Droplet Evaporation and Watermark Formation on Si Wafers with Various Films

The evaporation of droplets from Si wafers coated with low-dielectric-constant (Low-k), Cu, and thermally oxidized (Th-Ox) films was investigated to fundamentally study precision wafer cleaning. Ultrapure water was dropped onto the wafer, and the resulting droplet shapes and watermarks were observed using microscope cameras. These images were then used to measure the droplet's shape, namely its height and contact area, from which the contact angle and droplet volume were calculated. The watermark size was also measured after the evaporation was completed. Results showed that the droplet evaporation processes can occur in two modes, namely constant contact angle (CCA) and constant contact radius (CCR) modes. The watermark size was strongly affected by the evaporation process: for CCR-type evaporation, the watermark was almost the same size as the initial droplet contact area, whereas the watermarks produced via CCA-type processes were smaller. For the Low-k and Th-Ox films, the evaporation processes changed from CCR to CCA, whereas the droplet on the Cu film showed only CCR-type evaporation. The droplet evaporation process and watermark size were measured for a patterned wafer with on a Cu pattern on a Low-k film. Results showed only CCR-type evaporation, similar to that observed in the Cu film.