Mask Contamination Induced by X-Ray Exposure

Abstract

To investigate the growth of mask contamination induced by X-ray exposure, masks are exposed to X-rays using an X-ray stepper. Independent of the exposure conditions, and in the presence or absence of resist coating, the main component of the contaminant is the salt (NH4)2SO4. On some resist-coated wafers, the contaminant growth is governed by the volume of sulfuric materials which evaporate during exposure. Contamination growth can be suppressed by applying a top coat to the resist, as using a resist that does not evaporate as a sulfuric out-gas. Gases were irradiated by X-rays in order to examine the change in environmental pollutants. In humid air, NO, NO2 and NH3 form rapidly up on X-ray irradiation. SO2 also increases up on X-ray irradiation, and changes into H2SO4. H2SO4 easily combines with NH3 which is produced by X-ray irradiation in humid air, and turns into the salt (NH4)2SO4. Mask coating was introduced to suppress the contaminant growth. The mask coated with TiO2 layer does not show contaminant growth, showing a drastic curtailment of contamination by the photocatalysis effect.