GB2286200A - Plasma treatment of substrates having non planar surfaces; contoured electrodes - Google Patents

Abstract

A non-planar substrate surface is subjected to plasma treatment e.g. to provide a substantially uniform surface coating. The substrate 11 is disposed between first and second electrodes 12, 13 and contoured to match the respective substrate surface whereby to define a gap of uniform thickness between one electrode 13 and the respective substrate surface. A radio frequency or microwave plasma is generated in the gap to effect the plasma treatment which may comprise deposition of a surface film, e.g. diamond-like carbon. <IMAGE>

Description

PLASMA TREATMENT PROCESS This invention relates to a process and apparatus for surface treatment of a substrate with a radio-frequency or microwave frequency plasma.

Plasma deposition is a method for depositing on a substrate surface a wide variety of thin film materials in which film precursors contained in a low pressure gas are dissociated by an electrical discharge to form nonvolatile species which condense to form a film. This technique has been most widely used to deposit silicon nitride passivation coatings in the semiconductor industry, but recently has shown to be capable of depositing a wide variety of other materials including compound semiconductor films, diamond films and metal films. However, the standard deposition techniques, are capable of uniformly coating only flat substrate surfaces.Three dimensional objects, including curved surfaces, cannot be coated successfully via currently used techniques as the discharge intensity varies across the curved surface and leads to a corresponding variation in both deposition rate and film quality. This is a particular problem in the application of plasma processes to the treatment of optical components such as lenses and windows where it is essential that any surface coating is uniform both in thickness and in composition to avoid significant impairment of the optical qualities of the treated device.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a process for plasma treatment of a generally laminar substrate having non planar opposite surfaces, the method including providing first and second electrodes contoured each to match a respective substrate surface and providing between the electrodes a radio frequency or microwave plasma whereby to provide substantially uniform treatment of one or both of said substrate surfaces.

According to another aspect of the invention there is provided an apparatus for plasma treatment of a generally laminar substrate having non planar opposite surfaces, the apparatus including first and second electrodes contoured each to match a respective surface and so disposed with the substrate therebetween as to define a gap of substantially uniform thickness between one electrode and the respective substrate surface, and means for generating a radio frequency or microwave plasma between the one electrode and the respective substrate surface, whereby in use to provide substantially uniform treatment of that surface.

The electrodes are contoured such that there is a gap of substantially uniform thickness between one electrode and a corresponding substrate surface. The method is applicable to plasma systems in which the plasma is generated by a radio-frequency source, typically 1 to 100 MHz, or by a microwave frequency source.

An important parameter in the application of a radio frequency plasma to the deposition of surface coatings is the potential difference or bias that develops between the electrodes. A high bias can be advantageous in the deposition of hard coatings, such as diamond-like carbon, as the associated ion bombardment induces densification of the film. For the preparation of multilayer structures however, the effects of excessive ion bombardment can be severely destructive and hence a low bias is preferred for those applications.

The plasma may be a continuous plasma or a pulsed plasma. We prefer to employ a pulsed plasma as this has been found to give a very high degree of uniformity. Pulsed plasma techniques are described for example in our specification No. 2105729.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figs. 1 and 2 are cross-sectional views of plasma coating arrangements for treatment of a convex surface and a concave surface respectively; Figs. 3 and 4 are cross-sectional views of coating arrangements analogous to those of Figs. 1 and 2 but with the provision of a low bias between the electrodes; and Figs. 5 and 6 illustrate electrode structures for treatment of concave and convex surfaces respectively via a microwave plasma.

Referring to Fig. 1, this shows an electrode arrangement for plasma deposition in a high bias mode. Deposition may be effected on e.g. a generally cone shaped substrate 11 which is mounted between a pair of electrodes 12, 13 each of which is contoured to match the surface topography of the substrate. The electrode 121 on which the substrate 11 is supported, may be grounded. Further, as the surface configuration of this electrode is matched to that of the substrate, there is intimate contact between this electrode and the substrate. This ensures uniform and efficient cooling of the substrate 11 via the electrode 12 during the plasma treatment process.

The two electrodes 12 and 13 are so positioned within a deposition chamber 14 as to provide a gap of substantially uniform thickness between the substrate 11 and the electrode 13. During the plasma treatment process a substantially equal quantity of precursor gas can be provided at all points of the substrate surface this ensuring uniformity of surface treatment.

Plasma treatment, e.g. to effect film deposition, of the surface of the substrate 11 is effected by generating a plasma, under reduced pressure, within the chamber 14 between the electrodes 12 and 13. Radiofrequency power is applied to electrode 13 via a conductor 15. Reactant gases incorporating precursors of a material to be deposited are admitted to the chamber via inlet 16.

We prefer to employ pulsed radio-frequency power and to match the pulsing rate to the rate at which gas is exchanged in the region between the substrate 11 and the electrode 13. The time period between pulses should be sufficient to achieve a substantially uniform gas distribution over the substrate surfaces.

The deposited film material may comprise polycrystalline diamond deposited from a plasma incorporating e.g. methane. A plasma process for depositing diamond films is described in our specification No.

2,240,114 B.

The electrode structure of Fig. 1 is intended for treatment of a convex surface. An analogous structure for treating a concave surface is shown in Fig. 2. In this arrangement the grounded electrode 22 supporting the substrate 11 is concave in form to match the convex surface of the substrate, and the RF electrode 23 is given a convex surface.

