Process monitoring of etched fused silica phase shift reticles

摘要

Alternating Aperture Phase Shift Masks (AAPSMs) are becoming increasingly important as a reticle enhancement technique (RET) in meeting the key lithography requirements for the ITRS Lithography Roadmap. Fused silica etch depth is the critical parameter that determines the phase shift and must be monitored closely for depth and uniformity. At present the methods that measure phase-shift and etch depth, such as interferometry, profilometry, AFM, and SEM, are expensive, slow, and/or destructive, making alternate techniques attractive for a production environment. This paper will present a novel technique, which utilizes a special type of broadband spectrophotometry, specifically developed for high-throughput, accurate, and non-destructive real-time measurements of trench depth. The measurement takes only three seconds per measured site, so that a full uniformity map can be obtained in two to three minutes. The technique involves simultaneous measurement and mapping of reflectance (R) and transmittance (T) from 190-1000 nm and analysis based on the Forouhi-Bloomer dispersion equations, to efficiently and nondestructively monitor trench depth. This new measurement system exhibits excellent repeatability, correlates very closely with AFM, SEM, as well as direct phase shift measurements, and has been proven to perform very well in production environments. Data will be presented from the user's (Etec Systems, Applied Materials Mask Business Group) viewpoint.