WO2003029511A1

WO2003029511A1 - Method for deposition of transparent silver-containing metal layers

Info

Publication number: WO2003029511A1
Application number: PCT/EP2002/009798
Authority: WO
Inventors: Matthias Fahland; Christoph Charton; Patrik Karlsson
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e. V.
Priority date: 2001-09-27
Filing date: 2002-09-03
Publication date: 2003-04-10
Also published as: DE10147861A1

Abstract

In numerous technical applications, layers with high optical transparency and good electrical conductivity are required, whereby layers of the utmost thinness with said properties are often sought. Magnetron atomisation methods are frequently used for the production of said layers. By application of a discharge voltage greater than 700 V the layer thickness of silver-containing layers can be significantly reduced with the same planar resistance and improved optical transparency. Production of flat screens, thin-layer solar cells, electroluminescence systems, etc.

Description

Process for the deposition of transparent silver-containing metal layers

The invention relates to a method for the deposition of thin transparent silver-containing metal layers of high conductivity in a vacuum. Such layers can pass through

Evaporation of the metal or by cathode sputtering, also called sputtering, are deposited.

Thin transparent silver-containing metal layers of high conductivity are further understood to be layers of pure silver or a silver alloy with a layer thickness below 20 nm and a sheet resistance below 30 Ω _D. These are characterized by the fact that they are not optically dense, that is to say a considerable part of the incident

Let light through.

Such thin silver-containing metal layers are found as components of layer systems

Applications in which they have to ensure high optical transparency with the best possible electrical conductivity.

An important application for thin silver-containing layers is their use as transparent electrodes.

Transparent electrodes are used today in a wide variety of devices. Their use is particularly widespread in flat screens which are used both for displays at computer workstations and in various mobile devices. Regardless of the special technology of image generation, a transparent electrode is required, which is located between the viewer and the self-illuminating components of the flat screen, or components that modify the external light.

Other products in which transparent electrodes are used are solar cells, flat lamps based on organic or inorganic electroluminescence, electrochromic glasses or mirrors.

• For many applications of thin silver-containing metal layers, where the visual impression of a finished product is important, it is important that the layers are deposited evenly and with good reproducibility. This uniformity and reproducibility relates to the layer thickness, electrical and optical properties and to the structural parameters of the layer. This applies in particular to transparent electrodes. Due to these requirements, the use of the Magnetron sputtering necessary as a process for layer deposition. Evaporation processes do not allow layers with sufficiently uniform layer parameters to be deposited.

According to the current state of the art, transparent electrodes are usually made from the semiconductor indium tin oxide (90% ln ₂ 0 ₃ , 10% Sn0 ₂ : short name ITO). This material is transparent in the visible spectral range and has an electrical conductivity in the range 10 ^"4 ... ^ ^{" 3} Ωcm. Although this is far less than with the metals used in electrical engineering such as copper, aluminum and silver, it is nevertheless sufficient for most applications today.

It is known to apply ITO by means of solid-state atomization (sputtering) in a vacuum, with different magnetrons usually being used.

However, this method, which corresponds to the current state of the art, has considerable

Disadvantage.

Indium is required as the raw material for the production of the layer. This material is only available to a limited extent. Therefore and from the fact that an expected increase in demand can be expected, the price of the target material will increase in the next few years. This makes many economic assessments based on today's information uncertain or uneconomical from the start.

For the deposition of high-quality ITO layers with conductivity values of about 1 * 10 ^"4 Ohmcm, heating of the substrate is necessary. Since the optimal temperatures are above 200 ° C, various heat-sensitive substrates are not suitable for coating with such ITO layers.

For high-quality ITO, it is necessary to work with very low operating voltages of approx. 100 V in the case of sputtering. This can only be achieved by using a specially designed magnetic field or by coupling additional electrical energy.

In the case of very low-resistance layers (10 Ω _D and less), despite the measures mentioned, it is necessary to deposit a layer of several 100 nm, which can lead to problems with the internal stability of the layer, since internal forces, in particular the compressive layer stress, the layer structure influence negatively. In addition, large layer thicknesses always mean increased coating costs due to the increased coating time.

Efforts have been made to replace ITO with a semiconductor material that has properties similar to indium tin oxide and at the same time is made up of elements that are sufficiently available and inexpensive. An example of this is the use of zinc aluminum oxide. This material is much cheaper than ITO and there is no restriction on the availability of the raw materials. This material also has the disadvantage of having to set high substrate temperatures. Furthermore, only specific conductivities could be achieved, which are slightly above those of the ITO, which means layer thicknesses similar to those when using ITO layers. Thus, the problems of coating costs and layer stability remain, as arise from the application of very thick layers.

