Many piezoelectric microelectromechanical systems (MEMS) measure or generate acoustic signals via the motion of radially non-uniform, thin film composite plates. The composite layers provide piezoelectric actuation, structural support, electrode metallization, passivation, etc. Often, the layers are non-uniform over the plate and contain residual stresses introduced during the fabrication process. Accurate models of non-uniform composite plate mechanics are crucial for predicting and optimizing device performance. In this paper, an analytical solution for a radially non-uniform, piezoelectric, circular composite plate incorporating residual stress is derived. The analytical solution is compared to experimental measurements of a MEMS piezoelectric diaphragm. The results show the improved accuracy of the analytical model when including film stress, the speed of the analytical solution as compared to finite element analysis, the sensitivity of device performance to residual stress and the importance of accurate film stresses as model inputs. Finally, the analytical model presented is useful as a design optimization tool given the efficiency of the computational time, shown to be 275 times less than a comparable finite element analysis.
Localized stress variation in aluminum nitride (AlN) sputtered on patterned metallization has been monitored through the use of UV micro-Raman spectroscopy. This technique utilizing 325 nm laser excitation allows detection of the AlN E2(high) phonon mode in the presence of metal electrodes beneath the AlN layer with a high spatial resolution of less than 400 nm. The AlN film stress shifted 400 MPa from regions where AlN was deposited over a bottom metal electrode versus silicon dioxide. Across wafer stress variations were also investigated showing that wafer level stress metrology, for example using wafer curvature measurements, introduces large uncertainties for predicting the impact of AlN residual stress on the device performance.
A single-axis MEMS wall shear stress sensor with differential capacitive transduction method is produced. Using a synchronous modulation and demodulation interface circuit, the system is capable of making real time measurements of both mean and fluctuating wall shear stress. A sensitivity of 3.44 mV/Pa is achieved, with linearity in response demonstrated up to testing limit of 2 Pa. Minimum detectable signals of 340 μPa at 100 Hz and 120 μPa at 1 kHz are indicated, with a resonance of 3.5 kHz. Multiple full scale wind tunnel tests are performed, producing spectral measurements of turbulent boundary layers in wind speeds ranging up to 0.5 Ma (18 Pa of mean wall shear stress). The compact packaging allows for minimally invasive installation, and has proven relatively robust over multiple testing events. Temperature sensitivity, likely due to poor CTE matching of packaged materials, is an ongoing concern being addressed. These successes are being directly leveraged into a development plan for a dual-axis wall shear stress sensor, capable of producing true vector estimates at the wall.
After years in the field, many materials suffer degradation, off-gassing, and chemical changes causing build-up of measurable chemical atmospheres. Stand-alone embedded chemical sensors are typically limited in specificity, require electrical lines, and/or calibration drift makes data reliability questionable. Along with size, these "Achilles' heels" have prevented incorporation of gas sensing into sealed, hazardous locations which would highly benefit from in-situ analysis. We report on development of an all-optical, mid-IR, fiber-optic based MEMS Photoacoustic Spectroscopy solution to address these limitations. Concurrent modeling and computational simulation are used to guide hardware design and implementation.