Femtosecond laser processing of photomask glass for transmittance control

Abstract

Photolithography is the most important process in semiconductor manufacturing processes. To ensure high quality products, the uniformity of critical dimension (CD) of the circuit patterns produced by photolithography process is one of the important requirements. It has been reported that the degradation of CD uniformity occurred by various reasons such as the non-uniformity of light source, glass, coating, or etching and also by mask error. In this thesis, the improvement of light illumination uniformity on photomask by locally controlling the transmittance of the photomask glass is studied. The control of glass transmittance was achieved by creating scattering patterns inside the photomask glass by using a femtosecond laser. When the femtosecond laser was focused into the quartz using an objective lens, permanent deformation occurred, causing a decrease of refractive index. The created laser pattern serves as an internal scattering pattern, thereby lowering the transmittance of light. This femtosecond laser patterning was locally applied on the photomask glass where overexposure was observed in order to make the illumination at the exit side of the glass be uniform. The effects of various process parameters including laser pulse energy, spot-to-spot distance, focusing, and number of pattern layers on transmittance were investigated. It is shown that the transmittance of ultraviolet light (at 400 nm wavelength) can be controlled in the range of 46~92% with a single layer pattern by changing process conditions. It is also shown that the CD of developed patterns after photolithography varied proportionally with the glass transmittance.