JP2007073697A - Method of manufacturing thin film transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of stably manufacturing an oxide having an electron carrier concentration of less than the level of 10<SP>18</SP>/cm<SP>3</SP>. <P>SOLUTION: The method of manufacturing a thin film transistor having an active layer of a transparent oxide film containing In, Zn and O at an electronic carrier concentration of less than the level of 10<SP>18</SP>/cm<SP>3</SP>, a source electrode, a drain electrode, a gate insulation film, and a gate electrode comprises a step of forming the active layer by the sputtering method in an atmosphere gas containing water. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an oxide and a method for manufacturing a thin film transistor (TFT) using the oxide.

ITO is used as a transparent electrode in transmissive liquid crystal devices and the like, but In ₂ O ₃ which is the main raw material of ITO is a rare metal, and there is a concern whether it can be continuously supplied in the future. Has been.

  Therefore, research and development of materials that can be substituted for ITO have been actively conducted. For example, zinc oxide films (ZnO), zinc-indium oxides (Patent Document 1), zinc-indium oxides with a predetermined amount of gallium. There is an oxide (Patent Document 2) to which these are added.

  In recent years, there have been attempts to form not only electrodes but also channel layers of transistors, for example, with transparent films.

  For example, a TFT using a transparent conductive oxide polycrystalline thin film containing ZnO as a main component for a channel layer has been actively developed (Patent Document 3).

Since the thin film can be formed at a low temperature and is transparent to visible light, it is said that a flexible transparent TFT can be formed on a substrate such as a plastic plate or a film.
On the other hand, from the viewpoint of a manufacturing method, a method of forming an oxide containing Zn, Ga, and In as a highly conductive oxide by a sputtering method is disclosed. (Patent Documents 2 and 4)
In addition, a method of forming a dense and high-quality film by adding water vapor at the time of transparent conductive oxide sputtering film formation, or a method of manufacturing a stable transparent conductive oxide is disclosed. (Patent Documents 5 and 6)
JP 07-235219 A Japanese Patent Laid-Open No. 2000-044236 JP 2002-76356 A Japanese Patent Laid-Open No. 08-245220 JP-A 61-64874 JP 2001-342555 A

However, in the conductive transparent oxide containing ZnO as a main component, oxygen defects are likely to occur, a large number of carrier electrons are generated, and it is difficult to reduce the electrical conductivity.
For this reason, even when no gate voltage is applied, a large current flows between the source terminal and the drain terminal, and the normally-off operation of the TFT cannot be realized. It is also difficult to increase the on / off ratio of the transistor.

In addition, when an amorphous oxide film as described in Patent Document 2 is used for a channel layer of a TFT, the electron carrier concentration of the amorphous film is 10 ¹⁸ / cm ³ or more, which is normally It is not preferable as an off-type TFT channel layer.
Here, the amorphous oxide film disclosed in Patent Document _2, in _{Zn x M y In z O (} x + 3y / 3z / 2) ( wherein, M represents at least one element of Al and Ga Yes, the ratio x / y is in the range of 0.2 to 12, and the ratio z / y is in the range of 0.4 to 1.4.

Transparent Amorphous Oxide Film Conventionally, it has not been possible to obtain a film having an electron carrier concentration of less than 10 ¹⁸ / cm ^{3 using} such a transparent amorphous oxide film.

Therefore, an object of the present invention is to provide a production method capable of stably producing an oxide having an electron carrier concentration of less than 10 ¹⁸ / cm ³ .

  An object of the present invention is also to provide a manufacturing method capable of stably manufacturing a normally-off type TFT.

The inventors of the present invention vigorously advanced research and development on an InGaO ₃ (ZnO) _m film and film growth conditions related thereto. As a result, it was found that a transparent amorphous oxide film having an electron carrier concentration of less than 10 ¹⁸ / cm ³ can be produced by controlling the oxygen atmosphere conditions during film formation.

  The present invention provides further improvements to the film itself that achieves the above electron carrier concentration, and provides elements and devices using these films.

  The present invention will be specifically described below.

The present invention includes an active layer composed of a transparent amorphous oxide film including In—Ga—Zn—O and having an electron carrier concentration of less than 10 ¹⁸ / cm ³ , a source electrode, a drain electrode, and a gate insulating film And a gate electrode comprising a step of forming the active layer by a sputtering method, wherein the atmosphere gas used to form the active layer by the sputtering method includes water vapor. .

Further, the present invention is characterized in that the water vapor partial pressure in the sputtering method is 5.0 × 10 ⁻⁵ Pa or more and 1.0 × 10 ⁻¹ Pa or less.

Further, the present invention is characterized in that a water vapor partial pressure in the sputtering method is 7.5 × 10 ⁻⁵ Pa or more and 5.0 × 10 ⁻² Pa or less.

