DIAMOND FILM DEPOSITION

Abstract: The invention provides an improved method and apparatus for pre-treating a substrate surface (19) to prepare it for subsequent deposition upon it of a film of diamond. The method of pre-treating a surface according to the invention comprises the steps of positioning a quantity of diamond dust (16) in relation to the surface, producing a shock wave in gas adjacent the dust, subjecting the dust to the shock wave, and thereby causing the diamond dust to impinge at high velocity on to the surface. The apparatus according to the invention comprises means for holding a charge of diamond dust (15), means for generating a shock wave in gas adjacent the charge of diamond dust (17), and means for directing the shock wave at the diamond dust (12) and causing the diamond dust to be expelled at high velocity for impingement on the substrate surface. The invention also provides a method depositing a diamond film on a substrate, including pre-treatment by the aforesaid method or apparatus.

Description: DIAMOND FILM DEPOSITION This Invention relates to the deposition of diamond films on surfaces of substrates of suitable materials and, in particular, to the pre-treat ent of such surfaces to prepare them for subsequent diamond film deposition by any of a number of available chemical vapour techniques such as described, for example, in a review article entitled "Diamonds from the vapour phase", by P. Bachmann, published in "Physics World", April 1991, pages 32-36. Suitable pre-treatment of a substrate surface, to enhance the nucleation density and the rate of growth during film deposition, is of paramount importance for the success of all these vapour deposition techniques. Conventionally it has been accomplished by abrading the surface using, as the abrasive, dust of diamond or other hard materials, either dry or as a paste or suspended in a liquid, such as water in an ultrasonic bath. If selective film deposition is required, on only parts of the abraded surface, various techniques may be employed. For instance, it 1s known to provide those parts with a temporary resist coating and then expose the remainder of the abraded surface to etching by means of an Ar-ion beam. Such etching or sputtering of unmasked areas of the surface reduces the nucleation density there due to the abrasion, and it 1s found that after subsequent removal of the temporary masking it is only on those parts which have been masked, so that the abrasions have remained unaffected by the 1on-beam etching step, that diamond film deposition occurs when the whole surface is submitted to the diamond deposition step. It 1s an object of the present invention to provide an improved method and means of pre-treating a substrate surface to prepare it for subsequent deposition upon it of a film of diamond. According to one aspect of the invention, the method of pre-treating a surface comprises the steps of positioning a quantity of diamond dust in relation to the surface, producing a shock wave in gas adjacent the dust, subjecting the dust to the shock wave, and thereby causing the diamond dust to impinge at high velocity on to the surface. The shock wave may be produced by causing a build-up of gas under pressure contained in part by a rupturable membrane or diaphragm, and then causing the membrane or diaphragm to rupture. The rupture of the membrane or diaphragm may be caused either by progressively increasing the pressure of the gas or may be initiated by piercing it with a sharp point. Alternatively the shock wave may be produced in the gas by other means, such as by producing an electrical spark within the gas. The method may include the steps of positioning the diamond dust within a nozzle directed towards the substrate surface and causing the diamond dust to be expelled through the nozzle by the shock wave; and the diamond dust may be provided in the form of a friable pellet of compressed diamond dust. After this pre-treatment, followed by any desirable cleaning of the surface, by washing or blowing loose dust off the surface or by standard chemical means, and any desired selection of parts of the surface to be coated, such as by partial masking and etching as described above, the surface <or the selected part of 1t) is ready for deposition of diamond upon it by any of the known deposition techniques. Accordingly, the invention also provides a method of depositing a diamond film on a substrate surface, comprising pre-treating the substrate surface by the method outlined above and thereafter depositing a diamond film on all or part of the pre-treated surface by any known and suitable method of deposition. According to another aspect of the invention there is provided apparatus for pre-treating a surface prior to