JP2011144408A - Method of depositing transparent conductive film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively deposit a Nb-doped titanium oxide film having a desired composition in a short period of time. <P>SOLUTION: A substrate 101 constituted of a transparent glass is prepared. Then, a transparent conductive film 102 comprising niobium-doped titanium oxide is deposited on the substrate 101 by simultaneously performing reactive sputtering with oxygen gas using a first target comprising metal Ti and the sputtering using a second target comprising niobium pentaoxide (Nb<SB>2</SB>O<SB>5</SB>). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for forming a transparent conductive film used in flat panel displays, solar cells, and the like.

  Currently, ITO (Indium Tin Oxide) is mainly used as a material for transparent electrodes used in flat panel displays and the like, but alternative materials are being searched for due to the problem of depletion of In resources. So far, ZnO-based transparent conductive films such as Al-doped ZnO and Ga-doped ZnO have become promising candidates. However, these materials are only used in some devices such as solar cells due to chemical resistance problems and the fact that the wet etching process has not been established.

  On the other hand, Nb-doped titanium oxide (TNO) films are attracting attention as a new transparent conductive film that has recently appeared. If the same characteristics as those of the ZnO-based transparent conductive film can be obtained by the TNO film, and if excellent characteristics such as workability and film stability are obtained, the TNO film becomes an alternative material that can be changed to a ZnO-based material.

Until now, the TNO film has been mainly formed by the pulse laser deposition (PLD) method. The PLD method is a film forming technique capable of obtaining an oxide thin film with good characteristics. However, the film formation by the PLD method has a problem that the range of conditions under which a film having a good composition can be formed is narrow and the productivity is poor. On the other hand, attempts have been made to form a TNO film using magnetron sputtering having excellent productivity (see Non-Patent Documents 1 to 3). In Non-Patent Documents 1 and 2, a TNO film is formed by magnetron sputtering using a TiO ₂ oxide target doped with Nb. In Non-Patent Document 3, a TNO film is formed by magnetron sputtering with Nb ₂ O ₅ pellets placed on a TiO ₂ target.

Meagen A. Gillispie, et al., "Sputtered Nb- and Ta-doped TiO2 transparent conducting oxide films on glass", J. Mater. Res., Vol.22, No.10, pp.2832-2837, 2007. N. Yamada, et al., "Structural, electrical and optical properties of sputter-deposited Nb-doped TiO2 (TNO) coated films", Thin Solid Films, vol.516, pp.5754-5757, 2008. Y. Sato, et al., "Transparent conductive Nb-doped TiO2 films deposited by direct-current magnetron sputtering using a TiO2-x target", Thin Solid Films, vol.516, pp.5758-5762, 2008.

  However, the TNO film formation technique described above has a problem that it is not easy to change the ratio of the amount of Nb added.

The electrical conductivity of the TNO film strongly depends not only on the amount of Nb but also on the amount of oxygen constituting the film. For example, a TiO ₂ film having a stoichiometric composition does not exhibit electrical conductivity. Moreover, it is not realistic to separate oxygen from the produced TNO film to make it an oxygen-deficient state (reduced state) that exhibits electrical conductivity. For this reason, in forming the TNO film, it is important to deposit a reduced TNO film. For example, when an amorphous TNO film is deposited on a substrate in advance and crystallized by annealing in a hydrogen gas atmosphere or vacuum, the amorphous TNO film before crystallization is already reduced. It is necessary to be.

In order to form a reduced TNO film, an oxygen-deficient TiO _2-x target is used. However, the production of such a reducing target generally has a problem of high cost.

  Further, when a target that is an oxide is used even in the reduced state, as is well known, the sputtering rate is not so fast, and the film formation takes time. On the other hand, it is possible to deposit titanium oxide in a reduced state by sputtering a film by introducing a small amount of oxygen using a target of metal Ti. When the sputtering mode is in the material mode, the sputtering speed is rapidly reduced. In such a state, it takes a very long time to form a relatively thick film used for a flat panel display or the like. In either case, there is a problem that it takes time to form a film, and productivity is difficult.

