Vertical electrostatic force in MEMS cantilever IR sensor

Abstract

A MEMS cantilever IR detector that repetitively lifts from the surface under the influence of a saw-tooth electrostatic force, where the contact duty cycle is a measure of the absorbed IR radiation, is analyzed. The design is comprised of three parallel conducting plates. Fixed buried and surface plates are held at opposite potential. A moveable cantilever is biased the same as the surface plate. Calculations based on energy methods with position-dependent capacity and electrostatic induction coefficients demonstrate the upward sign of the force on the cantilever and determine the force magnitude. 2D finite element method calculations of the local fields confirm the sign of the force and determine its distribution across the cantilever. The upward force is maximized when the surface plate is slightly larger than the other two. The electrostatic repulsion is compared with Casimir sticking force to determine the maximum useful contact area. MEMS devices were fabricated and the vertical displacement of the cantilever was observed in a number of experiments. The approach may be applied also to MEMS actuators and micromirrors.

Micro electro mechanical cantilever with electrostatically controlled tip contact

“…Each conductor feels only the negative pressure due to the fields at its own surface. 15 Integration of this pressure over the surface gives the net force 18 and confirms the sign found here.) Fig.…”

“…An SEM image of one of these large cantilevers confirms this interpretation of the initial paddle deformation in null position. 18 When bias is applied, the fringes from the middle of the paddle are observed to shift toward the tip, increasing their spacing, while no change in the interference pattern is observed near the base of the paddle or arms. This indicates a lifting of the tip and flattening of the paddle with bias.…”

“…Optimization by 2 dimensional FEM calculations is presented in Ref. 18, which also presents visualization and discussion of the fringing fields.…”

Thermomechancial characterization in a radiant energy imager using null switching

“…The negative electrostatic pressure responsible for lifting the cantilever is proportional to the square of the electric field at the cantilever surface, and electric field should be proportional to the applied bias voltage V B . According to [3], the electrostatic lifting force F ES is inversely proportional to cantilever height z, and we find that FES can be approximated by the function…”

“…An upward electrostatic force F ES appears when the device is biased as shown 2,3 . This is opposed by an elastic restoring force F E and a sticking force F C such as Casimir force 3,4 . The latter force will be ignored here.…”