Field ion microscopy has a long history, going back to Mueller’s 1951 paper in Z. Phys.. A major advance leading to practical sub-nanometer application came in 2006 with the appearance of the ALIS paper in JVST B. This paper revealed the tri-atomic ionization source, which permits high brightness and very small source size.
Hlawacek et al. at Twente posted a review of helium ion microscopy to the arXiv yesterday. Despite its brevity (16 pages) this review covers many theoretical and practical aspects. With 136 references, it provides a jumping-off point for anyone wanting a serious introduction to this technology.
G Hlawacek, V Veligura, R van Gastel, and B. Poelsema, “Helium Ion Microscopy”, arXiv:1311.1711v1