Further, the present invention is characterized in that the water vapor partial pressure in the sputtering method is 1.0 × 10 ⁻⁴ Pa or more and 1.5 × 10 ⁻² Pa or less.

  According to the present invention, it is possible to provide a high-performance element in a TFT using a transparent amorphous oxide film, and to provide a circuit, a device, or the like using the element.

  In addition, according to the present invention, it is possible to provide a circuit, a device, or the like in which a TFT using a transparent amorphous oxide film is formed in a large area.

Transparent Amorphous Oxide Film First, a transparent amorphous oxide film having an electron carrier concentration of less than 10 ¹⁸ / cm ³ successfully produced by the present inventors will be described in detail.

Specifically, the transparent amorphous oxide film includes In—Ga—Zn—O, a composition in a crystalline state is represented by InGaO ₃ (ZnO) _m (m is a natural number of less than 6), and an electron carrier The concentration is less than 10 ¹⁸ / cm ³ .
In this film, it is also preferable that the electron mobility is 1 cm ² / (V · sec) or more.
If the above film is used for the channel layer, a TFT having transistor characteristics with a normally-off gate current of less than 0.1 μA and an on / off ratio of more than 10 ³ , which is transparent to visible light and flexible, can be obtained. Can be created.
The transparent amorphous oxide film has higher electron mobility as the number of conduction electrons increases.
As the substrate on which the transparent amorphous oxide film is formed, a glass substrate, a plastic substrate, a plastic film, or the like can be used.
In the present invention, the transparent amorphous oxide film can function as a gate insulating film by setting the electron carrier concentration to less than 10 ¹⁴ / cm ³ .
The present inventors have found that this transparent amorphous oxide film has a unique characteristic that the electron mobility increases as the number of conduction electrons increases.

  Then, a TFT was formed using the film, and it was found that transistor characteristics such as an on / off ratio, a saturation current in a pinch-off state, and a switch speed were further improved.

When a transparent amorphous oxide film is used as a channel layer of a thin film transistor, it is preferable that an electron mobility is 1 cm ² / (V · sec) or more and an electron carrier concentration is less than 10 ¹⁸ / cm ³ .

Further, it is preferably 5 cm ² / (V · sec) or more and less than 10 ¹⁶ / cm ³ .

  By controlling the electron mobility and the electron carrier concentration within this range, the current between the drain and source terminals when off (when no gate voltage is applied) can be made less than 10 μA, preferably less than 0.1 μA.

When the thin film is used, when the electron mobility is 1 cm ² / (V · sec) or more, preferably 5 cm ² / (V · sec) or more, the saturation current after pinch-off can be more than 10 μA. The on / off ratio can be 10 ³ or more.

  In the TFT, in a pinch-off state, a high voltage is applied to the gate terminal, and high-density electrons exist in the channel.

  Therefore, according to the present invention, the saturation current value can be increased more than the increase in electron mobility.

  As a result, almost all transistor characteristics such as an increase in on / off ratio, an increase in saturation current, and an increase in switching speed are improved.

In a normal compound, when the number of electrons increases, electron mobility decreases due to collisions between electrons.
As a structure of the TFT, a stagger (top gate) structure in which a gate insulating film and a gate terminal are sequentially formed on a semiconductor channel layer can be employed.
Further, an inverted stagger (bottom gate) structure in which a gate insulating film and a semiconductor channel layer are formed in order on the gate terminal can be employed.
(About film composition)
The transparent amorphous oxide thin film whose composition in the crystalline state is represented by InGaO ₃ (ZnO) _m (m is a natural number less than 6) is stable in an amorphous state up to a high temperature of 800 ° C. or higher when the value of m is less than 6. Kept.

However, as the value of m increases, the ratio of ZnO to InGaO ₃ increases, and crystallization becomes easier as the ZnO composition is approached.
Therefore, the value of m is preferably less than 6 for the channel layer of the amorphous TFT.

(Regarding control of deposition gas)
In order to obtain an amorphous oxide of In—Ga—Zn, a polycrystalline sintered body having InGaO ₃ (ZnO) _m (m is a natural number of less than 6) is used as a target, and argon gas and oxygen gas are used as atmospheric gases. The sputter deposition method was used.

The substrate temperature was controlled at 100 ° C., the sputtering pressure was 0.53 Pa, the partial pressure of water vapor was 1.2 × 10 ⁻³ Pa, and the oxygen gas ratio was 0.2% to 13%.

As shown in FIG. 1, when the oxygen gas ratio was 0.5% or more, the electron carrier density could be reduced to less than 10 ¹⁸ / cm ³ .

  However, when the oxygen gas ratio was 10% or more, it was not possible to measure well whether the resistance was too high.

