Determination of coupled acid catalysis-diffusion processes in a positive-tone chemically amplified photoresist

Abstract

Acid diffusion during postexposure baking is viewed to be a limiting factor in the extension of lithography using chemically amplified resists to formation of nanoscale features. Quantification of thermally activated reaction-diffusion kinetics in these materials is therefore an important step in understanding the extendability of this class of resist systems. Previous investigations have addressed this issue, however there is poor agreement among them, and too few data exist in the literature to allow the systematics of the effect of polymer, photoacid generator, added base or other resist components on the diffusion process to be understood. We describe in this article a combined experimental and modeling protocol that is designed to elucidate the chemistry and physics of the reaction-diffusion process. Because it is physically based, not phenomenological, it provides a means of developing a set of predictive, mutually comparable data that will allow new insights to be developed into the nanoscale behavior of chemically amplified resist materials. We apply the protocol to a p-t-butyloxycarbonyloxystyrene/bis͑t-butylphenyl͒iodonium perfluorobutanesulfonate positive-tone photoresist system. The resulting kinetics measurements show that diffusion is environment sensistive and describable with two limiting diffusion coefficients. The Arrhenius parameters for the coefficients in p-t-butyloxycarbonyloxystyrene are D 0 ϭ1.9ϫ10 8 cm 2 /s and E a ϭ36.5 kcal/mol; those for diffusion in the deprotected polymer product p-hydroxystyrene are D 0 ϭ9ϫ10 Ϫ3 cm 2 /s and E a ϭ22.1 kcal/mol. The coefficients are much smaller than previously reported, resulting in a very slow diffusion rate. The model indicates that the considerable image spreading observed during the postexposure bake process is attributable primarily to the efficiency of the catalytic chemistry. Our results suggest that numerical models currently used for prediction of imaging in chemically amplified resists may require refinement in order to be useful for feature sizes below 100 nm and for new classes of resist systems.

Stochastic methods for aerosol chemistry: a compact molecular description of functionalization and fragmentation in the heterogeneous oxidation of squalane aerosol by OH radicals

“…Similar approaches have been previously applied to kinetics modeling of gas-phase aerosol oxidation, 35 combustion, [36][37][38][39] and polymer chemistry. [29][30][31] Because of the symmetry of the molecule, squalane is broken up into a collection of hydrogen reactive sites as shown in Figure 3.. The concentration of each type of hydrogen is set to the initial concentration of squalane multiplied by the number of hydrogens for each type.…”

“…The program used for the simulations is called Kinetiscope and is an enhanced version of the CKS and VSIM codes used in previous work. [29][30][31] This simulation package has been used to successfully describe key aspects of materials chemistry, from rare events controlled by coupled equilibria 32,33 to coupled reaction-diffusion processes on surfaces. 30 The simulations generate an absolute time base, enabling direct comparison of the concentrations and other system properties as a function of time to experimental data.…”

“…[29][30][31] This simulation package has been used to successfully describe key aspects of materials chemistry, from rare events controlled by coupled equilibria 32,33 to coupled reaction-diffusion processes on surfaces. 30 The simulations generate an absolute time base, enabling direct comparison of the concentrations and other system properties as a function of time to experimental data. Most importantly, because the algorithm involves simple arithmetic to advance the simulation instead of integration of coupled differential equations, molecules can be represented as whole or partial species.…”

“…30 To initialize the calculation, the molecular volume of squalane (i.e. the inverse of density, ρSq) is divided equally among the 30 different reactive sites.…”

Resist Requirements and Limitations for Nanoscale Electron-Beam Patterning

“…Detailed models have been produced of the processes occurring during acid diffusion and acidolysis [23,24,25], incorporating effects such as the sharp decrease in acid diffusion that occurs when the acid is diffusing in a deprotected as opposed to protected area and the evolution of deprotected site density during post-exposure bake. Current estimates [26] put the acid diffusion range at 5 nm during a typical post exposure bake, indicating that it is the highly efficient acidolysis process that is a major cause of image blur during PEB at large feature sizes, while the two are equally important in the 40 -50 nm range, with acid diffusion dominating at the smallest feature sizes.…”