A Mechanical Approach to Overcome RF MEMS Switch Stiction Problem

摘要

MEMS switches provide high isolation when open and low insertion loss when closed. They also have the advantages of low power consumption. Therefore, RF MEMS switches are an attractive solution to switch antenna bands and transmit/receive switching for future multi-band, high bandwidth cell phones. However, Stiction is a major concern for resistive switches with metal-to-metal contact. An iterative-coupled electrostatic-structural analysis is utilized to evaluate the effect of design parameters on restoring force of MEMS switches. Parameters including metal thickness, dielectric thickness, beam-to-ground gap height, metal and dielectric width, and cantilever beam length can be evaluated. The electrostatic force is first calculated based on the electrical field component. A structural analysis is then performed to determine the cantilever beam deflection due to the electrostatic force. A unique integrated empirical-numerical method is used to qualitatively determine the Stiction force based on measured actuation voltages for real devices. The analysis can provide quick evaluation and screenings of proposed designs to determine if their actuation voltage falls in the acceptable range. Simulation prediction agrees very well with test measurements. Although increasing cantilever thickness and shortening cantilever length both increase restoring force, the actuation voltage will increase significantly as a result. The most favorable modification is to increase the electrode area. A short and wide structure with a large area can increase restoring force while maintaining low actuation voltage. Compared to similar bi-layer designs, sandwich designs can be actuated at further reduced voltages without changing the beam restoring force. In addition, the sandwich structure, being thermal-stress-balanced, is less sensitive to temperature excursion. With the properly selected design parameters, the new designs will be able to achieve the break away restoring force of the original design at much lower actuation voltages. Switches with good electrical as well as mechanical performances have been successfully fabricated.