Diamond windows for infrared and multispectral applications

The low pressure deposition of polycrystalline diamond and the preparation of diamond windows at the Fraunhofer-IAF is reported. Using microwave plasma CVD, large area (2-6'' diameter) diamond wafers with thicknesses of up to 2 mm have been grown. The deposition is carried out in a novel microwave plasma system which uses an ellipsoid cavity to generate very intense, spatially extended plasmas. Several of these plasma reactors with 6 to 60 kW microwave power are presently used at the IAF. The diamond wafers are ground and polished and laser-cut to the desired dimensions. To assess the optical and thermal properties, the residual absorption in the 10 mu m range is determined by CO2 laser calorimetry, and temperature dependent and spatially resolved measurements of the thermal conductivity are performed. In addition, thermal expansion and refractive index measurements as a function of temperature are reported. Using high-purity process gases and optimized CVD conditions, diamond windows with a residual absorption at 10.6 mu m below 0. 1 cm(-1) and a thermal conductivity over 20 W/cmK have been realized. In addition, low dielectric losses of tan delta = 0.6x10(exp -4) at 140 GHz have been measured