Electrodeposition of Ni from low-temperature sulfamate electrolytes

Article abstract of DOI:10.1149/1.2181447

The structure and properties of Ni deposited from sulfamate electrolytes is reported. Particulate filtering of the electrolyte has significant consequences with respect to the microstructure and resulting mechanical properties. Effects were most pronounced at low current densities, and gradually disappeared as current density increased. At low current density (3 mA cm2), deposits plated from a filtered electrolyte were fine-grained and exhibited 〈011〉 texture orientation, characteristic of inhibited growth. The strengths of these deposits ranged from 700 MPa to 1 GPa; increasing with increasing deposition temperature from 28 to 50°C. Low-current-density deposits from unfiltered electrolyte exhibited a temperature dependent instability in grain morphology and texture. At low temperatures (≤32°C) deposits were coarse grained and predominantly 〈001〉, while at 50°C, deposits were fine grained and 〈011〉. At intermediate temperatures, the deposits grew initially in the uninhibited, coarse grain 〈001〉 mode but then transitioned to the fine grain, 〈011〉 inhibited growth mode. At high current density (15 mA cm2), the structure and properties of electrodeposits were unaffected by particle filtering. Irrespective of deposition temperature or filtering condition, deposits had the grain morphology and crystallography characteristics of uninhibited growth - namely, coarse, columnar grains with preferred 〈001〉 texture. The measured strength of these deposits were 350- 400 MPa.

Full text of DOI:10.1149/1.2181447

Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
C325
0013-4651/2006/153͑5͒/C325/7/$20.00 © The Electrochemical Society
Electrodeposition of Ni from Low-Temperature Sulfamate
Electrolytes
II. Properties and Structure of Electrodeposits
a
c
a
S. H. Goods,a,z J. J. Kelly,b, A. A. Talin, J. R. Michael, and R. M. Watson
*
aSandia National Laboratories, Livermore, California 94551, USA
bIBM, T.J. Watson Research Center, Yorktown Heights, New York 10598, USA
cSandia National Laboratories, Albuquerque, New Mexico 87185, USA
The structure and properties of Ni deposited from sulfamate electrolytes is reported. Particulate filtering of the electrolyte has
significant consequences with respect to the microstructure and resulting mechanical properties. Effects were most pronounced at
low current densities, and gradually disappeared as current density increased. At low current density ͑3 mA/cm2͒, deposits plated
from a filtered electrolyte were fine-grained and exhibited ͗011͘ texture orientation, characteristic of “inhibited” growth. The
strengths of these deposits ranged from 700 MPa to 1 GPa; increasing with increasing deposition temperature from 28 to 50°C.
Low-current-density deposits from unfiltered electrolyte exhibited a temperature dependent instability in grain morphology and
texture. At low temperatures ͑ഛ32°C͒ deposits were coarse grained and predominantly ͗001͘, while at 50°C, deposits were fine
grained and ͗011͘. At intermediate temperatures, the deposits grew initially in the uninhibited, coarse grain ͗001͘ mode but then
transitioned to the fine grain, ͗011͘ inhibited growth mode. At high current density ͑15 mA/cm2͒, the structure and properties of
electrodeposits were unaffected by particle filtering. Irrespective of deposition temperature or filtering condition, deposits had the
grain morphology and crystallography characteristics of uninhibited growth – namely, coarse, columnar grains with preferred
͗001͘ texture. The measured strength of these deposits were 350–400 MPa.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2181447͔ All rights reserved.
Manuscript submitted October 6, 2005; revised manuscript received January 6, 2006. Available electronically March 27, 2006.
Electrodeposition of nickel from sulfamate electrolytes has found
wide use in many commercial and industrial applications,1-8 and the
rational for its use was described in Part I of this work. More re-
cently, electrodeposited nickel from sulfamate ͑hereafter referred to
as ED Ni-sulfamate͒ has become a favored material for the elec-
trodeposition of microsystem components using the LIGA process
͑an acronym for the German words for: lithography, electroplating,
and molding͒. While the LIGA process was introduced in Part I,
since the test specimens studied in this second part were fabricated
using the LIGA process, we describe it here in greater detail.
