Tris(phosphino)borato silver(I) complexes as precursors for metallic silver aerosol-assisted chemical vapor deposition

Article abstract of DOI:10.1021/ic701852x

A series of light- and air-stable tris(phosphino)borato silver(I) complexes has been synthesized, structurally and spectroscopically characterized, and implemented in the growth of low resistivity metallic silver thin films by aerosol-assisted chemical vapor deposition (AACVD). Of the four complexes in the series, [RB(CH₂PR′₂)₃]AgPEt₃ (R = Ph (1, 3), ⁿBu (2, 4); R′ = Ph (1, 2), ⁱPr (3, 4), complexes 1 and 2 have been characterized by single-crystal X-ray diffraction. Complex 2 represents a significant improvement over previously available nonfluorinated Ag precursors, owing to ease of handling and efficient film deposition characteristics. Thermogravimetric analysis (TGA) shows that the thermolytic properties of these complexes can be significantly modified by altering the ligand structure. Polycrystalline cubic-phase Ag thin films were grown on glass, MgO(100), and 52100 steel substrates. Ag films of thicknesses 3 μm, grown at rates of 14-18 nm/min, exhibit low levels of extraneous element contamination by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicate that film growth proceeds primarily via an island growth (Volmer-Weber) mechanism.

Full text of DOI:10.1021/ic701852x

Inorg. Chem. 2008, 47, 2534-2542
Tris(phosphino)borato Silver(I) Complexes as Precursors for Metallic
Silver Aerosol-Assisted Chemical Vapor Deposition
Matthew N. McCain,† Sven Schneider,‡ Michael R. Salata,† and Tobin J. Marks*,†
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145, Sheridan
Road, EVanston, Illinois 60208-3113
Received September 19, 2007
A series of light- and air-stable tris(phosphino)borato silver(I) complexes has been synthesized, structurally and
spectroscopically characterized, and implemented in the growth of low resistivity metallic silver thin films by aerosol-
assisted chemical vapor deposition (AACVD). Of the four complexes in the series, [RB(CH2PR′2)3]AgPEt3 (R ) Ph
n
i
(1, 3), Bu (2, 4); R′ ) Ph (1, 2), Pr (3, 4), complexes 1 and 2 have been characterized by single-crystal X-ray
diffraction. Complex 2 represents a significant improvement over previously available nonfluorinated Ag precursors,
owing to ease of handling and efficient film deposition characteristics. Thermogravimetric analysis (TGA) shows
that the thermolytic properties of these complexes can be significantly modified by altering the ligand structure.
Polycrystalline cubic-phase Ag thin films were grown on glass, MgO(100), and 52100 steel substrates. Ag films of
thicknesses 3 µm, grown at rates of 14–18 nm/min, exhibit low levels of extraneous element contamination by
X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron microscopy (SEM)
indicate that film growth proceeds primarily via an island growth (Volmer–Weber) mechanism.
Introduction
thermal evaporation,5 electron-beam evaporation,6 and chemi-
cal vapor deposition (CVD).7 Of these techniques, CVD
processes offer the attraction of conformal surface coverage
at closer to ambient conditions, adaptability to a wide range
of materials and manufacturing scales, simple apparatus, and
the possibility of creating metastable phases. For the deposi-
tion of metallic thin films, metal-organic (MO) CVD has
Metallic silver thin films have importance in a wide range
of applications including microelectronics as advanced
interconnects,1 components of high Tc superconductor struc-
tures for improved stress durability,2 and friction and wear
control via solid lubricant coatings.3 Various methods have
been used to deposit silver thin films, such as sputtering,4
(5) Uchil, J.; Mohan Rao, K.; Pattabi, M. J. Phys. D: Appl. Phys. 1996,
29, 2992–2996.
* E-mail: t-marks@northwestern.edu.
†
Northwestern University.
Current address: Technische Universität München, Anorganisch-
(6) Carter, A. C.; Chang, W.; Qadri, S. B.; Horwitz, J. S.; Leuchtner, R.;
Chrisey, D. B. J. Mater. Res. 1998, 13, 1418–1421.
‡
Chemisches Institut, Lichtenbergstrasse 4, 85747 Garching, Germany.
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Electrochem. Soc. 2000, 147, 3345–3349. (c) Manepalli, R.; Stepniak,
F.; Bidstrup-Allen, S. A.; Kohl, P. A. IEEE Trans. AdV. Packag. 1999,
22, 4–8.
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Metal CV; Kodas, T., Hampden-Smith, M., Eds.; VCH: Weinheim,
Germany, 1994.
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2534 Inorganic Chemistry, Vol. 47, No. 7, 2008
10.1021/ic701852x CCC: $40.75  2008 American Chemical Society
Published on Web 02/23/2008

Tris(phosphino)borato Ag(I) as AACVD Precursors
form monometallic silver(I) complexes. However, to date,
no silver tris(phosphino)borato complexes have been re-
ported. In this contribution, we report the synthesis and
structural/spectroscopic characterization of a series of tris-
(phosphino)borato silver(I) complexes which are both air-
and light-stable. The thermal stabilities and decomposition
pathways of these complexes are investigated, and their
implementation as effective precursors for the growth of
metallic silver thin films by AACVD is reported.
Figure 1. Generalized structure of the family of tris(phosphino)borato low-
valence metal complexes.
been widely established.8 However, a major limitation of
this method is the requirement of highly volatile metal-
organic precursors. Traditionally, the design of silver precur-
sors has focused mainly on ꢀ-diketonates8e,9 or structurally
related ligands,10 as well as carboxylates,7a,8d,11 featuring
tertiary phosphine ancillary ligands. Without the use of
expensive fluorinated substituents, these compounds suffer
from poor volatility, as well as from poor stability under
ambient conditions. Films deposited using complexes with
fluorinated substituents suffer from significant F contamina-
tion. In comparison, aerosol-assisted (AA) CVD has been
recently developed to utilize metal precursors with lower
volatilities, without sacrificing film quality.7b,8c,12 However,
compared to MOCVD, reports of silver thin film growth by
AACVD have been limited,13 with reported precursors also
suffering from instability under ambient conditions.
Peters and co-workers have recently developed a family
of tris(phosphino)borato ligands (Figure 1) that has been
shown to effectively stabilize monomeric complexes of low-
valence late transition metals with particularly interesting
reactivities toward small molecule activation.14 The strong
tendency of the monoanionic chelate ligand to enforce
pseudotetrahedral coordination geometries in combination
with the soft phosphorus donors should be ideally suited to
Experimental Section
Materials and Methods. All manipulations of air-sensitive
materials were carried out with rigorous exclusion of oxygen
and moisture in flame- or oven-dried Schlenk-type glassware on a
dual-manifold Schlenk line, or in a nitrogen-filled Vacuum Atmo-
spheres or MBraun glovebox with a high-capacity recirculator (<2
ppm O2). Hydrocarbon solvents (pentane and toluene) were dried
using an activated alumina column and Q-5 column according to
the method described by Grubbs,15 or by distilling from calcium
hydride (dichloromethane) or sodium (diethyl ether, tetrahyrofuran).
