Two modes of surface roughening during plasma etching of silicon: Role of ionized etch products

Abstract

Atomic- or nanometer-scale surface roughening has been investigated during Si etching in inductively coupled Cl{2} plasmas, as a function of rf bias power or ion incident energy E{i} , by varying feed gas flow rate, wafer stage temperature, and etching time. The experiments revealed two modes of surface roughening which occur depending on E{i} : one is the roughening mode at low E{i} < 200–300 eV, where the root-mean-square (rms) roughness of etched surfaces increases with increasing E{i} , exhibiting an almost linear increase with time during etching (t < 20 min). The other is the smoothing mode at higher E{i} , where the rms surface roughness decreases substantially with E{i} down to a low level < 0.4 nm, exhibiting a quasi-steady state after some increase at the initial stage (t < 1 min). Correspondingly, two different behaviors depending on E{i} were also observed in the etchrate versus √E{i} curve, and in the evolution of the power spectral density distribution of surfaces.Such changes from the roughening to smoothing modes with increasing E{i} were found to correspond to changes in the predominant ion flux from feed gas ions Cl{x}[+] to ionized etch products SiCl{x}[+]caused by the increased etch rates at increased E{i} , in view of the results of several plasma diagnostics. Possible mechanisms for the formation and evolution of surface roughness during plasma etching are discussed with the help of Monte Carlo simulations of the surface feature evolution and classical molecular dynamics simulations of etch fundamentals, including stochastic roughening and effects of ion reflection and etch inhibitors.