Ellipsometric determination of optical properties for ultrathin gate oxides

The silicon dioxide (SiO/sub 2/) layer thermally grown on a crystalline silicon is usually used in dielectric isolation of one device from another, acts as a component in MOS structures, and provides electrical isolation of multilevel metallization systems. The 1997 edition of the National Technology Roadmap for Semiconductors (NTRS) had envisioned the need for 3-4 nm gate oxides with a process tolerance of 4% at 180 nm technology node in 1999. The next technology node of 150 nm predicts gate oxides of 2 to 3 nm for the year 2001 with a process tolerance of 4%. An uncertainty of 1% is required at 95% or better confidence level. Because even a small degree of interfacial micro-roughness or nonuniformity can alter device performance, it is crucial to control the atomic scale structure. Ellipsometry is an optical technique that is widely used for the characterization of thin films. In recent articles, it was reported that, if the wavelength of a spectroscopic ellipsometer is adjusted to the antireflection condition where the s-polarized component of the incident light is transmitted through to the substrate with minimum reflected intensity, then an interface layer between the top layer and the substrate can strongly influence the antireflection condition established in the top layer. In this work, the experimental value of the phase shift and amplitude attenuation for oxide samples was obtained by a spectroscopic phase-modulated ellipsometer. To simulate the possibility of a non-abrupt interface an interlayer between the SiO/sub 2/ layer and the Si substrate was assumed. The dielectric function of the interlayer was obtained by evaluating the effective dielectric function of a physical mixture of SiO/sub 2/ and amorphous silicon (a-Si) in the Bruggeman effective medium approximation.