High-resolution direct-write patterning using focused ion beams
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Introduction The first focused ion beam (FIB) tools were built with the intent of direct-write milling and to expose resist,1–3 later evolving into an analytical tool as is mostly used today. Since then, FIB usage has exploded into a myriad of applications. A focused ion beam tool consists of an ion source that is at a voltage relative to ground, where the ions are accelerated down a tubular section that contains electrostatic optics used to focus and steer the beam of ions onto a sample in a chamber. The entire time the ions and sample are kept under high vacuum that can range from 10–9 Torr to 10–6 Torr. The reason for the high vacuum is to allow the ions to reach the sample without scattering off any other gas atoms that may get in the way. For a review on the wide range of ion beam micromachining applications, see the 2007 issue of MRS Bulletin,4 the article by Allen,5 and the Introductory article in this issue. This article discusses three main areas of advancement in ion beam direct-write technology: ion beam lithography and micromachining, gas-assisted etching applications, and novel ion sources and applications.
Ion beam lithography and micromachining Direct-write ion beam lithography (DWIBL) started as an alternative to electron beam lithography, exposing polymer films, known as resists, to ions in the keV6 or MeV energy range.7
The key feature highlighted by resist-based ion beam lithography is the relatively limited ion scattering when compared to electron scattering in electron beam lithography. Recent work on multiple ion sources shows that the scattering limit is only a few nanometers, see Figure 1.8 Resist-based lithography further evolved to projection ion beam lithography, where stencil masks and sophisticated projection optics were used to increase exposure throughput.9,10 Most DWIBL work was performed using either a scanning beam or an ion projection approach, almost always based on Ga+ ion sources except for a few studies involving lighter ions such as H+, H2+, Be+, and He+, as reviewed in articles by Melngailis et al.11,12 Modern FIB/SEM dual beam systems successfully combine the capabilities of particle beams into a powerful tool used for sample modification, analysis, and characterization. Depending on the dwell time, or the dose, direct-write lithography on a sample can be divided into surface/bulk amorphization (low dose) and direct milling (high dose). Recent work at low doses has been shown to lead to the intermixing of substrate target atoms. One of the most exciting new areas where this is being used is in changing material properties such that the target becomes etch resistant to chemicals that would normally etch the native untouched material such as the intermixing effect of Gallium on silicon or diamond.13–15 Intermixing can also enable etching in a preferential manner in H-Si 16
Leonidas E. Ocola, Center for Nanoscale Materials, Argonne National Laboratory; [email protected] Chad Rue, FEI Company; [email protected] Diederik Maas, TNO, Delft; [email protected] DOI: 10.1557/mrs.2014.56
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