GB2200654A - Heating enhancement of resistive evaporation sources in ionisation assisted physical vapour deposition - Google Patents

Abstract

The enhancement and control of evaporation rates from one or more resistive evaporation vapour sources in physical vapour deposition is achieved by the use of negative ion and/or electron bombardment of the source surface. An electron emitting source 9 is biased negatively with respect to resistive evaporation source 10 which is itself biased positively with respect to the vacuum vessel. <IMAGE>

Description

HEATING ENHANCEMENT OF RESISTIVE EVAPORATION SOURCES IN IONISATION ASSISTED PHYSICAL VAPOUR DEPOSITION This invention relates to a means of increasing and controlling evaporation rates during ionisation assisted Physical Vapour Deposition. In particular it refers to resistive evaporation.

In recent years there has been considerable commercial and scientific interest in the deposition technologies which are carried out under partial vacuum conditions, and which incorporate some degree of ionisation of the depositing species. Benefits which are claimed for these processes include improved adhesion, controlled coating structures, and,. in the case of ceramic deposition, reduced deposition temperatures. One problem, especially in the case of the refractory, high melting point materials (which are used particularly in the reactive processes producing ceramic films lies in identifying a suitable means of evaporating the metal. Early work was carried out using a resistively heated evaporant holder, often in the form of a coil of wire, a sheet metal boat or a solid graphite or intermetallic crucible which held the metal to be evaporated.Provided the melting point of the holder was above that of the evaporant then this method provided a convenient means of producing evaporation. However, the technique proved unsuitable for the evaporation of higher melting point metals such as tungsten, molybdenum and titanium, and so alternative techniques were developed, such as various types of electron beam guns, arc evaporation sources and sputter sources. These had the advantage that the source itself enhanced ionisation of the deposition species.

The purpose of the present invention is to permit the resistive evaporation of materials which could not previously be readily evaporated by this technique, and to increase the evaporation rates and degree of control for materials previously evaporated by this method.

Furthermore, the invention allows the enhancement and control of ionisation of the gaseous species within the coating chamber, particularly the evaporated metal species. These objectives are achieved by the incorporation of an electron emitting source or sources (9) biased negatively with respect to the resistive evaporation source (10), which is itself biased positively with respect to the vacuum vessel (Figure 1). This arrangement is intended to achieve several effects. Firstly electrons drawn through the vapour stream directly increase ionisation of the evaporant.

Secondly, on arriving at the melt surface they impart a heating effect, which can be enhanced by any production of negative ions that occurs. This allows materials to be evaporated in part by resistive-heating, which could not previously be vapourised by that technique.

The method- has the advantage that additional energy is imparted to the vapour source at its surface, rather than relying on indirect heating via a holder or crucible. It can thus be used alone, or to enhance other vapour sources, primarily resistively heated ones, for which this arrangement has not previously been used.

The process layout (a schematic view of which is shown in Figure 1 for a single source system) utilises a heated wire (usually of tungsten) to emit electrons, and these are preferentially attracted to the vapour source, rather than the rest of the earthed chamber, by biasing that source positively by the supply 1, typically at between 10 and 200 V DC.

The process would typically follow the following stages. Firstly the chamber would be evacuated to a pressure below 10-5 Torr. If this level of ultimate pressure is not used, then the chamber must be "flushed" comprehensively (eg with argon). Also it is advisable to heat the surfaces within the chamber to "bake-out" and drive off any contaminants that might otherwise be emitted during processing. This can conveniently be achieved utilizing 9, the electron emitter. The process would then typically proceed by reducing the pumping speed and introducing an inert gas such as argon, 6, to create a pressure suitable for sputter cleaning (typically 10-20 m Torr). This stage would involve the initiation of a bias voltage on the specimen, 8, through the supply 5.This voltage could be in the range from 200V to 3kV depending on the material of the specimen and the complexity of its shape. Lower voltages are generally preferred when sharp corners or edges are present as these can lead to so called 'field effects' leading to sputtering nonuniformity on the specimen. After a suitable sputtering period (typically 20 minutes), during which time the specimen will be heated by ion and energetic neutral bombardment, the electron emitting source, 9, will be heated by passing a current through it via supply 3. At this stage it can be at earth potential with the vapour source, 10, or an additional electrode (not shown) acting as an anode, biased by supply 1, and/or the electron emitter can be biased negatively with respect to earth, by supply 2. In the latter case it would not be necessary to initiate supply 1 at this stage.The effect of the electron emitter is then to increase ionisation of the inert gas and thereby increase the bombardment heating of the specimen.

