Amplitude-phase-shift masks for projection lithography of submicron technology

Submicrometer optical lithography is possible with conventional projection cameras when the mask controls the phase of the light at the object plane. The phase shifting mask consists of a normal transmission mask that has been with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI era.