Comparison of optical methods for surface roughness characterization

Nikolaj Agentoft Feidenhans'l, Poul Erik Hansen, Lukas Pilny, Morten H. Madsen, Giuliano Bissacco, Jan C. Petersen, Rafael J. Taboryski

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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.
Original languageEnglish
Article number085208
JournalMeasurement Science and Technology
Issue number8
Number of pages10
Publication statusPublished - 2015


  • Angle-resolved scattering (ARS)
  • Scatterometry
  • Surface roughness
  • Bi-directional reflection distribution function (BRDF)
  • Optical profilometry
  • Confocal microscopy

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