  It can be predicted that the number of electron carriers will be reduced by further increasing the oxygen gas ratio.

  Further, the relationship between the electron carrier density and the electron mobility is shown in FIG.

A TFT composed of In—Ga—Zn—O with an oxygen gas ratio of 0.5% to 10% and using a transparent amorphous oxide thin film as an active layer, and has a normally-off and on / off ratio exceeding 10 ³ Transistor could be constructed.

  Moreover, in the thin film produced by the sputtering method, as shown in FIG. 2, the electron mobility increases as the number of conduction electrons increases.

  As described above, by controlling the oxygen gas ratio, oxygen defects can be reduced, and as a result, the electron carrier concentration can be decreased without adding specific impurity ions.

  In the amorphous state, unlike the polycrystalline state, there is essentially no particle interface, so that an amorphous thin film with high electron mobility can be obtained.

  Furthermore, since the number of conduction electrons can be reduced without adding specific impurities, there is no scattering due to impurities, and electron mobility can be kept high.

As the gate insulating film used for the thin film transistor using the transparent amorphous oxide film described above, Al ₂ O ₃ , Y ₂ O ₃ , and HfO ₂ are preferable.

Furthermore, an oxide film containing In—Ga—Zn—O of the present invention and having an electron carrier concentration of less than 10 ¹⁴ / cm ³ or a mixed crystal compound containing at least two of these compounds is used as a gate insulating film. Is preferred.

If there is a defect at the interface between the gate insulating thin film and the channel layer thin film, the electron mobility is lowered and the transistor characteristics are hysteresis.
Further, the leakage current varies greatly depending on the type of the gate insulating film. For this purpose, it is necessary to select a gate insulating film suitable for the channel layer.
If an Al ₂ O ₃ film is used, leakage current can be reduced. Further, the hysteresis can be reduced by using a Y ₂ O ₃ film.
Further, if a high dielectric constant HfO ₂ film is used, the electron mobility can be increased.
Further, by using mixed crystals of these films, a TFT with small leakage current and hysteresis and high electron mobility can be formed.
In addition, since the gate insulating film formation process and the channel layer formation process can be performed at room temperature, both a staggered structure and an inverted staggered structure can be formed as the TFT structure.
A thin film transistor (TFT) is a three-terminal element including a gate terminal, a source terminal, and a drain terminal.
Then, a semiconductor thin film formed on an insulating substrate such as ceramic, glass, or plastic is used as a channel layer through which electrons or holes move.
In operation, the active element has a function of switching a current between a source terminal and a drain terminal by applying a voltage to a gate terminal to control a current flowing in a channel layer.

  Next, the present invention will be specifically described.

  When forming the amorphous oxide active layer or the gate insulating layer of the transparent amorphous oxide film TFT, not only the high frequency sputtering method but also the following method can be used.

  That is, the DC sputtering method, the method of superimposing a pulse on the DC sputtering method, the method of applying DC to the high-frequency sputtering method, or the method of electrically floating the opposite substrate is preferably performed according to the conditions.

  Thereby, an effect is obtained that an active layer or a gate insulating film having a quality equivalent to or higher than that of a normal sputtering method can be deposited on a large-area substrate.

  When forming the amorphous oxide active layer or the gate insulating layer of the transparent amorphous oxide film TFT, the atmosphere gas contains a small amount of water vapor in addition to argon gas and oxygen gas, thereby stabilizing the spatter discharge and increasing the sputtering target time. Stability can be maintained.

  In addition, the activation of the oxygen gas is also pulled out, so that the consumption power of the sputter discharge can be suppressed in addition to the suppression of the oxygen gas consumption, and the plasma damage or thermal damage to the deposited film due to the excessive input power is suppressed. be able to.

  Further, when introducing argon gas, oxygen gas, and water vapor into the film formation chamber, they are mixed before entering the film formation chamber so that at least three kinds of gases are well mixed.

According to the knowledge of the present inventors, the effective water vapor amount is 5.0 × 10 ⁻⁵ Pa or more and 1.0 × 10 ⁻¹ Pa or less.

Preferably, it is 7.5 × 10 ⁻⁵ Pa or more and 5.0 × 10 ⁻² Pa or less.

Furthermore, it is preferably 1.0 × 10 ⁻⁴ Pa or more and 1.5 × 10 ⁻² Pa or less.

  As a result, it is possible to obtain an effect that an active layer or a gate insulating film having a quality higher than that of a normal sputtering method can be uniformly deposited on a large area substrate.

  Further, when the transparent amorphous oxide film is manufactured, further improvements are made.

