ULSAN — Researchers have developed a technology that allows the sensitivity and precision of next-generation extreme ultraviolet (EUV) photoresists, a core material in ultrafine semiconductor processing, to be tailored to specific process requirements.
A team led by Professor Chang Ji-hyun of the Department of Energy and Chemical Engineering at the Ulsan National Institute of Science and Technology (UNIST) said on the 21st that it had developed a design technology that independently controls the reaction speed and circuit precision of photoresists by tuning the chemical properties of ligands in tin-based materials. The findings were selected as a back cover paper in Small, an international journal in the nanotechnology field.
Photoresists are light-sensitive materials that react to light to draw fine circuits on semiconductor wafers. With the recent adoption of EUV lithography, tin oxide cluster (SnOC) materials have drawn attention for their strong light absorption and high resistance during etching. But existing approaches struggled to overcome a trade-off known as the RLS trade-off: raising tin content to boost sensitivity caused metal contamination, or internal bonds broke down randomly during exposure, leaving pattern edges rough.
The team solved the problem by altering the electronic properties of the ligands, the organic molecules surrounding the tin atoms. By reacting adamantane-1-carboxylic acid, which donates electrons, and diphenylphosphinic acid, which withdraws electrons, with tin precursors, the researchers selectively synthesized a cluster containing six tin atoms (6-SnOC) and one containing three (3-SnOC).
Analysis and simulations showed clear differences in the exposure reaction paths of the two materials. When exposed to an electron beam, 6-SnOC first broke tin-carbon (Sn-C) bonds, which have low bond dissociation energy, and cross-linking between molecules proceeded rapidly. As a result, it showed high sensitivity, quickly forming ultrafine patterns 20 nanometers wide with a low beam dose (D₀ of 285 µC·cm⁻²).
By contrast, in 3-SnOC the robust aromatic ligands suppressed decomposition, while an inorganic network condensation reaction, in which tin and oxygen become densely intertwined, proceeded gradually. It required a larger electron beam dose (D₀ of 1,000 µC·cm⁻²), but produced smooth, straight circuits with line edge roughness (LER) of just 1.02 nanometers.
Using density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS), the team demonstrated the sequence in which chemical bonds break down before and after exposure at the atomic level.
The technology is seen as opening the way to process-specific molecular design of photoresists, with 6-SnOC applied to ultrafast processes that prioritize productivity and 3-SnOC used in precision processes where line width roughness and uniformity matter.
"This study showed that ligand properties, not just tin content, determine sensitivity and pattern precision," Chang said. "It will be used as a design standard for developing next-generation EUV photoresists that meet process requirements."
