Role of Anionic Backbone in NHC‐Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition**

Abstract

Cu and Ag precursors that are volatile, reactive, and thermally stable are currently of high interest for their application in atomic layer deposition (ALD) of thin metal films. In pursuit of new precursors for coinage metals namely Cu and Ag, a series of new N-heterocyclic carbene (NHC) based Cu(I) and Ag(I) complexes were synthesized. Modifications in the substitution pattern of diketonate-based anionic backbones led to five monomeric Cu complexes and four closely related Ag complexes with the general formula [M( tBu NHC)(R)] (M = Cu, Ag; tBu NHC = 1,3-di-tert-butyl-imidazolin-2-ylidene; R = diketonate). Thermal analysis indicated that most of the Cu complexes are thermally stable and volatile compared to the more fragile Ag analogs. One of the promising Cu precursors was evaluated for the ALD of nanoparticulate Cu metal films using hydroquinone as the reducing agent at appreciably low deposition temperatures (145-160 °C). This study highlights the considerable impact of the employed ligand sphere on the structural and thermal properties of metal complexes that are relevant for vapor phase processing of thin films.

In Pursuit of Next Generation N-Heterocyclic Carbene-Stabilized Copper and Silver Precursors for Metalorganic Chemical Vapor Deposition and Atomic Layer Deposition Processes

“…SCXRD provides a precise structural situation and confirmation of the molecular integrity of the complexes in the solid state. Figure 3 portrays the solid-state structures of three complexes synthesized in this work along with those of [Cu(tBuNHC) (acac)] and [Ag(tBuNHC) (acac)], previously reported by our group, for the sake of comparison and the trends seen in different structural parameters [35]. All the structures crystalize in monoclinic or orthorhombic crystal systems.…”

“…The tBuNHC (tertiary butyl N-Heterocyclic carbene) chloride salt was synthesized based on the literature reports with minimal modifications [35,43]. The complexes were synthesized utilizing a one-pot synthesis route published by Boysen et al [30].…”

“…Unlike the frequently reported salt metathesis reactions employed for the synthesis of a wide range of MOCVD and ALD precursors [44,45], in this case, we opted for a displacement reaction of the anionic ligand itself as seen in Scheme 1, a modified procedure previously reported by Boyson et al for [Cu(tBuNHC) (acac)] [35]. With a 1:1 ratio of the starting material to that of the ligands NacacMe/NacNacMe, this route was primarily chosen due to the higher basicity of HMDS (pka: 14.40) relative to that of both NacacMe (pka: 6.39) and NacNacMe (pka: 12.95), which is the driving force for the exchange of HMDS.…”

“…As mentioned earlier, similar NHC ligands can also be applied to silver, which significantly stabilizes the reactive complexes. As an example, [Ag(tBuNHC)(HMDS)] showed promising physico-chemical To investigate the influence of the anionic backbone in such NHC-stabilized complexes, further functionalization was carried out by Boysen et al, reporting a row of differently functionalized [M(tBuNHC) (dktacac)] (dkt = β-diketonate; M = Cu, Ag) complexes (tBuNHC = tertiarybutyl-N-Heterocyclic carbene, acac = acetylacetone) along with the proof of deposition of copper via thermal ALD at moderate temperatures of 145-160 • C and with hydroquinone as the co-reactant [35]. The resulting nanoparticulate films contained residual oxygen and carbon impurities that might be caused by the precursor and co-reactant at the same time.…”

Assessing the Environmental Impact of Atomic Layer Deposition (ALD) Processes and Pathways to Lower It

“…Generally, lower E-Factors are thus desirable for a precursor chemist as they consequently indicate less production of waste. For example, when considering the synthesis of Cu I and Ag I NHC-based precursors of the general type [M­(NHC)­(hmds)] (M = Cu, Ag) which have been successfully established in spatial ALD for the deposition of metallic Cu and Ag layers, ,, the E-Number can indeed be calculated to assess the amount of waste that is produced during the synthesis. Notably, over 16 different chemicals in three different synthetic steps are involved in the synthesis of [Cu­(NHC)­(hmds)] (Figure ).…”