depositing a film of diamond thereon, the apparatus comprising means for holding a charge of diamond dust, means for generating a shock wave in gas adjacent the charge of diamond dust, and means for directing the shock wave at the diamond dust and causing the diamond dust to be expelled at high velocity for impingement on an adjacent surface. Preferably the means for generating a shock wave comprises a burstable diaphragm and means for providing a build-up of gaseous pressure on a side of the diaphragm remote from the charge of diamond dust. The invention will be further disclosed and explained below in the following description with reference to the accompanying drawing, which shows, in longitudinal section and positioned with respect to a substrate whose surface is to be pre-treated by a method according to the invention in one of its aspects, one embodiment of pre-treating apparatus which itself embodies another aspect of the invention. The apparatus shown in the drawing comprises a pressure vessel 10 composed of two stainless steel tubular sections 11 and 12 formed with respective end flanges 11a and 12a by means of which the two sections are releasably held together end-to-end by an internally threaded collar 13 and a co-operating externally threaded collar 14. The sections 11 and 12 may be of about 10 mm internal diameter and of length 250 mm and 150 mm respectively. At its end remote from the section 11, the section 12 is internally threaded to receive a nozzle fitment 15 having a nozzle outlet portion 15a with an internal diameter of about 1.5 mm and a portion 15b adapted to receive and hold a friable pellet or disc 16 of compressed diamond dust. For use, the sections 11 and 12 of the device 10, constituting respectively a driver section and an expansion section of the device, are secured together with a burstable membrane or diaphragm 17 held between them so as to obturate the tubular interior. The driver section 11, at its end remote from the section 12, is fitted with a high-pressure hose 18 for connection via a control valve (not shown), to a source (also not shown) of high-pressure gas, preferably nitrogen or other inert gas. The device 10 is positioned with the nozzle 15a spaced, by a distance typically between 0 and 50 mm, from a surface which is to be pre-treated and which, as illustrated, may be one optically polished surface of a single crystal silicon wafer 19 of which the other face rests on a solidly mounted support 20. Controlled admission of gas through the hose 18 then allows a build-up of pressure within the driver section 11 until, at a pressure determined by the choice of diaphragm 17, the diaphragm ruptures. This generates a shock wave which travels down the interior of the expansion section 12 to impact on and pulverise the disc 16 and to project the resulting diamond dust through the nozzle 15a to impact at high velocity on the opposed surface of the wafer or substrate 19. Using the apparatus described above, several series of pre-treatments were carried out on silicon wafers 19 as described, with the nozzle 15a spaced in each case at a distance of 20 mm from the wafer surface, using nitrogen as the driver gas and with discs 16 made of commercially obtained synthetic or natural diamond dust having an average diameter of 0.75-1.5 micron. In a first series of experiments, six separate regions angularly spaced round a wafer 19 at equal distances from its centre were each subjected to a pre-treatment as above described, using discs 16 containing equal amounts of natural diamond dust in each case but with different bursting pressures of the diaphragms 17. The different bursting pressures were achieved by using different materials and thicknesses for the diaphragms 17, from aluminium foil of thickness 0.5 thou (O.Olrnm) for a bursting pressure of 1 bar pressure differential across the diaphragm to tracing paper of thickness 3.0 thou (0.075mm) for a bursting pressure differential of 5 bar. Additionally, for comparison purposes, two parts of the central region of the wafer surface were abraded in conventional manner using natural and synthetic diamond dust respectively. The wafer 19 was then cleaned by washing with deionised water and drying under nitrogen. The whole of the wafer surface to different parts of which the pre-treatments had been applied was then subjected to a diamond film deposition process using the known microwave plasma technique. In this, the wafer substrate was maintained at a temperature of 700°C while exposed within a deposition chamber to a plasma formed by subjecting a gas mixture of hydrogen with 0.5% of methane to microwave radiation while flowing through the chamber under reduced pressure. The pressure employed within the chamber was 11 torr, the flow rate