High reflecting dielectric mirror coatings deposited with plasma assisted reactive magnetron sputtering

Manufacturing all dielectric mirror coating with reflectivity values of more than 99.99 % is still a challenge to achieve. Losses caused either be transmittance, absorption or scattering have to be maintained well below 100ppm. Increasing the layer number for minimizing the transmittance losses usually increases the scattering by the growth of the roughness. High energy processes are required to minimize or avoid this behavior, but which are a challenge for avoiding unwanted contamination and interface absorption due to unwanted sputtering. As high energy process we used for the preparation of high reflecting dielectric mirrors plasma assisted reactive magnetron sputtering with a Helios 800 system. The machine was equipped with 3 cathode position for low and high index materials. We used metallic tantalum and hafnium targets for the preparation of the high index, silicon and silica targets for the low index. Metallic targets were powered with mid frequency, whereas the quartz target was sputtered by RF. As substrate we used either super polished fused silica or standard silicon wafer. The optical properties of the substrates we characterized by CRD, Laser calorimetry and spectrophotometric measurements. All combination allowed us to reach reflectivity values above 99.99%, with total deficit levels as low as 36ppm.