Prevention of Progressive Mask Defects through Residual Ammonium and Sulfuric Ion Control

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Progressive defect generation has been a serious issue in wafer lithography, as illumination wavelength becomes shorter with the introduction of the 248 and 193 nm wavelengths. Several research groups have reported that cleaning residues, mostly ammonium and sulfate ions, on a mask surface are critical sources of progressive defect generation. A new cleaning process has been developed and studied to minimize progressive defect generation sources. The new cleaning process consists of dehydration baking, dry etching, and H2O2 treatment. Dehydration baking was proposed to reduce the concentration of sulfate residues while H2O2 treatment and dry etching are proposed to control ammonium ions. Dehydration baking is employed at 230 °C for 600 s to reduce the sulfate ion concentrations from 32 to 1.6 ppb on chrome on glass (COG) or binary intensity mask (BIM). ArF embedded attenuate phase shift mask (EAPSM) is associated with a high concentration of ammonium ions (>5000 ppb), and is easily susceptible to progressive defect generation under ArF illumination. More than 90% ammonium extraction (removal) efficiency was achieved by H2O2 treatment for ArF EAPSM. Moreover, the optimized new cleaning process has reduced the concentration of ammonium ions up to 50 ppb compared with the 1500 ppb reported by conventional RCA cleaning in ArF EAPSM. In a strictly controlled sulfate environment (less than 6 ppb), threshold defect energy has improved from 25 to 100 kJ by reducing the concentration of ammonium ions down to 45 ppb and below. Therefore, the concentration of residual ammonium ions along with residual sulfate ions should be strictly controlled to improve wafer and photomask yields.