@whitequark Gas injection system, as used for circuit edits.

You put a needle in your vacuum chamber connected to a metered source of CVD/etchant precursor gas that can be pneumatically inserted ~200um from your sample.

Then you open up the valve, a monolayer of the gas coats your sample, and the beam causes it to decompose and deposit material on / etch the surface. After the raster beam passes, the monolayer reforms and it repeats.