Kinetics of the diamond-like film deposition on glass fibers

D#MOND RELATED MATERIALS Diamond and Related Materials 4 (1995) 1126-1130

Kinetics of the diamond-like film deposition on glass fibers B. TomCik a, A. Jelenak a, M.M. MitroviC b, Z.Lj. Petrovii: a a Institute of Physics, University of Belgrade, P.O. Box 68, 11080 Zemun, Minor Yugoslavia b Faculty of Physics, University of Belgrade, P.O. Box 550, 11001 Belgrade, Minor Yugoslavia Received 4 November 1994; accepted in final form 26 January 1995

Abstract In order to determine the mechanism of diamond-like film deposition in a r.f. discharge in methane, a tiny glass fiber was spanned along the discharge axis and perpendicular to the parallel-plate electrodes. The discharge was operated in a symmetric and slightly asymmetric regime. The film thickness and its surface topography were analyzed by means of scanning electron microscopy. Across the interelectrode space a non-uniform film thickness distribution on the fiber was observed. The films up to 15 km were deposited with excellent coverage uniformity and time stability on a 40 pm diameter glass fiber substrate. The film thickness distribution along the fiber is the result of the space-dependent density profile of methyl CH, radicals which contribute mostly to the film formation. It was concluded that the diffusion of methyl radicals in the discharge is responsible for the observed film thickness profile along the fiber. Keywords: Diamond-like

films

1. Introduction Over the last decade basic physical and mechanical properties of amorphous hydrogenated carbon films, also known as diamond-like carbon coatings, have been investigated. The high atomic density of diamond-like films can serve as a good moisture barrier for glass fibers. During the fiber drawing process there are always some surface microcracks that tend to spread owing to bending or stretching of the fiber. Water vapor additionally enhances breaking of the silicon-oxygen bonds in the fiber, reducing its strength. Water penetration into the fiber cannot be stopped even by using polymer cladding that gives a fiber overall mechanical strength. Up to now there were several reported attempts to deposit diamond-like films on a glass fiber [l-5]. Techniques were developed to match the film deposition rate to the large scale production drawing rate of the fiber. The film thickness in the in-line drawing and deposition technology is usually limited to 30 nm. By means of spectroscopic elipsometry it was shown that water passes only through the top ultra-thin surface layer that maintained the roughness [6]. Various methods were proposed to measure the concentration of methyl CH, and CH1 radicals in glow discharges. The density of the methyl radical has been determined by using infrared absorption spectroscopy Elsevier Science S.A. SSDI 0925-9635(95)00280-4

[ 7-91, resonance-enhanced multiphoton ionization [lo] and appearance mass spectrometry [ll,lZ]. The CH radical concentration has been mainly measured by optical emission spectroscopy and laser-induced fluorescence spectroscopy (LIF) [ 131. Insertion of a tiny insulating glass fiber perturbs the glow discharge very little and can serve as a probe for the investigation of the radical distribution [ 14,151. After completion of a full r.f. cycle the net current to the fiber is zero. The fiber charges negatively with respect to the plasma to be able to attract ions and repel the higher mobility electrons. The spatial distribution of the deposition rate obtained from the thickness of the film deposited on the thin fiber has been used successfully by Gallagher and coworkers [ 14-161 to prove that SiH3 is the deposition precursor for silane discharges. The aim of this paper is to present a technique for studying the spatial distribution of deposition rate and quality of the films which can be used in conjunction with a plasma chemical model to show the identity of the deposition precursor of the diamond-like films.

