Acoustic wave resonators based on bulk piezoelectric materials such as quartz crystal and lithium niobate have been the mainstay for many decades of timing and filtering devices in RF systems and in various sensing applications. Quartz-based Bulk Acoustic Wave (BAW) resonators have advantages in terms of temperature stability and high Q-factor however; one of the major limitations is that they cannot readily achieve frequencies in range of hundreds of MHz due to low acoustic velocity and processing limitations. SAW devices on lithium niobate substrates can operate in the range of GHz frequencies however, this technology can’t provide performance comparable to BAW devices. In addition, the manufacturing cost and size of these bulk piezoelectric devices remains relatively large versus other system components.
These limitations have led to the development of MEMS (Micro-Electro-Mechanical-Systems) version of piezoelectric resonator (i.e. Thin-Film Bulk Acoustic Resonator, FBAR) using a thin film of AlN (Aluminium Nitride) as the piezoelectric layer. The FBAR technology demonstrates higher operation frequencies and smaller form-factor than traditional bulk piezoelectric resonators, and is compatible with the IC fabrication.
The interest of Tyndall’s PiezoMEMS Group lies in research and development of thin film piezoelectric resonators such as SAW, BAW, FBAR and LWR, for RF applications (e.g. oscillators and filters) and sensing applications (e.g. gravimetric mass sensing). The figure shown demonstrates one of the resonator technologies being developed in Tyndall.
It is the AlN and AlScN based LWR resonator that combines the advantages of commercial resonator technologies: (i) the operation frequency of LWR is determined by the dimension of the IDT transducer which, as in SAW devices, is lithography determined and this provides the flexibility in terms of achieving various operation frequencies on a single wafer, (ii) similarly to FBAR resonators, the LWR is an air-suspended structure enabling low damping and high acoustic velocity that reflects in high Q-factors and high operation frequencies, (iii) the processing of LWR is CMOS compatible therefore advanced integration with IC circuitry is feasible.