Deposition of reflection layers on metal and polymer substrates for the CMS Experiment at CERN

In 2007 the Large Hadron Collider will be launched at CERN in Geneva. One of the most exciting experiments planned for this new equipment is the CMS experiment (Compact Muon Solenoid). This is a huge and sophisticated detector for high energy species created by the particle collisions under test. The conversion of the particle energy into Cherenkov radiation is one of the effects used for measurements. The light impulses are measured by a system of photodetectors. It is necessary to provide the light signals to these detectors with low losses. Therefore crystals and light guides are enveloped with flexible material of high reflectivity. It was the task of FEP to provide this material. The most challenging part of was the coating of the polymer film. Polyethylenterephtalate (PET) was used as the substrate material for the layer stack. The film was coated with an adhesion promoting layer and aluminium. But the reflectivity of the metal would not have been high enough for the requirements of CERN. Therefore a four layer stack consisting of silica and niobia as materials with low and high refractive index, respectively, was applied on top of the aluminium layer. The optical thickness was optimized in such a way that the stack provided a maximum reflectivity between 400 nm and 650 nm under an angle of incidence of 70°. Finally a reflectivity ? 95 % was achieved. This value met the requirements of CERN. The material was coated in the coFlex? 600 sputter roll coater at FEP site. A newly designed DMS system for the deposition of silica was used for these runs. Deposition rate and uniformity data for this equipment will be presented.