2.4.1 Binary Masks

A binary mask consists of a transparent plate called blank, covered with a patterned film of opaque material. The transmission characteristic is a binary one, i.e., ``1'' for transparent and ``0'' for opaque. The blank is made of soda lime, borosilicate glass, or fused quartz. The advantages of quartz are its good transparency even for DUV wavelengths at 246 nm or 193 nm and its very low thermal expansion coefficient. The low expansion coefficient becomes very important for small minimum feature sizes and large die sizes. The opaque material is typically a very thin ($\le$ 100 nm) film of chrome, covered with an anti-reflective coating, such as chrome oxide, to suppress interferences at the wafer surface. High quality photomasks must meet stringent requirements in flatness, accuracy of pattern placement, minimum feature size, linewidth control over the entire mask area, and defect density. The most common exposure tool to prepare binary masks is an electron-beam system (cf. Section 2.7.3). Up to 75% of electron-beam generated masks have been observed as being defective and require repair. Automatic mask inspection techniques are available for repairing defects larger than approximately 0.1 $\mu$m in size. Typical defects are missing chrome, pinholes in the chrome layer, or regions of unresolved chrome. Such defects can be repaired by locally adding opaque material or removing excess chrome with a laser beam or focused ion-beam.