Following the Acid: Effect of Acid Surface Depletion on Phenolic Polymers

This paper reports on the diffusion coefficients for acid loss for triflic acid (TFA), toluenesulfonic acid (TSA), camphorsulfonic acid (CSA), and methanesulfonic acid (MSA) in an hydrogenated poly(p-vinyl)phenol matrix. A direct method for following acid content was derived from the TBPB spectrometric titration method. In this manner, the Ea for acid loss for TSA, MSA, and TFA was found to be 76, 87, and 106 kJ/mol, respectively. Using the Ea values, acid loss could be predicted over a wide range of processing conditions. It was found that MSA had an order of magnitude greater D than TSA and TFA, which renders MSA an impractical acid for use in chemically amplified resists. For TSA and TFA, D was found to be 1.2 x 10-5 μm2/s and 4.8 X 10-6 μm2/s,respectively, at 90°C. These values make these acids usable at 90°C, but TSA will be more problematic than TFA. All of these acids show sufficient acid loss at room temperature so that long term staging of exposed resists is not recommended. For DUV lithography with chemically amplified resists, we recommend linked PEB tracks directly to the exposure tool so that the maximum postexposure delay is 15 minutes. The one acid that shows virtually no acid loss even at 150°C was CSA. This acid shows no evidence of 't-tops' and may be the acid of choice for high thermal processing.