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In response to a study conducted at Perkin-Elmer ALO, which showed that surface anomalies present on PBS-coated masks prior to exposure were responsible for more than 50 percent of the chrome spot defects observed on the finished masks, Perkin-Elmer EBT initiated a study focusing on the causes of the defects found in the original materials. This study found that chemical oxidation, not foreign particulate contamination, was the major cause of the discrete defects found on PBS films. Elemental analysis of the small defects on fresh PBS-coated quartz masks with a scanning electron microscope, energy-dispersive x-ray analysis, and Auger spectroscopy showed that 66 percent of the defects contained little or no elemental contamination other than excess oxygen. X-ray photoelectron spectroscopy and small-spot refective infrared analysis of larger oxidized defects on older plates and of defects induced by known initiators of oxidation in polymers yielded information on chemical functional groups in the defects. This information confirmed the finding that oxidation was responsible for the changes observed in discrete areas. Oxidation changes the physical and chemical properties of the reacting area and produces an embedded particle or pinhole that causes a chrome spot or pinhole defect in the finished mask. Perkin-Elmer EBT has matched chrome spot defects that are images of known oxidized areas on PBS films with chrome spot defects detected by a KLA on finished production masks at Perkin-Elmer ALO. That similar defects have been observed on PMMA, EBR-9, and several kinds of AZ optical resists allows us to predict that oxidation of resists increases defect densities in all types of microlithography. The mechanisms of oxidation that result in discrete defects are described, and ways of reducing or eliminating such defects are discussed.
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