Kinetic Analysis of Au deposition from Aqueous HF onto Si(111) by Surface Second Harmonic Generation

Article abstract of DOI:10.1149/1.1391645

Deposition of Au from aqueous HF onto Si(111) has been studied by Rutherford backscattering (RBS) and surface second harmonic generation (SHG), which at 532 nm is nearly resonant with the surface plasmon of Au nanoclusters. The RBS measurements indicate that Au deposition is rate-limited by diffusion, while the SHG measurements indicate that Au cluster growth is rate-limited by either a surface reaction involving a fluoride-containing species or electron transfer. This apparent contradiction can be reconciled by proposing that initial deposition in the form of AuCN is followed by a slow electroless reduction of Au(I) accompanied by Si oxidation. By addition of HCl and KF, the solution phase equilibria can be separately manipulated, motivating further SHG experiments which indicate that HF and not HF₂^- is the kinetically active fluoride-containing species. The apparent reaction order for Au cluster growth with respect to HF is approximately 1/2, and the reaction order for Au cluster growth with respect to Au(CN)₂^- is near zero in the concentration range 10^-4 to 10^-5 M.

Full text of DOI:10.1149/1.1391645

570
Journal of The Electrochemical Society, 146 (2) 570-573 (1999)
S0013-4651(98)05-003-3 CCC: $7.00 © The Electrochemical Society, Inc.
Kinetic Analysis of Au deposition from Aqueous HF onto Si(111) by
Surface Second Harmonic Generation
Ramanathan Srinivasan and Ian Ivar Suni*,z
Department of Chemical Engineering, Clarkson University, Potsdam, New York 13699-5705, USA
Deposition of Au from aqueous HF onto Si(111) has been studied by Rutherford backscattering (RBS) and surface second har-
monic generation (SHG), which at 532 nm is nearly resonant with the surface plasmon of Au nanoclusters. The RBS measurements
indicate that Au deposition is rate-limited by diffusion, while the SHG measurements indicate that Au cluster growth is rate-limit-
ed by either a surface reaction involving a fluoride-containing species or electron transfer. This apparent contradiction can be rec-
onciled by proposing that initial deposition in the form of AuCN is followed by a slow electroless reduction of Au(I) accompanied
by Si oxidation. By addition of HCl and KF, the solution phase equilibria can be separately manipulated, motivating further SHG
experiments which indicate that HF and not HF2Ϫ is the kinetically active fluoride-containing species. The apparent reaction order
for Au cluster growth with respect to HF is approximately 1/2, and the reaction order for Au cluster growth with respect to
Au(CN)Ϫ2 is near zero in the concentration range 10Ϫ4 to 10Ϫ5 M.
© 1999 The Electrochemical Society. S0013-4651(98)05-003-3. All rights reserved.
Manuscript submitted May 1, 1998; revised manuscript received October 30, 1998.
Deposition of noble metals onto Si is important from both a fun-
sified Princeton Instruments LN/1024 CCD. The SH signal was ver-
ified to depend quadratically on the laser power, with final experi-
ments performed at a laser power of 5 mJ/pulse. The p-type, 0.01 ⍀
cm Si(111) sample lay horizontally at the bottom of the virgin Teflon
cell. Prior to each experiment, the sample was cleaned by treatment
in a 0.50 MHF etchant and SC-1 and SC-2 process solutions. All
reagents were of very large-scale integrated (VLSI) grade purity.
Following each experiment the sample was replaced. The RBS
measurements were performed ex situ by Charles Evans and Associ-
ates following sample removal and rinsing.
All the experiments reported here were performed on H-termi-
nated Si(111), which was prepared by immersing a cleaned Si(111)
sample into 0.50 M HF, decanting the solution, and rinsing the sur-
face. The perfectly H-terminated Si(111) surface which is formed is
known to be stable in air for periods of several hours.14-18 This sur-
face is probably the most ideal solid-liquid interface which can be
prepared, exhibiting the lowest surface recombination velocity ever
observed for a semiconductor.19 Defect densities as low as 0.004%
have been reported.20 Preparation of H–Si(111) allows the deposi-
tion kinetics to be studied without the complication of an induction
period during which HF dissolves the native oxide.
