Depth profiling: RBS versus energy-dispersive X-ray imaging using scanningtransmission electron microscopy

Rutherford backscattering spectrometry (RBS) is known to be one of the tech niques ideal for analysis of thin films. Elemental concentrations of matrix components and impurities can be investigated as well as depth profiles of almost each element of the periodic table. Best of all, RES has both a hig h sensitivity and a high depth resolution, and is a nondestructive analysis technique that does not require specific sample preparation. Solid-state s amples are mounted without preparation inside a high-vacuum analysis chambe r. However, depth-related interpretation of elemental depth profiles requir es the material density of the specimen and stopping power values to be tak en into consideration. In many cases, these parameters can be estimated wit h sufficient precision. However, the assumed density can be inaccurate for depth scales in the nanometer range. For example, in the case of Ge nanoclu sters in 500 nm thick SiO2 layers, uncertainty is related to the actual pos ition of a very thin Ge nanocluster band. Energy-dispersive X-ray emission (EDX) spectroscopy, using a high-resolution scanning transmission electron microscope (STEM) can assist in removing this uncertainty. By preparing a t hin section of the specimen, EDX can be used to identify the position of th e Ge nanocluster band very precisely, by correlating the Ge profile with th e depth profiles of silicon and oxygen. However, extraction of the concentr ation profiles from STEM-EDX spectra is in general not straightforward. The refore, a combination of the two very different analysis techniques is ofte n the best and only successful way to extract high-resolution concentration profiles. (C) 2000 Elsevier Science B.V. All rights reserved.