8.3 Gas-phase etching
8.3.1 Fluorine-based etching of silicon
Given the noxious chemistry needed to etch silicon with a liquid, it is perhaps surprising that a gas can do the job at all. However, both xenon fluoride (XeF2) and chlorine trifluoride (ClF3) gases have been used successfully for just this purpose. Each acts as a source of fluorine atoms, which are just barely bound together into molecules and are easily rearranged around silicon atoms with which they form strong bonds, turning them into inert SiF4 gas. These highly corrosive gases can achieve very similar results to liquid etchants (etching at a few μm min−1) without the problems associated with drying.
XeF2 is a rare example of a compound of a noble gas. That it exists is a measure of the extreme reactivity of the fluorine atom.
Neither liquid nor gaseous etchants give much control over the etch profile. The etched holes grow sideways, undercut the mask, and before long join up with their neighbours. This is not usually the desired result. To pack devices densely together, we need to etch straight downwards.