Development of a transmission X-ray nanometer-resolution microscope based on laboratory light source
Authors: Liao Ke-Liang , He Qi-Li , Song Yang , Li Rong-Gang , Song Mao-Hua , Li Pan-Yun , Zhao Hai-Feng , Liu Peng , Zhu Pei-Ping ,   Ji’nan Key Laboratory of X-ray Optics, Ji’nan Hanjiang Opto-electronics Technology Ltd., Ji’nan 250000, China,   Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
Published: August 2024

Transmission X-ray microscope (TXM) is a high-precision, cutting-edge X-ray imaging instrument, which is a marvel of modern science and technology. It enables non-destructive imaging on a nanoscale, providing a powerful research tool for various scientific fields such as physics, life science, materials science, and chemistry. Although many synchrotron radiation facilities at home and abroad have established nano-CT experimental stations with TXM as the core, currently only a few companies internationally can provide commercial TXM instrument based on laboratory X-ray sources. The primary reason is that this instrument involves numerous engineering challenges, including high-brightness laboratory X-ray sources, high-resolution X-ray optical elements, high-precision sample stage systems, high-sensitivity detectors, and extremely strict requirements for environmental factors such as temperature and vibration. In order to promote the development of high-end X-ray imaging instruments, it is necessary to overcome the technological bottlenecks encountered in the development of X-ray nano-CT. Discussed in this work mainly are the instrument design of a laboratory transmission X-ray microscope with working energy of 5.4 keV and the results of full-field imaging experiments. To start with, the design of the TXM instrument is introduced in detail. The TXM instrument is equipped with several key components, including laboratory X-ray source, condenser, sample stage module, zone plate, and imaging detector. The TXM instrument adopts a modular vibration isolation design and is equipped with a dedicated temperature control system. The main imaging magnifications of the TXM instrument are 50×, 75×, and 100×, and the corresponding optical parameters and photos are introduced. The X-ray source used is a micro-focus X-ray source, operating in Cr target mode, with a focal spot size of 20 μm and a Ka characteristic spectrum brightness of<inline-formula><tex-math id="M1">\begin{document}$ 5\times {10}^{9}~\rm {photons}/({mm}^2\cdot {mrad}^2\cdot s)$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20240727_M1.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20240727_M1.png"/></alternatives></inline-formula>. The X-ray source provides illumination for the sample after being focused by an ellipsoidal condenser. The outer ring of the condenser's illumination ring corresponds to a numerical aperture (<i>NA</i>) of <inline-formula><tex-math id="M2">\begin{document}$ {NA}_{2} = 3.196~\rm mrad $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20240727_M2.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20240727_M2.png"/></alternatives></inline-formula>, and the inner ring corresponds to a numerical aperture of <inline-formula><tex-math id="M3">\begin{document}$ {NA}_{1} = 1.9086~\rm mrad $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20240727_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20240727_M3.png"/></alternatives></inline-formula>. Under these conditions, the limit resolution of this TXM instrument is 22 nm. The zone plate has a diameter of 70μm, a focal length of 8.7mm, and 616 zones. The TXM instrument uses a high-resolution optical coupling detector equipped with a scientific-grade CMOS camera with an effective pixel size of 7.52μm. The optical coupling detector is equipped with 2× and 10× high numerical aperture objectives. When the TXM instrument magnification is 50×, the effective pixel size of the TXM instrument is 15 nm. In addition <b>,</b> a gold resolution test card is used as the sample to determine the imaging field of view of the TXM instrument by observing the size of the imaging area of the test card on the detector, and to determine the imaging resolution of the TXM instrument by observing the line width of the star-shaped target in the center of the test card. Experimental results show that the TXM instrument has an imaging field of view of 26μm and can achieve the clear imaging of characteristic structure with a line width of 30 nm. The radial power spectrum curve of the Siemens Star shows this TXM instrument has the potential to resolve 28.6-nm half pitch line pair features. Finally, we draw some conclusions and present outlook. At present, imaging of 30-nm-wide line features has been realized, but the imaging of 30-nm half pitch line pair feature has not yet been achieved, and the limit resolution has not reached the design value, either. We will continue to explore the potential for upgrading the imaging resolution of the laboratory TXM in future work.