Phase-locked interferometry for automatic mask alignment in projection printers

This paper describes a mask alignment technique for a projection printer in which the mask-to-wafer offset is determined interferometrically by a phase-locked method. The alignment patterns used in this method are cross grids which are illuminated with a He-Ne laser beam. By diffraction and interference a harmonic intensity signal is obtained whose phase is representative for the relative displacement of mask and wafer. Phase modulation and fine alignment are done with a tiltable glass plate driven by a galvanometric scanner. The total alignment operation including registration and fine alignment is controlled by a PC. With a registration precision in the nanometer region and cycle times below 0.3 s, this method is particularly suited for each field alignment in projection steppers. Results of theoretical and experimental investigations are discussed.