Measuring the thickness of ultra-thin diamond-like carbon films

This paper examines the challenge posed by the measurement of thickness of sub-50 nm diamond-like carbon (DLC) films deposited onto silicon substrates. We compared contact profilometry (CP), optical profilometry (OP), contact atomic force microscopy (CAFM), tapping atomic force microscopy (TAFM) and X-ray reflectometry (XRR). Generally, CP, CAFM, TAFM and XRR give similar thickness values except for the case of the more compliant samples measured by CP and CAFM. Moreover, the theoretically precise XRR technique gives significant standard deviation due to the layering of the DLC film. For those transparent samples, OP always gives an erroneous measurement. These metrological artefacts are compared to calculations of mechanical deformation (CP and CAFM), energy dissipation (TAFM) and thin film interferences (OP). The OP artefact is used to extract the film’s refractive index, in good agreement with literature values. Finally, the comparative data obtained in this study also shows that the density and refractive index of the 10 nm thick films are constituently lower than those of the 50 nm thick films. This scaling effect, which is consistent with known growth mechanisms for DLC, further complicates the measurement of thickness by optical techniques.