Photoresist compositions and process for multiple exposures with multiple layer photoresist systems

A method of forming patterned structure, said method comprising:

• (a) dispensing a chemically amplified positive first photoresist formulation over a top surface of a substrate or a top surface of a fabrication layer formed on said substrate and removing a first casting solvent by pre-exposure baking said first photoresist formulation to form at first photoresist layer, said first photoresist formulation comprising a mixture of said first casting solvent, a first terpolymer or tetrapolymer dissolved in said first casting solvent, and a first photoacid generator;
• after (a), (b) patternwise exposing said first photoresist layer to first actinic radiation to form a first exposed photoresist layer;
• after (b), (c) developing said first exposed photoresist layer in a first aqueous base solution to selectively remove regions of said first photoresist layer exposed to said first actinic radiation to form a first patterned photoresist layer;
• after (c), (d) post-develop baking said first patterned photoresist layer to a temperature about 150° C. or greater, after said post-develop baking, said first terpolymer or tetrapolymer being substantially insoluble in said first casting solvent or a second casting solvent of a positive second photoresist formulation;
• after (d), (e) dispensing said positive second photoresist formulation over said patterned first photoresist layer, said second photoresist formulation comprising said second casting solvent, a second polymer dissolved in said second casting solvent, and a second photoacid generator, and removing said second casting solvent to form a second photoresist layer comprising said second polymer;
• after (e), (f) patternwise exposing said second photoresist layer to second actinic radiation to form a second exposed photoresist layer;
• and
• after (f), (g) placing said second exposed photoresist layer in a second aqueous base solution to selectively remove regions of said second exposed photoresist layer exposed to said second actinic radiation to form a second patterned photoresist layer.