Looking inside 3D nanostructures without a trace: the good, the bad and the ugly

19-12-2019

Discovering what is inside a three-dimensional (3D) nanostructure, is no longer a matter of cutting slices, and in this way destroying its functionality. Scientists from the University of Twente and the European Synchrotron (ESRF) have discovered a new method to look deep inside a 3D nanostructure. This ‘traceless X-ray tomography’ (TXT) can be used for photonics, electron chips and memories, for example. The results are published in the ‘ACS Nano’ journal.

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Today, nano-structures are complicated three-dimensional ‘buildings’, for example for manipulating light. Silicon electronics chips, as well, consist of multiple layers of material and interconnecting wires. But how do architects know if the full structure matches their expectations? Looking through a microscope, only the surface can be checked. At this surface level, a ‘good, bad or ugly’ sample may look perfectly the same. One way of looking deeper inside is cutting the material in thin slices and examining those. Unfortunately, it then loses its functionality. 

Although X-ray technology has been available for a long time, it is sometimes still necessary to open up a sample for reaching sufficient depth and contrast. The new traceless X-ray tomography (TXT) makes use of a higher X-ray energy level, making it possible to look into samples that have a silicon thickness of over 1 millimetre. The principal author of the ACS Nano paper, Diana Grishina, of the University of Twente, explains the advantages of the new technique: “In modern nanotechnology this is plenty sufficient to image through wafers. Indeed, all silicon devices remained untouched and ‘as is’ during our study. This is up to 20 times deeper than using existing technologies!” The new method also makes it possible to zoom-in on a desired region.

The experiments were carried out on beamline ID16A, with a beam focused to a spot of 23 nanometer by 37 nanometer. The sample rotated to create very sensitive transmission images from different viewpoints. At the end, intensive processing was done to combine all the separate images in one 3D image. Peter Cloetens, beamline responsible for ID16A, says: “One key feature of our TXT study is the use of X-rays with a much higher photon energy than before. Therefore, the attenuation length for silicon is 640 µm, which is 9 to 20× greater than before, and sufficient to traverse wafer-thick silicon substrates.”

As an example, the researchers took a so-called photonic bandgap crystal, a recent breakthrough in photonics. Its functionality depends on many deep pores in two directions, creating cavities in which light can be manipulated. Although at surface level the structures seem identical, looking inside only one of the three is ‘good’. Another has a large void inside due to an error in the manufacturing process: it is ‘bad’. A third one even lacks a 3D structure inside and is ‘ugly’: the pores are too shallow. According to the leader of the research team, Willem Vos, “TXT serves to non-destructively differentiate between the possible reasons of not finding the designed and expected performance. This is why we think that TXT is an original and powerful tool to critically assess 3D functional nanostructures.”

The Extremely Brilliant Source of the ESRF will improve this technique: “The EBS will obviously increase the through-put for this kind of measurements, but also the resolving power and image quality will be further improved by using even higher X-ray energy beams with sufficient coherence”, concludes Peter Cloetens. 

The research was done in the Complex Photonics Group of UT’s MESA+ Institute, together with the European Synchrotron Radiation Facility in Grenoble, France. It was made possible by the NWO programme ‘Stirring Light’, the Shell/NWO programme ‘Computational Sciences for Energy research’, the Descartes-Huygens Prize of the French Academy of Sciences, and contributions of MESA+ (Applied Nano Photonics) and ESRF (beamtime grants).

Reference:

Grishina, DA., et al,  ACS Nano 2019, https://doi.org/10.1021/acsnano.9b05519

Text and video by Montserrat Capellas Espuny.

Top image: Birds-eye view of reconstructed a 3D photonic crystal that reveals a broad photonic gap in agreement with theory: “the Good”. Credits: ACS Nano 2019.