Addressing Back-End Lithography Issues
Ruth DeJule, Associate Editor -- Semiconductor International, 1/1/2000
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Instead of etching lines in thin metal films, the damascene process deposits metal into trenches that are etched in a dielectric and subsequently CMP polished. Of the four damascene processes in development — trench first, buried etch stop, buried etch mask and via first processing —via-first appears to offer greater advantages, such as 30% fewer fabrication steps and fewer metal interfaces. However, this process poses lithography challenges such as via fill, which is spurring studies, including one performed at Brewer Science (Rolla, Mo.) to investigate the reaction of via fill to different AR coatings. Organic bottom, instead of top, anti-reflective coatings (BARC) were used because in addition to reflectivity control, they can serve as an etch block and protection against reflective notching. Top AR layers are removed prior to the resist. Organic BARCs at this point are spin-on materials and thus provide a planarizing coating that allows the BARC to flow into via holes. In contrast, inorganic BARCs are typically very thin and conformal, and therefore not suitable as an etch block.
Via-first dual-damascene patterns the via using standard lithography before the dielectric material is etched. To form the trench, an AR coating is spun onto the wafer partially or fully filling the via, which provides an etch stop during the trench etch step. Next, a standard resist process forms the trench over the via. Upon exposure/develop, the BARC is etched, then the dielectric. The resist and AR layers are stripped, and metal is deposited. Metal lines are formed when CMP removes all metal except that within the trenches. Because the trenches must be filled, in addition to substrate stack reflectivity due to thin film interference, the via fill profile defined by meniscus shape and via fill depth must be carefully controlled.
| Via fill profiles of two high molecular weight chemistries, DUV42 with low activation energy (top) and DUV44 with high (bottom), illustrate bubble formation due to cross-linking after thermal reflow. (Source: Brewer Science) |
This study outlined the factors that affect the fill properties of AR coatings for partial fill dual-damascene processing. While it may conclude that process conditions, material properties and via hole size all play important roles, there is still much work to be done in determining the impact each of these factors ultimately will have. •
Brewer Science
