Picosecond ultrasonics: Characterization of single crystal piezoelectric materials for advanced RF filters

Picosecond Ultrasonics (PULSE (TM)) is a rapid, non-contact, non-destructive first principles acoustic metrology technique for in-line metal film thickness measurements. In this paper, we discuss the application of the technique to the measurement of epitaxially grown AlN and AlGaN piezoelectric layers. Since the film is transparent, we analyze the oscillatory component of the signal described as Brillouin oscillations to calculate the longitudinal sound velocity. The additional parameter not only helps characterize the material and improve the accuracy of the reported thickness but also helps in process control. We have shown that by independently calculating the velocity, we are able to detect misprocessing and characterize film quality. Finally, we demonstrate that the PULSE data provides critical thickness measurements for the RF filter stack including individual layer thickness and this allows optimization of filter performance.