  The present invention has been made to solve the above-described problems, and it is an object of the present invention to form an Nb-doped titanium oxide film having a desired composition in a shorter time and at a lower cost. And

  The method for forming a transparent conductive film according to the present invention includes a step of performing reactive sputtering with oxygen gas using a first target made of metal Ti and a step of performing sputtering using a second target made of niobium pentoxide. By carrying out simultaneously, the transparent conductive film which consists of a titanium oxide with which niobium was doped is formed on a board | substrate.

  In the method for forming the transparent conductive film, reactive sputtering and sputtering using a second target are different forms of sputtering, and reactive sputtering may be electron cyclotron resonance plasma sputtering. Sputtering using a target made of niobium pentoxide may be RF magnetron sputtering.

  As described above, according to the present invention, reactive sputtering using oxygen gas using a first target made of metal Ti and sputtering using a second target made of niobium pentoxide are performed simultaneously. Thus, an excellent effect is obtained that an Nb-doped titanium oxide film having a desired composition can be formed in a shorter time and at a lower cost.

It is process drawing for demonstrating the formation method of the transparent conductive film in embodiment of this invention. It is process drawing for demonstrating the formation method of the transparent conductive film in embodiment of this invention. It is a block diagram which shows the structural example of the film-forming apparatus for enforcing the formation method of the transparent conductive film in embodiment of this invention. Carrier density (n: white circle), Hall mobility (titanium oxide (TiO _2-x ) film formed by reactive sputtering with oxygen gas using a target made of metal Ti when the supplied oxygen flow rate is changed. FIG. 6 is a characteristic diagram showing changes in μ (black circle) and resistivity (ρ: white square). Carrier density (n: white circle), Hall mobility (μ: black circle) with respect to RF power (sputter output) applied to a target made of Nb ₂ O ₅ when a TNO film is formed at an oxygen flow rate of 1.0 sccm, It is a characteristic view which shows the change of resistivity ((rho): white square). Carrier density (n: white circle) and Hall mobility (μ: black circle) with respect to RF power (sputter output) applied to the target 205 made of Nb ₂ O ₅ when a TNO film is formed at an oxygen flow rate of 1.4 sccm. FIG. 5 is a characteristic diagram showing a change in resistivity (ρ: white square). FIG. 5 is a characteristic diagram showing a change in transmittance from visible to near-infrared region of a TNO film having an oxygen flow rate of 1.4 sccm and an Nb concentration of 6 mol /% during film formation.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A and 1B are process diagrams for explaining a method of forming a transparent conductive film in an embodiment of the present invention. First, as shown in FIG. 1A, for example, a substrate 101 made of transparent glass is prepared.

Next, reactive sputtering by oxygen gas using a first target made of metal Ti and sputtering using a second target made of niobium pentoxide (Nb ₂ O ₅ ) are performed simultaneously, as shown in FIG. 1B. Thus, the transparent conductive film 102 made of titanium oxide doped with niobium is formed on the substrate 101.

  According to the present embodiment described above, in reactive sputtering using oxygen gas using the first target, even if the supply amount of oxygen gas is reduced within a range where the sputtering speed does not decrease, oxygen is sputtered by the second target. Therefore, the transparent electrode film 102 having a desired oxygen composition can be formed. Similarly, since Nb is doped by sputtering with the second target, the transparent electrode film 102 having a desired composition can be formed. Further, neither the first target nor the second target causes an increase in cost. Thus, according to the present embodiment, an Nb-doped titanium oxide film having a desired composition can be formed in a shorter time and at a lower cost.

Here, in the reactive sputtering using a metal Ti target, for example, the range of the oxygen flow rate in the metal mode is determined in advance by an experiment, and the oxygen flow rate is determined within the determined range. Sputtering of the reduced TiO _2-x film may be performed. Simultaneously with this sputtering, sputtering from the Nb ₂ O ₅ target (second target) may be superimposed. By changing the sputtering power of the Nb ₂ O ₅ target, a TNO film having a desired Nb amount and oxygen amount can be obtained.

