MEMS Material Microstructure and Elastic Property Modeling
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191 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
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Desired Deflection
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Uncertainty in load
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Figure 1: FEM predicted load vs. deflection response for a LIGA nickel MEMS component. The behavior depends strongly on Young's modulus. For many LIGA nickel MEMS structural components, such as springs, the component dimensions are on order of 20 gim while the grains have been observed to be columnar having diameters ranging from 51gm to 25 gim [7,8]. Consequently, the authors propose that LIGA nickel MEMS materials should be treated as "meso-scale" materials. The authors consider "1meso-scale" materials as materials having a limited number of grains within the volume of a component. Other MEMS materials, such as polysilicon, with grain sizes on the order of 0.25 jtm and spring widths on the order of 4jm may also be treated as "meso-scale" materials. The efforts presented establish a framework to develop a quantitative relationship between microstructure and properties based on finite element modeling (FEM) treating LIGA nickel MEMS materials as "meso-scale" materials. The approach combines microsample tensile testing methods, microstructural characterization techniques and finite element modeling. This approach is intended to develop the elastic property - microstructure relationship in electroplated LIGA nickel structures. The initial focus is on the elastic properties since MEMS devices are designed to operate in the elastic region. This approach is not specific to LIGA nickel structures and may be applied to structures made from other MEMS materials.
EXPERIMENTAL PROCEDURES The LIGA fabrication process was used to produce the microsamples and specimens studied. The LIGA fabrication process is an x-ray lithography and electroplating process. A description of the LIGA process has been presented previously [12]. The mechanical properties as reported by Xie, et al. [9] were measured using a tensile test method developed by Sharpe and co-workers [8]. Modeling of the tensile microsample used by Sharpe and Xie will provide validation for the model. Scanning electron microscopy/orientation image microscopy techniques were used to measure grain size and to provide local grain orientation (texture) information. Orientation image microscopy techniques utilize information obtained from electron back-scattered Kikuchi patterns to determine grain orientation and grain size [13]. Microstructural Characterization A cross-section of the gage section of an as-fabricated LIGA nickel tensile specimen is shown in Figure 2. From this optical micrograph it is observed that the microstructure is columnar. Additional results from optical microscopy studies on LIGA nickel specimens is presented by Xie, et al. [9].
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Figure 2: Photo of the gage section of a LIGA nickel tensile microsample cross-section. Scanning Electron Micros
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