Fig. 1 Reflectance spectra (700nm -1700nm) of a 5um polymer film for different surface roughnesses (simulation). There is characteristic, for light scattering, accelerated degradation (of intensity and interference) at shorter wavelengths
Fig. 2 Metal surface with 0.5um RMS roughness ( reticle diameter is 20um). There is significant roughness on a 20um scale but smooth areas are visible on a < 5um scale
Film thickness measurement on rough surfaces is, typically, challenging because of light-scattering. In fact, surface roughness and thickness nonuniformity are the two main factors that can degrade thickness measurement capability of spectroscopic reflectance based systems. Yet, these properties are commonly present in many real-life cases,like coatings on metals. offers a solution to overcome this limitation and to measure even the most challenging applications.
The key to the solution is the fact that both surface roughness and thickness nonuniformity are dependent on the measurement area. By reducing the measurement spot size one can reduce effective roughness and thickness nonuniformity (observed within this spot).
Surface roughness causes increased light-scattering. This leads to decreased and degradation of . The light scattering is more pronouced at a shorter weavelengths. For this reason, long Visible and Near-infrared (NIR) wavelength range (700-1700nm) is better suited for appication with surface roughness. Simulation of the effect of surface roughness on reflectance spectra (Fig.1) shows that interference pattern is significantly degraded for RMS> 100nm. And it becomes challenging to measure thickness in these conditions.
Rough surface exhibits peaks and smooth areas on the microscopic level (Fig. 2). It is clear that there is significant roughness on 20um scale but, there are smooth areas on a < 5um scale. So, it should be possible to select a small enough measurement spot to reduce the roughness effect. Indeed, results on Fig.3, 4 shows that using 2um measurement spot surface roughness effect can be practically eliminated and coating thickness can be reliably measured. Roughness topology is also leading to variation of the coating thickness on microscopic level - a potential source of measurement degradation(as we will review below). But the small spot size helps to reduce this as well. The smallest possible measurement spot size does not always gives best results. The spot size needs to be optimized based on the particular application.