Briefly, a thick ͑100s to 1000s of micrometers͒ polymer photo-
resist blank, bonded to a metallized substrate ͑viz., the plating base͒,
is lithographically patterned using a synchrotron X-ray source and
developed to yield a mold consisting of deep prismatic cavities hav-
ing feature sizes measuring in the 10s to 100s of micrometers. It is
into these cavities that an elemental metal or alloy is electrodepos-
ited. As these microcomponents may perform mechanical functions,
their microstructure and mechanical properties are of interest, as
well as the deposition conditions that may affect them. As stated in
Part I, while the process–structure–property relationships are gener-
ally known for ED Ni-sulfamate under typical operating
conditions,9,10 the successful integration of the electrodeposition
step with other steps in the LIGA process may necessitate deposition
conditions that differ from standard plating practice.
In particular, it may be necessary to operate plating cells at tem-
peratures well below those recommended by common practice in
order to reduce thermal distortions of the resist material that define
the lateral dimensions of a structure.11 Another aspect that must be
considered for the successful integration of the electrodeposition
step into the LIGA process is the deposition current density. Trans-
port limitations, particularly important in narrow, high-aspect-ratio
features where circulation of the bulk electrolyte is impeded, are
typically minimized by reducing deposition rates. This is most
straightforwardly done by depositing metal at low current densities.
In the first part of this work, we reported the electrochemistry and
process dependence of the intrinsic film stresses of ED Ni-sulfamate
over a range of current densities and bath temperatures. In this sec-
ond part, the effects of these conditions on the mechanical properties
and microstructure of net shape material plated using the LIGA
process are presented.
Experimental
Deposition.— The composition of the Ni-sulfamate electrolyte is
the same as that presented in Part I: 1.35 M Ni͑SO3NH2͒2, 30 g/L
boric acid, and 0.2 g/L sodium dodecyl sulfate ͑as a wetting agent
that reduces pitting and otherwise has no effect on the Ni͒.12 The
deposition parameters for net shape fabrication of test specimens
and structures are within the same range of those used for film stress
measurements in Part I. In this work, the influence of deposition
temperature was examined at 28, 32 ͑the minimum manufacturer
recommended deposition temperature͒, 40, and 50°C. A pH of
3.5 0.1 was used for all deposition runs; the typically recom-
mended range is from 3.5 to 4.5. Details related to the preparation of
the Ni anodes and the as-received sulfamate electrolyte are de-
scribed in Part I as well. In particular, we note here that new baths
that were purified by a carbon treatment before their initial use and
wound, 5-␮m polypropylene fiber filters ͑Floking͒ were used as the
particle and debris collecting media.
Mechanical testing.— Net shape tensile specimens were fabri-
cated using the LIGA process as described above. Tensile specimens
had a reduced gauge section measuring 0.76 ϫ 0.25 mm ͑width ϫ
thickness͒ and an overall gauge length of 6.2 mm. After final pla-
narization and release from the substrate, the mechanical test speci-
mens were tested in uniaxial tension in an Instron model 5848 mi-
crotester. Load was measured using
a 1-kN load cell and
displacement was measured using a noncontacting laser extensom-
eter ͑EIR model LE-01͒ having approximately 1-␮m resolution.
Tests were performed at room temperature at a constant extension
rate of 5 ϫ 10−4 mm/s.
Microscopy.— A FEI DB235 dual beam focused ion beam ͑FIB͒
scanning electron microscope ͑SEM͒ was used to characterize the
microstructure of as-deposited samples employing the ion beam-
induced channeling contrast imaging ͑ICI͒ capabilities of the FIB
instrument.13 The depth of penetration of the ions is related to the
crystallographic orientation of individual grains. As such, certain
crystallographic orientations result in stronger ion channeling and
therefore deeper penetration of the ions into the sample. Deeper ion
*
Electrochemical Society Active Member.
z E-mail: shgoods@sandia.gov
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Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
Figure 2. Tensile behavior of electrodeposited nickel from sulfamate elec-
trolytes at 50°C. ͑a͒ High strength is observed in low current density depos-
its independent of filtering condition. ͑b͒ Much lower strength is observed in
high current density deposits independent of filtering condition.
