AIN Thin Film Structures for High-Q Resonators

The development of wireless and mobile communication in the last decade led to the elaboration of new components based on thin films with continually increasing performance and reduced fabrication cost. A fundamental element for mobile communication is the passband filter defining the total bandwidth, the so-called RF filter. Thanks to outstanding characteristics, thin film bulk acoustic wave resonators (TFBAR's) based on piezoelectric AlN thin films have conquered parts of this market against surface acoustic wave devices based on piezoelectric crystals. AlN is today the most favoured material for such these devices, thanks to its excellent electro-acoustic properties. This thesis explores paths for a new type of application in oscillators for time and frequency control in frequency range of several GHz. In portable applications, this domain is largely dominated by quartz crystals. Their resonance frequency – typically around 33 kHz – is multiplied to reach higher frequency domains. For such applications, the main issues are high quality factors and temperature stability of the resonators. This exploration implied two research directions, the first focused on AlN thin film property-process relationships for films grown on amorphous substrates, and the second on the property-design relations of such new devices. High quality factors and temperature stability are key issues in oscillator applications. We proposed to apply symmetric composite resonators (composite TFBAR) including two amorphous SiO2 thin films sandwiching high quality piezoelectric thin films of AlN. This required the growth of AlN on amorphous and insulating surfaces. We thus studied the growth of AlN on such substrates to evaluate the impact on film microstructure and morphology, mechanical stress, quality of orientation, piezoelectric constant d33,f , and polarization uniformity. Substrate roughness and substrate bias power (voltage) were identified as crucial growth parameters. In parallel, we also optimized Mo-electrode thin films for optimal AlN/Mo thin film structures. This was necessary to derive properties from pure AlN TFBAR's, and to produce HBAR's for comparison with our composite BAR's. The impact of the general device design, comprising active area surface, apodization and lateral edge design, was investigated concerning the Q-factor, kt2 and parasitic behaviour of devices. The important role of excitation of parasitic resonances was highlighted. We could show that also the microstructure of AlN plays a key role in their excitation and propagation. AlN exhibits a polar structure with one polar axis (c-axis). The growth process has to provide mechanisms to align the polar axis of all grains. No reorientation is possible by external means. The stability of the piezoelectric AlN layer with applied electric field guarantees a linear response for sensors and actuators. The high stiffness, combined with low density leads to a high longitudinal acoustic velocity of AlN, thus