An illumination system includes a light source used to generate a light and an opaque plate. The opaque plate is disposed between the light source and a photomask and includes an annular aperture and an aperture dipole. The annular aperture has an inner side and an outer side. The aperture dipole includes at least one first aperture and at least one second aperture. The first aperture and the second aperture connected to the annular aperture respectively and protruding out from the outer side of the annular aperture are disposed symmetrically with respect to a center of the annular aperture.