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Scientists map invisible materials at the nanoscale with ultrasound


Researchers Gerard Verbiest, Ruben Guis en Martin Robin. Credit: Delft University of Technology

The increasing miniaturization of electrical components in industry requires a new nanometer-scale imaging technique. The Delft researcher Gerard Verbiest has developed a first proof-of-concept method together with ASML, which they are now going to further develop. The method uses the same principle as a pregnancy ultrasound scan, but on a much, much smaller scale.

Ultrasound

'The existing non-destructive techniques for imaging nanoelectronics, such as optical and electron microscopy, are not accurate enough or are not applicable for deeper-lying structures,' explains Gerard Verbiest of the 3mE Faculty of Delft. 'A well-known 3D technique on a macro scale is ultrasound. The advantage is that it works for every sample. Ultrasound is therefore a very good way of mapping the 3D structure of a non-transparent sample in a non-destructive way. '

Nevertheless, there was no nanoscale ultrasound technology yet. After all, the resolution of ultrasound imaging is strongly determined by the wavelength of the sound used, which is typically around a millimeter.

AFM

'To improve this, ultrasound has already been integrated into a so-called Atomic Force Microscope (AFM)', Verbiest continues. 'AFM is a technique that allows you to scan and map surfaces very precisely with a tiny needle. This has the advantage that it is not the wavelength but the size of the tip of the AFM that determines the resolution. But unfortunately we see that at the frequencies used so far (1-10 MHz), the response of the AFM is small and unclear. We do see something, but we don't really know exactly what we see. So the frequency of the sound used had to be increased further, to the GHz regime, and that's what we did. '

This increase in frequency has only recently been possible, Verbiest explains. 'We achieve this with photo acoustics. By using the photo-acoustic effect, you can generate extremely short sound pulses. We are integrating this technique into an AFM. With the tip of the AFM we can focus the signal. Our setup is ready and the first tests have been done.

Cell biology

As mentioned, the new method is especially interesting for nanoelectronics. 'If you want to be able to make even smaller chips with even smaller patterns in the future, you have to take this step,' says Verbiest. 'For example to make it possible to lay two layers on top of each other with nanometer precision.'

'But there are certainly also possible applications outside of electronics. In cell biology, you could use this to create a detailed 3D picture of a single living cell, for example of the way mitochondria are folded in a cell. And in materials science you can think of research into heat transport in a wonderful material such as graphene. '

Fast

Verbiest has achieved rapid success. 'A post-doc researcher has been working on this project since April last year and a PhD student since October. So within eight months we succeeded in taking the first measurements with our setup and we will expand this further in the coming period. In time, ASML, which also owns the intellectual property, will take over the research, hopefully on the way to industrial application of the new method. But that of course depends on the results achieved.

Nano Tv