Activation of the dimers and tetramers of metal amidinate atomic layer deposition precursors upon adsorption on silicon oxide surfaces

Abstract

The gas-phase structure of three copper amidinate compounds, copper(I)-N,N′-di-sec-butyl-acetamidinate (1), copper(I)-N-sec-butyl-2-iminopyrrolidinate (2), and copper(I)-N-tert-butyl-5,5-dimethyl-2-iminopyrrolidinate (3), and their initial adsorption on silicon oxide surfaces, were characterized by a combination of experimental measurements and density-functional theory (DFT) calculations. These compounds have previously been shown to crystallize in dimeric or tetrameric form, and liquid-injection field desorption ionization mass spectrometry data proved that such structures are retained upon vaporization into the gas phase (dimers for the first and third compounds, a tetramer for the second). Results from DFT calculations of the relative energies of formation of the monomers, dimers, and tetramers confirmed the experimental results. Adsorption on the surface of silicon oxide films was determined, based on additional DFT calculations, to lead to the binding of the copper amidinates preferentially as dimers; although the monomers form stronger bonds to the silicon surface because they bind directly through their copper atom, this cannot fully compensate for the large energy required to break the dimers apart. N 1s x-ray photoelectron spectroscopy data were used to corroborate both the presence of the dimers on the surface with the second (2) precursor and the threshold for their surface decomposition around room temperature. The behavior of the third compound is somewhat more complex, with some decomposition possibly happening immediately upon adsorption at 100 K.

Role of oligomer structures in the surface chemistry of amidinate metal complexes used for atomic layer deposition of thin films

“…Our previous work had indicated that these oligomers, typically dimers, are quite stable: they are well known to form in the solid phase [9,10,11] and are also thermodynamically favored in the gas phase (the DFT calculations in those reports did not include van der Waals interactions, but the new calculations reported here, which do, corroborate the general trends identified before) [19,22]. Moreover, we found that in the dimer of Cu(I)-(2-sec-butylimino)-pyrrolidinate, the copper ions appear to be sheltered to a sufficient degree that, on silica surfaces at least, the first interaction is not via the metal center but rather through one of the nitrogen atoms [19]. We speculate that such initial interaction may promote subsequent surface chemistry, leading to the decomposition of the ligands not foreseen when these precursors were designed for film growth applications.…”

“…We do not have at present an interpretation of these observations or a full assignment of all of the amus detected, but speculate that perhaps the species desorbing at 350 K is a dimer where some skeletal rearrangement has taken place. Another possibility is that one of the peaks may correspond to a dimer and the other to a tetramer, as the two are not too far in energy [19]. It is difficult to settle this point with the available data.…”

“…In order to better understand the energetics of the monomerdimer interconversion of the iminopyrrolidinate complexes when adsorbed on surfaces, complementary DFT calculations were performed. Because it is well known that copper amidinates tend to dimerize in the solid state [10,29,30,31] (and in the gas phase as well [19]), we first optimized the atomic structure of isolated monomers and dimers of (1) using a big unit cell (to simulate the gas phase, in which all molecules are separated by a large distance). Figure 5 shows schematic views Figure 5: Schematic views of the most stable monomer (a) and dimer (b) structures of (1).…”

“…To address this issue, we in our laboratory have focused on the characterization of copper precursors made with amidinate and related ligands used for atomic layer deposition (ALD) applications [12,13,14,15,16,17,18,19,20,21,22]. We rely on our extensive experience in the study of reaction mechanisms on solid surfaces using modern surface-sensitive techniques [13,23,24,25].…”

“…We rely on our extensive experience in the study of reaction mechanisms on solid surfaces using modern surface-sensitive techniques [13,23,24,25]. In our initial report on the specific subject of the behavior of copper amidinates, we provided mass spectrometry evidence for the preservation of the oligomeric structure of these compounds when vaporizing them from their solid state, and additional quantum mechanics calculations to explain the origin of the energetic stability gained by the formation of dimers or tetramers [19]. We also found that, on silica surfaces, the steric constraints added by the shielding of the metal center in the gas-phase dimers force adsorption via one of the nitrogen atoms rather than the Cu ions.…”