A method includes providing a photomask having a patterned absorption layer over a substrate. The photomask is irradiated with a beam having a mixture of transverse electronic (TE) waves and transverse magnetic (TM) waves. The irradiating includes generating surface plasmonic polaritons (SPP) on a sidewall of the patterned absorption layer. The SPP is used to suppress the TM waves while reflecting the TE waves. A target substrate is exposed to TE waves.