Abstract: A method for plasma etching substrates having high open area patterns is described. The method is useful in microelectrical mechanical system (MEMS) applications, and in the fabrication of integrated circuits and other electronic devices. The method can be used to etch strict profile control trenches with 89° +/−1° sidewalls on silicon substrates with high open area patterns such as patterns between about 50% and about 90%. The novel method plasma etches high open area substrates using a plasma formed from a gaseous mixture that includes an oxygen source gas, a fluorine source gas and a fluorocarbon source gas. In an alternative embodiment, the fluorocarbon source gas is a passivation gas. In another alternative embodiment, the fluorocarbon source gas consists essentially of a fluorocarbon having fluorine and carbon in a 2:1 ratio. In another particular embodiment, the oxygen source gas is O2, the fluorine source gas is SF6 and the fluorocarbon source gas is C4F8.