Development of a Transmission X-ray Microscope with Nanometer Resolution Based on Laboratory X-ray Source

Abstract

The transmission X-ray microscope (TXM) is a high-precision, cutting-edge X-ray imaging instrument and a marvel of modern science and technology. It enables non-destructive imaging at the nanoscale, providing a powerful research tool for various scientific fields such as physics, life sciences, materials science, and chemistry. Although many synchrotron facilities domestically and internationally have established Nano-CT experimental stations based on TXM, currently only few companies worldwide can offer 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 system, high-sensitivity detectors, and extremely strict requirements for environmental factors such as temperature and vibration. To push the development of high-end X-ray imaging instruments, it is necessary to overcome the technical bottlenecks encountered in the development of X-ray Nano-CT. This paper mainly discusses the instrument design of a laboratory transmission X-ray microscope with a working energy of 5.4keV and the results of full-field imaging experiments. Firstly, 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 50X, 75X, and 100X, and the optical path parameters and physical photos of the instrument at these three magnifications 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 5*10<sup>9</sup> <i>photos</i>/<i>mm</i><sup>2</sup>/<i>mrad</i><sup>2</sup>/s. 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 (NA) of <i>NA</i><sub>2</sub>=3.196<i>mrad</i>, and the inner ring corresponds to a numerical aperture of <i>NA</i><sub>1</sub>=1.9086<i>mrad</i>. Under these conditions, the TXM instrument's limit resolution is 22nm. 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 2X and 10X high numerical aperture objectives. When the TXM instrument magnification is 50X, the effective pixel size of the TXM instrument is 15nm. Secondly, a gold resolution test card was 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 clear imaging of line features with 30nm width. The radial power spectrum curve of the Siemens star test card imaging results shows that the TXM instrument's limit resolution is 28.6nm. Finally, we close with a conclusion and outlook. Currently, imaging of line features with 30nm width has been achieved, but the imaging of line-pair features with 30nm half pitch has not yet been achieved, and the limit resolution has not reached the design value. We will continue to explore the potential for upgrading the imaging resolution of the laboratory TXM in future work.