Specifically, when forming an amorphous oxide active layer or a gate insulating layer of a TFT, a desired electron carrier density can be obtained and stable for a long time by setting the sputtering pressure to 0.3 Pa or more and 6.65 Pa or less. A deposited film with the desired quality can be formed.
When the sputtering pressure is lower than this range, the active ion species attacks the deposited film, causing damage or extremely slowing the deposition rate.

  When the sputtering pressure is larger than this range, high-density plasma is biased toward the target side, and a high deposition rate cannot be obtained unless the distance between the target and the substrate is extremely close.

Therefore, by setting the sputtering pressure within this range, it is possible to obtain an effect that an active layer or a gate insulating film having a quality higher than that of a normal sputtering method can be deposited on a large area at a high speed.
Further, when the transparent amorphous oxide film is manufactured, further improvements are made.
Specifically, when forming the amorphous oxide active layer or the gate insulating layer of the TFT, when the sputtering target is exposed to the atmosphere, the following treatment is preferably performed.

  Specifically, the target surface is planarized by polishing to remove nodules and the like generated on the surface of the sputtering target.

  In addition, in order to desorb the adsorbed gas when exposed to the atmosphere, an adsorbed gas desorption process may be performed by heating the target surface, vacuum heating, hydrogen plasma treatment, or the like.

  At this time, it is preferable to remove the peeled deposited film falling on the target as necessary.

  Further, by performing pre-sputtering before the main sputtering, a more stable deposited film can be maintained.

  As a result, it is possible to obtain an effect that an active layer or a gate insulating film having a quality higher than that of a normal sputtering method can be deposited on a substrate in a large area at a high speed.

  It is important that the desired electron carrier concentration can be achieved by controlling the oxygen deficiency.

  Control of the oxygen amount (oxygen deficiency amount) of the transparent amorphous oxide film is controlled by the pressure during sputtering, the water vapor amount, the oxygen gas ratio, and the substrate temperature.

  However, it is also preferable to control (reduce or increase) the amount of oxygen vacancies after film formation by post-processing the oxide film in an atmosphere containing oxygen.

  In order to effectively control the oxygen deficiency, the temperature in the atmosphere containing oxygen is 0 ° C. or higher and 450 ° C. or lower, preferably 25 ° C. or higher and 400 ° C. or lower, more preferably 100 ° C. or higher and 350 ° C. or lower. Good.

The lower limit of the electron carrier concentration is, for example, 10 ¹⁴ / cm ³ or more, although it depends on what kind of element, circuit, or device the oxide film obtained is used for.

  In the above, an amorphous oxide including In—Ga—Zn is described as an example, but the present invention can be applied to an amorphous oxide including at least one element of Sn, In, and Zn.

Further, when Sn is selected as at least a part of the constituent elements of the amorphous oxide, Sn is replaced by Sn _1-x M4 _x (0 <x <1, M4 is Si of a group 4 element having an atomic number smaller than Sn, It can also be substituted with Ge or Zr.

In addition, when In is selected as at least a part of the constituent elements of the amorphous oxide, In is replaced with In _1-y M3 _y (0 <y <1, M3 is a group 3 element having an atomic number smaller than Lu or In. B, Al, Ga, or Y.) can also be substituted.

Further, when Zn is selected as at least a part of the constituent elements of the amorphous oxide, Zn is replaced by Zn _1-z M2 _z (0 <z <1, M2 is a group 2 element Mg or Ca having an atomic number smaller than Zn. It can also be substituted.

  Specifically, amorphous materials applicable to the present invention include Sn—In—Zn oxide, In—Zn—Ga—Mg oxide, In oxide, In—Sn oxide, In—Ga oxide, and In—Zn oxide. Materials, Zn—Ga oxide, Sn—In—Zn oxide, and the like.

  Of course, the composition ratio of the constituent materials is not necessarily 1: 1.

  In addition, Zn or Sn alone may be difficult to form an amorphous state, but an amorphous layer is easily formed by including In.

  For example, in the case of an In—Zn-based material, the atomic ratio excluding oxygen is preferably a composition containing In of about 20 atomic% or more.

  In the case of the Sn—In system, it is preferable that the ratio of the number of atoms excluding oxygen is such that In is included at about 80 atomic% or more. In the case of the Sn—In—Zn system, it is preferable that the ratio of the number of atoms excluding oxygen is such that In is included at about 15 atomic% or more.

  Amorphous is confirmed by the fact that a clear diffraction peak is not detected (ie, a halo pattern is observed) when X-ray diffraction is performed on a thin film to be measured at a low incident angle of about 0.5 degrees. it can.

  Note that the present invention does not exclude that when the above-described material is used for a channel layer of a field effect transistor, the channel layer includes a constituent material in a microcrystalline state.

