Contact layers of the DRAM manufacturing process can be printed well using alternating phase-shifting masks. State-of-the-art mask making tools have sufficient performance to manufacture defect free contact masks. The enlarged process window compared to conventional masks allows to shrink the contacts size or to substitute advanced scanners by older generation steppers for contact layer patterning. Using older generation exposure systems may cause problems originating in worse lens aberration performance. A method will be described how to overcome overlay problems by applying a specifically designed OL measurement target.