JP5846563B2 - Thin film transistor, thin film transistor manufacturing method, and semiconductor device - Google Patents

Description

The present invention relates to a thin film transistor, a method for manufacturing the thin film transistor, and a semiconductor device.
This application claims priority on June 14, 2012 based on Japanese Patent Application No. 2012-134940 for which it applied to Japan, and uses the content here.

  Thin film transistors (TFTs) are widely used as switching elements for liquid crystal displays and organic electroluminescence (EL) displays that employ an active matrix drive system.

  As the TFT, a semiconductor layer (channel layer) using amorphous silicon or polysilicon is known. In recent years, TFTs using In (indium) -Zn (zinc) -O-based metal oxides or In-Ga (gallium) -Zn-O-based metal oxides for semiconductor layers in order to improve various characteristics. (For example, refer to Patent Document 1). In the following description, a thin film transistor using a metal oxide as a material for forming a semiconductor layer may be referred to as an “oxide film transistor”.

  Such an oxide film transistor has n-type conductivity and exhibits higher channel mobility than amorphous silicon or polysilicon, and thus can be suitably used as a switching element for a high-definition display or a large-screen display. In addition, since the semiconductor layer made of a metal oxide does not exhibit p-type conduction in principle and the off current is extremely small, the use of an oxide film transistor has the advantage that power consumption can be reduced.

  However, In—Zn—O-based metal oxides and In—Ga—Zn—O-based metal oxides, which are metal oxides described in the above-mentioned patent documents, can easily react with moisture in the air. The characteristic change is likely to occur after the fabrication of the oxide film transistor. In order to prevent such a characteristic change, it is necessary to protect the semiconductor layer of the oxide film transistor, which increases the process load.

  Therefore, there has been a demand for a novel semiconductor material that has the characteristics of a conventionally known oxide film transistor and can suppress a change in characteristics.

JP 2011-4425 A

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a thin film transistor in which a change in characteristics is suppressed using a novel semiconductor material. It is another object of the present invention to provide a method for manufacturing a thin film transistor using a novel semiconductor material in which a change in characteristics is suppressed. It is another object to provide a semiconductor device having such a thin film transistor.

  In order to solve the above problems, a thin film transistor according to one embodiment of the present invention includes a source electrode and a drain electrode, a semiconductor layer provided in contact with the source electrode and the drain electrode, and the source electrode and the drain electrode. A gate electrode provided corresponding to a channel between the gate electrode and an insulator layer provided between the gate electrode and the semiconductor layer, wherein the semiconductor layer is formed of indium and an atomic number It is a complex metal oxide containing 73 or more metal atoms.

  In the thin film transistor according to one embodiment of the present invention, the surface of the semiconductor layer may have a root mean square roughness measured by an atomic force microscope of 1.0 nm or less.

  In the thin film transistor according to one embodiment of the present invention, the semiconductor layer may have a thickness of 20 nm or less.

  In the thin film transistor according to one embodiment of the present invention, the metal atom having an atomic number of 73 or more may be tungsten.

  The thin film transistor manufacturing method according to one embodiment of the present invention includes a composite metal oxide containing indium and the metal atom having an atomic number of 73 or more by a physical vapor deposition method using a target and a process gas. A step of forming a semiconductor layer, wherein the target is a sintered body including an indium oxide powder and an oxide powder of a metal atom having an atomic number of 73 or more, and the process gas is a rare gas It is a mixed gas of oxygen and oxygen and does not contain a compound having a hydrogen atom.

  In the method for manufacturing a thin film transistor according to one embodiment of the present invention, the metal atom oxide having an atomic number of 73 or more may be tungsten oxide.

  In the method for manufacturing a thin film transistor according to one embodiment of the present invention, the content of the tungsten oxide contained in the sintered body may be 10% by mass or less.

  In the method for manufacturing a thin film transistor according to one embodiment of the present invention, the step of forming the semiconductor layer may be performed at 10 ° C. or higher and 100 ° C. or lower.

  A thin film transistor according to another aspect of the present invention is manufactured by the above-described method for manufacturing a thin film transistor.

  In addition, a semiconductor device according to one embodiment of the present invention includes a substrate and the above thin film transistor.

