| AVS 65th International Symposium & Exhibition | |
| Advanced Ion Microscopy Focus Topic | Wednesday Sessions |
| Session HI-WeA |
| Session: | Novel Beam Induced Material Engineering & Nano-Patterning |
| Presenter: | John A. Notte, Carl Zeiss Microscopy, LLC |
| Authors: | J.A. Notte, Carl Zeiss Microscopy, LLC B.B. Lewis, Carl Zeiss Microscopy, LLC |
| Correspondent: | Click to Email |
The term “FIB Renaissance” has been applied to the recent period of ion source development which has brought forth many new species suitable for focused ion beam (FIB) instruments. Several of the new species are relatively light ions, including hydrogen, helium, lithium, and neon, which are appreciably lighter than the prevailing gallium FIB – by a factor of 3 or more. At the conventional energies (5 to 30 keV) these ions species interact with the sample differently, and warrant a reconsideration of the established understanding which is largely founded on the traditional gallium FIB.
The most marked distinction of these light ions is the ratio of electronic stopping power compared to nuclear stopping power. For example, for a 30 keV helium ion, the nuclear stopping power can be a decade lower than its electronic stopping power. While for 30 keV gallium, the nuclear stopping power is a decade higher than the electronic stopping power. Consequential to this, near the surface the light ions remain relatively collimated because Mion >> Melec, making angular deflections necessarily small. As the light ions gradually penetrate deeper, they lose their energy, and the electronic stopping power is correspondingly reduced until the nuclear stopping power dominates. Here, large angular deflections become dominant, and the majority of the lattice damage takes place at these greater depths for light ions. For the special case of thin films, nuclear stopping might never become predominant for light ions.
The heat transfer mechanisms are even more drastically different when comparing light ions to heavier ions. First, by virtue of their large penetration depth, the light ions have a larger volume in which their energy is dissipated – reducing the corresponding temperature rise. But more significantly, the light ions lose most of their energy through excitations to the electrons. These excited electrons have characteristic mean free paths which can be relatively long, providing an effective pathway for energy transfer to a much larger volume. Whereas for nuclear stopping power, the ion’s energy is transferred to the lattice much more locally. And since nuclear stopping is predominant for heavy ions, the energy is necessarily deposited locally, giving rise to appreciably higher temperature. Further, for the special case of thin films, the temperature rise from light ions is further reduced. Lastly, a special case of low beam currents is considered, where the time interval between ion arrivals may sometimes be longer than the time scale for thermal relaxation. This gives rise to non-overlapping temperature spikes which can be independent of probe current.