E. Landree, (CSTL, Gaithersburg)

Objective: Requirements established by the National Technology Roadmap for Semiconductors have dictated the need for a metrological technique capable of characterizing thin oxide and dielectric films that are on the order of nanometers thick. A promising technique that is both non-destructive and sensitive to structure and local chemistry is grazing incidence x-ray photoelectron spectroscopy (GIXPS). Utilizing the non-linear dependence of photoelectron yield on quantities such as film thickness, density, cross-section, and x-ray index of refraction, it is possible to characterize a given layer structure by fitting these parameters to photoemission peaks measured for varying angles of x-ray incidence upon the surface, see Figure 1.

Problem: The fundamental material parameters, such as cross-section and index of refraction, are not well known, particularly within the energy range of interest (1-2 keV). In addition the effect of interface/surface abruptness upon the photoemission yield has yet to be characterized. Each of these parameters will influence the perceived film thickness and density.

Figure 1. Photoemission yield for silicon oxynitride thin film

Approach: An amorphous-carbon/silicon oxide/silicon substrate model is constructed and the electron photoemission is calculated using values for the ionization cross-section and index of refraction that differ from their nominal values. The deviation in perceived film thickness and density relative to the constructed model is then characterized by fitting these parameters while assuming the nominal values for the ionization cross-section and index of refraction. Similarly, the photoemission yield is calculated for models that include varying degrees of interface/surface gradation. The film thickness and density are then fit to the simulated photoemission yield assuming an abrupt atomic interface in order to observe the influence of gradation upon the perceived thickness and density.

Results and Future Plans: Results indicate deviations in the index of refraction have a more dramatic effect on the perceived film thickness and density than corresponding deviations in the ionization cross-section. In addition, moderate interface/surface gradation produced only small deviations in the perceived film thickness and density. A comparison of this method, along with other techniques on an assortment of samples, should allow us to arrive at a set of acceptable values for the fundamental parameters that will be used in future analysis.

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