Chemical sputtering by H2+ and H3+ ions during silicon deposition

We investigated chemical sputtering of silicon films by Hy + ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H2 inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H2 inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3–0.4) ± 0.1 Si atom per bombarding Hy + ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions SF (SF(%) = [SiH4]/[H2]*100) ranging from SF = 0% to 20%. We experimentally observed that the SiHy + flux is not proportional to SF but decreasing from SF = 3.4% to 20%. This counterintuitive SiHy + flux trend was partly explained by an increasing chemical sputtering rate with decreasing SF and partly by the reaction between H3 + and SiH4 that forms SiH3 +.