The oxygen atoms supplied to the TNO film are oxygen atoms contained in the outermost TiO _x layer formed by oxidizing the metal Ti target (first target) during reactive sputtering, and the supplied oxygen gas is generated by plasma. Dissociated oxygen atoms and oxygen contained in the Nb ₂ O ₅ target are included. The amount of oxygen released from the Nb ₂ O ₅ target is independent of the amount of oxygen gas introduced, and depends only on the power applied to the Nb ₂ O ₅ target. As for the slightly reduced TiO _{2 -x} film that cannot be completely oxidized by sputtering of the metal Ti target, the electric conduction characteristics greatly change even with a slight difference in oxygen amount. The amount of oxygen in the TiO _2-x film can be controlled to oxygen supplied from the Nb ₂ O ₅ target.

Here, reactive sputtering with oxygen gas using the first target made of metal Ti can be performed by, for example, electron cyclotron resonance (ECR) sputtering. A good quality TNO film can be formed by depositing TiO _{2 -x} by an ECR sputtering method capable of forming a film with low damage. In the ECR sputtering method, first, plasma is generated in a position remote from the substrate surface. Second, since the cylindrical target is used, the emission direction of the high-energy particles is mainly directed toward the inside of the cylindrical target, and is incident on the substrate surface in order to emit in a direction parallel to the substrate surface. The amount is greatly reduced. As a result, according to ECR sputtering, a good quality TNO film can be formed. In addition, the ECR sputtering has an advantage that it is easy to ensure the reducing property of the thin film because the plasma flow is continuously applied to the TNO film during the film formation.

  Hereinafter, a film forming apparatus for carrying out the method for forming a transparent conductive film in the embodiment of the present invention will be described. FIG. 2 is a configuration diagram showing a configuration example of the film forming apparatus. This apparatus is based on a vacuum processing chamber 201 communicated with a vacuum exhaust device such as a turbo molecular pump (not shown), an ECR plasma source 202 provided in the vacuum processing chamber 201, and an ECR plasma generated from the ECR plasma source 202. And a target (first target) 203 made of metal Ti for performing sputtering. The ECR plasma source 202 and the target 203 constitute an ECR sputtering source (first sputtering source). The ECR plasma source 202 is operated, ECR plasma is generated using argon gas, and RF atoms are applied to the cylindrical target 203 so that Ti atoms are sputtered and adhere to the surface of the substrate W located downstream. . The substrate W is placed on the substrate table 210.

  The target 203 is not limited to the cylindrical type, and is disposed at a position surrounding the periphery of the plasma flow generated from the ECR plasma source 202, and has a shape in which the surface to be sputtered is parallel to the traveling direction of the plasma flow. It only has to be. For example, the target 203 may be formed in a polygonal shape. The target 203 may be composed of a plurality of plates.

In addition, the film formation apparatus in this embodiment mode includes an RF magnetron plasma generation unit 204 and a target (NibO pentoxide (Nb ₂ O ₅ )) for performing sputtering using plasma generated by the RF magnetron plasma generation unit 204 ( A second target) 205, which are connected to the vacuum processing chamber 201 by an introduction unit 206. The RF magnetron plasma generation unit 204 and the target 205 constitute an RF magnetron sputtering source (second sputtering source). The RF magnetron plasma generation unit 204 generates argon gas plasma and applies RF to the disk-like target 205 to sputter Nb and O atoms of the target 205 (RF magnetron sputtering), and these are located downstream. It adheres to the surface of the substrate W to be

With these configurations, the particles sputtered and sputtered from the target 203 and the particles sputtered and sputtered from the target 205 are deposited on the film forming surface of the substrate W to be processed disposed inside the vacuum processing chamber 201. It becomes possible. Further, the vacuum processing chamber 201 is provided with a gas inlet 207 for introducing oxygen gas. In FIG. 2, the introduction of a sputtering gas such as argon is omitted. As will be described later, an Nb-doped titanium oxide film can be formed by using a target 205 made of Nb ₂ O ₅ .

