Increasing the resolution of X-ray microscopy

X-ray microscopes have the advantage of penetrating most substances, so internal organs and skeletons can be observed non-invasively through chest X-rays or CT scans, but they are not so good at imaging at the nanoscale. Dr KyeoReh Lee and colleagues at the Korea Advanced Institute of Science and Technology (KAIST) have now used the results of previous studies to solve the lingering problem of how to overcome the resolution limitations of existing X-ray microscopes. Their research has been published in the journal Light: Science and Applications.

In an X-ray microscope, a circular grating called a concentric zone plate is used instead of a lens. The resolution of an image obtained using the zone plate is determined by the quality of the nanostructure that comprises the plate. There are several difficulties in fabricating and maintaining these nanostructures, which set the limit to the level of resolution for X-ray microscopy. The KAIST researchers developed a new X-ray nanomicroscopy technology to overcome this problem.

The X-ray lens proposed by the research team is in the form of numerous holes punched in a thin tungsten film, and generates random diffraction patterns by diffracting incident X-rays. The team mathematically identified that, paradoxically, the high-resolution information of the sample was fully contained in these random diffraction patterns, and actually succeeded in extracting the information and imaging the internal states of the samples. The imaging method using the mathematical properties of random diffraction was first proposed and implemented in the visible light band by Lee and colleague Professor YongKeun Park back in 2016.

The resolution of the image of the constructed sample has no direct correlation with the size of the pattern etched on the random lens used. Based on this idea, the research team succeeded in acquiring images with 14 nm resolution (approximately 1/7th the size of the coronavirus) by using random lenses made in a circular pattern with a diameter of 300 nm.

“In this study, the resolution was limited to 14 nm, but if the next-generation X-ray light source and high-performance X-ray detector are used, the resolution would exceed that of the conventional X-ray nanoimaging and approach the resolution of an electron microscope,” said Lee, who served as a co-corresponding author on the new study.

“Unlike an electron microscope, X-rays can observe the internal structure without damaging the sample, so it will be able to present a new standard for non-invasive nanostructure observation processes such as quality inspections for semiconductors.”

Co-corresponding author Dr Jun Lim, from the Pohang Accelerator Laboratory, added, “In the same context, the developed image technology is expected to greatly increase the performance in the fourth-generation multipurpose radiation accelerator, which is set to be established in Ochang of the Northern Chungcheong Province.”

Image caption: Images taken from the proposed randomness-based X-ray imaging (bottom) and the corresponding surface electron microscope (SEM) images (top).