Measurements of acid generation by extreme ultraviolet irradiation in lithographic films

Abstract

Extreme ultraviolet lithography based nanofabrication using a bilevel photoresistExtreme ultraviolet ͑EUV͒ lithography requires photoresist materials that incorporate highly efficient photoacid generators ͑PAGs͒ due to the low intensity of the currently available EUV light sources. It is therefore necessary to understand the parameters that control acid generation mechanisms in photoresists under EUV irradiation, such as photoacid generator and base quencher structure, polymer matrix effects, and the interaction of ionizing radiation with the resist components. In this study, a known acid detection technique based on a dye indicator ͑coumarin 6͒ has been optimized for the quantification of the amount of photoacid produced by irradiation of lithographic resist films. Incorporation of acid-base indicators to solid polymer matrices has already been employed in previous studies; however, the environmental stability of the reactive species and the influence of the polymer resin have not been considered to date. Here, the authors present a comprehensive evaluation of PAGs that were exposed to 13.5 nm radiation and compared to 248 nm exposures. For some specific cases studied, it was possible to correlate the acid generation efficiency to the PAG structure and polymer matrix effects. Moreover, it is found that environmental effects such as humidity have a significant impact on acid strength, acid detection, and acid generation efficiency. The acid generation efficiency experiments were complemented with dose to clear exposures for selected resist systems. The results obtained indicate that the use of acid detection methods implemented for the design of EUV resist compositions with optimized photospeed requires careful attention to the complex role of the reactivity of all formulation components and photoreaction products in the presence of ionizing radiation.

Acid generation efficiency: EUV photons versus photoelectrons

“…PAG-11 bears semi-fluorinated phenyl moieties with the resulting reduction in hydrogen content and increase in hydrophobicity compared to other PAGs. Aggregation might be effective for PAG-11 as it was shown for PAGs with high fluorine content blended in non-polar resist matrices [17], thus reducing the effective PAG concentration with the detrimental impact in photospeed. In the case of PAG-12, the unusual acid generator cation [37] was chosen in order to address the possible negative impact of outgassed iodine in EUV optics reflectivity and investigate the effect of the geometrically strained C-I + -C bond angle on photospeed response.…”

“…Ultimately, acid generation efficiency is described by the quantum yield (ϕ), defined as the total number of photogenerated acid molecules per absorbed EUV photon, which is dependent on the initial PAG loading. Reported quantum yield values are around 2-3 for PAG concentrations typically used in photoresist formulations [16][17][18][19], evidencing the important role that EUV photoelectrons play in image formation. …”

Radiation Chemistry in Chemically Amplified Resists

“…118,127,128) The relationship between reductive potential and relative acid yield has been reported for sulfonium and iodonium salts as well as for nonionic acid generators. 125,126) Also, the reactivity of acid generators with solvated electrons formed in solutions has been measured from the viewpoint of molecular structure dependence.…”

Charge Transfer in Photoacids Observed by Stark Spectroscopy

“…2 Photoacids also provide an efficient means of generating protons at a specific time, and this concept has been used extensively in the development of polymers for photolithography. [4][5][6] The change in acidity between the ground and excited states of photoacids can be extraordinary, with the acidity equilibrium constant (K a ) differing by more than 10 orders of magnitude in some cases. 7 Photoacids have been studied in both gas and condensed phases, with benzene and naphthalene derivatives examined most often in the gas phase, [8][9][10][11] and naphthalene and pyrene derivatives studied more extensively in condensed phases.…”