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Optical properties of laser-modified diamond surface

A.V. Khomich

Institute of Radio Engineering & ElectronicsAc.Vvedensky sql, 141120 Fryazino, Moscow region, Russia

v.v. Kononenko, S.M. Pimenov, VI. Konov

General Physics Institute38 Vavilova str., 117942 Moscow,Russia

S. Gloor, W. Luthy, H.P. Weber

Institute of Applied Physics, University of BernSidlerstrasse 5, CH-3012 Bern, Switzerland

ABSTRACT

The results of laser polishing of 350 tm thick free-standing diamond films are reported. The polishing wasperformed with a grazing beam of a copper vapor laser. It is shown that the laser polishing conditions and theresulting surface roughness are controlled by varying an angle of incidence of a scanning laser beam duringpolishing. The surface roughness of the as-grown films was reduced by an order of magnitude and a minimumroughness of RO.38 m was achieved as a result of the two-step polishing. Optical transmission in the UV-visible spectral range of the diamond films polished under the optimized conditions (two-step polishing) wasfound to be close to the optical transmission ofthe mechanically polished diamond film.

Properties of the laser-graphitized layer at the diamond surface were studied with optical spectroscopytechniques in the process of oxidative removal of the layer with increasing temperature of the oxidation inambient air. The optical properties and oxidation stability of the laser-modified surface layer were found tochange throughout its thickness from the surface to the diamond interface, depending on the laser polishingregime.

Key words: diamond films, laser polishing, surface roughness, graphitization, optical properties

1. INTRODUCTION

A laser polishing technique is known to be effectively applied for smoothing a very rough surface of thickdiamond films (i.e., the films of several hundred micron thick). ' Two aspects, roughness and surfacegraphitization, are equally important in characterization of optical properties of laser-polished diamond films.The value of initial surface roughness should strongly limit the achievable minimum roughness of the laser-polished surface that follows from the results obtained for thin diamond films (of up to 30 im thick)4'5 and fromthe analysis of experimental data available on laser polishing.3 In regard to a surface finish of thick diamondplates, the laser polishing has been used as a coarse polishing step followed by a conventional mechanicalpolishing2 to obtain a mirror-like surface and to provide high optical transmission in a wide spectral range. Usinglaser polishing, the optical transmission of thick films was found to be increased considerably in the infraredrange,3 and, for this, a laser-graphitized surface layer must be removed either by hydrogen plasma etching or byoxidation at elevated temperatures. This means that removing the graphitic layer "step-by-step' (e.g., byoxidation at increasing temperature) and measuring the optical transmission after each step can give valuableinformation on the structure and oxidation stability of the laser-modified diamond and, once the layer being

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removed, on the optical quality of the diamond films achieved by laser polishing. It should be emphasized thatthe properties of the laser-modified diamond surface are of particular interest and a mechanism of the formationofthe laser-modified layers is still not clear since the earlier findings by Geis et al.6 who reported on unidentifiedcomposite structure, optical anisotropy and high temperature stability of the laser-graphitized diamond.

In the present paper, studies are focussed on two particular goals. First, we aimed at a choice of optimumconditions for laser polishing of thick high-quality diamond films using a copper vapor laser (510 nmwavelength, 20 ns pulse duration, 15 kHz repetition rate) in an attempt to make the optical quality of the laser-polished diamond film surface be closer to that obtained by the abrasive mechanical polishing. The second goalwas to study the structure of the laser-modified diamond surface using UV, visible and JR optical spectroscopytechniques in the course of the 'step-by-step" oxidative removal of the graphitic layers with increasingtemperature of the oxidation in ambient air.

