The passivation step used in the “Bosch” process (alternating etching and deposition steps) to perform deep anisotropic silicon etching has been examined in detail. The effect of pressure, inductively coupled plasma power, temperature, flow rate, and bias power on both deposition rate and film composition has been explored over a relatively wide range. Deposition rate was found to vary significantly as a function of temperature, power, and pressure. In contrast, only two film composition regimes were observed: high fluorine-to-carbon ratio (F:C) films (∼1.6) at low pressure/high power versus low F:C films (∼1.2) at high pressure/low power. Optical emission spectroscopy of the deposition plasmas also show only two regimes: C2, C3, and F emission dominated (high F:C films) and CF2 emission dominated (low F:C films). A two-step deposition mechanism is assumed: carbon deposition followed by fluorination. Low F concentration and deposition from large fluorine-deficient CxFy species in the CF2-rich plasmas result in the low F:C ratio films. Films deposited during an actual Bosch cycle generally mirror these bulk films, with slight differences. Analysis of etch:deposition rate ratios as a function of film F:C ratio indicates that, for the conditions studied here, a F:C ratio of 1.45 is optimal for Bosch processing (i.e., has the lowest etch:deposition rate ratio). Further analysis is needed to determine the effect of passivant F:C ratio on feature profiles.