Ph2PCH3 (ACROS) and all other reagents (Aldrich) were used
without further purification. Ph2PCH2Li ·TMEDA,16 [Li(TMEDA)]-
17
18
i
PhB(CH2PPh2)3 and PrPCH2Li were prepared by literature
methods. Deuterated solvents (Cambridge Isotope Laboratories)
were dried over CaH2, transferred via filter cannula under inert
atmosphere, and stored over 4-A molecular sieves in vacuum-tight
storage flasks prior to use.
Physical and Analytical Measurements. Elemental analyses
were performed by Midwest Microlabs, Inc. NMR spectra were
1
recorded on a Varian Mercury-400 (FT, 400 MHz, H; 100 MHz,
13C, 162 MHz 31P) instrument. 1H and 13C chemical shifts (δ) were
referenced to residual solvent. Chemical shifts for 31P are reported
relative to an external 85% H3PO4 standard at room temperature.
Thermogravimetric analysis (TGA) was performed on a Mettler
Toledo TGA/SDTA851e ultramicro balance instrument at a ramp
rate of 2.5 °C min-1 and under a N2 flow rate of 50 mL min-1 at
atmospheric pressure.
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Alikhanyan, A.; Kuzmina, N.; Troyanov, S. I.; Kaul, A. R. J. Phys.
IV 2001, 11, Pr3645–Pr3652. (b) Chi, K.-M.; Lin, C.-T.; Peng
S.-M.; Lee, G.-H. Organometallics, 1996, 15, 2660–2663. (c) Yuan,
Z.; Dryden, N. H.; Li, X.; Vittal, J. J.; Puddephatt, R. J. J. Mater.
Chem. 1995, 5, 303–7. (d) Yuan, Z.; Dryden, N. H.; Vittal, J. J.;
Puddephatt, R. J. 1995, 7, 1696–702. (e) Serghini-Monim, S.; Yuan,
Z.; Griffiths, K.; Norton, P. R.; Puddephatt, R. J. J. Am. Chem. Soc.
1995, 117, 4030–4036. (f) Xu, C.; Corbitt, T. S.; Hampden-Smith,
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19, 2841–9. (g) Dryden, N. H.; Vittal, J. J.; Puddephatt, Richard J.
Chem. Mater. 1993, 5, 765–6.
Synthesis of [PhB(CH2PPh2)3Ag(PEt3)] (1). Using a mortar and
pestle, 0.29 g (2.0 mmol) AgCl was ground to a fine powder and
added to a 100 mL flask containing 1.62 g (2.0 mmol) of
[Li(TMEDA)PhB(CH2PPh2)3]. The flask was evacuated and back-
filled with N2 three times, covered with aluminum foil, and cooled
to -78 °C in an acetone/dry ice bath. After tetrahydrofuran (45
mL) was added, 0.30 g (2.0 mmol) of triethylphosphine was added
via syringe. The reaction mixture was stirred for 30 min and then
removed from the cooling bath and allowed to warm to room
temperature with stirring. The yellow solution was next separated
from the colorless solids via cannula filtration into another Schlenk
flask. The solvent was removed in vacuo and the residue washed
with 3 × 60 mL of pentane (vigorous stirring) to remove TMEDA.
The solid residue was then dissolved with 60 mL toluene and the
resulting yellow solution filtered from colorless solids via cannula.
(10) (a) Ngo, S. C.; Banger, K. K.; Toscano, P. J.; Welch, J. T. Polyhedron
2002, 21, 1289–1297. (b) Edwards, D. A.; Harker, R. M.; Mahon,
M. F.; Molloy, K. C. J. Mater. Chem. 1999, 9, 1771–1780.
(11) (a) Haase, T.; Kohse-Hoeinghaus, K.; Atakan, B.; Schmidt, H.; Lang,
H. Chem. Vap. Deposition 2003, 9, 144–148. (b) Szlyk, E.; Piszczek,
P.; Grodzicki, A.; Chaberski, M.; Golinski, A.; Szatkowski, J.;
Blaszczyk, T. Chem. Vap. Deposition 2001, 7, 111–116. (c) Lu
S.-Y.; Lin, Y.-Z. Thin Solid Films 2000, 376, 67–72. (d) Szlyk, E.;
Piszczek, P.; Lakomska, I.; Grodzicki, A.; Szatkowski, J.; Blaszczyk,
T. Chem. Vap. Deposition 2000, 6, 105–108.
(12) (a) Rodriquez-Castro, J.; Mahon, M. F.; Molloy, K. C. Chem. Vap.
Deposition 2006, 12, 601–607. (b) Peters, E. S.; Carmalt, C. J.; Parkin,
I. P.; Tocher, D. A. Eur. J. Inorg. Chem. 2005, 4179, 4185. (c) Schäfer,
P.; Waser, R. AdV. Mater. Optics Electron. 2000, 10, 169–175. (d)
Kodas T. T.; Hampden-Smith, M. J. Aerosol Processing of Materials;
Wiley: New York, 1999. (e) Hubert-Pfalzgraf, L. G.; Guillon, H. Appl.
Organomet. Chem. 1998, 12, 221–236. (f) Roger, C.; Corbitt, T.; Xu,
C.; Zeng, D.; Powell, Q.; Chandler, C. D.; Nyman, M.; Hampden-
Smith, M. J.; Kodas, T. T. Nanostruct. Mater. 1994, 4, 529–35.
(13) (a) Edwards, D. A.; Mahon, M. F.; Molloy, K. C.; Ogrodnik, V. J.
Mater. Chem. 2003, 13, 563–570. (b) Edwards, D. A.; Harker, R. M.;
Mahon, M. F.; Molloy, K. C. Inorg. Chim. Acta 2002, 328, 134–146.
(c) Xu, C.; Hampden-Smith, M. J.; Kodas, T. T. Chem. Mater. 1995,
7, 1539–1546.
(14) (a) Lu, C. C.; Peters, J. C. Inorg. Chem. 2006, 45, 8597–8607. (b)
MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg.
Chem. 2004, 43, 4645–4662. (c) Daida, E. J.; Peters, J. C. Inorg. Chem.
2004, 43, 7474–7485. (d) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. A.;
Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 15336–
15350.
(15) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–20.
(16) Peters, J. C.; Feldmann, J. D.; Tilley, T. D. J. Am. Chem. Soc. 1999,
121, 9871–9872.
(17) Fraenkel, G.; Winchester, W. R. Organometallics 1989, 8, 2308–2311.
(18) Betley, T. A.; Peters, D. M. Inorg. Chem. 2003, 42, 5074–5084.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2535

McCain et al.
