JP2006063365A - Transparent electrode manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transparent electrode manufacturing method for manufacturing a thin electrode film of low resistance which is transparent in a range from ultraviolet ray to visible ray by suppressing the high resistance effect caused by electron scattering on a film interface occurring when the film is very thin. <P>SOLUTION: The electrode film of high transparency in a range from ultraviolet ray to visible ray is provided by employing structure in which a metal layer having a layered structure of ≤30 nm thickness and a conductive transparent oxide semiconductor of a layered structure of a thickness close to the mean free path of the bulk value of the metal are laminated, thereby suppressing the increasing effect of the resistance of the film caused by scattering of conductive electrons and a reduction in the thickness of a continuous laminar part resulting in low resistance of the thin film, and by suppressing reflection of the light of a metal film. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a low-resistance electrode thin film having high transparency from the ultraviolet region to the visible region, and can be applied to increase the efficiency of optoelectronic devices.

With the advancement of information technology in recent years, improvement in recording density is demanded for light emitting elements such as LEDs and lasers, and further improvement in conversion efficiency is required for light receiving / converting elements. For that purpose, it is indispensable to use ultraviolet rays having a short wavelength, and the transparency of the electrode film on the surface necessary for electrical driving becomes very important in determining the efficiency of the device. For example, if light in a region that could not be transmitted by a solar cell or the like can be used, the efficiency is greatly improved. In general, in order to transmit light from the ultraviolet region with a thin film having a thickness of several hundreds of nanometers, a material having a band gap of 5 eV or more is required. As a material in this range, an insulator such as alumina or titanium oxide is used. Most of these oxides are used, and an ITO-based band gap of about 2.8 eV, which is mainly composed of indium oxide and is practically used as a semiconductor material exhibiting conductivity, is insufficient. As another material, an oxide such as gallium having a band gap of about 5 eV in Japanese Patent Laid-Open No. 2002-93243 is also promising, but a film having a very high processing temperature and a low resistance comparable to ITO is obtained. There is no current situation.
JP 2002-93243 A

  Therefore, as one solution, a method of increasing the transparency is effective by thinning the conductive metal film so that light absorption does not occur. However, since a metal film generally performs Volmer-Weber type three-dimensional nucleus growth in its initial growth stage, when the film is as thin as several nanometers, the film is often an island-like structure and thus does not exhibit conductivity. If the film is thickened to a layered structure that exhibits properties, the light transmittance will be significantly reduced. Therefore, a technique has been developed to suppress the generation of island-like structures of the metal film and to produce a film having a continuous layered structure. (Japanese Patent Application No. 2003-334683) However, in an actually produced metal thin film having a film thickness of several tens of nm or less, the average free path of electrons responsible for electrical conduction and the film thickness are almost the same. The electron scattering effect (size effect) at the interface between the film and the substrate cannot be ignored, and the resistance value tended to increase significantly compared to the bulk value. Furthermore, as shown in FIG. 2, an actual metal thin film has a considerably uneven surface on the atomic order scale, and even if the manufacturing method is improved, it cannot be completely flattened. It was one cause. In other words, when the film thickness is very thin, non-uniformity of the film structure due to the roughness of the substrate and non-uniformity of crystal grains due to random growth become non-negligible with respect to the film thickness. The uneven part of the film as shown in the left part of FIG. 1 occupies most of the film, and the effect of increasing the resistance value that the thickness of the layered part related to electrical conduction decreases (the effect of reducing the thickness of the layered structure) Will occur. Due to these two effects, the resistance of the ultra-thin metal film was greatly increased.

  Generally, in a flat display device such as a liquid crystal display or a plasma display, the sheet resistance value is 10Ω / sq. The following transparent electrodes are required. For example, when gold is selected as the metal, 10 Ω / sq. In order to achieve a sheet resistance of 2 to 3 nm, a film thickness of 2 to 3 nm should be sufficient. However, in reality, several hundreds Ω / sq. A high value is shown. The gold film having this thickness has a light transmittance of nearly 80% even in the ultraviolet region, but because of its high resistance value, it has been difficult to apply to a flat display device or the like that requires a low resistance.

