Fraunhofer IST: Precise measurement for better diamond coatings

The homogeneity of polycrystalline diamond layers is a key quality factor for their functionality and service life. However, conventional measurement methods reach their limits, especially with large-format substrates, such as silicon wafers with diameters of up to 300 millimeters. Material inhomogeneities, surface roughness and complex layer structures make reliable analysis difficult and process control complex and time-consuming. The Fraunhofer Institute for Surface Engineering and Thin Films IST therefore relies on a holistic approach and combines various measurement methods: Spectral photometry is used to examine how strongly a surface reflects or transmits light at different wavelengths. The spectral reflection and transmission curve changes depending on the material and coating thickness. Angle-dependent ellipsometry is also used. It records how the state of polarized light changes after reflection at different angles of incidence and, like spectral photometry, enables the refractive index, extinction coefficient and coating thickness to be determined.

Determination of optical constants of large-area diamond coatings

Both measurement methods are based on different measurement principles and react with different sensitivities to the coating parameters. In combination, they therefore provide a significantly more robust and precise characterization of the coatings. Instead of evaluating the measurements individually, they are analyzed simultaneously at the Fraunhofer IST using a global modeling method. The basis is a specially developed multi-layer model that realistically depicts the polycrystalline diamond layer including roughness and already contains known optical material data. "By simultaneously evaluating all measurement information, we achieve significantly higher accuracy than with conventional individual measurements and can precisely determine the optical dispersion and other relevant layer parameters," says Dr. Volker Sittinger, head of the "Diamond-based Systems" department at the Fraunhofer IST, explaining the unique selling point of the new simultaneous fit method. The decisive advantage here is that once the dispersion and all optical data have been reliably determined, the most time-consuming part of the analysis is done, as it is no longer necessary to determine all the data again in a complicated manner at every point. A simple and quick reflection measurement, which is applied to many measuring points on the entire surface, is sufficient to determine, for example, the coating thickness and the resulting uniformity of the coating. This so-called reflection mapping can be used to efficiently characterize large-area diamond coatings.

Coating thickness of silicon wafers becomes more uniform

For industrial customers, this means significantly shorter measuring times, improved process control and the ability to specifically evaluate and optimize coating homogeneity over large areas. Using the example of diamond-coated 300 mm silicon wafers, it has been shown that improved process control can significantly reduce fluctuations in coating thickness. The process therefore contributes directly to higher production quality and creates the basis for high-performance and durable diamond coatings. "With the measurement approach used, we offer our partners a powerful solution for quality and process control of modern diamond coating systems. The fast, robust and automatable method supports companies in accelerating development and production processes, reducing rejects and sustainably increasing the reliability of their products," explains Sittinger.