Volker Sittinger, Christian Beyen, Markus Höfer, Markus Armgardt, Thomas Melzig, Christian Stein, Fraunhofer Institute for Surface Engineering and Thin Film IST, Braunschweig, Germany
The homogeneity of large-area diamond coatings is crucial for various applications. To reliably evaluate and optimize coatings on wafers up to 300 mm in diameter, the complex refractive index of diamond must be known with high accuracy over a broad wavelength range. Its precise determination is challenging and can only be achieved to a limited extent with conventional, separate optical measurement and modeling approaches.
At Fraunhofer IST, optical measurements combining spectral spectrophotometry (reflection) and variable angle spectroscopic ellipsometry are used, and all data are evaluated simultaneously in a multi-spectral fitting procedure. Diamond-coated silicon substrates are characterized by reflection and ellipsometry in the 250–1200 nm range (ellipsometry at various angles, microspot size 200 µm). A dedicated multilayer model is employed, which includes the compact diamond layer and an additional roughness layer described by an effective medium, together with tabulated dispersion data for silicon and its native oxide. The dielectric function of the diamond layer is obtained using either a Lorentz-oscillator model or a semiconductor-type dispersion model. This combined, simultaneous fitting approach is a key distinctive feature of the Fraunhofer IST methodology.
Once the optical dispersion of the diamond layer has been determined, large-area wafers (e.g. 300 mm) can be characterized rapidly and efficiently by pure reflection mapping, requiring only the extraction of layer thickness and surface roughness from each spectrum. This leads to significantly reduced measurement time, improved process control and reliable assessment of thickness homogeneity over large areas, enabling higher production quality and durable high-performance diamond coatings for industry and research.