Visualizing slurry flow in chemical mechanical planarization via high-speed videography

A method using high-speed videography is developed for visualizing and quantifying the width of the slurry bow wave formed at the leading edge of a retaining ring during polishing. This new method overcomes many limitations of previous techniques as it employs an inert dye thereby bypassing any photobleaching issues encountered in the past. This allows the use of pads of all types and colors in a normally-lit environment. Image analysis is used to quantify and classify pixel brightness in regions of interest on the CMP pad. By employing suitable baselines, evolution of the slurry bow wave (specifically its width) as a function of polish time was calculated and analyzed. This new method was then applied to retaining ring polishing experiments at two different flow rates and in two different regions where the bow wave was formed. Results consistently show that the bow wave fluctuated with time and its width grew wider at higher flow rates, especially in region closer to the slurry injection point. Fast Fourier Transformation (FFT) was employed to convert the fluctuating component of the bow wave width from time domain to frequency domain. When looking at the spectral signature of the slurry bow wave width, major peaks were observed at 1, 2, 4, 8-9 and 12-14 Hz all of which could be theoretically explained by taking into account ring geometry and polisher kinematics.