A quantitative study of dry etch behavior in deep silicon trenches in high density plasmas (electron cyclotron resonance, inductively coupled plasma) at low temperatures (160-210 K) is presented. The quantitative approach implies etch behavior being studied in relation to the relevant particle fluxes (atomic F and O and ions) as measured by in situ diagnostics, Two etch modes are observed. In one mode faceting shows up as due to crystallographic orientation preference, i.e., Si[111] being etched slower than Si[100]. In the other mode the normal anisotropic ion-induced behavior is observed. Controlled switch from one mode to the other is studied under influence of process parameters like pressure, ion energy, and substrate temperature. The second part of this study deals with aspect ratio dependent etching (ARDE). Both vertical and horizontal trenches have been taken into account as to distinguish between radical and ion-induced effects. The flux of radical species into the deep trench is governed by Knudsen transport, with a reaction probability of atomic fluorine of about 0.5. As a consequence depletion of the fluorine content at the bottom is the main reason for ARDE. With the bottleneck identified, the plasma process has been readily tuned to the aspect ratio independent etch regime. This regime coincides with the crystallographic preference mode where surface reaction kinetics form the rate limiting step. Detailed surface analysis studies by x-ray photoelectron spectroscopy, in situ ellipsometry, and transmission electron microscopy have been used Co characterize the surface reaction process.