Plasma-surface interactions of nanoporous silica during plasma-based pattern transfer using $C4F8$ and $C4F8∕Ar$ gas mixtures

We have investigated plasma surface interactions of nanoporous silica (NPS) films with porosities up to 50%, and $SiO2$ with $C4F8∕Ar$ discharges used for plasma etching. The pore size was about $2–3nm$ for all films. In highly polymerizing plasmas (e.g., pure $C4F8$ discharges), the porous structure of NPS material favors surface polymerization over etching and porosity-corrected etching rates (CER) were suppressed and lower than $SiO2$ etching rate for the same conditions. The etching rates of NPS were dramatically enhanced in ion rich discharges (e.g., $C4F8∕90%Ar$⁠) and the CER in this case is greater than the $SiO2$ etching rate. Both x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (static SIMS) show that fairly thick $(∼2–3nm)$ fluorocarbon layers exist on the NPS surface during $C4F8$ etching. This layer blocks the direct interaction of ions with the NPS surface and results in a low etching rate. For $C4F8∕90%Ar$ discharges, little fluorocarbon coverage is observed for NPS surfaces and the direct ion surface interaction is significantly enhanced, explaining the enhancement of CER. We can deduce from analysis of angular resolved XPS data that the surface of NPS materials and $SiO2$ remain smooth during $C4F8$ etching. For $C4F8∕90%Ar$ etching, the NPS surfaces became rough. The surface roughening is due to angle-dependent ion etching effects. These surface models were directly verified by the transmission electron microscopy. Depth profiling study of NPS partially etched using $C4F8$ or $C4F8∕90%Ar$ discharges using dynamic SIMS indicates that the plasma induced modification of NPS was enhanced significantly compared with $SiO2$ due to the porous structure, which allows the plasma attack of the subsurface region. The modified layer thickness is related to the overall porosity and dramatically increases for NPS with an overall porosity of 50%. The distinct etching behavior of high porosity NPS $(∼50%)$ in fluorocarbon-based discharges relative to NPS material with lower overall porosity is possibly due to interconnected pores, which allow plasma species to more easily penetrate into the subsurface region.