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Monitoring the Thickness of Thin MgF2 and LiF Films on Al by Reflectance Measurements Using the 1216-Å Line of Hydrogen

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Abstract

In order to obtain highest reflectance with Al + MgF2 and Al + LiF coatings at λ2161 Å and λ1026 Å, respectively, the thickness of the dielectric films must be precisely controlled. This paper presents a method of monitoring the thickness of thin MgF2 and LiF films on freshly evaporated Al films by measuring the reflectance of the Al surface at λ1216 Å as the dielectric films are being deposited. A specially designed hydrogen discharge lamp with a MgF2 window is used as the light source, and an NO ionization chamber, also with a MgF2 window, serves as the detector. This combination results in an essentially monochromatic response at λ1216 Å. Reflectances of about 85% at λ1216 Å were obtained by overcoating freshly deposited Al with MgF2 films that had an effective optical thickness of λ/2 at λ1216 Å. 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 λ1026 Å and λ1216 Å were consistently prepared.

© 1972 Optical Society of America

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