Vacuum Ultraviolet Spectroscopic Ellipsometry Study of Porous Low-k SiOC Thin Films

Jin Yong Kim1,2, Nhan V. Nguyen1, Hae-Jeong Lee3, and Debra L. Kaiser2

1Semiconductor Electronics Division, 2 Ceramics Division, 3Polymers Division, 

National Institute of Standards and Technology, Gaithersburg, MD 20899

Yoon-Hae Kim and Hyeong Joon Kim

School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea

In a few years, semiconductor industry will have no choice but to replace SiO2-based gate dielectrics and aluminum-based interconnects with novel alternative materials,according to the ITRS. In pursuit of low-k interlayer dielectrics (ILD) and Cu interconnect technology, only porous low-k films (both inorganic and organic or hybrid) enables a decrease in permittivity to below 2. Film porosity and pore size distribution define electrical, mechanical, thermal and chemical properties of the film and itsfeasibility to be adoptedin fabrication process. Considerable efforts have already been made to evaluate those properties by using different measurement techniques such as positronium annihilation lifetime spectroscopy, ellipsometric porosimetry, x-ray reflectivity (XR), small angle neutron scattering (SANS). Spectroscopic ellipsometry (SE) is a fast, non-destructive technique used for the routine product monitoring of single layered and multi-layered thin film structure. Especially, vacuum ultraviolet (VUV) SE with extended spectral energy up to 9 eV is a promising tool for structural and optical studies of the films having high energy absorption tails such as SiOC, spin-on hybrid organic-siloxane, silicon carbide. 

In this study, SiOC thin films were prepared on p-type (100) Si substrates by plasma enhanced chemical vapor deposition using a bis-trimethylsilymethane liquid precursor. Using a commercial rotating analyzer ellipsometer with a computer-controlled retarder, we measured the ellipsometric angle Y and D from 1 eV to 9 eV (138 nm to 1240 nm) at various angles of incidence (65o to 75o). The VUV-SE results show the SiOC film is transparent in the region of the energies lower than 5 eV. Fitting the measured spectra in the transparent region gave us the accurate thickness and porosity of the SiOC films. After getting those parameters, we could obtain complex pseudo-dielectric functions of the SiOC films by using the fixed parameters. The Tauc-Lorentz (TL) or Generalized Tauc-Lorentz (GTL) dispersion functions were adopted to characterize the dielectric function of the SiOC films. The inversions of the film dielectric functions were obtained by fitting the experimental ellipsometric D and Y data at each photon energy by using the layer thickness and film porosities as known values. It was found that a simple model based on the GTL dispersion gave good fit to the ellipsometric data in low photon energy region. The VUV-SE study in conjunction with XR and SANS facilitated the precise characterization of both pore structures and dielectric functions of porous low-k SiOC thin films.