 Solution

A compatible, reaction-limited chemical vapor deposition process can give nearly conformal coverage of tops, sidewalls and floor. Gas-phase precursors reach recessed surfaces and react there; the coating is not restricted to the line of sight of a solid source. The result is a continuous film over the resist pattern, which is unfavorable for ordinary lift-off process.

If each wall receives the same $0.20\mu \mathrm{m}$ coating as a horizontal surface, the nominal $0.50\mu \mathrm{m}$ gap narrows toThat numerical result is the conformal idealization. Precursor depletion or rapid surface reaction can instead give poorer bottom coverage or premature closure near the mouth. Actual silver chemistry and deposition temperature must also be compatible with the resist; a process that decomposes the resist does not preserve the illustrated mask.