  In the above description, the transparent amorphous oxide film has been mainly described when the TFT channel layer is used. However, the present invention is not limited to the case where the transparent amorphous oxide film is used for the channel layer.

(Example 1: Preparation of amorphous In-Ga-Zn-O thin film)
A case where a film is formed by a high frequency sputtering method including argon gas, oxygen gas, and water vapor as an atmospheric gas will be described.

  The sputtering method was performed using the apparatus shown in FIG. In FIG. 3, reference numeral 301 denotes a film formation chamber (chamber), 302 a target, 303 a shutter, and 304 a film formation substrate.

  305 is a substrate holding means with a heating / cooling mechanism, 311 is a gas introduction tube, 306 is a Pirani vacuum gauge, 307 is an ion vacuum gauge, 308 is a gate valve, 309 is a turbo molecular pump, and 310 is a rotary pump.

A SiO ₂ glass substrate (1737 manufactured by Corning) was prepared as a film formation substrate. As pre-deposition treatment, the substrate was subjected to ultrasonic degreasing and cleaning with acetone, IPA, and ultrapure water for 5 minutes each and then dried at 100 ° C. in air.

As a target material, a polycrystalline sintered body (size 98 mmΦ5 mmt) having an InGaO ₃ (ZnO) ₄ composition was used.

In this sintered body, as a starting material, In ₂ O ₃ : Ga ₂ O ₃ : ZnO (each 4N reagent) is wet-mixed (solvent: ethanol), pre-sintered (1000 degrees: 2 h), dry pulverization, main sintering (1500 ° C .: 2 hours).

  The electric conductivity of the target 302 was 10 (S / cm), and it was in a semi-insulating state.

The ultimate vacuum of the film formation chamber 301 is 4.0 × 10 ⁻⁴ Pa, the water vapor partial pressure during film formation is 8.0 × 10 ⁻⁴ Pa, and the total pressure during film formation is 0.5 Pa. The oxygen gas ratio was changed in the range of 0.2% to 13% with a constant value.

  Water vapor, argon gas, and oxygen gas were mixed before entering the film formation chamber 301 and introduced from the gas introduction pipe 311.

  The substrate temperature was 100 ° C., and the distance between the target 302 and the deposition target substrate 304 was 40 (mm). The input power was RF 350 W, and the film formation rate was 7 (nm / min).

  With respect to the obtained film, X-ray diffraction measurement was performed by making X-rays incident on the surface to be measured at an incident angle of 0.5 degree (thin film method). As a result, a clear diffraction peak was not recognized, and thus the manufactured In—Ga—Zn—O-based thin film can be said to be amorphous.

  Furthermore, as a result of measuring the X-ray reflectivity of the sample having an oxygen gas ratio of 3.5% and analyzing the pattern, the mean square roughness (Rrms) of the thin film was about 0.5 nm, and the film thickness was It was found to be about 110 nm.

  As a result of X-ray fluorescence (XRF) analysis, the metal composition ratio of the thin film was In: Ga: Zn = 0.97: 1.03: 4.

The electrical conductivity was less than about 10 ⁻² S / cm. The electron carrier concentration is estimated to be about 10 ¹⁶ / cm ³ or less, and the electron mobility is estimated to be about 7 cm ² / (V · sec).

  From the analysis of the light absorption spectrum, the forbidden band energy width of the produced amorphous thin film was found to be about 3.2 ev.

From the above, the produced In—Ga—Zn—O-based thin film is an amorphous layer close to the composition of crystalline InGaO ₃ (ZnO) ₄ and is a transparent flat thin film with little oxygen deficiency and low electrical conductivity. I found out.

(Preparation of MISFET device)
The top gate type MISFET device shown in FIG. 4 was produced.

First, a 50-nm-thick semi-insulating amorphous InGaO ₃ (ZnO) ₄ film used as the channel layer (2) was formed on the glass substrate (1) by the above-described method for producing an amorphous In—Ga—Zn—O thin film. .

  Further thereon, 5 nm and 30 nm of Ti and gold were laminated by electron beam evaporation, respectively, and a drain terminal (5) and a source terminal (6) were formed by photolithography and lift-off methods.

Finally, a Y ₂ O ₃ film used as the gate insulating film (3) was formed by a desired sputtering method. (Thickness: 120 nm, relative dielectric constant: about 15, leakage current density: 10 ⁻³ A / cm ² when 0.5 MV / cm is applied) A gold terminal is formed thereon, and a gate terminal is formed by a photolithography method and a lift-off method (4) was formed.

(Characteristic evaluation of MISFET device)
FIG. 5 shows the current-voltage characteristics of the MISFET element measured at room temperature.

It can be seen that the channel is an n-type semiconductor because the drain current I _DS increases with an increase in the drain voltage V _DS .

