Overview of imaging using FIB

Scanning electron microscopes (SEM), transmission electron microscopes (TEM), and scanning TEM (STEM) have been used for structural observation of microdevices, advanced materials, and biological specimens. In recent years, a gallium (Ga) focused ion beam (FIB) has been used for preparing their cross-sectional samples [1, 2, 3, and Chapter 9 in this book]. Here, FIB works both as a milling beam and as a probe for a scanning ion microscope (SIM). SIM images are used during the whole milling processes, that is, drawing the milling area, milling monitoring, confirmation of the final milling, and observation of the FIB milled sections of interest. As SIM image resolution has been improved and about 5 nm is achievable at present, SIM observation has been increasingly used in place of SEM observation when there is no especial need for high image resolution.

However, we have found that the properties of SIM images are somewhat different to SEM images. For example, SIM and SEM images of identical FIB milled cross sections are shown in Figures 4.1(a)–(d): (a) and (c) show a solder (Pb-Sn) on copper (Cu) and (b) and (d) show a Si device (static random access memory) [4]. Black-and-white contrast among materials is opposite between the SIM and SEM images. In addition, grain contrast (or channeling contrast) can be observed more clearly in the SIM image and its contrast is sometimes stronger than the material contrast. Figure 4.2 shows SIM and SEM images of FIB cross-sectioned human hair [5].