Ellipsometry with polarisation analysis at cryogenic temperatures inside a vacuum chamber

摘要

In this paper we describe a new variant of null ellipsometry to determinethicknesses and optical properties of thin films on a substrate at cryogenictemperatures. In the PCSA arrangement of ellipsometry the polarizer and thecompensator are placed before the substrate and the analyzer after it. Usually,the polarizer and the analyzer are rotated to find the intensity minimumsearched for in null ellipsometry. In our variant we rotate the polarizer andthe compensator instead, both being placed in the incoming beam before thesubstrate. Therefore the polarization analysis of the reflected beam can berealized by an analyzer at fixed orientation. We developed this method forinvestigations of thin cryogenic films inside a vacuum chamber, where theanalyzer and detector had to be placed inside the cold shield at a temperatureof T approx. 90K close to the substrate. All other optical components wereinstalled at the incoming beam line outside the vacuum chamber, including allcomponents which need to be rotated during the measurements. Our nullellipsometry variant has been tested with condensed krypton films on a highlyoriented pyrolytic graphite substrate (HOPG) at a temperature of T approx. 25K.We show that it is possible to determine the indices of refraction of condensedkrypton and of the HOPG substrate as well as thickness of krypton films withreasonable accuracy.