  In order to solve the above problems, in the present invention, in order to reduce the resistance of a metal thin film having a layered structure as shown in the right part of FIG. 1, a metal thin film used as a layer for main electron conduction, By laminating a highly transparent conductive oxide semiconductor film of about several tens of nanometers, which is about the mean free path of electrons close to the bulk value of the metal to suppress strong electron scattering at the metal film surface and grain boundaries, It is characterized in that an electrode film having high transparency can be realized in a wide range from the ultraviolet region to visible light by constituting a thin electrode film as a whole while simultaneously achieving low resistance.

  By adopting the structure of the present invention, it is possible to suppress the size effect due to strong electron scattering on the film surface that occurs when the film is thin and the effect of reducing the thickness of the layered structure, so that the volume resistivity close to that of bulk metal It is possible to manufacture a film having a very low resistance value of a metal layer showing Furthermore, by making the thickness of the conductive oxide of the present invention in the range of 8 to 30 nm, it is possible to prevent the reflection of light of the visible metal film, and an electrode having a very high transmittance as compared with a single layer. A film can be made. Further, in the structure of the present invention, the unevenness of the metal film is filled with the conductive oxide, and at the same time, the film surface is covered with a hard and smooth conductive oxide as compared with the metal, so that the metal film alone is not very smooth. It is possible to form a hard film surface, and in this structure, the thickness of the entire film is in the range of several tens of nanometers. Therefore, it is necessary when manufacturing with a conventional low-resistance conductive oxide semiconductor film. Compared with a thick film of several hundreds of nanometers, material and processing costs can be greatly reduced.

  Specifically, there are three forms of the metal thin film: the island structure shown in FIG. 3, the state where the islands and layers are mixed as shown in FIG. 4, and the state where the layers are completely layered as shown in FIG. The island portions on the left side of FIGS. 3 and 4 are portions that are not related to electrical conduction, and merely reduce the transparency of the film. When a conductive oxide semiconductor film having a high transparency of about 30 nm or less according to the present invention is stacked on a metal thin film, the portion can also be incorporated into electric conduction and the resistance value of the film can be lowered. On the other hand, even in the structure of FIG. 5 having an ideal layered structure, strong electron scattering at the film interface cannot be avoided, and the electron mean free path is limited to the length of the film thickness and exhibits high resistance. Since the average free path of electrons can be increased by the film thickness without significantly degrading optical characteristics by adopting the laminated structure of the present invention, a low resistance value close to the bulk value can be realized. According to Example 1, a sheet resistance value of 90 Ω / sq. Although only a transparent electrode having a transmittance value of 70% at a wavelength of 550 nm can be obtained, the sheet resistance value is 18 Ω / sq. A transparent electrode having a transmittance value of 80% at a wavelength of 550 nm can be produced.

  As the metal conductive layer, a metal having a small specific resistance such as gold, silver or copper is used as the electron conductive layer. The reason why a metal having a small specific resistance is selected is to minimize the film thickness for producing a film having the same sheet resistance in order to increase the transmittance. If oxidation of the metal film is a concern, a gold film may be used. In order to produce a film with low sheet resistance and high transparency, it is important that a metal ultrathin film has a dense film structure and a layered structure with a smooth surface. FIG. 6 shows the film thickness dependence of the volume resistivity ρ of a gold film produced by dual ion beam sputtering (NDIBS) of ion beam sputtering (IBS), RF magnetron sputtering (RFMS), and (Japanese Patent Application No. 2003-334683). . It can be seen that ρ increases as the film thickness decreases due to the two effects described above. Among these, IBS and NDIBS films exhibit superior characteristics because the metal thin film has a smooth layered structure than normal RF magnetron sputtering.

