Characterization of diamond fluorinated by glow discharge plasma treatment

Diamond and Related Materials 10 Ž2001. 490᎐495

Characterization of diamond fluorinated by glow discharge plasma treatment Steven F. Durrant a,U , Vıtor ´ Baranauskas a , Alfredo C. Peterlevitz a , Sandra G. Castro b, Richard Landers b, Mario ´ A. Bica de Moraes c a

Departamento de Semicondutores, Instrumentos e Fotonica, Faculdade de Engenharia Eletrica e Computac ¸ao, ˆ ´ ˜ Uni¨ ersidade Estadual de Campinas, A¨ . Albert Einstein N. 400, 13083-970, Campinas, SP, Brazil b ´ Departamento de Fısica Instituto de Fısica ´ Aplicada, Grupo de Fısica ´ de Superficies, ´ Gleb Wataghin, Uni¨ ersidade Estadual de Campinas, 13083-970, Campinas, SP, Brazil c Departamento de Fısica ´ Aplicada, Laboratorio ´ de Processos de Plasma, Instituto de Fısica ´ Gleb Wataghin, Uni¨ ersidade Estadual de Campinas, 13083-970, Campinas, SP, Brazil

Abstract The surface fluorination of diamond by treatment in glow discharge plasmas of CF4 for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition ŽHFCVD.. Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF4 for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy ŽAFM., Raman spectroscopy and X-ray photoelectron spectroscopy ŽXPS., respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy ŽSEM.. Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy ŽEPRS., revealing that untreated diamond possesses an appreciable density of free radicals Ž6 = 10 12 gy1 . which initially falls with treatment time in the CF4 plasma but increases for long treatment times. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluorine; Diamond films; HFCVD; XPS

1. Introduction Fluorinated hydrogenated carbon films with characteristics ranging from relatively soft amorphous plasma polymers w1᎐3x, to fluorinated diamond-like carbon w4,5x and fluorinated diamond w6,7x have attracted much research attention because of their interesting physical properties such as hydrophobicity, chemical inertness U

Corresponding author. Tel.: q55-19-242-0731; fax: q55-19-2420731. E-mail address: [email protected] ŽS.F. Durrant..

and low coefficient of friction. Concerning diamond, several methods have been employed for the introduction of fluorine atoms either into the bulk or onto the surface of the material. For bulk fluorination of films ion implantation may be employed and has potential in dopant applications w8,9x. In the chemical vapor deposition ŽCVD. of diamond, halogens have been introduced into the chamber feed with the aim of improving diamond quality and reducing the deposition temperature w10,11x but this constitutes a research area distinct from the present work, whose aim is the surface fluorination of diamond fabri-

0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 3 4 - 3

S.F. Durrant et al. r Diamond and Related Materials 10 (2001) 490᎐495

cated by HFCVD using treatments in radiofrequency plasmas of CF4 . Other methods employed for surface fluorination of diamond include direct treatment using F2 w12x, exposure to F atoms at low pressure in a microwave plasma w13x, and X-ray irradiation of perfluoroalkyl radicals chemically bound to the diamond surface w14x. A wide-ranging review of the fluorination of diverse carbon materials has recently been published w15x.

2. Experimental Diamond of high quality was deposited in a cylindrical quartz hot filament CVD reactor described more fully in the literature w16x. Films were grown from ethanolrhydrogen mixtures Ž99.5% H 2 . at a total flow rate of approximately 100 sccm Žstandard cubic centimeters per minute. and a total pressure of approximately 27 mbar. The deposition temperature at the substrate surface was approximately 990 K. Deposition times of approximately 6.5 h were employed. Typical film dimensions were 10 mm= 6 mm= 8 ␮m. Fluorination was accomplished in a stainless steel radiofrequency plasma deposition system with a few modifications from that described previously w17x. The system employs horizontal parallel-plate electrodes, the upper electrode being fed radiofrequency power at 40 MHz, the lower electrode being earthed. The changes made to the system involve the vacuum pumping, which is now accomplished by a Roots pump backed by a rotary-vane pump. Carbon tetrafluoride Ž99.9% pure. was fed to the chamber via a precision flowmeter at 47 sccm. By throttling the gas flow by partially closing the exit valve, a constant pressure of 7 = 10y2 mbar was maintained prior to the plasma treatment. For treatment, the diamond samples were placed in the middle of the lower electrode Žanode.. Treatment times of 5, 10, 15, 30 and 60 min were employed, each at a plasma power of approximately 60 W. Atomic force microscopy images were obtained using a Nanoscope II ŽDigital Instruments . microscope. For Raman spectroscopy, a Raman Jobin Yvon T64000 spectrometer was used with excitation at 514.5 nm at a power of approximately 8 mW. The XPS analyses were carried out using a VSW HA100 spectrometer employing the Al K ␣ radiation Ž1486.6 eV. for excitation. Fixed analyzer transmission was used with a pass energy of 44 eV. The radius of the electrostatic analyzer was 100 mm. A base pressure of 2 = 10y9 torr was obtained. Carbon 1s spectra were deconvoluted using a computer program developed by the Surface Physics Group. Electron paramagnetic resonance spectroscopy was

