After etching in a helical resonator HBr plasma, polysilicon (poly‐Si) micron‐size features masked with photoresist (PR) were investigated with angle‐resolved x‐ray photoelectron spectroscopy and scanning electron microscopy to spatially resolve surface chemical compositions. The poly‐Si sidewalls and trench bottoms, and the PR sidewalls are covered with one to two monolayers of SiBr and SiBr2. No SiBrxspecies are present on top of the PR surfaces. No carbon or oxygen were detected on the poly‐Si sidewalls, suggesting that line‐of‐sight deposition of carbon from sputter erosion of the corners and sides of the PR does not occur. Because Br atoms react very slowly with poly‐Si, relative to the ion‐assisted etch rates, anisotropic etching does not require sidewall passivation from products of PR erosion. A SiBrxlayer does form on the PR sidewall, however, and could play a role in suppressing lateral erosion of the mask and improving profile control with HBr versus Cl2plasma etching.