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New method for determining the optical properties of thin films by reflectometry

The thickness and complex refractive indices of the thin films on a silicon wafer during lithographic imaging are critical factors affecting the processing of integrated circuits. The inorganic materials involved, such as the silicon substrate and inorganic anti-reflection coatings, are usually well characterized or present few difficulties. The optical properties of organic materials, such as photoresists and anti-reflection coatings have been more difficult to determine with confidence. In general, for each material on the substrate, three values need to be determined; they include the thickness and the real and imaginary parts of the refractive index at the exposure wavelength. Single measurements of reflectance or ellipsometric parameters do not provide sufficient degrees of freedom for determining these three unknowns. Typically, this problem is resolved by collecting reflectance or ellipsometric data over a range of wavelengths. However, because n and k functions of wavelength, two additional unknowns are present for each additional wavelengths. This problem is typically resolved by fitting n and kn to various spectral functions in order to reduce the number of unknowns. Unfortunately, the functional forms used are frequently inappropriate for the organic materials of interest. The essence of the new method is the use of data from coatings having different thicknesses in order to provide the degrees of freedom necessary for a solution. This is achieved at a single wavelength and thereby avoids the spectral model fits that are frequently fraught with problems. The new method is demonstrated by determining thicknesses and n and k values at the exposure wavelength for a photoresist and an anti-reflection coating designed to be used with 193 nm exposures. The optical properties of two 248 nm anti-reflection coatings are also determined over a spectral range.