LONG-TERM STABILITY OF SILICON BRIDGE OSCILLATORS FABRICATED USING THE BORON ETCH-STOP

Micromachined silicon cantilevers and bridges are prepared using anisotropic etching in conjunction with boron doping. The residual curvature and resonant response of the structure is studied as a function of fabrication conditions, and then they are driven into resonant vibration for periods of over 2000 h. Their natural frequencies and quality factors are recorded as a function of ageing time. It is found that ageing has occurred in all of the samples tested and this is manifested as an increase in the quality factor and a small decrease in resonance frequency, with both values eventually stabilizing. The magnitude of this effect is found to increase with increasing initial boron diffusion temperature. The mechanism of ageing is not clear, but may be linked to dislocation damage and activity of interstitial boron within the crystals. In addition, a softening effect has been identified in the cantilevers fabricated using relatively low diffusion temperatures.