Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2020 / Paper 1 / Section A(ii) / 4

Use thermal evaporation or electron-beam evaporation: evaporated Au travels approximately ballistically to the Si wafer. For line-of-sight deposition, the gas mean free pathshould greatly exceed the source-wafer distance $L$. Increasing the chamber's useful transport length therefore requires a proportionately lower pressure to retain the same $\ell /L$, for fixed gas and temperature. There is no universal pressure requirement determined by chamber volume alone.

A distant compact source gives a more uniform angular flux across the wafer, but the deposition rate falls roughly as $L^{-2}$. For an ideal cosine-emitting point source and a parallel wafer, the relative thickness at lateral radius $r$ is approximately $[1+(r/L{)}^2{]}^{-2}$. Mitigate the trade-off with wafer rotation and planetary motion, multiple or extended sources, shaped masks and suitable source/wafer geometry. Rotation averages azimuthal variation; ordinary rotation alone does not remove a purely radial thickness gradient. A thin Ti or Cr adhesion layer may be needed for robust Au adhesion to oxidized Si.

The arrangement is diode sputter deposition: the Au target, a, is attached to the upper cathode assembly c; positive Ar ions bombard it and sputter Au onto the wafer. The lower wafer-holder electrode b is the anode in the simple two-electrode arrangement shown.

Unlike evaporation, sputtering needs enough inert gas to sustain a discharge. Over a low-pressure operating interval, raising Ar pressure can increase ionization and ion current and hence the sputtering rate at comparable applied voltage. This is not a universal monotonic law: at higher pressure, scattering and thermalization of sputtered Au reduce the fraction reaching the wafer; at fixed regulated power, target voltage and current also adjust. A pressure increase can initially increase the rate, but excessive pressure lowers transport efficiency.