Dual trench alternating phase shift mask fabrication

A dual-trench alternating phase shift mask (PSM) fabricated at least in part by performing a method comprising:

• patterning an chromium layer over a mask layer over a quartz layer of the PSM according to a semiconductor design;
• for each of a first plurality of times,
• applying the first photoresist layer over the chromium layer;
• beam writing the deep trenches of the alternating PSM design in the first photoresist layer;
• developing the first photoresist layer to remove beam-written parts of the first photoresist layer;
• and,
• dry etching the quartz layer through the exposed parts of the first photoresist layer to initially form the deep trenches of the PSM;
• for each of a second plurality of times,
• applying the second photoresist layer over the chromium layer;
• backside ultraviolet exposing the second photoresist layer;
• developing the second photoresist layer to remove exposed parts of the second photoresist layer;
• dry etching the quartz layer through the exposed parts of the second photoresist layer to completely form the shallow trenches and the deep trenches of the PSM;
• and,
• removing the second photoresist layer.