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undertaken on the samples using a Varian V2502 spectrometer, operating at X-band microwave frequencies and 100 kHz magnetic field modulation. For measurement, fragments of known mass Žfew milligrams. of the diamond films deposited onto silicon were placed inside thin Ž2-mm internal diameter. quartz tubes in the spectrometer cavity. Measurements using fragments of the silicon substrate alone gave no appreciable signals under the conditions used. Absolute spin densities were obtained by comparison with a diphenylpicrylhydrazyl ŽDPPH. standard, which was also used to calibrate the magnetic field of the spectrometer.

3. Results and discussion Fig. 1a᎐f show AFM images of the untreated diamond and the films fluorinated for 10 min and 1 h, respectively. Two magnifications are shown in each case. Fig. 1a,b shows the surface of the untreated diamond. The angular grains and their smooth appearance are characteristic of high quality diamond. Following plasma treatment, new features are observed as indicated in Fig. 1c,d,f,g. A plasma fed CF4 can be expected to contain free radicals, CFx Ž xs 1᎐3., together with atomic fluorine. The latter is an effective etchant. There is a clear contrast in the surface texture from Fig. 1a,c, i.e. of the untreated diamond surface compared to that treated for 10 min. Fig. 1c and particularly Fig. 1d suggest that etching has begun over the grain surfaces, leaving closely-spaced ‘islands’ of material. Inspection of Fig. 1e,f suggests that with longer exposure to the etching plasma, the islands are further eroded and the spacing between them increases. The mechanism tentatively proposed above to give rise to the surface morphologies of the films is also supported by scanning electron micrographs of the samples. Fig. 2a᎐c shows, at a magnification of 5000, the surfaces of the untreated diamond and samples treated for 30 and 60 min, respectively. Again, the ‘islands’ on the surfaces are absent from image Fig. 2a, present in image Fig. 2b, and present but more sparse in image Fig. 2c. Moreover, SEM cross-sectional images of the samples Žnot shown. revealed contiguous films with no indication of modifications to any appreciable depth produced by the plasma. Fig. 3 shows Raman spectra in the range 1200᎐1700 cmy1 for the untreated film and those treated for 5 or 30 min. Each spectrum shows a narrow sharp peak at approximately 1332 cmy1 , characteristic of sp 3 bonds. No peak at approximately 1580 cmy1 , characteristic of graphite, is observed in any of the spectra. Fig. 4 shows long-scan X-ray photoelectron spectra of a diamond film before and after fluorination. Spec-

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Fig. 1. Atomic force microscopy images of the surface of untreated diamond Ža,b., and diamond treated in a CF4 plasma for 10 min Žc,d. and 1 h Že,f..

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Fig. 3. Raman spectra of untreated diamond Ža., diamond treated in a CF4 plasma for 5 min Žb., and for 30 min Žc..

