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

Reactions of Ag2O with 2 equiv of perfluoro-tert-butanol, followed by the addition of 2 equiv of a trialkylphosphine
afforded [(Ag{OC(CF3)3}{PR3})n] (PR3 = PMe3 (1), PEtMe2 (2), PMe2(CH2SiMe3) (3), PEt3 (4), PiPr2Me (5), PiPr3 (6), PtBuiPr2 (7), PiPr2(CH2SiMe3) (8), and PtBu3 (9)). Compounds 1–7 are dimers (n = 2)
in the solid state, with bridging OC(CF3)3 groups,
whereas 8 and 9 crystallized as monomers.
Cryoscopic solution molecular weight determination and 109Ag{1H} 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 T50% 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 = OCMe2CMe2O) proceeded to completion within minutes at
room temperature, affording a silver mirror, (F3C)3COBpin, free PiPr3,
and H2. 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 SiO2/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 SiO2/Si
and H–Si, and comparable film morphologies for films of similar
thickness deposited at 143 and 199 °C.