Figure 1. Tensile behavior of electrodeposited nickel from sulfamate elec-
trolytes at 28°C. ͑a͒ High strength is observed in low current density depos-
its fabricated from particle filtered electrolyte. ͑b͒ Filtration has little effect
on mechanical properties when deposition occurs at higher current density.
Results and Discussion
penetration, in turn, produces fewer collectable secondary electrons,
so strongly channeling orientations appear dark as compared to
more weakly channeling orientations. The amount of channeling is
quite sensitive to small variations ͑1–2°͒ in orientation so that not
only can grains of different orientations be imaged, but small mis-
orientations within a grain can be visualized as well.
Electron backscatter diffraction ͑EBSD͒, performed on a Zeiss
Supra 55VP field emission gun SEM using an HKL Technology
camera and Channel 5 software, was used to map the through-
thickness grain orientation ͑texture͒ from metallographically pre-
pared cross sections of the as-deposited test specimens.
Filtering effects on mechanical properties.— In Part I of this
work, it was shown that the most pronounced effect of filtering on
blanket film stress occurred at low current densities and low plating
temperature. Not surprisingly, the as-plated mechanical properties
parallel this effect. Figure 1a and b illustrates the typical tensile
behavior for electrodeposited test specimens, all plated at 28°C and
at the current densities and filtering conditions indicated. Stress-
strain curves for the material deposited at 3 mA/cm2 are shown in
Fig. 1a. For each filtering condition, the tensile behavior is quite
reproducible. Material deposited from an unfiltered bath exhibit
yield and ultimate tensile strengths of 350 and 500 MPa, respec-
tively, similar to the values reported for nickel deposits from a sulfa-
mate bath in the literature.16,17 It is clear from Fig. 1a that particle
filtering has a significant effect on both the yield and ultimate tensile
strength ͑UTS͒, with each increasing to 730 and 950 MPa, respec-
tively.
Conversely, particle filtering has little effect on the mechanical
properties of the electrodeposits when plated at a current density of
15 mA/cm2 as illustrated in Fig. 1b, where tensile curves are pre-
sented for each filtering condition. It is evident that, at this higher
current density, filtering induces only a small increase in the yield
and UTS: 360 and 475 MPa ͑yield strength͒ for the material depos-
ited from an unfiltered bath vs 410 and 540 ͑UTS͒ MPa for the
X-ray characterization.— Specimen texture was determined us-
ing a Philips X’Pert MRD X-ray diffractometer with a Cu anode
operated at 45 kV and 40 mA. Pole figures corresponding to ͑111͒,
͑200͒, and ͑220͒ reflections were collected for azimuthal rotation of
0 ഛ ␾ ഛ 360° and tilt angle of 0 ഛ ␺ ഛ 90° in 5° increments. The
experimental pole figures were corrected for absorption and defo-
cusing using a Ni powder specimen.14 Based on the experimental
pole figures ͑data for only ␺ ഛ 80° was used͒, orientation distribu-
tion functions ͑ODFs͒ were computed using the BEARTEX
program;15 the ODFs were in turn used for calculation of inverse
pole figures.
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Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
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Figure 3. Summary of process parameter effects on strength ͑average of 3–4
tensile tests͒ of electrodeposited nickel from sulfamate electrolytes. High-
current-density deposits always exhibit low strength. Low-current-density
deposits from filtered electrolyte always exhibit high strength. Low-current-
density deposits from unfiltered electrolyte exhibit monotonically increasing
strength as deposition temperature increases.
