Thermal decomposition mechanisms of tungsten nitride CVD precursors on Cu(1 1 1)

Abstract

Chemisorption and thermal decomposition of metallorganic chemical vapor deposition precursors, ( t-BuN) 2W(NHBu- t) 2, bis( tert-butylimido)bis( tert-butylamido)tungsten (BTBTT) and ( t-BuN) 2W(NEt 2) 2, bis( tert-butylimido)bis(diethylamido)tungsten (BTBDT), on Cu(1 1 1) have been investigated by means of thermal desorption spectroscopy (TDS) and synchrotron-based X-ray photoelectron spectroscopy (SR-XPS) under ultrahigh vacuum conditions. The precursors remain intact upon chemisorption on Cu(1 1 1) at 100 K, and at 300 K both precursors decompose readily via the characteristic hydride abstraction/elimination pathways to produce two stable surface intermediates for each precursor. For BTBTT, one species is W(=NBu- t) 3 and the other is proposed to be a bridged amido complex, [( t-BuN) 2W(μ-NBu- t)] 2. In comparison, a W-imine complex and a W–N–C metallacycle are two intermediates produced from BTBDT. Annealing toward 800 K further decomposes the intermediates and the detectable desorption species are completely derived from the ligands. The desorption products from BTBTT include t-butylamine generated from α-H abstraction, isobutylene from γ-H elimination, acetonitrile from β-methyl elimination, and molecular hydrogen. In addition to these desorption species, BTBDT produces hydrogen cyanide and imine (EtN = CHMe) via β-H elimination, not possible with BTBTT due to the absence of β-H in the ligands. Eventually, tungsten nitrides incorporating oxygen atoms and a small amount of graphitic carbons are formed and the stoichiometry is approximated as WN 1.5O 0.1. Oxygen incorporation, driven by a large oxide formation enthalpy, is sensitively dependent on the moisture exposure in UHV environment.