Effect of Deposition Rate and Substrate Temperature on the Vacuum Ultraviolet Reflectance of MgF(2)- and LiF-Overcoated Aluminum Mirrors.

Freshly deposited aluminum prepared under optimum conditions was overcoated with MgF(2) and LiF films at various deposition rates and substrate temperatures. The thickness of the dielectric films was precisely controlled by reflectance measurements at lambda = 1216 A. The thicknesses of the MgF(2) and LiF films were chosen to give highest reflectance at 1216 A and 1026 A, respectively, and the reflectance was measured from 1000 A to 2000 A. Highest reflectance for aluminum overcoated with MgF(2) was obtained with a MgF(2) deposition rate of about 45 A/sec. When the MgF(2) deposition rate was decreased from 45 A/sec to 5 A/sec, the reflectance at 1216 A decreased from 85.7% to 78%. A decrease in reflectance was also noticeable for deposition rates higher than 45 A/sec. No improvement in reflectance was noted for substrate temperatures up to 100 degrees C, and at temperatures above 100 degrees C the reflectance decreased. The reflectance of aluminum overcoated with LiF did not show a measurable dependence on the LiF deposition rate, but the reflectance was strongly influenced by the substrate temperature. Highest reflectance at 1026 A was obtained when the LiF was deposited at a substrate temperature of 100 degrees C. The effects of aging on the reflectance of the mirrors are discussed.