Following the bulk Metal 4 Cu deposition, a thick copper overburden remains across the entire wafer surface P4.The primary objective of this Cu CMP step is to remove this bulk copper overburden and achieve a globally planarized surface, stopping precisely on the underlying Ta-based liner P4.Within the 40nm BSI CMOS Image Sensor integration scheme, MET4 typically functions as a higher-level routing or power distribution layer, which fundamentally distinguishes this specific step from earlier Cu C