2. Experiment A glass chamber with parallel-plate electrodes separated by 40 mm which were 7 cm in diameter is shown

B. TomCik et al.lDiamond and Related Materials 4 (1995) 1126-1130

in Fig. 1. A 13.6 MHz r.f. power supply was capacitively coupled to the electrodes through the matching network which allowed the discharge operation either in symmetrical or in asymmetrical mode. A uniformly thin glass fiber 40 nm in diameter was spanned across the interelectrode space. On the bottom powered electrode the fiber rests freely in a small center hole and at the other end it passes through the central hole in the upper grounded electrode. Treatment time varied from 2 h up to 19 h. The base pressure in the chamber was 1 x 1O-5 mbar. At 2 seem flow rate of methane, regulated by the thermal mass flow controller, the working pressure in the chamber was kept constant at 0.1 mbar. The residence time of neutral molecular species in the chamber was approximately 3 s. The mean r.f. energy deposited in the discharge was 0.4 W cme2. The maximum observed deposition rate of the diamond-like film on the glass fiber was 0.3 A s-l. The r.f. discharge was operated in a symmetrical regime and in a slightly asymmetrical regime. In the latter case the ratio of the d.c. offset voltage V,,,. developed on the powered electrode to the peak-applied r.f. voltage V,., was less than 0.2 (V,,JV,,,
3. Results and discussion The film thickness distribution along the 40 pm thick glass fiber shown in Fig. 2 was determined by scanning electron microscopy (SEM). Previously, it was necessary to deposit up to 10 nm thin gold layers on the fiber to avoid charging effects caused by the primary beam of electrons of SEM. The highest film deposition rate was observed not close to the electrodes, as one would expect [17], but in the middle of the discharge, in the plasma volume. Under the assumption of the constant sticking coefficient, the observed non-uniform film thickness along the fiber indicates that the distribution of the precursors of deposition may be determined by diffusion. It is well known that under the applied power and pressure conditions the most abundant radical of the cracking reaction of the CH, molecule is the methyl CH3 radical. The CHJ and CH2 radicals are produced mainly through the primary dissociation of methane by electron impact. CH3 has a higher abundance than CH, partly due to the abstraction reaction of H atoms, H + CH4 -+H, + CH3, and partly due to the loss of CH, radicals through the reaction with methane forming the higher order alkanes [ 7,161. The CH3 radical density in the steady-state discharge is determined by the balance of CH, production through: electron impact dissociation of CH4 (CH_, +e-+ CH, + H+e); the ion-molecule reaction CH: + CH4 --+CH: + CH,); and losses through: recombination (CH3 + CH3 +C2Hs); neutral-neutral reactions (CH, + CH2 -+C,H, + H; CH, + H+CH2 + H,); the ion-molecule reaction (CH: + CH4 -+C,H: + H,); and diffusion in conjunction with surface collisions. The time scale for the CH, radical diffusion to the center of a discharge, z = L2/D (where L is a distance from the peak position of the CH* emission profile given in Fig. 3 to the place on a fiber with the largest film thickness observed, LFZ1 cm), is approximately 0.7 ms. A value of D = 140 cm2 Torr s-l was used for the diffu-

.

input and pressure meter

, ~_

electrodes *j

ids _-;

matching unit

‘RF’

Fig. 1. Vacuum

chamber

l

4

~~ glass fiber

i

;

r,

.

q

,

1127

‘glass chamb

. . . . .* l* . . f

to vacuum pump

\ 20 distance

for the glass fiber deposition.

Fig. 2. Film thickness

30 [mm]

distribution

along

.

*

40

the fiber.

50

1128

B. Tom&k --

.

cw431.2nm sscond dcriv. of film thicket

0.4

0.3

d .* i d

0.2

a g ‘L1 f

0.1

i

8 17L 0

0.0 IO

20 diatMcs

30

I

[mm]

Fig. 3. Spatial profile of the optical emission intensity of CH* and the second derivative of the film thickness.

sion coefficient D which is appropriate for the pressure of 100 mTorr [ 13,171. The corresponding time scale is about two orders of magnitude larger then that corresponding to the recombination rate of CHJ. The time variation of CH, radical density n can be described by the equation $=

-2k,n’

- k,n

where kl is the rate coefficient of methyl radical recombination and kz is the surface loss constant (k, ~svA/4V, where s is the sticking coefficient of the CH3 radical and v the thermal velocity of radical on the surface area A within the volume I’ of a discharge [ 13,171). The spatial profile of the optical emission intensity of an excited CH* radical, as shown in Fig. 3, reflects its production rate distribution between two parallel electrodes. Also, the second derivative of the film thickness distribution represents the distributed source of the radical species in a discharge, if the diffusion of radicals determines their spatial distribution [ 181. The CH ground-state radical distribution has been measured by LIF [14] and is expected to be close to that of excited CH, since both are dominantly formed by the dissociation of methane. The peaks closer to the powered electrode, as shown in Fig. 3, of both the emission profile and the second derivative of the thickness are in very good agreement. The peak of the second derivative closer to the grounded electrode however was difficult to obtain since it consisted of several peaks owing to numerical differentiation, and in the present figure those oscillations were removed. In general, in all measurements both in the quasi-symmetrical and asymmetrical mode, peaks of the emission agreed well with the second derivative of the deposition profile. The CH, radicals are non-emissive in the visible and near-infrared range. Their spatial distribution has been established by using mass spectrometry with threshold