The solvent was removed in vacuo, giving 1.50 g (83% yield) of
pale-yellow microcrystalline powder 1. 1H NMR (400.2 MHz, THF-
d8): δ 7.48 (d, 2H, ortho-PhB), 7.26 (m, 12H, PhB(CH2P Ph2)3),
7.10 (m, 18H PhB(CH2P Ph2)3), 6.87 (m, 2H, meta-PhB), 6.46 (t,
1H, para-PhB), 1.92 (dq, 6H, PC H2CH3), 1.35 (s, 6H, PhB(C
H2PPh2)3), 1.21 (dt, 9H, PCH2C H3). 13C NMR (100.6 MHz, THF-
d8): δ 147.22, 141.39, 132.62, 132.56, 132.53, 132.47, 131.06,
127.79, 127.70, 19.15 (P(CH2CH3)3), 8.34 (P(CH2 CH3)3). 31P NMR
(162.0 MHz, THF- d8, 25 °C): δ -1.15 (dd, 1J(P-109Ag) ) 216.92
31P NMR (162.0 MHz, THF- d8, 25 °C): δ -0.80 (dd, 1J(P-109Ag)
) 206.71 Hz, 1J(P-107Ag) ) 179.88 Hz, 2J(P-P) ) 30.57 Hz), -4.59
(dq, 1J(P-109Ag) ) 464.09 Hz, 1J(P-107Ag) ) 402.51 Hz, 2J(P-P) )
32.24 Hz),. 31P NMR (162.0 MHz, THF- d8, -103 °C): δ -1.47
(dd, 1J(P-109Ag) ) 215.06 Hz, 1J(P-109Ag) )186.62 Hz, 2J(P-P) )
1
1
32.24 Hz), -2.94 (dq, J(P-109Ag) ) 466.02 Hz, J(P-107Ag) )
406.94 Hz, 2J(P-P) ) 30.68 Hz). Anal. Calcd for C49H60AgBP4: C,
66.01; H, 6.78; P, 13.90. Found: C, 65.96; H, 6.80; P, 13.66.
Synthesis of [Li(TMEDA)-PhB(CH2PiPr2)3]. A stirring suspen-
1
2
i
Hz, J(P-107Ag) ) 188.41 Hz, J(P-P) ) 30.94 Hz), -4.25 (dq,
1J(P-109Ag) ) 463.00 Hz, 1J(P-107Ag) ) 401.77 Hz, 2J(P-P) ) 30.94
Hz). 31P NMR (162.0 MHz, THF- d8, -103 °C): δ -1.64 (dd, 1J(P-
sion of 5.0 g (36.2 mmol) of Pr2PCH2Li in 45 mL of ether and
5.43 mL (36.2 mmol) of anhydrous TMEDA was cooled to -78
°C in a acetone/dry ice bath. Next 1.62 mL (12.1 mmol) PhBCl2
was slowly added. The cooling bath was then removed and the
solution was allowed to warm to room temperature with stirring,
during which time colorless solids precipitated from the orange
solution. The solution was then filtered and the solvent removed
from the filtrate in vacuo, affording a thick orange oil. The oil was
then washed with 3 × 60 mL pentane (vigorous stirring) affording
7.16 g (88% yield) of an orange oil upon drying in vacuo. 1H NMR
(400.2 MHz, THF- d8): δ 7.63 (d, 2H, ortho-PhB), 6.86 (m, 2H,
meta-PhB), 6.78 (t, 1H, para-PhB), 2.31 (s, 4H, Me2NC H2C
H2NMe2), 2.16 (s, 12H, Me2NCH2CH2N Me2, 1.43 (s, 6H, PhB(C
1
2
109Ag) ) 218.17 Hz, J(P-107Ag) ) 189.99 Hz, J(P-P) ) 25.76
Hz), -2.51 (dq, J(P-109Ag) ) 465.91 Hz, J(P-107Ag) ) 405.97
Hz, 2J(P-P) ) 25.76 Hz). Anal. Calcd for C51H56AgBP4: C, 67.20;
H, 6.19; P, 13.59. Found: C, 67.23; H 6.27; P, 13.44.
1
1
Synthesis of [Li(TMEDA)nBuB(CH2PPh2)3]. To a stirring
suspension of 7.08 g (22.0 mmol) Ph2PCH2Li·TMEDA in 45 mL
of ether, cooled to -78 °C in an acetone/dry ice bath, was slowly
added 7.3 mL of a 1.0 M hexane solution of nBuBCl2. The cooling
bath was then removed and the reaction mixture was allowed to
warm to room temperature with stirring, during which time a
colorless solid began to precipitate from the yellow solution. The
solution was next filtered and the solvent removed in vacuo,
affording a thick yellow oil. The oil was washed with 3 × 60 mL
of pentane (vigorous stirring). The resulting solid residue was then
dissolved in 60 mL of toluene and the resulting yellow solution
filtered from the colorless solids via cannula. The solvent was
removed in vacuo, giving 4.56 g (79% yield) of a deep-yellow
i
H2P Pr2)3), 1.04 (m, 3H, PhB(CH2P(C HMe2)2)3), 0.83 (d, 36H,
P(CH(C H3)2)2). 13C NMR (100.6 MHz, THF- d8): δ 135.6, 135.0,
130.0, 127.7, 127.1, 57.0 (Me2N CH2 CH2NMe2), 44.1
(Me2NCH2CH2N Me2), 25.0, 14.7. 31P NMR (162.0 MHz, THF-
d8): δ -9.91. Anal. Calcd for C33H69BLiN2P3: C, 65.56; H, 11.50;
P, 15.37. Found: C, 65.48; H, 11.46; P, 15.20.
