Frequency Trimming of Aluminum Nitride Microresonators Using Rapid Thermal Annealing

To transition aluminum nitride (AlN) microresonator filters into a manufacturable technology, accurate control of the resonator frequency across a wafer is required. This paper describes a postfabrication rapid thermal anneal approach to trim resonator frequency over 27000 ppm with an accuracy of 500 ppm. Measurements made on 22.4 MHz resonators indicate that the effect of annealing on the resonators saturates in 5 min and upshift the resonator frequency super linearly with temperature. We replicate the frequency trimming effect on hermetically sealed wafer level packaged devices to reduce the across-wafer frequency distribution from 22000 to 4000 ppm. We confirm that this postannealing technique is permanent by temperature cycling the resonators from 50°C to 125°C. This technique provides a method to trim AlN microresonator frequency overcoming effects such as thin film variations, which are inherent to microsystems fabrication.