Figure 4. Comparison of intrinsic film strength ͑FS͒ and electrodeposit yield
strength ͑YS͒ for ED Ni–sulfamate deposited at 50°C. Film stress and
strength of electrodeposits do not correlate.
these 15-mA/cm2 specimens, yield strength is universally low and
ranges from about 350 MPa to no higher than 410 MPa. At the low
current density, however, specimens plating in unfiltered cells show
a progressive increase in strength as the plating temperature in-
creases. For these specimens, yield strength ranged from 350 MPa at
28°C to 950 MPa at 50°C. Lastly, the specimens deposited at the
low current density from the filtered cells show universally high
strength, with some indication that the strength increases with in-
creasing deposition temperature. In this instance, the yield strength
increased from 750 MPa ͑at 28°C͒ to 900 MPa ͑at 50°C͒.
Figure 4 compares the intrinsic film stress results from Part I of
this work to the current yield strength measurements for ED Ni–
sulfamate deposited at 15 mA/cm2. The previous film stress mea-
surements revealed relatively high stresses ͑70–75 MPa͒ for films
deposited at the lowest temperature. Figure 4 shows that these film
stresses decrease monotonically with increasing deposition tempera-
ture for both the filtered and unfiltered electrolyte, so that at a depo-
sition temperature of 50°C the film stresses are quite low. In con-
trast, the yield strengths of the deposits are low for all deposition
temperatures and filtering conditions. It is evident that there is no
correlation between the intrinsic film stress of these deposits and the
resultant mechanical properties. Similarly, for other plating condi-
tions ͑temperature and filtering condition͒, there is no correlation
between the deposit film stress and the strength of the deposited
material.
material plated from a filtered bath. It is also clear from comparing
Fig. 1a and b that current density plays no role in determining me-
chanical properties in unfiltered baths, both baths yielding material
with virtually the same mechanical properties.
Figure 2a and b illustrates the typical tensile behavior for speci-
mens deposited at the highest temperature, 50°C. For specimens
deposited at the lowest current density, 3 mA/cm2, the presence or
absence of filter has no effect on the mechanical properties. Indeed,
the tensile curve for filtered and unfiltered specimens are essentially
indistinguishable from each other. However, these deposits have ex-
tremely high strength ͑note the scale change on the strength axis͒, as
high or higher than that reported in previous literature.9,16,17 For
specimens deposited at 15 mA/cm2, filtering ͑or its absence͒ again
has no measurable effect on properties, but in this instance strength
was low and comparable to that of the specimens deposited at 28°C
and at the same current density, 15 mA/cm2.
Figure 3 and Table I summarize the strength measurements for
specimens deposited at all temperatures, current densities, and bath
filtering conditions. Both indicated average strength values ͑yield
and UTS͒ from 3–4 individual tensile tests. It is evident from Fig. 3
that the mechanical properties of ED Ni–sulfamate are insensitive to
the presence or absence of particle filtration when plated at the high
current density over the entire temperature range examined. For
Filtering effects on microstructure.— Figure 5a shows an ICI of
a cross section for a specimen plated at 3 mA/cm2 from an unfil-
tered electrolyte where the plating direction is from the bottom of
the image to the top. The coarse, columnar microstructure is typical
of ED Ni–sulfamate plated under conditions where deposition is
said to be “uninhibited.”18 We note that the microstructure revealed
in this figure is characteristic of the electrodeposits plated from both
15 mA/cm2 plating cells as well, and for the sake of brevity those
micrographs are not shown. The very bottom of the image includes
the initial deposition of nickel upon the metallized LIGA mold sub-
strate. The grain size is initially extremely fine over the first mi-
crometer or two, but evolves rapidly to the much coarser grain struc-
ture that is characteristic of the bulk deposit. This initiation zone
where grain size and structure evolve rapidly is commonly observed
in electrodeposits.5 Figure 5b shows an EBSD map of this deposit in
Table I. Deposition parameter effects on average mechanical
properties.
Filtered
Unfiltered
Current
density
Temperature
͑C͒
͑mA/cm2͒
YS
UTS
YS
UTS
28
32
3
15
3
15
3
750
390
740
380
860
900
375
925
550
980
350
360
450
350
505
950
410
490
470
660
490
720
1350
540
500
40
50
1205
1340
525
3
15
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Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
Figure 5. ͑Color͒ ͑a͒ ICI micrograph of electrodeposit cross section from
specimen deposited at 28°C, 3 mA/cm2 in unfiltered electrolyte. Deposition
direction is vertical and a finer grain initiation zone is seen at the bottom of
the micrograph. ͑b͒ EBSD map indicates that the microstructure is predomi-
nantly ͗001͘. The coarse-grain ͗001͘-textured microstructure is indicative of
uninhibited growth. Specimens from the 15-mA/cm2 deposits at this tem-
perature exhibited similar grain size and texture.
