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23 June 2000 Chemistry of photoresist reclamation III
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We have studied the chemical changes that result in photoresist used for LCD device manufacturing due to various stresses that the photoresist is subjected to during the recovery and reclamation process. (1) When the roll coater recovery photoresist is kept open at room temperature for 50 days, needle-shaped crystals formed in the recovery photoresist. Examination by X-ray diffraction revealed that those crystals are ammonium zinc sulfate hexahydrate that was grown owing to the dissolution of the roll material of coater. It is possible to avoid this problem by appropriate selection of the material that comes in contact with photoresist. (2) When new, tightly sealed photoresist was subjected to thermal stress at 40 degrees Celsius for 100 days, a bead-shaped coagulated material appeared. NMR analysis showed that the coagulated material was mainly a photosensitive ester tetramer formed by a thermal change. Furthermore, HPLC analysis showed that the tetramer consisted of mainly tetraesters, but contained diesters, triesters, even monoesters. (3) In the case of the spin coating, although there is virtually no damage from physical stress, water content of about 1% is absorbed from the air during the spin-coating process, and the photo-active compound is decomposed by the thermal stress on the photoresist, lowering the photosensitivity. This change in the photosensitivity of the photoresist can be virtually eliminated if the photoresist contains now water so that the photo-active compound chemical change does not occur during the distillation process. (4) In the reclamation of photoresist, (a) the photoresist is coated in a dry nitrogen atmosphere with no water content, and next (b) the photoresist in a coater cup is dissolved by a photoresist solvent and recovered, and then (c) the viscosity of the recovered photoresist is adjusted by distillation, and it is filtered at the end. By adding fresh photoresist, this type of reclamation can be continuously performed.
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Hideki Nishida, Akihiko Igawa, Kenji Ohshiro, and Itaru Shiiba "Chemistry of photoresist reclamation III", Proc. SPIE 3999, Advances in Resist Technology and Processing XVII, (23 June 2000);

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