Another possibility for producing transparent electrodes is to store a thin metal layer, in most cases made of silver, or a silver-containing layer in two transparent layers, for example consisting of ITO. This increases the conductivity of the overall system with the same thickness of the overall system, since the metal, as already explained above, has a significantly higher conductivity than the transparent material. With the same conductivity, significantly lower layer thicknesses of the ITO layers are required, since the conductivity of the layer system is essentially determined by the silver layer. Therefore, such layer systems can gain importance in display technology, for example on glass substrates, because they help to save the expensive coating material ITO. [M. Bender et al. Thin Solid Films 326 (1998), 76-71).

In addition, this layer system can also be used as a high-quality transparent electrode on plastics, for example polyethylene terephalate (PET) (M. Fahland et al. Proceedings of ICCG 2000). A disadvantage of this method, however, is that the transparency of the overall system is reduced by using a pure metal layer. For this reason, it is important that the metal layer itself has the highest possible transparency in order to reduce the transparency of the electrode as little as possible.

Common to the applications with a silver layer is that a thin silver layer is deposited by sputtering. A magnetron with a Silver target installed in a coating chamber. The chamber is then evacuated and argon is introduced into the chamber as the working gas. The target is connected to a sputter power supply. A gas discharge is ignited by the energy supply. Due to the potential conditions in the discharge plasma, individual atoms are knocked out of the target material by ion bombardment and can deposit as a layer on the substrate. The power of the sputtering power supply is usually fixed. Then current and voltage are set according to the impedance conditions in the discharge chamber. A voltage between 200 and 500 V is usually set for silver targets. The exact value depends on the size of the target area, the argon pressure and the magnetic field of the magnetron.

It is known that when a thin metal layer is deposited, a few separate metal islands initially form on the substrate. This is called the aggregate state of the layer. With increasing coating time, i.e. H. increasing number of metal atoms on the substrate, these islands grow together and form a closed layer.

In the aggregated state, the optical and electrical parameters of the layer differ significantly from those of the closed layer. The aggregated state is characterized by an increased electrical resistance and an increased optical absorption. For all the applications listed above, it is advantageous to deposit layers that are as thin as possible and that are already closed. Layer thicknesses of silver-containing layers of over 20 nm are unsuitable for transparent electrodes due to their naturally low transparency. It is known that the transition from the aggregated state to the closed film can vary depending on the substrate and sublayer. In particular, the

Roughness of the surface plays a crucial role (Abeles e.al. Solid State Problems XXIV (1984), 93-1 17)

It is also known that the transition can be influenced by superimposing an RF discharge and a DC discharge. The disadvantage of this method is that one station has to be equipped with several sputter power supplies. Furthermore, it is technically extremely complex to provide large-area magnetrons with RF sputtering power supplies.

It is also known that the set coating rate influences the transition. (Journal of Optical Society V40 No. 4 (1950)). However, since in a continuous system This process has the disadvantage that it has to maintain constant substrate speed in all stations, that it creates boundary conditions for the other deposition processes, which can have an unfavorable effect on investment costs or productivity of the plant.

It is also known to influence the transition to the closed layer by adjusting the substrate temperature (Journal of Optical Society V40 No. 4 (1950)), (Abeles et al. Solid State Problems XXIV (1984), 93-117). The disadvantage of this method is that it means additional effort for the substrate heating and subsequent process steps can be influenced. Furthermore, the method can only be used to a limited extent, in particular for polymer substrates, which must not be exposed to high temperatures.

A reduction in the layer thickness of a silver-containing layer is only sensible or economical to a certain degree according to the prior art for the applications mentioned. Below a certain transition layer thickness, which for silver can be between 10 and 18 nm depending on the process conditions, there is no continuous layer, but a conglomerate of individual layer islands. This shows itself on the one hand in a drastic increase in the electrical resistance and on the other hand also leads to an increasing absorption within the layer despite the further decrease in the layer thickness. It follows from this that there is an optimal thickness of the silver-containing layer for achieving the maximum transparency of a silver-containing layer, which is largely determined by the transition layer thickness at which the formation of a closed layer occurs for the first time. The transparency of the silver-containing layer also determines the transparency of the entire layer system which contains the silver-containing layer, that is to say, for example, a transparent electrode for electroluminescent systems.

