Challenges in the areal measurement of surface roughness and shape at the micro and nanoscale

Measuring surface roughness accurately at the micro and nano scale presents several challenges. While optical techniques can be used to rapidly measure large areas, significant variations can be found between results from different techniques on similar samples. In the present work, a comparison has been made between the results of two different systems using interference microscopy and AFM to make measurements at the same place on the same sample. Two samples were prepared on silicon wafers by marking them with a multi-scale pattern using a photoresist process of lithography from an optical mask, followed by reactive ion etching. One was left bare and the other was prepared with a rough layer of hydroxyapatite before measuring at the chosen positions. Comparison of the results showed that while the general shapes of the measured surface microstructures were similar, several differences were found. For example, there was a variation of up to 7% between techniques in the measurement of the depths of the etched features and artefacts were also visible at square edges. These results show the need to pay careful attention to instrument calibration and probe/surface interactions in order to improve the accuracy of surface characterization of surface roughness and topography.