  This is consistent with the fact that the amorphous In—Ga—Zn—O-based semiconductor is n-type.

I _DS shows the behavior of a typical semiconductor transistor that saturates (pinch off) at about V _DS = 6V.

When the gain characteristic was examined, the threshold value of the gate voltage V _GS when V _DS = 4 V was applied was about 1.2V. Further, when V _G = 5 V, a current of I _DS = 1.2 × 10 ⁻⁴ A flowed.

  This corresponds to the fact that carriers can be induced in the In—Ga—Zn—O-based amorphous semiconductor thin film on the insulator side by the gate bias.

The on / off ratio of the transistor was more than 10 ⁵ .

Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 8 cm ² (VS) ⁻¹ was obtained in the saturation region.

  A similar measurement was performed by irradiating the fabricated device with visible light, but almost no change in transistor characteristics was observed.

(Example 2: Production of amorphous In-Ga-Zn-O thin film)
Next, the case where a film is formed by DC sputtering will be described. As in Example 1, the sputtering method was performed using the apparatus shown in FIG.

  In FIG. 3, a high frequency power source (not shown) connected to the target 302 was changed to a DC power source, and DC sputtering was performed.

As a target material, a polycrystalline sintered body (size 98 mmΦ5 mmt) having a lower resistance than that of Example 1 having an InGaO ₃ (ZnO) ₄ composition was used. The electric conductivity of this target 302 was 100 (S / cm) and was a conductor.

The ultimate vacuum of the film formation chamber 301 is 5.0 × 10 ⁻⁴ Pa, the water vapor partial pressure during film formation is 2.0 × 10 ⁻³ Pa, and the total pressure during film formation is 0.54 Pa. A constant value was set, and the oxygen gas ratio was 8%.

  Both the partial pressure of water vapor and the oxygen gas ratio were made higher than in Example 1. The substrate temperature was 120 ° C., and the distance between the target 302 and the deposition target substrate 304 was 50 (mm). The input power was DC 550 W, and the film formation rate was 8 (nm / min).

With respect to the obtained film, X-ray diffraction measurement was performed by allowing X-rays to enter the measurement target surface at an incident angle of 0.5 degrees.
As a result, a clear diffraction peak was not recognized, and thus the manufactured In—Ga—Zn—O-based thin film can be said to be amorphous.
Furthermore, as a result of measuring the X-ray reflectivity and analyzing the pattern, it was found that the mean square roughness (Rrms) of the thin film was about 0.5 nm and the film thickness was about 120 nm.
As a result of X-ray fluorescence (XRF) analysis, the metal composition ratio of the thin film was In: Ga: Zn = 0.98: 1.02: 4.
The electrical conductivity was less than about 10 ⁻² S / cm. The electron carrier concentration is estimated to be about 10 ¹⁶ / cm ³ or less, and the electron mobility is estimated to be about 7 cm ² / (V · sec). From the analysis of the light absorption spectrum, the forbidden band energy width of the produced amorphous thin film was found to be about 3.2 eV.
From the above, the produced In—Ga—Zn—O-based thin film is an amorphous layer close to the composition of crystalline InGaO ₃ (ZnO) ₄ , and is a transparent flat thin film with little oxygen deficiency and low electrical conductivity. I found out.
(Preparation of MISFET device)
As in Example 1, the top gate type MISFET element shown in FIG. First, a 45-nm-thick semi-insulating amorphous InGaO ₃ (ZnO) ₄ film used as the channel layer (2) was formed on the glass substrate (1) by the above-described method for producing an amorphous In—Ga—Zn—O thin film. .

  Further thereon, 5 nm and 40 nm of Ti and gold were laminated by electron beam evaporation, respectively, and a drain terminal (5) and a source terminal (6) were formed by photolithography and lift-off methods.

Finally, a Y ₂ O ₃ film used as the gate insulating film (3) was formed by a desired sputtering method. (Thickness: 130 nm, relative dielectric constant: about 15, leakage current density: 10 ⁻³ A / cm ² when 0.5 MV / cm is applied) A gold terminal is formed thereon, and a gate terminal is formed by a photolithography method and a lift-off method (4) was formed.

(Characteristic evaluation of MISFET device)
Similar to Example 1, the current-voltage characteristics of the MISFET element were measured at room temperature.

It can be seen that the channel is an n-type semiconductor because the drain current I _DS increases with an increase in the drain voltage V _DS .

  This is consistent with the fact that the amorphous In—Ga—Zn—O-based semiconductor is n-type.

I _DS shows the behavior of a typical semiconductor transistor that saturates (pinch off) at about V _DS = 6V.

When the gain characteristic was examined, the threshold value of the gate voltage V _GS when V _DS = 5 V was applied was about 1.1 V.