Figure 6. ͑Color͒ ͑a͒ ICI micrograph of electrodeposit cross section from
specimen deposited at 28°C, 3 mA/cm2 in filtered electrolyte. Deposition
direction is vertical and the initiation zone of the deposit is seen at the
bottom of the micrograph. ͑b͒ EBSD map indicates that the microstructure is
predominantly ͗011͘. The fine grain size and ͗001͘-textured microstructure
is indicative of inhibited growth.
inset legend reveals that the texture is predominantly ͗001͘, which is
also indicative of “uninhibited” growth for ED Ni. Additional tex-
ture mapping indicated that all deposits had a fiber texture, typical of
thick EDs.
Figure 6a shows a similar ICI image of the cross section of a
specimen plated at 3 mA/cm2 in a filtered electrolyte. We note that
the initiation zone is still apparent at the bottom of the micrograph
but it is evident that the columnar grain size in the bulk is much finer
than in the previous instance. This finer grain size is responsible for
the considerably higher yield strength19 of the deposit as shown in
cross section ͑except for the initiation zone͒. The false color inset is
an inverse pole figure ͑IPF͒ schematic and indicates that the deposit
exhibits a high degree of texture ͑here texture is referenced to the
growth direction normal, that is, the “out-of-plane” texture͒.d The
dThe EBSD image in Fig. 5 and subsequent figures are not purported to be of the precise
region shown in the ICI images, as both images are obtained from different microscopes
requiring intermediate surface preparation. Rather, the EBSD images are taken from the
same surfaces as represented by the companion grayscale ICI image.
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Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
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Figure 7. ͑Color͒ Planview image of deposit shown in Fig. 5. XRD-
generated IPF inset indicates that the ͗001͘ texture is 17 times random.
Figure 9. ICI micrograph of electrodeposit cross section from specimen
deposited at 40°C, 3 mA/cm2 in unfiltered electrolyte. The micrograph in the
vertical direction spans the entire thickness of the deposit. The deposit is
initially coarse-grained but abruptly transitions to a fine-grain size. The win-
dowed region is shown at higher magnification in Fig. 10.
Fig. 1 and 3 and is characteristic of inhibited film growth. Inhibited
growth in electrodeposits results not only in finer grain size material,
but also in a change in the characteristic texture from ͗001͘ to
͗011 ͘. Indeed, the EBSD-derived map shown in Fig. 6b confirms
this change in out-of-plane texture to ͗011͘.
Figure 7 shows a planview orientation of the same deposit shown
in Fig. 5a. Line intercept analysis performed on this SEM image
indicated that the average grain size for this electrodeposit was ap-
proximately 1.0 ␮m. The inset in Fig. 7 is an X-ray diffraction
͑XRD͒-generated IPF that confirms that the preferred texture is
͗001͘. Since XRD samples a larger planar area of the deposit than is
the case for EBSD, the degree of preferential texture can be assessed
more quantitatively. For this deposit, the texture is ϳ17 times ran-
dom, indicating a high degree of texture. Figure 8 shows a similar
planview image for a specimen plated under conditions as for the
specimen shown in Fig. 6a. Line intercept analysis performed on
this SEM image indicates that the average grain size for this elec-
trodeposit is nearly five-fold smaller than for the electrodeposit
shown in the previous figure, ϳ200 nm. The IPF inset in Fig. 8
confirms the EBSD texture analysis as being ͗011͘, and the degree
of texturing in this instance was 5 times random. The IPF also gives
an indication there is a small amount of texturing associated with
higher index directions.
