Improving High Resolution TEM Images using Low Energy Ion Milling

Ion beam milling has become a widespread specimen preparation technique for non-biological materials over the last two decades, particularly for cross-sectional and plan-view transmission electron microscope (TEM) specimens. The basic principle of ion milling involves bombarding a specimen with energetic ions or neutral atoms acclerated and formed into a tightly focused ion beam. Material is sputtered from the specimen resulting in electron transparent areas around the area of interest. Geometrical arrangements from system to system vary, but typically the specimen is rotated or oscillated relative to the ion beam during the milling process. The ion beam is tilted at a given angle with respect to the specimen surface during the ion beam milling process and is adjusted from 0 to 15 degrees. The disadvantage of ion beam milling specimens for TEM are the artifacts produced during the process. Artifacts include preferential sputtering (one material sputtering at a different rate than another), specimen heating, and radiation damage. Minimizing the incident ion angle to the surface of the sample has been found to be one of the most effective techniques for reducing these effects on the specimen. However, the major artifact that still remains directly resulting from ion milling is the amorphous damage created due to high energy ion bombardment. It has been well known that this process of ion bombardment (typically from 2 – 10 keV) causes damage in the crystalline structure (Ishiguro, et al, 1987). In the case of semiconductor crystals (such as Si and GaAs), this damage appears mostly as amorphous material on the surfaces of the thinned sample. Reducing the incident ion energy used during the milling process has been found to dramatically reduce and eliminate the amorphous damage produced by traditional ion milling techniques (G. Radnoczi, A. Barna, 1996).