Further, when V _G = 5 V, a current of I _DS = 1.0 × 10 ⁻⁴ A flowed.

  This corresponds to the fact that carriers can be induced in the In—Ga—Zn—O-based amorphous semiconductor thin film on the insulator side by the gate bias.

The on / off ratio of the transistor was more than 10 ⁵ . Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 7 cm ² (Vs) ⁻¹ was obtained in the saturation region.

  A similar measurement was performed by irradiating the fabricated device with visible light, but almost no change in transistor characteristics was observed.

(Example 3)
In Example 1, the amount of the introduced water vapor was changed as the atmospheric gas, and a TFT was fabricated in the same manner as in Example 1.

The results are shown in FIG. From this result, if the water vapor partial pressure is not 1.5 × 10 ⁻² Pa or less, the electron mobility is drastically decreased because of excessive oxidation.

In addition, the lower the water vapor partial pressure, the electron mobility tends to be improved, but the on / off ratio tends to decrease due to an increase in the electron carrier concentration, which is 1.0 × 10 ⁻⁴ Pa or less. Then, the on / off ratio becomes 10 ² or less, which is not realistic.

Therefore, the water vapor partial pressure is preferably 1.5 × 10 ⁻² Pa or less, and the lower limit should be set from the optimum values of electron mobility and on / off ratio.

Example 4
In Example 1, a sample TFT in which the oxygen gas ratio was changed in the range of 0.2% to 13% was prototyped and evaluated.

  The result is shown in FIG. From this result, if the oxygen gas ratio is not less than 10%, the electron mobility drops sharply because the oxidation proceeds too much.

  Also, the lower the oxygen gas ratio, the higher the electron mobility tends to be, but the higher the electron carrier concentration, the lower the on / off ratio. Therefore, the oxygen gas ratio is preferably 0.5% or more and 10% or less.

(Example 5)
In Example 1, the total pressure during film formation was changed in a range of 0.1 Pa to 10 Pa, and a TFT was prototyped and evaluated. The result is shown in FIG.

  From this result, the total pressure during film formation is good in the range of 0.3 Pa to 6.65 Pa. If it is less than 0.3 Pa, the plasma damage is too strong or the deposition rate is too high, Mobility will drop sharply.

  Further, when the total pressure during film formation is increased, plasma damage is alleviated and the deposition rate tends to decrease.

  However, the reaction with oxygen in the gas phase increases, rather the oxidation proceeds and a high resistance film is formed, so that the electron carrier density decreases and the electron mobility tends to decrease.

  Therefore, if the total pressure during film formation exceeds 6.65 Pa, the desired electron mobility cannot be obtained.

(Example 6)
In Example 1, the same film formation was repeated 7 times, and 700 TFTs were manufactured as a prototype.

At this time, when the film was formed by treating the surface of the sputtering target in contact with the atmosphere, and when the film was formed without any treatment, the electron mobility was 5 or more and the on / off ratio was 10 ⁵ or more. did.

In addition, the surface treatment of this time was performed by polishing for removing joules and the like generated on the surface of the sputter target and removing the peeled deposited film falling on the target. The result is shown in FIG.
From this result, the number of samples treated with the surface did not change even if the number of times increased, and the total rate of acceptance was 99.7%, while the number of samples without surface treatment increased with the number of times. There was a tendency to decrease.
And in the seventh experiment, only 65 pieces could pass, and the total pass rate stayed at 89.3%. From this, it was found that the surface treatment of the sputtering target in contact with the atmosphere is effective for maintaining a high yield.

(Example 7)
In Example 1, the same film formation was repeated 8 times, and 800 TFTs were prototyped.

At this time, when the sputtering target is exposed to the atmosphere, the electron mobility is 5 or more and the on / off ratio is about when the film is formed by pre-sputtering before the main sputtering and when the film is formed without any pre-sputtering. There were compared in 10 ⁵ or more of the yield.

  The pre-sputtering this time was 7 minutes. The result is shown in FIG. From this result, it can be seen that a stable yield can be obtained by performing pre-sputtering before the main sputtering when the sputtering target is exposed to the atmosphere.

  When the sputter target is exposed to the atmosphere, it is considered that the initial deposited film is too low in resistance or too high in resistance due to the influence of humidity and the like at that time.

(Example 8)
In Example 1, a target containing slightly more Ga ₂ O ₃ than the In—Ga—ZnO sputter target used for the channel layer was used, and the In—Ga—Zn—O content was changed while changing the oxygen gas ratio. The configured gate insulating layer was formed instead of the Y ₂ O ₃ film.

In the sample, the electron carrier density was higher than that of 1 × 10 ⁻¹⁴ , and a gate insulating layer was formed.

