Gold Films by Atomic Layer Deposition: Precursor Design, Synthesis, Evaluation and Optimization in Deposition

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Gold films are of significant interest in the thin film community. Applications for gold thin films range from sensor interfaces (due to gold's chemical inertness and enhanced surface plasmon) to metamaterials (gold's negative permeability at visible wavelengths). Gold is typically produced as a thin film by sputtering or evaporation, but these techniques suffer line-of-sight deposition issues: shadow masking and lack of control over the deposition rate at nanometre thicknesses. Chemical vapour deposition (CVD) and atomic layer deposition (ALD) have been main themes of research in my group for a decade. We have had great success designing ligand systems for the deposition of oxides (aluminium, gallium, and indium) as well as metals (particularly copper). My group's experience with synthesis of group 11 compounds and their thermolysis pathways gives us the occasion to understand the mechanistic chemisorption and decomposition of similar compounds. Our ever-developing catalogue of ligands gives us an opportunity to explore some novel gold precursors and their thermal and surface chemistries, as well as to undertake the study of existing gold compounds that have been developed for purposes other than vapour deposition. For example, catalysis chemistry is rich with Au(I) carbene complexes, similar to the formulation our known gold precursor. One research project will undertake the testing of previously-synthesized and known compounds of Au(I). As an example, our research has defined a class of N-heterocyclic carbene (NHC) compounds: R2-NHC-M-N(SiMe3)2, where the “R” group is bound to the nitrogen of the NHC. The compound iPr2-NHC-Au-N(SiMe3)2 was found to deposit a gold film with a growth rate of 0.10 Å per cycle when subjected to a plasma of hydrogen in argon. The problem with this deposition is that the precursor only provides sufficient vapour pressure for the deposition process at 110°C, but starts to undergo thermal decomposition (to gold metal) at 140°C. This makes a reasonably good CVD precursor, but the precursor's “thermal window” needs to be improved as an ALD precursor. Limitations to our gold deposition research have prevented a systematic study of the effect of altering this R-group on thermal stability and deposition conditions. This grant will allow us to undertake such a study. Similarly, we have previously synthesized a series of iminopyrrolidinate (IP) ligands (R-IP, where R is any propyl or butyl group) that limit the thermal decomposition by preventing two known thermal decomposition pathways: carbodiimide elimination and ß-hydrogen elimination. This has proven to be very successful at extending the thermal stability of both Al(III) and Cu(I) compounds. It is necessary to make this series of compounds with Au(I) to test both thermal stability and deposition conditions. This project will also study the effect of incorporating a coordinating ligand (producing a monomeric compound) or allowing the IP to bridge to produce higher-order oligomers. Ultimately, the research program will have a larger scope, envisioning both Au(I) and Au(III) precursors. For anionic ligands, focus will be on nitrogen-based groups such as amides, amidinates, guanidinates, and iminopyrrolidinates. The coordination sphere of gold (2 for Au(I) and 4 for Au(III)) will also necessitate the inclusion of coordinating ligands. Here, the focus will be on phosphines, NHCs, and acyclic aminocarbenes. The program will include the synthesis and thermal testing of these precursor compounds, as well as development of deposition processes and film characterization. The goal of this research program will be to enable the robust and repeatable deposition of nanometre-thick films of gold over highly complex geometries.