Proceedings Article | 21 November 2023
KEYWORDS: Contamination, Photomasks, Inspection, Standards development, Defect inspection, Pellicles, Particles, Optical spheres, Manufacturing, Extreme ultraviolet
Photomask contamination standards are used by leading-edge equipment suppliers to develop, qualify, and calibrate defect mask inspection and review systems. Mask shops at prominent device manufacturers also use mask standards to qualify inspection/review tools during process development and for periodic quality checks during manufacturing. Although perhaps widely overlooked, these standards are essential for a healthy mask ecosystem. Contamination standards are produced by depositing soft defects of known composition, size, and morphology at prescribed locations on a mask or pellicle surface (front side and/or back side). A typical standard consists of multiple circular spot deposits each with hundreds to thousands of uniformly dispersed particles. The electrophoretic deposition process is virtually indifferent to the substrate, having been applied to mask blanks, films (metal, nitride, resist, absorber), and patterned masks, both DUV and EUV. Challenges met over the past decade include sub-nanometer repeatability of nanoparticle size, deposition on mask edges (bevel and sidewall), deposition of large particles up to 20μm in diameter, and deposition on pellicles. Inspection of deposits on wafers are used as a proxy for inspection of masks and pellicles, enabled by repeatable deposition processing. Particle materials have evolved with inspection capabilities and challenges. Polystyrene latex (PSL) spheres, used ubiquitously as size standards for decades, have largely been replaced by SiO2 spheres, which better withstand intense DUV and EUV energies. Recently, we have refined processes to deposit different defect materials such as alumina, silicon nitride, tin, and tungsten to emulate process-induced defects found in the real world.
