WO1995012006A1

WO1995012006A1 - Process and device for electron beam vapour deposition

Info

Publication number: WO1995012006A1
Application number: PCT/DE1994/001210
Authority: WO
Inventors: Siegfried Schiller; Klaus Goedicke; Jonathan Reschke; Bert Scheffel; Christoph Metzner; Heinz Kern
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1993-10-27
Filing date: 1994-10-11
Publication date: 1995-05-04
Also published as: DE4336680A1; DE4336680C2

Abstract

In the electron beam vapour deposition using an arc, cathode spots occur which lead to the formation of droplets. These very adversely affect the quality of the coatings. Accordingly to the invention, a periodically deflectable, high-energy electron beam is moved over the deposition material to form a virtually uniformly heated region in which there is a high rate of evaporation. An arc discharge is struck in this region which impinges in a diffuse manner and extends over said region. A high proportion of the vapour is ionised. No process gas is fed in during the process. The invention is applicable even to the reactive coating of components, tools and steel strip.

Description

METHOD AND DEVICE FOR EVAPORATING ELECTRON BEAMS

The invention relates to a method for evaporating materials in a vacuum by means of an electron beam in connection with an arc discharge and the device for carrying out the method. The method is preferably suitable for plasma-assisted vapor deposition of large areas and is used in particular for reactive coating of components, tools and strip steel.

In electron beam evaporation, electrons are emitted from a hot cathode in an electron gun and accelerated by a high voltage (10 ... 50 kV). The high-energy electron beam can be deflected and focused by means of electrical or magnetic fields and is directed directly onto the surface of the material to be evaporated. The decisive advantage of electron beam evaporation lies in the fact that the vapor-emitting surface is heated directly without the energy being supplied via the crucible or the evaporation material. This also enables the use of water-cooled copper crucibles, thereby preventing reactions between the crucible wall and the material to be evaporated. Another important reason for using this method is the good local and temporal controllability of the electron beam, which enables the use of large-area evaporator crucibles. The highest evaporation rates are achieved with powerful electron guns of the axial type.

A disadvantage of this evaporation process lies in the relatively low proportion of excited and ionized particles and the low energy (E <leV) of the evaporated particles. This is due on the one hand to the relatively low process temperature (evaporation temperature) on the vapor-emitting surface and on the other hand to the fact that the cross section for the impact between high-energy beam electrons and vapor particles is very small.

Essential requirements for the deposition of high-quality, dense layers - even with low substrate temperatures - temperature and for the production of connecting layers by reactive process control - are just high degrees of excitation and ionization of the vapor cloud.

Various methods for plasma-assisted layer deposition are described which are intended to overcome the disadvantages mentioned.

It is known to accelerate the scattering and secondary electrons generated on the molten bath surface by the primary high-energy electron beam by means of an additional electrode. This is set to a low, positive potential (20 ... 100 V) and arranged in the evaporation zone in such a way that many impacts between electrons, vapor and reactive gas particles can occur on the path of the electrons to the anode (US Pat. No. 3,791,852). A glow discharge forms and the substrate reaches an ion current. The energy of the positively ionized particles can additionally be increased by a negative bias voltage applied to the substrate.

However, this method has not yet become technically successful since attempts to use the process on an industrial scale by using large evaporator crucibles with high evaporation rates have failed.

A device is known which is intended to improve the above-mentioned principle in such a way that stoichiometric layers can also be deposited at higher coating rates. This device is characterized in that the evaporator crucible is surrounded by a chamber which has an aperture in the direction of the substrate. A positively biased electrode is arranged outside the chamber and sucks off the charge carriers generated in the inner chamber, a glow discharge burning in the area of the aperture opening and electrode being generated (DE 36 27 151 AI). With this method, coating rates around 5 μm / min can be achieved without further notice. The disadvantages of the method lie in the fact that the reactive gas is let into the inner chamber and leads to an increased pressure therein. The interactions between electrical Although the electron beam and reactive gas are wanted, they lead to strong scattering of the electron beam at higher coating rates and the higher gas quantities required. In addition, a large proportion of the vaporized material is deposited unused on the wall of the inner chamber.

