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Arnout van den Bos (promotion date: 14 November 2003)

CantiClever – Planar fabrication of probes for magnetic imaging.

Promotion Date: 14 novmeber 2003

Arnout van de Bos

Basically my thesis is about the invention of a new sensor for a Magnetic Force Microscope (MFM).

Such a sensor consists of a small spring, with a magnetic tip at the end. The MFM is, amongst other applications, used to visualize the bits on hard disks. You could think of a bit on a hard disk as a very small magnet. When the microscope scans the disk, the magnetic tip is either attracted or repulsed by the bits. This is moving the small spring (cantilever) of the microscope.

What was your thesis about?

Basically it is about the invention of a new sensor for a Magnetic Force Microscope (MFM).

Such a sensor consists of a small spring, with a magnetic tip at the end. The MFM is, amongst other applications, used to visualize the bits on hard disks. You could think of a bit on a hard disk as a very small magnet. When the microscope scans the disk, the magnetic tip is either attracted or repulsed by the bits. This is moving the small spring (cantilever) of the microscope. This movement is measured, resulting in a high resolution image of the bits. If the magnetic microscope is to be of any use for this purpose, it has to be able to resolve the bits, that are getting smaller all the time. Since the invention of the hard disk by IBM in 1956, the bits have become around 17 million times smaller. And the density of bits still increases every year with a factor 2… Existing sensors for MFM consist have a magnetic tip that has the shape of a pyramid. This shape of the tip is not ideal for high resolution imaging, and cannot be easily varied or made smaller. The new sensor has a magnetic tip with an improved shape, that closely resembles the ideal tip shape for MFM, that of a very small bar magnet, while it can be controlled much better than on existing sensors. With our new sensor, it should be possible to keep up with the incredible speed at which the bits get smaller, at least for some time.

Is the purpose of this magnetic imaging hard disk quality control then?

No, it will be used at a previous stage in the development of hard disks to determine the recording performance of the medium, in which the bits are written, and the heads, that are used to write the bits. MFM measurements could give an answer to questions such as: how do the bits look, what is the minimum bit size, are the boundaries between the bits smooth, how long are the bits stable, and so on. The hard disks nowadays are approaching phyiscal limits in terms of bit density and lifetime. The bits consist of so-called grains, that are very tiny, indepent magnets. To get a decent signal when reading the bits, you need at least 100 of these grains. If you want to make smaller bits, while keeping at least 100 grains, you have to make the grains smaller. However, if the grains are getting smaller, each grain contains less magnetic energy and can therefore more easily switch its magnetization direction. At a certain point, the grains get so small that the thermal energy at room temperature is sufficient to switch the magnetization direction of the grains. This greatly reduces the lifetime of the bits. The MFM can be used to help to understand this process and find a solution.

You would expect industry to jump at the opportunity to get such a piece of innovative equipment.

Yes, you would expect that. This was an STW project, which means that there is a user committee with representatives from industry to whom we submit our results. It is for them to decide whether or not to apply for a patent.

We worked with Nanosensors, for instance. It is one of the largest manufacturer in this area. They were interested, but not very keen on making them a commercial product. Our sensor needs an entire different method of construction, involving huge investments for them.

Were you very disappointed?

No, not really. We demonstrated that our sensor works very well for MFM. Besides of that, other sensors besides a magnetic tip can be integrated on the spring as well. This means that apart from measuring magnetic bits, our sensor can be adapted to suit many other purposes. Others will continue to adapt the sensor for other purposes, such as writing bits or reading with different methods. That is really a completely new field.

You are not going to continue?

No, I got a job with Texas Instruments in Almelo. It took some thought, but I finally decided that I spent enough time at this university (10 years). I did however get an offer to go to France and work on similar techniques in a European project. But you get a postdoc position for two years only and there are hardly any fixed positions at universities available at this time. So I opted for more certainty.

What will your job be?

It is the Texas Instruments’ branch for Automotive Sensors. They are producing sensors measuring pressure and air quality for cars. I’ll be a design engineer there.

What did you like best in your research?

Everything is in your own hands. For a large part it is you who decides the direction of the research. You also make things yourself, you check if it works, and you do it again in order to improve. All these different aspects of the research made it very rewarding. And in the group I worked in, the Systems and Materials for Information storage (SMI), there is a very pleasant atmosphere, the colleagues are also your friends.

Did you have many international or national external contacts for your research?

On the STW user committee, there were several people from industry. Besides two Dutch members, there were also two members from Germany, as scanning probe microscopy is a very big research topic in there. During the project, two Japanese researchers worked in our lab on the sensor. Furthermore, part of the research was carried out in a European project, where we worked together with groups from the U.K. and France.

What didn’t you like about your research?

Working in a cleanroom can be terrible. It is great when it all goes well, but when it doesn’t it is very tedious. You can make an error in the first few days of processing which at a later stage spoils all your work. Besides that, writing my thesis was also not always pleasant. It becomes a routine after a couple of months, making you want to do some ‘real’ work again. But I’m very pleased with the end result.

If I understand you correctly: what you did was very innovative. Don’t you mind that nothing of the honours that may come of it go your way?

Yes, a bit. But on the other hand I am very happy that somebody is going to continue. Two years ago our invention was patented, STW being the owner, and Daan Bijl is going to start up a spin-off company to manufacture and sell these sensors. We have shown that the manufacturing process works; we are very close indeed to a commercial product. It is only a small step. The product is patented, and there are commercial possibilities. There is enough satisfaction in that.