Rapid and precise monitor of reticle haze

Abstract

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 1 .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 2 . 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. INTRODUCTIONReticle haze is a problem that occurs in DUV and lower wavelength environments. Reticle haze was initially observed as a reticle back-side phenomenon but has recently been recognized as a contaminant that can appear on the chrome or 'object side of the reticle. Haze is a contaminant that is caused by mask-making chemicals, process residue, reticlecontainer out-gassing or through localized high-energy interactions with environmental gases in the exposure tool environment.Reticle haze on DUV process tools is rising in significance as a significant source of yield and capacity loss in device manufacturing. Reticle front or back-side haze causes non-uniform transmission loss within and near the hazed-areas across the reticl...