The near ultraviolet spectroscopy and photodissociation dynamics of two families of asymmetrically substituted thiophenols (2- and 3-YPhSH, with Y = F and Me) have been investigated experimentally (by H (Rydberg) atom photofragment translational spectroscopy) and by ab initio electronic structure calculations. Photoexcitation in all cases populates the 11ππ* and/or 11πσ* excited states and results in S–H bond fission. Analyses of the experimentally obtained total kinetic energy release (TKER) spectra yield the respective parent S–H bond strengths, estimates of ΔE(A∼−X∼), the energy splitting between the ground (X∼) and first excited (A∼) states of the resulting 2-(3-)YPhS radicals, and reveal a clear propensity for excitation of the C–S in-plane bending vibration in the radical products. The companion theory highlights roles for both geometric (e.g., steric effects and intramolecular H-bonding) and electronic (i.e., π (resonance) and σ (inductive)) effects in determining the respective parent minimum energy geometries, and the observed substituent and position-dependent trends in S–H bond strength and ΔE(A∼−X∼). 2-FPhSH shows some clear spectroscopic and photophysical differences. Intramolecular H-bonding ensures that most 2-FPhSH molecules exist as the syn rotamer, for which the electronic structure calculations return a substantial barrier to tunnelling from the photoexcited 11ππ* state to the 11πσ* continuum. The 11ππ* ← S0 excitation spectrum of syn-2-FPhSH thus exhibits resolved vibronic structure, enabling photolysis studies with a greater parent state selectivity. Structure apparent in the TKER spectrum of the H + 2-FPhS products formed when exciting at the 11ππ* ← S0 origin is interpreted by assuming unintended photoexcitation of an overlapping resonance associated with syn-2-FPhSH(v33 = 1) molecules. The present data offer tantalising hints that such out-of-plane motion influences non-adiabatic coupling in the vicinity of a conical intersection (between the 11πσ* and ground state potentials at extended S–H bond lengths) and thus the electronic branching in the eventual radical products.
Hydrogen Silsesquioxane (HSQ) photoresist has shown extremely high-resolution performance for Electron-Beam Lithography (EBL) and Interference Lithography (IL) and can be a potential photoresist candidate for Extreme Ultraviolet Lithography (EUVL). To optimize this system for sub-10 nm patterning, it is important to understand the EUV and electron-induced chemistry underpinning the functionality of this resist material. Here we present a EUVprintability study on HSQ photoresist at a resolution of 16 and 22 nm combined with a mechanistic study on EUV and electron-induced desorption of HSQ film. Firstly, patterning results showed that the simple HSQ cages require a high EUV-dose and an aggressive developer to print dense features. EUV-and electron-induced desorption experiments revealed that hydrogen and silane are the dominant species fragmented from HSQ, indicating dehydrogenation and redistribution pathways as the crosslinking mechanism. Quantum chemical calculations suggested that the Neutral Dissociation (ND) is the dominant mechanism in HSQ cross-linking at low energies, i.e., below its ionization threshold, whereas Dissociative Ionization (DI) contributes significantly at higher energies. A distinct structure is observed at about 8 eV and a clear peak at about 11 eV indicating a significant contribution through Dissociative electron attachment (DEA) at these energies. Based on these results, an engineered HSQ system is designed by adding silanol or carbinol (R-CH3OH)-groups to the partially crosslinked HSQ-cages to increase its Tetramethylammonium hydroxide (TMAH)-developer and EUV-sensitivity. Finally, 2.38% v/v TMAH is used to develop a 16 nm printed dense line-space (L/S) with a line-edge-roughness (LER) of 6.4 nm but requiring an EUV-dose of over 100 mJ/cm 2 .
Secondary electrons generated during the Extreme Ultraviolet Lithography (EUVL) process are predominantly responsible for inducing important patterning chemistry in the photoresist film. Therefore, it is crucial to understand the electron-induced...
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