Oven-Based Thermally Tunable Aluminum Nitride Microresonators

Frequency tuning of aluminum nitride (AlN) microresonators has been demonstrated via localized heating (ovenization) of the resonator. Specifically, piezoelectrically driven ~ 100 MHz microresonators were heated by embedded joule heaters in vacuum. Three different designs with three different film stacks were tested, and among the tested devices, thermal resistances as large as 92 K/mW have been demonstrated, which corresponds to 1-mW power consumption to yield a temperature increase of 92°C. To minimize heat loss, the devices were suspended from the substrate by high thermal isolation beam-type supports. The beams exhibit very high thermal resistance not only due to their high length to cross-sectional area ratio but also because they are made of thin-film-deposited polycrystalline aluminum nitride. Film-deposited AlN has been shown to have thermal conductivity much lower than that measured in bulk materials. Thermal time constants for these devices were measured ranging from submilliseconds to 10 ms depending on the design and film stacks, and frequency tunability was measured as high as 2548 parts per million/mW. The availability of a power-efficient frequency tuning method, coupled with all other performance benefits, makes AlN microresonators a promising candidate for the next-generation timing devices and tunable filters for multiband communication systems.