  Next, it is necessary to stack a highly transparent conductive oxide semiconductor film having an electron mean free path close to the bulk value of the metal for suppressing the size effect and the like. FIG. 7 shows the film thickness dependence of the volume resistivity ρ and the mean free path λe of free electrons of a gold film produced by IBS in which the substrate is placed at a 45 ° direction from the target surface. The film showed good electrical conductivity only when it showed a layered structure, and ρ tended to decrease monotonously as the film thickness increased within a thin film thickness of 100 nm or less. Thus, when the λe of the gold film is roughly estimated, the value is smaller than the film thickness and tends to increase as the film thickness increases to 40 nm in the bulk. This indicates that the resistance value of the film is almost determined by the size effect. In other words, a metal film having a low resistance value cannot be obtained without a bulk thickness of 40 nm or more. However, when the film thickness is increased to this value, the transparency of the film is remarkably reduced and it cannot be used as a highly transparent electrode.

  Therefore, in order to increase the mean free path of the electroconductivity of the film, it may be above or below the metal film, but by laminating a highly transparent conductive film with a film thickness of several tens of nm, Strong electron scattering at the interface can be suppressed, the mean free path of the metal film conductor can be increased, and the resistance of the entire film can be reduced. At this time, in order to aim at widening the band from the visible region to the ultraviolet region as the electrode film, the conductive transparent oxide semiconductor to be laminated naturally needs to have a wide band. However, as shown in FIG. 11, the film thickness is about 60 nm or less. Then, light absorption due to the band gap of an oxide semiconductor such as ITO is not a problem.

  Up to now, GaO and ZnO-based materials have been studied as conductive transparent oxide semiconductors that can be used in the ultraviolet region, but transparent oxidation with low resistance comparable to ITO doped with tin indium oxide by low-temperature synthesis. No physical semiconductor has been obtained. In order to aim at the effect of lowering the resistance of the present invention, the oxide semiconductor to be stacked is required to be a low resistivity film close to a metal film rather than light absorption due to the band gap. Therefore, in order to generate the effect of the present invention, ITO is most suitable as the conductive transparent oxide semiconductor film.

  Also, the conductive oxide film having a film thickness on the order of the mean free path of the underlying metal is required to have a smooth layer structure like the metal thin film. If this structure cannot be realized, the structure shown in the right part of FIG. 1 becomes incomplete, so that the effect of reducing the resistance and the antireflection effect of the present invention cannot be expected.

  An ITO film is usually produced by plasma sputtering, but its film characteristics are very sensitive to production conditions, and recoil Ar in sputtering often damages the film along with the influence of plasma. Therefore, in IBS, a target and a substrate are opposed to each other by a normal sputtering method, and a film and a substrate having a manufacturing condition with less impact due to recoil Ar to the substrate have a large impact due to recoil Ar in which the film and the substrate are set at the incident angle of the ion beam. A film under fabrication conditions was fabricated on a water-cooled substrate. The SEM image of the film | membrane surface form of ITO with a film thickness of 300 nm is shown in FIG. It can be seen that a large grain boundary appears on the surface of the film under a production condition with a large amount of recoil Ar, which is close to that of a normal sputtering method such as RFMS, and the surface is considerably uneven. Even if a film having a thickness of several tens of nanometers is produced in this state, a low-resistance film having good electrical characteristics cannot be obtained. On the other hand, a film with very little unevenness could be produced under the production conditions with little IBS recoil Ar. The surface roughness Ra was a value of 0.4 nm or less, which is equal to or less than that of the substrate, with a film thickness of 300 nm or less.

  FIG. 9 shows the film thickness dependence of the carrier density Nc and the mobility μ of the ITO film produced under these conditions, and FIG. 10 shows the film thickness dependence of the sheet resistance ρs and volume resistivity ρ of the film. It can be seen that when the film thickness is 10 nm or less, Nc and μ are small, and ρ increases rapidly. That is, it was confirmed that the thickness of the ITO film needs to be 10 nm or more in order to aim at the effect of reducing the resistance of the present invention by this IBS manufacturing method.

  FIG. 11 shows a transmittance T spectrum using the thickness of the produced ITO film as a parameter. Judging from the transmittance in the wavelength region of 300 to 400 nm, it can be seen that a film thickness of about 60 nm or less is desirable in order to achieve a broad band. Therefore, in order to produce a transparent electrode film having a low resistance and a wide bandwidth, an ITO film having a thickness in the range of 10 to 60 nm is effective as the conductive transparent oxide semiconductor to be laminated.

