The characteristics of oxide and photoresist masked polysilicon trench etching has been studied by real‐time HeNe laser ellipsometry and quasiinsitux‐ray photoelectron spectroscopy (XPS). Poly‐Si films on SiO2‐covered Si (100) substrates were masked either with a 1‐μm‐thick photoresist or a 200‐nm‐thick oxide hard mask. The 200‐mm‐diam wafers were etched downstream in a helicon high density plasma source using a chlorine‐based gas chemistry. When using an oxide hard mask instead of a photoresist mask, the selectivity of polysilicon over oxide was improved by a factor greater than 3. A new approach to the surface characterization of semiconductor submicron structures by XPS is presented. Photoelectron signals originating from the gate oxide film and the underlying silicon substrate were measured in regular arrays of trenches. The ratio between the SiO2peak area of the gate oxide film and the Si 2ppeak area of the silicon substrate was correlated with the thickness of the SiO2film. The thickness determined was obtained by calibrating peak area ratios with oxide thickness measurements using spectroscopic ellipsometry. Consequently, the gate oxide thicknesses derived from the area ratios were calculated in patterned areas after etching of photoresist and oxide hard masked polysilicon features as a function of the aspect ratio of the features and mask coverage on the wafer. It was found that the gate oxide consumption is enhanced in high aspect ratio features masked with photoresist as well as in areas with high photoresist coverage; the carbon coverage on the gate oxide film was found to scale with the increased gate oxide etching in the small features. Similar effects were not observed with the oxide masked sample.