2. EXPERIMENTAL

Diamond films 350 pm thick were grown on ground Mo substrates with microwave plasma-enhancedchemical vapor deposition (MW-PECVD) using a conventional ASTeX CVD diamond reactor.7 The diamondfilms were separated from the substrate during cooling of the substrate upon the CVD diamond growth isstopped. The resulting free-standing diamond plates were examined by Raman and optical spectroscopies andthen used in laser polishing experiments. Raman spectra were obtained with a "Jobin Yvon S-3000' triplemonochromator in backscattering geometry; the radiation of an Ar+-ion laser (514.5 nm wavelength) was usedfor excitation and the laser spot size was 2 jim. The linewidth of the diamond peak at 1 332 cm' was about 2.5cm' and nondiamond carbon phases were not detected in the spectra. Optical spectroscopy measurements werecarried out using UV-VIS and JR 'Specord" spectrophotometers in two spectral ranges 0. 1 85-0.9 tm and 2.5-50rim. The concentration of carbon-hydrogen groups is 2 iO' cm3 as estimated from the integral intensity of CHstretching mode vibrations in the JR spectra. The concentration of nitrogen impurities in the form of singlesubstitutional N atoms is estimated to be 3lO' cm3 resulting from the amplitude of the 4.6 eV (270 nm)absorption band.

A copper vapor laser (510 nm wavelength, 20 ns pulse duration, 1 5 kHz repetition rate) was used in thepolishing experiments. The mean laser power was about 2 W. The choice of the laser was due to its high pulserepetition rate. A laser beam focussed with a 2-cm-focal-length lens was incident onto the film surface at anangle B�75° with respect to the surface normal. As is known,"8'9 laser polishing with a grazing beam (atgives the best smoothing results. A diamond film was placed on a computer-controlled X-Y translation stage toprovide the beam scanning with a speed of 5 mm/s along the X-axis with successive translations in a 5 jim stepalong the Y-axis. Two laser polishing regimes were used. In the 1st regime, the polishing was performed at a 75°angle of incidence. The 2nd regime was a two-step polishing: the first step was the polishing at the 75° incidentangle and, at the second step, the polishing of the same surface area was continued at a 85° incident angle. Thevalues of the mean laser fluence corresponding to the two angles of incidence are given in Table 1, the laserfluence of 2.2 J/cm2 (i.e., at 85°) being just above the ablation threshold of the smoothed diamond film surface.The laser-polished area at the film surface was SxS mm2 for the regime #1 and half of that for the regime #2. Themorphology of the laser-polished surface was examined with scanning electron microscopy and the surfaceroughness was measured with a stylus profilometer with a diamond tip of a S jim radius.

After the laser polishing, the diamond films were annealed in ambient oxygen to remove a laser-graphitizedlayer on the diamond surface. The annealing (oxidation) experiments were performed at 400-590°C in air (for 0.5hour at each temperature). The oxidation under these conditions leaded to removal of theupper part of the laser-modified layer and did not change the properties throughout the layer thickness. Optical spectra were taken fromthe samples after each successive annealing procedure.

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RESULTS

The surface morphology of a 350 tm thick diamond film polished with a copper vapor laser under theexperimental conditions described above is shown in SEM images in Fig. 1 in comparison with the surface reliefof the as-grown film. The SEM images of the laser-polished films were obtained after sequential annealing inambient air in the temperature range 400-590°C. While a long-period waviness (a result of scanning the laserbeam) is observed at the surface after the one-step polishing, the film surface after the two-step polishing looksas an extremely flat one. Whatever the polishing mechanism (consideration of which is out of the goals of thispaper and some ideas on the mechanisms of laser polishing are reviewed elsewhere3), it is concluded that evenfor thick diamond films having a very rough growth surface the laser polishing conditions and the resultingroughness can be controlled by varying the angle of incidence in the course of the polishing process. The resultsof the surface roughness (Rn) measurements are presented in Table I both for the original and laser-polished filmsurface. The scanning length of a stylus during recording a surface profile is 450 m. The roughness data foreach film are averaged over I 0 measurements. Obviously, the profilometry results are in complete agreementwith the SEM images of the laser-polished surface, and the roughness of the as grown film has been reduced byan order of magnitude using the two-step polishing regime.

