NOVEL EXTRA-ACCURATE METHOD FOR 2-SIDED ALIGNMENT ON SILICON-WAFERS

An accurate method for double-sided alignments with less than 2 mum er ror across a 4 in silicon wafer is devised and demonstrated. In this m ethod, only alignment marks on the front side of the wafer are require d and formed in a low-stress insulating layer. This insulating layer w ith the alignment marks is later bulk-micromachined into a free-standi ng diaphragm with a cavity underneath it. This cavity in our design is etched through the whole wafer so the front-side alignment marks are visible from the back side. This allows visual alignment from both sid es of the wafer, so better than 2 mum alignment accuracy is achieved f rom our GCA 4800 stepper.