Coatings for the protection of diamond in high-temperature environments

Diamond and Related Materials 8 (1999) 916–919

Coatings for the protection of diamond in high-temperature environments S.P. McGeoch a,*, F. Placido b, Z. Gou b, C.J.H. Wort c, J.A. Savage d a Pilkington Optronics, Barr and Stroud Ltd., 1 Linthouse Road, Glasgow, G51 4BZ, UK b Department of Electronic Engineering and Physics, University of Paisley, High Street, Paisley, PA1 2BE, UK c De Beers Industrial Diamond Division, Charters, Sunninghill, Ascot, Berkshire, SL5 9PX, UK d Defence Evaluation and Research Agency, St. Andrews Road, Malvern, Worcestershire, WR14 3PS, UK Received 23 September 1998; accepted 10 November 1998

Abstract Diamond is an ultra-durable material with high thermal conductivity and good transmission in the visible, near-infrared and far-IR (8–12 mm) wavebands. Recent advances in the development of synthetic diamond made by chemical vapour deposition promise an expanding range of applications for the material. An example is in advanced airborne windows and domes for highspeed flight, either as a window or as a protective coating for other infrared window materials, where the diamond has sufficient durability to withstand high-speed impact by particles and raindrops, and a high level of thermal conductivity to minimise the effect of thermal shock due to frictional heating. However, diamond is subject to oxidation in air at temperatures greater than 750 °C. After only a few seconds of exposure, the diamond surface becomes severely etched, and the optical transmission is degraded. Very high-speed flight can lead to temperatures in excess of 800 °C. For this and other high-temperature applications, therefore, it is essential to protect the diamond surface from exposure to air. We have demonstrated that CVD diamond can be protected from oxidation for extended exposure (≥10 s) in air at temperatures up to 1000 °C by a single-layer anti-reflection coating of d.c. magnetron-sputtered aluminium nitride. The coatings have excellent mechanical durability. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Coating; Diamond; Protection; Temperature

1. Introduction Diamond is attractive as an optical material for highspeed applications such as in windows and domes of missile seekers because it has good multi-spectral transparency in the visible, near-infrared and far-infrared wavebands, and is an ultra-durable material which can withstand the erosive conditions of particle and raindrop impact [1]. Further, the high thermal conductivity of diamond over a wide temperature range gives resistance to the thermal shock encountered in the high-acceleration phase of missile flight [2]. The rapidly advancing technology of diamond growth by chemical vapour deposition will be able to provide windows and domes of practical sizes (~100 mm diameter) for airborne use in the near term [3]. However, diamond oxidizes at temperatures in excess of 750 °C, which can be reached by aerodynamic frictional heating at speeds projected for future missiles. In * Corresponding author. Tel.: +44 141 4404000; fax: +44 141 4404001.

these conditions, the surface of the diamond is degraded by etching, and there is optical loss due to scatter. It has been the aim of this work to provide a durable protective coating for diamond at elevated temperatures. The coating is also required to survive the thermal shock environment. Additionally, it is beneficial if the coating can provide anti-reflection of the diamond substrate, and films of ~2.5 mm in optical thickness are required for this in the far-IR (8–12 mm) waveband. Previously disclosed coatings for the protection of diamond infrared windows at elevated temperatures have involved two-stage processes with an adhesionpromoting layer [4] or ion implantation of the diamond surface [5]. We have set our targets based on the perceived requirements for future missiles. The maximum stagnation temperature at the seeker dome is likely to be ~800 °C: however, in order to have some safety margin, we have set the target of survivability at 1000 °C. In order to simulate the thermal shock, we need to achieve the test temperature from ambient in 10 s. Our baseline criterion for assessment of the coatings has been protec-

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tion of the substrate from etching. However, we have additionally aimed at providing a durable coating which itself survives the conditions intact.

2. Experimental 2.1. General Samples were prepared by coating diamond substrates with oxide and nitride compounds of aluminium. Aluminium nitride in particular has shown good adhesion to diamond in a range of conditions, for example as a buffer layer in diamond growth [6 ] and in composite coatings [7]. The coated samples were tested by exposure to elevated temperature in a rapid thermal annealer (RTA) in an air atmosphere. In some cases, the coating was removed (chemically) from part of the substrate surface to allow comparison between the protected and unprotected regions. The samples were assessed before and after testing using techniques including Nomarski microscopy and infrared spectroscopy. 2.2. Substrates The substrates were CVD diamond supplied by Diamanx. The sample sizes varied from ~10 mm× 10 mm×0.3 mm to 30 mm in diameter and 1.5 mm in thickness. No correlation was observed between sample size and performance. Throughout the programme, samples were coated and tested, then the coating was removed and the sample reused. The considerable effort involved in polishing diamond precluded the reworking of surfaces, except in the case of gross damage.

