.
• the presence of a predetermined amount of magnesium provides an effect of reducing carrier density. Therefore, when the crystalline oxide semiconductor thin film of the present invention is used in TFTs, the on/off ratio of TFTs can be increased.
• the oxide sintered body is substantially composed of; an In 2 O 3 phase having a bixbyite-type structure and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase, and thus an oxide semiconductor thin film having an excellent carrier mobility of 10 Cm 2 V ⁇ 1 sec ⁇ 1 or more can be stably obtained by sputter deposition. Therefore, the oxide sintered body, the target, and the oxide semiconductor thin film obtained by using the target in the present invention are industrially very useful.
• An oxide sintered body of the present invention contains indium, gallium, and magnesium as oxides.
• 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 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.
• the gallium content is less than 0.08 in terms of Ga/(In+Ga) atomic ratio, this effect is not sufficiently obtained.
• 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.
• 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.
• 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.
• the on/off ratio of TFTs can be increased.
• 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.
• M are positive divalent elements other than magnesium and positive trivalent to positive hexavalent elements other than indium and gallium.
• substantially free of M 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.
• 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.
• 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
• positive hexavalent elements such as W and Mo.
• the oxide sintered body of the present invention is composed of an In 2 O 3 phase having a bixbyite-type structure and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase.
• Nodules are generated when the oxide sintered body is composed only of an In 2 O 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.
• an In(GaMg)O 4 phase, an MgGa 2 O 4 phase, and an In 2 MgO 4 phase are all phases having a high resistance so as to be a cause of arcing or nodules.
• the In 2 MgO 4 phase has a specific resistance of about 10 ⁇ 2 ⁇ cm (Non-Patent Document 1) so as to have an electrical resistance higher by one to two orders than that of the In 2 O 3 phase or the GaInO 3 phase, and thus nodules are likely to be generated as the In 2 MgO 4 phase is likely to remain by sputter deposition.
• the In(GaMg)O 4 phase has a higher specific resistance of about 10 0 ⁇ cm (Non-Patent Document 2) so as to be a cause of the nodule generation.
• the MgGa 2 O 4 phase has an even higher specific resistance since it does not contain In so as to be a cause of arcing.
• an oxide semiconductor thin film obtained by sputter deposition using an oxide sintered body in which these phases are formed includes an In 2 O 3 phase exhibiting low crystallinity and the carrier mobility thereof tends to decrease.
• Gallium and magnesium are dissolved in the In 2 O 3 phase.
• gallium makes up the GaInO 3 phase or the (Ga, In) 2 O 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 2 O 3 phase because of unsuccessful sintering or the like, and as a result, a Ga 2 O 3 phase having a ⁇ -Ga 2 O 3 -type structure is formed. Since the Ga 2 O 3 phase has low conductivity, abnormal discharge arises.
• the oxide sintered body of the present invention preferably includes only a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure other than the In 2 O 3 phase having a bixbyite-type structure, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase other than the In 2 O 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.
• I [In 2 O 3 phase (400)] represents a (400) peak intensity of the In 2 O 3 phase having a bixbyite-type structure
• I [GaInO 3 phase (111)] represents a (111) peak intensity of the ⁇ -GaInO 3 phase that is a composite oxide having a ⁇ -Ga 2 O 3 -type structure.
• 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.
• 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.
• the structure of the oxide sintered body of the present invention is preferably composed of; an In 2 O 3 phase having a bixbyite-type structure; and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 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.
• the oxide sintered body includes, in addition to the In 2 O 3 phase, the GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure or the GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and the (Ga, In) 2 O 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.
• 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.
• gallium oxide powder or magnesium oxide powder used 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.
• ordinary-pressure sintering is preferably employed.
• Ordinary-pressure sintering is a simple and industrially advantageous method, and is also an economically preferable means.
• the ball mill mixing is preferably performed for 18 hours or longer in order to suppress excessive formation of the GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure, or the GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and the (Ga, In) 2 O 3 phase other than the In 2 O 3 phase or to avoid formation of the Ga 2 O 3 phase having a ⁇ -Ga 2 O 3 -type structure.
• hard ZrO 2 balls are used as mixing balls.
• 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 2 ) to 294 MPa (3 ton/cm 2 ) by cold isostatic pressing to form a compact.
• 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%.
• 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.
• 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.
• 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.
• the oxide sintered body that is composed of; an In 2 O 3 phase having a bixbyite-type structure; and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase is obtained.
• 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 3 .
• the formation of the (Ga, In) 2 O 3 phase is remarkable when the sintering temperature is higher than 1550° C.
• the (Ga, In) 2 O 3 phase has a higher electrical resistance than the GaInO 3 phase so as to cause a decrease in deposition rate.
• the sintering temperature is 1550° C. or lower, namely, the (Ga, In) 2 O 3 phase is formed in a small amount.
• 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.
• 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 2 O 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.
• 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.
• the density of the oxide sintered body of the present invention is preferably 6.4 g/cm 3 or more, and it is preferably 6.8 g/cm 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 3 since nodules are generated during use in mass production.
