Decomposition of Hexamethyldisilazane on Hot Metal Filaments and its Gas-phase Chemistry in a Hot-wire Chemical Vapor Deposition Reactor
Creators: name: Ampong, Eric
Publisher: name: Science
Year: 2019
Subjects: subject: HWCVD
subject: Education--Sciences

description: Hot-wire chemical vapor deposition (HWCVD) has been used to produce silicon-containing thin films, nanomaterials, and functional polymer coatings for applications in microelectronic and photovoltaic devices. Silicon carbonitride (SiCyNz) thin films, deposited by HWCVD, have found a wide range of applications due to their nonstoichiometric component that exhibits unique properties from a combination of SiC and Si3N4 binary compounds. Most CVD growth of SiCyNz proceeds through the use of separate Si-containing (SiH4), C-bearing (CH4) and NH3 precursors. Handling of pyrophoric silane is difficult, and the process optimization with multiple source gases is extremely complex. This has urged interest in exploring alternative single-source precursors for SiCyNz deposition. 1,1,1,3,3,3-Hexamethyldisilazane (HMDSZ), a non-pyrophoric and non-corrosive molecule, is one of the single-source precursors for use in HWCVD of SiCyNz thin films. In this work, single-photon ionization using vacuum ultraviolet wavelength at 118 nm coupled with time-of-flight mass spectrometry is employed to examine the products from primary decomposition on tungsten and tantalum filaments under collision-free conditions and from secondary gas-phase reactions in a HWCVD reactor. It has been shown that HMDSZ decomposes on the heated metal filaments to produce methyl radicals via Si-CH3 bond cleavage. The methyl radical formation is controlled by surface reactions at filament temperatures ranging from 1600 to 2400 oC. The activation energy for the formation of methyl radicals on the W and Ta filament has been determined to be 71.2 ± 9.1 to 76.7 ± 8.1 kJ/mol, respectively. A comparison with the theoretical energy (363 kJ/mol) required for the homolytic cleavage of Si-CH3 bond in the gas phase indicates that the dissociation of HMDSZ on the W and Ta surfaces to produce methyl radicals is a catalytic cracking process. Aside from the homolytic cleavages, other decomposition routes of HMDSZ, both concerted and stepwise ones, have been systematically explored in this work. The concerted formation of trimethylsilylamine and 1,1-dimethylsilene was found to be the most kinetically favorable route of all, with an activation barrier of 278 kJ/mol. It is also interesting to find that both the elimination of CH3 radical from a methylated silylamino radical and elimination of H atom from the C atom attached to a Si atom in a silyl radical site proceed without an activation barrier. In the secondary gas-phase reactions, radical-radical and radical-molecule reactions are dominant. Formation of 1,1-dimethylsilanimine ((CH3)2Si=NH) was detected from the HWCVD reactor when HMDSZ was introduced. 1,1-dimethylsilanimine undergoes head-to-tail cycloaddition and nucleophilic addition reaction with the abundant HMDSZ molecules to form 1,1,3,3-tetramethylcyclodisilazane and octamethyltrisilazane that were both detected in this work. The secondary gas-phase reactions are also characterized by a free radical short-chain reaction initiated by the primary decomposition of HMDSZ on the metal filaments to produce methyl radicals. Hydrogen abstraction by methyl radical is the main propagation step in the reactor, and biradical recombination reactions terminate the chain reaction to form various stable products with high molecular masses that were also detected in this work.