Numerical analysis of the sub-wavelength fabrication of MTMO grayscale photomasks by direct laser writing.

Metal-transparent-metallic-oxide (MTMO) grayscale photomasks fabricated by direct laser writing have been proposed in recent years. The fabrication mechanism is attributed to light-induced melt-oxidization. The temporal-spatial distribution of temperature fields of indium film-glass samples under a laser pulse have been calculated by the Finite-Difference Time-Domain method. The laser action area of the indium film is studied based on the oxidation theories and the absorbed laser power density distribution in molten indium films. The calculated average sub-wavelength fabrication diameter of 302 nm is consistent with the experimental fabrication size under a laser power of 6.0 - 8.0 mW.