= Solution
Use \b[thermal evaporation] or electron-beam evaporation: evaporated Au travels approximately ballistically to the Si wafer. For <line-of-sight deposition>, the <gas> <mean free path>
$$
\ell=\frac{k_BT}{\sqrt2\pi d_g^2p}
$$
should 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.
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