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Ellipsometric monitoring of multilayer coatings

The spectral performance requirements of optical thin film coatings continue to be increasingly demanding. Optical thin film design programs are now so sophisticated that they can meet the majority of these specifications. However, producing these designs is often very difficult. Many of the resulting designs have many layers (>50) of unequal optical thicknesses (non-quarter-waves) including both very thin layers (<5 nm) and thick layers. The tolerances on layer thicknesses and refractive indices are usually very tight (<1 nm per layer). Techniques for the monitoring of optical coatings range from crystal oscillator thickness and rate monitoring, time monitoring for sputtered films, single- wavelength turning point monitoring (particularly narrow-band interference filters such as DWDMs), and multiwavelength monitoring for multilayers having complex designs with possible real time re-optimization. This presentation will describe the use of ellipsometry for optical monitoring. Advantages of the technique are that both refractive index and thickness are measured during deposition, meaning that accurate design re-optimization is possible after each layer is terminated. Very thin layers can be measured accurately, and unlike photometric monitoring the technique has high sensitivity for all layer thicknesses. Examples will be given that illustrate the advantages of ellipsometric monitoring, including a laser notch plus band-blocker filter and a very broadband antireflection coating.