CMOS–MEMS Lateral Electrothermal Actuators

In this paper, a type of lateral electrothermal (ET) actuator fabricated with post-CMOS micromachining is presented. The actuator is a beam with a multimorph structure, composed of CMOS dielectric and metal interconnect. Following structural release, the actuators demonstrate self-assembly under the moments arising from residual stress. Actuation is achieved through the imbalanced thermal expansion of internal interconnect members, whose relative positions and widths determine the magnitude and direction of actuation. Joule heating in discrete polysilicon resistors is used to convert energy from the electrical to thermal domain. For a 1.3-mum-wide 100-mum-long 4.2-mum-thick actuator composed of two driving metal layers, thermal sensitivities up to 18 nm/K are demonstrated with a force of 0.27 muN, given a 112-K temperature change. The analytic model and finite-element-analysis simulation output for thermal sensitivity agree with experiment to within 6%. Increasing thermal isolation is shown to give a diminishing return on thermal sensitivity and to reduce the thermal cutoff frequency of an actuator from 800 to 150 Hz. A figure of merit called the efficiency-volume ratio is presented and used to compare this paper with several actuators taken from the literature. Lateral ET multimorph actuators are shown to provide an advantage in applications where area is constrained and the load is small.