Real-time sensors are key for flexible manufacturing environments where variable layer structures are processed concurrently. We have developed a high speed spectral ellipsometer capable of precisely measuring thicknesses and compositions of multilayer structures in situ and in real-time (i.e., the time frame of process changes or about one second). This spectral ellipsometer has been integrated into several vacuum chambers of a flexible process flow. Utilizing phase modulation, multichannel detection, and digital signal processing techniques, less than 0.5 s is required for acquisition of 46 spectral points. Fast numerical algorithms and processor are used for reducing measured spectral ellipsometric data to physical parameters (i.e., thicknesses and compositions), in real time based on a "standard model" approach. Implementations of spectral ellipsometric sensing in rapid thermal oxidation and remote plasma etch are described. For thermally activated processes such as oxidation, temperature dependent dielectric functions have been employed for analyzing samples at process temperatures as high as 1100-degrees-C. Multilayer combinations Of Si3N4, SiO2, Si3N4 plus SiO2 mixtures, and polycrystalline Si have been monitored and thicknesses controlled.