Uniform diamond coatings are essential when these layers are used in demanding applications such as power electronics, optics and sensors. Fraunhofer IST has developed a measurement and analysis method that determines optical constants and coating parameters on large substrates more efficiently. The approach is especially relevant for 300 mm silicon wafers, where conventional measurement routines can become slow and difficult to interpret.
Polycrystalline diamond films combine useful functional properties with challenging measurement conditions. Their quality is determined not only by the average layer thickness, but also by how consistently that thickness and the optical behavior are maintained across the coated surface. Variations can affect functionality and service life, which makes reliable process control important during development and production.
Large substrates make this task more complex. Material inhomogeneities, surface roughness and multilayer structures influence the optical response of the coating. Measuring every relevant point with detailed methods would take considerable time, while isolated measurements may not provide enough information to describe the full surface. Fraunhofer IST addresses this by combining complementary optical measurement techniques with a global analysis model. The result is a route that first establishes the optical data of the coating in detail, then uses faster mapping measurements to assess uniformity over the full area.
Combining optical methods for robust layer data
The method combines spectral photometry and angle-dependent ellipsometry. Spectral photometry measures how strongly a surface reflects or transmits light at different wavelengths. These reflection and transmission curves change with material properties and layer thickness, so they provide information about the coating structure. Ellipsometry adds a second perspective by recording how polarized light changes after reflection at different angles of incidence.
Both techniques can be used to determine the refractive index, extinction coefficient and film thickness, but they do not respond to every parameter in the same way. That difference is important. If the methods are evaluated separately, uncertainties in roughness, layer structure or optical dispersion can limit the reliability of the result. Fraunhofer IST instead evaluates the measurement data simultaneously.
For this purpose, the institute uses a specially developed multilayer model that represents the polycrystalline diamond layer, including surface roughness, and incorporates known optical material data. According to Fraunhofer IST, this simultaneous fitting approach improves the determination of optical dispersion and other coating parameters compared with conventional individual measurements.
Faster mapping across large coated surfaces
The most time-consuming part of the workflow is the detailed determination of the optical constants. Once these values and the dispersion behavior of the diamond coating are known, they do not have to be fully remeasured at every point on the substrate. That changes the practical measurement strategy.
Instead of repeating complete photometry and ellipsometry routines across the entire wafer, Fraunhofer IST uses quick reflectance measurements at many positions on the surface. This reflection mapping is then linked to the previously established optical model. In practice, it allows parameters such as film thickness and thickness uniformity to be determined efficiently over large coated areas.
This is particularly relevant for silicon wafers with diameters up to 300 mm. At that scale, local deviations can be missed if the measurement grid is too sparse, while a very detailed conventional analysis would slow down process feedback. Reflection mapping offers a way to increase spatial coverage without making the measurement process impractical for production-related work.
Better process feedback for diamond coating systems
For coating processes, measurement data are valuable only if they can be turned into useful feedback. The approach developed at Fraunhofer IST is aimed at making that feedback faster and more reliable. By reducing the effort required for full-surface characterization, operators can evaluate layer homogeneity more often and in more detail.
Fraunhofer IST reports that the method was demonstrated on diamond-coated 300 mm silicon wafers. In that example, improved process control helped reduce variations in film thickness. For manufacturers, the practical effect is not limited to documentation of final quality. More reliable thickness and uniformity data can support adjustments in the coating process and help identify deviations before they lead to avoidable scrap.
The method is also described as automatable, which is relevant for production environments where repeatability and measurement speed matter. A faster measurement route can shorten development cycles and make coating processes easier to monitor, especially when diamond films are used in applications where durability and consistent optical or functional behavior are critical.
