Metrology of very thin silicon epitaxial films

Abstract

In this thesis, non-destructive thickness measurement of sub-0.5 µm silicon epitaxial films has been successfully performed using spectroscopic ellipsometry (SE) in the visible to near infrared (NIR) range (0.73 - 3.3 eV). The undoped epitaxial .films are grown on heavily doped substrates by chemical vapor deposition at 700 - 900° C. The effect of heavy dop­ing on the optical properties of crystalline silicon in the spectral range 0.73 - 3.3 eV is precisely described by the Drude free carrier model. It is shown that visible-NIR SE can simultaneously determine the substrate dopant concentration, the thicknesses of epitaxial film and native oxide, and if present, the thickness of the transition layer between the epi­taxial film and the substrate. The epitaxial film thicknesses measured by visible-NIR SE are in excellent agreement with results of secondary ion mass spectrometry (SIMS). The sub­strate dopant concentrations measured by SE also agree well with SIMS results for n-type substrates, but are consistently higher than SIMS values for p-type substrates. It is also demonstrated that visible-NIR SE is very sensitive to the epi/substrate interface quality therefore can be used for process monitoring in low temperature silicon epitaxy. Non-destructive determination of thickness and composition of strained Si1-xGex (0 < x 0.30) heteroepitaxial layers has also been successfully carried out in this thesis using visible-NIR SE. The dielectric function of the Sii-xGex layers in the spectral range 0. 75 - 2. 75 e V is fitted to a self-consistent empirical formula with only five fitting parameters. Ac­curate ellipsometry measurement of thickness and composition is successfully demonstrated using this formula. This thesis has developed a non-destructive technique for the measurement of both sili­con and Sii-xGex epitaxial films. This technique is suitable and can be easily implemented for in-line, in situ and real-time measurement. This study also provides a numerical expression for the optical constants of strained Si1-xGex in the spectral range of interest for most optoelectronic applications.