AGING PHENOMENA IN HEAVILY DOPED (P+) MICROMACHINED SILICON CANTILEVER BEAMS

Micromachined (100) silicon cantilever beams with and without coating materials have been excited to study their aging process. The center frequency and the full width at half amplitude (FWHA) of the spectra are obtained as a function of time to study internal friction. The dynamic Young's modulus is determined from the resonance frequency of the cantilever beams. In uncoated silicon cantilever beams, the dynamic Young's modulus increases as a function of aging from 129 to 136 GPa in 2 x 10(9) cycles. The Young's modulus in these heavily boron-doped beams is smaller than the Young's modulus in silicon with low doping. In coated cantilever beams, the effective dynamic Young's modulus decreases as function of aging and the amount of this change depends on the coating material, being largest in the Al/Si system. In all the cantilever beams the FWHA increases as a function of time, indicating that the amount of energy dissipated internally increases as the cantilevers age. Also, the observed FWHA values of the resonances in these systems are too large to be caused by thermoelastic effects; microcracks are probably generated as the beams age. This is the first study of aging in micromachined p+ heavily boron-doped single-crystal silicon.