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Reticle Haze results from the deposition of a chemical residue of a reaction that is initiated by Deep Ultra Violet (DUV)
or higher frequency actinic radiation. Haze can form on the backside of the reticle, on the chrome side and on the pellicle
itself.
The most commonly reported effect of haze is a gradual loss in transmission of the reticle that results in a need to
increase the exposure-dose in order to maintain properly sized features. Since haze formation is non-uniform across the
reticle, transmission loss results in an increase in the Across Chip Linewidth Variation (ACLV) that is accompanied by a
corresponding reduction in the manufacturing process window. Haze continues to grow as the reticle is exposed to
additional low wavelength radiation through repeated use.
Early haze formation is a small-area phenomenon in comparison to the total area of the reticle and may initiate
simultaneously in separate areas. The early stages of reticle haze therefore results in a degradation of Best Focus, Depth
of Focus and the Exposure latitude of individual features in the "hazed" area prior to any noticeable large area
transmission loss. Production lots subject to reticle hazing on critical layers will experience a direct loss of lithographic
yields, loss of capacity, an increase in rework rates and an ultimate loss in overall final-test yield long before the need for
an overall image exposure-dose increase is detected.
Feature profiles and process response are degraded at the earliest stages of haze formation. While early hazing may occur
in a small area of the reticle, the area influenced by the initial deposition is relatively large in comparison to the size of
an individual circuit feature. A sampled metrological inspection of a regular array of points across the exposure field is
therefore able to detect any form of reticle haze if the analysis monitors the feature-profile response rather than simply
feature widths. A model-driven method for the early detection of reticle-haze using basic feature metrology is developed
in this study. Application results from a production reticle are used to demonstrate validation of the technique that
employs a highly accurate method of calculation of the uniformity of the reticle exposure-response for individual
features across the exposure.
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