Fundamentals of Focused Ion Beam Nanostructural Processing: Below, At, and Above the Surface
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5/7/07
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Fundamentals of
Focused Ion Beam Nanostructural Processing: Below, At, and Above the Surface
Warren J. MoberlyChan, David P. Adams, Michael J. Aziz, Gerhard Hobler, and Thomas Schenkel Abstract This article considers the fundamentals of what happens in a solid when it is impacted by a medium-energy gallium ion. The study of the ion/sample interaction at the nanometer scale is applicable to most focused ion beam (FIB)–based work even if the FIB/sample interaction is only a step in the process, for example, micromachining or microelectronics device processing. Whereas the objective in other articles in this issue is to use the FIB tool to characterize a material or to machine a device or transmission electron microscopy sample, the goal of the FIB in this article is to have the FIB/sample interaction itself become the product. To that end, the FIB/sample interaction is considered in three categories according to geometry: below, at, and above the surface. First, the FIB ions can penetrate the top atom layer(s) and interact below the surface. Ion implantation and ion damage on flat surfaces have been comprehensively examined; however, FIB applications require the further investigation of high doses in three-dimensional profiles. Second, the ions can interact at the surface, where a morphological instability can lead to ripples and surface self-organization, which can depend on boundary conditions for site-specific and compound FIB processing. Third, the FIB may interact above the surface (and/or produce secondary particles that interact above the surface). Such ion beam–assisted deposition, FIB–CVD (chemical vapor deposition), offers an elaborate complexity in three dimensions with an FIB using a gas injection system. At the nanometer scale, these three regimes—below, at, and above the surface—can require an interdependent understanding to be judiciously controlled by the FIB.
Introduction The focused ion beam (FIB) is becoming an ideal tool for growing, sculpting, infusing, and observing small shapes in an ever-widening range of applications. In conjunction with scanning electron
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microscopy (SEM) as well as an increasing variety of add-on tools (micromanipulators, gas-injection systems, and spectroscopic and crystallographic analysis), the FIB can prototype devices, characterize
structures in situ, and provide site-specific extractions for further ex situ processing or as sample preparation for other analyses.1–3 Ion beam processing predates FIB, ranging from processing films for semiconductor devices to the preparation of transmission electron microscopy (TEM) samples,4 and the ion/surface interaction has been understood and optimized in one-dimensional processing at doses up to roughly 1015/cm2 (e.g., semiconductor doping). The FIB has extended the applications to a regime in which localized, three-dimensional (3D) ion/surface interactions are important, with doses of ~1018/cm2 and unprecedented current densities. The FIB as a processing or analysis tool has commonly used a fixe
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