ionization [ 191. However, from the knowledge of the potential diagrams for the CH, molecule and the energy dependence of the cross-sections, the region of the CH, production should coincide with the region of intense HE and CH (AZA+X217) emission, which maps the region where electrons have sufficient energy to induce dissociative excitation of methane. In our measurements the region of excitation coincides with the region of the large second derivative of the spatial deposition profile. It is the proof that the deposition profile is determined by diffusion. The effective lifetime of radicals other than CH,, is much shorter because of their efficient removal by the ground-state methane. Thus only CH3 can lead to the deposition profile as observed in our experiment. Under the assumption that the optical emission profile from Fig. 3 is close to the generation pattern of the CH, radical it is possible to explain the film thickness distribution along the fiber by diffusion of CHJ radicals. In symmetric and slightly asymmetric discharges the most intense source of radicals is located in the vicinity of the cathode fall region. Therefore, the central region of the fiber is exposed to the contributions of the radicals from the sheath regions of both electrodes through the mechanism of radical diffusion. Since the time-averaged concentrations of ionic species may be three orders of magnitude smaller than the neutral species [20,21], the main contribution to the film thickness distribution comes from the neutral particles in the plasma. The neutral CH, methyl radical possesses a non-vanishing dipole moment in the physisorbed state and mainly contributes to the adsorption process. The influence of the neutral methane molecules is a minor one because of its low polarizability and symmetrical shape [ 221. We thus conclude that the dominant precursor of the deposition is the CH, radical, which is consistent with observations based on different diagnostic techniques. The conclusion is based on the fact that the spatial distribution of light emission is different from the distribution of film deposition, but is similar to the distribution of its second derivative. Only the CH, radical can survive in sufficient numbers over a long enough time to lead to the observed profile, based on the available data for the chemistry of the radicals produced by methane dissociation [21]. The present results, however, appear to be inconsistent with the results of Mutsukura et al. [ 173. These authors used a pile of glass plates placed on the powered electrode and they studied both the deposition rate and the quality of the film as a function of the distance from the powered electrode. As mentioned earlier the maximum deposition rate was observed close to the electrode. This may not be in disagreement with our results and the two sets of data could indicate that directed component of motion of radicals becomes increasingly important for deposition closer to the electrode. On the other hand the two sets of data cannot be easily compared

B. Tom&k et al./Diamond and Related Materials 4 (1995) 1126-1130

because in the experiment by Mutsukura et al. [ 171 the discharge was significantly perturbed by the glass plates and charging of the surface could play a more significant role than in the case of the glass fiber. Diamond-like films deposited on the fiber in the vicinity of electrodes showed, on SEM, a compact and even structure with uniform film growth (Fig. 4). In the middle of the discharge a uniform film growth around the fiber was observed as well. Film topology is slightly modified with preferential film growth through the islands formation. The film deposition rate on the glass fiber in the middle of a discharge was up to two times the value achieved on the silicon substrate placed on the powered electrode, under the same deposition conditions. In all cases, from run to run, the uniform film thickness with a good coverage all around the glass fiber was observed. No peeling or spontaneous delaminations occurred even for the extreme film thickness coverage

1129

of 15 urn. Along the fiber, additional building in the form of large segmented clusters can be easily recognized as well as prismatic and needle-like crystals with a linear dimension of a couple of micrometers. A smooth structure prevails with the droplet roughness of an irregular shape and distribution. On the cross-section of a fiber, shown in Fig. 5, a compact columnar film growth was found at different positions along the interelectrode space. No holes in the film or incorporation of the soot particles, that tend to appear, especially on the glass chamber walls at higher deposition pressures, were observed. Raman spectra were recorded for the samples deposited on the flat substrate placed on the powered electrode. They indicate amorphous carbon structure without the lines specific for diamond crystals. However, it was not possible to obtain a sufficient signal from the fiber with the available system and we could not analyze

(4

C-9 Fig. 4. Topography of the film along the fiber at various from the powered electrode; (c) fiber at a 28 mm distance the powered electrode.

distances from the powered electrode: (a) non-treated fiber; (b) fiber at a 1,mm distance from the powered electrode; (d) enlarged compact film portion at a 1 mm distance from

1130

B. TomEik et al.lDiamond and Related Materials 4 (1995) 1126-1130

Acknowledgements Partial funding for the work presented in this paper came from the projects of the Ministry of Science and Technology of Serbia. One of the authors, Z.Lj. Petrovic, is grateful to Dr. A. Gallagher and Dr. G. Stetson of JILA, University of Colorado, for ideas, useful discussions and providing us with optical fibers.