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have long-wavelength absorption edges within the waveband. However, they have the merit of transparency in the visible waveband, and so are prospective multispectral materials. It is noted that no significant increase in absorption was observed for either material upon heating to 600 °C. It is suggested that these materials may be considered as readily available ‘‘models’’ for oxides and nitrides of heavier metals, such as yttrium, with longer-wavelength absorption edges. 2.4. Test method The samples were heated in the RTA by contact with the front surface of a relatively large (100 mm diameter) silicon wafer susceptor. The susceptor was heated radiatively from the front surface using quartz lamps. The temperature of the rear surface was monitored using a pyroelectric detector, which was calibrated against thermocouples in contact with the susceptor in steady-state conditions. The set temperature was achieved in 10 s, and maintained for 10 s. In our experiments, set temperatures of 600, 800 and 1000 °C were used. The diamond samples were all of a small thermal mass relative to the susceptor, and were expected to reach the same temperature. The high thermal conductivity of diamond meant that the samples would experience a rapid temperature rise and reach the equilibrium temperature in the timescales of the test. It was noticed throughout the test programme that we seldom observed damage to the rear surface (i.e. the surface in contact with the silicon), even when the front surface was severely degraded. We suggest that this may be due to protection from the atmosphere by a partially entrapped layer of oxidized carbon gas.

2.3. Coatings

3. Results

The coatings were prepared by d.c. magnetron sputtering of an aluminium target in a reactive atmosphere. No substrate heating was used. Coatings of a physical thickness in the range 0.5–3.8 mm were used in this study. The coating work was carried out using proprietary processes at the University of Paisley. Aluminium oxide deposited directly onto diamond showed little durability even at moderate temperatures (~600 °C ). Accordingly, we used a thin (10 nm) layer of aluminium nitride as a barrier when preparing oxide films. It is noted that aluminium is an efficient getter for oxygen. Small, uncontrolled leaks in the coating chamber mean that it is unlikely that the deposition atmosphere is entirely oxygen-free. Therefore, although we speak of nitride films, it is understood that these may contain some oxide. Aluminium oxide and nitride are not ideal materials for coatings used in the 8–12 mm spectral region as they

3.1. Initial process In our earliest work, we used a non-optimised process to produce films of aluminium oxide and nitride on CVD diamond. Both films damaged extensively on exposure in the RTA to a plateau temperature of 600 °C. The oxide failed by blistering, and we felt that this was indicative of compressive stress in the film. The nitride failed by crazing in a way typical of a film in tensile stress. These observations were supported by measurements of the induced curvature of thin silicon beams (35 mm×5 mm×0.7 mm), which also confirmed that there was compressive stress in the oxide and tensile stress in the nitride. An attempt was made to produce a more durable coating by depositing it in an atmosphere containing oxygen and nitrogen to give a homogenous oxynitride film. The samples failed by complete delamination on exposure to 800 °C in the RTA. Samples with four alternating layers of equal thicknesses of oxide and

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Fig. 1. Summary of results for improved process aluminium nitride coatings on CVD diamond.

nitride proved more durable since they adhered substantially to the substrate, although extensively damaged, after exposure at 800 °C. 3.2. Improved process A Taguchi analysis of the coating deposition parameters was carried out, and new process conditions selected which produced films of an improved visual appearance. For our first experiments with the new process, we investiTable 1 Initial results with the improved deposition process Film structurea

Relative thicknessesb

Conditionc after testd

N N N:O N:O N:O N:O N:O N:O:N:O N:O:N:O N:O Oe Oe

100 100 67:33 67:33 50:50 50:50 50:50 25:25:25:25 25:25:25:25 33:67 100 100

5* 5* 5* 2* 3* 3* 4* 3* 2* 3* 1* 1*

aLeft to right from substrate to air; N for aluminium nitride; O for aluminium oxide. bRelative physical thickness of layers. All films have a total physical thickness of ~1.3 mm. c5*: Coating intact with little or no damage; 4*: coating intact with local damage (usually at the edge of the sample); 3*: coating intact, but with widespread damage; 2*: limited delamination; 1*: extensive delamination. d10 s ramp from ambient and 10 s exposure at 800 °C. eWith a thin (~10nm) barrier layer of nitride at the CVD diamond surface.

gated layer structures of nitride and oxide. All of the films had a total physical thickness of ~1.3 mm. Most of the films consisted of a single nitride layer at the substrate and an oxide layer. Two samples with four layers, and two oxide-only (with a thin nitride keying layer) samples were also included. The samples were tested at 800 °C in the RTA. The results are summarised in Table 1. We used a ranking system as a measure of survivability: films which showed no damage were assessed as 5*; films which remained intact but showed damage sites in local areas (usually near the sample edge) were assessed as 4*; films which showed extensive damage were called 3*; films which partially delaminated were given a 2* rating; and films which (near-) completely delaminated were given 1*. The results strongly indicated that nitride-only films showed most promise for meeting the requirement with this process. Accordingly, we decided to concentrate our efforts on these.

