Sticking Coefficient of CH3 Radicals

It was emphasized in the previous sections that the cavity probe technique is sensitive to the surface loss probability of radicals rather than their sticking probability. The sticking probability is only accessible by experiments which allow to directly compare the rate of sticking to the flux of incoming species. The ability of the MAJESTIX experiment to perform such a task was described in Sect. 11.3.2. As a first example, we shall review our results on the sticking coefficient of methyl radicals on a-C H surfaces. 

A methyl radical flux of j 2 x 1015 cm 2s 1 is directed to the surface of a pre-deposited a-C D film. For film deposition, CD4 was used as the plasma source gas instead of CH4. Isotopic labelling of the substrate film is very advantageous for in-situ infrared spectroscopic measurements since it 

At a substrate temperature of 340 K, the adsorption of CH3 is observed with ellipsometry. The change of the ellipsometric signal is depicted in Fig. 11.7. The points 1, 2, and 3 label consecutive stages in the evolution of the ellipsometry data points in the F, A plane point 1 represents the C D layer at the beginning of the experiment. With the onset of the CH3 flux, the ellipsometric angles shift at first to larger values of and A (point 1 to 2), followed by a shift to larger values of but smaller values of A (point 2 to 3). 

This variation of the ellipsometric angles is compared with a two-layer optical model the open squares correspond to a hard a C H film with refractive index h = 2.1 — i0.13 and variable film thickness d. At point 1, the film thickness is d = 31.62 nm while d = 31.71 nm at point 1. A polymer-like film with refractive index n = 1.62 — 0 on top of a 31.62 nm thick hard film is represented by open circles. In both model curves, film thickness increases by Sd = 0.01 nm between consecutive model points in the direction of decreasing values for A. 

The measured shift of A from point 1 to 2 in Fig. 11.7 can be understood by assuming that a surface layer of the hard C D film (point 1 to 1) is transformed into a polymer-like C H D film (point 1 to 2) due to adsorption of CH3 radicals at sp2-coordinated carbon groups at the initial film surface it is assumed, that this reaction is similar to the hydrogenation of sp2 carbon groups by atomic hydrogen, which changes the hybridization of the sp2 CC bond to sp3 [53]. The extinction coefficient at 632.8 nm is related to the 

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Fig. 11.8. Temperature-dependent sticking coefficient of CH3 radicals at an a-C H surface. The CH3 flux density is j 2 x 101B cm-2s-1. The absolute growth rate is given on the left vertical axis...

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