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A capacitive accelerometer detects acceleration through a change in capacitance caused by motion of a suspended proof mass. In the ADXL335, a micromachined polysilicon structure and differential capacitors form the mechanical sensor and its electrical readout.^[ref]

For an ideal pair of parallel plates with gaps $g-x$ and $g+x$, $C_{+}-C_{-}\approx 2\epsilon Ax/g^2$ when $|x|\ll g$. Combining this with $x\approx -ma/k$ gives an approximately linear acceleration signal. The approximation requires small displacement and operation sufficiently below mechanical resonance. A differential measurement identifies the direction of displacement and rejects some common changes affecting both capacitors similarly.

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 microelectromechanical system combines small mechanical structures with electrical sensing or actuation. Fabrication methods related to semiconductor device fabrication can create suspended beams, membranes and movable masses together with their electrodes. Sensors and actuators can consequently be integrated into compact devices.

A capacitive accelerometer illustrates the mechanism. A suspended proof mass moves relative to its support under acceleration. In a simple spring model, a slowly varying support acceleration $a$ produces a relative displacement $x\approx -ma/k$, where $m$ is the mass and $k$ is the spring constant. Measuring a change of capacitance then measures that displacement. The ADXL335 uses suspended polysilicon structures and a differential capacitor to turn motion into an electrical signal.^[ref]

For an idealized pair of parallel-plate capacitors with equal area $A$ and gaps $g\mp x$, the capacitance difference isThis model explains why a small mechanical displacement can produce a signed electrical measurement; it is not a detailed model of every commercial MEMS geometry.

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.

A temporary material supporting another structure during fabrication, then removed selectively. In an oxide-supported polysilicon beam, selective oxide removal makes an air gap while protected substrate contacts retain anchors. Removal must reach beneath the structure without destroying it; a purely vertical etch may not release a lateral overhang.