Residual Stress of Focused Ion Beam-Exposed Polycrystalline Silicon
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Residual Stress of Focused Ion Beam-Exposed Polycrystalline Silicon Kim M. Archuleta1,2, David P. Adams1, Michael J. Vasile1, and Julia E. Fulghum2 1 Sandia National Laboratories, Albuquerque, NM, 87185 2 University of New Mexico, Albuquerque, NM, 87106 ABSTRACT Medium energy (30 keV) focused gallium ion beam exposure of silicon results in a compressive inplane stress with a magnitude as large as 0.4 GPa. Experiments involve uniform irradiation of thin polysilicon microcantilevers (200 µm length) over a range of dose from 1 x 1016 to 2 x 1018 ions/cm2. The radii of curvature of microcantilevers are measured using white light interferometry before and after ion beam exposure. The residual stress is determined from these radii and other measured properties using Stoney’s equation. The large residual stress is attributed to ion beam damage, microstructural changes and implantation.
INTRODUCTION Focused ion beam (FIB) systems are increasingly utilized to fabricate tools, instruments, sensors and devices on the micrometer and nanometer scales. It is thus critical to understand the impact of FIB bombardment on the relevant properties of different materials. Despite many investigations of implanted gallium concentrations, surface roughening and microstructural changes, few studies[1] have quantified the residual stress that results from FIB exposure. The residual stress that accompanies focused ion beam bombardment can potentially influence the physical characteristics (i.e., shape) of modified components, such as the flatness of optical microelectromechanical systems (MEMS) mirrors or the morphology of porous membranes used for microbiological investigations[1]. Furthermore, changes in stress may impact surface and microstructural analysis by transmission electron microscopy (TEM) or other methods. Recent studies of microstructure about a nanoindent claim that focused ion beam-based sectioning methods can influence the density of observed cracks about the site of mechanical loading[2]. The authors of this previous work[2] have attributed these effects, in part, to a large residual stress accompanying FIB sectioning. The bombardment of a sample material can, in general, cause an intrinsic stress, closely related to the microstructure of the exposed volume. Implanted particles and defects can also contribute to the observed stress. The effects of concurrent low energy ion bombardment on thin film stress (e.g., that developed during physical vapor deposition) have been identified for many years[3]. Goals of this study include 1) development of techniques for quantifying stress resulting from focused ion beam exposure, 2) measurement of polysilicon cantilever beam curvature changes resulting from uniform ion beam exposure, and 3) determination of residual stress with increasing ion dose. The Stoney’s equation[4] is used to relate measured changes in beam curvature to residual stress. We therefore include a brief description of the assumptions inherent to the derivation of Stoney’s equation and compare these wit
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