Optical superlattices as phase-shift masks for microlithography

Optical lithography is one of the key enabling technologies in semiconductor microcircuit fabrication. As the demand for devices with higher performance and speed continue, the need for patterning circuits with finer features is driving optical microlithography to shorter and shorter wavelengths (248 nm yields 193 nm yields 157 nm). This is because the resolution with traditional Cr masks, that either block or pass light for imaging, is limited by optical diffraction. At any wavelength, however, phase-shift masks can enhance resolution beyond the wavelength-imposed diffraction limit. Phase-shift masks work by employing destructive optical interference to enhance contrast. This paper discusses a novel, systematic materials approach--optical superlattices- -to design embedded attenuating phase-shift masks, the most versatile and common type phase-shift mask, for any optical wavelength. These superlattices are comprised of alternating, ultrathin ( 3 N 4 and TiN. Film structure, optical properties, etching, chemical stability, and radiation durability are discussed.