MEMS sensor structure and microfabrication process therefor

DETAILED DESCRIPTION The present invention provides a micro-electro-mechanical sensor structure with an improved design comprising rigid interdigitated projections forming capacitive plate elements and, in a preferred embodiment, flexible projections forming a rotationally compliant suspension.
According to the invention, the micro-electro-mechanical structure basically comprises a semi-conductor layer which is micromachined to define a proof mass suspended relative to a support substrate by one or more flexible suspension projections extending from the proof mass to a substrate-based support area.
Between these suspension projections and also extending outwardly from the proof mass are sets of additional rigid, spaced apart projections which move with the proof mass according to a compliance mode established by the suspension elements, e.
g.
, at right angles to the longitudinal axes of the finger-like projections.
Interdigitated with such projections are complemental projections extending from the support area toward the proof mass and defining, in combination with the rigid body projections, narrow sensor gaps of uniform width and larger, parasitic capacitive gaps.
The sensor gaps are formed to exhibit essentially constant gap widths such that the etch process is easily geared to their formation with no loss of accuracy due to different etch rates in other areas of the film.
In the illustrative embodiment, the proof mass is generally circular and the suspension elements and interdigitated capacitance elements are radially arranged.
The compliance mode in this embodiment is circular or rotary.
However, linear devices using the principles hereafter explained are readily designed.
The present invention further provides an improved process for fabricating the micro-electro-mechanical structure with its improved design for opposing, interdigitated projections consistent with general bond/etch-back methods of fabrication.
The process basically includes the steps of providing a first substrate, etching a cavity within the first substrate, and forming an isolation layer on the first substrate