Synthesis of [PhB(CH2PiPr2)3Ag(PEt3)] (3). Using a mortar and
pestle, we ground 1.65 g (11.6 mmol) of AgCl to a fine powder
and added it to a 100 mL flask containing 6.98 g (11.6 mmol) of
[Li(TMEDA)PhB(CH2PiPr2)3]. The flask was evacuated and back-
filled with N2 three times, covered with aluminum foil, and cooled
with stirring to -78 °C in an acetone/dry ice bath. Next, tetrahy-
drofuran (45 mL) was added via syringe, followed by 1.70 g (11.6
mmol) of triethyl-phosphine. The reaction mixture was stirred for
30 min, and the flask was then removed from the cooling bath and
allowed to warm to room temperature with stirring. The orange
supernatant solution was then separated from the colorless solids
via cannula filtration into another Schlenk flask, and the solvent
was removed in vacuo. The resulting solid was washed with 3 ×
60 mL pentane (vigorous stirring) to remove TMEDA, the solid
residue was then extracted with 60 mL dichloromethane, and the
resulting orange solution was filtered via cannula. The solvent was
removed in vacuo, affording 6.03 g (74% yield) of an orange oil
1
microcrystalline powder. H NMR (400.2 MHz, THF- d8): δ 7.31
n
n
(m, 12H, BuB(CH2P Ph2)3), 7.01 (m, 18H, BuB(CH2P Ph2)3),
2.31 (s, 4H, Me2NC H2C H2NMe2), 2.16 (s, 12H, Me2NCH2CH2N
Me2, 1.30 (t, 2H, CH3CH2CH2C H2B), 0.98 (s, 6H, nBuB(C
H2PPh2)3), 0.78 (m, 4H, CH3C H2C H2CH2B), 0.56 (t, 3H, C
H3CH2CH2CH2B). 13C NMR (100.6 MHz, THF- d8): δ 135.8, 133.1,
132.5, 131.7, 55.2 (Me2N CH2 CH2NMe2), 42.7 (Me2NCH2CH2N
Me2), 22.6, 18.4, 15.8, 8.1. 31P NMR (162.0 MHz, THF- d8): δ
-7.99 (s). Anal. Calcd for C49H61BLiN2P3.: C, 74.62; H, 7.80; P,
11.78. Found: C, 74.71; H, 7.55; P,11.79.
Synthesis of [nBuB(CH2PPh2)3Ag(PEt3)] (2). Using a mortar
and pestle, 0.83 g (5.8 mmol) AgCl was ground to a fine powder
and added to a 100 mL flask containing 4.56 g (5.8 mmol)
[Li(TMEDA)nBuB(CH2PPh2)3]. The flask was evacuated and back-
filled with N2 three times, covered with aluminum foil, and cooled
to -78 °C in an acetone/dry ice bath. With stirring, tetrahydrofuran
(45 mL) was added via syringe, followed by 0.85 g (5.8 mmol) of
triethylphosphine. The reaction mixture was stirred for 30 min, then
the flask was removed from the cooling bath and allowed the
reaction mixture to warm to room temperature with stirring. The
yellow solution was next separated from the colorless solids via
cannula filtration into another Schlenk flask. The solvent was
removed in vacuo and the resulting solids washed with 3 × 60 mL
pentane (vigorous stirring) to facilitate TMEDA removal. The solid
residue was then extracted with 60 mL dichloromethane. The yellow
solution was filtered from the colorless solids via cannula filtration.
The solvent was removed from the filtrate in vacuo, giving 4.07 g
1
(3). H NMR (400.2 MHz, THF-d8): δ 7.71 (d, 2H, ortho-PhB),
7.19 (m, 2H, meta-PhB), 6.61 (t, 1H, para-PhB), 1.96 (m, 6H, PC
H2CH3), 1.68 (m, 9H, PCH2C H3), 1.21 (m, 3H, PhB(CH2(P(C
i
HMe2)2)3), 1.16 (s, 6H, PhB(C H2P Pr2)3), 0.89 (d, 36H, P(CH(C
H3)2)2). 13C NMR (100.6 MHz, THF-d8): δ 135.4, 134.4, 127.3,
126.8, 125.0, 24.4, 19.7, 18.1 (P(CH2CH3)3), 9.5 (P(CH2 CH3)3).
31P NMR (162.0 MHz, THF-d8, 25 °C): δ 11.66 (br s, ∆υ1/2
)
33.78 Hz), 2.30 (br s, ∆υ1/2 ) 44.62 Hz). 31P NMR (162.0 MHz,
1
1
THF-d8, -103 °C): δ 8.21 (dd, J(P-109Ag) ) 486.08 Hz, J(P-
107Ag) ) 425.33 Hz, 2J(P-P) ) 33.26 Hz), -1.02 (dq, 1J(P-109Ag)
) 602.65 Hz, 1J(P-107Ag) ) 528.20 Hz, 2J(P-P) ) 33.26 Hz). Anal.
Calc. for C33H68AgBP4: C, 56.02; H, 9.69; P, 17.51. Found: C,
56.01; H, 9.84; P, 17.78.
1
(79% yield) of a yellow powder (2). H NMR (400.2 MHz, THF-
d8): δ 7.21 (m, 12H, nBuB(CH2P Ph2)3), 6.99 (m, 18H nBuB(CH2P
Ph2)3), 1.87 (dq, 6H, PC H2CH3), 1.35 (s, 6H, nBuB(C H2PPh2)3),
1.30 (t, 2H, CH3CH2CH2C H2B) 1.16 (dt, 9H, PCH2C H3), 0.99
(m, 4H, CH3C H2C H2CH2B), 0.34 (t, 3H, C H3CH2CH2CH2B).
13C NMR (100.6 MHz, THF-d8): δ 142.2, 133.4, 130.7, 129.7,
128.5, 30.5, 28.7, 20.0, 18.4 (P(CH2CH3)3), 9.4 (P(CH2 CH3)3). 7.6.
Synthesis of [Li(TMEDA)nBuB(CH2PiPr2)3]. To a stirring
i
suspension of 1.90 g (13.8 mmol) Pr2PCH2Li in 45 mL of ether
was added 2.07 mL (13.8 mmol) anhydrous TMEDA. The mixture
was cooled to -78 °C in an acetone/dry ice bath with stirring, and
n
4.6 mL (4.6 mmol) of a hexane solution of BuBCl2 was slowly
2536 Inorganic Chemistry, Vol. 47, No. 7, 2008

Tris(phosphino)borato Ag(I) as AACVD Precursors
added. The cooling bath was then removed and the solution was
allowed to warm to room temperature with stirring, during which
time a colorless solid precipitated from the yellow solution. The
solution was then filtered via cannula and the solvent removed in
aerosol injection and at atmospheric pressure (740–760 Torr) with
a solvent trap and bubbler for the exhaust gas. Precursor solutions
were nebulized with a TSI 3076 collision-type nebulizer and were
deposited on substrates downstream in the reactor. Carrier gas (Ar)
flow rates were controlled at 1.65 L/min with a mass flow controller.
The 52100 steel for substrates was purchased from McMaster-Carr,
then cut into 1 cm × 1 cm squares and polished to ∼20 nm
roughness as determined by atomic force microscopy. The 1′′ ×
1′′ 1737F Corning glass substrates were purchased from Precision
Glass and Optics. MgO(100) (1 cm × 1 cm) substrates were
purchased from MTI Corporation. Substrates were cleaned with
acetone, then placed on the graphite susceptor in the AACVD
reactor, and heated during film growth with an infrared lamp. The
reactor was evacuated and backfilled with Ar three times prior to
use, and the precursor reservoir loaded with freshly prepared
precursor solutions in tetrahydrofuran (c ) 0.015 mol/L) via cannula
filtration (Whatman GF/B glass fiber filters). Susceptor temperatures
were varied from 250–550 °C to determine optimum growth
temperatures. Film thicknesses were measured with a Tencor P-10
profilometer and film chemical compositions were assayed with
an Omicron ESCA Al KR probe X-ray photoelectron spectrometer.
Samples were sputtered with an Ar+ ion beam before spectra were
recorded. The phase purity of the Ag films was also examined using
θ-2θ scans on a computer-interfaced Rigaku DMAX-A powder
diffractometer using Ni-filtered Cu KR radiation, and glancing X-ray
diffraction (GXRD; angle of incidence R ) 0.3°) θ-2θ scans were
recorded on a computer-interfaced Rigaku ATX-G diffractometer.
Film microstructure and morphologies were assessed with an FEI
Quanta 600sFEG scanning electron microscope and a Veeco TM
CP II research atomic force microscope operating in the contact
mode. Four-probe resistivity data were acquired on a Bio-Rad
HL5500 instrument at room temperature.
