JP5918631B2 - ZnO film forming method and ZnO film forming apparatus - Google Patents

Description

  The present invention relates to a ZnO film forming method and a ZnO film forming apparatus.

  An atomic layer growth method (hereinafter also referred to as ALD (Atomic Layer Deposition) method, which is abbreviated as an atomic layer deposition method) for forming a thin film in units of atomic layers on a substrate has two kinds of elements mainly composed of elements constituting the film to be formed. In this thin film formation technique, a gas having a desired thickness is formed by repeatedly supplying a gas onto a film formation target substrate and forming a thin film in units of atomic layers on the substrate a plurality of times. For example, when a ZnO film is formed on a substrate, a source gas composed of diethyl zinc or dimethyl zinc and an oxidizing gas containing O are used.

  In recent years, the ZnO film has a wide band gap of about 3.37 eV, and thus has transparency in visible light, and has a higher mobility than the conventionally used amorphous Si. Various proposals have been made to use a flat panel display as an active channel of a TFT (Thin Film Transistor).

  Under such circumstances, ZnO improves the on / off ratio of the thin film transistor by suppressing an increase in leakage current due to an increase in the amount of carriers accompanying the increase in crystallinity using atomic layer deposition. A film forming method is known (Patent Document 1).

  The forming method includes the steps of injecting a zinc precursor into the chamber and adsorbing the zinc precursor on the substrate, injecting nitrogen or an inert gas into the chamber, and removing the remaining zinc precursor, For example, an oxygen precursor using water plasma is injected into the chamber so as to react with the zinc precursor formed on the substrate to form a ZnO semiconductor film, and nitrogen or an inert gas is injected into the chamber. And removing the remaining oxygen precursor. Thereafter, the surface treatment of the ZnO semiconductor film formed using oxygen plasma or ozone is repeatedly performed. Thereby, the mobility of the semiconductor film can be increased by increasing the crystal of the semiconductor film, the leakage current can be reduced by adjusting the amount of carriers, and a transparent ZnO semiconductor film having excellent characteristics can be formed. It is said that.

Japanese Patent No. 4616359

  In the above ZnO film formation method, a transparent ZnO semiconductor film having excellent characteristics can be formed, but it is necessary to generate water plasma and oxygen plasma, and the process is complicated. For this reason, there exists a problem that a ZnO film cannot be formed efficiently in a short time.

  Therefore, an object of the present invention is to provide a ZnO film forming method and a ZnO film forming apparatus capable of forming a ZnO film having excellent characteristics by a simple process in order to solve the conventional problems.

One embodiment of the present invention is a ZnO film formation method. The method is
A first step of adsorbing the organic metal on the substrate by flowing an organic metal gas containing Zn as a source gas above the substrate disposed in the film formation space;
After the organic metal is adsorbed on the substrate, gaseous water is introduced into the deposition space and oxygen gas is introduced as a purge gas to generate plasma in the deposition space, thereby forming a ZnO film. Forming a second step.
The first step and the second step are continuously repeated until the thickness of the ZnO film reaches a preset thickness.

  At this time, it is preferable that the oxygen gas is introduced into the film formation space as a purge gas except for the period of the first step.

  In the first step and the second step, the substrate is preferably heated and held at 95 ° C. or lower.

Another embodiment of the present invention is a ZnO film forming apparatus. The device is
A film formation container having a film formation space in which a substrate is disposed;
A plasma generation unit that is provided in the film formation space and generates plasma;
A gas supply unit for introducing each of an organic metal source gas containing Zn, gaseous water, and oxygen gas into the film formation space;
A controller that controls the timing of introduction of each of the source gas, the water, and the oxygen gas and the timing of feeding power to the plasma generation unit.
Wherein the controller, the substrate is controlled so as to introduce the raw material gas to arranged the film forming space, after stopping the introduction of the raw material gas, together with introducing the gaseous water in the film forming space The oxygen gas is introduced as a purge gas, and the plasma generation unit is controlled to generate plasma in the film formation space .

  The apparatus further includes a heater for heating and holding the substrate, and the heater preferably heats the substrate at a temperature of 95 ° C. or lower.

  With the above-described ZnO film forming method and ZnO film forming apparatus, a ZnO film having excellent characteristics can be formed by a simple process.

