Focused ion beam (FIB) technology

About this technique

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A focused ion beam (FIB) instrument uses an ion beam (typically gallium ions), which is focused to an extremely fine probe size (<10 nm) onto the surface of a specimen. This beam is very powerful and can cut through very hard specimens. As such it is used to selectively mill precise areas of material from the specimen surface. This technique can be used for cross-sectioning and imaging, along with the fabrication of specimens for other microscopy and microanalysis techniques such as transmission electron microscopy (TEM) and atom probe tomography (APT). It can cut ultra-thin sections of very hard materials that conventional microtomy cannot manage. This allows transmission electron microscopy of these samples where it would otherwise not be possible. FIB can shape materials for a variety of purposes such as the needle-shaped samples for use in an atom probe instrument. Many current-generation focused ion beam platforms consist of both ion and electron columns on a single instrument (called dual beam instruments), allowing the specimen to be imaged in detail using the electron beam, without damaging the surface of the specimen with the ion beam.

Additionally, various conducting, non-conducting and semiconducting materials may also be deposited onto selected areas of the surface (of a diameter of approximately 100 nm) by applying specific gases to the surface, which are decomposed onto the surface only in areas to which the ion beam is applied.

FIB tomography

The focused ion beam can cut trenches into hard materials to allow viewing of a cross-section. By sequentially removing slices from the surface of a sample a dataset of adjacent surface SEM images can be created and reconstructed into a 3-D image. In addition to regular SEM imaging of the exposed surface, it is possible to carry out energy dispersive spectroscopy (EDS) to identify elements present in the sample, and electron backscatter diffraction (EBSD) to give information on its crystalline structure. Therefore it is now possible to generate 3-D compositional and structural maps using FIB although this is still a newly emerging technique.

FIB tomography allows acquisition of 3-D image data from regions of interest between 100 nm and 10 µm in size. That is, in the gap in feature size that exists between high-resolution techniques such as atom probe tomography and TEM, and hard X-ray techniques such as X-ray microtomography.

Modern dual-beam instruments have automated procedures, whereby the ion beam is used to cut a series of slices of material from the specimen and the electron beam is used to collect consecutive images. The 2-D image dataset is supplied to a dedicated software package for reconstruction of the slices into a 3-D representation of the original feature(s). 3-D visualisation of the features often allows more complex interpretations to be made and reveals morphologies that may not necessarily have been apparent when only referring to the 2-D images.

Nanofabrication

FIB instruments are often equipped with micromanipulator systems. They may be located ex-situ (used for example to lift out FIB-prepared electron transparent membranes and transfer to a grid for TEM analysis) or in-situ. The combination of precise milling, deposition of various materials and in-situ specimen manipulation allows the fabrication of complex 3-D structures on small (micro-to-nano-) size scales. For example, a series of high aspect ratio holes were milled into a surface to test a pattern for a photonic crystal. The holes are approximately100 nm in diameter and 400 nm deep.

FIB investigations of biological specimens

Another emerging field in focused ion beam technology is its use for the examination of biological specimens. It can be used to cut ultra-thin section from materials that are difficult to cut by traditional microtomy. Cryo-stages allow the examination of various biological specimens, as well as food, emulsions etc., solidifying the specimen to allow ion beam milling and also minimising ion beam damage in the specimens.