Diffractive techniques for lithographic process monitoring and control

Abstract

Diffraction-based techniques have been shown to provide convenient, nondestructive, rapid metrology for lithography steps during semiconductor fabrication. Monitoring diffraction from latent images provides the capability to determine exposure tool focus to an absolute accuracy of 0.1 μm. Exposure dose and subsequently post-exposure bake have also been monitored using diffraction from latent images. Moiré alignment and overlay measurement techniques with nm-scale precision are demonstrated. Using 0.47-μm pitch gratings, a 1-nm translational sensitivity is demonstrated. A novel double-period moiré grating is used to provide both coarse (∼10 μm) and fine (∼1 μm) capture ranges for integration with existing stage positioning systems. A new diffraction-order interferometry technique for nm-precision remote overlay readout is demonstrated, with potential application to latent image structures immediately after exposure. We also present diffraction-based techniques to measure (critical dimensions) of ≥0.4-μm linewidth photoresist gratings. The results obtained are in excellent agreement with scanning electron microscope measurements.