Triple-exposure method for fabricating triangular-lattice photonic crystals

We have developed a triple-exposure method for defining a triangular-lattic e pattern to form two-dimensional (2D) photonic-crystal (PC) waveguides ope rating at a 1.55-mu m wavelength. This method, in which three line-and-spac e (L/S) patterns are projected onto a wafer in three different directions, allows us to define a smaller-pitch triangular-lattice pattern compared to that definable by the conventional exposure method. When 0.61-mu m-pitch L/ S masks were employed, an i-line stepper successfully resolved a 0.7-mu m-h ole-pitch triangular-lattice pattern, which was at the resolution limit. A sharply bent line defect was introduced as a waveguide into the pattern by using US photomasks with the corresponding parts of the space areas removed . We applied this method to fabricate a triangular-lattice 2D PC with a hol e pitch of 0.8 mu m and a minimum feature size of 0.05 mu m. The measured p hotonic bandgap (PBG) of this PC indicated that it operates at a near-1.55- mu m wavelength.