MINERALOGICAL MAGAZINE VOLUME 60 NUMBER 403 DECEMBER 1996 Orientation contrast imaging of microstructures in rocks using forescatter detectors in the scanning electron microscope DAVID J: PRIOR, PATRICK W. TRIMBY, URSULA D.WEBER Department of Earth Sciences, Liverpool University, L69 3BX, UK AND DAVID J. DINGLEY Department of Physics, University of Bristol, UK Abstract We have developed a system using 'forescatter detectors' for backscattered imaging of specimen surfaces inclined at 50-80 ~ to the incident beam (inclined-scanning) in the SEM. These detectors comprise semi- conductor chips placed below the tilted specimen. Forescatter detectors provide an orientation contrast (OC) image to complement quantitative crystallographic data from electron backscatter patterns (EBSP). Specimens were imaged using two detector geometries and these images were compared to those collected with the specimen surface normal to the incident beam (normal-scanning) using conventional backscattered electron detector geometries and also to an automated technique, orientation imaging microscopy (DIM). When normal-scanning, the component of the BSE signal relating to the mean atomic number (z) of the material is an order of magnitude greater than any OC component, making OC imaging in polyphase specimens almost impossible. Images formed in inclined-scanning, using forescatter detectors, have OC and z-contrast signals of similar magnitude, allowing OC imaging in polyphase specimens. OC imaging is purely qualitative, and by repeatedly imaging the same area using different specimen-beam geometries, we found that a single image picks out less than 60% of the total microstructural information and as many as 6 combined images are required to give the full data set. The DIM technique is limited by the EBSP resolution (1-2 ~ and subsequently misses a lot of microstructural information. The use of forescatter detectors is the most practical means of imaging OC in tilted specimens, but it is also a powerful tool in its own right for imaging microstructures in polyphase specimens, an essential asset for geological work. KEYWORDS: contrast images, scanning electron microscopy, backscattered electrons. Introduction patterns (Alam et al., 1954; Venables and Harland, 1972; Dingley, 1981; 1984; Dingley and Baba-Kishi, SELECTED area electron channelling patterns (Coates, 1990; Dingley and Randle, 1992; Randle, 1992; Day, 1967; Joy, 1974; Joy et al., 1982; Davidson, 1984; 1993) both provide quantitative data concerning Lloyd, 1985; 1987; Schmidt and Olesen, 1989; Lloyd spatial variation of crystallographic orientations of et al., 1991; Lloyd, 1995) and electron backscatter minerals. In many problems relevant to geologists, Mineralogical Magazine, December 1996, Vol. 60, pp. 859-869 9 Copyright the Mineralogical Society860 D.J. PRIOR ET AL. the electron channelling approach has had an advantage in that it is easy to switch between a backscattered electron (BSE) image which clearly shows the specimen microstructure as orientation contrast (OC), to a selected area electron channelling pattern (SAECP) which can be used to index the crystallographic orientation of grains located using the OC image (Lloyd, 1987; 1995; Lloyd et al., 1987). Orientation Contrast is generated where there are differences in the diffraction geometry of an incident beam and the crystal lattice of the specimen (Hirsch et al., 1962; Newbury et al., 1973; 1974), such as between grains or sub-grains of different crystallographic orientations or different crystallo- graphic structure (Davidson, 1984; Lloyd, 1985; 1987). OC has been a valuable tool, with and without supporting SAECP data, in the study of textures in rocks (Lloyd et al., 1987; Prior et al., 1990; Lloyd et al., 1991; 1992; Burnley et al., 1991; Lloyd and Knipe., 1992; Prior, 1992; Mainprice et al., 1993). Orientation Contrast has also been referred to as electron channelling contrast (ECC: Newbury et al., 1974) and electron channelling contrast imaging (ECCI: Wilkinson et al., 1993). To collect an electron backscatter pattern (EBSP), the specimen needs to be tilted so that the EBSP can be recorded directly on film, or on a phosphor screen imaged by a video camera. Typically angles of 50 ~ to 80 ~ are required, between the normal to the specimen surface and the incident electron beam (Dingley, 1984). With this specimen geometry, conventional imaging methods, using secondary electron or pole- piece BSE detectors, do not provide an equivalent to the OC image so that in practical terms the EBSPs have been of little use to the geologist since the EBSP data cannot be related back to observed microstructures. A backscatter detector positioned on the pole piece can be used to image a specimen tilted at a high angle but the collection geometry is far from ideal so that the resultant image is of poor quality and in practice contains no resolvable OC component. In some materials (e.g. metals, calcite) it is possible to etch or decorate the specimens to reveal aspects of the microstructure and then to use the secondary electron image of a tilted specimen to locate EBSP data. However, these techniques are not generally applicable to all minerals and do not carry as much information as an OC image. Etching is often selective (Day, 1993) and it is not clear that etching of some materials (quartz for example) reveals microstructurally significant features (Prior, 1988; M. Handy, pers. comm.). An alternative approach is to automate EBSP collection and to reconstruct the specimen micro- structure from a grid-work of EBSP data-points (Adams et al., 1993; Dingley and Randle, 1992; Randle, 1992; Kunze et al., 1995). This technique, named orientation imaging microscopy (OIM: Adams et al., 1993), is undoubtedly powerful, but represents a significant overkill for many geological problems and at present is only developed for monominerallic aggregates. The technique is also limited by the angular resolution of EBSPs; this limitation will be highlighted later on. In early BSE studies, specimens