The etching of poly‐Si (n+and undoped) in a radio‐frequency‐biased electron cyclotron resonance plasma of HBr was studied. Etch rates of Si, oxide, and photoresist were obtained by ellipsometry as a function of the bias voltage and two substrate temperatures (15 and 50 °C) at 5 mTorr pressure. The etch rate of poly‐Si depends on the doping level, withn+Si etching faster than intrinsic Si. High selectivities of Si over both oxide and photoresist can be achieved at low bias voltages. Using angle‐resolved x‐ray photoelectron spectroscopy, it is shown that a carbon and bromine containing layer is deposited on the sidewall of the poly‐Si during the etching process when a photoresist patterned wafer is used. The thickness of the sidewall film decreases with increasing substrate temperature and increases at the bias voltage is raised. The thickness of this sidewall film influences the anisotropy of the process, with some undercutting occurring at high temperature and low bias voltage.