Polyatomic Focused Ion Beams – Origin and Applications

In the last four 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 the FIB systems works 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 slight penetration depth, enhanced sputtering efficiency and the huge energy deposition due to the simultaneous impact of several atoms in the same point of the surface.
Beside others heavy elements or alloys, those containing Au but in particular Bi are very appropriate for the emission of polyatomic ions. Such projectiles with masses up to about 1000 amu have an energy spread in the range of ΔEFWHM = 30 … 150 eV, which restrict the final FIB resolution (spot size) due to chromatic aberration to 10 to 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 on inorganic as well as organic specimen [2]. A second 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 energy transfer by the cluster ions to the surface a self-organization process of hexagonal ordered dot arrays on Ge and Si could be found surprisingly for pure elemental targets at normal incidence, described by the formation of tiny melt pools [3] shown in the figure.

[1] L. Bischoff, Nucl. Instr. Meth. B 266 (2008) 1846.
[2] F. Kollmer, Appl. Surf. Sci. 231-232 (2004) 153.
[3] R. Böttger, L. Bischoff, K.-H. Heinig, W. Pilz and B. Schmidt, JVST B 30 (2012) 06FF12.