Use of fluorescent probes to determine catalytic chain length in chemically amplified resists

Abstract

Acid-catalyzed deprotection of tert-butoxycarbonyl (t-Boc) pendant groups present in a polymer film is one of the most common chemical reactions in photolithography. We present here a novel method to determine the catalytic chain length of this reaction under different development conditions. We demonstrate this in model PMMA thin films containing triphenylsulphonium triflate as a photoacid generator, acid sensor coumarin 6, t-Boc protected coumarin 4, and base 2-piperidin-1-yl-ethanol (2-PE). Deprotection of the t-Boc group, catalyzed by the photogenerated acid, during the post-exposure bake leads to formation of coumarin 4, a strongly fluorescent molecule whose concentration is monitored using fluorescence. We take advantage of the change in the emission spectra of coumarin 6 upon protonation to monitor acid formation. To quantify the amount of acid, an acid–base titration is done in the polymer film using 2-PE. Knowing the amount of deprotected probe and the amount of acid, we are able to calculate the number of moles of deprotected C4-t-Boc per mol of acid, which is the actual catalytic chain length.Key words: photolithography, chemical amplification, catalytic chain length, fluorescent sensors, photoacid generator.

Photoimaging and Lithographic Processes in Polymers

“…parameter critical for CA resists is the catalytic chain length (CCL), which is given by the number of deprotected t-Boc groups per proton generated. Literature reports suggest that a molecule of acid is capable of catalyzing from tens to hundreds of t-Boc deprotection reactions, for example, 800-1000 [18,19].…”

“…The reported values describe a diffusion sphere of the H þ with a diameter varying between 27 and 50 A , depending on the system studied, the PEB time, and temperature [18,19,33].…”

“…The detection and quantification of the acid formed have been done in two ways: in situ within the photoresist film, and in solution, the exposed film being developed with an appropriate solvent, and the acid titrated with a base in the presence of an acid-base indicator whose spectroscopic properties were easily monitored [18,[34][35][36][37][38][39].…”

“…In Scheme 12.8, we present a selection of acid sensors employed both in solution and in films [18,34,37,39,40].…”

“…A more convenient way of detecting and quantifying the acid formed is in situ, using fluorescent pH sensors that clearly mark the areas where the acid was formed [18,36,[38][39][40][41][42]. These sensors are introduced into the photoresist that is spin coated, then following exposure they image the areas where acid is formed.…”