In order to overcome the disadvantages mentioned, intensive electron sources have been developed that work with low acceleration voltages. These include the low-voltage arc evaporators and hollow-cathode electron beam evaporators with heated or cold cathodes. Characteristic of these processes is the design of the crucible as an anode and the inlet of a working gas (eg argon) in a separate cathode chamber. Since a low gas pressure is desired in the coating chamber, pressure stages must be arranged between the cathode and coating chamber. A method for evaporating metals is also described, an electron beam from a transversal evaporator and a low-voltage arc being directed onto the crucible connected as an anode (DE 28 23 876 C 2). Compared to the individual processes, the evaporation rate and ionization or excitation can thus be implemented more independently of one another.

The plasma activation is very intensive in these processes, in practice these processes provide layers with very good properties, but high coating rates of around 1 μm / s were not achieved with these processes. The transfer of these processes to extensive crucibles for coating large areas has not yet been solved.

In addition, arc evaporators are also used for coating purposes. For this purpose, an arc is ignited on the cathode of the arc evaporator using suitable means. The arc burns in the self-generated steam between the cathode and the anode, the discharge being constricted on the cathode side in so-called cathode spots with very high current densities (j = 10 ⁵ ... 10 ⁸ A / cm ² ) (DE 31 52 736). The cathode spots in which the melting and evaporation of the target material take place move stochastically and abruptly over the cathode surface. The average drift speed and the direction of this movement is influenced by the target material, the arc current, external magnetic fields and the presence of additionally introduced gases. The decisive advantage of this process lies in the very high degree of ionization of the generated steam cloud (10 ... 90%) with a high proportion of multi-ionized particles.

Layers applied with this process are very dense and have a high adhesive strength. In many applications, however, these advantages are severely limited by the undesired incorporation of numerous microparticles up to approximately 50 μm in size, the so-called droplets. Because of the extremely high current density, these droplets are thrown out of the melting craters on the surface of the cathode. The frequency and size of these droplets can be reduced by various measures. These include methods for the subsequent filtering of droplets using magnetic or electrical deflection fields for charged particles. However, this again requires additional equipment.

Another direction of development of the arc evaporators assumes that droplets are suppressed or avoided if the high intrinsic dynamics of the cathodic arc base point is restricted by guiding the base point via additional means on the target surface. The base point is guided through the field of a permanent magnet on a closed path. The target can be removed in a defined manner by moving the magnet relative to the target surface (US Pat. No. 4,673,477).

Uniform removal of the target surface is also achieved by using an electron or laser beam to guide and stabilize the cathode base (DE 40 06 456 C 1). The arc discharge is operated in an area where a substantial part of the arc current flows through small spots on the target surface and the target is connected as a cathode. The laser or electron beam creates a local vapor cloud over the target. The cathode base is concentrated in the local vapor cloud and can be guided over the target surface by deflecting the laser or electron beam. A method for evaporating material in a vacuum is also known, in which the material to be evaporated is bombarded with electrons from a low-voltage arc discharge, as a result of which the material to be evaporated is additionally subjected to a high-energy electron beam. This combination of an arc discharge with an electron beam evaporator is said to achieve high depletion rates for refractory metals and electrically non-conductive materials (DE 32 06 882 AI).

The disadvantage of this method is that high-energy electron beams (energy> 1 keV) can penetrate metal vapors well, but there is considerable scattering in the gases. However, gases, preferably argon, are absolutely necessary to maintain the arc discharge. This leads to a practical limitation. High-melting metals and electrically non-conductive materials can be evaporated; the desired activation. However, the vapor or reaction gas takes place in a small volume range above the crucible. This lack stands in the way of many applications.

Finally, an arc discharge for coating purposes is also known, in which the anode is designed as a thermally insulated crucible and is heated with the electron current of an arc which burns on a separate, cold cathode. The arc automatically concentrates on the hot evaporation material of the anode into so-called anode spots and leads to its intensive evaporation, excitation and ionization. The arc burns mainly in the vapor of the anodized material. (DE 34 13 891 C 2) This form of arc evaporation completely avoids the formation of droplets. Compared to the processes with cathodic evaporation, the disadvantage is the comparatively less ionization of the steam, the difficulty in separating the vapors from the anode and cathode which arise at the same time, and the energy consumption required for the evaporation of cathode material, which does not contribute to the formation of layers.

The invention has for its object to provide a method for vacuum coating by electron beam evaporation and the associated device with which a very high Coating rate is reached. A high degree of ionization of the generated steam cloud should be possible and the spectrum of separable materials should be very large. The coating of large areas should be possible through large evaporator areas and the coating rate, ion current density on the substrate and the average energy of the deposited particles should be easy to control. In contrast to the known methods with arc discharge, no cathode spots may occur.

