Understanding the internal friction of a silicon micro-mechanical oscillator

In order to understand the consensus that mechanical loss of an oscillator increases as its size decreases, we studied the internal friction of a 1.5 μ m thick micro-mechanical oscillator with laser Doppler vibrometry. We separated the loss due to attachment by studying mode shape dependence of different vibration modes, the loss due to defects generated during plasma etching by rapid thermal annealing, loss due to surface adsorbates by its post-anneal time dependence, loss due to surface roughness by sand-blasting of a macro-mechanical oscillator. We conclude that the near surface lattice defects caused by plasma etching and surface adsorbates are the main source of the observed internal friction at room temperature. While the loss caused by plasma etching can be annealed away, the effect associated with surface adsorbates will come back after annealing. We further studied the internal friction at low temperatures and by in situ laser-flash annealing. We found that the loss associated with the surface adsorbates is responsible for an internal friction peak at about 155 K. The reversible time dependence of internal friction at room temperature is attributed to the broad tail of the peak at 155 K.