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[Featured Article] The recent development of soft x-ray interference lithography in SSRF

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Release Date: 2020-03-26 Visited: 

REVIEW  OPEN ACCESS                                                                                                Read More

Jun Zhao, Shumin Yang, Chaofan Xue, Liansheng Wang, Zhaofeng Liang, Lei Zhang, Yong Wang, Yanqing Wu and Renzhong Tai

1. Article Guide

Synchrotron radiation not only plays an important role in the research of physics, materials, life sciences, etc., but also plays its own unique role in the extreme manufacturing field. Prof. Yanqing Wu introduced the recent progress in extreme manufacturing technology of Shanghai Synchrotron Radiation Facility (SSRF) with soft X-ray interference lithography (XIL) as the core. In the field of micro/nanomanufacturing, XIL is a unique parallel fabrication technique independent of mainstream extreme manufacturing techniques. It focuses on the manufacture of strictly periodic patterns. Moreover, large areas of high-resolution nanostructures can be achieved efficiently. This technology has been widely used in research fields such as extreme ultraviolet photoresist evaluation, nano-optics and nano-magnetism. Prof. Yanqing Wu, Prof. Renzhong Tai, Jun Zhao, Shumin Yang, Chaofan Xue, from the Shanghai Institute of Applied Physics, Chinese Academy of Sciences / Shanghai advanced Research Institute, Chinese Academy of Sciences, published a review of "The recent development of soft X-ray interference lithography in SSRF" in "International Journal of Extreme Manufacturing" (IJEM) , which systematically introduced the research background, the latest progress and the future outlook of soft X-ray interference lithography at SSRF-XIL beamline.

2. Research Background

Soft X-ray interference lithography (XIL) technique is a novel micro/nano manufactruing technique that uses the interference fringe of spatial coherent soft X-ray beams to expose the photoresist. XIL has the characteristics of strict periodicity, large-area fabrication, large depth of focus, and no need for substrate conduction. The XIL beamline (BL08U1B) in SSRF applies a high brilliance undulator source and an achromatic diffraction scheme to obtain high quality interference fringes and practical throughput for a variety of scientific research and industrial applications. Internationally, the XIL-II beamline of the Swiss Light Source and the BL-9 beamline of the new SUBARU light source have also performed a lot of related research based on this technique. Professor Yanqing Wu gave a detailed introduction to the recent progress of SSRF XIL beamline in the development of this technique.

3. The Latest Progress

The article mentioned that: at the XIL beamline, a laser interferometer was installed on the exposure system to monitor the displacement between the mask stage and the wafer stage in the horizontal and vertical directions, thus realizing a precise real-time vibration evaluation system, as shown in figure 1. Based on this evaluation system, we determined we could monitor and further improve the stability of the exposure system and the surrounding environment and selected appropriate experimental conditions. In the current experiment, the relative position fluctuation of the mask and the sample can be controlled below 2.5 nm root mean square during a ten-minute exposure, which provides a good experimental guarantee for the subsequent XIL experiment. Subsequently, we developed experimental methods such as interference lithography with high-order diffraction beams, large-area stitching deep exposure, and parallel direct-write Talbot lithography. We improved the line station's exposure resolution, exposure depth, and exposure area, and also achieved the exposure of sub-micron periodic patterns composed of  complex cells.


Figure 1. The schematic and experimental setup of the vibration evaluation system. 

Interference lithography with high-order diffraction beams

The high-order diffraction interference lithography method is performed by using second-order or above-order diffracted beams, and the exposure pattern period can be reduced by 1/4 of the original mask lithography period, as shown in figure 2. This technology has been used for EUV photoresist evaluation.


Figure 2. (a) The principle of higher order diffraction interference lithography technology and (b) the scanning electron microscopy (SEM) image of the photoresist pattern after second order diffraction interference lithography with about 25 nm half-pitch


Large-area stitching exposure method capable of deep exposure

The large-area stitching exposure method capable of deep exposure is a stitching exposure technique based on high-order harmonic inline alignment, as shown in figure 3. Using this method, users can quickly fabricate large-area (square centimeter-level) nanoperiod patterns on substrates such as silicon (Si), yttrium aluminum garnet (YAG), and silicon dioxide (SiO2).


Figure 3. (a) The principle of large-area stitching deep exposure and (b) the corresponding experimental setup (c) exposure results


Parallel direct writing achromatic Talbot lithography (ATL)

The principle and preliminary experimental results of DW-ATL are shown in figure 4. The light spot arrays obtained by ATL are employed as the basic exposure units. Periodic patterns with complex cells can be achieved by scanning the wafer stage with nanometer precision.


Figure 4. (a) The principle of achromatic Talbot lithography (ATL) and an ATL mask consists of a hexagonal chromium (Cr) lattice with a 520 nm pitch and 130 nm hole diameter. (b) A 520 nm pitch lattice with a spot diameter of approximately 50 nm obtained directly from ATL, and further complex patterns obtained by direct writing achromatic Talbot lithography (DW-ATL) scanning.

4. Future Outlook

EUV photoresist is an important part of EUV lithography technology, and it is also one of the domestic research hotspots and one of the "bottleneck" technologies that need urgent solution. EUV interference lithography based on a 92.5 eV synchrotron radiation source is recognized as the most effective EUV photoresist evaluation tool, which can greatly accelerate the development of EUV photoresists. The XIL beamline at SSRF has established a complete evaluation platform for EUV photoresist sensitivity, resolution, line edge roughness, and outgassing analysis. A large amount of EUV photoresist research work has been conducted based on the platformshown in figure 5.


Figure 5. Performance evaluation of EUV photoresists based on SSRF-XIL beamline


5. Author Introduction


Prof. Yanqing Wu has long been engaged in the research of soft X-ray interference lithography technology, synchrotron radiation beamline theory and technology research. His research fields also include X-ray imaging detector research. He is the principal of SSRF XIL beamline, and currently undertakes and participates in a number of key R & D projects of the National Natural Science Foundation and the Ministry of Science and Technology.

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