MESA+ University of Twente
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Wabe Koelmans (promotion date: 17 June 2011)

Parallel probe readout

Promotion date: June 17th, 2011

Promotor: Prof. dr. Miko Elwenspoek

Co-promotor: Dr. ir. Leon Abelmann

In this thesis techniques are developed to read out nanoscale probes and arrays of probes. The main targeted application area is probe-based data storage. The work also contributes to other areas, such as metrology, biological sensing, materials research and nano-electro-mechanical switches.

We show that it is possible to optically read out many cantilevers operating in parallel at the nanoscale by addressing the cantilevers at different frequencies.

A second readout technique makes use of the diffraction patterns, generated by an array of cantilevers. Different configurations of cantilevers give rise to different, distinguishable diffraction patterns.

Also, a novel fabrication process is developed to produce probe arrays with sharp tips that are self-aligned on the cantilever. Cantilevers and tips are producible using a single mask.

Finally, a force modulation scheme is introduced for conductive AFM. Force modulation is shown to lower the wear of the AFM tip and also aid the formation of the nanoscale electrical contact.

Was the thesis research application driven?

Especially for long term data storage - longer than ten years - the cantilever principle is attractive,

because it allows remote readout of the cantilevers by using laser light. In this way, the readout can be separated from the storage medium, as is the case in a DVD player. With cantilevers one can achieve ultra-high densities, because the density is not limited by the wavelength of the laser light.

The principle has already been proven as a high-density data storage technique. Now, within the Transducers Science and Technology group, new readout techniques were investigated, doing a lot of experimental work and using various cleanroom optical techniques. The work was part of a large European project, called Protem, including eight international partners. I spent five month in Zürich at IBM Research.

Do you recall some special moments during your thesis period at Twente?

In 2008 I had some surprising results in an experimental setup. Instead of measuring an ordinary resonance peak distribution, some dip was detected, much to my surprise. This turned out to be due to interference of light, usable for a new kind of readout technique. On this subject I was able to have some articles published later on in the project.

In what journals did you publish?

For example, I published in the Journal of Applied Physics and in Nanotechnology.

How did you develop personally, as a scientist and researcher?

I really learnt a lot of different kinds of skills, for example many cleanroom fabrication techniques such as using the focused ion beam (FIB). With this equipment apparatus it is possible to drill on a nanoscale level, being able to produce arrays of three or more cantilevers.

Furthermore, I am planning research in a far more clever way now. For example by making use of intermediate stages during the process which are of interest on their own, even leading to new publications from time to time.

In general I gained a deeper understanding, leading to a balanced approach wherein strategic thinking and performing experiments are more and more interlinked. Nowadays I have a better feel for which parameters are of prime importance in practical situations.

The main aim of a thesis period is to become a researcher, and now I am one, acting independently, with a critical attitude and focused on scientific results.

What are your future plans?

I am looking for a job position where university and industry are meeting one another. This can be in a post-doc position or in industry, working closely together with scientific researchers.

In what way can Mesa+ remain successful in future you believe?

The institute has a lot to offer from a wide variety of disciplines.

The cleanroom culture is very good in my opinion. Specialists are eager to know about your project and are able to think of the best possible solutions. For example, Hans Mertens, was of great help when I fabricated magnetic layers on the cantilevers.

I think, transparent procedures are necessary to deliver the best outcome of the apparatus and specialists present at Mesa+. When all of these conditions are met, every researcher can bring the best out of him, especially when the new building is in use and the cleanroom facilities are functioning at their top capacity.