Simple post-processing technique to tune resonant frequency of film bulk acoustic resonators and stacked crystal filters

This paper describes a simple post-processing technique to tune the resonant frequency of an Al-ZnO-Al thickness-excitation film bulk acoustic resonator (FBAR) and stacked crystal filters built on a low-stress silicon nitride membrane after fabrication. The post-fabrication technique uses a reactive ion etching (RIE) to remove a portion of the silicon nitride membrane. This simple, mask-less, technique tunes the resonant frequency by about 10% with a variation of within 0.2% over a 1.2 cm-square area (a typical chip size) in our RIE system, and also allows us to match the resonant frequency within 0.01% of the desired value. For a typical resonator with the fundamental resonant frequency around 1 GHz (that uses 0.8 μm thick silicon nitride membrane, 0.2 μm thick Al electrodes, 2.3 μm thick ZnO), a 10% frequency tuning is obtained by removing about 0.45 μm the nitride support membrane from the backside of the wafer. The nitride removal is typically done in about 12 minutes with CF₄ RIE at 100 Watts. The fQ factor of the resonator increases by about 5-8% after the nitride removal from a value of about 8.5×10¹¹ with quality factors in the range of 900. With this technique, we have achieved about 8% tuning of the center frequency (550 MHz) of the stacked crystal filters which have the loaded insertion loss of less than 5 dB in a 50 Ω system. We have accurately simulated the tuning effect by modeling the resonator with cascaded transmission lines derived from the Mason´s model