Electron trapping: a mechanism for acid production in extreme ultraviolet photoresists

The photomechanism of extreme ultraviolet (EUV) exposures in chemically amplified photoresists is much different than that of previous lithographic wavelengths. Electrons generated during EUV exposure are demonstrated to be a source of acid production through a process referred to as electron trapping. Density functional theory modeling indicates that it is energetically favorable for the photoacid generator (PAG) molecule to decompose if an electron is trapped. Low-energy electrons (<10 eV) that are unlikely to produce holes and secondary electrons generate acid-indicating electron–PAG interactions that are capable of inducing decomposition. Additionally, solution phase reduction in PAGs via electrolysis is shown to produce acid. Furthermore, a more easily reduced PAG (i.e., higher likelihood of trapping an electron) produces a higher acid yield, further supporting electron trapping as a process of acid production regardless of the polymer matrix. An acid indicator, Coumarin 6, was used to determine the number of acids generated per absorbed EUV photon. The results of these measurements indicate that electron–PAG interactions are a source of acid production through electron trapping; thus, an increase in the number of electron-hole pairs available to induce chemical reactions would improve sensitivity.