Polymer dissolution. I. On the dissolution behavior of copolymers of methyl methacrylate and methacrylic acid

Copolymers of methyl methacrylate with methacrylic acid P(MMA/MA)] are interesting resist materials for microlithography. At moderate baking temperatures, these copolymers undergo an intramolecular cyclization, yielding terpolymers containing anhydride moieties. This process has striking consequences for the dissolution behavior and results in higher ketone solubilities. The dissolution rates in methyl ethyl ketone (MEK) and in mixtures with ethyl glycol (EG) correlate in an Arrhenius-like manner with the reciprocal baking temperatures. Two distinct temperature ranges with different slopes between 130–180°C and 180–230°C are found. This is in good agreement with other findings, indicating a different mechanism of anhydride formation in these temperature regions. The activation energies for the dissolution of P(MMA/MA) in EG or EG/MEK mixtures are about 17 kcal/mol and those of the thermally treated material in MEK or MEK/EG or mixtures of MEK with methyl isobutyl ketone (MIBK) are in the range of 20–30 kcal/mol. For the investigation of the M_n dependence, γ-irradiated probes of the copolymer were used. We obtained the usual exponential M_n dependence with an exponent of 0.7–0.8 in MIBK and 0.3–0.4 in EG. Our findings are in agreement with a “relaxation-controlled” dissolution behavior, especially for the anhydride-containing terpolymer. No residual layers or pronounced inhibition periods indicative for gel-layer formation, however, could be found. We suggest a normal dissolution process with a very small gel layer. For the copolymer in an alcohol-containing solvent, a stress-driven dissolution behaviour is more likely. © 1994 John Wiley & Sons, Inc.