Progress in KrF Antireflective Coatings
By Semiconductor International Staff -- Semiconductor International, 5/1/2000
At 0.18 µm, antireflective coatings are widely used to enhance CD control by suppressing reflective notching, standing wave effects and the swing ratio caused by thin film interference. One of the major challenges for bottom antireflective coatings (BARC) today is maintaining efficient light reflection reduction from the substrate as photoresists and BARCs get thinner with decreasing feature size (see Figure). Dual-damascene structures also require BARC films with gap-filling capability. At the recent SPIE meeting, several companies presented results from characterizing new BARC materials that address these concerns.
 Reflectivity is a function of film thickness, refractive index (n) and extinction coefficient (k). (Source: Brewer Science) |
Researchers from Tokyo Ohka Kogyo Co. (Kanagawa, Japan) evaluated the effectiveness of SWK-EX6 BARC used with DP-308AL KrF photoresist and WINNELLI simulation software from Sopra Co. (Bois-Colombes, France). Simulations determined refractive index (n) and extinction coefficient (k) values of 1.80 and 0.64, respectively, for KrF lithography, regardless of thickness variation in the layers. Evaluation with 430 nm of photoresist on 40 nm of BARC baked at 180degC for 90 seconds enhanced depth of focus to 1.0 µm for 150 nm lines and spaces. Gap-fill properties were achieved by substituting anthracene as dye and low molecular oligomer of melamine derivatives as binders. The study showed the BARC can contain the effects of reflectivity, making thin layer application possible for 150 nm design rules.
Researchers from Novellus (San Jose), MIT (Cambridge, Mass.), Shipley (Marlboro, Mass.) and Nanometrics (Sunnyvale, Calif.) teamed to develop a universal antireflective layer (UARL) inorganic film stack for transparent dielectric films in multilayer metal stacks. This situation is particularly challenging because "a reflection minimized for one local film stack may result in a high reflection for another local film stack." The UARL, deposited by PECVD, is designed to not contribute to interface reflection, making interface reflectivity inside the photoresist (Rsub) the same across the wafer. The UARL also reduces Rsub to a few tenths of 1%, eliminating CD variation caused by resist thickness non-uniformity. The group determined optical design requirements using Prolith/2 simulations on deep sub-quarter-micron damascene structures, measured leakage current behavior at device electrical field levels and determined PECVD oxynitride film characteristics as a function of key process parameters. The group demonstrated statistical linewidth CD control on different substrate types and also presented marathon data.
Two concerns in the use of organic BARC films are slow etch rate and incompatibility with a variety of photoresists. Compatibility is especially critical with chemically amplified resists that can negatively interact with the acidity levels of the BARC. Researchers from Hyundai Electronics (Kyoungki-do, Korea) developed an organic BARC material for KrF and ArF lithography containing chromophore and cross-linkable functional groups in the same copolymer as the photoresist to prevent the formation of acidic or basic byproducts during thermal hardening. The researchers tested lithographic performance and compatibility of the organic BARC with ESCAP, acetal and hybrid types of resist. The material also demonstrated compatibility with several alicyclic and acrylate-based resists for ArF (193 nm) exposure.
Also facing the challenges of developing more robust BARC films, researchers from LSI Logic (Santa Clara, Calif.) and Brewer Science (Rolla, Mo.) tested a new BARC, DUV44, a material designed to overcome problems of track and coat bowl contamination and coagulation in drain lines. DUV44, developed by Nissan Chemicals, has the same optical properties of its predecessor, DUV42 (k=1.49, n=0.42), so the conversion in manufacturing should introduce minimal process variation with respect to depth of focus or exposure latitude. The new BARC has 70% less catalyst to address spin bowl coagulation issues, and the polymer liner was changed to a more basic crosslinking reagent to resolve resist profile issues. Modification to a weaker acidic system means DUV44 will not process well with strong acid acetal-based resists. •
— Laura Peters
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