Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 1 / Section A(ii) / 4 / i / a

Expect appreciable but nonuniform sidewall coverage, rounded upper edges and possible narrowing near the opening mouth. In sputtering, gas collisions and the extended source distribute arriving material over a broader range of directions than simple high-vacuum evaporation. At the specified pressure, the extent of that redistribution also depends on source-to-substrate distance and particle energies; pressure alone does not determine a unique sidewall thickness.

The top surfaces receive the nominal film. Oblique rays coat accessible walls, while the tall opening shadows some directions from its lower portions. Growth at the mouth can further shadow the floor or form an overhang. The sidewall film is therefore neither guaranteed zero nor guaranteed as thick as the horizontal film. This step coverage is generally unfavorable for clean lift-off process if it connects the film on the resist to the film on the substrate.

Normal evaporative deposition gives approximately horizontal films on the resist tops and exposed substrate, with little coating of vertical walls. A small distant electron-beam-heated source in high vacuum gives nearly line-of-sight arrival. Near-normal rays reach the opening floor, while vertical walls present almost no projected area.

The floor film and top film can remain disconnected, which helps a later lift-off process. A finite source size, tilted incidence, scattering or sample rotation changes that idealization and may coat one wall or shadow part of the floor. The stated $200\mathrm{nm}$ thickness is well below the $1\mu \mathrm{m}$ resist height, but this alone does not guarantee a clean break at every real edge.

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.