  FIG. 12 shows the film thickness of sheet resistance ρs of an ITO / Au film obtained by laminating a gold thin film simple substance Au produced under the same sputtering conditions as the ITO film and an ITO film having a thickness of 29 nm (280 Ω / sq. Of ρs). Indicates dependency. In the case of Au alone, the film was continuous from 9.4 nm and exhibited conductivity, and the resistance tended to decrease as the film thickness increased. On the other hand, the ITO / Au film exhibits conductivity even when the Au is 2.1 nm, but since the Au portion has an island shape, the conductivity is 100 Ω / sq. The value of the stand was high. The film thickness at which the island-like structures start to be connected to each other is 7 nm to 18 Ω / sq. And showed a low value. This indicates that the ITO film of the ITO / Au film has an effect of electrically connecting island structures even when the metal film is not completely continuous.

Next, FIG. 13 shows the film thickness dependence of the volume resistivity ρ of an ITO / Au film in which a single gold thin film (Au) and an ITO film are laminated. The ρ of the ITO / Au film is calculated using only the thickness of the Au film. The ρ of Au tended to decrease rapidly as the film thickness increased from 9.4 nm. On the other hand, ρ of the ITO / Au film tends to gradually increase in the range of 1.2 to 1.5 × 10 ⁻⁵ Ωcm in the range of the continuous film thicker than 7 nm, but is lower than that of the Au film. Is shown.

By the way, since this effect includes a phenomenon in which the ITO film enters the Au film electrically in parallel, it is necessary to consider the influence. Therefore, FIG. 14 shows the sheet resistance value R _Au // R _ITO of the film and the actually measured sheet resistance value R _{ITO / Au} of the laminated film, calculated simply by placing the ITO film and the Au film in parallel. R _Au // R _ITO calculated at a film thickness of 7 nm is about 100Ω / sq. The measured RITO / _Au is 18Ω / sq. It was confirmed that the measured values showed much smaller values. This difference shows the effect of reducing the resistance of the present invention.

FIG. 15 shows the relationship between the sheet resistance value ρs and the transmittance T ₅₅₀ having a wavelength of 550 nm. Although _T550 is 75% at maximum in the case of gold alone, it is possible to produce a film having a characteristic of nearly 90% by taking a laminated structure, and a film having a high transmittance even with the same ρs.

FIG. 16 shows the transmittance T spectrum when the thickness t _ITO of the ITO film laminated on the gold thin film (Au) having a thickness of 12 nm is changed. In the visible region of 400 to 800 nm, the effect of increasing the transparency as the thickness of the ITO film increased was observed. In particular, the effect is remarkable in the wavelength range near 650 nm, and the film thickness of about 29 nm is the most excellent for increasing the transparency of visible light. On the other hand, the transmittance in the ultraviolet region with a wavelength of 400 nm or less is reduced due to the characteristics of the ITO film, but has a transmittance of 20% or more even at 250 nm. 10Ω / sq. The low resistivity of the stand was realized. This effect did not appear when Au had a complete island structure.

  From the above, in order to obtain high transparency in the ultraviolet region in the present invention, the thickness of the ITO film may be reduced to about 60 nm or less, and the oxide has a relatively high hardness, so it functions as a protective film for the metal layer. It can be seen that the film thickness may be controlled according to the purpose of use.

In order to reduce the resistance, it is important to produce a layered film even when the metal thin film portion is thin. Therefore, considering the film growth process in a general PVD method, a refractory metal is considered suitable as a metal material. Therefore, three types of films of tantalum (Ta), platinum (Pt) and tin (Sn), which have a low melting point but are used as a dopant for ITO, are prepared by IBS, and sheet resistance ρs and wavelength of these metal thin films. The relationship between transmittances T ₅₅₀ and T ₃₀₀ for light at 300 and 550 nm was examined. The result is shown in FIG. Ta and Pt can have a layered structure from a region having a smaller film thickness than Au, but in order to obtain the same sheet resistance as Au due to the high specific resistance of the material itself, a thicker film of the difference is necessary, and It was found that since the film was microcrystallized, the resistance value of the film was very high, and a film having a lower resistance and higher transmittance than Au could not be produced. On the other hand, with respect to Sn having a low melting point, the film was layered from a very thick region of several tens of nm or more, and there was almost no transparency in the film thickness at which conductivity was obtained. From the above results, it was considered that Au having a small specific resistance was the most excellent as the metal thin film. As other materials, silver Ag, copper Cu and the like having a small specific resistance are also effective. Among them, Au, which is difficult to oxidize and is stable, is considered to be the most reliable and promising material.

