Triple-exposure method for fabricating triangular-lattice photonic crystals

We have developed a triple-exposure method for defining a triangular-lattice pattern to form two-dimensional (2D) photonic-crystal (PC) waveguides operating at a 1.55-μm wavelength. This method, in which three line-and-space (US) 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-μm-pitch L/S masks were employed, an i-line stepper successfully resolved a 0.7-μm-hole-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 L/S photomasks with the corresponding parts of the space areas removed. We applied this method to fabricate a triangular-lattice 2D PC with a hole pitch of 0.8 μm and a minimum feature size of 0.05 μm. The measured photonic bandgap (PBG) of this PC indicated that it operates at a near-1.55-μm wavelength.