Maskblanks

Mask blanks are the substrate portions of photomasks used in optical and extreme ultraviolet lithography. They serve as the base on which a circuit pattern is defined and subsequently transferred to semiconductor wafers. The blank must be manufactured with extreme flatness, parallelism, and surface quality to minimize printing errors during exposure.

Construction and materials: In conventional optical lithography, mask blanks are typically made from high-purity fused silica (quartz) or similar transparent substrates. An opaque metal layer, usually chromium, is deposited on the surface to form the light-blocking regions. Additional layers may be used for anti-reflective properties or to support different mask types. For phase-shift masks, the blank incorporates design features such as etched trenches or variable thickness to introduce phase differences that improve image contrast. Attenuated masks reduce transmission in the patterned areas.

Extreme ultraviolet (EUV) lithography masks use a different blank: a reflective multilayer coating, commonly molybdenum/silicon, deposited on a substrate such as silicon or high-quality glass, with a patterned absorber to define the image. EUV masks require stringent defect control because defects can directly imprint on wafers.

Quality and metrology: Mask blanks are specified for substrate thickness uniformity, surface roughness, defect density, and coating uniformity. Cleanliness and defect-free surfaces are critical, as even small defects can cause printing errors.

Manufacturing and use: After blank fabrication, the pattern is written by lithography tools (e-beam or laser writers) and then etched or deposited to create the final mask. Mask blanks are used in photolithography tools such as steppers and scanners to fabricate integrated circuits.