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Institutional fabrication tutorial. The surface micromachining section describes sacrificial support, oxide/polysilicon release, HF selectivity and capillary stiction. No separate publication date is established.

A sacrificial layer temporarily supports the intended moving structure and determines its clearance above the substrate. One workable surface micromachining sequence deposits silicon dioxide on silicon, opens anchor windows in that oxide, deposits a structural polysilicon film and patterns the beam and release openings. The structural film contacts the substrate at the anchors. Selective removal of the oxide beneath the beam leaves the beam suspended while retaining its attachment. An access opening or exposed perimeter is essential; an etchant cannot instantly reach sealed material.

The release needs substantial lateral removal, so an isotropic etch with high etch selectivity is useful. Hydrofluoric-acid oxide release is compatible with a suitable polysilicon structure; a directional reactive-ion etching step alone would mainly clear exposed vertical paths and leave material beneath the beam. Choose the structural material, anchor design, sacrificial material and etchant together, and allow for transport-limited undercut and structural residual stress. Wet release followed by ordinary evaporation can pull a compliant structure onto the substrate through capillary force and cause stiction. critical point drying or a compatible vapor-phase release etch avoids the receding liquid interface. Neither method removes the need to check residues, selectivity and anchor survival. Pattern the support and structure first, then selectively undercut the support while preserving the anchors.^[ref]

Polycrystalline silicon, containing many crystalline grains. Deposited polysilicon can form structural layers in surface micromachining; grain structure, doping and residual stress influence its properties.

Unwanted sticking that prevents intended relative motion. In surface micromachining, a receding liquid meniscus can pull a compliant released beam onto the substrate; adhesion can then keep it there. Avoiding a liquid-vapor drying interface addresses one route to stiction, but does not remove contact adhesion during use.