Hybrid photomask registration measurement system for high-precision and high-speed registration metrology

Abstract

Legacy-process semiconductors continue to support a wide range of electronic devices used in daily life. In semiconductor manufacturing, accurate mask pattern registration is essential, as it directly affects lithographic overlay performance and device yield. Registration systems for legacy nodes therefore require high accuracy, stability, throughput, and cost efficiency. Laser interferometers are commonly used for mask registration due to their high measurement precision; however, their accuracy can be degraded by air refractive index fluctuations, requiring strict environmental control. As an alternative, two-dimensional (2D) glass scales offer high repeatability and low sensitivity to environmental disturbances, although they include inherent nonlinear errors and thus require calibration. In this study, a hybrid photomask registration tool incorporating both a laser interferometer and a 2D glass scale was developed for the 90 nm process node, supporting photomask sizes from 5 to 9 inches. Calibration was performed in two stages: stage self-calibration using the laser interferometer, followed by calibration of the 2D glass scale referenced to the interferometer. Performance evaluation based on photomask mark measurements demonstrated that the calibrated 2D glass scale achieved mask shape measurement accuracy comparable to that of laser-interferometer-only measurements, while improving short-term repeatability by approximately 1 nm. These results confirm that a laser-calibrated 2D glass scale enables high-accuracy and high-stability mask registration measurement.