C. J. Powell and A. Jablonski (Institute of Physical Chemistry, Warsaw, Poland)

Objective: To enable measurements of film thicknesses to be made with improved accuracy by Auger-electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) for nanotechnology, semiconductor metrology and other applications.

Problem: AES and XPS are used extensively in the semiconductor and other industries for determining thicknesses of thin (< 10 nm) overlayer films on substrates. Such measurements depend on the effective attenuation lengths (EALs) for the relevant electron energy and film material. The EAL differs from the corresponding electron inelastic mean free path (IMFP) because of elastic scattering of the signal electrons in the specimen. While the IMFP is a material property, the EAL depends on the film thickness and the XPS or AES measurement configuration. The extent to which the EAL differs from the IMFP has not been previously documented for films of SiO2 on Si that are of high technological importance. Recent comparisons of SiO2 film thicknesses measured with different techniques have shown significant differences. It is clearly desirable to identify the measurement uncertainties of each technique in greater detail.

Approach: Calculations have been made of Si 2p photoelectron currents for a Si substrate and SiO2 films of varying thickness, for Al and Mg characteristic x rays, and for three common XPS instrumental configurations. The calculations were made with an algorithm based on solution of the kinetic Boltzmann equation within the transport approximation to account for elastic scattering along the electron trajectories.

Results and Future Plans: The Figure shows an example of the results. The ratio of the average EAL in the SiO2 film, <EAL>, to the IMFP in the film is plotted against the SiO2 film thickness for Mg Ka X rays and for an XPS configuration in which the angle y between the X-ray source and the electron energy analyzer is 54º. The solid lines show plots of this ratio for different electron emission angles a. The dashed lines indicate values of the <EAL>/IMFP ratio for constant attenuations of the substrate-signal current of 1%, 2%, 5%, and 10%. It is clear that for a less than about 60º, the <EAL>/IMFP ratio is approximately constant (within 3%) for film thicknesses of practical relevance. The average value of this ratio varies between 0.906 and 0.935 depending on the X-ray source, the film-thickness range, and the particular value of y. For a greater than about 60º, the <EAL>/IMFP ratio changes appreciably with film thickness, and the <EAL> for the specific conditions needs to be determined from the calculated curves. It is planned to extend these calculations to other materials for which film thicknesses are determined by XPS or AES.

Publications: Powell, C. J. and Jablonski, A., "Measurements of Silicon Dioxide Film Thicknesses by X-ray Photoelectron Spectroscopy," in Characterization and Metrology for ULSI Technology-2000 (in press).

Powell, C. J. and Jablonski, A., "Effects of Elastic-Electron Scattering on Measurements of Silicon Dioxide Film Thicknesses by X-ray Photoelectron Spectroscopy," J. Electron Spectrosc. (in press).

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