Fig. 2. Transmittance spectra of a CVD diamond sample with a ~1–3 mm aluminium nitride coating after exposure at 800 and 1000 °C.

S.P. McGeoch et al. / Diamond and Related Materials 8 (1999) 916–919

Fig. 3. Nomarski micrograph of protected and unprotected areas of a CVD diamond sample after exposure for 10 s at 800 and 1000 °C and chemical removal of the protective coating.

A series of 12 coating runs (A–L) was carried out and nitride films of thicknesses in the range 0.5–3.8 mm were deposited. The samples were tested in the RTA at 800 and 1000 °C. The results are summarised in Fig. 1. Samples were tested one or more times. The 1000 °C part of the vertical axis gives the total exposure of the sample, e.g. the samples in run A were exposed at 800 °C for 10 s, then for 10 s at 1000 °C, and then for 20 s (total 30 s) at 1000 °C. A three-level grading system was used this time: the open symbol corresponds to the 5* and 4* ratings used previously (i.e. coating intact), the closed symbol to coating delamination (2* and 1*), and the half-closed symbol to the case of extensive damage (3*). The infrared transmittance spectra of the samples were assessed before and after testing. An example is shown in Fig. 2 of a CVD diamond sample with an aluminium nitride anti-reflection coating (~9 mm) before and after testing at 800 and 1000 °C. The transmittance was not affected by the high-temperature exposure. Fig. 3 shows the contrast between protected and unprotected regions of a sample. The sample was coated, then the coating was chemically removed from half of its area, so that there were protected and unprotected regions of the sample. The sample was then exposed at 800 and 1000 °C in the RTA. The coating was then removed chemically from the protected region and the sample examined in a Nomarski microscope. The protected region of the sample is shown in the upper part of the micrograph, and the unprotected in the lower part. It can be seen that the unprotected region has been severely etched, while the protected region is undamaged. 4. Discussion A number of observations can be made. By far the majority of the aluminium-nitride coated samples made with the improved process pass the requirement of survival at 800 °C. Failures (at 1000 °C ) seem mostly to be

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attributable to particular runs. This suggests that the tolerances on the deposition parameters are critical to the success of the coating in these extreme conditions, since the coating conditions are nominally identical within the present limits of our process control diagnostics. Within otherwise successful runs, some samples failed. We believe that this reflects different standards of surface preparation and condition (samples very substantially because of their differing histories and reuse). However, it is clear that the process produces films which can satisfy not only the basic requirements of the application, but which can, in some cases, greatly exceed this baseline. A large number of samples passed the maximum target requirement of 10 s exposure at 1000 °C, and, for example, one sample from batch G was tested to a total of 90 s exposure at 1000 °C without degradation. 5. Conclusions Aluminium nitride can be deposited directly onto diamond in a single process to give a durable coating which protects the diamond surface from oxidation at 1000 °C. The coating can survive the thermal shock of reaching this plateau temperature in 10 s from ambient (~20 °C ). The process conditions appear to be critical to the success of the coating, and it will be necessary to increase the level of control and characterise the diamond–aluminium nitride interface if a reliable process is to be realised. The coating can be used to provide antireflection in the 8–12 mm waveband, and is also transparent at visible wavelengths, which gives rise to the possibility of application to multi-spectral optical systems.

Acknowledgements The authors would like to thank Suzanne Kelly, Eddie Crossan and Caspar Clark, who were students at the University of Paisley and were employed by Pilkington Optronics during this project, for their efforts with this work. Catriona Bryce and Joan Carson of the University of Glasgow are also due our thanks for their assistance with the high-temperature testing. S.P.McG. would like to thank the Board of Directors of Pilkington Optronics for their support of this programme.

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R.S. Sussmann et al., Diamond Relat. Mater. 3 (1994) 303. C.J.H. Wort et al., Diamond Relat. Mater. 3 (1994) 1158. R.S. Sussmann et al., Paper #4.1, Diamond 1998. L.F. Johnson et al., US Patent 5 472 787, 1995. K.A. Klemm et al., in: Paul Klocek (Ed.), Window and Dome Technologies and Materials, vol. IV, Proceedings SPIE 2286, 1994, pp. 347–353. [6 ] V.P. Godbole, J. Narayan, J. Mater. Res. 11 (7) (1996) 1810–1818. [7] V.P. Godbole, J. Narayan, Mater. Sci. Eng. B 39 (1996) 153–159.