through the chamber was 1000 standard cubic centimetres of gas mixture per minute, and the microwave power applied was 620W with a frequency of 2.45 GHz. The deposition process was continued for a period of 5.5 hours, after which the wafer was allowed to cool and was then examined. It was found that deposition of a diamond film had occurred, but only on those parts of the surface to which a pre-treatment had occurred. The film thickness was greater (implying higher growth rates) for the outer regions which had been pre-treated in accordance with the invention than for the two parts of the central region which had been conventionally abraded; and, as between different areas of the outer pre-treated regions, the film thickness was greater, the higher the pressure at which the diaphragm had burst during carrying out of the pre-treatment. Two further series of experiments were carried out, Identical with that just described except that the discs 16 were made not of diamond dust but of silicon carbide dust 1n one case and aluminium oxide dust in the other. In both these series of experiments, however, the final plasma deposition step exhibited no enhancement of the nucleation and growth of diamond compared with that achieved using the previously known pre-treatments. It is conjectured that, in all three series, dust particles from the pulverised discs 16 impacted at high velocity against the wafer 19 and became partially embedded in its exposed surface, while remaining partially exposed to the plasma during the subsequent film deposition stage; but that only when those partially embedded particles were themselves of diamond did they constitute satisfactory nuclei for diamond deposition, so as to promote the formation of a diamond film on the surface. A further series of experiments were carried out on a further wafer 19, eight spaced-apart regions of which were subjected to pre-treatment in accordance with the invention as previously described, equal discs 16 of natural diamond dust being used in all cases but the driving pressures of nitrogen gas being increased sequentially in this series all the way from 0.5 bar to 15 bar by suitable choice of diaphragm material and thickness. In this case, confirming the previous results, it was again found that, after the wafer had been cleaned and subjected to the microwave plasma treatment as already described, diamond film had formed only on the surface areas which had been subject to the pre-treatment and the films were of increasing thickness with increasing required pressure to rupture the corresponding diaphragm 17. Although the above-described experiments appear to show that diamond powder is required for pre-treatment in accordance with the invention for promoting the deposition of diamond film, it 1s believed that the invention is not limited to substrates of silicon as above described but may also be applied to promote the deposition of diamond film on substrates of other materials. It will be understood that within the scope of the invention many variations or modifications of the embodiment described above for purposes of Illustration are permissible. For example, a diaphragm 17 may be used in combination with a gas pressure which is not itself sufficient to rupture the diaphragm, some other means being provided, such as a sharp point which can be triggered to pierce the diaphragm, as a means of Initiating the rupture. Or, indeed, the shock wave may be generated by other means than a bursting diaphragm, for example by producing an electrical spark within gas adjacent the diamond powder. Then again, the diamond dust is not necessarily provided in the form of a pellet or disc 16 as described above: particularly if the apparatus 10 is operated in a vertical position as illustrated 1n the drawing, the diamond dust may be provided simply as a charge of loose powder in the bore of the fitment 15, in the lower part of its portion 15b and/or in the bore of its nozzle portion 15a. In the experiments described above, the discs 16 each contained 7mg of diamond dust; but it will be understood that larger or smaller quantities may also be used, whether as loose powder or formed into a disc. The dust expelled from the nozzle 15a issues as a generally conically-expanding jet and impinges on a generally circular area of the substrate 19, the diameter of such area increasing with increased spacing of the nozzle from the substrate. With a given nozzle 15a, increasing its spacing from the substrate in order to pre-treat a larger area of the substrate surface will usually be best accompanied by an increase in the amount of diamond dust used and also, probably, in the energy generated and transmitted by the shock wave, if an equal quality of performance is to be maintained.

