Phenomenological Model of Non-Evaporated Getter for Micro-electromechanical Systems (MEMS) Applications
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Phenomenological Model of Non-Evaporated Getter for Micro-electromechanical Systems (MEMS) Applications Caroline A. Kondoleon and Thomas F. Marinis Electronics Packaging and Prototyping Division, The Charles Stark Draper Laboratory, Inc., Cambridge, MA 02139 ABSTRACT There has been increasing interest in the development and use of micro-electromechanical systems (MEMS) for various applications. Some MEM devices such as gyroscopes, accelerometers and bolometers, must be sealed under vacuum, at pressures below 10 millitorr, for efficient operation. A gas absorbing material (getter) is placed in the packages of these devices, to help maintain vacuum levels over service lives of many years. Heating under vacuum, just prior to sealing the package, activates getter, of this type. This study was undertaken to develop a model that could be used to estimate the quantity of getter needed as well as optimize the activation process subject to process constraints on time and temperature. The material studied was a titanium and zirconium-based alloy (7:3 by weight) nonevaporable getter in the form of strips produced by SAES Getters. The zirconium alloy consisted of zirconium (70.0%), vanadium (24.6%) and iron (5.4%) by weight. The getter was analyzed under different ambient conditions of temperature, time and atmospheric pressure. Auger electron spectroscopy (AES) depth profiling was used to analyze the diffusion depth of the contaminant gases absorbed by the getter material under each condition. The data acquired from the depth profiles were fit to a simple diffusion model. This model is currently being validated, by activating the getter material under various ambient conditions, and measuring pressures and gas compositions inside packages using a residual gas analyzer (RGA). The utility of this model for optimization of getter activation and estimating package vacuum levels over time will be discussed. INTRODUCTION Micromechanical gyroscopes and oscillating accelerometers are sealed in vacuum to eliminate air damping energy losses [1]. A critical component of the vacuum package is a nonevaporated, thick-film getter, which serves two purposes. First, it absorbs gases that are released by dissolution of metallization within the package, which occurs when a lid is brazed on to seal it under vacuum. Even when the dissolved gas content of this metallization is only 1 part per million, sufficient gas is released to degrade the package vacuum. The second function of the getter is to absorb gases that diffuse out of the package walls into the vacuum cavity [2]. Most applications require that the package pressure be held below a few millitorr over a 20-year life. It is unlikely that this requirement could be met without a functioning getter. Non-evaporated getter is a multi-component material that interacts with its surrounding atmosphere in a complex and dynamic way. When it is placed in a vacuum and heated, gas molecules that are adsorbed on the surface diffuse into the interior to expose reactive metal, which is the basis of the act
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