Etching of photoresist in an O2plasma generated by an electron cyclotron resonance source (ECR) was investigated. The ECR source was a microwave multipolar reactor at 2.45 GHz, and the stage was connected to a rf power supply at 13.56 MHz. Effects of microwave power, rf power, ECR source to sample distance, and pressure on photoresist etch rate were characterized. It has been found that the photoresist etch rate increases with microwave and rf power, but decreases with source to sample distance. With microwave power at 1000 W and rf power at 300 W, smooth morphology and fast etch rate at 1.61 μm/min were obtained. Self‐induced dc bias voltage increases with rf power and source to sample distance, but decreases with microwave power. Etch rate uniformity better than 0.5% was obtained across 7.5 cm diam wafer even with a very close source to sample distance of 3 cm. Etch profile can be varied depending on the etch conditions. Vertical profile in polyimide has been obtained using a trilayer resist scheme.