Following preparation of the H-terminated surface, Au was
deposited from aqueous solutions containing various concentrations
of KAu(CN)2 and HF. These solutions were prepared by mixing con-
centrated HF with highly purified H2O, then dissolving KAu(CN)2.
They were stable for the duration of the experiments but would de-
posit a yellowish material, probably AuCN, on the container walls if
left overnight. Appreciable formation of HCN was not observed. The
pH of these solutions, as determined from equilibrium calculations
described later, ranged from about 0.4 to 1.9.
damental and a practical viewpoint. Interesting fundamental ques-
tions arise due to the complex interactions between Si surfaces and
noble metals, including rapid interdiffusion and metastable silicide
formation.1 For example, deposition of macroscopic Au films onto
Si substrates yields a complex layered structure in the following
sequence: Si substrate, a diffuse 15-20 Å thick alloy mixture of Au
and Si, a region of pure Au, and a 1-2 monolayer thick metal-rich
Au–Si alloy on top.1 Many aspects of this system are not well under-
stood, including the complicated reaction dynamics at elevated tem-
peratures, which remain controversial.
Deposition of noble metals is also important in the electronics
industry in two different respects, growth of conducting films and un-
wanted metallic contamination. Future generations of microelectron-
ic devices are expected to have Cu interconnects due to its lower re-
sistivity and better electromigration resistance with respect to Al.2-4
Electro- and electroless deposition of Cu are already in widespread
use in printed circuit manufacture, particularly for filling high-aspect-
ratio through-holes.5,6 Electro- and electroless Au deposition have a
number of applications, including fabrication of the absorber layer in
X-ray masks,7 conductive coatings,8 and electrical contacts.9-12
Noble metal deposition onto Si is also important during aqueous
processing of Si wafers during microelectronics manufacture. Since
bare Si is a strong reducing agent, metals more noble than hydrogen
ion can be reduced and deposited from a HF etchant following dis-
solution of the SiOx protective layer.13 This type of electroless
process is usually described as galvanic displacement to distinguish
it from autocatalytic deposition. This report describes fundamental
studies of Au deposition and cluster growth from aqueous HF onto
Si(111) using surface second harmonic generation (SHG) and
Rutherford backscattering (RBS). The SH signal is strongly en-
hanced by resonance with the surface plasmon of Au nanoclusters.
This should be generally applicable to studies of noble metal nucle-
ation and cluster growth on insulating and semiconducting surfaces
as surface plasmon–enhanced SHG.
Experimental
SHG experiments were performed during Au deposition from
aqueous HF using the experimental setup shown in Fig. 1. A Con-
tinuum Powerlite 6050 Nd:YAG laser operating at 1064 nm, 50 Hz,
and 8 ns/pulse was filtered to remove light at harmonic frequencies
and reflected from the Si surface at an incident angle of approxi-
mately 26Њ. After filtering out the fundamental and third harmonic
frequencies, the SH signal at 532 nm was detected by a gated, inten-
Figure 1. Experimental schematic for second harmonic generation (SHG)
studies.
* Electrochemical Society Active Member.
E-mail: isuni@clarkson.edu
z
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Journal of The Electrochemical Society, 146 (2) 570-573 (1999)
571
S0013-4651(98)05-003-3 CCC: $7.00 © The Electrochemical Society, Inc.
