B. Ohler, F.T. Limpoco, R. Proksch
Oxford Instruments Asylum Research Inc,
Keywords: aluminum nitride, atomic force microscopy, piezoresponse force microscopy, acoustic resonator filters
Summary:Faster wireless communication and efficient use of the RF spectrum are made possible by continuing advances to bulk acoustic wave (BAW) filters. These MEMS devices use a piezoelectric film to interconvert between electrical and mechanical acoustic energy, where the material properties and device geometry determine the frequency response. Aluminum nitride (AlN) is the most used piezoelectric material based on several advantages including compatibility with CMOS processing. However, its electromechanical response is marginal, which began to limit filter performance at 5G frequencies. The discovery that doping AlN with scandium can increase the piezoelectric coefficient by up to 40% has allowed the industry to extend beyond those limits. However, at high Sc concentrations, defects commonly occur in the films and therefore processing conditions must be optimized. Fabricating complete devices to test piezoelectric performance adds significant cost, so tools to evaluate the films earlier in the process are desirable. Piezoresponse force microscopy (PFM), an atomic force microscopy (AFM) technique, is one such tool. Unlike tools like Berlincourt piezometers and dual-beam light interferometers that measure only a bulk spatially-averaged response, PFM measures nanoscale piezoelectric response at the level of film grains and defects. However, when implemented on conventional AFM hardware, it is difficult to make quantitative repeatable measurements of the piezoelectric coefficient, d33, because of artifacts introduced by electrostatic interactions between the sample and AFM cantilever. Here, we demonstrate that using a new AFM design with interferometric sensing of the cantilever tip displacement (Cypher IDS) allows us to make highly repeatable measurements of piezoelectric response by eliminating these artifacts. PFM measurements with the Cypher IDS were made on Sc-doped AlN with scandium concentrations up to 40%. Topography images clearly show the film morphology and defects while the simultaneously acquired PFM amplitude images indicate the piezoelectric response. These PFM measurements were able to reproduce a Sc-concentration-dependent trend similar to that first observed by Akiyama et al. using a Berlincourt piezometer, wherein the piezoelectric response increases with Sc concentration up to a point but then drops sharply at higher concentrations. We suggest that this quantitative PFM capability should assist in optimizing processing conditions for Sc-doped AlN by providing a quantitative measure of piezoelectric response that can be directly correlated with observation of grain structure and film defects.