  In the film forming apparatus configured as described above, the angle formed between the normal line of the surface of the target 205 (surface to be sputtered) and the normal line of the surface of the substrate W is set to 60 ° or more and less than 90 °. In FIG. 2, the angle θ is set to 60 ° or more and less than 90 °. The direction in which the plasma from the ECR plasma source 202 flows is the normal direction of the substrate W, and the angle between the normal of the surface of the target 205 and the direction in which the plasma from the ECR plasma source 202 flows is θ, which is 60 More than 90 ° and less than 90 °. In this embodiment, a cylindrical target 203 is used, and a line passing through the center of the hollow portion of the target 203 is a direction in which the plasma from the ECR plasma source 202 flows.

  If the angle is within the above-described range, the Nb deposition rate is not excessively increased, and the amount of Nb introduced can be controlled within a desired range. On the other hand, when the angle exceeds 90 °, the target 205 cannot be expected from the film forming surface of the substrate W, and particles from the target 205 hardly reach. For this reason, the said angle shall be less than 90 degrees. Actually, if the angle exceeds 80 °, the region (width) of the surface of the substrate W that can be expected from the target 205 becomes too narrow. Therefore, the angle formed by the normal line on the surface of the target 205 and the normal line on the surface of the substrate W is better in the range of 60 ° to 80 °. For example, by setting the above-described angle to 70 °, a TNO film having an RF power of about several tens of watts and an Nb content of several mol% can be formed in the RF magnetron plasma generation unit 204.

  In the plane direction parallel to the surface of the substrate W, the substrate W is disposed at a position that falls within a region (range) where the sputtered particles from the target 205 reach. Further, the substrate stage 210 is provided with a substrate rotation function for rotating the substrate W about the normal line passing through the central portion. By rotating the substrate W by this function, the film thickness and each composition in the plane of the substrate W are provided. Can be ensured.

  Further, the direction in which the plasma from the ECR plasma source 202 flows is the normal direction of the substrate W, and this state maximizes the deposition rate of the titanium oxide film by the ECR sputtering source. The density per unit area of the ECR plasma flow (sputtered particles) decreases and the deposition rate (film formation rate) decreases as the normal direction of the substrate W is shifted with respect to the direction in which the plasma from the ECR plasma source 202 flows. . If the normal direction of the substrate W is shifted too much with respect to the direction in which the plasma from the ECR plasma source 202 flows, the deposition rate of the titanium oxide film decreases too much, and the amount of Nb doping increases relatively, and the desired Nb The amount of dope may not be obtained. Therefore, the angle of the normal direction of the substrate W with respect to the direction in which the plasma from the ECR plasma source 202 flows should not be so large.

  Next, a method for forming an Nb-doped titanium oxide film using the film forming apparatus in the present embodiment described above will be briefly described. A substrate made of transparent glass is prepared, and the prepared substrate is carried into the film forming apparatus described above.

  Next, an ECR (Electron Cyclotron Resonance) plasma is generated in the ECR plasma source 202. For example, first, the inside of the film forming apparatus is decompressed to a high vacuum pressure, and then, for example, a rare gas such as argon (Ar) gas and oxygen gas is introduced. In this state, a 2.45 GHz microwave (about 500 to 600 W) and a 0.0875 T magnetic field are supplied to the ECR plasma source 202 to obtain an electron cyclotron resonance condition, whereby Ar and oxygen are contained in the plasma generation chamber. A state in which plasma (ECR plasma) is generated is obtained. The magnetic field is supplied by passing a current 14A through the electromagnet. T (Tesla) is a unit of magnetic flux density and is 1T = 20000 Gauss.

  The ECR plasma generated as described above is emitted from the plasma generation chamber to the side of the processing chamber communicating therewith by the divergent magnetic field of the magnetic coil of the ECR plasma source 202. In this state, a high frequency power (target bias) of, for example, 13.56 MHz · 600 W is supplied (applied) to the target 203 disposed at the outlet of the ECR plasma source 202 to which the ECR plasma is supplied. As a result, the particles generated by the generated ECR plasma collide with the target 203 to cause a sputtering phenomenon, and particles (Ti atoms) constituting the target 203 jump out.

