Mask contamination induced by X-ray exposure

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 expo sure conditions, and in the presence or absence of resist coating the main component of the contaminant is the salt (NH4)(2)SO4. On some resist-coated wafers, the contaminant growth is governed by the volume of sulfuric mater ials which evaporate during exposure. Contamination growth can be suppresse d by applying a top coat to the: resist, as using a resist that does not ev aporate 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 N H3 form rapidly up on X-ray irradiation. SO2 also increases up on X-ray irr adiation, and changes into H2SO4. H2SO4 easily combines with NH which is pr oduced by X-ray irradiation in humid air. and turns into the salt (NH4)(2)S O4. Mask coating was introduced to suppress the contaminant growth. The mas k coated with TiO2 layer does not show contaminant growth, showing a drasti c curtailment of contamination by the photocatalysis effect.