Claims: CLAIMS 1. A method of pre-treating a substrate surface to prepare it for subsequent deposition upon it of a film of diamond, comprising the steps of positioning a quantity of diamond dust 1n relation to the surface, producing a shock wave in gas adjacent the dust, subjecting the dust to the shock wave, and thereby causing the diamond dust to impinge at high velocity on to the surface. 2. A method of pre-treating a substrate surface, as claimed 1n Claim 1, wherein the shock wave is produced by causing a build-up of gas under pressure contained in part by a rupturable membrane or diaphragm, and then causing the membrane or diaphragm to rupture. 3. A method as claimed in Claim 2, wherein the membrane or diaphragm is caused to rupture by progressively increasing the pressure of the gas. 4. A method as claimed in Claim 2, wherein rupture of the membrane or diaphragm is Initiated by piercing it with a sharp point. 5. A method of pre-treating a substrate surface, as claimed in Claim 1, wherein the shock wave 1s produced in the gas by producing an electrical spark within the gas. 6. A method of pre-treat1ng a substrate surface, as claimed 1n any of Claims 1 to 5, including the steps of positioning the diamond dust within a nozzle directed towards the substrate surface and causing the diamond dust to be expelled through the nozzle by the shock wave. 7. A method as claimed in any of Claims 1 to 6, including the step of providing the diamond dust in the form of a friable pellet of compressed diamond dust. 8. A method of depositing a diamond film on a substrate surface, comprising pre-treating the substrate surface by the method claimed in any of Claims 1 to 7 and thereafter depositing a diamond film on the pre-treated surface by any known and suitable method of deposition. 9. A method as claimed in Claim 8, wherein the diamond film is deposited on the pre-treated surface by a microwave plasma deposition process Including the steps of subjecting a flow of hydrogen at subat ospheric pressure and containing a minor proportion of methane to microwave Irradiation to generate a carbon-containing plasma, and exposing the pre-treated substrate to the plasma while maintaining the substrate at elevated temperature. 10. A method as claimed in Claim 9, wherein the substrate is maintained at a temperature of approximately 700°C and the plasma to which it is exposed is produced in a flow of hydrogen containing approximately 0.5% of methane and at a pressure of approximately 11 torr. 11. A method as claimed in Claim 10, wherein the gas mixture is caused to flow over the substrate surface at a rate of approximately 1000 standard cubic centimetres per minute and is irradiated with microwave power of a frequency of approximately 2.45 GHz applied at a rate of approximately 620 W. 12. A method of depositing a diamond film on a substrate surface, as claimed in any of Claims 8 to 11, including selectively treating parts of the pre-treated surface before deposition and thereby rendering it selectively receptive to the subsequent deposition of the diamond film thereon. 13. Apparatus for pre-treating a surface prior to depositing a film of diamond thereon, the apparatus comprising means for holding a charge of diamond dust, means for generating a shock wave in gas adjacent the charge of diamond dust, and means for directing the shock wave at the diamond dust and causing the diamond dust to be expelled at high velocity for impingement on an adjacent surface. 14. Apparatus as claimed in Claim 13, wherein the means for generating a shock wave comprises a rupturable diaphragm and means for providing a build-up of gaseous pressure on a side of the diaphragm remote from the charge of diamond dust. 15. Apparatus as claimed in Claim 14, comprising first and second tubular parts securable together and to end with the rupturable diaphragm or membrane secured between them and separating their interiors from one another, means for supplying gas under pressure to the interior of one of the parts and nozzle means opening out of the other of the parts and adapted to hold a charge of diamond dust for discharge through the nozzle under impulsion by a shock wave generated in said gas under pressure on rupture of the membrane or diaphragm. 16. A method of pre-treating a substrate surface substantially as described herein. 17. A method of depositing a diamond film on a substrate surface, substantially as described herein. 18. Apparatus for pre-treating a surface substantially as described herein with reference to the accompanying drawing.

Inventors: DRUMMOND IAIN CAMERON, JOHN PHILLIP, WILSON JOHN IVOR BARRETT, DRUMMOND, IAIN, CAMERON, JOHN, PHILLIP, WILSON, JOHN, IVOR, BARRETT
Applicants: BRITISH TECH GROUP, DRUMMOND IAIN CAMERON, JOHN PHILLIP, WILSON JOHN IVOR BARRETT, BRITISH TECHNOLOGY GROUP LTD.
Application year: 1993
Main classification: C23C16
IPC codes: B24C3, B24C5, B24C11, C23C16