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An inductively coupled plasma receives power through electromagnetic induction. An alternating coil electric current produces a changing magnetic field, which induces an electric field in the gas. Electrons accelerated by that field transfer energy through collisions, sustaining ionization and driving atomic transitions.

A typical analytical source surrounds a gas-carrying dielectric torch with a radiofrequency coil. Excited atoms emit spectral lines useful for atomic spectroscopy, while extracted ions can feed mass spectrometry. The coil supplies energy without requiring it to touch the plasma. Coupling, gas composition and particle losses determine the discharge response. ^[ref]

Use a patterned resist or hard mask and anisotropic chlorine-based reactive-ion etching, for example an appropriate $\mathrm{C}{\mathrm{l}}_2/BC{\mathrm{l}}_3$ inductively coupled plasma process for GaAs/AlGaAs. Ion-assisted removal gives a well-defined mesa with less lateral undercut than an isotropic wet etch. Control etch time or endpoint to obtain the specified approximately $1.2\mu \mathrm{m}$ depth.

The mask must survive the etch and protect the active aperture. Control ion damage and sidewall roughness, remove residues and passivate the sidewall where required. Compatibility with the metal stack determines whether the mesa is etched before or after front-contact deposition.