The invention is based on the object of specifying a method which is improved compared to the prior art and which permits the production of a thin silver-containing layer with the highest possible optical transparency and good conductivity.

According to the invention, this object is achieved by a method according to claim 1. Claims 2 to 8 contain further advantageous refinements.

According to the invention, the atomization of a silver-containing target ensures that the magnetron discharge at voltages between the cathode and the anode of more than 700 V. is operated. It is irrelevant whether a special electrode or another magnetron connected as an anode is used as the anode. It was surprisingly found that thin silver-containing metal layers, which are deposited under these circumstances, have a significantly higher transparency than silver-containing metal layers of the same layer thickness, which have been deposited by conventional sputtering processes, in particular by magnetron sputtering at lower discharge voltages. The invention is apparently based on the effect that a significant increase in the discharge voltage compared to the values customary in the atomization of silver targets apparently leads to closed silver-containing layers even with smaller layer thicknesses.

Such a high voltage can advantageously be realized at low plasma impedances, such as are present in the case of silver sputtering, if the power supply is pulsed, i. H. if it is interrupted before the current has reached a value that can damage the target. Furthermore, it was surprisingly found that the transparency can be increased even further if in addition to the

Sputter processes customary noble gas, preferably argon, yet another, multi-atom gas is admitted. Under the influence of plasma, this apparently forms free radicals on the substrate or already present sub-layers, which influence the aggregation behavior. The inlet of nitrogen has proven to be particularly effective. A further increase in the discharge voltage to 850 V or 900 V also led to a further increase in the transparency with the same layer thickness of the silver-containing layer. Technological requirements for transparent electrodes, namely surface resistances between 10 Ω _D and 30 Ω ₀ for layer thicknesses that do not yet cause damage to the layer due to internal stresses, can be met with the method according to the invention, with a high optical transparency being achieved. It has proven to be particularly advantageous if a pure silver layer is deposited as a silver-containing layer by sputtering a silver target. It is also particularly advantageous if the coating is carried out in such a way that the thickness of the silver-containing layer is less than 15 nm.

The invention will be explained in more detail using an exemplary embodiment.

A polymer film made of polyethylene terephthalate PET is coated in a vacuum recipient by magnetron sputtering. For this purpose, the film is wrapped in this recipient from an unwinding roll over a chill roll to a winding roll. While the slide passed over the chill roll, it passes through three coating stations one after the other in which it is coated with the ITO-Silver-ITO coating system. The two ITO layers have a layer thickness of 40 nm.

According to the invention, the silver layer is applied by magnetron sputtering. The magnetron in the coating station, in which the silver coating takes place, is connected to a DC sputtering power supply, which is operated in constant voltage mode. 900 V are permanently set as the output voltage. An electrical circuit between the DC sputter power supply alternately connects the DC sputter power supply to the magnetron for 10 μs and then the connection is interrupted for 80 μs. It is thereby achieved that the three-layer system ITO-silver-ITO deposited on polyethylene terephthalate has a transparency of 84% at a wavelength of 550 nm and an area resistance of 14 Ω _D. These layers are particularly suitable as transparent electrodes for use in flat screen technology or electroluminescent systems.

Claims

claims

1. A method for the deposition of silver-containing metal layers with layer thicknesses below 20 nm by magnetron sputtering in an argon-containing atmosphere, characterized in that the coating process is regulated so that at least temporarily between at least one magnetron operated as a cathode with a target made of the material to be atomized and at least one anode is at a voltage greater than 700 V.

2. The method according to claim 1, characterized in that the plasma discharge of the sputtering process is operated in a pulsed manner.

3. The method according to claim 1 or 2, characterized in that a target of pure silver is atomized.

4. The method according to any one of claims 1 to 3, characterized in that nitrogen is supplied to the argon-containing atmosphere.

5. The method according to any one of claims 1 to 4, characterized in that the deposition of the silver-containing metal layer is carried out on a polymer substrate.

6. The method according to any one of claims 1 to 4, characterized in that the deposition of the silver-containing metal layer is carried out on a glass substrate.

7. The method according to any one of claims 1 to 6, characterized in that the plasma discharge is operated at least temporarily with a voltage which is greater than 850 V.

8. The method according to any one of claims 1 to 7, characterized in that the coating is carried out so that the thickness of the silver-containing layer below

Is 15 nm.