Origin of plasma-induced surface roughening and ripple formation during plasma etching: The crucial role of ion reflection

摘要

Plasma-induced surface roughening and ripple formation has been studied based on Monte Carlo simulations of plasma-surface interactions and feature profile evolution during Si etching in Clbased plasmas, with emphasis being placed on the role and effects of ion reflection from microstructural feature surfaces on incidence. The simulation model included the effects of Cl+ ion reflection (and/or its penetration into substrates) through calculating the momentum and energy conservation in successive two-body elastic collisions with substrate Si atoms every ion incidence. The "reflection coefficient r(i)" was then further introduced in the model (0 = r(i) = 1), representing the fraction of ions incident on surfaces with the reflection/penetration calculation scheme turned on. The coefficient r(i) is, in a sense, a measure of the reflection probability for impacts of an ion species onto Si surfaces relative to that for Cl+ impacts. Simulations for ion incidence angles of.i = 0 degrees, 45 degrees, and 75 degrees onto substrate surfaces with incident energies in the range Ei = 20-500 eV showed that as r(i) is slightly decreased from unity, the roughness decreases substantially, and the ripple formation fades away: the roughness remains at the low level of stochastic roughening during etching for decreased r(i) = r(i)* approximate to 0.95-0.75 (the critical r(i)* tends to be lower at higher Ei and.i) with no ripple structures at off-normal.i. This elucidates that the ion reflection is indispensable in surface roughening and rippling during plasma etching, and their degree relies significantly on the reflectivity of ions. Simulations further showed that at intermediate off-normal.i = 45 degrees, the ripple wavelength increases significantly with decreasing r(i), while the increase in amplitude is relatively less significant; thus, sawtooth-like ripple profiles pronounced for r(i) = 1 tend to be collapsed with decreasing r(i). These effects of reduced ion reflection on plasma-induced surface roughening and ripple formation are discussed in terms of effectively enhanced smoothing due to neutral reactants, which competes with the roughening and rippling caused by ion bombardment. Published by AIP Publishing.