High aspect ratio etching of polyimide in an O2plasma generated by an electron cyclotron resonance (ECR) source was investigated. The dependence of etch rate, profile, and selectivity on microwave power, rf power, pressure, etch time, and gas composition were characterized. Etch rate was found to increase with microwave and rf power. Etch anisotropy increases with rf power but decreases with pressure and etch time. With 750 W microwave power, 300 W rf power, and 0.5 mTorr pressure, the polyimide etch rate was 0.91 μm/min and the anisotropy was 0.92. Etch selectivity between polyimide and the Ti mask was 3150:1 at 50 W and 536:1 at 300 W rf power. Compared to reactive ion etching, etching using an ECR source provides etch rate that is typically 10× faster and etch selectivity that is 4–6× higher. High aspect ratio (≳15:1) structures in polyimide that were 50 μm deep and 3 μm wide have been obtained using an O2plasma generated by an ECR source.