Time-resolved synchrotron XPS monitoring of irradiation-induced nitrobenzene reduction for chemical lithography.

The irradiation-induced reduction of electrochemically grafted nitrobenzene films on Si(111) was monitored by high-resolution photoelectron spectroscopy. The experiments were performed using synchrotron soft X-ray irradiation at the BESSY II synchrotron facility. The evolution of different chemical species was monitored as a function of time. Careful fitting of the Si2p, C1s, N1s, and O1s core level spectra allowed us to follow this process in detail and to determine the constants of growth and decay of the specific components. The chemical changes were caused by the X-ray irradiation-induced secondary electron current through the aryl layer. A minor fraction (approximately 25%) of the initial nitro groups was split off and desorbed. The bulk of the NO2 groups was reduced to species in an amino-like chemical environment. A desorption of carbon fragments was not observed, and benzene ring specific shakeup satellites indicated that the aromatic ring structure remained intact. Irradiation-induced line-shape changes suggest a polymerization via -NH- bridges, which were formed after the irradiation-induced N-O bond splitting. A significant part of the released oxygen appeared to contribute to an oxidation of the silicon substrate at the Si(111)/benzene interface. The irradiation-induced aryl layer modification can be exploited for chemical lithography (i.e., a lateral structuring of functionalized silicon surfaces).