(Invited) AlN and ScAlN Contour Mode Resonators for RF Filters

Abstract

A very strong technological market pull for wide bandgap semiconductor devices comes from the piezoelectric properties of aluminum nitride (AlN). Portable wireless products are expanding in their use of AlN films as RF devices in bulk acoustic wave (BAW) and surface acoustic wave (SAW) filters with substantial performance properties over other semiconductor materials. To maintain market dominance properties such as size, integration costs, and filter performance metrics continue to improve. Metrics closely coupled to material properties such as piezoelectric coupling coefficient (k2 eff) and quality factor (Q) arising from material sputtering conditions directly impact key filter parameters such as insertion loss, bandwidth, and rejection, and these properties have been intensely investigated as have device and filter architecture. To improve versatility, alternate resonator designs are emerging that combine micromachining and different piezoelectric coefficients such as d31 instead of d33. These contour mode resonators (CMR) differ slightly from BAW and SAW since resonator frequency is determined lithographically and devices are suspended which provides the opportunity to provide multiple frequencies on a single die with reduction of cross talk and integration on top of CMOS. This work will discuss our efforts in utilizing CMRs to achieve a wide range of operating frequencies on a single die with improved performance. In this presentation we will discuss our RF sputter deposition process for creating high-performance AlN films, as well as plasma etching and integration techniques for fabricating CMRs. Fabricated devices spanning a range of 400 MHz to 3 GHz will be demonstrated with spur modal interaction discussed. Further, early investigations into scandium aluminum nitride (ScAlN) suggest that metrics such as Q and K2 eff can substantially improve simultaneously while still providing a wide frequency range of devices.