## 3 Results
### CMP Pad Surface Measurements Using System  
Furthermore, we tested the surface-mapping performance of our system using a perforated anodized aluminum plate with a surface structure similar to that of the pad surface. The surface-mapping results are shown in Figure 4a-d. Figure 4a is a microscope image (300 \(\times\) 300 \(\upmu\)m) of the aluminum plate used in our set-up, showing the microporous surface. Figure 4c is a phase-shift contrast image obtained after a series of post-processing steps using the phase-shifted interference images (Figure 4b), where the darker areas represent deeper valleys in the surface profile. A topology of the mean-referenced surface map is displayed in pseudo color in Figure 4d, delineating the microscopic pores (arrow heads) and asperities (asterisks) on the surface, which correspond to the symbols in positions in Figure 4a.  
Finally, CMP-pad imaging was performed using the system. A fully processed (worn out) commercial pad was prepared for imaging; its strip was immersed in diluted water to simulate the use of the polishing pad in a turbid medium, i.e., a slurry mixture. The  
Figure 4: Surface mapping of a pad-like sample and a used CMP pad. (**a**) A microscope image (300 \(\times\) 300 \(\upmu\)m) of the porous surface of an anodized aluminum plate. (**b**) Phase-shifted interference images of the plate. (**c**) A phase-shifted contrast image. (**d**) A topology of the surface map. (**e**) A microscope image of the processed CMP pad surface immersed in a diluted emulsion, where arrows indicate the top of air bubbles trapped in the pores, which were removed before PSI. (**f**) A phase-shifted contrast image of the CMP pad surface. (**g**) The topology of the CMP pad surface map. (**h**) A 1-D surface profile taken along the dotted line in (**g**).  
vibration and tapping on the optical table were repeated to mimic the typical industrial environment. Figure 4e shows a microscope image of the surface of the pad strip in the emulsion. The bright spots (indicated by arrows) in the image result from significant reflectance caused by air bubbles in the pad pores. These air bubbles were removed using a micropipette prior to experiment. Without this step, the highly reflective liquid-air interface may have prevented the generation of interference at the pores, resulting in an error in the measurement of the pad asperity. It has been carefully speculated that the cause of bubble formation in the pores may be air that is accidentally trapped in the immersed pores when the pad with the air-filled pores is immersed in water or subjected to pouring water. This is evidenced by the fact that the gas bubbles in the liquid-solid surface interface tend to stay on the hydrophobic surface, and particularly on the hydrophobic pit sites [20]. Because the pad used in the current study was made of hydrophobic polyurethane, which naturally repels water, the bubbles may preferentially attach to the pores. However, the bubbles may not appear on the pad during CMP because of constant polishing and the influx of chemical slurry particles into the pores. Figure 4f shows a phase-shifted contrast image of the pad surface, from which a pad-surface profile was topographically reconstructed, as shown in Figure 4g. Figure 4g clearly shows the pores and neighboring asperities, which were colder and hotter in color, respectively. These were smoothed and glazed on the surface, implying that the pad surface roughness may be reduced due to the large amount of polishing. For surface analysis, a surface profile of the pad (Figure 4h) was taken along the dotted line in Figure 4g, and its average roughness (Ra) was measured to be 303 nm. This result indicates the applicability of our approach in examining the pad surface during CMP.