This work presents a photomask lithography for mass manufacture of 100 nm photonicbandgap structures with designed defects. The photomask is composed of 300 nm thickdielectric rod arrays in order to generate edge diffracted beams. As the rod sizes are smallerthan one wavelength, diffracted beams from the edges merge together and formsubwavelength beaming arrays at rod regions. Theoretical calculations and opticalnear-field measurements verify the existence of subwavelength focused beams. Furthermore,the focused beams are not affected by the neighbouring defects in the near-fieldregion. Using the transparent photomask, a simple laser exposure system andreactive dry etching method, we demonstrate the fabrication of subwavelengthphotonic bandgap structures with designed channel defects in a silicon substrate.