Electrostatic actuators with intrinsic stress gradient.

Electrostatic actuators based on microelectromechanical systems ͑MEMS͒ have many attractive features for use as variable capacitors in high-frequency applications. The devices consist of two electrodes, one fixed and the other movable. In this study, a curved, cantilever beam was used as the movable electrode. A novel process has been developed for fabricating an all-gold, curved beam. The cantilever beams were curved due to an intrinsic stress gradient in the metal. Electroplating and conventional lithography were used to metallize the cantilever beam electrodes. The internal stress gradient in the gold was obtained by changing the electroplating conditions during fabrication. Stiction during release and operation of the variable capacitors was alleviated by treating the gold with an alkane thiol self-assembled monolayer. The intrinsic stress gradient and the stress-induced bending moment were calculated using a generalized model for the stress gradient in the films. Compared to bimetallic, cantilever beams, the curvature of the all-gold beam was found to be independent of temperature. This implies that the operation of the single-metal variable capacitor will be more reproducible and stable with temperature than a comparable bimetallic device. Electrostatically actuated beams are one of the fundamental building blocks in microelectromechanical systems ͑MEMS͒ and find applications in a variety of fields. They are used in a number of areas such as communications, sensing, optics, microfluidics, and measurement of materials properties. 1-4 The principle of electrostatic actuation is used in the fabrication of optical switches for optical networking circuits. 5 The electrostatically actuated rf capacitors provide greater isolation and more linear response compared to solid-state varactor diodes. The MEMS rf switches and variable capacitors also have higher quality factors and provide higher on/off capacitance ratio compared to traditional p-n junction varactors. A MEMS variable capacitor with a movable electrode and a fixed electrode is studied here. A curved cantilever beam design is considered for the movable electrode. A schematic diagram of the electrostatic actuator is shown in During the operation of bimetallic actuators, temperature has to be controlled precisely, as any variation in temperature leads to a change in the deflection of the actuator, which in turn changes the capacitance of the device. In addition, the mechanical and electrical properties, such as resistive loss and the electrical conductivity of the movable electrode, can be compromised due to the presence of a different material in the bimetallic film stack. The initial curvature of the movable electrode can also be obtained by using a single material, stress gradient approach. Deposited films with intrinsic residual stresses have been used previously to create nonplanar beams. 2,4 Polysilicon beams using the Multi-User MEMS Processes ͑MUMPs͒ have been reported to have residual compressive stress and stress gradients leading to deflection of the beam. 9 The single material, stress gradient approach for fabricating the movable electrode has considerable advantages over the conventional bimetallic film approach. Precisely controlled stress gradients in the films can be used to achieve a desired level of deflection in the devices. The deflection of the movable electrode deposited with an intrinsic stress gradient is expected to be largely independent of time and temperature. The mechanical and electrical properties of the material are not compromised, as this approach uses a single material. Furthermore, the material issues such as intermetallic reaction between the two materials are also avoided. The long-term reliability and reproducibility of the device could be greatly improved using this approach. The goal of this study is to select a materials set, which is highly reproducible and stable, and to develop a simple process technique to obtain a controllable stress gradient in the movable electrode. Gold has been considered as the material for the movable electrode due to its high electrical conductivity, low loss, ease of deposition, and excellent flexibility. The fixed electrode was made of aluminum. The deposition conditions during metallization were varied to investigate the effect on deflection of the movable electrode. The devices were then subjected to time and temperature aging. Finally, using the generalized model for the stress gradient in the films, the intrinsic stress gradient and the stress-induced bending moment were calculated