COMPOSITION AND CHEMICAL WIDTH OF ULTRATHIN AMORPHOUS FILMS AT GRAIN-BOUNDARIES IN SILICON-NITRIDE

Two different electron energy loss spectroscopy (EELS) quantitative an alytical methods for obtaining complete compositions from interface re gions are applied to ultrathin oxide-based amorphous grain boundary (G B) films of similar to 1 nm thickness in high-purity HIPed Si3N4 ceram ics. The first method, 1, is a quantification of the segregation exces s at interfaces for all the elements, including the bulk constituents such as silicon and nitrogen; this yields a GB film composition of SiN 0.49+/-1.4O1.02+/-0.42 when combined with the average film thickness f rom high resolution electron microscopy (HREM). The second method, II, is based on an EELS near-edge structure (ELNES) analysis of the Si-L- 2.3 edge of thin GB films which permits a subtraction procedure that y ields a complete EELS spectrum, e.g., that also includes the O-K and N -K edges, explicitly for the GB film. From analysis of these spectra, the film composition is directly obtained as SiN0.63+/-0.19O1.44+/-0.3 3, close to the one obtained by the first method but with much better statistical quality. The improved quality results from the fewer assum ptions made in method II; while in method I uniform thickness and illu mination condition have to be assumed, and correction of such effects yields an extra systematic error. Method II is convenient as it does n ot depend on the film thickness detected by HREM, nor suffer from mate rial lost by preferential thinning at the GB. in addition, a chemical width for these films can be deduced as 1.33 +/- 0.25 nm, that depends on an estimation of film density based on its composition. Such a che mical width is in good agreement with the structural thickness determi ned by HREM, with a small difference that is probably due to the diffe rent way in which these techniques probe the GB film. The GB film comp ositions are both nonstoichiometric, but in an opposite sense, this di screpancy is probably due to different ways of treating the surface ox idation layers in both methods.