Monotonic and fatigue testing of spring-bridged freestanding microbeams application for MEMS

Abstract

An electroplating spring-bridge micro-tensile specimen is fabricated to carry out a series of monotonic tensile testing on it. Freestanding thin films were loaded by performing monotonic loading/unloading and tension-tension fatigue experiments. Loading was applied using a piezoelectric actuator with 0.1 μm resolution connected through pin hole into the test chip specimen. Loads were measured by connected a capacitor load cell with a resolution of less than 0.1 mN. We found the modulus of gold, copper and tantalum nitride thin films with thickness of 200~800nm at ambient temperature. Displacement controlled tension-tension fatigue experiments have also been performed and a trend of decreasing cycles to failure with increasing displacement amplitude and increasing mean displacement has been noted. 1. INTRODUCTION Microelectromechanical systems (MEMS) technologies are developing rapidly with increasing study of the design, fabrication and commercialization of microscale systems and devices. Accurate knowledge on the mechanical behaviors of thin film materials used for MEMS is important for successful design and development of MEMS. One of the MEMS application are primary used in RF switch, which has a potential use in tunable transmission lines for cellular telephone applications, or phase array radars. Advantages of RF switches are excellent linearity up to high switching speeds and low power consumption. In an RF switch, schematically depicted in Figure 1, an electrostatic force applied between the substrate and a bridge structure pulls the bridge down. This causes a change in capacitance by a factor of 100 or more. The state in which the electrostatic force is applied corresponds to a closed circuit (the " on " position) and the state with no electrostatic force applied corresponds to an open circuit, or the " off " position. During operation of an RF switch, the bridge structures deflected as far as the gap permits. Since these deflections are anticipated to reach kHz frequencies, very high cycle numbers can be attained over short time periods. Fig. 1. The Schematic of RF switches operation. Thus, understanding of fundamental observed failure mode and mechanical response respected to external loads plays an important role in products design and lifetime prediction and needs to be established. Here an electroplating micro-tensile specimen integrates pin-pin align holes, misalignment compensate spring, load sensor beam and freestanding thin film is demonstrated and fabricated. The specimen is fit into a specially designed micro-mechanical apparatus to carry out a series of monotonic tensile testing as well as displacement controlled tension-tension …

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