Ellipsometry with polarisation analysis at cryogenic temperatures inside a vacuum chamber

TL;DR

This paper introduces a novel null ellipsometry method suitable for cryogenic thin film analysis inside vacuum chambers, enabling accurate measurement of film thickness and optical properties at low temperatures.

Contribution

A new ellipsometry variant rotating the polarizer and compensator before the substrate, allowing fixed analyzer measurements suitable for cryogenic vacuum environments.

Findings

Successfully measured krypton film indices and thickness at 25K.

Demonstrated method's effectiveness for cryogenic thin film analysis.

Achieved accurate optical property determination in challenging low-temperature conditions.

Abstract

In this paper we describe a new variant of null ellipsometry to determine thicknesses and optical properties of thin films on a substrate at cryogenic temperatures. In the PCSA arrangement of ellipsometry the polarizer and the compensator are placed before the substrate and the analyzer after it. Usually, the polarizer and the analyzer are rotated to find the intensity minimum searched for in null ellipsometry. In our variant we rotate the polarizer and the compensator instead, both being placed in the incoming beam before the substrate. Therefore the polarization analysis of the reflected beam can be realized by an analyzer at fixed orientation. We developed this method for investigations of thin cryogenic films inside a vacuum chamber, where the analyzer and detector had to be placed inside the cold shield at a temperature of T approx. 90K close to the substrate. All other optical components were installed at the incoming beam line outside the vacuum chamber, including all components which need to be rotated during the measurements. Our null ellipsometry variant has been tested with condensed krypton films on a highly oriented pyrolytic graphite substrate (HOPG) at a temperature of T approx. 25K. We show that it is possible to determine the indices of refraction of condensed krypton and of the HOPG substrate as well as thickness of krypton films with reasonable accuracy.