The microstructure of the electrodeposits plated at 50°C and
15 mA/cm2 in both filtered and unfiltered electrolyte exhibit the
same characteristics as that shown in Fig. 5 and 7, that is, coarse-
grained and ͗001͘-textured, characteristic of an uninhibited growth
mode for Ni. The only difference observed at 50°C ͑derived from
the filtering condition͒ is a greater degree of preferred orientation for
deposit plated from unfiltered electrolyte, 50 times random, vs 16
times random for the deposit from filtered electrolyte. The coarse
grain structure of both of these deposits is reflected in their low yield
strength, as shown in Fig. 3. The 50°C deposits plated at 3 mA/cm2
have very high yield strengths and not surprisingly exhibit a fine
grain size and ͗011͘ texture very similar to that shown in Fig. 6 and
8. XRD-derived texture revealed that, for the filtered deposit, the
͗011͘ texture was 7 times random while for the unfiltered deposit it
was 15 times random.
Figure 3 shows that the electrodeposit plated at 40°C and at
3 mA/cm2 from an unfiltered electrolyte exhibits a yield strength
that is intermediate between that of the high current density deposits
and that of the low current density deposit from the filtered electro-
lyte. It is reasonable, therefore, to imagine that the microstructure of
the deposit would consist of a mixture of the both the coarse-
grained, ͗001͘-textured material and the fine-grained, ͗011͘-textured
material. Figure 9 shows that this is indeed the case. Here, as in
earlier figures, the growth direction is from the bottom to the top of
the image. Rather than a uniform mixture of fine- and coarse-
grained material, leading to the increased strength, Fig. 9 reveals the
deposit to be duplex in nature, consisting of an initially coarse-grain
columnar structure ͑disregarding the initiation zone at the very bot-
tom of the micrograph͒ which abruptly transitions to a fine-grained
structure at a point very near the mid-thickness of the deposit. Fig-
ure 10a shows a higher magnification image of the region outlined
in Fig. 9. The abrupt nature of the transition from coarse grain to
fine grain is evident. Figure 10b shows an EBSD map of this tran-
sition region and reveals that, in concert with the change in grain
size, the characteristic texture changes from the ͗001͘ free-growth
Figure 8. ͑Color͒ Planview image of deposit shown in Fig. 6. XRD-
generated IPF inset indicates that the ͗011͘ texture is approximately 5 times
random.
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Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
indicate that for the 3 mA/cm2 current density, the switch in growth
modes occurs in approximately 2 h out of a total plating time of
roughly 3 days. Sudden changes in film structure, sometimes peri-
odic, that result in nonuniform films or laminate-like films have
been previously observed in cross-sectional micrographs of elec-
trodeposited Ni.9 The behavior in Fig. 10 demonstrates that, under
certain conditions, the microstructure of electroplated Ni may
quickly change during the deposition, resulting in a nonuniform
through-thickness microstructure. This is presumably due to a
change in the nature or type of adsorbed species at the solid–liquid
interface, such as hydrogen and hydroxides ͑see below͒, electrolyte
aging species, or species from other chemical reactions occurring in
the bulk electrolyte, all of which may result in a change in the film
nucleation and growth.