1
vacuo, affording 1.84 g (86% yield) of a yellow solid. H NMR
(400.2 MHz, THF-d8): δ 2.31 (s, 4H, Me2NC H2C H2NMe2), 2.16
(s, 12H, Me2NCH2CH2N Me2, 1.47 (t, 2H, CH3CH2CH2C H2B),
n
n
1.36 (m, 3H, BuB(CH2P(C HMe2)2)3), 1.22 (s, 6H, BuB(C H2P
iPr2)3), 0.90 (d, 36H, P(CH(C H3)2)2), 0.70 (m, 4H, CH3C H2C
H2CH2B), 0.34 (t, 3H, C H3CH2CH2CH2B). 13C NMR (100.6 MHz,
THF-d8): δ 56.2 (Me2N CH2 CH2NMe2), 43.3 (Me2NCH2CH2N
Me2), 26.6, 21.5, 17.6, 15.8, 14.1, 7.8. 31P NMR (162.0 MHz, THF-
d8): δ -7.53. Anal. Calcd for C31H73BLiN2P3: C, 63.69; H, 12.59;
P, 15.89. Found: C, 63.76; H, 12.26; P, 16.05.
Synthesis of [nBuB(CH2PiPr2)3Ag(PEt3)] (4). Using a mortar
and pestle, 0.56 g (3.9 mmol) AgCl was ground to a fine powder
and added to a 100 mL flask containing 1.84 g (3.9 mmol) of
[Li(TMEDA)nBuB(CH2PiPr2)3]. The flask was evacuated and back-
filled with N2 three times, covered with aluminum foil, and cooled
to -78 °C with stirring in an acetone/dry ice bath. Next, tetrahy-
drofuran (45 mL) was added via syringe, followed by 0.58 g (3.9
mmol) triethylphosphine. The reaction mixture was stirred for 30
min., and then the flask was removed from the cooling bath and
allowed to warm to room temperature with stirring. The yellow
solution was separated from colorless solids via cannula filtration
into another Schlenk flask, and the solvent was removed in vacuo.
The resulting solid was then washed with 3 × 60 mL pentane
(vigorous stirring) to remove TMEDA. The solid residue was then
dissolved in 45 mL of dichloromethane and the resulting yellow
solution was filtered from colorless solids via cannula filtration.
The solvent was removed from the filtrate in vacuo, giving 1.82 g
(68% yield) of a thick yellow oil (3). 1H NMR (400.2 MHz, THF-
d8): δ 1.90 (m, 6H, PC H2CH3), 1.62 (m, 9H, PCH2C H3), 1.49 (t,
n
2H, CH3CH2CH2C H2B), 1.40 (m, 3H, BuB(CH2P(C HMe2)2)3),
Results
n
i
1.25 (s, 6H, BuB(C H2P Pr2)3), 1.03 (d, 36H, P(CH(C H3)2)2),
0.78 (m, 4H, CH3 CH2CH2CH2B), 0.44 (t, 3H, C H3CH2CH2CH2B).
13C NMR (100.6 MHz, THF-d8): δ 27.6, 20.0, 19.1 (P(CH2CH3)3),
17.2, 16.1, 15.2, 14.1, 8.0 (P(CH2 CH3)3), 7.4. 31P NMR (162.0
MHz, THF-d8, 25 °C): δ 14.57 (br dd), 3.49 (br dq). 31P NMR
(162.0 MHz, THF-d8, -103 °C): δ 11.53 (dd, 1J(P-109Ag) ) 708.45
Hz, 1J(P-107Ag) ) 617.27 Hz, 2J(P-P) ) 44.19 Hz), 4.35 (dq, 1J(P-
In this section, the synthesis of a new series of tris(pho-
shino)borato silver(I) AACVD precursors is presented, as
well as a discussion of molecular structure and volatility
characteristics. Next, two of these precursors are selected
for thin film growth of metallic silver thin films on glass,
MgO(100), and steel substrates, and their subsequent com-
positional and microstructural analysis is presented.
1
2
109Ag) ) 726.25 Hz, J(P-107Ag) ) 629.53 Hz, J(P-P) ) 44.19
Hz). Anal. Calcd for C31H72AgBP4: C, 54.16; H, 10.56; P, 18.02.
Found: C, 54.25; H, 10.58; P, 17.87.
AACVD Precursor Synthesis and Characterization.
Four tris(phosphino)borato silver(I) complexes, in which
substituents on the boron and phosphorus atoms are varied
from phenyl to alkyl groups, were synthesized in a three-
step procedure (Scheme 1) modified from that of Peters et
al.,14 using commercially available reagents. Because of the
light sensitivity of AgCl, its reaction with lithiated tris(pho-
sphoryl)borate and PEt3 was performed under aluminum foil.
However, the subsequently formed new silver complexes are
found to be air- and light-stable.
For complexes 1 and 2, 31P NMR spectroscopy indicates
that exchange of the phosphine ligands is sufficiently slow
at room temperature on the NMR time scale to give
detectable 31P-107,109Ag spin–spin coupling. However, the
31P NMR spectra of complexes 3 and 4 at room temperature
show broadened peaks for the two distinct phosphorus
chemical environments in the structure (tripodal ligand and
PEt3). Thus, low temperature (-103 °C) 31P spectra were
recorded in THF-d8 for all complexes, and their coupling
X-ray Single Crystal Diffraction Characterization of Com-
plexes 1 and 2. Pale yellow crystals of [PhB(CH2PPh2)3Ag(PEt3)]
(1) suitable for X-ray analysis were grown by slow cooling of a
toluene solution to -40 °C. Slow cooling of a diethyl ether solution
to -40 °C produced pale yellow crystals of [nBuB(CH2PPh2)3-
Ag(PEt3)] (2) suitable for X-ray analysis. X-ray diffraction data
were collected for single crystals of complexes 1 and 2 on a
SMART-1000 CCD area detector instrument at 153(2) K with
graphite monochromated Mo KR X-ray radiation. Data were
corrected for Lorentz and polarization effects. The structures were
solved by direct methods and expanded using Fourier techniques.
All nonhydrogen atoms were refined anisotropically. All calculations
were performed using the Bruker SHELXTL crystallographic
software. The higher R value for complex 2 is attributed to increased
thermal disorder associated with the nBu alkyl group and the
triethylphosphine ethyl groups. Relevant data collection parameters
are summarized in Table S1 of the Supporting Information.
Film Growth and Characterization. Thin films of metallic Ag
were grown in a horizontal cold-wall quartz reactor with liquid
Inorganic Chemistry, Vol. 47, No. 7, 2008 2537

McCain et al.
