A Novel Optical Technique to Measure Pad-Wafer Contact Area in Chemical Mechanical Planarization

Abstract

AbstractReal contact area between a CMP pad and wafer is a key factor in local contact pressure, friction and pad wear, all of which impact material removal and defect formation. A new optical method is introduced that quantifies the real contact area during polishing. Confocal reflectance interference contrast microscopy (C-RICM) uses a single focal plane to image the pad-wafer contact interface. A sapphire cover slip is used to provide optical transparency and to match the refractive index of the pad. Imaging the pad surface through the cover slip reveals areas of no reflection (pad-cover slip contact), areas of reflection (non-contact), and interference fringes (near contact). The C-RICM method was validated using micro-fabricated pads having uniform arrays of cylindrical surface structures of known contact area. Experiments conducted on porous polyurethane pads revealed that the real contact area is less than 10% of the total presented area. However Greenwood-Williamson (G-W) theory, widely used in CMP material removal models, predicts a contact area at least a factor of ten smaller. The discrepancy was found to result at least in part because the individual contact zones are not elliptical as assumed in G-W theory. In fact many contacting structures are crescents located at the perimeter of individual pores. These findings underscore the need for accurate control of pore density and morphology in polishing pads, in both initial manufacture and surface conditioning during CMP. The C-RICM method allows non-destructive benchmarking of polishing media in terms of pad-wafer contact, essential for developing improved pad architectures that achieve lower CMP defect levels.