According to the invention, the object is achieved according to the features of claim 1. Refinements of the method and the device are described in the subclaims.

It has been found that, with a very high vapor density in the space between the anode and the cathode, the non-stationary cathode spot phenomena on the cathode surprisingly disappear due to a correspondingly high surface temperature of the material to be evaporated. The arc discharge can be maintained in a parameter range in which an intense and distributed plasma is formed over the hot and evaporating parts on the surface of the material to be evaporated. The base of the arc extends over the area that is almost uniformly heated by the deflected electron beam. The material to be evaporated is a cathode in that it is contacted via the crucible wall or an additional electrode in the crucible.

Even at a low voltage (10 to 100 V) between the cathode and the anode, a current-intensive diffuse arc discharge with a current strength of more than one hundred, preferably several thousand amperes is produced under these conditions without an additional ignition device. The current density of this new type of arc is relatively low compared to known vacuum arcs and is 10 to 1000 A / cm ² .

This creates a stable working method with a high evaporation rate and controllable degree of excitation and ionization of the steam. The upper limit of the excitation and ionization that can be achieved depends on the type of evaporation material. It can be determined by typical values for the ion current density characterize substrates to be coated in the range of 100 mA / cm ² . For some materials, much higher values can be achieved.

A special feature of the method according to the invention is that the transient phenomena which occur in known vacuum arcs, such as rapid movement of the cathode spot, frequent quenching of the discharge, fluctuations in the operating voltage and plasma intensity do not occur.

The main advantage is that the diffuse arc discharge with its large-area cathode attachment does not cause any droplets because the droplet emission in the case of arc discharges is closely coupled to the cathode mechanisms with very high current densities. In fact, the droplet freedom could be demonstrated on layers produced by the described method.

The special possibilities of the method are that the location as well as the area and consequently the current density of the cathodic approach of the diffuse arc can be controlled. They result from the fact that the electron beam can be deflected very quickly and at the point of impact can draw any figures and areas on the material to be evaporated, which form the base area for the diffuse arc. To achieve a specific power density, the electron beam can be focused or defocused. The use of separate power controls for the electron beam and arc discharge results in the possibility of largely independently adjusting evaporation parameters (evaporation rate) and plasma parameters (degree of ionization and excitation).

The process is suitable for the deposition of layers of pure metals and alloys under the influence of ions (ion plating) and leads to very good layer properties. However, it is also particularly suitable for reactive evaporation for the production of layers from chemical compounds.

For the deposition of connecting layers with the method according to the invention, a reactive gas is introduced into the in a known manner Vacuum chamber embedded. Reactions take place between reactive gas particles and vapor particles deposited on the substrate. Stoichiometric layers can be deposited if the evaporation rate and the reactive gas inlet are correctly coordinated. Reactions with the reactive gas also occur at the cathode, but these hardly affect the stability of the diffuse arc discharge when the electron beam strikes the vaporized material with a high power density and thus keeps the surface free of contamination.

The problem of extensive plasma sources in plasma-assisted coating has so far mostly been solved by stringing together a number of individual plasma sources with associated separate power supplies. This procedure is technically very complex and expensive. It was found that the process according to the invention is particularly suitable for plasma-assisted large-area vaporization. The reason for this is that it is surprisingly possible to divide the arc on the cathode surface into a plurality of spatially separate areas, in that different areas of the surface are acted upon successively by a high alternating frequency at the same time by the electron beam. This means that spatially extended, large evaporator crucibles can also be used for the coating of large-area substrates. Several separate areas on the surface of the vaporized material can be activated by a single evaporator system (electron beam gun with deflection system) and a common plasma power supply system.

The invention is explained in more detail using an example. In the accompanying drawing, a top view of a device for electron beam vaporization is shown.

