Porous silicon as an acoustic material for BAW

Layers of porous silicon with a varying degree of porosity between 30% and 75% were prepared by electrochemical etching from bulk-Si. Thickness of the layers was in a range of 2 to 4 μm. Acoustic impedance and velocity for longitudinal waves in a frequency range of 2 GHz were evaluated utilizing a BAW resonator built on top of the porous Si layers. Material parameters were extracted from the electrical impedance characteristics of the BAW resonators. Special preprocessing of the frequency dependent impedance data was applied to simplify an initial fit to a Mason-Model. A subsequent fit to full data was performed to refine acoustic material parameters and estimate the propagation loss occurring in the porous Si. TEM cross-sections were prepared to verify the thickness of the porous Si layers. The longitudinal acoustic impedance was found to be in the range between 4.6 and 11.1 Mrayl while velocity was 4900 to 6950 m/s. Propagation loss was found to be lower than one would expect from a porous film.