Optical monitoring of nonquarterwave multilayer filters

Many spectral filtering problems require the use of assemblies of layers having thicknesses which bear no obvious relationship to each other. Successful production of these multilayers requires films with thicknesses approximating theoretical values. We show that the optical methods currently used in the production of film assemblies of quarterwave layer thicknesses, which are based on the use of just one single wavelength, are poorly adapted to monitoring the deposition of nonintegral thickness multilayers. In contrast we show that with an optical control system utilizing a broad spectral bandwidth, thickness errors can be reduced. Transmittance measurements with the precision necessary to achieve this improved thickness control are attainable with existing instrumentation. This result is established by a computer simulation of the construction of a specific multilayer and remains valid for other nonquarterwave multilayer filters.