Avoiding amorphization during ion beam irradiation and critical dimension reduction of nanostructures

Ion beam induced collateral damage is becoming an issue in FIB processing, as it limits the
application of ion beams for nanostructure fabrication. This is of special importance for the
application of focused ion beams for nanostructure fabrication.
Here, we present an approach to mitigate the ion beam induced damage inflicted on semi -
conductor nanostructures during ion beam irradiation. Nanopillars (with a diameter of
35 nm and a height of 70 nm) have been irradiated with both, a 50 keV Si + broad beam and
a 25 keV focused Ne + beam from a helium ion microscope (HIM). Upon irradiation of the
nanopillars at room temperature with a medium fluence (2x10 16 ions/cm2), strong plastic
deformation has been observed which hinders further device integration. The shape and
crystallinity has been studied by HIM and TEM. This differs from predictions made by
Monte-Carlo based simulations using the TRI3DYN. However, irradiation at elevated tem-
peratures with the same fluence not only preserves the shape of the nanopillars but allows
for controlled diameter reduction by as much as 50 % without significant change in pillar
height.
It is well known that above a critical temperature amorphization of silicon is prevented in-
dependent of the applied fluence. At high enough temperatures and for not too high flux
this prevents the ion beam hammering and viscous flow of the nanostructures. These two
effects are responsible for the shape change observed at low temperature. We find that ir-
radiation above 650 K preserves the crystalline nature of the pillars and prevents viscous
flow. In addition, a steady thinning process of the nanopillars to a diameter of 10 nm with-
out a significant change in height is observed for higher fluencies at elevated temperatures.
As the original pillar diameter is smaller than the size of the collision cascade, enhanced
forward sputtering through the sidewalls of the pillar is responsible for this pillar-thinning
effect. Results for various ion beam energies, fluencies, fluxes and temperatures will be
presented and compared to TRI3DYN simulations. Such a reliable and CMOS-compatible
process could serve as a potential down scaling technique for large-scale fabrication of
nanostructure based electronics and many other FIB based milling applications.