Surface modification with focused polyatomic ion beams

In the last decades Focused Ion Beams (FIB) have evolved from a sophisticated idea to a distinguished standard technique for sample preparation for SEM and TEM, prototyping in research and development and analytics in fields like microelectronics or nanotechnology. Most of FIB systems work with Ga beams, but liquid metal ion sources (LMIS) provide a much broader spectrum of other ion species using different source materials and an ion optical column equipped with an ExB mass separator [1]. From the source tip, beside single and double charged monatomic ions also dimers, trimers and heavier projectiles are extracted, which play an increasing role due to their special properties, like shallow penetration depth, enhanced sputtering efficiency and the huge energy deposition due to the simultaneous impact of several atoms in the same point of the sample surface.
Beside others heavy elements or alloys, those containing Au but in particular Bi are very suitable for the emission of polyatomic ions. Such projectiles with masses up to about 1000 amu have an energy spread in the range of EFWHM = 30 … 150 eV, which restrict the final FIB resolution (spot size) due to chromatic aberration to 10 … 100 nm. This is a result of the complex appearance of polyatomic species in the area around the emission point.
One of the main application fields at present is SIMS, which increasingly works with polyatomic Bi beams for defined surface erosion of inorganic as well as organic specimens. Another exciting field of application is the surface modification in terms of surface patterning by heavy dimer and trimer ions (e.g. Aunm+, Binm+). Due to the enormous, surface-near energy density deposition of cluster ions a self-organization process of hexagonally ordered dot arrays on Ge and Si based on transient melt pool formation can be triggered (Fig. 1) [2].
The authors would like to thank S. Facsko and B. Schmidt from HZDR for fruitful contributions to this work.

[1] L. Bischoff, Nucl. Instr. Meth. B 2008, 266, 1846.
[2] R. Böttger, L. Bischoff, K.-H. Heinig, W. Pilz, B. Schmidt, J. Vac. Sci. Technol. B 2012, 30, 06FF12.