Scheme 1. Three-step synthesis of complexes 1–4
Table 1. Low-temperature (-103 °C) 31P NMR Coupling Constant and Chemical Shift Data for Tripodal Ligand Skeletal Phosphorus (PL) and
Triethylphosphine Phosphorus (PT) nuclei
1
2
3
4
1
1J(109Ag-PL), J(107Ag-PL) (Hz)
218, 190
466, 406
25.8
215, 187
466, 407
30.7
486, 425
603, 528
33.3
708, 617
726, 630
44.2
1
1J(109Ag-PT), J(107Ag-PT) (Hz)
2J(P-P) (Hz)
δ (ppm)
-1.64, -2.51
-1.47, -2.94
8.21, -1.02
11.53, 4.35
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for Complexes 1 and 2
bond distances (Å)
bond angles (deg)
complex 1
bond
complex 1
complex 2
bond
complex 2
Ag(1)-P(1)
Ag(1)-P(2)
Ag(1)-P(3)
Ag(1)-P(4)
2.4065(8)
2.5426(7)
2.5275(7)
2.5321(7)
2.439(2)
2.551(2)
2.512(2)
2.5287(19)
P(1)-Ag-P(2)
P(1)-Ag-P(3)
P(1)-Ag-P(4)
P(2)-Ag-P(3)
P(2)-Ag-P(4)
P(3)-Ag-P(4)
124.47(2)
125.19(3)
127.58(3)
89.91(2)
89.31(2)
88.73(2)
124.04
125.59
125.91
90.03(6)
90.56(7)
89.73(7)
constants are summarized in Table 1. Pale yellow single
crystals suitable for diffraction studies were obtained by slow
cooling of toluene (1) or ether (2) solutions. The crystal
structures (Figure 2) confirm the mononuclearity of the
molecular structures, with distorted tetrahedral coordination
geometries about the Ag+ centers similar to the geometries
found in other low-valence transition metal complexes.14
Metrical parameters are summarized in Table 2. The angles
between the phosphorylborate P-Ag vectors, P-Ag-P,
range from 88.72(2) to 89.91(2)° in complex 1 and from
89.73(7) to 90.56(7)° in complex 2. The angles between the
Et3P-Ag and phosphorylborate-Ag vectors range from
124.47(3) to 127.57(3)° in complex 1 and from 124.04(6)
to 125.91(7)° in complex 2, indicating significant distortions
from idealized tetrahedra. The Ag-P(phosphorylborate) bond
lengths in complex 1 range from 2.5275(7) to 2.5426(7) Å.
The Ag-PEt3 bond length for the triethylphosphine P is
shorter, at 2.4065(8) Å. In complex 2, the Ag-P(phosphoryl-
borate) bond lengths range from 2.512(2) to 2.551(2) Å. The
Ag-PEt3 bond length for the triethylphosphine P in complex
2 is 2.439(2) Å. Compared to the AgP4 tetrahedra present in
Ag3P11 (P-Ag ) 2.535(3) Å),19 the only other reported Ag(I)
compoundsurroundedbyfourPligands,theAg-P(phosphoryl-
borate) bond lengths present in complexes 1 and 2 agree
well (Figure 3). The Ag-PEt3 distances in complexes 1 and
2 agree well with another solid state structure that involves
a four-coordinate Ag(I)-triethylphosphine bond, [Et3PAgBr]4,
where P-Ag ) 2.402(3) Å (Figure 4).20
Differences in the Ag-PEt3 bond lengths between com-
plexes 1 and 2 can be explained as arising from the different
substituents on the borate ligand. The phenyl substituent in
the aryltrialkylborate (1) results in better anionic charge
delocalization as compared with the tetraalkylborate in
complex 2. Hence, a stronger Coulombic interaction within
the zwitterionic complex 2 between borate anion and silver
cation can be anticipated, given the fact that the Ag-B
distance is only slightly larger than the sum of the Ag and
B van-der-Waals radii (3.42 Å).21We speculate that this ionic
interaction may account for a stronger bond between Ag+
and the PEt3 ligand in complex 1, resulting in a shorter
Ag-PEt3 bond length. This picture is supported by the greater
Ag-B interatomic distance in complex 1 (3.656(3)) vs complex
2 (3.619(9)) and the slightly larger Ag-P coupling constant with
the PEt3 ligand found for complex 1 in the solution NMR (218
Hz (1), 215 Hz (2)). Likewise, Peters has recently interpreted
spectroscopic results within a series of zwitterionic platinum
and rhodium boratocomplexes in terms of Coulombic metal-
borato interactions at similar M-B distances.22
Atmospheric pressure thermogravimetric analysis (TGA)
scans for complexes 1–4 under N2 are compared in Figure
(20) Churchill, M. R.; Donahue, J.; Rotella, F. J. Inorg. Chem. 1976, 11,
2752–2758.
(21) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(22) (a) Betley, T. A.; Peters, J. C. Angew. Chem., Int. Ed. 2003, 42, 2385.
(b) Thomas, C. M.; Peters, J. C. Organometallics 2005, 24, 5858.
(19) Moller, M. H.; Jeitschko, W. Inorg. Chem. 1981, 20, 828–833.
2538 Inorganic Chemistry, Vol. 47, No. 7, 2008

Tris(phosphino)borato Ag(I) as AACVD Precursors
Figure 4. P4Ag4Br4 core of [PEt3AgBr]4, showing 3-fold disorder of the
silver atoms. Reprinted with permission from ref 20. Copyright 1976
American Chemical Society.
Figure 5. Atmospheric pressure thermogravimetric analysis (TGA) of the
volatility characteristics of complexes 1-4. The weight loss data were
recorded at a ramp rate of 2.5 °Cmin-1 and 50 mL min-1 N2 flow rate.
atures, again resulting in dark gray residues. Complex 2
exhibits the lowest residual weight percent (12.5%), which
can be compared to the calculated weight percentage for pure
silver of 12.0%, with the additional 0.5% attributable to
decomposed ligand contamination. Complexes 3 and 4 give
residual weights of 25.0 and 19.9%, respectively. These
residual weights correspond to calculated weight percentages
for silver of 15.1 and 15.6%, respectively. Again, the
additional weight percentages appear to be due to decom-
posed ligand contamination (9.9 and 4.3%, respectively).
Ag Thin Film Growth and Characterization. Complex
1 was initially chosen for silver metal film growth process
experiments on glass and Mg(100) (advanced interconnect
applications), and 52100 steel substrates (lubricant coating
applications) because of the sharp, two-step volatilization
process determined by TGA. AACVD film growth was
carried out with 0.015 M solutions of complex 1 in THF.
The optimum susceptor temperature for high growth rates
and crystallinity was found, with some experimentation, to
be 300 °C. However, on glass and MgO(100) substrates,
under these conditions, only minute quantities of transparent
brown films are deposited. Glancing X-ray diffraction
(GXRD) reveals the (111) and (200) reflections of cubic
phase silver metal (PDF 04–0783, Figure 6) for the films
grown on MgO(100). On 52100 steel, growth rates are
0.5–0.6 nm/min, depositing brown-tinted adherent films
which are visibly reflective. The X-ray diffraction (XRD)
θ-2θ scan carried out on 100 nm films shows only
reflections from polycrystalline iron (PDF 06–0696). How-
ever, X-ray photoelectron spectroscopy (XPS) does show the
Figure 2. ORTEP drawings (ellipsoids at the 50% confidence level) of
(A) [PhB(CH2PPh2)3Ag(PEt3)] (1) and (B) [nBuB(CH2PPh2)3Ag(PEt3)] (2)
molecular structures showing the Ag and P atom labeling schemes.
Hydrogen atoms are omitted for clarity.
Figure 3. Core structure of Ag3P11. Reprinted with permission from ref
19. Copyright 1981 American Chemical Society.
5. Complex 1 shows the most abrupt weight loss features,
occurring in two distinct steps at 160 and 205 °C. The first
decomposition step (-12.8%) corresponds to the calculated
weight percent loss for triethylphosphine of -12.9%. The
second decomposition step yields dark gray material with a
residual weight percent of 15.3%. This can be compared to
the calculated weight percentage for pure silver of 11.8%,
where the additional 3.5% would then be attributable to
decomposed ligand fragments. For the other complexes, the
decomposition range for each complex is wider than for
complex 1, and final decompositions are at higher temper-
Inorganic Chemistry, Vol. 47, No. 7, 2008 2539

McCain et al.
