We present two developments in low energy ion beam etching. First, we have extended the usable ion energy range for broad beam ion etching down to tens of electron volts by using a single extraction grid of very fine aperture size. High ion current densities (up to 1.0 mA/cm2) are achieved below 100 eV ion energy while maintaining a collimated beam with low energy spread (about 10 eV). Second, we have applied this capability to reactive ion beam etching (RIBE) of Si and and we introduce a quantitative parameter which characterizes the effectiveness of ion bombardment in enhancing chemical etching. This parameter is obtained from etch rate measurements using Ar and gases over the ion energy range 20–1500 eV. Step edge profiles are also examined.