It is the schematic showing the structure of the ZnO film forming apparatus of this embodiment. It is a timing chart explaining the flow rate control of the gas which the controller of this embodiment performs, and the timing of the electric power feeding to a parallel plate electrode. (A), (b) is a figure which shows the measurement result of the impurity contained in a ZnO film | membrane. It is a figure which shows the schematic structure of the semiconductor element produced for evaluation of a ZnO film | membrane. It is a figure which shows the transfer characteristic of the semiconductor element shown in FIG.

Hereinafter, the ZnO film forming method and the ZnO film forming apparatus of the present invention will be described in detail.
FIG. 1 is a schematic diagram showing a configuration of a ZnO film forming apparatus 10 of the present embodiment. A ZnO film forming apparatus 10 shown in FIG. 1 applies an ALD method to form a ZnO film to be formed by using an organic metal source gas mainly composed of Zn and gaseous water as a substrate in a film formation space. Supply alternately on top. At that time, the ZnO film forming apparatus 10 generates plasma in order to enhance the reaction activity and forms an oxide film (ZnO film) of the source gas on the substrate in units of atomic layers. The ZnO film forming apparatus 10 forms a film having a predetermined thickness by repeating the process continuously for a plurality of cycles with the above process as one cycle. At that time, the ZnO film forming apparatus 10 simultaneously introduces oxygen gas into the film formation space in addition to gaseous water used for plasma generation. Oxygen gas is used to purge gaseous water. By introducing this oxygen gas, the carbon component contained as an impurity in the ZnO film can be kept lower than the oxygen component and the hydrogen component. As a result, when a ZnO film is used as a semiconductor, transfer characteristics and mobility representing drain current with respect to gate voltage can be enhanced. That is, a ZnO film having excellent characteristics can be formed by a simple process.
In the following description, Zn (C ₂ H ₅ ) ₂ is used as the organic metal source gas, but Zn (CH ₃ ) ₂ can also be used in addition to Zn (C ₂ H ₅ ) ₂ as the source gas.
That is, the ZnO film forming method of the present embodiment includes a first step of adsorbing an organic metal on a substrate by flowing an organic metal gas containing Zn as a source gas above the substrate disposed in the film formation space. Then, after adsorbing the organic metal to the substrate, gaseous water is introduced into the film formation space, and oxygen gas is introduced as a purge gas to generate plasma in the film formation space, thereby forming a ZnO film. And a second step. The first step and the second step are continuously repeated until the thickness of the ZnO film reaches a preset thickness.

  Further, the ZnO film forming apparatus 10 of the present embodiment is a capacitively coupled plasma generating apparatus using parallel plate electrodes as a plasma generating unit, but in addition to this, an electromagnetically coupled plasma generating apparatus using a plurality of antenna electrodes, An ECR type plasma generation apparatus using electron cyclotron resonance or an inductively coupled plasma generation apparatus can also be used.

(ZnO film forming apparatus)
The ZnO film forming apparatus 10 includes a film forming container 12, parallel plate electrodes 14, a gas supply unit 16, a controller 18, a high frequency power supply 20, a matching box 22, and an exhaust unit 24.
The film formation container 12 maintains the film formation space in the film formation container 12 in a constant reduced pressure atmosphere by the exhaust performed by the exhaust unit 24.
A parallel plate electrode 14 is provided in the film formation space. The parallel plate electrode 14 includes an upper electrode 14a and a lower electrode 14b, and is provided in the film formation space to generate plasma. The upper electrode 14a of the parallel plate electrode 14 is provided so as to face the substrate mounting surface of the susceptor 30 provided in the film formation space. The upper electrode 14 a is connected to the high frequency power source 20 via the matching box 22 by a power supply line extending from above the film forming container 12. The matching box 22 adjusts the inductance of the inductor and the capacitance of the capacitor in the matching box 22 so as to match the impedance of the parallel plate electrode 14. The upper electrode 14a is supplied with high frequency power of 13.56 to 27.12 MHz from the high frequency power supply 20 for 10 milliseconds to 10 seconds.
The surface of the lower electrode 14b is a substrate mounting surface of the susceptor 30 and is grounded. The susceptor 30 has a heater 32 therein, and the substrate during film formation is heated and held at, for example, 50 ° C. or more and 95 ° C. or less by the heater 32. That is, the heater 32 of the ZNO film forming apparatus 10 heats and holds the substrate at a temperature of 95 ° C. or lower. Preferably, since the ZnO film can be formed by heating and holding at 70 ° C. to 80 ° C., it is not necessary to use a large amount of energy for heating, and the ZnO film can be manufactured more easily.

