Thermal Atomic Layer Deposition of Silver Metal Using Silver Alkoxide Precursors in Combination with a Hydroborane Co-reactant

Reactions of Ag 2 O with 2 equiv of perfluoro- tert -butanol, followed by the addition of 2 equiv of a trialkylphosphine afforded [(Ag{OC(CF 3 ) 3 }{PR 3 }) n ] (PR 3 = PMe 3 ( 1 ), PEtMe 2 ( 2 ), PMe 2 (CH 2 SiMe 3 ) ( 3 ), PEt 3 ( 4 ), P i Pr 2 Me ( 5 ), P i Pr 3 ( 6 ), P t Bu i Pr 2 ( 7 ), P i Pr 2 (CH 2 SiMe 3 ) ( 8 ), and P t Bu 3 ( 9 )). Compounds 1 – 7 are dimers ( n = 2) in the solid state, with bridging OC(CF 3 ) 3 groups, whereas 8 and 9 crystallized as monomers. Cryoscopic solution molecular weight determination and 109 Ag{ 1 H} NMR spectroscopy (with accompanying density functional theory (DFT) calculations) suggest that in solution, 1 remains as a dimer, whereas 6 and 9 exist predominantly as monomers. Compounds 1–9 sublimed at 45–80 °C with minimal residue at 5 mTorr, and melted at 80–165 °C. Thermogravimetric analysis (TGA) was carried out on all compounds, and 6, 7, and 9 afforded low residual masses (2–4%) indicative of clean volatilization, with T 50% temperatures (the temperature at which 50% mass loss is reached) from 226 to 242 °C. Compound 6 was selected for further studies, and solution reactions with excess pinacolborane (HBpin; pin = OCMe 2 CMe 2 O) proceeded to completion within minutes at room temperature, affording a silver mirror, (F 3 C) 3 COBpin, free P i Pr 3, and H 2 . Atomic layer deposition (ALD) studies were carried out using 6 and HBpin, delivered at 115 °C ( 6 is partially melted at this temperature) and 8 °C, respectively, affording highly reflective films, with self-limiting growth on H-terminated Si and SiO 2 /Si between 124 and 199 °C. The growth rate (for films deposited using 50–500 cycles) ranged from 1.5 to 0.9 Å/cycle, depending on the temperature and substrate, and X-ray photoelectron spectroscopy (XPS) (of films deposited at 143 °C) shows that the films are comprised of silver, with ≤4% C, O, and Si content (likely arising from atmospheric contamination and exposed substrate) and no detectable phosphorus, fluorine, or boron. X-ray diffraction (XRD) confirms the presence of crystalline silver, although the films are effectively nonconductive until they exceed a thickness of around 100 nm, at which point they begin to coalesce into a continuous layer; a resistivity of 7.17 μΩ·cm was achieved for a 109 nm thick film. Films deposited at 143 °C using 50, 125, 250, and 500 ALD cycles are comprised of crystallites/grains with an average diameter of ∼15 nm after 50 cycles, increasing to over 100 nm after 500 cycles. Atomic force microscopy (AFM) images indicate near-identical Ag film morphologies on SiO 2 /Si and H–Si, and comparable film morphologies for films of similar thickness deposited at 143 and 199 °C.