Residual Stress Analysis Of Aluminum Nitride Piezoelectric Micromachined Ultrasonic Transducers Using Raman Spectroscopy

In this study, the Raman biaxial stress coefficients K-II and strain-free phonon frequencies omega(0) have been determined for the E-2 (low), E-2 (high), and A(1) (LO) phonon modes of aluminum nitride, AlN, using both experimental and theoretical approaches. The E-2 (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 E-2 (high) Raman biaxial stress coefficient of -3.8cm(-1)/GPa and strain-free phonon frequency of 656.68cm(-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 similar to 150MPa 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 similar to 20 mu m directly outside the outer edge of the pMUT electrode, a large lateral spatial variation in residual stress of similar to 100MPa was measured, highlighting the impact of metallization structures on residual stress in the AlN film. Published under an exclusive license by AIP Publishing.