An etch method includes etching a masked substrate to form a recess with a first sidewall in the substrate. A thin surface layer of the substrate on the first sidewall is then converted into a passivation layer. The masked substrate is etched again to deepen the recess in the substrate. A surface layer of the substrate on the second sidewall of the recess is then converted into a passivation layer. In one embodiment, upon removal of the passivation layers from both sidewalls, the first and second sidewalls of the high aspect ratio recess are aligned to within 10 Å of each other to provide a high aspect ratio recess having a vertical profile.