  According to the embodiment of the present invention, it is possible to provide a thin film transistor in which a change in characteristics is suppressed using a novel semiconductor material. In addition, a method for manufacturing a thin film transistor in which a change in characteristics is suppressed using a novel semiconductor material can be provided. In addition, a semiconductor device including such a thin film transistor can be provided.

It is a schematic sectional drawing of the thin-film transistor and semiconductor device which concern on this embodiment. It is a schematic sectional drawing of the thin-film transistor of an Example. 4 is a graph showing the evaluation results of the characteristics of the thin film transistor of Example 1. It is a graph which shows the relationship between the oxygen partial pressure in the process gas of Example 1, and the electrical conductivity of the formed semiconductor layer.

  Hereinafter, a thin film transistor, a method for manufacturing a thin film transistor, and a semiconductor device according to embodiments of the present invention will be described with reference to the drawings. In all of the following drawings, the dimensions and ratios of the constituent elements are shown as appropriately different from those of actual products in order to make the drawings easy to see.

  The thin film transistor of this embodiment includes a source electrode and a drain electrode, a semiconductor layer provided in contact with the source electrode and the drain electrode, and a gate provided corresponding to a channel between the source electrode and the drain electrode. An electrode, and an insulator layer provided between the gate electrode and the semiconductor layer. A material for forming the semiconductor layer is a composite metal oxide containing indium and a metal atom having an atomic number of 73 or more.

  Further, in the method for manufacturing a thin film transistor of this embodiment, a semiconductor layer made of a composite metal oxide containing indium and the metal atom having the atomic number of 73 or more is formed by physical vapor deposition using a target and a process gas. Forming. The target is a sintered body containing indium oxide powder and metal atom oxide powder having an atomic number of 73 or more. The process gas is a mixed gas of a rare gas and oxygen and does not include a compound having a hydrogen atom.

  In addition, the thin film transistor of this embodiment is manufactured by the above-described method for manufacturing a thin film transistor.

The semiconductor device of this embodiment includes a substrate and a thin film transistor provided on the substrate. The thin film transistor is a thin film transistor manufactured by the above-described thin film transistor or the above-described thin film transistor manufacturing method.
Hereinafter, it demonstrates in order.

(Thin film transistor, semiconductor device)
FIG. 1 is a schematic cross-sectional view of a thin film transistor 1 and a semiconductor device 100 according to this embodiment.
As shown in the figure, the semiconductor device 100 of this embodiment includes a substrate 2 and the thin film transistor 1 of this embodiment formed on the substrate 2. In addition, the semiconductor device 100 may have a wiring or an element (not shown) that is electrically connected to the thin film transistor 1.

As the substrate 2, a substrate formed of a known forming material can be used, and any of those having light transmission properties and those having no light transmission properties can be used. For example, an inorganic substrate made of alkali silicate glass, quartz glass, silicon nitride, or the like; a silicon substrate; a metal substrate whose surface is insulated; acrylic resin, polycarbonate resin, PET (polyethylene terephthalate), or PBT (polybutylene) Various substrates such as a resin substrate made of a polyester resin such as terephthalate) or a paper substrate can be used. Further, the substrate may be a composite material formed by combining a plurality of these materials.
The thickness of the substrate 2 can be appropriately set according to the design.

  The thin film transistor 1 is a so-called bottom gate type transistor. The thin film transistor 1 includes a gate electrode 3 provided on a substrate 2, an insulator layer 4 provided to cover the gate electrode 3, a semiconductor layer 5 provided on the upper surface of the insulator layer 4, It has a source electrode 6 and a drain electrode 7 provided in contact with the semiconductor layer 5 on the upper surface. The gate electrode 3 is provided corresponding to the channel region A of the semiconductor layer 5 (at a position overlapping the channel region A in a plan view).

  As the gate electrode 3, the source electrode 6, and the drain electrode 7, those formed of a generally known material can be used. Examples of the material for forming these electrodes include aluminum (Al), gold (Au), silver (Ag), copper (Cu), nickel (Ni), molybdenum (Mo), tantalum (Ta), and tungsten (W). Examples thereof include metal materials such as these and alloys thereof; conductive oxides such as indium tin oxide (ITO) and zinc oxide (ZnO). Moreover, these electrodes may have a laminated structure of two or more layers, and this laminated structure may be formed, for example, by plating the surface with a metal material.

