This article presents a study of the characteristics of radicals in high-aspect-ratio deep Si etching by continuous-type SF6/O-2 plasmas. A parametric study of etched depths clearly shows that the Si etch rates are dependent on concentrations of F atoms but independent of ion energy and substrate temperature. Results of Monte Carlo simulation based on a Knudsen transport model provide a remarkably good fit for experimental results on aspect-ratio-dependent etching. Comparison of the experimental data and results of simulation shows that the probability of a F atom reacting with the Si surface is 0.4-0.45 if the probability of loss at sidewall surfaces for F atoms is negligible. Results also indicate that the latter probability is, in fact, extremely small (<0.005). (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3466794]