In the arrangement of Figs. 1 and 2, the two electrodes are electrically isolated so that a DC bias can be developed by the plasma process. This results in ion bombardment of the substrate with consequent densification of a deposited surface film. In the electrode arrangement of Fig. 3, the development of a bias voltage between the RF electrode 33 and the counter electrode 32 is inhibited by the provision of opposite conductive studs 37 which provide a DC current path between the electrodes (Fig. 3A shows a cross-sectional view of the electrode structure along the line X-X of Fig. 3).In this case the chamber walls 38 provide a ground for the discharge The reduction in the DC bias between the plasma and the substrate to an insignificant level allows complex multilayer film structures to be produced, for example, in the fabrication of optical or infra-red interference filters.

The electrode arrangement of Fig. 3 is adapted for the coating of a convex surface. A corresponding arrangement for coating a concave surface is shown in Fig. 4.

As discussed above, the technique can be extended to the employment of a microwave plasma. Suitable electrode structures for use at microwave frequencies are shown in Figs. 5 and 6 of the drawings which again illustrate the treatment of convex and concave surfaces respectively. In Fig. 5,. the substrate 51 is mounted in proximity to a grounded or counter electrode 52 whose surface is contoured to match the concave surface contour of the substrate whereby to define a gap of substantially uniform thickness therebetween. A second electrode 53, which matches the contour of the convex surface of the substrate is disposed in abutment with the substrate. This second electrode 53 may comprise a preformed conductive sheet or it may comprise a conductive film formed e.g. by sputtering, on the convex surface of the substrate 11. The second electrode 53 is isolated from ground.

In use, microwave energy is directed towards the electrode 53 which electrode functions as an antenna and couples the microwave energy into a plasma formed between the substrate 11 and the grounded electrode 52. The plasma extends uniformly across the substrate surface.

A corresponding arrangement for coating a concave surface is shown in Fig. 6. The microwave technique is particularly adapted to the coating of substrates of a generally conical configuration.

In many applications, such as the deposition of crystalline materials including diamond-like carbon, an additional form of heating of the substrate may be required above that provided by the discharge. This can be provided by resistive heating of the metal film or sheet electrode 53.

Alternatively, an infra-red heater may be used to heat the sample from the rear provided the metal is coated with an infra-red absorbing medium.

In the pulsed microwave plasma technique, the coating uniformity can be further improved by adjusting the dimensions and position of the grounded counter electrode 52 in order to limit the volume of the discharge near the areas to be covered. This is because in this technique, the intensity of the discharge is kept sufficiently high in order to completely dissociate the gas between the object to be coated and the counter electrode, and the flow of gas is adjusted so that the volume of gas in the discharge is replenished between each pulse. As a result of this, the volume of the discharge determines the thickness of the deposited film.Consequently, a uniform film is deposited over a curved surface when the counter electrode is configured such that the volume between the counter electrode and the electrode is proportional to the sum of the surface area of counter electrode and substrate at each point on the substrate surface to be coated. Conversely, if it is desired to vary the coating thickness so that certain area of the object are coated more thickly, the counter electrode can be configured to increase the volume of gas over the desired area.

Applications of the technique include coating of observation domes and instrumentation windows with protective coatings, ant-reflection coatings on lenses and the direct deposition of optical filters on curved surfaces.

Both internal and external filter coatings can be produced, and by coating both the inside and outside of the object with reflective dielectric filters, an etalon structure can be produced with the object as the spacer.

In addition, by depositing a thick coating on an expendable former, and then removing the former by etching away the former in a second step, three dimensional solids can be produced. A typical application of this is the production of diamond domes and cones by the microwave discharge of methane and hydrogen. This is readily done by depositing on thin flexible silicon substrates secured tightly over a metal former which is then placed in the microwave deposition equipment described above. After depositing a suitable thickness of diamond, the silicon can be removed from the former and the silicon removed using standard etching techniques.

Claims (12)

1. A process for plasma treatment of a generally laminar substrate having non planar opposite surfaces, the method including providing first and second electrodes contoured each to match a respective substrate surface, and providing between the electrodes a radio frequency or microwave plasma whereby to provide substantially uniform treatment of one or both of said substrate surfaces.

2. A process for plasma treatment of a generally laminar substrate having non planar opposite surfaces, the method including providing first and second electrodes contoured to match the respective opposite surfaces, supporting the substrate in one said electrode whereby to provide a gap of substantially uniform thickness between the substrate and the other electrode, and forming between the said other electrodes and the substrate a radio frequency or microwave plasma whereby to provide substantially uniform treatment of that substrate surface exposed to the plasma.

3. A process as claimed in claim 1 or 2, wherein said plasma is a pulsed plasma.

4. A process as claimed in claim 1, 2 or 3, wherein an electrical bias is maintained between said electrodes.

5. A process as claimed in claim 1 or 2, wherein an electrical coupling is provided between the electrodes.

6. A process as claimed in any one of claims 1 to 5, wherein said plasma treatment comprises deposition of a film of substantially uniform thickness on said substrate.

7. A process as claimed in claim 6, wherein said film comprises polycrystalline diamond.

8. A process as claimed in claim 4, wherein one of said electrodes is grounded.

9. A plasma treatment process substantially as described herein with reference to the accompanying drawings.

10. A substrate treated by a process as claimed in any one of claims 1 to9.

11.. Apparatus for plasma treatment of a generally laminar substrate having non planar opposite surfaces, the apparatus including first and second electrodes contoured each to match a respective surface and so disposed with a substrate therebetween as to define a gap of substantially uniform thickness between one electrode and the respective substrate surface, and means for generating a radio frequency or microwave plasma between the one electrode and the respective substrate surface, whereby in use to provide substantially uniform treatment of that surface.

12. Apparatus for plasma treatment of a substrate substantially as described herein with reference to and as shown in the accompanying drawings.