GB2215739A

GB2215739A - Ionisation assisted chemical vapour deposition

Info

Publication number: GB2215739A
Application number: GB8807261A
Authority: GB
Inventors: Of Hull He University; Allan Matthews
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1988-03-26
Filing date: 1988-03-26
Publication date: 1989-09-27
Also published as: GB8807261D0

Abstract

The substrate 5 to be coated is supported within a vacuum chamber 1 comprising a thermionic filament 7, an anode 8 and means 2 and 3 for introducing gas into the chamber 1 and venting gas from the chamber 1, and the thermionic filament 6 and the substrate are negatively biased with respect to the plasma generated thereby producing an intensified plasma which acts as a source of ions and/or neutrals which bombard the substrate. <IMAGE>

Description

DESCRIPTION IMPROVEMENTS IN AND RELATING TO IONISATION ASSISTED CHEMICAL VAPOUR DEPOSITION The present invention relates to a method cl and apparatus for ionisation assisted chemical vapour deposition of a coating species onto a substrate or workpiece.

Vapour deposition techniques can be divided into those which utilisea solid source of evaporant, in other words physical vapour deposition (PVD) and those which utilise a gaseous source, in other words chemical vapour deposition (CVD). In recent years there has been much interest in the concept of improving vapour deposition techniques by incorporating some degree of ionisation in the depositing species. This has been shown to improve structure control and has made possible a reduction in the deposition temperature at which certain phases are formed.

However, whilst ionisation assisted vapour depositIon has been developed and applied in relation to PVD techniques, enabling industrial adaption and exploitation, the same cannot be said for CVD tehcniques, especially in the mechanical engineering sector.

It is an object of the present invention to provide an ionisation assisted chemical vapour deposition technique which enables deposition of films or coatings at lower temperatures, with greater uniformity and with better structures than have been obtainable with previously known techniques.

According to a first aspect of the present invention there is provided an ionisation assisted chemical vapour deposition process for producing films or coatings on a substrate in which the substrate to be coated is supported within a vacuum chamber comprising a thermionic filament, an anode and means for introducing gas into the chamber and venting gas from the chamber, wherein a plasma discharge is generated within the chamber and the thermionic filament and the substrate are negatively biased with respect to the plasma, thereby producing an intensified plasma which acts as a source of ions and/or neutrals which bombard the substrate.

Conveniently, the process of the present invention is initially carried out in the presence of an inert or nonreactive gas, thereby sputter cleaning the surface of the substrate.

The supply of gases to the chamber can be pulsed to periodically purge the chamber of unwanted species.

Advantageously, a shroud is provided around the thermionic filament which shields it from the workpieces.

This prevents or at least reduces sputtering of the filament. Conveniently, the shroud comprises a stainless steel tube which encloses the filament and opens at a point removed from the substrate. Conveniently, an inert or non-reactive gas is introduced into the tube to reduce the likelihood of reactive gases reaching the filament.

Preferably, a magnetic directional system is provided within the chamber to extend the path length of electrons emitted from the filament and proceeding to the anode.

Preferably, the substrate is heated by ion and neutral bombardment, although this can be supplement by a heating device within the chamber.

The electrical bias to the anode and cathode elements of the system may be provided by DC or RF power supplies.

According to a second aspect of the present invention there is provided apparatus for producing films or coatings on a substrate by ionisation assisted chemical vapour deposition, comprising a vacuum chamber, means for supporting the substrate within the chamber, a thermionic filament, an anode, means for introducing gas into the chamber and venting gas from the chamber, and power supply means for negatively biasing the thermionic filament and the substrate, thereby producing an intensified plasma within the chamber which acts as a' source of ions and/or neutrals which bombard the substrate.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a schemmatic view of a system for ionisation assisted chemical vapour deposition of a film or coating onto a substrate in accordance with the present invention; and, Pig. 2 shows a detailed schemmatic view of a confinement tube for use in the system of Fig. 1.

Referring to Fig. 1 of the accompanying drawings there is shown an ionisation assisted chemical vapour deposition system comprising a vacuum chamber 1 with a pumping port 2 in the base and a gas inlet 3 to one side.

A support platform 4 is provided within the chamber 1 on which is supported a pair of workpieces 5. The support platform 4 is connected to a power supply 6 which maintains it at a negative potential. The discharge is supported within the chamber 1 by means of a thermionic filament 7 and a positively biased anode 8. The power supplies for the filament 7 and the anode 8 are shown separately and indicated by reference numerals 9 and 10 respectively.