  The gate electrode 3, the source electrode 6, and the drain electrode 7 may be formed of the same forming material, or may be formed of different forming materials. Since manufacture becomes easy, it is preferable that the source electrode 6 and the drain electrode 7 are the same formation material.

The insulator layer 4 is formed using either an inorganic material or an organic material as long as it has insulating properties and can electrically insulate the gate electrode 3 from the source electrode 6 and the drain electrode 7. May be. Examples of the inorganic material include generally known insulating oxides such as SiO ₂ , SiN _x , SiON, Al ₂ O ₃ , and AlN, nitrides, and oxynitrides. Examples of the organic material include acrylic resin, epoxy resin, silicone resin, and fluorine resin. The organic material is preferably a photocurable resin material because it is easy to manufacture and process.

The semiconductor layer 5 is made of a composite metal oxide containing indium and a metal atom having an atomic number of 73 or more. That is, the semiconductor layer 5 contains the composite metal oxide. The metal atom having an atomic number of 73 or more contained in the semiconductor layer 5 is preferably a metal atom belonging to the sixth period in the periodic table. Specific examples of such metal atoms include tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), and mercury (Hg). ), Thallium (Tl), and lead (Pb). Among these, tungsten is more preferable.
In the composite metal oxide, the ratio (percentage) of the content (number of atoms) of metal atoms having an atomic number of 73 or more to the total amount (number of atoms) of metal atoms having an atomic number of 73 or more and indium is preferably 0.00. 5 to 10 atomic%. The remainder other than the metal atom having an atomic number of 73 or more is indium, oxygen atoms, and inevitable impurities.
When the metal atom having an atomic number of 73 or more is tungsten, the ratio (percentage) of the tungsten content (number of atoms) to the total amount (number of atoms) of tungsten and indium in the composite metal oxide is preferably 0.00. It is 5-10 atomic%, More preferably, it is 0.5-2 atomic%.

  In the semiconductor layer 5 of this embodiment, the metal atom having an atomic number of 73 or more suppresses local crystallization of indium oxide during the formation of the semiconductor layer, and the indium oxide tends to be in an amorphous state. As a result, the flatness of the film surface is also increased. Therefore, the semiconductor layer 5 is in a homogeneous amorphous state.

  In addition, in an In—Zn—O-based metal oxide or an In—Ga—Zn—O-based metal oxide, which are generally known oxide semiconductors, zinc oxide contained in a large amount is easily altered by water. Therefore, a thin film transistor in which a semiconductor layer is formed using these oxide semiconductors needs to appropriately protect the semiconductor layer and suppress deterioration.

  On the other hand, since the semiconductor layer 5 of the thin film transistor 1 of the present embodiment does not contain zinc oxide, which is easily altered by water, protection is unnecessary. In addition, when etching is necessary in forming the semiconductor layer 5, both dry etching and wet etching can be employed, so that the degree of freedom of the process is increased.

  In addition, since the semiconductor layer 5 of the thin film transistor 1 of the present embodiment does not contain Ga with a high raw material cost, the cost related to the target at the time of manufacturing can be reduced, and the manufacturing cost of the thin film transistor can also be reduced.

The surface of the semiconductor layer 5 has a root mean square roughness (Rq) measured by an atomic force microscope of 1.0 nm or less. Since Rq is ideally 0 nm, the lower limit value of Rq is 0 nm.
The upper limit of Rq is preferably 0.7 nm or less, more preferably 0.5 nm or less, and still more preferably 0.3 nm or less.

  Here, in this specification, the root mean square roughness (Rq) is an observation image obtained by measuring a measurement area of 5 μm × 5 μm using an atomic force microscope (manufactured by SII, model number SPI5000, tapping mode observation). The value calculated using.

  Further, the thickness of the semiconductor layer 5 is preferably 20 nm or less, and more preferably 10 nm or less. In the present embodiment, the thickness of the semiconductor layer 5 is measured by using a crystal oscillation type film thickness meter disposed mainly in the sputtering chamber for forming the semiconductor layer 5 for the purpose of film thickness calibration.

  An In—Zn—O-based metal oxide or an In—Ga—Zn—O-based metal oxide tends to be polycrystalline when a semiconductor layer is formed. For this reason, in a conventionally known oxide film transistor, the surface of the semiconductor layer does not become so flat as to exhibit the above-mentioned root mean square roughness due to crystal grains contained in the semiconductor layer. In addition, the normally known semiconductor layer of an oxide film transistor has a reduced electrical conductivity in the plane direction due to such crystal grains.

