Applying laser Doppler vibrometry to probe anchor losses in MEMS AlN-on-Si contour mode resonators

Abstract

This paper reports the application of high frequency laser Doppler vibrometry to observe the surface profile of waves at the boundaries of contour mode aluminum nitride (AlN) on silicon (Si) resonators. The findings provide better understanding of the nature of the losses at the anchoring regions. These losses set the quality factors of these resonators. We have applied the technique to three groups of resonators to correlate the findings from laser vibrometer measurements to the measured quality factors (and thus also anchor loss). The three groups of resonators differ in their resonant frequencies, which are determined by the resonator dimensions, in order to provide a range of frequencies for enhanced testing reliability. Each group includes a flat-edge resonator and a biconvex resonator of the same resonant frequencies but largely different quality factors (at least by a factor of six: e.g. 12000 vs. 2100) in order to accentuate differences observed by vibrometry. Experimental vibrometer results for all resonators consistently show starkly different surface wave profiles (i.e. out of plane direction) despite the fact that the vibration modes are primarily lateral. The vibrometer wave profiles show strong confinement in the biconvex resonators while the waves in the normal flat edge resonators are spread out uniformly to the boundaries. The vibrometer results match well with the finite-element models. In summary, we find a strong correlation between the out-of-plane displacement profiles and anchor loss, which allows us to verify and consequently exploit the simulations in the course of analyzing anchor losses in AlN-on-Si contour mode resonators. Finally, our results show that varying the support tether length has neither any effect on the wave profile nor the quality factor.