Reference-free Characterization Of Semiconductor Surface Contamination And Nano layers By X-Ray Spectrometry

X-Ray Spectrometry (XRS) is a wide spread technique for revealing reliable information concerning the elemental composition and binding state in various materials. Reference-free quantitation in x-ray spectrometry is based on the knowledge of both the instrumental and fundamental atomic parameters. In different configurations, both matrix and trace constituents of a sample or layer thicknesses can be determined, even providing lateral or depth-profiling elemental information. With respect to very flat samples, such as semiconductor wafers or structures, the photon energy and the angle of incidence of the exciting radiation determines the probing depth of XRS analysis. In total-reflection geometry, i.e. having an angle of incidence smaller than the critical angle of total external reflection, only surface contamination and the surface-near layer of a few nm contributes to the fluorescence spectra. Allowing the angle of incidence to be varied from close to zero up to about four times the critical angle of total external reflection, the probing depth ranges from a few up to several hundreds of nm. The methodological development of XRS at the Physikalisch-Technische Bundesanstalt (PTB), Germany's national metrology institute, is, among other issues, dedicated to high-end investigations in the R&D of semiconductor samples requiring reference-free methods, in particular for new materials where not enough appropriate reference materials are available. PTB can handle 200 mm and 300 mm silicon wafers, employing an EFEM module, as well as smaller semiconductor wafers in its XRS instrumentation. The use of undulator radiation in the PTB laboratory at BESSY is advantageous for off-line contamination control on semiconductor surfaces as it provides very high photon fluxes for the efficient excitation of light elements. Optimizing the respective excitation conditions such as the angle of incidence and the incident photon energy, detection limits of light elements in the fg range can be achieved. Grazing incidence investigations demonstrated the capability for depth profiling of light elements in nanolayers. Reference-free XRS has the potential to contribute to the thickness and composition analysis of nearly vertical sidewalls of semiconductor test structures. This technique is also able to contribute to the elemental depth-profiling of ultra-shallow junctions (USJ), i.e. near-surface implantation profiles in wafers, as a complementary approach to other analytical techniques. At a given incident angle, XRS can be combined with x-ray absorption spectroscopy (NEXAFS, XANES or EXAFS), revealing information on the depth profile of the chemical structure in a sample, e.g. about buried nanolayers or interfaces with varying chemical state.