  The susceptor 30 is configured such that an elevating shaft 30a provided at a lower portion of the susceptor 30 moves up and down in the vertical direction in the drawing through an elevating mechanism 30b. The substrate mounting surface of the susceptor 30 moves to an upper position so as to be flush with the upper surface of the protruding wall 12a provided in the film forming container 12 during the film forming process. Before or after the film forming process, the susceptor 30 is moved to a lower position, a shutter (not shown) provided in the film forming container 12 is opened, and the substrate is carried in from the outside of the film forming container 12, or the film is formed. It is carried out of the container 12.

The gas supply unit 16 introduces Zn (C ₂ H ₅ ) ₂ (hereinafter referred to as DEZ), oxygen gas, and gaseous water, which are source gases, into the film formation space.
Specifically, the gas supply unit 16 includes a DEZ source 16a, an O ₂ source 16b, an H ₂ O source 16c, a diaphragm valve (hereinafter referred to as a valve) 17a, mass flow controllers 17b and 17c, and a DEZ source 16a. A tube 18a connecting the film formation space in the film formation container 12 through the valve 17a, an O ₂ source 16b, a tube 18b connecting the film formation space in the film formation container 12 through the mass flow controller 17b, and an H ₂ O source 16c. And a pipe 18c for connecting the film forming space in the film forming container 12 through the mass flow controller 17c.
Each of the valve 17a and the mass flow controllers 17b and 17c is operated under the control of the controller 18 to introduce DEZ source gas, O ₂ gas, and gaseous water into the film formation space at a predetermined timing.

  The exhaust unit 24 exhausts the DEZ, oxygen gas, and gaseous water introduced from the left wall of the film formation container 12 into the film formation space through the exhaust pipe 28 in the horizontal direction. A valve 26 is provided in the middle of the exhaust pipe 28 and is appropriately opened and closed according to instructions from the controller 18.

The controller 18 controls the timing of introducing each of the raw material gases DEZ, oxygen gas, and gaseous water, and the timing of feeding power to the parallel plate electrodes 14. Furthermore, the controller 18 controls the opening and closing of the valve 26.
Specifically, the controller 18 controls power feeding to the upper electrode 14a of the parallel plate electrode 14 so that the parallel plate electrode 14 generates plasma using water in accordance with the introduction of water into the film formation space. .

  More specifically, the controller 18 controls the amount of gas input according to the opening / closing time of the valve 17a when introducing DEZ into the film formation space, and uses the mass flow controller 17b when introducing oxygen gas into the film formation space. When the gas flow rate is controlled and gaseous water is introduced into the film formation space, the gas flow rate is controlled by the mass flow controller 17c.

FIG. 2 is a timing chart for explaining the gas flow rate control performed by the controller 18 and the power supply timing to the parallel plate electrodes 14.
First, the controller 18 controls the opening / closing time of the valve 17a so that a predetermined amount of DEZ is introduced into the film forming space where the substrate is placed on the substrate mounting surface. By controlling the flow rate, DEZ is introduced into the film formation space for 0.1 seconds, for example.
Thereafter, when the controller 18 stops the introduction of DEZ into the film formation space using the valve 17a, the controller 18 then controls the flow rate of oxygen gas using the mass flow controller 17b to introduce the oxygen gas into the film formation space. Start. The introduction of oxygen gas is continued until DEZ is introduced again. Further, for a certain period during the introduction of oxygen gas, the controller 18 controls the flow rate of gaseous water using the mass flow controller 17c and starts introducing water into the film formation space. The gaseous water is introduced, for example, for 2 seconds. During a certain period of time, the controller 18 controls the high frequency power supply 20 to supply power to the upper electrode 14a through the matching box 22. This power supply is performed for 0.2 seconds, for example. By supplying power to the upper electrode 14a, the parallel plate electrode 14 generates plasma using gaseous water in the film formation space.

  After the power supply to the upper electrode 14a is stopped and the introduction of water into the film formation space by the mass flow controller 17b is stopped, purging of water, reaction products, etc. is performed by the continuously introduced oxygen gas. Thereafter, the controller 18 stops the flow rate of oxygen gas using the mass flow controller 17c. Furthermore, the controller 18 again controls the flow rate of the valve 17a so as to introduce DEZ into the film formation space. Thus, the introduction of the DEZ film formation space, the introduction of oxygen gas into the film formation space, the introduction of gaseous water into the film formation space, and the generation of plasma using water as one cycle, multiple cycles By repeating continuously, a ZnO film having a predetermined thickness can be formed on the substrate.