At a recipient 1 is an electron gun 2 of the axial type with a power of max. 300 kW and an adjustable acceleration voltage from 10 to 50 kV flanged. The recipient 1 is evacuated to a pressure of 10 ^{~ 3} Pa using a vacuum puncture system 3. In the recipient 1 there is a water-cooled evaporator crucible 4 made of copper with a diameter of 25 cm, which is connected to the negative pole of the arc power supply 8. A static magnetic field, which is generated by two opposing electromagnets 5, deflects the high-energy electron beam 6, which is injected into the recipient 1 in the horizontal direction, onto the evaporator crucible 4. By means of a known deflection device on the electron gun 2, the electron beam can draw any figures on the material to be evaporated at the point of impact. A U-shaped electrode 7 is arranged at a distance of 5 cm above the evaporator crucible 4, the opening of which electrode points in the direction of the incident electron beam 6. The electrode is water-cooled and connected to the positive pole of the arc power supply 8, which is capable of delivering regulated currents of 20 ... 2000 A at voltages of 0 ... 70 V. The negative pole of the power supply is connected to the mass of the device. The substrate is connected to negative potential against ground by means of a bias current supply. A reactive gas can be admitted into the recipient via the gas inlet system 9.

The process for coating a substrate with titanium nitride is carried out as follows.

The evaporator crucible 4 is filled with titanium. The electron beam 6 is focused on the surface of the material to be evaporated and deflected in such a way that a spiral-shaped path with an outer diameter of 6 cm is formed on the material to be evaporated. This spiral figure rotates periodically on a circular path, so that its centroid describes a circle of 1 cm in diameter and passes through it at a frequency of 2 Hz. The electron gun 2 is set to a power of 60 kW at a voltage of 45 kV. The material to be evaporated is in the liquid state and the area of the spiral-shaped web is heated up virtually uniformly. If a stationary vapor density has arisen in said area, the arc power supply is switched on. An arc can be observed on the target surface which burns exclusively in titanium vapor. At a current of 300 A, the discharge changes into the diffuse form described. The current will now increased to 1000 A, the discharge voltage is 15 V.

The substrate to be coated is at a distance of 20 cm above the edge of the evaporator crucible 4. The bias voltage is 50 V. If the arc discharge in the described diffuse form burns with a current of 1000 A, a current flows to the substrate with a current density of 100 mA / cm ² .

The plasma-assisted deposition of stoichiometric titanium nitride layers with a high deposition rate on the substrates is achieved by admitting reactive nitrogen up to a pressure of 1.2 Pa.

Claims

PATENT CLAIMS

1. A method for electron beam evaporation in a vacuum by means of a very fast and high-frequency deflection of the high-energy electron beam deflected on the material to be evaporated in a vaporizer and an arc discharge burning simultaneously between the material to be evaporated and an anode, characterized in that the rapid deflection of the electron beam, part of the entire surface of the material to be evaporated is heated almost uniformly in such a way that a high evaporation rate occurs in this area and that the arc discharge burns in this area and forms a diffuse arc, the base of which approximates the area on the material to be evaporated, which is uniformly heated by the electron beam. . Speaking and thereby ionizing a substantial part of the steam that no process gas is supplied during the process.

2. The method according to claim 1, characterized in that on the material to be evaporated by the local and temporal deflection of the electron beam several quasi uniformly heated surfaces are formed and a diffuse arc is formed in each of these surfaces.

3. The method according to claim 1 and 2, characterized in that the substrates to be vaporized are brought to a freely selectable, relative to the evaporative potential negative.

4. The method according to claim 1 to 3, characterized in that a reactive gas is admitted for the production of connecting layers during the Verdamp¬ fungsprozesses.

5. Device for carrying out the method according to claim 1, consisting of a recipient with an evaporator crucible arranged therein and a magnet system for deflecting an electron beam, which generates an electron gun arranged on the recipient, characterized in that the evaporator crucible (4) to the negative pole of an arc current supply device 8 is placed at a short distance above the evaporator crucible (4) outside the surface of the evaporation gas tes and within the magnets (5) of the magnet system for deflecting the electron beam (6) an electrode (7) is connected to the positive pole of the arc current supply device 8, and that this electrode (7) in the area in which the Electron beam (6) enters, is open.

6. Device according to claim 5, characterized in that the electrode (7) acting as an anode is water-cooled and / or thermally insulated.

7. Device according to claim 5 and 6, characterized in that an additional electrode for supplying the arc current is arranged in the evaporator crucible (4).

8. Device according to claim 5 to 7, characterized in that on the recipient (1) a plurality of electron guns (2) are angeord¬ net, each associated with a specific area of the Verdampfertie¬ gel (4) to this with the electron beam ( 6) to act upon.