Focused ion beam sectioning of micro-optics as a tool for destructive testing for optical material

In previous research we introduced an experimental methodology in which focused-ion-beam (FIB) sectioning, followed by secondary ion (SI) and secondary electron (SE) imaging, was used for testing the internal material homogeneity of silica and chalcogenide glass microspheres. The methodology is readily applied to micro-optics with dimensions of a few microns. The use of both SI and SE imaging of the sequentially sectioned samples was shown to allow accurate assignment of inhomogeneities, voids and other imperfections as being within the footprint of the micro-optic. On larger micro-optics FIB sectioning can become prohibitively time intensive and can require the use of too much platinum in sample preparation for evaluation of the bulk of the micro-optic. However, improved sample preparation and image analysis has enabled high magnification and high sensitivity study of the glass near the surface of chalcogenide microspheres with diameter of order 70μm. The chalcogenide glass is Ga ₂S ₃/La ₂S ₃, in a 70/30 weight percent ternary (GLS) and the microspheres had been kept in air, in normal laboratory conditions, for about two years prior to testing. Evidence of an altered layer with a width of the order of 0.1μm near the surface and then an outer porous layer at the surface was found. Lower resolution studies are then reappraised in light of the high resolution measurements.