Helium Ion Microscopy: A high resolution tool for the nano-world

Helium Ion Microscopy (HIM) [1] utilizes a subnanometer He+ beam to scan the sample surface. Besides the name giving helium also the heavier neon is available in the latest generation of tools. High-resolution imaging—also of uncoated insulating samples—has become a routine task in HIM. However, the combination of neon and helium also allows for efficient and precise milling of nano-structures. In HIM several signals are available to form an image and obtain information on sample properties. Besides secondary electrons and backscattered helium atoms also photons can be utilized. The latter phenomenon is referred to as ionoluminescence (IL).
We will present examples of high resolution imaging and nano-fabrication always highlighting the important physical aspects relevant for the contrast generation. Dechanneling contrast [2], subsurface imaging and the high surface sensitivity [3] of the method will be discussed. Particular emphasis will be given to the role of defects created by the energetic ion beam and how they can be exploited. We used IL together with HIM to understand the creation of defects and their behaviour in ionic crystals [4]. IL has also been used to directly measure the surface projected interaction volume of the beam with sodium chloride. The results are compared to different theoretical calculations. We find a minimum density of 3 emission centres per nm² is needed for a detectable IL signal [5].
Acknowledgements:
This research is supported by the Dutch Technology Foundation STW, which is the applied science division of NWO, and the Technology Programme of the Ministry of Economic Affairs.
References:
[1] Gregor Hlawacek, et al. Helium ion microscopy. J. Vac. Sci. Technol. B, 32(2):020801, (2014).
[2] Hlawacek, G. et al. Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism. Beilstein J. Nanotechnol. 3, 507–512 (2012).
[3] Hlawacek, G., Ahmad, I., Smithers, M. A. & Kooij, E. S. To see or not to see: Imaging surfactant coated nano-particles using HIM and SEM. Ultramicroscopy 135C, 89–94 (2013).
[4] Vasilisa Veligura, Gregor Hlawacek, R van Gastel, Harold J. W. Zandvliet, and Bene Poelsema. A high resolution ionoluminescence study of defect creation and interaction. J. Phys. Condens. Matter., 26(16):165401, (2014).
[5] Vasilisa Veligura, Gregor Hlawacek, Uwe Jahn, Raoul van Gastel, Harold J. W. Zandvliet, and Bene Poelsema. Creation and physical aspects of luminescent patterns using helium ion microscopy. J. Appl. Phys., 115(18):183502, (2014).