Comparison of optical methods for surface roughness characterization

Nikolaj A Feidenhans’L,Rafael Taboryski,Morten H Madsen,Giuliano Bissacco,Lukáš Pilný,Poul-Erik Hansen,Jan C Petersen

doi:10.1088/0957-0233/26/8/085208

Nikolaj A Feidenhans’L, Rafael Taboryski + Show 5 more

Open Access

https://doi.org/10.1088/0957-0233/26/8/085208

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Abstract

We report a study of the correlation between three optical methods for characterizing surface roughness: a laboratory scatterometer measuring the bi-directional reflection distribution function (BRDF instrument), a simple commercial scatterometer (rBRDF instrument), and a confocal optical profiler. For each instrument, the effective range of spatial surface wavelengths is determined, and the common bandwidth used when comparing the evaluated roughness parameters. The compared roughness parameters are: the root-mean-square (RMS) profile deviation (Rq), the RMS profile slope (Rdq), and the variance of the scattering angle distribution (Aq). The twenty-two investigated samples were manufactured with several methods in order to obtain a suitable diversity of roughness patterns.Our study shows a one-to-one correlation of both the Rq and the Rdq roughness values when obtained with the BRDF and the confocal instruments, if the common bandwidth is applied. Likewise, a correlation is observed when determining the Aq value with the BRDF and the rBRDF instruments.Furthermore, we show that it is possible to determine the Rq value from the Aq value, by applying a simple transfer function derived from the instrument comparisons. The presented method is validated for surfaces with predominantly 1D roughness, i.e. consisting of parallel grooves of various periods, and a reflectance similar to stainless steel. The Rq values are predicted with an accuracy of 38% at the 95% confidence interval.

Highlights

Accurate characterization of nanoscale surface roughness is important in many applications, and a number of techniques exist for this purpose [1, 2]
We report a study of the correlation between three optical methods for characterizing surface roughness: a laboratory scatterometer measuring the bi-directional reflection distribution function (BRDF instrument), a simple commercial scatterometer, and a confocal optical profiler
Three instruments for characterization of nanoscale surface roughness have been examined for correspondence between the parameters root-mean-square (RMS) profile deviation (Rq), Rdq and Aq

Summary

Introduction

Accurate characterization of nanoscale surface roughness is important in many applications, and a number of techniques exist for this purpose [1, 2]. Techniques, are often optimal for different applications or stages of a process [1], hereby requiring that the measured values are comparable between the instruments. Comparison of values obtained with different instruments is not a simple task, as the design of each instrument imposes different limitations to the measurement bandwidth [1, 3]. This is an often overlooked effect when comparing roughness values [2, 4]. To perform a reliable comparison of values obtained with different instruments, a study of the accuracy and limits of each method is required

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