4.3. The set of surface reactions

The loss or production of species on the reactor walls is captured by a set of heterogeneous surface reactions described in table 2. F atoms and SFx (x = 3,4,5) radicals adsorb on the reactor walls and cover a certain fraction of each elementary surface (reactions (1)–(4) in table 2, (S1)–(S4). F atoms also join the fluorination of the adsorbed SF3, SF4, and SF5 ((S5)–(S7)). We consider that only saturated species, i.e. only SF6, return to the gas phase by the fluorination of adsorbed SFx . Unsaturated SFx radicals (x = 4,5) return to the gas phase by recombination of SFx−1 radicals with adsorbed F ((S9)–(S11)). F atoms can also recombine with the adsorbed F and produce F2 (S8).

SF+x (x = 3,4,5) and F2+ ions, due to their low energy (plasma potential), are considered to be directly adsorbed on the surface ((S12)–(S15)); they also contribute to the sputtering of unsaturated SFx radicals and F atoms ((S16)–(S31)). Finally SFx radicals (x = 3,4,5) can be deposited on the fraction of the surface covered by SFx radicals ((S32)–(S40)) and form a fluoro-sulfur film.

The rate coefficient for the loss of a neutral species by a surface reaction depends on the probability of this reaction and the diffusion coefficient of the neutral species. For the simple case where the only surface reaction is the adsorption

2 − s(1 − θB) 4 LN

where s is the probability of the reaction, uth,B is the thermal velocity of B, A is the area of the reactor wall and V is the volume of the reactor. θB is the surface coverage by B, DB is the diffusion coefficient of species B and LN is the diffusion length for the neutral species.

The rate coefficient for the loss of a positive ion i by a surface reaction is

j

where Ub,i is the Bohm velocity of ion i, Aj is a surface area of the reactor. hj,i is the ratio of the sheath (formed before surface Aj ) edge density to the average bulk density of ion i. It depends on several quantities such as the ratio of negative ion to electron density, ratio of ion energy to electron energy, the dimensions of the reactor, and is calculated by the approximation described in the work of Lee and Lieberman [45].

The values for the sticking and recombination probabilities which reproduce the available experimental measurements are included in table 2. Note that the values shown in table 2 are not the quantities usually measured. The measurements refer to the effective sticking coefficient of the species (see section 5.3). The probabilities of the surface reactions in table 2 express the fundamental interaction between the reactant species, while the effective sticking coefficients stand