Sensitizers in extreme ultraviolet chemically amplified resists: mechanism of sensitivity improvement

Abstract

Extreme ultraviolet (EUV) lithography utilizes photons with 92 eV energy to ionize resists, generate secondary electrons, and enable electron driven reactions that produce acid in chemically amplified photoresists. Efficiently using the available photons is of key importance. To increase photon absorption, sensitizer molecules, containing highly absorbing elements, can be added to photoresist formulations. These sensitizers have gained growing attention in recent years, showing significant sensitivity improvement. Aside from an increasing absorption, adding metal salts into the resist formulation can induce other mechanisms, like higher secondary electron generation or acid yield, or modification of the dissolution rate that also can affect patterning performance. In this work, we used different sensitizers in chemically amplified resists. We measured experimentally the absorption of EUV light, the acid yield, the photoelectron emission, the dissolution rate, and the patterning performance of the resists. Addition of a sensitizer raised the acid yield even though a decrease in film absorbance occurred, suggesting an apparent increase in chemically resonant secondary electrons. While patterning results confirm a significant sensitivity improvement, it was at the cost of roughness degradation at higher sensitizer loading. This is hypothesized by the chemical distribution of the sensitizer in the resist combined with a modification of the dissolution contrast, as observed by dissolution rate monitor measurements.

Sensitizer for EUV Chemically Amplified Resist: Metal versus Halogen

“…Our previous study showed that mixing metal salt sensitizers composing of a metal cation and organic anions into chemical amplified resists (CAR) improved EUV sensitivity [7,8]. These metal salt sensitizers carry high EUV absorption, leading to high photoelectron yield enhanced acid yield [9]. However not only metals have high absorption, halogen atoms like iodine and fluorine also provide high absorption which would be suitable alternative sensitizer to the metal salt sensitizer.…”

Photoresist Absorption Measurement at Extreme Ultraviolet (EUV) Wavelength by Thin Film Transmission Method

“…Several methods have been investigated to achieve high photoresist sensitivity. For instance, addition of high EUV absorption metal species such as Hafnium (Hf), Zirconium (Zr) [8] or other sensitizers [9] in the base resin of the EUV photoresist. By increasing the EUV absorption of the base resin, secondary electron generation can be increased and thus photoresist sensitivity [10,10].…”

“…By increasing the EUV absorption of the base resin, secondary electron generation can be increased and thus photoresist sensitivity [10,10]. Nevertheless, calculated and measured absorption values are not always perfectly matching, and deviations were reported [9]. Furthermore, the exact composition and density of a photoresist is not always available, especially in the case of resist screening of state-ofthe-art photoresists from different suppliers.…”