Figure 9. X-ray diffraction (XRD) θ-2θ scan of Ag film grown on 52100
steel by AACVD using precursor 2. Peaks are labeled with the corresponding
(hkl) reflections from cubic phase Fe (PDF 06-0696) and cubic phase Ag
(PDF 04-0783).
Figure 6. Glancing X-ray diffraction (GXRD) θ-2θ scan (R ) 0.30°) of
a Ag film grown on MgO(100) by AACVD using precursor 1. Peaks are
labeled with the corresponding (hkl) reflections from cubic phase Ag (PDF
04-0783).
Figure 10. Glancing XRD θ-2θ scan (R ) 0.30°) of Ag film grown on
52100 steel by AACVD using precursor 2. Plotted underneath are the peak
positions and relative intensities for the powder pattern of cubic phase Ag
(PDF 04-0783).
Figure 7. X-ray photoelectron spectroscopy (XPS) spectrum of a Ag film
deposited on 52100 steel by AACVD using precursor 1. Peaks are labeled
with their corresponding element. The inset is enlarged portion of the
spectrum showing the Ag 3d5/2 and 3d3/2 ionizations.
Figure 11. XPS spectrum of a Ag film deposited on 52100 steel by
AACVD using precursor 2. Peaks are labeled with their corresponding
element. The inset is enlarged portion of the spectrum showing the Ag 3d5/2
and 3d3/2 ionizations.
Figure 8. Glancing X-ray diffraction (GXRD) θ-2θ scan (R ) 0.30°) of
Ag film grown on MgO(100) by AACVD using precursor 2. Peaks are
labeled with the corresponding (hkl) reflections for cubic phase Ag (PDF
04-0783).
presence of metallic silver (Figure 7). The Ag 3d5/2 and 3d3/2
peak positions match precisely those of Ag metal.23 However,
other features are evident corresponding to 3% B, 9% C,
3% P versus Ag, as well as Fe and O attributable to the
substrate.
Precursor 2 was next investigated for the growth of Ag
films by AACVD, because of its cleaner decomposition
characteristics in comparison to the other precursors as
determined by the atmospheric pressure TGA experiments
(Figure 5). As conducted for precursor 1, AACVD growth
was carried out with 0.015 M solutions of complex 2 in THF.
For this precursor, the optimum temperature for conformal
Ag film growth was determined to be 500 °C. The glass and
Mg(100) substrates again yielded minute quantities of
transparent gray/brown films. Thicknesses ranged between
25 and 30 nm for a 2.5 h growth time. GXRD reveals the
same diffraction pattern as with precursor 1 with increased
intensities (Figure 8). For 52100 steel as the substrate, a dark
gray, reflective film was deposited at a growth rate of 14–18
nm/min. The XRD θ-2θ scan carried out on 3 µm thick
films (Figure 9) exhibits strong iron reflections, however the
weak reflection at 38.1° can be assigned to the cubic phase
Figure 12. SEM images of Ag films grown on a 52100 steel substrate
using precursor 2 with growth times of (A) 5 min and (B) 3.5 h.
of Ag. Furthermore, GXRD of these samples reveals reflec-
tions for polycrystalline cubic-phase Ag with slightly pre-
ferred orientation for the (111) direction (Figure 10). XPS
also confirms the presence of Ag metal (Figure 11). Traces
of contamination (1% B, 4% C, 1% P), doubtless arising
from decomposed ligand are also detected, although at
substantially lower levels than observed with precursor 1.
Absent from the XPS spectrum are features due to the
underlying substrate (Fe, O).
The microstructure and the growth mechanism of the silver
films were analyzed using scanning electron microscopy
(SEM) and atomic force microscopy (AFM). Figure 12
shows SEM images of two films grown by AACVD using
precursor 2. In the first image (Figure 12A), growth of the
silver films was conducted for 5 min under the conditions
described above, revealing small (<100 nm) particles with
little apparent ordering and incomplete coverage of the
(23) Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., King,
R. C. Eds.; Physical Electronics: Eden Prairie, MN, 1995.
2540 Inorganic Chemistry, Vol. 47, No. 7, 2008

Tris(phosphino)borato Ag(I) as AACVD Precursors
developed in literature for other low-valent late transition
metals,16–18 using commercial reagents. This new family of
precursors exhibits good solubility in a variety of common
organic solvents (toluene, THF, ether, acetone, dichlo-
romethane, pyridine).
1
The J(107Ag-31P) parameters in tetrahedral silver com-
plexes of tertiary phosphines generally range from 190 to
235 Hz,27,28 with higher magnitudes found for
phosphites27a,29 and aminophosphines.28 The low temperature
(-103 °C) values for the 1J(107Ag-31PT) in complexes 1 (406
Hz) and 2 (407 Hz) are rather large for tetrahedral Ag(I)
complexes and more closely match values associated with
two- and three-coordinate Ag+ phosphine complexes in
Figure 13. Atomic force microscopy scan of 5.0 µm × 5.0 µm of the Ag
film shown in Figure 12B. Rrms ) 55.4 nm; Rp-v ) 463 nm.
1
solution. The J(107Ag-31PT) values for complexes 3 and 4
are also very large (528 and 630 Hz, respectively) for
tetrahedral Ag(I) complexes and closer in range to one-
coordinate Ag+ phosphine complexes in solution. The 31P NMR
spectra patterns suggest that the solid state structures of
complexes 1–4 are essentially maintained in solution. The ratios
of the isotopomeric Ag-P coupling constants (1J(109Ag-P)/
1J(107Ag-P) ) 1.14–1.15) are in excellent agreement with
the theoretical ratio γ(109Ag)/γ(107Ag) ) 1.15. The resonance
due to Et3P is a pair of doublets owing to splitting by 107Ag
[I ) ½; 51.82% natural abundance] and 109Ag [I ) ½;
48.18% natural abundance], and each component is further
split into a 1:3:3:1 quartet by coupling to the three magneti-
cally equivalent phosphorus nuclei of the tripodal ligand. The
couplings are such that the patterns overlap, giving a 1:4:
6:4:1 pattern for complexes 1 and 2, and multiplets for
complexes 3 and 4. The tripodal ligand 31P nuclei appear as
a pair of doublet-of-doublets with 107Ag and 109Ag couplings,
and a two-bond 31P-31P coupling to the triethylphosphine
phosphorus. At low temperature (-103 °C), the resonances
are generally sharper, owing to a decrease in the rate of
ligand exchange.
Thermal analysis shows that by varying the ligand
framework substituents, decomposition temperatures can be
altered to provide cleanly decomposing precursors. Introduc-
tion of an alkyl substituent on the boron results in thermal
decomposition products more closely approaching metallic
Ag for complexes 2 and 4 vs complexes 1 and 3, where there
is a B-Ph substituent. When comparing alkyl vs phenyl
phosphorus substitution on the tripodal ligand, the opposite
trend is observed. Thus, thermal decomposition products of
complexes 1 and 2 with phenyl-substituted phosphines more
closely approach metallic Ag.
substrate. The next image (Figure 12B) is of a silver film
grown under the same conditions for a 3.5 h period. Here
the substrate has full coverage by the Ag film with little
ordering and significant apparent roughness as indicated by
lighter shaded peaks rising from the film. AFM data (Figure
13) agree with this assessment, with an rms roughness (Rrms
)
of 55.4 nm and a peak-to-valley roughness (Rp-v) of 463
nm.
