Development of damage resistant sputtered oxide optical coatings for use at 248 NM
This report summarizes the results of a six-month effort to develop damage-resistant Kr*F laser mirrors by using and refining reactive sputter deposition techniques for the fabrication of multilayer oxide optical coatings. Mirror performance goals included a reflectivity of 99% at 248 nm and a laser damage threshold of 5 J/cm 2 for 20 ns pulses. Above and beyond these goals, however, particular attention was to be paid to improvement of the physical and optical properties of the coatings with
... the coatings with the goal of extending to shorter wavelengths the ultraviolet absorption edge of candidate multilayer combinations. Oxide multilayer coating combinations selected for development were Si0 2 /A1 2 0 3 , Si0 2 /Hf0 2 and Si0 2 /Y 2 0 3 . Selection was based on review and compilation of the optical properties of oxide materials reported in the recent literature. Twenty-eight coatings of selected designs were fabricated on LLNL substrates for laser damage testing by LLNL. Forty other coatings were fabricated on PNL substrates for optical, microstructural and topographical characterization by PNL aimed at optimization of their performance. Specimens for damage testing consisted of single layers of A1 2 0 3 , Hf0 2 and Y 2 0 3 in thicknesses of A/2, 3A/2 and 2A at 248 nm plus high reflectors of the design LL (HL)m HLL. In this design each layer is A/4 at 248 nm, L is always Si0 2 , and m = 19, 10 and 6 when H is A1 2 0 3 , Y 2 0 3 and Hf0 2 , respectively. Highest reflectivities at 248 nm were obtained for the Hf0 2 /Si0 2 combination for which the multipoint average of readings taken across the face of the 2-inch substrates was 94-96%. Reflectivities in excess of 99% were obtained in some regions of some of these mirrors. Average reflectivities for A1 2 0 3 /Si0 2 and Y 2 0 3 /Si0 2 were 80-83% and 62-80%, respectively.