MESA+ University of Twente
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Sumy Jose (promotion date: 13 December 2011)

Reflector stack optimization for Bulk Acoustic Wave resonators

Promotion date: 13. December 2011

Promotor: Prof.dr. Jurriaan Schmitz

Assistant promotor: Dr. ir. Ray Hueting

The Thin-film bulk-acoustic-wave (BAW) devices are used for RF selectivity in mobile communication system and other wireless applications. Currently, conventional RF filters are getting replaced by BAW filters in all major cell phone standards. In this

thesis, solidly mounted BAW resonators (SMR) are studied 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.

This thesis presents several novel design approaches for optimizing the reflector stack for dual reflection of longitudinal and

shear acoustic waves, in view of an 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

experimentally 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.

What were the main research activities in your thesis project?

In order to prevent interference of voices in mobile phones, filters are crucial. With respect to bulk-acoustic wave devices, a quandary exists for many years already. I estimate back to 2005. Two types of acoustic waves, longitudinal and shear, seemed to ask for different approaches in order to control them. Although, there were some brute force optimization techniques available, systematic design approaches were never reported.

In collaboration with NXP Semiconductors and in a later stage a US company, devices were fabricated following my theoretical and numerical analyses. I was able to follow an extensive training at NXP after my supervisor here came up with the idea to go into the topic of BAW resonator in great detail.

The devices were tested by the companies and I interpreted these measurements, eventually coming up with a set of optimization skills to be used by the designers in industry. Novel design approaches involved different materials combinations, optimization techniques and making the theories known in general optics, useful for engineering approaches in this field.

Telephone discussions were held with involved experts. I guess that was quite a specific feature of my thesis project. Actually perceiving in simulations that the longitudinal and shear acoustic waves were filtered both, following my design principles, was a great feeling.

Did you succeed to publish in well-known magazines or have your findings spread in conferences?

One important publication was in IEEE Transactions on Ultrasonics, Ferro-electricity and Frequency Control.

I was a regular speaker at the Ultrasonics Symposia. Around a thousand experts of the Ultrasonics community come to speak there. In the 2011 conference even two of my abstracts were accepted. I was also an invited speaker at a conference in 2011.

The work received the attention of Netherlands technical magazine “Bits and chips” in October 2010.

How did you develop personally during your PhD research period?

Working abroad with a well-known institute like Mesa+ is a great opportunity for me as a scientist, developing many skills. The hands-on cleanroom experiments were not part of my project, which I regret a little bit.

Also I collaborated with International Office at the University of Twente. I succeeded to establish progress in collaboration with three universities in India. It is a great feeling to do something back to my home country and also be of interest for the UT and Mesa+ simultaneously. I hope in the future new Indian faces will be seen in Twente to perform some great research.

What are your future plans?

I am focussing on a job in industrial R&D. Right now I interact with companies, both in the Netherlands and in the US. Perhaps in a later stage of my career I return to academic research again. Right now I am very curious what industrial research will be like. Interdisciplinary research involving new nanotechnologies and transducers have my special interest.

What in your opinion is important for Mesa+ to stay successful in future?

Mesa+ has a great international call everywhere. It is a quite famous institution.

I guess the way of collaborating with industry is of a high level. This approach suits me well. Results from PhD research can be brought to industry, and the devices can be fabricated there. This can accelerate the PhD research if still more working relationships with industry take place.