  Therefore, in the case of forming a semiconductor layer using a generally known oxide semiconductor, in order to form a continuous layer in the plane direction (in order not to form a discontinuous region) and to ensure conductivity, a film is used. The thickness is 30 to 80 nm.

  On the other hand, in the semiconductor layer 5 of the present embodiment, the surface exhibits the root mean square roughness as described above, and becomes a very flat surface. For this reason, even if the semiconductor layer 5 is formed thin, it is difficult to be discontinuous. In addition, unlike the above-described semiconductor layer made of a generally known oxide semiconductor, a phenomenon in which crystal grains locally grow inside the formed semiconductor layer 5 hardly occurs, and the electric conductivity caused by the crystal grains is not generated. There is no decline.

Therefore, in the thin film transistor 1 of the present embodiment, the thickness of the semiconductor layer 5 can be reduced as described above. Thereby, the material cost which forms the semiconductor layer 5 can be suppressed. Furthermore, since the time required for forming the semiconductor layer can be shortened as compared with a conventionally known oxide film transistor, the manufacturing time can be shortened.
The thin film transistor 1 and the semiconductor device 100 of the present embodiment are configured as described above.

(Thin Film Transistor Manufacturing Method)
Next, a method for manufacturing the thin film transistor 1 of the present embodiment will be described. The semiconductor layer of the thin film transistor of this embodiment can also be formed by using physical vapor deposition (or physical vapor deposition).

  Here, examples of physical vapor deposition include vapor deposition and sputtering. Examples of the vapor deposition method include vacuum vapor deposition, molecular beam vapor deposition (MBA), ion plating, and ion beam vapor deposition. Examples of the sputtering method include conventional sputtering, magnetron sputtering, ion beam sputtering, ECR (electron cyclotron resonance) sputtering, and reactive sputtering. When plasma is used in the sputtering method, a film forming method such as a reactive sputtering method, a DC (direct current) sputtering method, or a radio frequency (RF) sputtering method can be used.

  Furthermore, it is preferable to manufacture a thin film transistor using the following manufacturing method. When the following manufacturing method is used, a higher quality thin film transistor can be manufactured.

  In the method for manufacturing the thin film transistor 1 of this embodiment, the gate electrode 3 and the insulator layer 4 are formed on the substrate 2 by a generally known method, and then the semiconductor layer 5 is formed. In the manufacturing method of the present embodiment, the semiconductor layer 5 is manufactured by physical vapor deposition using a target and a process gas. The target is a sintered body containing indium oxide powder and metal atom oxide powder having an atomic number of 73 or more. The process gas is a mixed gas of a rare gas and oxygen and does not include a compound having a hydrogen atom. Here, it demonstrates as using a sputtering method as a physical vapor deposition method.

In the sintered body, the ratio (percentage) of the content (number of atoms) of the metal atom having an atomic number of 73 or more to the total amount (number of atoms) of the metal atom and indium having an atomic number of 73 or more is preferably 0.5. -10 atomic percent. The remainder other than the metal atom having an atomic number of 73 or more is indium, oxygen atoms, and inevitable impurities.
The composition of the semiconductor layer 5 to be formed (the composition of elements other than oxygen atoms) and the composition of the target (the composition of elements other than oxygen atoms) are the same or substantially the same. For this reason, it is preferable to prepare a target having the same composition (composition of elements other than oxygen atoms) as the composition of the target semiconductor layer 5 (composition of elements other than oxygen atoms).
For example, when an In—W—O-based metal oxide is employed as the semiconductor layer 5, a sintered body of indium oxide powder and tungsten oxide powder may be employed as the target. Note that tungsten oxide includes W ₂ O ₃ , WO ₂ , and WO ₃ . Of these, WO ₃ is the most stable and inexpensive to manufacture. In the manufacturing process of WO ₃ , W ₂ O ₃ and WO ₂ may be mixed in a trace amount, but the composition of tungsten oxide is WO ₃ without changing. In this embodiment, a tungsten oxide, WO _3, or _W 2 _{O 3} and WO ₂ means a WO ₃ were mixed in trace amounts.
The target may be mixed with impurities such as an additive (metal oxide, etc.) in an amount equal to or less than the content (% by mass) of tungsten oxide. For example, a metal oxide (such as zinc oxide) other than indium oxide and tungsten oxide may be mixed into the target at a ratio equal to or lower than the content of tungsten oxide as an unintended unavoidable impurity.