The optical transmittance (T) of the laser-polished diamond films before oxidation was rather low and variedbetween 1 and 0.01% in the spectral range 700-250 nm, corresponding to the values of 2-4 in the optical densityspectra in Fig. 2. The optical density (D) is taken as D=lg[100/T(%)]. A non-monotonous modification of theoptical spectrum was observed in dependence on the oxidation temperature and the regimes of laser polishing.The laser-modified layer after the two-step polishing was characterized by a lower transmission than that afterthe one-step polishing as well as by a lower oxidation stability. So, after the oxidation runs at 400°C and 430°Cthe transmittance of the two-step polishing part of the film became higher than that after the one-step polishing.Such tendency persists at higher oxidation temperatures (460 and 5 10°C) and the optical transmission remains tobe higher for the film polished in the regime #2. However, the oxidation stability of the interface layer was ratherhigh, so the oxidation step at 5 10°C did not result in complete removal of the laser-modified layer. Furtherincrease in the oxidation temperature up to 590 °C leaded to slightly lowering the optical density spectrum of thefilm polished in the regime #1 , whereas the spectrum of the film polished in the regime #2 was shifted to highervalues of the optical density. The latter is likely to evidence that after removal of the adsorbing surface layer thecontinued oxygen etching ofthe diamond surface (preferentially in grain boundaries, local defects, etc.) increasesthe surface roughness and, hence, is responsible for the surface light scattering losses.

To exclude the intrinsic and defect-induced diamond absorption and light scattering from the substratesurface, the changes in the optical densities (AD) were analyzed between each oxidation run (Fig. 3), whichreflect the changes both in the optical absorption in the laser-modified layer and in the surface reflection andscattering on its interface. At the first approximation, the difference optical density (AD) spectra can beconventionally divided into two spectral components: the first component reflects a monotonous growth of theoptical density from long to short wavelengths and the second one is attributed to a characteristic 255 nm-centered absorption band. The first structureless component dominates in the D spectra for the upper part of agraphite-like layer for both polishing regimes, while the second component prevails for layers near the interfacebetween diamond and laser-modified layer.

The structure of the laser-modified layers on the diamond surface is not clear. In graphite-like materials thepeak at 255 nm is due to the interband absorption between itbands near the M point in the Brillouin zone.'°"So, the observed enhancement of such band in the spectra indicates the dominance of a graphite component in thelower part (i.e., close to the diamond-graphitic interface) of the laser-modified layer. To determine an effectivethickness of this graphitic phase, the spectral shape of the AD spectra were compared with the calculated spectrafor carbon materials from the literature data. Shown in Fig. 4 are i) the AD spectrum corresponding to the


changes (between 460°C and 510°C oxidation runs) in the optical density of the film polished in the regime #2,and ii) the optical density spectrum for highly-oriented pyrolytic graphite (HOPG) layer of 7 nm thick. Thespectrum of HOPG was calculated according to T=(1-R2)e/(1-R2 e2) using experimental data from Ref.1 1,where R is the reflectivity, a is the absorption coefficient and d is the thickness of a HOPG layer. The spectralshape and amplitude (after the background correction) of the 255-nm band for both spectra in Fig. 4 are veryclose. The shift of the AD spectrum relative to the HOPG spectrum is attributed to absorption and scatteringlosses in the laser-modified layer. The structureless optical losses in carbon materials are characteristic of non-crystalline or even amorphous material.'2 Another interpretation of such structureless spectra is the enhancedoptical losses due to light scattering at the surface (and/or in the volume) of the upper part of the laser-modifiedlayer. Probably, these two reasons are responsible for the spectral shape of the optical density spectra of theupper layer for both polishing regimes.

