The characteristics of poly‐Si trench etching in high density plasma processes was studied by x‐ray photoelectron spectroscopy. Poly‐Si films on SiO2‐covered Si(100) substrates were masked with a 200‐nm‐thick oxide hard mask. The 200 mm wafers were then etched downstream using a helicon high density plasma source and a chlorine‐based gas chemistry. After etching, samples were transferred under ultrahigh vacuum to a surface analysis chamber equipped with an x‐ray photoelectron spectrometer. Regular arrays of trenches were used to determine the photoelectron signals originating from the tops, sidewalls, and bottoms of the features. A thin oxide film was found on the sides of the oxide masked poly‐Si trenches. The origin of this film can be related to the sputtering and redeposition of oxide from the quartz tube of the helicon source located in the plasma generation region. A substantial amount of chlorine was present on the poly‐Si sidewall of the features, whereas less chlorine was found on the oxide surfaces. The poly‐Si sidewalls were covered by a small amount of oxygen.