Novel metrology methods for image quality control

Abstract

New metrology methods have been developed to characterize exposure tool performance: SPIN for monitoring lens aberration, Z-SPIN for focus measurement, and EFSCP for checking effective light source profile. These methods, with design intention directed to measurement accuracy, measurement speed, and process robustness, provide increased accuracy and reduced time by a factor of 15 and 1/6, respectively, compared to traditional SEM metrology. SPIN allows direct measurement of lens aberration, achieving reproducibility of 0.8 mλ RMS. Z-SPIN translates focus (i.e. displacement in Z-axis direction) into an X– Y shift. Reproducibility of Z-SPIN measurement mostly depends on repeatability of the metrology tool, i.e. overlay metrology tool, which is ∼1 nm, and the measurement time of Z-SPIN is 1/8 of the conventional SEM measurement. EFSCP is a function to measure effective light source profile with high precision, achieving the repeatability of 0.00028 NAill for light source shift. While the traditional exposure tool performance inspections using SEM provide measurement reproducibility of 20 nm for IFD, 15 nm for astigmatism, and 2 nm for LR-CD, the new metrology methods provide 7 nm, 1 nm, and 0.3 nm, respectively, when estimated on the same scale. Also, the measurement result using the new methods shows very good agreement with the average of five SEM measurements for all the three inspection items. From this fact, it can be said that the traditional SEM-based inspection can be replaced by the new methods. To insure exposure tool quality in an appropriate manner, we at Canon recommend employing the new metrology methods in characterizing the exposure tool performance.