Results and Discussion
Deposition of Au from aqueous HF onto Si may be described by
the following half-reactions
face plasmon-enhanced SHG from Na nanoclusters grown on dielec-
tric substrates in vacuum has been studied as a function of cluster
size, and maxima similar to those shown in Fig. 2 have been seen for
100-250 nm diam clusters.36-38 Atomic force microscopy (AFM)
studies indicate that the maxima in Fig. 2 arise from Au nanoclusters
with diameters in the range 70-110 nm, 39 which is the range in which
the absorbance of Au nanoclusters reaches a maximum in aqueous
solution.40,41 Previous AFM studies have observed the dependence of
Au cluster growth rate on HF concentration.42 The decline of the
SHG signals shown in Fig. 2 back to their original levels was previ-
ously believed to arise from interdiffusion between Au and Si.43 We
now expect that this decline arises primarily from the onset of radia-
tion damping, which arises due to nonuniformity of the electric field
inside larger clusters, but we still expect that interdiffusion may also
play a role. The variation in the intensities of the maxima in Fig. 2
arise from random noise in the SHG signal, from differences in
nucleation density, and from differences in the cluster aspect ratios.
Au(CN)Ϫ2 ϩ eϪ r Au ϩ 2CNϪ
Si ϩ 6FϪ r SiF26Ϫ ϩ 4eϪ
[1]
[2]
with standard reduction potentials of approximately Ϫ0.595 and
Ϫ1.37 V NHE, respectively.21 Reaction 2 is often written with HF
rather than FϪ as the reactant. As discussed later, the detailed mech-
anisms of reactions 1 and 2 are addressed by the present study.
Surface plasmon-enhanced SHG.— Figure 2 shows the sharp in-
crease, which depends on HF concentration, of the SH signal during
deposition of Au onto H-Si(111) from solutions containing 10Ϫ4
M
KAu(CN)2. A temporal maximum is observed for each HF concen-
tration shown in Fig. 2, although this is not always apparent due to
the scale chosen. Although the time at which each maximum occurs
is reproducible, the intensity of each maximum may vary by as much
as two times between repeat measurements. These maxima arise due
to resonance between the surface plasmon of Au nanoclusters and the
second harmonic photon at 532 nm. Neglecting surface scattering,
dynamic depolarization, and radiation damping, the electric field (Ei)
inside spheroidal clusters can be related to the applied field (E0)
according to22
Mechanism of Au reduction.—Both SHG measurements andAFM
studies seem to indicate that Au deposition is kinetically limited by
surface reaction. However, a more complex picture is revealed by sur-
face coverage measurements by RBS near the SHG maxima for
0.500 M and 2.50 M HF.43 The measured surface coverages yield
average fluxes of 1.5 ϫ 1013 and 1.2 ϫ 1013 atoms/cm2 for 0.500 and
2.50 M HF, respectively. These values can be compared to the diffu-
sion flux expected from
1
Ei ϭ
E0
[3]
C0
1 ϩ (⑀ Ϫ 1) A
[4]
J ϭ D
␦
where A is a constant describing the spheroid eccentricity. A classi-
cal resonance is observed when the denominator vanishes, at which
point the surface dipolar charge distribution can absorb maximum
electromagnetic radiation, behaving like a resonantly driven cavity.
The presence of noble metal hemispheres and hemispheroids and the
accompanying strong surface plasmon resonance can produce enor-
mous signal enhancement in surface spectroscopies such as surface-
enhanced Raman spectroscopy (SERS) and SHG.23-27
The surface plasmon resonance occurs in the range 510-540 nm
for Au colloids of 2.5-10 nm diam, while the resonance is red-shift-
ed for larger Au colloids.28-30 The surface plasmon resonance is dif-
ficult to model, depending upon a variety of factors in a complex
manner. The peak position and intensity can depend on the cluster
size and shape, and the resonance can be as broad as 100 nm.30 In
addition, the surface plasmon resonance of embedded Au clusters
shifts according to the dielectric function of the surrounding medi-
um,31,32 so the Si surface will affect the spectrum in a manner which
Assuming a diffusion coefficient (D) of 10Ϫ5 cm2/s and a diffusion
layer thickness (␦) of 350 ␮m yields a diffusion flux (J) of 1.7 ϫ
1013 atoms/cm2 for a Au(I) concentration (C0) of 10Ϫ4 M. Given the
uncertainty in the assumed values, this agrees well with the observed
fluxes. The agreement between the observed fluxes and those ex-
pected for diffusion-limited deposition does not alone establish kinet-
ic limitation by diffusion. However, coupled with the strong depen-
dence of the Au cluster growth rate on HF concentration, these results
strongly suggest that Au deposition is kinetically limited by diffusion.
