Picosecond Ultrasonics: The prefered tool for BAW characterization

Since the 80's, RF-filtering has carried the developments of Bulk Acoustic Wave (BAW) resonators. At the same time, picosecond ultrasonics, a non-contact and non-destructive technique for mechanical characterization, sees the light of day as an on-product process control measurements technique. The operating principle of a BAW resonator is the excitation of the first thickness mode of a piezoelectric layer sandwiched between two electrodes, which is the natural geometry in picosecond ultrasonics. This technique uses a pulsed laser source to excite and detect longitudinal acoustic waves at very high frequencies (100 GHz to 1 THz). Thus, it enables the measurement of decisive parameters of materials in thin films for the modeling of BAW resonators (sound velocity, thickness, density, acoustic attenuation, temperature coefficients). Here we present the capability of the technique that has been applied to BAW materials (AlN, Mo, SiO₂, W, SiN) for mechanical characterization. Then we present results obtained on BAW stacks that enables modeling of resonant frequencies and coupling coefficient. These results show that more than just a process control tool, picosecond ultrasonics can replace RF-tests for frequential characterization.