References [l] Fig. 5. Cross-section of a fiber with a film thickness of 1.8 urn.

the spatial dependence of the Raman spectrum. The present technique, whilst having the advantage that it does not perturb the discharge significantly, yields samples that can be analyzed by diagnostic techniques with greater difficulty.

[2] [ 33

[4] [S] [6] [7]

4. Conclusious It is possible to deposit diamond-like films over 15 urn thick on tiny glass fibers with excellent coverage uniformity and time stability. No peeling or spontaneous delamination of the film was observed. The film thickness distribution along the fiber is the result of the spacedependent creation density of the methyl CH, radical and its diffusion through the discharge volume. By means of a CCD camera a space-resolved optical emission study of the CH and Ha radiation showed an increased light emission in the regions close to the electrodes. It was concluded that the diffusion of methyl radicals in a discharge is mainly responsible for the observed film thickness distribution along the fiber. The glass fibers were deposited at low temperature (below 100 “C) at which the surface mobility of the adsorbed reactive neutral and ionic species is expected to be very low. Therefore, the film thickness development along a fiber is also determined by the mass transport distribution in the gas phase.

[S] [9] [lo] [11] [12]

S. Aisenberg and M.L. Stein, Apparatus for coating optical fibers, US Patent No. 4,530,750, July 23, 1985. S. Aisenberg and M.L. Stein, Process for coating optical fibers, US Patent No. 4,402,993, September 6, 1983. MStein, S. Aisenberg and J.M. Stevens, in Bendow and Mitra (eds.), Advances in Ceramics, American Ceramic Society, Columbus, OH, 1981, Vol. 2, p. 124. S. Aisenberg, Radiation Curing, VI Conf. Proc., Society of manufacturing Engineers, Dearborn, MI, 1982, Chap. 12, p. 14. S. Aisenberg, J. Vat. Sci. Technol. A, 2 (1984) 369. S. Orzesko, N. De Bhala, J. Wollam, J.J. Pouch, S.A. Alterowitz and D.C. Ingram, J. Appl. Phys., 64 (1988) 4175. F.G.Celii, P.E. Pehrsson, H.T. Wang and J.E. Butler, Appl. Phys. Lett., 52 (1988) 2043. S. Naito, H. Nomura and T. Goto, Rev. Laser Eng., 20 (1992) 746. S. Naito, N. Ito, T. Hattori and T. Goto, Proc. 2nd lnt. Conf. on Reactive Plasmas, Yokohama,, 1994. p. 375. F.G. Celii and J.E. Butler, Appl. Phys. Lett., 54 (1989) 1031. W.L. Hsu, Appl. Phys. Lett., 59 (1991) 1427. H. Toyoda, H. Kojima and H. Sugai, Appl. Phys. Lett., 54

(1989) 1507. [ 131 K. Tachibana, T. Mukai, A. Yuuki, Y. Matsui and H. Harima, Jpn. J. Appl. Phys., 29 (1990) 2156. [ 141 R. Robertson and A. Gallagher, J. Appl. Phys., 59 (1986) 3402. [15] D. A. Doughty and A. Gallagher, Phys. Rev. A, 42 (1990) 6166. [ 161 R. Robertson, D. Hils, H. Chatham and Gallagher, Appl. Phys. Lett., 43 (1983) 544. [17] N. Mutsukura, S. Inoue and Y. Machi, J. Appl. Phys., 72 (1992) 43. [ 18) D.A. Doughty and A. Gallagher, J. Appl. Phys., 67 ( 1990) 139. [ 191 H. Sugai, H. Toyoda, H. Kojima, Y. Yamashita and T. Nakano, in R. Itatani (ed.), Progress Report on Control of Reactive Plasmas in 1989, FY, Kyoto, 1990, p. 84.

[20] K. Tachibana, M. Nishida, H. Harima and Y. Yrano, J. Phys. D, 17 (1984) 1727. [21] A. Rhallabi and Y. Catherine, IEEE Trans. Plasma Sci., 19 (1991) 270. [22]

A. von Keudell and W. Mbller, J. Appl. Phys., 75 (1994) 7718.