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growth of the films in order to enhance the film adherence. In fact, without the buffer layer, the film peels off. The film composition was determined by ion beam analyses: RBS, ERDA and nuclear reaction, A 4 MV Van de Graaff accelerator provided He+ and proton beams. RBS was employed to determine the carbon and fluorine content and to identify any contamination. For these measurements, we used a 2 MeV He+ beam and a particle detector positioned at 1650 with respect to the incident beam. ERDA measurements determined the hydrogen content by using a 2.2 MeV He+ beam. From the 19F(p,y (X)160 reaction (Ep = 880 keV) the fluorine content was checked. By combining the areal atomic density provided by ion beam techniques and the thickness values of the samples, as determined by stylus profilometry, the atomic density was evaluated based on RUMP simulations. The as-deposited films were investigated by infrared transmission spectroscopy carried out in air by using a Perkin Elmer 2000 FT-IR spectrophotometer in the 370-4000 cm' range. The stress determination was made by measuring the curvature of the films and by applying Stoney's equation, as described in detail elsewhere [12]. Vickers hardness was obtained by micro-indentation with 15 seconds long 5 g-loads by employing a Shimadzu lMV-2000 equipment. For these measurements, 1.5-2.0 jtm thick films were used in order to minimize the effect of the substrate on the final hardness values. The indentation diagonals were measured by atomic force microscopy (AFM) in contact mode. The hardness values presented in this work correspond to the average of 10 independent indentations. RESULTS AND DISCUSSION The behavior of the deposition rate as a function of the CF 4 partial pressure is shown in Figure 1. An increase by a factor of 2 was observed within the entire range of mixtures employed. On the contrary, no film deposition but substrate erosion was found if using CF 4 partial pressures richer than -80 %. These facts are tentatively explained as follows. According to the adsorbed layer model [13], the growth of a-C:H films is due to the physisorption of hydrocarbon radicals onto the surface followed by chemisorption of such radicals induced by energetic processes such as ion bombardment. The existence of dangling bonds on the surface is expected to enhance the probability of hydrocarbon radical sticking. As the CF 4 partial pressure

25-

E

,

"E =E20,

Figure 1: Deposition rate of a-C:F:H films as a function of the CF 4 partial pressure.

A

a)

01,

10 0

20

40

60

80

CF 4 partial pressure (%)

348

0

20 -

F concentration

H concentration .

1"

Figure 2: Hydrogen and fluorine contents as functions of the CF 4 partial pressure as determined by ion

A

A

1C

.

beam analysis.

A

Zt1 Cu 10"

0

0-

."

0

20

4'0

6'0

8'0

CF 4 partial pressure (%) increases, the hydrogen concentration in the plasma decreases together with the increase of the fluorine one. Fluorine atoms can react on the surface by chemically eroding H from hydrocarbon radicals, forming volatile HIF and