A novel approach for site-specific atom probe specimen preparation by focused ion beam and transmission electron backscatter diffraction.

Abstract

Atom probe tomography (APT) is a suitable technique for chemical analyses with almost atomic resolution. However, the time-consuming site-specific specimen preparation can be improved. Recently, transmission electron backscatter diffraction (t-EBSD) has been established for high resolution crystallographic analyses of thin foils. In this paper we present the first successful application of a combined focused ion beam (FIB)/t-EBSD preparation of site-specific APT specimens using the example of grain boundary segregation in technically pure molybdenum. It will be shown that the preparation of a grain boundary can be substantially accelerated by t-EBSD analyses in-between the annular milling FIB procedure in the same microscope. With this combined method, a grain boundary can easily be recognized and positioned in the first 220nm of an APT sample much faster than e.g. with complementary investigations in a transmission electron microscope. Even more, the high resolution technique of t-EBSD gives the opportunity to get crystallographic information of the mapped area and, therefore, an analysis of the grain boundary character to support the interpretation of the APT data files. To optimize this newly developed technique for the application on needle-shaped APT specimens, a parameter study on enhanced background correction, acceleration voltage, and tilt angle was carried out. An acceleration voltage of 30kV at specimen surface tilt angles between -45° and -35° from horizontal plane leads to the best results. Even for molybdenum the observation of crystal orientation data up to about 200nm specimen thickness is possible.