The PMD 4 Etch step is a critical intermediate stage in the sequential opening of bondpads for 40nm Backside Illuminated (BSI) CMOS Image Sensors A2.In BSI architecture, accessing the frontside metal pads from the thinned backside requires etching through a thick, multi-layered dielectric stack A1.By dividing the total etch depth into multiple stages (PMD 1 through 5), the process avoids photoresist budget exhaustion and severe profile degradation (Engineering Practice).PMD 4 Etch specifically c