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Mubassira Syed Nawazuddin (promotion date: 22 November 2013)

Micromachined parallel plate structures for Casimir force measurement and optical modulation.

Promotion date: November 22.

Promotor: Prof.dr. Miko Elwenspoek

Assistant Promotor: Remco J. Wiegerink

In microelectromechanical systems (MEMS), parallel plate structures with submicron separation have been of much use in various types of sensors and actuators. In this thesis such parallel plate structures are employed for two applications viz. the Casimir force measurement and to develop a mechano-optical modulator. Both research studies are focused on realizing parallel plate structures with a separation distance in the order of 1 μm or less.

The plate-plate configuration gives the largest interaction area for quantifying the Cazimir force, but keeping the plates exactly parallel at sub-micrometer separation is a very demanding task. In this thesis, a methodology to measure the Casimir force using parallel plate structures is described, in which MEMS technology is used to improve the parallelism at sub-micrometer separations.

The fabrication of these parallel plate structures is described and a dynamic measurement methodology to determine Casimir force using a scanning laser vibrometer is developed. To realize the parallel plates separated at ~1 μm distance, two fabrication processes based on two different substrates: <111>-oriented silicon, providing a very smooth surface, and silicon-on-insulator (SOI) wafers. Only devices based on the SOI process were suitable for performing measurements.

The second part of the thesis deals with the design and realization of an IONM (Integrated Optical Nano-Mechanical) based mechano-optical modulator. The integrated mechano-optical modulator device is characterized for both the electrical and optical measurements. The Capacitance-Voltage (CV) measurements successfully demonstrated the bi-directional electrostatic actuation of the device. This measurement also confirmed the movement of the mechanical beam in close vicinity of the waveguide core. From these results, it is shown that the mechanical structure can be actuated towards and away from the waveguide, demonstrating successful self-aligned assembly.

Can you mention some important moments during your thesis period?

The parallel plate fabrication methods, using a <111>-oriented silicon substrate separated at sub-micron distance, proved to give very smooth surfaces, as expected. The contactless measurement principle to determine the Casimir Force using these two parallel plates, was carried out successfully. In this principle one plate was fixed and the other one was allowed to move independently. All these stages proved to be quite challenging. Another important result was the demonstration of the bi-directional electrostatic actuation of the mechanical structure developed for the mechano-optical modulator.

The thesis project consisted of big engineering questions as well as a scientific challenge, as this type of measurement, for the experimental investigation of the Casimir force, was quite cumbersome to do. Using MEMS-technology will open up new possibilities in this field.

Did you manage to publish some nice articles?

Two articles appeared in: the Journal of Micromechanics and Microengineering and in Challenges. Also I had two international conference papers, one on the IEEE MEMS conference for the optical modulator part and one on the IEEE Transducers for the Casimir force measurement part.

In what way did you develop personally, as a researcher and scientist?

There was personal development and growth on many different levels in this period. I learned to be a patient experimentalist in this period, combining design, fabrication and experimental work. The experimental side of the work I choose deliberately right from the start, in order to gain confidence in using cleanroom techniques and learning various fabrication processes and techniques.

The experts and technicians in the group played a significant role in building up these skills which I believe are important for my future career. Here I leave all possibilities open. Living in London, I am applying for jobs both in academics and in industry. Also the multidisciplinary experience which I gained in the PhD period will be of use, I believe.

Did you feel part of the Mesa+ institute during your thesis project?

Yes, I did. Mesa+ is a renowned institute that is well recognised all over the world. I enjoyed the Mesa+ Days and poster presentations, and the invitation talks. These activities mean a great deal to the PhD students working at Mesa+ who constitute the intern audience.

Also the staff and organization proved efficient and supporting all the way through my PhD period. Especially when the cleanroom labs transmigrated from one building to the other the planning was such that I didn’t experience a drag in progression. We had access to the right equipment whenever it really mattered. It’s a great feeling to have worked in such sophisticated cleanroom and institute.