Gas Phase and Surface Kinetics of Diamond-Like Carbon Films Growth in PECVD Reactors
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described in [3] was considered more adequate to describe the physics of the discharge and was consequently adopted. GAS PHASE KINETIC SCHEME The realization of a gas-phase kinetic scheme for plasma requires the consideration of reactions between electrons, ions and neutral species. Since in the plasmas here considered the ionization degree is very low (i.e., neutral species are present in a concentration at least 105 times higher than that of the charged species) reactions between ions and electrons and ions and ions are not likely to happen. Therefore, we considered only the reactive events between electrons and neutral species (electronic reactions), ions and neutral species (ionic reactions) and among neutral species (due to the low gas temperatures only neutral-radical and radical-radical reactions were considered). The ionic and neutral reaction scheme here adopted is described in detail in [4], therefore here we will limit our description to the process followed to determine the kinetic constants of electronic reactions. The rate of the electronic reactions was assumed to be proportional to the product of the precursor and the average electron concentration (i.e., r = keNeCi). The kinetic constants of these processes were calculated, as described in [3], as a function of Te adopting experimental electron impact cross sections [5,6] and the Druyvestein electron distribution function [7]. The considered reactions together with the calculated kinetic constants in function of Te are reported in Table I. Table I. Electron impact reactions for a methane gas. (k = AT4eReaction el CH4 +e-- CJH4 +2e e2 CH 4 +e--+ CHt + H+2e e3 CH4 +e- CH++H 2 +2e e4 CH4 +e- CH+ +H 2 +H+2e e5 CH4 +ee C+ +2H 2 +2e e6 CH4 + e CH 3 + H + e e7 CH4 +e- CH 2 +H 2 +e e8 CH4 +e- CH+H 2 +H+e e9 CH4 +e->•C+2H2 +e elO H 2 +e->2H+e C2 H+ +H 2 +2e ell C2H6 +e-+
e12
C 2 H 6 +e->-C2 H 4 +IH2+e
e13
C 2 H 4 +e-*C2 H2
FUi ):
units are in mol, cm3 , s. E (eV)
18.2
n -1.31 -1.31 -1.31 -1.31 -1.31 -0.99
Log OA 18.8 18.7 18.0 17.7 17.2
39.0 39.0 39.0 39.0 39.0 25.0
17.4
-0.99
25.0
17.1 14.1 18.6
-0.99 -0.99 -1.98
25.0 25.0 29.0
18.7 19.1
-0.89 -1.20
32.2 36.6 26.7
17.8
-0.28
e14 C2 H 4 -+e-->C2 H 2-+H -2+e 2
18.2
-0.51
31.1
el5 C2 H 2+e->C 2H+•2e
17.5
-0.16
26.5
+2e
322
SURFACE KINETIC SCHEME DLC films are amorphous solids characterized by the presence of sp 3 and sp 2 bonded carbon and hydrogen whose relative amount greatly influences their properties. The relationship linking the H/C ratio to the sp 3/sp 2 ratio can be obtained by applying the random covalent theory [8]. For H/C ratio close to unity, the structure of the film is much closer to that of diamond, while for lower H/C ratio the structure is more graphite-like. In this framework, our surface kinetic model is mainly conceived to describe the incorporation of hydrogen and the film growth rate, being the sp 3/sp 2 ratio obtained as described in [8]. Eight different chemical species were assumed to be present on the growing surface: C:H, CH 3 *, H*, C:2H, C:H*, C:
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