Measurement of thin film interfacial surface roughness by coherence scanning interferometry

H Yoshino,R Smith,D Mansfield,J M Walls,P M Kaminski,A Abbas

doi:10.1063/1.4978066

Abstract

Coherence Scanning Interferometry (CSI), which is also referred to as scanning white light interferometry, is a well-established optical method used to measure the surface roughness and topography with sub-nanometer precision. One of the challenges CSI has faced is extracting the interfacial topographies of a thin film assembly, where the thin film layers are deposited on a substrate, and each interface has its own defined roughness. What makes this analysis difficult is that the peaks of the interference signal are too close to each other to be separately identified. The Helical Complex Field (HCF) function is a topographically defined helix modulated by the electrical field reflectance, originally conceived for the measurement of thin film thickness. In this paper, we verify a new technique, which uses a first order Taylor expansion of the HCF function to determine the interfacial topographies at each pixel, so avoiding a heavy computation. The method is demonstrated on the surfaces of Silicon wafers using deposited Silica and Zirconia oxide thin films as test examples. These measurements show a reasonable agreement with those obtained by conventional CSI measurement of the bare Silicon wafer substrates.

Highlights

Three dimensional inspection of transparent/semi-transparent thin film layers together with roughness measurements of their upper and lower interfacial topographies would be useful to many optical applications such as those involved with optical coatings, semiconductors, photovoltaics (PV) and flat-panel displays
One of the challenges Coherence Scanning Interferometry (CSI) has faced is extracting the interfacial topographies of a thin film assembly, where the thin film layers are deposited on a substrate, and each interface has its own defined roughness
Application of the conventional CSI techniques to thin film metrologies such as film thickness and interfacial surface roughness, is limited by the film thickness because the interferogram is generally analysed in the time domain where the peaks of the signal should be separated

Summary

Introduction

Three dimensional inspection of transparent/semi-transparent thin film layers together with roughness measurements of their upper and lower interfacial topographies would be useful to many optical applications such as those involved with optical coatings, semiconductors, photovoltaics (PV) and flat-panel displays. Spectroscopic ellipsometry has been used for the non-destructive measurement of thin film thickness with the area of interest typically averaged over a large area of the order of a millimeter square. The stylus profilometers and spectroscopic ellipsometry are well-established techniques, there is a need for a nondestructive method for the measurement of thin film thickness in three dimensions with a higher horizontal resolution. Debnath et al, have proposed a method of measuring the tissue layer thickness and underlying topography using spectrally resolved OCT.. Debnath et al, have proposed a method of measuring the tissue layer thickness and underlying topography using spectrally resolved OCT. The layer thickness is typically over the micro-meter range

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