Assessment of digital cameras for micro-structural sensing of low contrast surface features

A. Temun, L. Mattsson

Department of Production Engineering, KTH – the Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Abstract

Evaluation of three digital cameras are presented focusing on their capabilities and applicability in the detection of low-contrast surface features on the micrometer level. The key to a successful identification of surface flaws on micro-structured surfaces, such as silicon wafers is the ability to determine minor deviations in the reflectance of an object surface. Micro-topography of a surface, surface structure and surface roughness has a strong influence on the amount and direction of scattered light. Different portions of the surrounding illumination are reflected on different parts of an uneven surface, hence the resulting contrast-variation on the illuminated surface has a strong correlation to the surface structure itself.

In our work we discuss the possibilities of using either of the investigated cameras for the automated visual inspection of micro-structured surfaces. The cameras - two of them equipped with CCD and one with CMOS image sensors - are studied in an environment which is similar to standard surface appearance measurements, involving human observers. The specimens, featuring different surface structures are imaged in a well-controlled environment under varying illumination conditions. Experimental results of spatial resolution and contrast sensitivity are presented.