In the coating procedure the workpieces 5 are mounted on the support platform 4. The chamber 1 is then pumped down to an ultimate pressure of less than uTorr to remove any background impurities from the chamber 1. If this level is not reached then the chamber 1 can be purged with an inert gas such as argon and/or "baked out" by heating it, conveniently with the filament 7. It is often desirable to sputter clean or condition the workpieces 5 prior to coating and conveniently this is achieved by inletting an inert gas, such as argon and/or hydrogen, into the chamber 1 through the gas inlet 3 and reducing the pumping rate through the pumping port 2 until the pressure within the chamber reaches a level at which a glow discharge can be initiated within the chamber 1, typically above 7 mTorr.A negative bias of between 50 - 200V is applied to the samples, enabling current densities of several mA/cm to be otained, limited primarily by workpiece heating. Under these conditions the surfaces of the workpieces 5 are bombarded with ions and/or neutrals, thereby clearing the surface of impurities.

The arrangement of the filament 7 and of the anode 8 within the chamber 1 ensures that the workpieces 5 are intensely bombarded, thereby accelerating this sputter cleaning stage. Indeed, the electron emission within the chamber allows for close control of the electical field around the workpieces 5, thereby improving bombardment uniformity and enabling operation at lower pressures. In fact, it is possible to dispense with the supply 9 and simply earth the filament 7, provided a positive bias is applied to the anode 8. In this respect, typical DC voltage for the anode would be between 50 - 200V, with a current rating of 80A, which ratings also apply to the actual deposition process.

It will be understood by those skilled in the art that the size and location of the anode 8 within the chamber 1 determines the distribution of the plasma and its electrical potential. If it is comparatively larger than the cathode, i.e. those surfaces within the chamber 1 which are negatively biased, then the plasma potential will be slightly positive with respect to the anode 8.

Accordingly, workpieces 5 can be held at a negative potential, or the supply 6 can be dispensed with and the workpieces connected to earth or allowed to float.

However, the supply 6 is preferably retained as it allows greater control of the workpiece potential and, hence, of the power thereto and the heating thereof. Moreover, it enables the energies of ions and neutrals arriving at the workpiecesto be controlled, with important consequences on subsequent film growth factors, as for example, is known to be the case in the deposition of carbon-based films from hydrocarbon gases.

After the sputter cleaning stage has been completed, film depostion may be carried out. In this respect, the anode 8 and the support platform 4 are biased as described above, that is to say with the anode 8 at a bias potential of 50 - 200V and the support platform 4' and the workpieces 5 supported thereon at earth or negative potential bias. Under these conditions a discharge is initiated between the filament 7 and the anode 8. At this point a gas or gases appropriate to the coating species are introduced into the chamber 1 through the gas inlet 3. The pressure of deposition will influence the level of coating uniformity achieved and the gas throughput will influence the film growth rate.

Coatings have, for example, been produced at pressures of less than 1 mTorr to over 50 mTorr. An important factor to consider in this is the workpiece temperature which can have a profound influence on the degree of diffusion of the depositing species upon the workpieces 5 and also on the coating morphology.

The distribution of plasma within the chamber 1 can be controlled by the number of thermionic filaments 7, as well as the location, size and number of anodes in the chamber 1. In order to extend the useful life of the thermionic filament 7 the inert gas feed 3 is in the vicinity of the filament 7, thereby preventing or minimising reaction of the filament with any reactive gas present in the chamber 1. In addition, the supply of gases to the chamber 1 can be pulsed, in the case of the reactive gases, to produce a layering effect in the coating deposited on the workpieces, and, in the case of the inert gases, to periodically purge the chamber 1 of unwanted gases.

To further prevent contamination of the filament 7 by the reactive gases introduced into the chamber 1 it may be shrouded from the workpieces 5 and the main plasma within the chamber 1. In this respect, there is shown in Fig. 2 an appropriate shroud comprising a tube 11 which encloses the filament 7 and has a 90 degree bend in it.

The tube 11 is made of stainless steel and is cooled by, for example, trace water cooling. The water feed, not shown, to the tube 11 is electrically insulated and the tube 11 itself is floating electrically in order to reduce the heating effect of the plasma and the filament.

