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Useful protocol for evaluating subtle and important differences between photoresist formulations
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15.We have measured the resist C parameter by the standard addition technique for the ND-Tf/PTBOCST system in previous work (see Ref. 5) using the 1PE base quencher exposed to the same source of x rays. Interestingly, the results from that experiment determined the C parameter to be with a 95% confidence interval of The results from the present study determined a greater C parameter than earlier work for the ND-Tf/PTBOCST system. There were several key differences in these experiments that likely explain the discrepancy between measurements. In the prior work, the concentration of ND-Tf was 80 μmol/g in the resist, the postapplication bake was at 90 °C for 60 s, the initial film thickness was approximately 800 nm, and the postexposure bake occurred at 90 °C for 90 s. It is important to recognize that the resist C parameter is a measurement of the overall efficiency of the photoacid generation process in the resist film, and is not a true measurement of the quantum yield of the PAG. It is entirely possible that factors in addition to the radiation chemistry during exposure may affect the magnitude of the C parameter. For example, in Ref. 11 we demonstrated that the concentration of PAG in the resist impacts the apparent efficiency of the photoacid generation process, with greater concentrations of PAG resulting in less efficient photoacid generation.
16.One caveat to the standard addition technique is that the determination of photoacid concentrations is made indirectly from the neutralization of photoacid by base. The major assumption of the technique is that the base acts only to neutralize a stoichiometric equivalence of photogenerated acid prior to the postexposure bake. The verification of this assumption for the 1PE base quencher in PTBOCST can be found in Refs. 4 and 5.
17.The removal of tBOC protecting groups from the PTBOCST polymer can be monitored using infrared spectroscopy by absorption peaks at 1149 and 1735 cm−1. The absorbance at 1149 cm−1 is characteristic of the tert-butyl group, and the absorbance at 1735 cm−1 is characteristic of the carbonyl. The absorbance of both peaks was monitored and found to exhibit nearly equivalent results. Only the absorbance at 1149 cm−1 is presented.
18.Figure 2 presents data for resist formulations containing 7 μmol/g of base (10% base loading). See Refs. 4 and 5 for verification that systems with and without the 1PE base in PTBOCST exhibited the same dependence of the infrared absorbance on the calculated concentration of photoacid.
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