Lundh, James S.; Coleman, Kathleen; Song, Yiwen; Griffin, Benjamin A.; Esteves, Giovanni E.; Douglas, Erica A.; Edstrand, Adam E.; Badescu, Stefan C.; Moore, Elizabeth A.; Leach, Jacob H.; Moody, Baxter; Trolier-Mckinstry, Susan; Choi, Sukwon
In this study, the Raman biaxial stress coefficients KII and strain-free phonon frequencies ω0 have been determined for the E2 (low), E2 (high), and A1 (LO) phonon modes of aluminum nitride, AlN, using both experimental and theoretical approaches. The E2 (high) mode of AlN is recommended for the residual stress analysis of AlN due to its high sensitivity and the largest signal-to-noise ratio among the studied modes. The E2 (high) Raman biaxial stress coefficient of -3.8 cm-1/GPa and strain-free phonon frequency of 656.68 cm-1 were then applied to perform both macroscopic and microscopic stress mappings. For macroscopic stress evaluation, the spatial variation of residual stress was measured across an AlN-on-Si wafer prepared by sputter deposition. A cross-wafer variation in residual stress of ∼150 MPa was observed regardless of the average stress state of the film. Microscopic stress evaluation was performed on AlN piezoelectric micromachined ultrasonic transducers (pMUTs) with submicrometer spatial resolution. These measurements were used to assess the effect of device fabrication on residual stress distribution in an individual pMUT and the effect of residual stress on the resonance frequency. At ∼20 μm directly outside the outer edge of the pMUT electrode, a large lateral spatial variation in residual stress of ∼100 MPa was measured, highlighting the impact of metallization structures on residual stress in the AlN film.
This paper reports on a near-zero power inertial wakeup sensor system supporting digital weighting of inputs and with protection against false positives due to mechanical shocks. This improves upon existing work by combining the selectivity and sensitivity (Q-amplification) of resonant MEMS sensors with the flexibility of digital signal processing while consuming below 10 nW. The target application is unattended sensors for perimeter sensing and machinery health monitoring where extended battery life afforded by the low power consumption eliminates the need for power cables. For machinery health monitoring, the signals of interest are stationary but may contain spurious mechanical shocks.
Edstrand, Adam E.; Sun, Yiyang S.; Schmid, Peter S.; Taira, Kunihiko T.; Cattafesta, Louis C.
Designing effective control for complex three-dimensional flow fields proves to be non-trivial. Often, intuitive control strategies lead to suboptimal control. To navigate the control space, we use a linear parabolized stability analysis to guide the design of a control scheme for a trailing vortex flow field aft of a NACA0012 half-wing at an angle of attack $\unicode[STIX]{x1D6FC}=5^{\circ }$ and a chord-based Reynolds number $Re=1000$. The stability results show that the unstable mode with the smallest growth rate (fifth wake mode) provides a pathway to excite a vortex instability, whereas the principal unstable mode does not. Inspired by this finding, we perform direct numerical simulations that excite each mode with body forces matching the shape function from the stability analysis. Furthermore, relative to the uncontrolled case, the controlled flows show increased attenuation of circulation and peak streamwise vorticity, with the fifth-mode-based control set-up outperforming the principal-mode-based set-up. From these results, we conclude that a rudimentary linear stability analysis can provide key insights into the underlying physics and help engineers design effective physics-based flow control strategies.