The on / off ratio of the transistor was more than 10 ⁴ . Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 3 cm ² (Vs) ⁻¹ was obtained in the saturation region.

From this, it was confirmed that the gate insulating layer including In—Ga—Zn—O and having an electron carrier density of less than 1 × 10 ⁻¹⁴ operates effectively.

  In the transparent amorphous oxide film according to the present invention, a transistor using the film as a channel layer can be used as a switching element of an LCD or an organic EL display.

  In addition, a thin film of semiconductor is formed on a flexible material such as a plastic film, so that it can be widely applied to flexible displays, IC cards and ID tags.

It is a graph which shows the relationship between the electron carrier density | concentration of an In-Ga-Zn-O type | system | group amorphous film | membrane, and the oxygen gas ratio during film-forming. It is a graph which shows the relationship between the electron mobility and electron carrier density | concentration of an In-Ga-Zn-O type | system | group amorphous film. It is a schematic diagram which shows the typical sputtering device used for this invention. FIG. 3 is a schematic diagram showing a top gate type MISFET element structure fabricated in Example 1. 3 is a graph showing current-voltage characteristics of a top gate type MISFET device fabricated in Example 1. FIG. It is a graph which shows the relationship between the electron mobility and water vapor partial pressure of an In-Ga-Zn-O type | system | group amorphous film. It is a graph which shows the relationship between the electron mobility of an In-Ga-Zn-O type amorphous film, and oxygen gas ratio. It is a graph which shows the relationship between the electron mobility of an In-Ga-Zn-O type amorphous film, and sputtering pressure. It is a table | surface which shows the relationship between the film-forming frequency | count of an In-Ga-Zn-O type | system | group amorphous film | membrane, and the number of TFT with which the desired characteristic was acquired. It is a table | surface which shows the relationship between the film-forming frequency | count of an In-Ga-Zn-O type | system | group amorphous film | membrane, and the number of TFT with which the desired characteristic was acquired.

Explanation of symbols

301 Deposition Chamber 302 Target 303 Shutter 304 Deposition Substrate 305 Substrate Holding Unit 306 Pirani Vacuum Gauge
307 Ion vacuum gauge 308 Gate valve 309 Turbo molecular pump 310 Rotary pump 311 Gas introduction chamber

Claims (13)

In a method for manufacturing a thin film transistor including an active layer including In, Zn, and O and including a transparent amorphous oxide film, a source electrode, a drain electrode, a gate insulating film, and a gate electrode,
A method of manufacturing a thin film transistor, comprising a step of forming the transparent amorphous oxide film by a sputtering method, wherein water is contained in an atmospheric gas when the transparent amorphous oxide film is formed by the sputtering method. 2. The method of manufacturing a thin film transistor according to claim 1, wherein the sputtering method is a DC sputtering method. 3. The method of manufacturing a thin film transistor according to claim 1, wherein a partial pressure of water in the sputtering method is 5.0 × 10 ⁻⁵ Pa or more and 1.0 × 10 ⁻¹ Pa or less. 4. The method of manufacturing a thin film transistor according to claim 3, wherein a partial pressure of water in the sputtering method is 7.5 × 10 ⁻⁵ Pa or more and 5.0 × 10 ⁻² Pa or less. 5. The method of manufacturing a thin film transistor according to claim 4, wherein a partial pressure of water in the sputtering method is 1.0 × 10 ⁻⁴ Pa or more and 1.5 × 10 ⁻² Pa or less. 2. The method of manufacturing a thin film transistor according to claim 1, wherein at least argon gas and oxygen gas are used as the atmospheric gas in the sputtering method. 2. The method of manufacturing a thin film transistor according to claim 1, wherein a sputtering pressure of the sputtering method is 0.3 Pa or more and 6.65 Pa or less. The sputtering method has an oxygen gas ratio of 0.5 vol. % Or more and 10 vol. 2. The method of manufacturing a thin film transistor according to claim 1, wherein the thin film transistor is not more than%. 2. The method of manufacturing a thin film transistor according to claim 1, wherein the temperature of the atmosphere during sputtering in the sputtering method is 450 ° C. or lower. 2. The method of manufacturing a thin film transistor according to claim 1, further comprising a step of performing a planarization process or an adsorption gas desorption process on the surface of the sputtering target before the step of forming the active layer by the sputtering method. 2. The method of manufacturing a thin film transistor according to claim 1, wherein pre-sputtering is performed before the main sputtering in the sputtering method. 2. The method of manufacturing a thin film transistor according to claim 1, further comprising a step of forming the gate insulating film by a sputtering method. 11. The method of manufacturing a thin film transistor according to claim 1, wherein the gate insulating film contains In, Zn, and O and has an electron carrier concentration of less than 10 ¹⁴ / cm ³ .

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