Thermal Chemistry of Metal Organic Compounds Adsorbed on Oxide Surfaces

Abstract

The thermal chemistry of several metal organic compounds with amidinate, diketonate, or cyclopentadienyl (Cp) ligands on oxide surfaces, mainly on silicon dioxide but also on aluminum oxide, was studied by using surface sensitive techniques, including temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), reflection–absorption infrared spectroscopy (RAIRS), and static secondary ion mass spectrometry (SSIMS), and those studies were complemented with quantum mechanics calculations. With the amidinates, TPD experiments revealed complex decomposition pathways, starting with a ligand-exchange step with OH surface groups. That is followed by migration of the remaining ligands from the metal center to the surface, where decomposition occurs mainly via bond scission at the terminal alkyl groups and β-hydride elimination steps to produce the corresponding olefins. At that stage, XPS data show that the metal is partially, but not completely, reduced, indicating a partially ionic metal–oxygen surface bond. The diketonates display only limited reactivity on silica, again via an initial ligand exchange step, but much higher reactivity is seen on alumina, where subsequent decomposition and olefin production are observed. The Cp ligands proved to be the most stable, hindering in fact the effective adsorption of these metal organic compounds unless prior activation in the gas phase via electron-impact excitation is carried out. After adsorption, the Cp proved to be quite sturdy, surviving long exposures to outside atmospheres (as shown by SSIMS). They can, however, be protonated (deuterated) upon high-temperature exposure to H2O (D2O). All this chemistry is discussed in general terms and contrasted with known reactivity in solution.

Density Functional Theory Study of the Adsorption and Dissociation of Copper(I) Acetamidinates on Ni(110): The Effect of the Substrate

“…It is experimentally known that, during the adsorption and dissociation process of copper­(I)– N , N ′ dimethylacetamidinate on metal surfaces, the Cu atoms from the precursor become reduced to their metallic state. ,,,,, It is also evident from those experimental studies that the rate-limiting step is the activation of the ligands, as the temperature threshold for Cu reduction and acetamidinate dehydrogenation is approximately the same. ,, It would be valuable to determine exactly at what stage of the conversion this metal reduction occurs. To that end, we have mapped here the evolution of the charges associated with the key elements of the system upon adsorption and dissociation of the dimer on the Ni(110) surface using Bader’s charge population analysis.…”

“…Fortunately, the advance of density functional theory (DFT) provides an alternative way to get that information. We ,− and others ,− have embarked in complementing studies on the mechanism of metalorganics on surfaces with this type of theoretical approach.…”

“…Our group has been focusing on metal acetamidinates as prototypical systems to extract general information on chemical systems relevant to CVD and ALD. ,,,,,,− Acetamidinate ligands have proven to be quite ubiquitous, given that they can be used to make complexes with a large number of metals − , and are promising as film deposition precursors because their physical properties (stability, vapor pressure, etc.) can be tuned by tinkering with the substitution groups added. − Our experimental work has shown that copper acetamidinates, stable under most conditions in solid, liquid, or gas phases, are nevertheless fairly reactive on solid surfaces, especially on those of late transition metals.…”

“…can be tuned by tinkering with the substitution groups added. − Our experimental work has shown that copper acetamidinates, stable under most conditions in solid, liquid, or gas phases, are nevertheless fairly reactive on solid surfaces, especially on those of late transition metals. Specifically, we have identified the beta-hydrogen position on the side moieties of most acetamidinates as a particular labile and reactive site leading to undesired decomposition. ,, We have also addressed the additional complication that arises from the fact that many metal acetamidinates form dimers or other oligomers when in solid or gas phases. ,,,,,, Dissociation of those into monomers upon adsorption onto solid surfaces compete with molecular desorption and/or unique decomposition routes stabilized by the dimeric structures.…”

Thermal Chemistry of Nickel Diketonate Atomic Layer Deposition (ALD) Precursors on Tantalum and Silicon Oxide Surfaces

“…In most of these compounds, the ligands are bound to the Ni centers in a bidentate configuration, a type of complexation that confers additional stability and presumably minimizes detrimental side decomposition pathways that may lead to the deposition of undesirable impurities. Nevertheless, recent surface-science studies by us − and others − have shown that even with bidentate ligands there are chemical avenues on solid surfaces for the organic moieties to fragment and leave carbon and other elements behind. In order to optimize the synthesis of metal organic precursors for ALD it is important to block these side reactions, and that in turn requires a better understanding of the fundamental mechanism of the thermally activated reactions that different types of organic ligands can follow when adsorbed on surfaces.…”