The shroud provided by tube 11 reduces the possibility of the filament 7 being sputter eroded, and ensures that if -any material is sputtered from the filament 7 it is less likely to reach the workpieces 5. Although not shown in the accompanying drawings the inert gas feed may actually be inlet directly into the tube 11 to reduce and prevent any possibility of the filament reacting with the reactive gases present in the chamber 1. The present invention also envisages the possibility of a magnetic system for directing the electrons leaving the filament 7 to increase their path length and thereby increase their ionising effect. This may conveniently be achieved by incorporating an axial magnetic field between the tube opening and the anode 8.

In conventional CVD techniques, workpiece heating is achieved by direct means, such as radiant heaters placed in the vacuum chamber. With the ionisation assisted CVD technique of the present invention such heaters can still be used, but they can be augmented or replaced altogether as a result of the heating effect produced by the bombarding ions and neutrals incident upon the workpieces. Indeed bombardment heating is more preferable because it is energy efficient and can also enhance diffusion through the "pseudo-diffusion" effect which occurs when the substrate atoms are mixed with those arriving from the reactive gas. In some applications though this self heating effect is not desirable, as, for instance, during the coating of optical components, when interface compounds can have a detrimental effect and needs to be minimised.Of course, because of the separate power supply 6 to the support platform 4 complete control of the workpiece potential is obtained and, as a direct result, ion energies, and thus sputtering rates, can be carefully determined.

A further benefit of the system according to the present invention, as compared with certain other "hot filament" ionisation assisted CVD techniques, is that it does not rely upon a directional ion beam and thereby reduces the "line of sight" constraints which they imply.

In effect the plasma surrounds the workpiece to be coated or diffusion treated. Other "hot filament" techniques do not provide a glow discharge or plasma, but instead rely upon a heating arrangement for the chamber. This means that the filament must be close to the workpieces, thus reducing the area over which acceptable deposits can be made. In addition, because these known techniques require that the system be operated at high temperatures, greater than 800 C, they are not suitable for use with substrates which are temperature sensitive. Moreover, within the plasma most of the energy required for deposition is present within the actually depositing species themselves. Thus, a plasma based system such as the one described hereinabove provides excellent results on samples which are themselves at relatively low temperatures, for example less than 200 C.

Claims

1. An ionisation assisted chemical vapour deposition process for producing films or coatings on a substrate in which the substrate to be coated is supported within a vacuum chamber comprising a thermionic filament, an anode and means for introducing gas into the chamber and venting gas from the chamber. wherein a plasma discharge is generated within the chamber and the thermionic filament and the substrate are negatively biased with respect to the plasma1 thereby producing an intensified plasma which acts as a source of ions and/or neutrals which bombard the substrate.

2. A process according to Claim 1. wherein the gas introduced into the chamber is an inert or non-reactive gas.

thereby sputter cleaning the surface of the substrate.

3. A process according to Claim 1 or 2. wherein an inert or non-reactive gas is introduced into the chamber with the reactive gas.

LL. A process according to any preceeding Claim1 wherein the supply of gas to the chamber is periodically turned on/off.

5. A process according to any preceeding Claim1 wherein the filament is shrouded from the substrate to be coated.

6. A process according to Claim 5. wherein an inert or non-reactive is introduced into the tube to prevent reactive gases reaching the filament.

7. A process according to any preceeding Claim, wherein a magnetic directional system is provided within the chamber to extend the path length of electrons emitted from the filament and proceeding to the anode.

8. A process according to any preceeding Claim, wherein the substrate is heated by ion and/or neutral bombardment.

9. Apparatus for producing films or coatings on a substrate by ionisation assisted chemical vapour deposition, comprising a vacuum chamber1 means for supporting the substrate within the chamber. a thermionic filament, an anode1 means for introducing gas into the chamber and venting gas from the chamber. and power supply means for negatively biasing the thermionic filament and the substrate. thereby producing an intensified plasma within the chamber which acts as a source of ions and/or neutrals which bombard the substrate.

10. Apparatus according to Claim 9, wherein a shroud is provided between the filament and the workpieces.

11. Apparatus according to Claim 10. wherein the shroud comprises a stainless steel tube which encloses the filament and opens at a point removed from the substrate.

12. Apparatus according to any of Claims 9 to 11, wherein the electrical bias to the anode and cathodic elements of the apparatus is provided by DC or RF power supplies.

13. An ionisation assisted chemical vapour deposition process for producing films or coatings on a substrate substantially as hereinbefore described with reference to the accompanying drawings.

ILL. Apparatus for producing films or coatings on a substrate by ionisation assisted chemical vapour deposition substantially as hereinbefore described with reference to the accompanying drawings