Solution
ID: past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2021/paper-1/9/i/b/solution
Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge 2021 Paper 1 9 i b Solution by 
Codex 0 2026-10-20
Codex 0 2026-10-20The contact dimensions are readily accessible to UV photolithography followed by directional evaporation and lift-off. A possible sequence is:
- Clean the semiconductor contact surface and coat it with a positive-tone lift-off resist stack. A bilayer giving a small undercut is useful. Soft-bake and align the mask to the intended mesa.
- Use a transparent annular opening in an otherwise opaque mask to expose the contact ring, leaving the central optical aperture covered by resist. Develop away the exposed positive resist. Contact or proximity exposure is adequate for the micrometre-scale ring; projection exposure also works.
- Evaporate the gold contact layer directionally onto the wafer. Keep the resist appreciably thicker than the metal and preserve the undercut so that metal on top of the resist does not bridge continuously to the desired ring. Evaporation is more convenient for lift-off than a strongly conformal coating.
- Dissolve the remaining resist and lift away its overlying metal. The annulus on the exposed semiconductor remains, while the central emitting aperture is free of metal.
A real device must also have appropriate ohmic contact metallurgy. gold alone on arbitrary GaAs is not automatically an ohmic contact: surface preparation, heavy contact-layer doping and, where required, a suitable alloyed contact stack are needed. The schematic gold label specifies the visible electrode, not all such process details. Order the mesa etch and contact steps so that the etchant and any anneal do not damage an already formed contact.
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