The ability to machine and polish materials controllably on the micro- and nanoscale, and to observe the morphological evolution of the products being fabricated, is essential for today’s advances in nanotechnology. Recently, state-of-the-art helium ion microscope (HIM) has become a popular tool in this regard due to its high resolution, non-contamination and ability of precise nanofabrication. One hitherto unexplored problem, however, is the potential alteration of the structure and properties of the small-volume material being micro-machined or examined.
Recently, the PhD student Yuecun Wang of CAMP NANO, XJTU-MSE used silicon as a prominent case study and utilized state-of-the-art in situ quantitative nanomechanical testing system, to demonstrate the dramatic effects of helium ion irradiation. Structurally, a sub-micron Si pillar can turn completely amorphous at helium ion doses typically used for micromachining, forming nanobubbles at higher doses. In terms of mechanical properties, the flow stress decreases markedly with the increasing dosage, and the softened amorphous Si exhibits spread-out plastic flow. These findings are important, considering the wide application of Si in computer and MEMS devices, and the increasing use of nanotechnologies in these industries.
The structure (left) and mechanical behaviors (right) change of Si micropillars with the increasing helium ions dosage
The project is supervised by Prof. Zhiwei Shan and Prof. Evan Ma. Besides Dr. Lin Tian and Dr. Yuanbin Qin from our faculty, Prof. Jingtao Wang and his PhD student Fan Liu, Gong Zheng from Nanjing University of Science and Technology also made a significant contribution to this work. This work has been published on the top research journal, Small (Impact factor 8.315).
近年来，氦离子显微镜因其较高的纳米级成像质量、精细的加工能力及损伤小污染少等优点而在当今纳米科技中异军突起，受到了广泛关注。但是迄今为止，氦离子显微镜对样品的结构和性能造成的可能影响还从未被报导过。
为了填补这方面的研究空白，西安交大材料学院微纳中心博士生王悦存在导师单智伟教授的指导下以当今半导体工业的支柱材料单晶硅为对象，借助先进的原位定量纳米力学测试技术，系统地研究了氦离子显微镜在纳米加工过程中离子剂量对样品结构及力学性能造成的影响。研究发现，在正常的加工条件下，氦离子就能将晶体硅完全地非晶化，并且随着离子剂量的增加，样品中开始出现氦气泡，并且样品结构明显发生疏松化。结构的变化也造成了材料性能的变化：随着离子剂量增加，硅微柱的屈服强度显著降低并且与剂量成线性负相关。这项研究成果对氦离子显微镜的进一步发展及其今后在半导体工业上的应用具有一定的指导意义。
上述研究成果近日在线发表于材料学国际知名期刊Small（影响因子8.315）上。(文章链接：http://onlinelibrary.wiley.com/doi/10.1002/smll.201601753/full。西安交通大学是第一作者与第一通讯单位。参与该工作的还有微纳中心的田琳博士、秦元斌博士、南京理工大学的王经涛教授以及其博士生刘帆、郑功。该研究得到了国家自然科学基金（51231005，51321003及51501144）和中国博士后自然基金（2015M580842）的支持。
