Working on the nanometre scale
Imagine a sheet of paper, A4 size. Take a pencil and draw something – a house, for example, or your favourite comic figure. Anyone with experience in drawing things is not likely to find that much of a challenge. Or a second idea to play with: imagine, once again, that you’re doing your drawing – but this time on a surface which is around 200 times smaller than the sheet of A4 paper. Even an artist working with the utmost precision and the sharpest pencil will come up against his or her limits. In the Münster Nanofabrication Facility (MNF), such dimensions are part of everyday work. This Facility at the University of Münster provides researchers with an opportunity to fabricate microscopically tiny materials and carry out research on and with them – in the cleanrooms of the Center for Soft Nanoscience (SoN) and the Center for Nanotechnology (CeNTech). Physics students also enjoy the benefits, for example in the “Fundamentals of Nanofabrication” course which I am sitting in on today as an observer. The aim of the seven-day period of practical training is to teach the students more about how to create structures on a nanoscale. Cleanliness here is the non plus ultra, because even just one single hair or speck of dust could ruin an experiment.
Proper preparation is, therefore, all the more important. For entry to the cleanroom, there are precise regulations regarding clothing, which everyone has to comply with meticulously: first there is the hairnet, then the overshoes and a face mask. We use isopropyl to clean all the items we bring with us. Ten minutes later, I have put on my new clothing – and only now am I allowed to enter the cleanroom. A laborious process for anyone who enters and leaves the cleanroom several times a day. “That can certainly be a bit tedious at times,” admits Mohammad Bilal Malik, a doctoral student who is leading the practical training and who sometimes goes into the cleanroom up to four times a day. However, he has more of a routine than I do and he only needs about three minutes to go through all the safety steps.
“Welcome to the cleanroom,” says Malik as the door opens. Yellow lamps bathe the cleanroom in warm light. This is necessary, he says, because the ultraviolet components in the blue and white light is harmful for the specimens. The way the cleanroom is divided up reminds me of the layout of an apartment. From the lobby in the middle, the physics lab is on the left and the right, and straight ahead is the etching room. I start off by following the students to the left into the preparation room, in which the specimens are prepared for further treatment.
During the first days of their practical training, the students carry out minor experiments to learn more about the methodological basics of nanofabrication. Today, the third day, sees the beginning of the first major experiment – the production of an organic light-emitting diode (OLED). Electronics fans will likely know about this. These microscopically tiny light sources are incorporated in mobile displays and TV screens, for example. The principle of manufacturing an OLED is easily explained. It consists of four layers which are applied one after the other to a thin glass plate. Aluminium contacts arranged in a rectangular fashion, wires made of indium tin oxide (ITO), an organic semiconductor and aluminium wires arranged vertically to the ITO wires. Where the wires overlap, a pixel is formed. A conventional television screen consists of thousands of pixels, but the students begin at a lower level: their aim is an OLED consisting of eight pixels by twelve.
To produce the aluminium contacts, they irradiate the photoresist-covered glass plate with UV light through a photomask. As a result of this process of photolithography, the light only hits the photoresist at the spots where the contacts are set to be made. The UV light causes the photoresist to be broken up and removed, and a template is created into which the students apply aluminium in a so-called sputter machine. The power source is set to 300 watts and 500 volts for one minute – and the aluminium contacts are soon completed. When the students also carry out this process with the ITO wires, I have to look twice. The reason is that, in contrast to the aluminium contacts, the wires cannot initially be seen with the naked eye. For the first time, I begin to understand the dimensions in which the budding physicists work here – because an ITO wire is just about 100 nanometres thick. For purposes of comparison: an average hair measures around 0.07 millimetres, making it 700 times thicker than the ITO wire. So it’s hardly surprising that one of the students’ specimens has already got broken. “Happens to me sometimes, too,” says Mohammad Bilal Malik. “It means that after a while you become very careful indeed.” This makes sense when you think that a moment’s lack of attention can destroy the work done in the previous few days.
After the students have completed the remainder of the steps, they test the functionality of their LED. For this purpose, they set up a power source at two wires lying vertically to each other. When the organic semiconductor previously applied begins to light up at the spot where they overlap, the relief felt by the students is palpable. Their specimen functions!
“The students really appreciate their practical training in the cleanroom – but it’s not offered very much at German universities,” Malik tells me as we go over to the equipment used for electron-beam lithography. In comparison to photolithography, this process is much more precise and, by means of a focused electron beam, it can write structures just a few nanometres wide in photoresist. This also forms part of the practical training. Beforehand, the students had sent their lecturers a digital black-white drawing and a QR code of their choice. The lithography device reads this file and transfers it to the specimen. The students look at the results under the scanning electron microscope – and now the six-micrometre-wide pixels can be recognised.
A microscopically tiny light-emitting diode and an equally tiny QR code – nice to play around with, I think after the training session. But where’s the benefit? “There are endless possibilities for applications from nanofabrication,” says Dr. Juan Navarro-Arenas, a postdoc in Prof. Carsten Schuck’s working group at the Department of Quantum Technology. Large quantities of data, for example, can be transmitted much faster using minute optical fibres than via cables. In this process, the light is absorbed in small detectors and converted into electrical impulses. It is very important in the fields of communications and sensor technology with individual photons.
I am not the only one to be impressed by the possibilities – hard to grasp but, at the same time, awesome – which nanofabrication offers. The students are, too. “It was a privilege to have done this practical training,” says a delighted masters student Mehrzad Movafagh. “I now understand the theoretical side better, and I have also acquired some routine in handling the equipment,” says the 28-year-old, who hopes this wasn’t the last time that he’s entered a cleanroom. “The training has triggered my curiosity regarding research projects at CeNTech,” he adds.
MNF Day
The Münster Nanofabrication Facility (MNF) is a machine park in the west of Münster which is used by researchers in the fields of natural sciences, geosciences and medicine. Every year, researchers and companies working the field of nanofabrication and nanoanalytics, as well as other interested parties, can get networked on MNF Day and take part in workshops and tours of laboratories. There are a range of talks given which provide information about current research trends and fields of application in nanotechnology. Among the speakers are representatives from companies which manufacture the equipment used in the cleanrooms. The next date is November 7 at CeNTech. The number of places is limited, and registration is necessary.
Author: Linus Peikenkamp
This article is from the University newspaper wissen|leben No. 6, 2 October 2024.