  FIG. 18 shows SEM images of the surface of the ITO film having film thicknesses of 165 nm and 350 nm. When the film thickness was 165 nm or less, the film surface was very smooth and the grain boundary could not be observed even with FE-SEM. It was found that a large grain boundary appeared when the film thickness was 350 nm or more, which became clear as the film thickness increased and the film surface was raised. Therefore, when the surface roughness Ra was measured by AFM, when the film thickness was 165 nm or less, it was about 0.46 nm or less, which was almost the same as the substrate, and showed a tendency to increase from that thickness, and the film thickness became 1.4 nm at 350 nm. . This means that when a low resistance film is made of ITO alone, the film needs to be thickened to some extent, but in that state there is an electrode surface, so application to a device that requires high flatness cannot be expected. Show.

  In order to show the effect of the ITO film having a very smooth surface, FIG. 19 compares the AFM images of the 25 nm-thickness Au film made of IBS and the ITO film laminated. . In the film of Au alone, particle growth with a diameter of about 20 nm occurs, and large irregularities appear on the film surface, but the irregularities are filled by laminating the ITO film of the present invention, which is shown in the right part of FIG. Such a very smooth film surface was formed. Therefore, it is shown that the film of the structure of the present invention is excellent as a low-resistance transparent electrode film having a smooth surface.

  FIG. 20 shows the relationship between the film thickness Ra and the surface roughness Ra of a film obtained by laminating a single gold thin film (Au) and ITO. Ra is obtained from an AFM image of a square region having a side of 1 μm. In both cases, when the Au film having a film thickness of up to 9.4 nm has an island-like structure, Ra tends to increase as the film thickness increases. Showed a tendency to decrease. In particular, the Ra of the ITO laminated film was very small, showing a value of 0.14 nm, which is smaller than 0.4 of Ra on the substrate surface. This indicates that the cross-sectional structure of the film of the present invention is completely in the state shown in FIG. Therefore, in the case of a single metal thin film, if the substrate surface is rough, the film itself may be locally cut, and it is difficult to form a continuous electrode film over a wide range. However, since the effect can be reduced in the laminated structure of the present invention, even a substrate such as a resin film that cannot reduce the surface roughness of the substrate can produce a smooth surface film. This indicates that the film of the present invention is very effective as a transparent electrode such as organic EL. Further, since the film is very thin as a few tens of nanometers as a whole, it is difficult to peel even a flexible substrate, and application to electronic paper or a flexible display is easy.

  FIG. 21 shows an ITO film, a gold thin film alone, and ρs of 18 and 10 Ω / sq. The transmittance | permeability characteristic of this film | membrane is shown. ρs is 10Ω / sq. It can be seen that with the ITO film alone, T decreases rapidly at a wavelength of 400 nm or less, but with the film of the present invention, it decreases gently. Further, since the film is thin, no light interference occurs, and the transmittance is the same as that of the ITO film in the visible region. Among them, ρs is 18Ω / sq. In this case, a film having a transmittance of 60% or more at 400 nm and 90% or more at 650 nm could be produced.

  As described above, the present invention suppresses the electron scattering effect at the film interface or grain boundary from the embodiment, and also incorporates the island-like region on the layered structure that is not related to the electric conduction in the metal thin film single layer into the electric conduction region. Therefore, it was shown that this is a very effective method that can realize a smooth electrode film having a low resistance and a high transparency in a wide range from the ultraviolet region to the visible region.

  In the transparent electrode of the present invention, since it has a feature of high transparency and low value in a wide range from ultraviolet light to visible light, it is an energy conversion type such as a light emitting device such as a flat display or a solar cell used in the wavelength region. It can be applied to improve the characteristics of light receiving electrical devices. Moreover, since a very smooth film | membrane surface can be formed, it is optimal also as a transparent electrode for organic electroluminescent displays on a resin film.

