Excited States of Pt(PF3)4 and Their Role in Focused Electron Beam Nanofabrication

Electron induced chemistry of metal-containing precursor molecules is central in focused electron beam induced deposition (FEBID). While some elementary processes leading to precursor decomposition were quantitatively characterized, data for neutral dissociation is missing. We provide this data for the model precursor Pt(PF3)4 by using the available cross sections for electronic excitation and characterizing fragmentation of the excited states theoretically by TDDFT. The potential energy curves for a number of states visible in the experimental electron energy loss spectra are dissociative, either directly or via conical intersections, indicating that the quantum yield for dissociation is close to 100%. Taking into account typical electron energy distribution at the FEBID spot reveals that the importance of neutral dissociation exceeds that of dissociative electron attachment, which has been so far considered to be the dominant decomposition process. We thus established neutral dissociation as an important, albeit often neglected, channel for FEBID using Pt(PF3)4. The calculations revealed a number of other phenomena that can play a role in electron induced chemistry of this compound, e.g., a considerable increase of bond dissociation energy with sequential removal of multiple ligands. ■ INTRODUCTION Focused electron beam induced deposition (FEBID) is a method for producing 3D metallic structures of sub-10 nm dimensions on planar or even nonplanar surfaces. The often metal-containing precursor molecules are physisorbed on a substrate and exposed to focused high-energy (keV) electron beam which degrades them to yield a metal deposit. The challenges of the technique are to increase the metal content of the deposits which is often less than 50%, yielding a conductivity far below that of bulk metal and improving deposit widths which are usually a multiple of the electron beam diameter. An important prerequisite for resolving the above-mentioned problems is the understanding of the fundamental gas phase electron-driven chemistry of the volatile metal complexes used as precursors. Four classes of processes contribute to the electron-driven degradation of the precursors + → → + → * + → + + → + → + + → ** + → + + − − −