High energy backscattered electron signals have been measured from 1-, 3-, and 10-μm-wide gold alignment marks on silicon and gallium arsenide substrates as a function of electron-beam energy (5–30 keV) and gold film thicknesses (650–10 000 Å) using an annular silicon-diode detector. Thin films of both silicon dioxide (3500 Å) and polymethyl methacrylate (5200 Å) on gold alignment marks (2600 Å) on silicon reduced the original signal contrast at 30-keV incident-beam energy by only 16% (10% and 6%, respectively) and degraded the original edge acuity by only about a factor of two. Signal contrast maxima for gold on silicon and gold on gallium arsenide were found to be 1.64 and 0.86, respectively, while silicon and silicon dioxide steps (∠4000 Å) produced no more than 0.08 and 0.04 contrast, respectively. The gold on silicon results are presented with the full realization of the general processing incompatibility of gold on silicon devices and circuits at high temperatures. These results are applicable to other high-atomic-number materials which can be optimized for a particular electron-beam lithography process. The advantage of efficient high-energy backscattered electron detection and high-atomic-number alignment marks to produce high-contrast video signals which are not significantly degraded by the addition of low-atomic-number thin films (e.g., SiO2, resists) will become increasingly important when very accurate alignment is required (±0.05 μm).
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