It is a conceptual diagram of the conventional metal film and the film | membrane of the laminated structure of this invention. It is a surface SEM image of a gold thin film. It is sectional drawing which showed the motion of the electroconductivity of the metal thin film (left) with the conventional island-like structure, and the film | membrane (right) of the laminated structure of this invention. It is sectional drawing which showed the motion of the electroconductivity of the metal thin film (left) which has the conventional island shape and layered structure, and the film | membrane (right) of the laminated structure of this invention. It is sectional drawing which showed the motion of the electroconductivity of the metal thin film (left) with the conventional perfect layered structure, and the film | membrane (right) of the laminated structure of this invention. This is the film thickness dependence of the volume resistivity ρ of the gold Au film produced by various methods. This is the film thickness dependence of the volume resistivity ρ and the mean free path λe of electrons of an Au film produced by ion beam sputtering IBS with the substrate placed at a direction of 45 degrees from the target surface. It is a SEM image of the ITO film | membrane of Au film | membrane produced by IBS. This is the film thickness dependence of the carrier density Nc and the mobility μ of the ITO film produced by IBS. This is the film thickness dependence of the sheet resistance ρs and volume resistivity ρ of the ITO film. It is a transmittance T spectrum using the thickness of the ITO film as a parameter. This is the film thickness dependence of the sheet resistance ρs of an (ITO / Au) film in which a single gold thin film (Au) and an ITO film are laminated. This is the film thickness dependence of the volume resistivity ρ of an (ITO / Au) film in which a single gold thin film (Au) and an ITO film are laminated. FIG. 6 is a comparison diagram of a measured value _RITO / _RAu and a measured value RITO _{/ Au} when a measured sheet resistance value ρs and an upper ITO film are connected in parallel. This is the relationship between the sheet resistance ρs of the Au single layer film and the laminated film and the transmittance T _{550 at} a wavelength of 550 nm. This is a spectrum of transmittance T when the film thickness of the ITO film laminated on a single gold thin film (Au) is changed. This is the relationship between the sheet resistance value ρs of various metal thin films and the light transmittances T ₃₀₀ and T ₅₅₀ at wavelengths of 300 and 550 nm. It is a SEM image of the ITO film | membrane surface with a film thickness of 165 nm and 350 nm. It is an AFM image of a film in which an Au film having a film thickness of 25 nm and an ITO film are further laminated. This is the relationship between the film thickness Ra and the surface roughness Ra of a film obtained by laminating a single gold thin film (Au) and ITO. The ITO film, the gold thin film alone, and the rho of the present invention is 18 and 10Ω / sq. It is the spectrum of the transmittance | permeability T of this film | membrane.

Explanation of symbols

1 Metal thin film 2 Transparent oxide semiconductor 3 Conductor 4 Substrate 5 Mean free path of electrons in metal 6 Mean free path of electrons in transparent oxide semiconductor 7 Thickness of island structure 8 Thickness of layer structure

Claims (5)

  A structure in which a metal layer having a layer structure of 30 nm or less and a conductive transparent oxide semiconductor having a layer structure having a thickness close to the mean free path of the bulk value of the metal are stacked, and the electron on the film surface and grain boundary of the metal layer Sheet resistance with suppressed resistance increase due to scattering is 50 Ω / sq. An electrode thin film characterized by being transparent in the range of ultraviolet to visible light having the following low resistance value:   The transparent electrode thin film according to claim 1, wherein gold is used for the metal layer.   2. The transparent electrode thin film according to claim 1, comprising indium oxide as the transparent oxide semiconductor.   The transparent electrode thin film according to claim 1, wherein reflection of light of the metal film is suppressed by setting the thickness of the conductive transparent oxide semiconductor to 10 to 30 nm.   4. A transparent electrode thin film having a transparent oxide semiconductor according to claim 3, which is fabricated by using an ion beam sputtering method in which a substrate and a target are opposed to each other and a recoil Ar is suppressed.

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