Application of Focused Ion Beam Miller in Fracture Characterization

Abstract

Abstract: Focused ion beam (FIB) miller is a new tool used for the examination of micro-fracture in ceramics. In this work, a FIB miller was employed to investigate fracture events involved in indentation and scratch tests of two differentα-sialon microstructures of the same chemical composition. The subsurface cross-sections were prepared via FIB milling and crack characteristics in both the surface and subsurface were identified. Crack interactions and the effect of microstructure on cracking were revealed. Keywords: Focused ion beam (FIB) miller, microstructure, fracture, indentation Introduction FIB millers are commonly found in the semiconductor industry, where they are used to deposit, etch, mill and image specimens during circuit modification and defect analysis. However, the FIB is becoming increasingly popular in studying fracture behaviour of ceramics, particularly for analysing subsurface damage following surface contact [1, 2]. The FIB uses a liquid metal ion source to emit gallium ions in a high vacuum environment. These particles are accelerated by an energy of between, typically, 5 and 50 keV, forming a fine (~10 nm diameter) energetic beam of gallium ions, which is then focusedonto the specimen surface by electrostatic lenses. When the ions impact and/or implant into the specimen, secondary electrons, secondary ions and atoms are sputtered from the surface. If the beam current is large (~ 10nA), through the insertion of a large beam-limiting aperture, the sputtering rate is very high and sections can be milled rapidly into the surface of the specimen. This is achieved by using the system software to draw a “mill-box” on the area of interest, so that milling occurs locally around specific microstructural features. After milling the beam current can then be reduced to a lower level (~ 10pA), by the insertion of a smaller aperture, and either the positive secondary ions or secondary electrons can be detected and used to form images similar to those acquired by a SEM. By using this method, subsurface features can be rapidly and precisely imaged. However, the FIB can be used in other ways, for example rastering the ion beam over the surface of specimen, at intermediate beam currents (~350 pA) will lead to localised surface etching and the generation of high contrast from multiphase materials. In this way, the relationship between surface features, such as cracks, and microstructure, can be readily assessed [2]. These features of the FIB can be utilized for the characterization of cracks in ceramics. In the past, cracks induced in ceramics by either indentation [3] or scratching [4] were identified typically from direct observation on the sample surface using either optical microscopy or SEM. When asubsurface cracks became critical, either ceramographic polishing [5] or a bonded-interface technique [6] was used for the preparation of subsurface cross-sections. It is noted that both techniques may introduce damage into the original cracks and consequently affect the reliability of identification. Furthermore, the two techniques could not reveal the interaction of microstructure and cracks, which is vital to fracture analysis and design of tougher ceramics. However, using the FIB, the subsurface cross-sections of ceramics can be prepared rapidly with minimal damage. Further, the area of interest can be etched to reveal the effect of microstructure on cracking. In this study,Ca α-sialon ceramic was used as a model material. he FIB was first sed toT u characterize the indentation-induced cracks in the samples, and underscore the effect of