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Studies of Cu Etchrate Variation in Dry-Film Photoresist Remover Containing Tetramethylammonium Hydroxide
Keywords: Cu Etch rate, mechanism, resist removal
Quaternary ammonium hydroxides are strong organic bases that are widely used in place of inorganic alkaline bases in semiconductor fabrication where metal ion contamination is of great concern. For example, tetramethylammonium hydroxide (TMAH) is effective for the dissolution of photoresist polymers, and therefore is commonly used for photoresist developing and stripping. A solvent mixture containing TMAH was formulated and has shown superb dry-film photoresist removal capability for the bumping process. Upon dry film dissolution, the UBM copper is exposed to the solution, in which case Cu corrosion may occur to different extents depending on the chemical conditions. One set of tests was carried out to monitor blanket Cu etchrate in the remover solution every 6 hours over a course of 48 hours. The results showed an increase in Cu etchrate with bathlife, i.e., the longer the solution is kept heated, the faster the copper film is etched away. In another set of tests, patterned wafers with dry-film photoresist and Cu UBM were loaded into the solution every 6 hours for 48 hours and Cu etchrate was measured. The results showed, however, an opposite trend in Cu etchrate as a function of bathlife, in which Cu etchrate decreased and stabilized as more wafers were loaded for cleaning. One chemical species was found by IC-TOFMS to play a key role in the Cu etchrate difference between these two tests. It was identified to be trimethylamine-n-oxide (tMAO), which is believed to be from trimethylamine, one of the decomposition products of TMAH. A possible mechanistic scheme is that TMAH and other chemical species in the solution decompose upon heating, and the decomposition products cause copper corrosion. As time goes on, more decomposition products, e.g. tMAO, are produced, causing Cu etchrate to increase. On the other hand, when dry-film photoresist is introduced into the solution, TMAH takes part in the dissolution of the resist polymer and is consumed. This in turn leads to fewer amounts of decomposition products that corrode Cu, such as tMAO, therefore lower Cu etchrate was observed. In conclusion, although an increasing trend of Cu etchrate is observed for a TMAH-containing remover solution over the course of long bathlife, when this remover solution is used as intended for dry-film photoresist removal, Cu etchrate is low and stable over the same period of time with continuous wafer loading.
David Maloney, Global R&D Director, EKC
DuPont WLP Solutions
Hayward, CA

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