Charge transport measurements can provide a qualitative
assessment of silver film quality, with measured resistivities
that are closer to bulk silver indicating higher film quality/
purity.11a,24 Resistivities measured on the films grown for
3.5 h on 52100 steel (3 µm thick) were 2–3 µΩ cm, very
close to the resistivity reported for bulk silver (1.6 µΩ cm).25
The current flow on the bare 52100 steel substrates was
below the instrumental detection limit.
Discussion
Precursor Design and Synthesis. The goal of this
investigation was to design and realize efficient AACVD
precursors for metallic silver thin films. Previous studies have
relied on fluorinated precursor ligand substituents to provide
ambient atmosphere stability and volatility. The complexes
developed in the present study utilize a tridentate phosphi-
noborate ligand, along with a neutral ancillary triethylphos-
phine ligand, to securely saturate the metal ion coordination
sphere. This results in a new series of silver complexes that
are both light- and air-stable, as well as excellent AACVD
precursors for metallic silver films.
An ideal AACVD precursor is soluble in common organic
solvents, should have sufficient volatility to form aerosol
particles capable of being delivered to the desired substrate,26
and should decompose cleanly to the target material. The
synthesis of complexes 1–4 is accomplished in three
straightforward steps (Scheme 1), modified from a procedure
Of the four new precursors, complexes 1 and 2 were
chosen for film growth experiments. Complex 1 displays a
sharp decomposition temperature in the atmospheric pressure
(27) (a) Alyea, E. C.; Malito, J.; Nelson, J. H. Inorg. Chem. 1987, 26,
4294–4296. (b) Price, S. J. B.; Brevard, C.; Pagelot, A.; Sadler, P. J.
Inorg. Chem. 1985, 24, 4278–4281. (c) Alyea, E. C.; Dias, S. A.;
Stevens, S. Inorg. Chim. Acta 1980, 44, L203–L204. (d) Goel, R. G.;
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Inorganic Chemistry, Vol. 47, No. 7, 2008 2541

McCain et al.
TGA, whereas complex 2 decomposes to Ag relatively free
of contamination. Complex 2 was found to provide higher
growth rates and Ag film purity on steel substrates, thus
proving to be the most effective precursor of the four
complexes for the efficient growth of metallic Ag thin films.
Silver Film Growth and Characterization. Silver films
were grown by AACVD using both precursors 1 and 2 on
three different substrates, amorphous glass, single crystal
cubic phase MgO (100), and polycrystalline 52100 steel
(cubic phase iron). Only the (111) and (200) growth
orientations are observed for the films grown on MgO(100)
substrates, while more orientations are observed for films
grown on 52100 steel. Other work has shown that substrate
texture can significantly affect growth orientation in metallic
silver films.30 In the present study higher film growth rates
are observed for films deposited on 52100 steel than for that
on MgO(100) or amorphous glass. To the best of our
knowledge, there have been no reports comparing substrate
effects on Ag CVD film growth rates. One hypothesis is that
the greater steel substrate surface roughness (∼20 nm) vs
the MgO(100) substrates (∼5 nm) promotes nucleation, thus
resulting in higher deposition rates. A recent study compared
growth rate and substrate surface roughness in the electro-
cyrstallization of Ag on carbon electrodes, however found
no direct correlation.31 Better wetting of Ag on the ferrous
steel substrate vs MgO might also be responsible for the
observed increase in growth rate. Silver is known to wet
oxide substrates poorly,32 and adhesion and wetting of Ag
has been shown to improve by adding transition metal
interlayers.33 In a study of carbonaceous deposits on iron
substrates, it was concluded that the better surface wetting
of the deposits on iron contributed to the observed higher
growth rates and surface coverage.34
the present silver films are metallic Ag, from the position of
the 3d photoelectron peaks, with shifting to higher binding
energies expected for silver in higher oxidation states.20 The
resistivities of films grown with precursor 2 approach those
of pure bulk Ag,1a suggesting that contamination is minimal.
The SEM and AFM images of the silver films grown with
precursor 2, measured after only 5 min (Figure 12A) of
deposition and after 3.5 h (Figure 12B), provide information
on the film growth mechanism. The first image (Figure 12A)
shows small Ag particles on the steel substrate. After longer
deposition times (Figure 12B), these “islands” have coalesced
into a conformal film, relatively rough as evidenced by AFM
(Figure 13). This island growth (Volmer–Weber) mechanism
is a common mode of Ag film growth on oxide and
semiconductor surfaces,8d,36 because of weak interaction
energies between the adsorbed metal atom and the substrate.
Conclusions
Four new light- and air-stable Ag(I) coordination com-
plexes have been synthesized, characterized, and evaluated
as potential AACVD precursors. Silver film growth has been
demonstrated using complexes 1 and 2, with the latter giving
higher growth rates of silver films having electrical resis-
tivities approaching that of bulk silver. Film growth is
suggested to occur through an island growth that coalesces
to conformal coverage. These complexes represent a sig-
nificant improvement in nonfluorinated Ag precursors, of-
fering ease of handling and efficient film deposition. While
marginal wettability of the films on untreated amorphous
glass and Mg(100) substrates makes application as intercon-
nects unlikely, the conformal coverage on 52100 steel makes
the Ag films grown in this study promising as solid lubricant
coatings. Future studies will focus on the tribological
characteristics of these films.
For Ag film growth on 52100 steel substrates with
precursor 1, the degree of decomposed ligand contamination
(3% B, 9% C, 3% P, as determined from XPS) adversely
affects film growth rate and crystallinity. However, precursor
2, with a significantly lower degree of decomposed ligand
contamination (1% B, 4% C, 1% P), can be used to deposit
metallic silver films at higher growth rates with better surface
coverage and higher crystallinity. An XRD θ-2θ scan shows
a small feature corresponding to the Ag (111) reflection,
while GXRD reveals the full polycrystalline pattern for the
cubic phase of silver in the films. The cubic phase is by far
the most common phase for metallic Ag, although a
metastable hexagonal phase is known.35 XPS confirms that
Acknowledgment. We thank the NSF (grant CMS-
0510895) for support of this research, and the Northwestern
Materials Research Center (NSF MRSEC grant DMR-
0520513) for providing characterization facilities. We thank
Prof. Q. Jane Wang for stimulating discussions about
tribology, as well as Dr. Yuyang Wang for assistance with
VTNMR data collection and Dr. J. Carsello for assistance
with X-ray diffraction measurements. We also thank Mr.
M. T. Russell for assistance with SEM image collection.
Supporting Information Available: Detailed crystallographic
information for complexes 1 and 2, room and low temperature 31
P
NMR for all complexes and VT 31P NMR for complex 3 (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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2542 Inorganic Chemistry, Vol. 47, No. 7, 2008