Electrostatic actuation in electroformed Ni microstructures

Electrostatic comb drive actuation is by far the simplest method of achieving actuation in planar resonators such as gyroscopes and accelerometers. Nickel electroplating of patterned photoresist layers provides a cheap and low temperature process when compared with silicon based alternatives. However UV patterning of thick photoresists can lead to devices with relatively low aspect ratios. Furthermore, the electroformed comb drives produced are trapezoidal in cross-section rather than rectangular or square, and this leads to unwanted out-of-plane components in the actuation force. This paper reports on the performance of trapezoidal comb drives in electrostatic actuation. A combination of conformal mapping and elementary field theory is used to calculate the out-of-plane forces in a sample cross-section, and the results are compared with finite-element calculations