Defect-Induced Shifts in the Elastic Constants of Silicon
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Defect-Induced Shifts in the Elastic Constants of Silicon Clark L. Allred1,2,3, Jeffrey T. Borenstein1, Marc S. Weinberg1, Xianglong Yuan2, Martin Z. Bazant2, Linn W. Hobbs2 1.
The Charles Stark Draper Laboratory, Inc., 555 Technology Square, Cambridge, MA 021393563, U.S.A. 2. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 021394307, U.S.A. 3. Air Force Institute of Technology, 2950 P Street, Wright-Patterson AFB, OH 45433-7765, U.S.A. ABSTRACT As MEMS devices become ever more sensitive, even slight shifts in materials properties can be detrimental to device performance. Radiation-induced defects can change both the dimensions and mechanical properties of MEMS materials, which will be of concern to designers of MEMS for applications involving radiation exposure, such as those in a reactor environment or in space. We have performed atomistic simulations of the effect that defects and amorphous regions, such as could be produced by radiation damage, have on the elastic constants of silicon. We have then applied the results of the elastic constant shift calculations to a hypothetical MEMS device, and calculated the difference that would be generated by this effect. INTRODUCTION It is well known that environmental factors such as temperature can adversely impact the performance of MEMS devices. In this study, we will demonstrate that radiation is also a potential source of performance degradation for sensitive devices through the alteration of mechanical properties of common MEMS materials. Space-based electronic devices are notoriously susceptible to degradation due to radiation, since the electrical properties of materials can be dramatically altered by relatively low radiation doses. Mechanical properties such as density and elasticity can also be altered by radiation damage. While these changes are slight, they can be of critical importance to a MEMS device operating in space or in a reactor environment. We assume a reactor environment involving fast (1 MeV equivalent) and thermal neutron fluxes, as well as gamma flux. Gamma radiation will be present in space, but protons, rather than neutrons, will be of primary concern in terms of ballistic collision displacement damage. It is convenient for the present analysis to examine the neutron case, since the mean free path of a 1 MeV neutron in Si is about 4.5 cm. This is a long mean free path on the MEMS scale, and will result in recoil ions that are uniformly distributed in silicon MEMS components. By contrast, a 1 MeV proton was shown in a SRIM-2003 [1] simulation to penetrate to only about 17 µm, which would complicate the analysis of elastic effects in MEMS parts due to the sharp gradient in damage with depth from the surface. With neutrons, we will also not have to account for the electronic stopping that is so important for protons. Fast neutrons cause damage by displacing atoms from their lattice sites through collisions with atomic nuclei. A 1 MeV neutron has an elastic scattering cross-section [2] of about 4.5
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