Multiple-layer blank structure for phase-shifting mask fabrication

A multiple-layer blank structure with two shifter layers coated on a quartz substrate is proposed for the fabrication of phase-shifting masks. The thickness of these shifter layers is such that they induce a 180 degrees phase shift of light compared to air. On top of these shifter layers, an opaque layer is coated. The top shifter layer is patterned during the fabrication of the phase-shifting mask. The bottom shifter layer is patterned only to repair missing shifter defects. But the main purpose of this bottom layer is to act as an etch layer or give an end point during the repair of the phase defects or during the fabrication of the mask. Using yttrium fluoride etch stop/end point layer (bottom shifter layer), we have demonstrated the feasibility of multiple-layer blanks. This layer acts as an etch stop during the patterning of the main shifter layer and has a good refractive index match to that of quartz. The phase-shifting masks made using this multiple-layer blank structure have shown lithographic performances comparable to those of standard phase-shifting masks. (C) 1996 American Vacuum Society.