  In that case, in the sintered body, the ratio (percentage) of the content (number of atoms) of tungsten to the total amount (number of atoms) of tungsten and indium is preferably 0.5 to 10 atomic%, more preferably 0. .5 to 2 atomic%.

  In-Zn-O-based metal oxides and In-Ga-Zn-O-based metal oxides, which are commonly known oxide semiconductors, indium oxide is used as a “host material”, and zinc oxide or gallium oxide is used as a “guest”. As the “material”, a guest material (zinc oxide or gallium oxide) in an amount of 20 to 30% (20 to 30%) is mixed with the amount of the host material (indium oxide).

  On the other hand, the semiconductor layer 5 of the thin film transistor 1 of the present embodiment is formed using the sintered body as described above as a target. Therefore, in the thin film transistor 1 manufactured by the manufacturing method according to the present embodiment, the oxide semiconductor of the semiconductor layer 5 has a guest material (indium oxide) content relative to the content of the host material (indium oxide) as compared with a conventionally known oxide semiconductor. The ratio of the content of (tungsten oxide) is extremely small.

In the method for manufacturing the thin film transistor 1, a mixed gas of a rare gas and oxygen is used as a process gas. Examples of the rare gas include helium, neon, argon, krypton, and xenon.
The amount (volume ratio) of oxygen gas in the process gas is preferably 8.5 to 12% by volume.

Further, the process gas does not include a compound having a hydrogen atom. Here, “not containing a compound having a hydrogen atom” means that a compound having a hydrogen atom such as water (H ₂ O) or hydrogen gas (H ₂ ) is not intentionally mixed in the process gas. . In this way of thinking, it is not excluded that the gas to be used or moisture or hydrogen gas existing in a minute amount in the work environment is unintentionally mixed in the process gas.

  In the manufacturing process of a semiconductor layer formed of an In—Zn—O-based metal oxide or an In—Ga—Zn—O-based metal oxide, the above-described local crystal grain growth is suppressed, and favorable film formation is achieved. To achieve this, a small amount of water may be included in the process gas. Further, hydrogen may be contained in the process gas.

  When sputtering is performed using such a gas containing hydrogen atoms as a process gas, hydrogen atoms (protons) are mixed into the film formed by sputtering. When the semiconductor layer contains such hydrogen atoms, the hydrogen atoms move in the semiconductor layer due to the current supplied when the transistor is driven, and the properties of the semiconductor layer are not stable. Therefore, in a thin film transistor having a semiconductor layer containing hydrogen atoms, the threshold voltage, which is the minimum gate voltage at which current starts to flow between two electrodes (source-drain), varies, and the operation is difficult to stabilize.

  In contrast, in the method for manufacturing the thin film transistor of this embodiment, as described above, the formed semiconductor layer includes metal atoms having an atomic number of 73 or more, and the metal atoms undergo local crystal growth. Suppress. Therefore, unlike the In—Zn—O-based metal oxide and the In—Ga—Zn—O-based metal oxide, it is not necessary to include water in the process gas. Therefore, in the method for manufacturing a thin film transistor of this embodiment, a gas that contains a rare gas and oxygen and does not contain a gas component containing hydrogen atoms such as water and hydrogen can be used as a process gas.

  In the method of manufacturing a thin film transistor of this embodiment, when the semiconductor layer is formed using a target including indium oxide and tungsten oxide, the metal oxide constituting the semiconductor layer is an amorphous film, as studied by the inventors. Is known to require no high temperature. Therefore, in the method for manufacturing a thin film transistor, the semiconductor layer can be suitably formed by performing the step of forming the semiconductor layer at 10 ° C. or higher and 100 ° C. or lower. Further, the step of forming the semiconductor layer is preferably performed at room temperature. Here, “implemented at room temperature” means that the semiconductor layer is not heated for the step of forming the semiconductor layer, and the temperature adjustment of the working environment is unnecessary.

  As a sputtering method employed in the method for manufacturing the thin film transistor of this embodiment, a known method such as RF sputtering, AC sputtering, or DC sputtering can be used.