At the same time, in the RF magnetron plasma generation unit 204, a predetermined RF power (for example, 10 W) is applied to the target 205, and RF plasma is generated in the magnetron sputtering source (target 205) so that sputtering is generated. The target 203 to which RF power is applied is sputtered by the plasma generated in this way in the magnetron sputtering source, and particles (Nb atoms, O atoms) constituting the target 205 made of Nb ₂ O ₃ jump out. It becomes a state.

  Here, in general, the operating pressure in RF magnetron sputtering is about 1 Pa, and stable discharge cannot be performed in a low pressure state where ECR plasma generation is performed. However, in this apparatus, ECR plasma is generated in a region communicating with the region of the magnetron sputter source, and a part of the ECR plasma enters the vicinity of the target 205 (the region of the magnetron sputter source). . For this reason, even in a magnetron sputtering source that is in a low pressure state as magnetron sputtering, plasma is supplied, and discharge (plasma) can be stably continued even with an RF power of about 10 W.

  As described above, the Nb-doped titanium oxide film is formed on the substrate by generating plasma and setting the sputtering state so that the particles sputtered by the target 203 and the target 205 are deposited on the substrate W. Can be formed.

Next, an evaluation result of a film actually produced using the above-described apparatus will be described. First, a TiO _2-x film is formed without doping from a target (second target) made of Nb ₂ O ₅ . The film formation is performed under the sputtering conditions where the microwave power of the ECR sputtering source is 530 W and the RF power to the target 203 is 600 W.

FIG. 3 shows the carrier density (n: white circle), Hall mobility (μ: black circle), and resistivity (ρ of the formed titanium oxide (TiO _2−x ) film when the supplied oxygen flow rate is changed. : White square). The substrate temperature is 400 ° C.

When the oxygen flow rate is increased, the deposited TiO _{2 -x} film becomes more oxidized, and the carrier density n gradually decreases. However, the Hall mobility μ is almost constant regardless of the oxygen flow rate. Therefore, it is considered that the change in resistivity ρ mainly reflects the change in carrier density. In particular, the resistivity changes by two orders of magnitude while the oxygen flow rate changes from 1.2 sccm to 1.4 sccm. Note that sccm is a unit of flow rate, and indicates that a fluid at 0 ° C. and 1 atm flows 1 cm ³ per minute.

The resistance change described above means that the TiO _2-x film is close to the stoichiometric TiO ₂ film. However, since the TiO ₂ film is an insulator, having conductivity means that it is in a reduced state.

At this time, the deposition rate of the TiO _2-x film decreases from 7.6 nm / min when the oxygen gas flow rate is 0.9 sccm to 5.4 nm / min when the oxygen gas flow rate is 0.9 sccm, but is still sufficiently large. I keep it. However, when the oxygen flow rate is set to 1.5 sccm, the film formation rate is reduced by one digit or more to 0.36 m / min. Further, the potential of the target 203 changes abruptly. From these facts, it can be seen that under the above-described conditions, the oxide mode is obtained when the oxygen flow rate is 1.5 sccm. In addition, the conductivity of the titanium oxide film formed in this oxide mode is lost. From the above results, it can be seen that when the oxygen flow rate is lower than 1.4 sccm, the film formation in the metal mode can be continued.

FIG. 4 shows carrier density (n: white circle) and Hall mobility (RF) with respect to RF power (sputter output) applied to a target 205 made of Nb ₂ O ₅ when a TNO film is formed at an oxygen flow rate of 1.0 sccm. FIG. 6 is a characteristic diagram showing changes in μ (black circle) and resistivity (ρ: white square). FIG. 4 shows the niobium concentration (mol%) corresponding to the sputter output. The mol concentration of Nb atoms was calculated by ICP-AES analysis.