Fig. 2. Scanning electron micrographs Žmagnification 5000 = . of untreated diamond Ža., diamond treated in a CF4 plasma for 10 min Žb., and for 1 h Žc..

trum A Žof the untreated film. exhibits characteristic peaks due to carbon and oxygen, and small peaks associated with silicon. Oxygen is usually observed on the surfaces of films exposed to the atmosphere; silicon derives from the substrate. Similar peaks are visible in spectrum B Žof the fluorinated film. together with others due to the contaminants copper and aluminum, possibly derived from the sample holder in the spectrometer. The prominent F1s peak confirms the surface fluorination of the film. As shown in Fig. 5, the XPS analyses also revealed that the ratio of the number of fluorine to carbon atoms in the films increased with the treatment time. Thus, some control over the degree of fluorination of the diamond surface was achieved. The maximum flu-

Fig. 4. Long-scan X-ray photoelectron spectrum of diamond ŽA. before and ŽB. after 60 min of plasma treatment.

orination reached was approximately 7 F atoms for every 100 C atoms. As shown in Fig. 6, deconvolution of a typical C1s spectrum of a fluorinated film revealed the presence of three peaks, which may be attributed to C᎐H Ž284.6 eV. w18x, diamond Ž286.0 eV. w18x and C᎐F Ž287.7 eV. w13x groups, respectively. There was no evidence for the presence of ᎐CF2 or more fluorinated groups. The free-radical density in the films, calculated from the EPR responses from the known diamond masses and the DPPH standard, is shown as a function of the fluorination time in Fig. 7. Clearly, the density falls at

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Fig. 5. F-to-C atom ratios in the films Ždetermined by XPS. as a function of plasma-treatment time.

Fig. 6. Deconvoluted C 1s spectrum of the film fluorinated for 60 min.

low plasma treatment times, reaches a minimum, and then rises at long treatment times. The free-radical density is thus quenched by low to moderate exposure to CF4 plasmas but increases when the surface becomes well fluorinated. Spin concentrations observed here are approximately 10 6 smaller than those typical of nitrogenated diamond w19x, but this may simply reflect the fact that fluorination is restricted to the diamond surface. The mean g-value observed for our samples, 2.0027" 0.0002, is also consistent with literature values for various types of diamond, 2.0025" 0.0002 w19x and 2.0024" 0.0005 w20x. In summary, good quality diamond produced by HFCVD has been fluorinated in r.f. plasmas of CF4 . To a degree, fluorination may be controlled through the choice of the plasma treatment time. In addition to fluorination, as detected by AFM and SEM images, etching occurs in the treatment plasmas. Electron paramagnetic resonance spectroscopy revealed that spin densities of approximately 10 12 gy1 , probably due to the presence of free radicals, are present in the films. The influence of other plasma treatment conditions Že.g. gases, applied power, substrate bias. on the diamond surface remains to be addressed.

Acknowledgements

Fig. 7. Spin density in the deposited films as a function of the plasma treatment time.

For help with various aspects of this work we thank our colleagues from UNICAMP: Jing Guo Zhao, Dailto Silva, Alberto Garcıa ´ Quiroz and Prof. Edson C. da Silva. The authors acknowledge financial assistance

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from the Brazilian agencies Fundac¸˜ ao de Amparo ` a Pesquisa do Estado de Sao ˜ Paulo ŽFAPESP. and Conselho Nacional de Desenvolvimento Cientıfico e Tec´ ŽCNPq.. We are also grateful to the nologico ´ Laboratorio Nacional de Luz Sincrotron ŽLNLS., ´ Campinas, SP, for use of its scanning electron microscope. References w1x R. D’Agostino, F. Cramarossa, V. Coloprico, R. d’Ettole, J. Appl. Phys. 54 Ž1983. 1284. w2x S.F. Durrant, R.P. Mota, M.A. Bica de Moraes, J. Appl. Phys. 71 Ž1992. 448. w3x S.F. Durrant, S.G. Castro, L. Bolivar-Marinez, D.S. Galvao, ˜ M.A. Bica de Moraes, Thin Solid Films 304 Ž1997. 149. w4x R. D’Agostino, R. Lamendola, P. Favia, A. Giquel, J. Vac. Sci. Technol. A 12 Ž1994. 308. w5x R.S. Butter, D.R. Waterman, A.H. Lettington, R.T. Ramos, E.J. Fordham, Thin Solid Films 311 Ž1997. 107. w6x B.E. Scruggs, K.K. Gleason, J. Phys. Chem. 97 Ž1993. 9187. w7x T. Ando, J. Tanaka, M. Ishii et al., J. Chem. Soc. Faraday Trans. 89 Ž1993. 3105.

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