The dry development of Plasmask 301-U resist was investigated using O2 plasmas generated in an experimental electron cyclotron resonance reactor. An rf source operating at 13.5 MHz was used to independently bias the wafer holder. The dc component of the rf bias was used as a rough indication of the bombardment energy of ions incident on the wafer surface. The ECR plasma source was characterized using Langmuir probes and optical emission spectroscopy. Etch rates were measured as a function of reactor pressure, O2 flow rate, wafer bias, and wafer distance from the ECR source chamber. Etch rates correlated most closely with ion current density over the range of variables studied. Etched profiles generated at different wafer positions, rf biasing conditions, oxygen gas flow rates, and gas pressures were investigated using scanning electron microscopy. The anisotropy of etched profiles improved with increasing rf bias and decreasing pressure. At very low oxygen flow rates, mask formation was inhibited; at high rf biases, mask sputtering was enhanced. Development of high resolution anisotropic profiles depended on a fine balance between silylation temperature, wafer bias, and etch pressure. Anisotropic, well-defined profiles of 0.30 micrometers lines and spaces were obtained at 0.5 m Torr and an applied bias of -60 volts.