Reflector stack optimization for bulk acoustic wave resonators


Jose, Sumy (2011) Reflector stack optimization for bulk acoustic wave resonators. thesis.

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Abstract:Thin-film bulk-acoustic-wave (BAW) devices are used for RF selectivity in mobile
communication system and other wireless applications. Currently, the conventional RF
filters are getting replaced by BAW filters in all major cell phone standards. In this
thesis, we study solidly mounted BAW resonators (SMR) which are the building blocks
of these filters. The good selectivity offered by the BAW resonators makes them
excellent components for inter-stage filters and duplexers for mobile applications.
Modelling and development of high performance thin-film BAW devices is a topic of
research gaining attention from BAW specialists around the world. The quality factor of
an SMR is limited by acoustic substrate losses, because the reflector stack is traditionally
optimized to reflect longitudinal waves only. This thesis presents several novel design
approaches for optimizing the reflector stack for dual reflection of longitudinal and
shear acoustic waves in view of achieving high acoustical quality factor.
Two main concepts are studied in this thesis. The first one is the optimization of the
reflector stack and the second one is the influence of reflector stack design on the
acoustic dispersion of the resonators. The realized devices utilizing the concepts are
experimental validated.
Two main reflector stack design approaches – the stop-band theory and the diffraction
grating based approaches – derived from the optics background were presented. The
approaches were successfully verified with 1D-model and 2D FEM simulations. The
approaches had been demonstrated for different material combinations and in all the
cases a minimum transmission of −25 dB and −20 dB at resonance frequencies were
obtained for longitudinal and shear waves respectively, for various practical reflector
material combinations. The standard quarter wave stack shows similar transmission for
longitudinal waves, but very high transmission (e.g. −0.59 dB) for shear waves. Hence
the optimized reflector stacks show an efficient reflection of both longitudinal and shear
The reflector stack design affects the acoustic dispersion of the resonators. The
resonators adopting reflector stack designs with stop-band theory based approaches
resulted in type II dispersion. On the positive side, the ones adopting the diffraction
grating based stacks exhibited type I dispersion which is the preferred dispersion type for frame region functioning. We derived a rule of thumb for flipping the dispersion
relation of SMRs to type I, by assuming the shear confinement in the top-oxide layer of
the reflector stack. The rule was also extended for free standing bulk acoustic wave
resonators (FBARs).
The experimental verification of the design approaches was presented. The extracted
1-D acoustical quality factor for the realized shear optimized devices with a stop-band
theory based SiO2/Ta2O5 reflector stack was increased to around 3300. The versatility as
well as credibility of the approaches was verified by another set of experiments on
SiO2/W stacks. The experiments established the theoretical prediction of the optimum
reflection when the optimization parameter is between 1 and 2. It was found that the
quarter-wave like stacks with an increased top-oxide layer thickness, showed a higher
extracted 1D quality factor. With DGM stacks the highest experimental quality factor
was obtained. FEM simulations were performed for further understanding of the
experimental results and the extracted values were corroborated with the trend
observed in the simulations.
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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