• 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.
• 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 2 O 3 phase having a bixbyite-type structure and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure, is important.
• 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.
• the oxide sintered body is composed only of an In 2 O 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.
• sputtering is used in the process for forming the amorphous thin film.
• direct current (DC) sputtering is industrially advantageous because the thermal effects are minimized during film deposition and high-rate deposition is achieved.
• 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.
• the substrate is typically a glass substrate and is preferably an alkali-free glass substrate.
• any resin sheet and resin film that withstands the temperature for the above process can be used.
• presputtering can be performed as follows: for example, after evacuation to 2 ⁇ 10 ⁇ 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 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.
• 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).
• 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.
• the magnesium content is more preferably 0.01 or more and 0.03 or less in terms of Mg/(In+Ga+Mg) atomic ratio.
• the crystalline oxide semiconductor thin film of the present invention is preferably composed only of an In 2 O 3 phase having a bixbyite structure.
• In the In 2 O 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.
• the carrier density decreases to less than 1.0 ⁇ 10 18 cm ⁇ 3 because magnesium doping has an effect of neutralizing carrier electrons generated mainly by oxygen defects, and more preferably a carrier density of 3.0 ⁇ 10 17 cm ⁇ 3 or less is obtained.
• 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.
• the carrier mobility tends to decrease together with a decrease in carrier density, but the carrier mobility is preferably 10 cm 2 V ⁇ 1 sec ⁇ 1 or more and more preferably 15 cm 2 V ⁇ 1 sec ⁇ 1 or more.
• 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.
• micromachining can be performed by wet etching using a weak acid after the amorphous film formation.
• weak acids can be used, but a weak acid composed mainly of oxalic acid is preferably used.
• ITO-06N available from Kanto Chemical Co., Inc., or the like can be used.
• 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.
• 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.
• the thickness is less than 10 nm, unfavorable crystallinity is obtained, and as a result, high carrier mobility is not achieved.
• the thickness is more than 500 nm, it is disadvantageous in that a problem associated with productivity arises.
• 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.
• 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.
• 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.
• 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.
• 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 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 2 .
• 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 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.
• 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.
• 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 2 O 3 phase having a bixbyite-type structure or only the diffraction peaks attributed to an In 2 O 3 phase having a bixbyite-type structure, a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure, and a (Ga, In) 2 O 3 phase were confirmed as shown in Table 1 and Table 2.
• the X-ray diffraction peak intensity ratio of the GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure defined by Formula 1 below is shown in Table 1 and Table 2.
• 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.
• 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.
• the target-substrate (holder) distance was fixed at 60 mm.
• 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 2 ).
• 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.
• 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.
• Example 2 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.
• 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 1 ⁇ 10 ⁇ 4 Pa so that the moisture pressure in the chamber is high or a hydrogen gas is added.
• the direct current power was set to 2500 W (direct current power density: 5.17 W/cm 2 ) in consideration of that the direct current power density employed in mass production is generally about 3 to 6 W/cm 2 .
• 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.
• the oxide sintered bodies are composed of; an In 2 O 3 phase having a bixbyite-type structure; and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase.
• 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 2 O 3 phase having a bixbyite-type structure.
• the oxide sintered body of the present invention that is composed of; an In 2 O 3 phase having a bixbyite-type structure; and a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure as a formed phase other than the In 2 O 3 phase, or a GaInO 3 phase having a ⁇ -Ga 2 O 3 -type structure and a (Ga, In) 2 O 3 phase as a formed phase other than the In 2 O 3 phase is not obtained.
• 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 2 O 3 phase having a bixbyite-type structure includes an In(GaMg)O 4 phase and an MgGa 2 O 4 phase, and the desired oxide sintered body of the present invention is not obtained.
• Example 2 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 2 O 3 phase having a bixbyite-type structure although the density of the sintered body is high.
• Comparative Examples 4 and 6 the fact that the density of the sintered body is low and the In(GaMg)O 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 2 O 4 phase.
• 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.
• the oxide semiconductor thin films of Examples are all found to be composed only of an In 2 O 3 phase having a bixbyite-type structure.
• the oxide semiconductor thin films of Examples are found to have a carrier density of less than 1.0 ⁇ 10 18 cm ⁇ 3 and a carrier mobility of 10 cm 2 V ⁇ 1 sec ⁇ 1 or more.
• 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 3.0 ⁇ 10 17 cm ⁇ 3 or less and a carrier mobility of 15 cm 2 V ⁇ 1 sec ⁇ 1 or more.
• 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 1.0 ⁇ 10 18 cm ⁇ 3 although they are an oxide semiconductor thin film composed only of an In 2 O 3 phase having a bixbyite-type structure.
• 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 2 V ⁇ 1 sec ⁇ 1 , and thus the desired oxide semiconductor thin film of the present invention is not obtained.
• the gallium content is 0.20 or more in terms of Ga/(In+Ga) atomic ratio and the carrier mobility is below 10 cm 2 V ⁇ 1 sec ⁇ 1 , and thus the desired oxide semiconductor thin film of the present invention is not obtained.

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