Flexibly shaped mirror improves X-ray microscopes

Japanese researchers have engineered a mirror for X-rays that can be flexibly shaped, resulting in high levels of precision at the atomic level and increased stability. The new technology, described in the journal Optica, improves the performance of X-ray microscopes and other technologies that use X-ray mirrors.

An X-ray microscope is an advanced imaging tool that bridges the gap between electron and light microscopy by using X-rays, which can provide better resolution than light and penetrate samples too thick for electrons to penetrate; this allows for the imaging of structures that are difficult to see with other microscopy techniques. X-ray microscopes have high resolution, which makes them especially valuable in fields such as materials science and biology because they can observe the composition, chemical state and structure inside a sample.

Mirrors play a vital role in X-ray microscopes as they reflect X-ray beams, allowing for high-resolution imaging of complex structures. However, the small wavelength of X-rays makes them vulnerable to distortion from minor manufacturing flaws and environmental influences. This creates wavefront aberrations that can limit image resolution. Satoshi Matsuyama and his colleagues at Nagoya University, in collaboration with RIKEN and JTEC Corporation, solved this problem by creating a mirror that can deform, adjusting its shape according to the detected X-ray wavefront.

To optimise their mirror, the researchers looked at piezoelectric materials. These materials are useful because they can deform or change shape when an electric field is applied. This allows the material to reshape itself to respond to even minor aberrations in the detected wave.

After considering various compounds, the researchers chose a single crystal of lithium niobate as their shape-changeable mirror. Single-crystal lithium niobate is useful in X-ray technology because it can be expanded and contracted by an electric field and polished to make a highly reflective surface. This allows it to serve as both the actuator and the reflective surface, simplifying the device.

“Conventional X-ray deformable mirrors are made by bonding a glass substrate and a PZT plate; however, joining dissimilar materials is not ideal and results in instability,” Matsuyama said. “To overcome this problem, we employed a single crystal piezo material, offering exceptional stability because it is made of a uniform material without bonding. Due to this simple structure, the mirror can be freely deformed with atomic precision. Moreover, this precision was maintained for seven hours, confirming its extremely high stability.”

When they tested their new device, the team found that their X-ray microscope exceeded expectations. Its high resolution makes it especially suitable for observing microscopic objects, such as semiconductor device components. Compared to the spatial resolution of conventional X-ray microscopy (typically 100 nm), their technique has the potential to develop a microscope that provides a resolution about 10 times better (10 nm) because the aberration correction brings it closer to the ideal resolution.

X-ray microscopic images showing the higher resolution using the new deformable mirror; the left and right were obtained before and after shape correction, respectively. Image credit: Matsuyama lab, Nagoya University.

“This achievement will promote the development of high-resolution X-ray microscopes, which had been limited by the precision of the fabrication process,” Matsuyama said. “These mirrors can be applied to other X-ray apparatus, such as lithography devices, telescopes, CT in medical diagnostics and X-ray nanobeam formation.”

Top image caption: The new type of deformable mirror for X-ray microscope, which achieved high image resolution by wavefront correction. Image credit: Matsuyama lab, Nagoya University.