As can be seen from FIG. 4, the resistivity increases as the sputtering output is increased to increase the amount of sputtering from Nb ₂ O ₅ . Under the condition of an oxygen flow rate of 1.0 sccm, it is considered that oxygen vacancies formed due to an insufficient oxygen composition ratio are responsible for most of the electrical conductivity. This shows that the effect of increasing the carrier due to the addition of Nb and the effect of reducing the oxygen vacancies due to the addition of oxygen from the target 205 cancel each other, and the effective number of carriers is not increased.

FIG. 5 shows carrier density (n: white circle), Hall mobility (with respect to RF power (sputter output) applied to the target 205 made of Nb ₂ O ₅ when a TNO film is formed at an oxygen flow rate of 1.4 sccm. FIG. 6 is a characteristic diagram showing changes in μ (black circle) and resistivity (ρ: white square).

  Under this condition, more oxygen is introduced into the titanium oxide film than in the case described above. In this state, when the amount of Nb added is increased, the resistivity ρ of the formed film is minimized when the concentration is around 3 to 4 mol /%. The carrier density n increases to 3 to 4 mol /%, but the Hall mobility μ does not change. Further, when Nb is doped at a high concentration, both the carrier density n and the Hall mobility μ are gradually decreased. This is presumably because crystal disturbance is induced by Nb introduced at a high concentration. Compared with the results shown in FIG. 4, the resistivity is increased by one digit or more by the amount of oxygen.

FIG. 6 is a characteristic diagram showing a change in transmittance from visible to near-infrared region of a TNO film having an oxygen flow rate of 1.4 sccm and an Nb concentration of 6 mol /% during film formation. It has a peak at a wavelength of 500 nm and has an average transmittance of 65% in the visible range. The resistivity of this TNO film was 2.7 × 10 ⁻² Ωcm.

As described above, the present invention is characterized in that a binary sputtering system is used to control both the amount of Nb and the degree of oxidation. As the above-described experimental results show, the electrical conductivity of the TNO film is generated by the generation of carriers from oxygen vacancies and Nb atoms, and by controlling both independently, the desired resistivity and transmittance can be obtained. A TNO film can be obtained. Further, since TiO _{2 -x} can be formed within the range of the metal mode using a metal Ti target, high-speed sputter film formation is possible. In the present invention, the oxidation of the TNO film is slightly changed by the oxygen supplied from the Nb ₂ O ₅ target. In reactive sputtering using a metal Ti target, the amount of oxygen does not need to be increased. Reactive sputtering from the target is not affected. According to the present invention, a TNO film having a resistivity of 10 ⁻² Ωcm to 10 ⁻³ Ωcm can be easily formed on a glass substrate in a sputtered state without performing post-annealing or the like.

  It should be noted that the present invention is not limited to the embodiment described above, and that many modifications can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, in the above description, the ECR sputtering method is used for reactive sputtering using a target made of metallic Ti, and the RF magnetron sputtering method is used for sputtering using a target made of niobium pentoxide. Instead, both may use the RF magnetron sputtering method. On the other hand, due to magnetic field interference, it is not easy to apply the ECR sputtering method to both. Further, although argon gas is used as the inert gas, the present invention is not limited to this, and the same applies when xenon gas is used. In the above description, the case where the TNO film is formed on the glass substrate has been described. However, the present invention is not limited to this. For example, the TNO film may be formed on a silicon substrate.

  101 ... substrate, 102 ... transparent conductive film.

Claims (3)

Performing reactive sputtering with oxygen gas using a first target made of metal Ti;
A step of performing sputtering using a second target made of niobium pentoxide to form a transparent conductive film made of niobium-doped titanium oxide on the substrate. Forming method. In the formation method of the transparent conductive film of Claim 1,
The reactive sputtering and the sputtering using the second target are different forms of sputtering,
The method of forming a transparent conductive film, wherein the reactive sputtering is electron cyclotron resonance plasma sputtering. In the formation method of the transparent conductive film of Claim 2,
Sputtering using a target made of niobium pentoxide is RF magnetron sputtering, wherein the transparent conductive film is formed.