Further, the target may be a sintered body of a mixture of these powders as long as it uses indium oxide powder and metal oxide oxide powder having an atomic number of 73 or more. It may be a sintered body. When a sintered body is formed for each metal oxide powder, the semiconductor layer can be formed by co-sputtering using a plurality of sintered bodies.
The manufacturing method of the thin film transistor of this embodiment is as described above.

  According to the thin film transistor having the above-described configuration, a change in characteristics is suppressed by using a novel semiconductor material.

  Further, according to the semiconductor device having the above-described configuration, the thin film transistor in which the change in characteristics is suppressed has high reliability.

  Further, according to the method for manufacturing a thin film transistor as described above, a thin film transistor in which a change in characteristics is suppressed can be easily manufactured using a novel semiconductor material.

  Note that although a so-called bottom-gate thin film transistor has been described in this embodiment, the present invention can also be applied to a so-called top-gate thin film transistor.

  In the present embodiment, a so-called top contact type thin film transistor has been described. However, the present invention can also be applied to a so-called bottom contact type thin film transistor.

  The preferred embodiments according to the present invention have been described above with reference to the accompanying drawings, but it is needless to say that the present invention is not limited to such examples. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the requirements of the present invention.

  The present embodiment will be described below by way of examples, but the present embodiment is not limited to these examples.

(Example 1)
In this example, the thin film transistor 10 shown in FIG. 2 was manufactured and the operation was confirmed.
The thin film transistor 10 shown in FIG. 2 has the same structure as that of the thin film transistor 1 shown in FIG. 1, and instead of the gate electrode 3 included in the thin film transistor 1 shown in FIG. 1, a Si layer 8 doped with a large amount of p-type impurities is used. It has been.

  The thin film transistor 10 of the example was manufactured as follows. The insulator layer 4 was formed by oxidizing the surface using a Si substrate doped with a p-type impurity. Next, the semiconductor layer 5 was formed on the surface of the insulator layer 4 using a method described later. The source electrode 6 and the drain electrode 7 were formed by mask vapor deposition on the surface of the semiconductor layer 5.

  The source electrode 6 and the drain electrode 7 used gold (Au) as a forming material and had a thickness of 40 nm. Further, the separation distance (gate length) between the source electrode 6 and the drain electrode 7 was 350 μm, and the length of the facing portion was 940 μm.

The semiconductor layer 5 is formed by a sputtering method (DC sputtering) under the following sputtering conditions using an IWO target (manufactured by Sumitomo Metal Mining Co., Ltd.) as a target material using a sputtering apparatus (manufactured by Shinko Seiki Co., Ltd., STV4321 type). did. As an IWO target, an In ₂ O ₃ -based sample product to which 1% by mass of tungsten oxide (WO ₃ ) was added was used. The thickness of the deposited semiconductor layer 5 was 10 nm.

(Sputtering conditions)
DC power: 50W
Degree of vacuum: 0.06 Pa
Process gas flow rate: Ar 3 sccm / O ₂ 0.5 sccm
(Sccm: Standard Cubic Centimeter per Minute)
Substrate temperature: 23 ° C. Without heating

  The root mean square roughness of the surface of the formed semiconductor layer was measured by an atomic force microscope (manufactured by SII, model number SPI5000, tapping mode observation). The root mean square roughness of the surface was 0.24 nm.

  The characteristics of the thin film transistor 10 thus manufactured were measured under the evaluation environment of 23 ° C. in a dark place and in a vacuum. FIG. 3 is a graph showing the results of measuring the characteristics of the thin film transistor 10. FIG. 3A shows the transfer characteristics, and FIG. 3B shows the output characteristics.

In the thin film transistor 10 manufactured in this example, the field-effect mobility was 1.18 cm ² / Vs.
Further, the ratio (Ion / Ioff) between the drive current (Ion) and the leak current (Ioff) was 10 ⁷ or more, and it was found that the leak current was very small.
Furthermore, the threshold voltage was 26V. The threshold voltage can be adjusted by adjusting the characteristics of the semiconductor layer by changing the film formation conditions, the film thickness, and the like of the semiconductor layer.
The subthreshold coefficient (S value) was 0.44 V / decade.
Therefore, the operation of the thin film transistor of this example was confirmed.