Thus, two consequences of the observed modifications in the optical density spectra are important. The first isthat the structure ofthe laser-modified layer is changed throughout the layer, and the second is the dependence ofthe layer properties on the laser irradiation parameters. Under the laser ablation of diamond with nanosecondpulses, the conditions of surface graphitization are far from the stationary conditions for direct transformation ofdiamond to graphite which occurs at about 1900 K in high vacuum (or at lower temperatures in case of catalyticgraphitization). Typical of this phase transformation is the formation of a graphite layer at the diamond surface,specific properties of which are not limited to the graphite-diamond interface region but maintained throughoutthe whole layer thickness of up to 150-200 nm.'3 During the laser ablation, there exists a temperature gradient inthe surface layer due to the difference between the surface temperature (�4000 K) and graphitization temperatureat the diamond-graphite interface (1 900 K), resulting in the structure variation (a 'structure" gradient) throughoutthe layer thickness, i.e., from the surface to the diamond-graphitic interface.

Upon removal of the laser-graphitized layer the optical transmission spectra of the laser-polished films weremeasured in the UV-VIS spectral range and compared with the optical transmission of the same diamond filmwhich was mechanically polished (the roughness of the mechanically polished surface is about a few nm). Thespectra of the laser- and mechanically polished films are presented in Fig. 5 . It is very important that the opticaltransmission of the film polished with the copper vapor laser by the two-step polishing procedure (aftersubsequent oxidation at 5 10°C) is close to that of the mechanically polished diamond film. Note also, that thevalue of the optical transmission for all the films is influenced by the intrinsic defect-induced absorption and thesurface roughness ofthe rear side ofthe free-standing diamond film as the film was deposited on the ground (i.e.,not polished) Mo substrate. It is expected therefore that the optical transmission of the polished diamond filmscan be further increased upon polishing and reducing the roughness ofthe rear side ofthe films.

4. CONCLUSIONS

Laser polishing of 350 im thick free-standing diamond films has been performed using acopper vapor laserto reduce the surface roughness of the as-grown films by an order of magnitude. It is shown that the laserpolishing conditions and the resulting surface roughness are controlled by varying an angle of incidence of ascanning laser beam in the course of polishing process, resulting in a minimum achievable roughness (Ra=O.38tm) under the two-step polishing regime. Optical transmission in the UV-visible spectralrange of the diamondfilms polished under the optimized conditions (two-step polishing) was found to be close to the opticaltransmission of the mechanically polished diamond film.

Structure and optical properties of the laser-graphitized layer at the diamond surface have been studied withoptical spectroscopy techniques in the process of oxidative removal of the layer with increasing temperature ofthe oxidation in ambient air. It is found that the optical properties and oxidation stability of the laser-modified

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surface layer are varied throughout its thickness from the surface to the diamond interface, depending on the laserpolishing regime.

5. ACKNOWLEDGMENTS

The authors are thankful to Dr. E.D. Obraztsova for Raman characterization of diamond films. The work wassupported by the Swiss National Science Foundation under the Project 75UPJ048239.

6. REFERENCES

I . M. Yoshikawa, "Application of CVD diamond to tools and machine components", Diamond Films andTechnology, 1 (1), pp. 1-29, 1991.

2. T. Chein, C. Cutshaw, C. Tanger and Y. Tzeng, "Polishing ofthick CVD diamond by an excimer laser and acast iron wheel", Proc. 3rd mt. Conf. on Applications of Diamond Films and Related Materials, Eds. A. Feldman,Y. Tzeng, W. A. Yarbrough, M. Yoshikawa and M. Murakawa, NIST Spec. Publ. 885, pp. 257-260, WashingtonDC, 1995.

3. V.G. Raichenko and S.M. Pimenov, "Laser processing ofdiamond films", Diamond Films and Technology,7 (1), pp. 15-40, 1997.

4. A. Blatter, U. Bogli, L.L. Bouilov, N.I. Chapliev, VI. Konov, S.M. Pimenov, A.A. Smolin, and B.V.Spitsyn, "Excimer laser etching and polishing of diamond films", Proc. of the 2nd mt. Symp. on DiamondMaterials, Vol. 91-8, pp. 357-364, The Electrochem. Soc., Pennington, 1991.

5. SM. Pimenov, VI. Konov, E.D. Obraztsova, U. Bogli, P. Tosin, E.N. Loubnin, "Effects of excimer laserpolishing on surface properties and tribological performance ofdiamond films", Diamond Films and Technology,7 (1), pp. 61-78, 1997.

