Application Example 21: Preparation of “Highly Flat and Damage-Free” Surfaces for High-Resolution Channeling BSE Imaging

Abstract

From the previous examples, it is evident that rf-GD sputtering is an extremely powerful technique to generate “ideal” surfaces for SEM examination. Sputtering time is very short, normally less than 10 s, with a distinct advantage, that damages to the sample surface are limited to the first atomic layer. For metals, sputtering generates surface with steps and terraces. Depending on the orientations of grains, various surface textures are developed after sputtering, as shown in Fig. E21.1a. Due to such textures, grains are now revealed clearly in both in-lens SE and SE 2 imaging modes. Fine precipitates are also revealed clearly. Through high-angle BSE imaging at lower accelerating voltages of less than 2 kV, compositional differences in the precipitates can be known in advance which greatly assists subsequent compositional analysis by EDX. Further and importantly, the presence of such fine steps, with their heights of normally less than 50 nm, does not significantly affect the EBSP analysis. For structural imaging of surfaces by channeling BSE imaging mode, however, surface roughness associated with such steps are very detrimental, as shown in Fig. E21.1b. In the grain, pictured left, where steps are prominent, contrast of the image appears to be totally dominated by the “roughnessrelated” contrast associated with steps. Over the relatively flat grain, pictured right, subgrain texture is observed with other fine features, such as narrow lines or fine dots. Precise interpretation of such features, however, is hindered due to possible contribution from roughness of exceedingly fine dimensions.