Imaging and milling resolution of light ion beams from helium ion microscopy and FIBs driven by liquid metal alloy ion sources.

Wolfgang Pilz,Paul Mazarov,Fabian Meyer,Lothar Bischoff,Gregor Hlawacek,Nico Klingner

doi:10.3762/bjnano.11.156

Abstract

While the application of focused ion beam (FIB) techniques has become a well-established technique in research and development for patterning and prototyping on the nanometer scale, there is still a large underused potential with respect to the usage of ion species other than gallium. Light ions in the range of m = 1–28 u (hydrogen to silicon) are of increasing interest due to the available high beam resolution in the nanometer range and their special chemical and physical behavior in the substrate. In this work, helium and neon ion beams from a helium ion microscope are compared with ion beams such as lithium, beryllium, boron, and silicon, obtained from a mass-separated FIB using a liquid metal alloy ion source (LMAIS) with respect to the imaging and milling resolution, as well as the current stability. Simulations were carried out to investigate whether the experimentally smallest ion-milled trenches are limited by the size of the collision cascade. While He+ offers, experimentally and in simulations, the smallest minimum trench width, light ion species such as Li+ or Be+ from a LMAIS offer higher milling rates and ion currents while outperforming the milling resolution of Ne+ from a gas field ion source. The comparison allows one to select the best possible ion species for the specific demands in terms of resolution, beam current, and volume to be drilled.

Highlights

In modern nanotechnology, focused ion beam (FIB) techniques are well-established for nanoscale structuring, local surface modification, doping, prototyping, as well as for ion beam analysis
Helium and neon ion beams from a helium ion microscope are compared with ion beams such as lithium, beryllium, boron, and silicon, obtained from a mass-separated FIB using a liquid metal alloy ion source (LMAIS) with respect to the imaging and milling resolution, as well as the current stability
The introduction of the helium ion microscope (HIM) [4], working with a gas field ion source (GFIS), about ten years ago solved a lot of problems in this field

Summary

Introduction

In modern nanotechnology, focused ion beam (FIB) techniques are well-established for nanoscale structuring, local surface modification, doping, prototyping, as well as for ion beam analysis. In a LMAIS, in contrast, the source material is a complex alloy delivering several ion species simultaneously in a process of field evaporation This initially provides a beam containing ions with different masses and charge states, whose fractions depend on the composition of the alloy and the ionization probability. This is important to determine the chromatic aberration, which strongly influences the achievable resolution of the FIB. For measuring the trench width milled into the gold layer, either the same primary ion beam microscope or a scanning electron microscope have been used to image the Figure 2: Mass spectrum of a Ga33Bi57Li10 LMAIS at an acceleration potential of 10 kV scanned by the ExB voltage using a constant magnetic field. Details of fabrication and operation principle are given in the corresponding references

Results and Discussion

Conclusion

15. Calculated r50 probe size He GFIS beam