6. MW. Geis, M. Rothschild, R.R. Kunz, R.L. Aggarwal, K.F. Wall, C.D. Parker, K.A. McIntosh, N.N.Efremow, J.J. Zayhovski, D.J. Ehrlich, and J.E. Butler, "Electrical, crystallographic, and optical properties of ArFlaser modified diamond surfaces", Appl.Phys.Lett., 55 (22), pp. 2295-2297, 1989.

7. V.G. Ralchenko, A.A. Smolin, V.1. Konov, K.F. Sergeichev, l.A. Sychov, II. Vlasov, V.V. Migulin, S.V.Voronina, A.V. Khomich, "Large-area diamond deposition by microwave plasma", Diamond Relat. Mater., 6, pp.417-421, 1997.

8. SM. Pimenov, A.A. Smolin, V.G. Raichenko, V.1. Konov, G.A. Sokolina, S.V. Bantsekov, and B.V.Spitsyn, "UV laser processing of diamond films: effects of irradiation conditions on the properties of laser-treateddiamond film surface", Diamond and Related Mater., 2, pp. 291-297, 1993.

9. P. Tosin, A. Blatter and W. LUthy, "Laser-induced surface structures on diamond films", J. Appl. Phys., 78(6), pp. 3797-3800, 1995.

10. L.G. Johnson and G. Dresselhaus, "Optical properties of graphite", Phys. Rev.B., 7 (6), pp. 2273-2285,1973.

11. J.M. Zhang and P.C. Ekiund, "Optical properties of graphite compounds", J.Mater.Res., 2 (6), pp. 858-863, 1987.

12. G. Compagnini, "Optical constants of hydrogenated and unhydrogenated amorphous carbon in the 0.5- 12-eV range", Applied Optics, 33 (11), pp. 7377-7381, 1994.

13. W.J.P. van Enckevort, "The effect of crystallographic orientation on the optical anisotropy of graphitelayers on diamond surfaces", J. Appl. Cryst., 20 (1), pp. 11-15, 1987.


lable I. Experimental conditions of two polishing regimes and surface roughness (Ra) of the original and laser-polished diamond filnis.

Original surface 3.4±1.6Experimental conditions Roughness

Ra. urn.N 0 E, J!cm75 6.5 0.59±0.18

0.38±0.102 75+35 6.5+2.2

x130 150pm

-

;...-&. :.

— t.,_. .

'_&

a

x500 40 pm

f; . ,,.

x130 150pm

,— *• ..—

'—-

1)

xSOO 40 pm

C

Polishing regime #1d

Polishing regime ft2

Fig. 1. SEM images of the surface morphology of a 350 p.m thick diamond film polished with a copper-vaporlaser: (a.c) - polishing regime #1 and (h,d) - polishing regime #2.

171


1 72

H

I

Fig. 2. Modification of the optical density spectra of the laser-polished films with the temperatureof oxidation in ambient air: (1) polishing regime #1, and (2) polishing regime #2.

Wavelength, nm

Wavelength, nm


; 1,0

0,5

200 400 600 800

Fig. 3. Modification of the difference optical density spectra (deduced from Fig. 2) of the laser-polished films with the temperature of oxidation in ambient air: (1) polishing regime 1, and(2) polishing regime #2.

Wavelength, nm

1,0

2% %

, .. - {430° q

_'%' %..\% {43Pq-{q

{46Q-

Wavelength, nm

173


0,4

0,2

400 600 800

Wavelength, nm

Fig. 4. Correlation between the optical density spectra of the laser-polished film (polished in theregime #2) and of a 7 nm thick layer of highly-oriented pyrolytic graphite.

{ {510°q

/ HOPGI

7nmthick

200

Ij

Fig. 5. Optical transmission spectra of a 350 tm thick laser-polished diamond film (for twopolishing regimes #1 and #2) after oxidation at 510°C in comparison with the spectrum of the samediamond film polished by an abrasive polishing technique.

Wavelength, nm

174


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