We investigate the reduction of transition loss across the star coupler boundary in a silicon arrayed waveguide grating (AWG) by suppressing multimode generation and scattering near the boundary of a star coupler. Eight-channel silicon AWGs were designed with optimal conditions based on enhanced field matching in combination with ultrashallow etched structures. The fabricated AWG demonstrates an insertion loss down to 0.63 dB with a cross talk of -23 to -25.3 dB, exhibiting ~0.8 dB improvement of insertion loss and ~4 dB improvement of cross talk compared to the Si AWG fabricated with a conventional double-etch technique.