  As described above, the oxygen gas is continuously introduced into the film formation space during the period excluding the DEZ introduction period, and functions as a purge gas for purging the gas in the film formation space. The reason why the oxygen gas is not introduced into the film formation space during the DEZ introduction period is to prevent the DEZ and oxygen gas from reacting in the gas phase. On the other hand, since oxygen gas is introduced into the film formation space even during plasma generation using water, it is considered that a part of the introduced oxygen gas is ionized to generate plasma. As will be described later, it is considered that the content of carbon components contained as impurities in the ZnO film can be reduced, as will be described later.

FIG. 3A shows the carbon atoms (C), oxygen atoms (O), and hydrogen atoms (H) contained in the ZnO film when the ZnO film formed by the method shown in FIG. 2 is formed. The quantity is measured as a distribution in the depth direction. For this measurement, a ZnO film having a width of 20 μm, a length of 10 μm, and a thickness of 50 nm was used. Examples of the conditions in the ZnO film forming apparatus 10 include the following contents.
In the film formation space, DEZ was introduced for 0.1 second while maintaining the pressure substantially at 10 to 100 Pa. The time T ₁ (see FIG. 2) from the start of DEZ introduction to the start of DEZ introduction in the next cycle was 5.9 seconds. The oxygen gas introduction time T ₂ (see FIG. 2) was 6 seconds. In the flow rate adjustment by the mass flow controllers 17b and 17c, the flow rate was adjusted so that the ratio of oxygen gas to gaseous water was 8: 2. The introduction time T ₃ of gaseous water (see FIG. 2) was set to 2 seconds. The feeding time T ₄ (see FIG. 2) to the parallel plate electrode 14 was set to 0.5 seconds. The electric power supplied to the parallel plate electrode 14 was 200 W at a high frequency of 13.56 MHz. Thereby, a 50 nm thick ZnO film was formed on the Si substrate. The Si substrate was always maintained at 80 ° C. during the ZnO film formation process.

Carbon atoms (C), oxygen atoms (O), and hydrogen atoms (H) are measured by irradiating the surface of the ZnO film with beam-form primary ions (Cs ions), and the primary ions and molecules and atoms of the ZnO film. A method of detecting secondary ions generated by collision at the level with a mass spectrometer, that is, secondary ion mass spectrometry was used. In the measurement, particles corresponding to the masses of carbon atoms (C), oxygen atoms (O), and hydrogen atoms (H) were detected and counted.
The vertical axis in FIG. 3A is the count value (CPS (Count per Second)) of each atom with respect to the depth of the ZnO film. A lower count value indicates a smaller amount of the corresponding atom.

  On the other hand, FIG. 3B shows a count value measured by the same method as FIG. 3A when a ZnO film is formed using oxygen gas instead of gaseous water in the method shown in FIG. Is the result of This ZnO film is not a plasma using gaseous water, but a ZnO film formed using only oxygen plasma and DEZ.

As can be seen from the results shown in FIGS. 3A and 3B, since the count value of carbon atoms is low in the results shown in FIG. 3A, the ZnO film formed by the method of the present embodiment contains impurities. It shows that the content of carbon atoms is low.
As a result, during the plasma generation using water, a part of the oxygen gas introduced into the deposition space as a purge gas is ionized to generate a plasma, and the oxygen radical atoms activated by this are generated in the ZnO film. It is considered that the carbon atoms contained as impurities are purged as carbon dioxide gas or carbon monoxide gas. Therefore, the ZnO film formed by the method of this embodiment, in which carbon atoms are hardly contained as impurities, can be effectively used as a semiconductor film with few impurities.

  FIG. 4 is a diagram showing a schematic configuration of a bottom gate type semiconductor element 50 formed by using the ZnO film forming method used in FIGS. 3A and 3B. In the semiconductor element 50, a gate electrode 54 is formed on the insulating layer 52 formed on the glass substrate, the gate electrode 54 is covered with a gate insulating layer 56, and a ZnO film 58 is formed thereon. In addition, a source electrode 60 and a drain electrode 62 are formed, and a protective film 64 is formed on the uppermost layer.

