MESA+ University of Twente
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Phuong Le Minh (promotion date: 11 December 2002)

Nanoscale light sources and hot spots for applications in μ-TAS

Interview: 11 December 2002

Phuong le Minh

The research I have been conducting has been about light emitting anti-fuse diodes. It was quite difficult in the beginning because the investigations about silicon light emitters were rather exploratory but the assignment was more applied oriented. It was a bit of a problem to join those two very different parts of the work.

What did you work on the last four years?
The research I have been conducting has been about light emitting anti-fuse diodes. It was quite difficult in the beginning because the investigations about silicon light emitters were rather exploratory but the assignment was more applied oriented. It was a bit of a problem to join those two very different parts of the work.
To explain a little bit what it was all about, I built a device that consists of two layers of polysilicon. In between there is an isolating layer of about 6 nm thick, so that the semiconductors form the plates of some kind of a capacitor. When you force a current through the isolating layer by applying a potential difference of about 10 V between the two plates, the capacitor dielectric breaks down. What happened is that at the spot of the breakdown, the so-called anti-fuse is formed, which has dimensions smaller than 100 nm. The dielectric at the breakdown link has been removed, it has molten, because of the high temperatures that came with the forcing through of the current. If you bias this spot with a current in reverse direction, now, it acts as a diode and it emits light. This light is in the visible range, it is rather perfectly white.

What kind of applications can you think of for these devices?
Thinking about applications was a very important part of my assignment. In an enhanced version of my device I used a channel, to send a liquid through it and on top I put an anti-fuse. Of course these were isolated from each other. That way we hoped we could do electro-osmotic speed measurements, to find out the speed of a liquid sent through the channel by osmotic forces. That can be done by sending phosphorescent beads with the liquid through the channel. The light of the anti-fuse diode makes the beads illuminate so that they can be detected. Another application I tried out was the measurement of the refractive index of a liquid in the channel.
And because of the really small size of the light source, its integration in silicon and the white light output, there are many other applications you can think of. Examples are investigations on illumination of photoresist layers that are normally used to print structures in chips. In photochemistry there might be more interesting applications. And surely there exist many other interesting fields where the anti-fuse diode can be used.

Where do you think further research will lead?
Actually I am rather concerned about the lack of interest from other research institutes in the Netherlands and abroad. But in my own group at MESA+ we will continue working in this field. At the moment they are working on a thermal anti-fuse. Its basic principle is the same as that of my device, but they make a very small spot very hot, so that it will emit heat. Really interesting.


What other activities did you like doing at MESA+?
I had a tutoring job in the Clean Room. I was an instructor of the practical Realization In Materials training. I had teaching experience before and I think it is a really nice thing to do. You let other people hear what you have to say. I had about two students at a time and I think they enjoyed it as much as I did. Photolithography is like magic to them.

You said you had teaching experience before. Where did you get that?
In Vietnam, during my Master’s studies I had a left-handed job of teaching English. Left-handed is a way of saying part-time in Vietnamese.

And how did you get to know the MESA+ institute when you were in Vietnam?
Through ITIMS (Hanoi International Training Institute for Materials Science) which is a cooperation project with the University of Amsterdam, the University of Twente, the University of Hanoi and the Hanoi University of Technology, where I studied. The official cooperation dates back to early 1990 but before that there already existed cooperation under another name. ITIMS was formed when the University of Twente joined. The University of Twente however did have good contacts in Hanoi before that as well. In the late 1970s a professor of the university helped to build a cleanroom there.
When I graduated for my Bachelor’s degree I already applied for a post at Twente through this collaboration but I was not qualified yet. When I got my Master’s degree, I finally could come over.

And how do you like the Netherlands?
I love the country. It is very peaceful here and the people are nice. I do not like the cold weather we have right now very much, but in summer it is nice. And the view of the campus is great.
The people work in a different way here, however. Everybody works independently. In Vietnam a professor often tells his student what to do and even sometimes hints at how it can be done. In the Netherlands I cannot rely on my supervisor very much. But to have both experiences is a good thing, I think.

How do you like MESA+?
The working atmosphere in the cleanroom is quite nice. I got a lot of support from many technicians. Of course sometimes they were not available and I had to wait and that’s what they have to work on. My own group is very international. We could talk a lot about many things. But scientific interaction I only had with my supervisor and during the biweekly meetings. And in the beginning there was post-doc who had some experience on my project. I liked the scientific discussions we had together. With other colleagues we only talked about other things. That was because everybody was working on quite different subjects.

Did you have some cooperation with other institutes or companies?
Not officially. But through personal contacts we were able to use some equipment of other research groups. We did some measurements in the Philips research laboratory. And we could also use the spectrometer of the optical group at the university through a personal contact. In our group we do not work much on optics, therefore we do not have any optical equipment. But all these contacts were not official; we were the last ones on the list to be able to use the equipment. That is not very useful; it was bad for our research.

And what are you going to do after you get your PhD?
I have different possibilities. If STW grants the funds, my project is going to be continued and I will have the possibility to stay on it as a post-doc. Or I could go back to Vietnam. There I would have good possibilities in institutes for example. But I could also go to another country. I would like to be a bit closer to Vietnam, however, the last 4 years I was home only 4 times.