This apparent contradiction can be reconciled by proposing that
Au deposition is kinetically limited by diffusion of Au(I) species but
that growth of Au nanoclusters is kinetically limited by a surface
reduction involving fluoride-containing species. This is consistent
with initial deposition occurring as the adsorbed intermediate AuCN,
as postulated for electrodeposition of Au onto a variety of metal sub-
strates.44-48 The reduction of Au(CN)Ϫ2 has been proposed to occur
by two parallel processes, direct reduction by reaction 1 at high over-
potentials and a two-step reduction process at low potentials44-48
is difficult to predict. Au nanoclusters lack clear visible absorption
32-35
features below a diameter of approximately 2 nm,
so clusters
must reach this size before resonant SH enhancement can occur. Sur-
Au(CN)Ϫ2 r AuCN(ads) ϩ CNϪ
AuCN(ads) ϩ eϪ r Au ϩ CNϪ
[5]
[6]
Surprisingly, SERS has only identified adsorbed Au(CN)Ϫ2 , 49,50 al-
though evidence for incorporation of nanocrystalline AuCN into Au
deposits has been obtained by transmission electron microscopy
(TEM) and Auger electron spectroscopy (AES).51 An alternative ex-
planation to the two-step reduction process described would be ini-
tial deposition into a partially discharged species, followed by full
reduction and incorporation into a Au nanocluster.
Mechanism of Si oxidation.—Kinetic analysis can be performed
on the data in Fig. 2 by noting that rapid attainment of a SH maxi-
mum corresponds to rapid growth of Au nanoclusters. The goal is to
identify the mechanistically important fluoride-containing species
involved in reaction 2. The aqueous chemistry of HF has been thor-
oughly studied and is complicated by the existence of polymer-type
solution phase reactions involving HF.52,53 In the present study, only
the following two equilibria are considered, since these are known to
predominate in moderately concentrated aqueous HF
Figure 2. Second harmonic signal as a function of time during Au deposition
from 10Ϫ4 M KAu(CN)2 at various nominal HF concentrations.
HF r Hϩ ϩ FϪ
[7]
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572
Journal of The Electrochemical Society, 146 (2) 570-573 (1999)
S0013-4651(98)05-003-3 CCC: $7.00 © The Electrochemical Society, Inc.
entially at steps,55 these are the likely sites of attack by HF, consis-
HF ϩ FϪ r HFϪ2
[8]
tent with AFM studies showing preferential deposition at defects.42
Figure 5, which shows the reaction rate as a function of ionic
strength, was constructed as a test to determine whether colloidal
solution phase phenomena might play an important kinetic role.
Once again, the cluster growth rate does not vary monotonically with
ionic strength. The results reported here are qualitatively unaffected
by the particular choice of equilibrium constants.
Disagreement exists in the literature regarding the equilibrium con-
stants for these two reactions, in part due to further polymerization
at high concentrations of HF.52 As the nominal HF concentration is
increased, the concentrations of Hϩ, FϪ, HF, and HF2Ϫ all increase,
so the effects of different species are difficult to isolate. For this rea-
son these two equilibria were manipulated by performing experi-
ments in which HCl and KF were added to aqueous HF. One exper-
iment was performed with 1.71 M HF and 0.780 M HCl and a sec-
ond with 0.249 M HF and 0.0516 M KF. The results of both are
included in Fig. 2.
