Kinetic study of low energy argon ion-enhanced plasma etching of polysilicon with atomic/molecular chlorine

Surface kinetics of ion-enhanced chlorine plasma etching in the low ion energy regime was studied by utilizing Ar+ for ion bombardment and Cl and Cl2 as reactants. The argon ion and chlorine atom (molecular) fluxes were controlled independently over more than an order of magnitude and at flux levels within an order of magnitude of that typically used in high density plasma processes. The ion-enhanced etching yield was characterized as a function of Ar ion energy, ion flux, neutral-to-ion flux ratio, and the ion incident angle. Possible reaction pathways are proposed and reduced into a two-parameter model which is useable in a profile simulator. The etching yield increases with the increase of flux ratio but gradually saturates at higher flux ratios as the ion flux limits the etching yield. The ion energy dependence was found to scale linearly with (Eion1/2−Eth1/2), where the threshold energy Eth is found to be 16 eV. The etching yield of Cl is found to be similar to that of Cl2 at flux ratios below 10, but 4–5 times higher than that of Cl2 at higher flux ratios. The sticking coefficient of Cl2 is similar to that of Cl for smaller neutral-to-ion flux ratios where the ion bombardment produces highly reactive surface silicon sites for both atomic and molecular chlorine. At high neutral-to-ion flux ratios, however, the dissociative adsorption of molecular chlorine is slower than the adsorption of atomic chlorine on the highly chlorinated surfaces. The angular dependence of ion-enhanced etching yield was also measured. The etching yield was reduced by approximately 35% when an ion impingement angle was changed from normal impingement to 60° off-normal.