With respect to this semiconductor element 50, a transfer characteristic indicating a change in the drain current Id with respect to the gate voltage Vg was measured. The width W (dimension in the direction perpendicular to the paper surface in FIG. 4) of the ZnO film 58 was 20 μm, and the length L (dimension between the source electrode and the drain electrode) was 10 μm. The drain current Id was measured with a drain voltage Vd of 5V. Therefore, the higher the drain current Id, the higher the mobility of the ZnO film. The drain current Id of the semiconductor element made of the ZnO film produced by the method of the present embodiment shown in FIG. 3A is the drain current of the semiconductor element made of the ZnO film produced by the method of the form shown in FIG. It is higher than the current Id, indicating that the mobility is high.
From this, it can be seen that the ZnO film manufactured by the method of this embodiment has few carbon atoms as impurities and exhibits excellent transfer characteristics and mobility of the semiconductor element.

In the formation of the ZnO film of Patent Document 1 described above, a ZnO film having a large crystal grain size is formed using plasma (water plasma) using gaseous water, and then the carrier of the ZnO film using oxygen plasma. The amount is reduced and the mobility in the semiconductor element is improved. For this reason, a complicated process of generating plasma using oxygen gas in addition to water plasma is used. However, in this embodiment, when a ZnO film is formed by generating plasma using water, oxygen gas is simultaneously introduced into the film formation space, so that it is contained in the ZnO film as shown in FIG. The amount of carbon atoms generated can be reduced. Therefore, a ZnO film that realizes a semiconductor device with high mobility can be easily formed by a simple process without using a conventional complicated process.
In this embodiment, although it can be used suitably for semiconductor elements, such as TFT used for a flat panel display, it can apply also to the cell for solar cells besides a flat panel display.

  As described above, the ZnO film forming method and the ZnO film forming apparatus of the present invention have been described in detail. However, the present invention is not limited to the above-described embodiments and modifications, and various improvements and modifications can be made without departing from the gist of the present invention. Of course, you may do it.

10 ZnO film forming apparatus 12 the film forming container 12a protruding wall 14 parallel plate electrodes 14a upper electrode 14b lower electrode 16 the gas supply unit 16a TMA source 16b N ₂ source 16c O ₂ source 17a diaphragm valves 17b, 17c mass flow controllers 18 controllers 18a, 18b, 18c Tube 20 High frequency power supply 22 Matching box 24 Exhaust unit 26 Valve 28 Exhaust tube 30 Susceptor 30a Lifting shaft 30b Lifting mechanism 32 Heater 50 Semiconductor element 52 Insulating layer 54 Gate electrode 56 Gate insulating layer 58 ZnO film 60 Source electrode 62 Drain electrode 64 Protective film

Claims (5)

A method for forming a ZnO film, comprising:
A first step of adsorbing the organic metal on the substrate by flowing an organic metal gas containing Zn as a source gas above the substrate disposed in the film formation space;
After the organic metal is adsorbed on the substrate, gaseous water is introduced into the deposition space and oxygen gas is introduced as a purge gas to generate plasma in the deposition space, thereby forming a ZnO film. A second step of forming,
The ZnO film forming method, wherein the first step and the second step are continuously repeated until the thickness of the ZnO film reaches a predetermined thickness.   2. The ZnO film formation method according to claim 1, wherein the oxygen gas is introduced into the film formation space as a purge gas except for the period of the first step.   The ZnO film forming method according to claim 1 or 2, wherein in the first step and the second step, the substrate is heated and held at 95 ° C or lower. An atomic layer growth apparatus,
A film formation container having a film formation space in which a substrate is disposed;
A plasma generation unit that is provided in the film formation space and generates plasma;
A gas supply unit for introducing each of an organic metal source gas containing Zn, gaseous water, and oxygen gas into the film formation space;
A controller that controls the timing of introduction of each of the source gas, the water, and the oxygen gas and the timing of feeding power to the plasma generation unit, and the controller includes the substrate. the controlled so as to introduce the raw material gas into the film forming space was the after stopping the introduction of the raw material gas, and introducing the oxygen gas as the purge gas is introduced to the gaseous water in the film forming space A ZnO film forming apparatus , wherein the plasma generating unit is controlled to generate plasma in the film forming space .   The ZnO film forming apparatus according to claim 4, further comprising a heater that heats and holds the substrate, wherein the heater heats the substrate at a temperature of 95 ° C. or less.

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