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Joined 2026-09-21 Message
Incoming links: Photolithography
Use optical photolithography for the two masks and a resist profile compatible with the selected process. A controlled wet mesa etch or a calibrated reactive-ion etching process must pass completely through the p–n interface, with limited sidewall damage. Use profilometry or cross-section controls to verify depth and a test device to check leakage. For the front lift-off process, directional evaporation and an undercut resist profile discourage a continuous metal fence bridging the mask sidewall.
The cap-removal step requires a separately qualified selective etchant: its selectivity between gallium arsenide and the particular aluminium gallium arsenide alloy depends on alloy composition, chemistry and processing conditions. Establish etch rate and endpoint on a witness stack; a universal etch time cannot be inferred from a diagram without thicknesses. The front metal must resist that etchant. Back metallization and activation must establish an ohmic contact rather than an unintended rectifying junction. Verify specific contact resistance on test structures and avoid an anneal that degrades the p–n profile. The final dielectric can be deposited by a low-damage evaporation or other compatible thin-film method.
The 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.