New imaging technique at nanometer scale is based on ultrasound

Existing non-destructive imaging techniques for nanoelectronics such as optical and

electron microscopy are not accurate enough andapplicable to deeper structures. A well-known three-dimensional technique at the macro level is ultrasound. Its advantage is that it works for every sample. This makes ultrasound an excellent way to map three-dimensional structure. Yet ultrasound technology at the nanoscale did not exist until now. Indeed, the resolution of ultrasound imaging is largely determined by the wavelength of the sound used and is typically around a millimeter. In turn, the nanoscale implies a range of particle sizes from 1 to 100 nm. At the same time, a nanometer is equal to one billionth of a meter, and a millimeter is equal to one thousandth.

Today ultrasound is already integratedin atomic force microscope (AFM). AFM is a technique that can very accurately scan and map surfaces using a tiny needle. The advantage here is that it is not the wavelength, but the size of the AFM tip that determines the resolution. Unfortunately, the frequencies used so far (1–10 MHz) are not enough. “We do see something, but it’s not entirely clear what it is. Therefore, the frequency of the sound used had to be further increased to the GHz range. This is what we did,” explains Gerard Verbiest from TU Delft.

Increasing the frequency has only recently become possible.The use of photoacoustics helped. The use of the photoacoustic effect generates extremely short sound pulses. Scientists have succeeded in integrating this technique into AFM. Using the AFM tip, the scientists managed to focus the signal. The installation has already passed preliminary tests.

As mentioned, the new method especiallyinteresting for nanoelectronics. In the future, this will help to make even smaller chips with fine patterns. For example, so that you can place two layers on top of each other with nanometer precision.

There are also potential applications foroutside of electronics. For example, in cell biology to create a detailed three-dimensional image of a single living cell. This will allow you to see how mitochondria fold in the cell. In materials science, the development will be useful for studying the process of heat transfer in graphene. 

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