Furthermore, by supporting the discharge, the chamber pressure can be lowered, reducing the level of background contaminants and preparing the system for the deposition stage. In some cases it is possible to avoid the initial (non thermionically supported) sputter-clean stage described above. The next stage is coating, which it is often desirable to carry out at reduced pressure as coating structures are thereby improved. This part of the cycle will normally involve the use of supplies 1, 2, 3, 4 and 5, though supply 5 can be dispensed with - the sample being either earthed or allowed to 'float' to a self-bias potential.Also if sufficient power is input to 10 from the bombarding electrons and/or negative ions then supply 4 can be dispensed with - making this process layout particularly flexible and efficient in terms of the materials that can be evaporated, the number of power supplies and the power consumed. With the latter modification it can be necessary to improve the cooling in the region of the vapour source. The system has particular benefits for the evaporation of materials which sublime. It is even possible to utilise direct resistance heating for some materials, without needing a holder or boat.Supplies 1 and 2 would typically operate at 10-200V, the exact value of 1 being determined primarily by the heating required of the source, and supplies 2 and 3 being adjusted to give the desired level of ionisation for the process (those skilled in this technology know that ionisation influences specimen heating and coating morphological properties). Further control over the system can be achieved by the incorporation of the additional positive electrode mentioned earlier (not shown) which can also be biased by supply 1. This has the effect of determining the plasma potential and the spatial distribution of the plasma in the desposition chamber.

The deposition stage may be accompanied by the inletting of a reactive gas or gases, 7, to produce ceramic or cermet films, for example, by the reaction of a reactive gas with a suitable metal species. The main novel feature of the arrangement is that it permits resistively heated sources to be used in the evaporation of high melting point metals at quite high rates (e.g. > 0.5 g/min). Examples of such metals are titanium, molybdenum, chromium, zirconium and tungsten - materials not previously considered suitable for commercial deposition by resistive evaporation methods.

Also the layout is particularly flexible and suited to exploitation since it avoids the necessity for magnetic confinement, deflection or focussing of the electron source, though of course these modifications can be incorporated if desired. Electrically biased surfaces surrounding the vapour source itself may (if necessary) be protected from contaminating the coating either by covering them with the coating material or by incorporating an earthed or biased shield close to but not touching these surfaces. The latter approach protects the surfaces from electron or ion bombardment and simultaneously prevents unwanted discharges occurring in that region.

It will be clear to those skilled in this technology that the new process and layout described will lend itself to continuous or pulsed vapour source replenishment systems, and to the deposition of alloys by the use of multiple sources or mixed element evaporation from an individual source or sources. Also it is possible to further reduce the number of power supplies (eg by using the same supply to bias 8 and 9, if necessary with suitable resistances in the circuit to regulate the voltage levels).

Figure 1 shows the process layout schematically.

In practice the vacuum feedthroughs which carry the electrical current can be water cooled to avoid unwanted heating. Also earthed shields can be incorporated close to the specimen and electron emitter feedthroughs to prevent build up of deposit on the insulators and to avoid electrical breakdown in those regions. By placing heat shields (typically made from a refractory material, and biased with the electron emitter) between the specimen and the electron emission source or sources (or otherwise placing the latter out of 'line of sight' of the specimen) the direct radiant heating of the specimen can be reduced, thereby helping to keep its temperature down. This can also reduce contamination of the specimen by the electron emitter material (e.g. tungsten). Feeding the inert gas into the chamber in the vicinity of the electron emitter can be beneficial in two ways. Firstly it can increase the ionizing effect of the electrons - also it can help to prevent reaction of any reactive gases present with the emitter. This is particularly effective when the filament is confined or shrouded to maintain a higher pressure in its vicinity.

Claims (10)

1. A process for the enhancement and control of evaporation rates from one or more vapour sources in physical vapour deposition by the use of negative ion and/or electron bombardment of the source surface.

2. A process according to claim 1, wherein the vapour source is resistively heated.

3. A process according to claim 1, whereby the -ionisation of the species is enhanced and controlled in ionisation assisted physical vapour deposition.

4. An apparatus for carrying out the process of claim 1, comprising an electron emitting source or sources biased negatively with respect to the evaporation source which in turn is biased positively with respect to the vacuum vessel.

5. An apparatus according to claim 4 in which all or part of a crucible, boat or other evaporant holder, or parts connected thereto, are manufactured from, coated with, or otherwise covered by the material to be evaporated, or other suitable material to prevent unwanted coating contamination.

6. An apparatus according to claim 4 or 5, in which there is a shield close to, but not touching, areas of the positively biased source and holder.

7. An apparatus according to claim 6, wherein the shield at or near earth potential is for preventing vapourisation and contamination from surfaces other than the vapour source itself.

8. The use of the apparatus according to claims 4 to 7 in the deposition of metallic coatings, including alloys by the use of alloy sources or multiple sources of different materials.

9. The use of the apparatus according to claims 4 to 7 in the deposition of ceramic and. cermet coatings, for example by reactive deposition through the evaporation of a metallic material in a reactive gas, to deposit onto surfaces -which are negative with respect to the vapour source.

10. A method or apparatus substantially as herein described, with or without reference to the accompanying drawing.