Development of CMOS integrated AlN based SAW-Filter and the role of Si substrate resistivity

Integrated AlN/SiO₂/Si(100) delay lines for Rayleigh surface acoustic waves (SAWs) with resonant frequencies up to 3.4 GHz were fabricated using a new CMOS compatible concept. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images were done to characterize the piezoelectric AlN layer. The roughness of the AlN surface is less than 3 nm. Due to the extremely flat AlN surface, it is possible to produce frequency filters with low losses due to the surface roughness. Furthermore, S-parameter measurements of SAW filters with different substrate resistivities were performed. Large substrate resistivities of 4000 Ωcm lead to a high out-of-band insertion loss due to the reduced capacitive coupling to the Silicon substrate, which is an important parameter for SAW applications. An additional SiO₂ layer can further increase the out-of-band insertion loss.