Methods and apparatus for etching substrates such as silicon wafers are provided. In one specific approach, a surface of the substrate assembly is covered with a resist that is patterned to define features to be etched. In this approach, the surface is then exposed to a plasma in a plasma etcher so that surface areas not covered with the resist are etched, while the thickness of the resist increases or etches at a rate that is at least ten times slower than that of the exposed areas of the surface. This etching process can be followed with a conventional plasma etch. By combining the etching that increases the resist thickness with the conventional etching of resist in which the resist thins during etching, features having high aspect ratios can be etched.