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SURFACE ROUGHNESS AND COATING THICKNESS MEASUREMENT   

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. MProbe MSP 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

         Surface roughness causes increased light-scattering. This leads to decreased specular reflectivity and degradation of interference. 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.  

Fig.3 Measurement reflectance spectrum 700 -1100nm              (sample Fig. 2) using 2um measurement spot. Pronounced interference fringes are clearly visible

Fig. 4  Measurement results (data Fig. 3). The peak indicates the coating thickness. It is strong and pronounced, showing excellent selectivity (absence of other thicknesses)

Fig. 5  Different thickness areas within the measurement spot 

Fig. 6 Transformation of measurement signals (convolution and deconvolution) from different thickness area, during the measurement process

THICKNESS NONUNIFORMITY

Coating thickness nonuniformity (within the measurement spot) causes the reflectance spectra similar to a multilayer filmstack. If the thickness variation is large enough – the light phase (and interference) will be scrambled and thickness measurement would not be possible. An example of a spot size covering two area S1, S2 with different thicknesses T1,T2 is shown on Fig. 5. 

When reflectances from area S1, S2 reach photodetector – reflectances are combined and converted to intensity (reflectances as vectors/ complex numbers). This process is called convolution. The optical phase is not lost – it is converted to the signal amplitude. During data analysis the signal is decomposed (using FFT) and thicknesses are extracted. Reflectance transformation during the measurement is illustrated on Fig. 6. The optical phase is never averaged and if the light beam has several areas with different phases – they  will be convoluted and de-convoluted independently.

As a result if the thickness is changing randomly (or continuously) within the measurement spot – it would create a continium of thicknesses after deconvolution. As a result, it would be impossible to determine the coating thickness.

By reducing the measurement spot size – we reduce the thickness variation and make thickness measurement possible.

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