Monitoring the Thickness of Thin MgF_2 and LiF Films on Al by Reflectance Measurements Using the 1216-Å Line of Hydrogen

Abstract

In order to obtain highest reflectance with Al +/- MgF(2) and Al +/- LiF coatings at lambda2161 A and lambda1026 A, respectively, the thickness of the dielectric films must be precisely controlled. This paper presents a method of monitoring the thickness of thin MgF(2) and LiF films on freshly evaporated Al films by measuring the reflectance of the Al surface at lambda1216 A as the dielectric films are being deposited. A specially designed hydrogen discharge lamp with a MgF(2) window is used as the light source, and an NO ionization chamber, also with a MgF(2) window, serves as the detector. This combination results in an essentially monochromatic response at lambda1216 A. Reflectances of about 85% at lambda1216 A were obtained by overcoating freshly deposited Al with MgF(2) films that had an effective optical thickness of lambda/2 at lambda1216 A. With the same monitoring system, the thickness of LiF on Al could also be precisely controlled and LiF-overcoated Al mirrors with reflectances higher than 71% at both lambda1026 A and lambda1216 A were consistently prepared.