The spatially resolved surface chemical composition or ‘‘chemical topography’’ of submicron features [polycrystalline silicon (poly‐Si) masked with photoresist (PR) lines] etched in high density, low pressure helical resonator Cl2/O2plasmas has been quantitatively determined using angle resolved x‐ray photoelectron spectroscopy (XPS). The chemical topography of plasma etched microelectronic materials is important in understanding how impinging ions and neutrals interact with surfaces to influence etched profiles. The spatial origin of XPS signals was determined from a combination of geometric shadowing of photoelectrons by adjacent features, electrostatic charging of insulating surfaces, XPS signal calibration versus take‐off angle, x‐ray attenuation, and geometric modeling. Equal line and space width (0.75–2.0 μm) features, unmasked poly‐Si, and unpatterned PR surfaces were examined following plasma etching and vacuum sample transfer. For pure Cl2plasmas, Cl surface concentration was found to be similar for horizontal and vertical surfaces of the poly‐Si and PR. Only a small amount of Si was found on the PR sidewall, and similarly, little C or O was observed on the side of the poly‐Si features, indicating that sidewall passivation is not occurring. O coverage on all surfaces increased with O2addition to the plasma. For Cl2/5% O2plasmas, a small amount of O was found on the poly‐Si trench bottom, and more (but still submonolayer) on the poly‐Si sidewall. Also, more Cl, O, and Si were found on the PR sidewall with 5% O2. For Cl2/10% O2plasmas, rough surfaces were observed by scanning electron microscopy (SEM). On poly‐Si trench bottoms, O coverage is comparable to Cl at roughly a monolayer. On poly‐Si sidewalls, O and Cl coverages are again comparable, but the O coverage is about double that found on the trench bottoms. The most dramatic effect by far at 10% added O2is the formation of a thick SiOxClylayer (wherex≊y≊1) on the side of the PR, detected by both XPS and SEM. The quantitative analysis method developed in this study is readily applicable to other etching gases and materials.