The data in Fig. 2 are analyzed by taking the time at each maxi-
mum to be proportional to the inverse reaction rate. The equilibrium
constants for reactions 7 and 8 are taken as 1.30 ϫ 10Ϫ3 and 9.62,
respectively,54 and the concentrations of HF, HFϪ2 , FϪ, and Hϩ in
solution are calculated. Logarithmic plots for the reaction rate as a
function of the calculated values of [HF] and [HFϪ2 ] are shown in
Fig. 3 and 4. Similar plots using calculated values of [Hϩ] and [FϪ]
are nonmonotonic. Comparing Fig. 3 and 4, the two additional meas-
urements with HCl and KF indicate that HF and not HFϪ2 is involved
in the rate-limiting step for Au cluster growth and simultaneous Si
dissolution. The slope of the plot in Fig. 3 is 0.54, suggesting that the
apparent reaction order with respect to HF is 1/2. Since infrared
spectroscopic results indicate that fluorine atoms are found prefer-
In order to determine the reaction order with respect to Au(CN)Ϫ2 ,
further SH observations were made during deposition from 0.500 M
aqueous HF containing 10Ϫ4, 3 ϫ 10Ϫ5, and 10Ϫ5 M KAu(CN)2.
The temporal maximum was found to be only weakly dependent on
the concentration of Au species. A logarithmic plot of the reaction
rate as a function of Au(CN)Ϫ2 concentration yields a reaction order
with respect to Au(CN)Ϫ2 of approximately 0.10. This suggests that
during cluster growth there is an excess of Au(I) species on the sur-
face. Future studies may investigate the kinetics with respect to
Au(CN)Ϫ2 by performing experiments at lower concentrations.
Comparison to other results.—The conclusion that HF and not
HFϪ2 is mechanistically active should be compared to previous stud-
ies. At first this might not seem surprising, since reaction 2 is often
written with HF as the reactant. However, this conclusion is in dis-
agreement with recent results for Si dissolution into HF, which con-
clude on the basis of solution phase equilibria that HFϪ2 is the active
species.56,57 The current system is chemically similar to electro-
chemical dissolution of Si in aqueous HF to form porous Si, although
the chemical reaction mechanism involved in porous Si formation
has been less thoroughly studied. Several competing mechanisms
have been proposed for porous Si formation, although two are most
commonly cited.58 The first proposes complete surface oxidation to
Si4ϩ, which is believed to desorb as SiF4 and react in solution to
form SiF26Ϫ. However, the chemical nature of the attacking fluoride
species in this mechanism is unclear. The second mechanism pro-
poses partial oxidation and dissolution of Si(II) as the transient
species SiF2, which immediately disproportionates to form SiF62Ϫ
and deposit Si on the pore walls.58 Although we are aware of no
direct evidence for the existence of SiF2 in solution, its importance in
at least a side reaction during porous Si formation is indirectly sup-
ported by the observation of amorphous Si deposited on pore walls.58
Again, the nature of the fluoride species involved in this mechanism
is unclear. Strong experimental support exists for neither of the
mechanisms discussed.
A more thoroughly studied, related system is dissolution of SiO2
in aqueous HF, which has attracted attention in part to enable pre-
diction of the time at which dissolution is complete. This may allow
prevention of metal deposition onto the bare Si surface during aque-
Figure 3. Reaction rate in 10Ϫ4 M KAu(CN)2 as a function of HF concen-
tration determined from equilibrium calculations.
Figure 4. Reaction rate in 10Ϫ4 M KAu(CN)2 as a function of HF2Ϫ concen-
Figure 5. Reaction rate in 10Ϫ4 M KAu(CN)2 as a function of calculated
tration determined from equilibrium calculations.
ionic strength.
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Journal of The Electrochemical Society, 146 (2) 570-573 (1999)
573
S0013-4651(98)05-003-3 CCC: $7.00 © The Electrochemical Society, Inc.
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Acknowledgments
This research has been supported by NSF grant CTS-9527497.
Purchase of the Nd:YAG laser was supported by the Center for
Advanced Materials Processing (CAMP) at Clarkson University.
Thanks to Charles Evans and Associates for the RBS analyses.
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