In addition, the thin film transistor 10 was fabricated in the same manner as described above, except that the oxygen partial pressure in the process gas was adjusted to various values when forming the semiconductor layer 5. Then, the conductivity of the semiconductor layer 5 was measured.
FIG. 4 is a graph showing the relationship between the partial pressure of oxygen in the process gas and the conductivity of the formed semiconductor layer 5.
The electrical conductivity (σ) and the oxygen partial pressure (P (O ₂ )) of the semiconductor layer 5 were expressed by the following relational expression.
σ = exp {−449 × P (O ₂ ) −2.44}
It can be seen that the higher the oxygen partial pressure in the process gas, the lower the conductivity of the semiconductor layer 5. When the oxygen partial pressure in the process gas was more than 0.04 Pa, the conductivity of the semiconductor layer 5 was less than 10 ⁻⁹ S / cm. When the oxygen partial pressure in the process gas was less than 0.028 Pa, the conductivity of the semiconductor layer 5 was about 10 ⁻⁶ S / cm or more. When the oxygen partial pressure in the process gas was 0.03 to 0.038 Pa, the conductivity of the semiconductor layer 5 was about 10 ⁻⁸ S / cm.
Thus, it was found that the semiconductor layer 5 can be formed with good reproducibility by adjusting the oxygen partial pressure in the process gas.

(Example 2)
The semiconductor layer 5 is formed in the same manner as in Example 1 except that a target composed of ₃ % by mass or 5% by mass of tungsten oxide (WO ₃ ) and indium oxide (In ₂ O ₃ ) is used as the IWO target. Thus, the thin film transistor 10 was produced.
As a result of measuring the electrical characteristics of the thin film transistor 10, it was found that the field effect mobility was a good value and the leakage current was very small. Therefore, the operation of the thin film transistor of this example was confirmed.

  From the above results, the operation of the thin film transistor of this embodiment could be confirmed, and the usefulness of this embodiment was confirmed.

  In the thin film transistor of this embodiment, the characteristic change is suppressed. Further, in the method for manufacturing the thin film transistor of this embodiment, it is not necessary to protect the semiconductor layer in order to prevent the characteristic change, and the process load is small. Furthermore, since Ga is not included, the cost concerning a target can be reduced. For this reason, this embodiment is preferably applicable to the manufacturing process of the switching element of a liquid crystal display or an organic electroluminescence (Electro Luminescence (EL)) display.

1, 10: thin film transistor, 2: substrate, 3: gate electrode, 4: insulator layer, 5: semiconductor layer, 6: source electrode, 7: drain electrode, 8: Si layer, 100: semiconductor device.

Claims (7)

A source electrode and a drain electrode;
A semiconductor layer provided in contact with the source electrode and the drain electrode;
A gate electrode provided corresponding to a channel between the source electrode and the drain electrode;
An insulator layer provided between the gate electrode and the semiconductor layer,
The semiconductor layer forming material is a complex metal oxide containing indium and a metal atom having an atomic number of 73 or more,
The surface of the semiconductor layer has a root mean square roughness measured with an atomic force microscope of 1.0 nm or less,
A thin film transistor in which the semiconductor layer has a thickness of 20 nm or less.   The thin film transistor according to claim 1, wherein the metal atom having an atomic number of 73 or more is tungsten. A method of manufacturing a thin film transistor according to claim 1 or 2,
Forming a semiconductor layer made of a composite metal oxide containing indium and a metal atom having an atomic number of 73 or more by a physical vapor deposition method using a target and a process gas;
The target is a sintered body containing indium oxide powder and metal oxide oxide powder having an atomic number of 73 or more,
The process gas is a method of manufacturing a thin film transistor that is a mixed gas of a rare gas and oxygen and does not include a compound having a hydrogen atom.   4. The method of manufacturing a thin film transistor according to claim 3, wherein the oxide of a metal atom having an atomic number of 73 or more is tungsten oxide.   The method for manufacturing a thin film transistor according to claim 4, wherein a content of the tungsten oxide contained in the sintered body is 10% by mass or less.   The method for manufacturing a thin film transistor according to claim 3, wherein the step of forming the semiconductor layer is performed at 10 ° C. or higher and 100 ° C. or lower. Semiconductor device comprising a substrate, and a thin film transistor according to claim 1 or 2 provided on the substrate.

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