It is clear that in all instances, coarse-grained deposits are asso-
ciated with the ͗001͘ texture indicative of free growth while fine-
grained deposits are associated with the characteristic ͗011͘
inhibited-growth mode. Rationalizing all of the various observations
presented above with respect to growth mode, deposition param-
eters, and electrolyte condition is difficult. Inhibition during elec-
trodeposition is usually attributed to the presence of an adsorbate,
suppressing growth of a certain orientation and promoting the
growth of other orientations.18,20,21 The degree to which such an
adsorbate is bound to the growing surface determines, at least to
some extent, the temperature and current density regime in which
inhibited growth is observed. The insensitivity of the 15 mA/cm2
deposits with respect to plating conditions can be rationalized in a
relatively straightforward manner in that at this higher current den-
sity, any inhibiting species that may be present ͑hydrogen or ad-
sorbed boric acid species, for example͒ is simply overwhelmed by
the high deposition rate. Deposition at this current density, therefore,
always occurs in the free-growth mode, regardless of the other plat-
ing parameters investigated here ͑temperature and electrolyte filtra-
tion͒. Similarly, for the electrodeposits plated at 3 mA/cm2 from
filtered electrolyte, deposition is sufficiently slow so as to allow
even a weakly bound adsorbate to remain effective in inhibiting
growth. Amblard et al. investigated texture evolution in Ni–Watts
electrodeposits plated at 50°C and observed a similar transition
from fine-grain, ͗011͘-oriented ͑at pH Ͻ 2.5͒ and ͗112͘-oriented
͑pH Ͼ 2.5͒ growth to coarse-grain, ͗001͘-oriented growth as cur-
rent density increased above ϳ5 mA/cm2.20 These authors argued
that at lower current density adsorbed hydrogen or Ni͑OH͒ inhib-
2
ited ͗001͘ growth while stabilizing ͗001͘ and ͗112͘ orientations,
leading to deposits with these textures. In the present case it is
unclear why particle filtering should affect the growth mode, since
no change in pH near the cathode upon filtering the electrolyte was
observed in Part I of this work.
It is difficult to postulate a single mechanism that can account for
the structure and texture of the deposits plated from the unfiltered
electrolyte at 3 mA/cm2. At the lowest deposition temperature, the
coarse-grain ͗001͘ texture observed in the 3 mA/cm2 deposit from
the unfiltered electrolyte implies free growth and the absence of any
inhibiting species, much like both of the deposits plated at the same
temperature but at the higher current density. However, compared to
the companion deposit from the filtered electrolyte at 28°C and
3 mA/cm2, it is apparent that the deposition conditions at the metal/
electrolyte interface are directly related to the presence or absence of
particle filtration in a way that is not obvious. It appears that particle
filtration of the sulfamate/boric acid electrolyte promotes the forma-
tion of a species that effectively inhibits deposition. Absent the fil-
tration at this temperature and current density, uninhibited growth is
the preferred mode.
Most problematic, though, is rationalizing the character of the
50°C, 3-mA/cm2 deposit from the unfiltered electrolyte. The fine-
grain microstructure and ͗011͘ preferred orientation of this deposit
suggests the presence of an effective adsorbed inhibiting species at
the higher temperature that is not present at the lower temperature.
Lin et al., who investigated effects of increased ammonium ion con-
Figure 10. ͑Color͒ ͑a͒ Higher magnification micrograph of region outlined
in Fig. 9. The abrupt change in grain size is apparent and occurs over about
a
10-␮m region of deposit growth. ͑b͒ EBSD map indicates that the texture
changes from ͗001͘ to ͗011͘, indicating that the growth mode changes from
uninhibited to inhibited during the deposition run.
texture to the ͗011͘ texture characteristic of inhibited growth. XRD-
derived texture of planview sections indicate that the ͗001͘ free-
growth region of the deposit was highly textured ͑27 times random͒
and the ͗011͘ inhibited-growth region was less strongly, but still
significantly, textured ͑6 times random͒.
It seems evident, therefore, that the plating parameters estab-
lished for fabricating this set of specimens result in unstable depo-
sition conditions, allowing the deposit to switch abruptly from
coarse-grain, highly ͗001͘-textured, uninhibited, free growth to a
fine-grain ͗011͘-textured, fully inhibited growth mode. Figure 10
shows that the transition from one growth mode to the other occurs
over a narrow region of about 10 ␮m, and faradaic considerations
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Journal of The Electrochemical Society, 153 ͑5͒ C325-C331 ͑2006͒
centration on sulfamate Ni deposits, also observed a similar trend in Acknowledgments
C331
texture and microstructure as bath temperature was increased from
40 to 50°C.22 It is not evident why higher deposition temperatures
would lead to increased inhibition at low current densities. One is
left, then, with postulating the presence of a second, additional
mechanism, perhaps the increased adsorption of a certain sulfamate
electrolyte species only at high temperatures and low deposition
rates, resulting in inhibited growth under such conditions.
Such a mechanism would help one understand the instability in
growth mechanism observed in the 40°C, 3-mA/cm2 deposit from
the unfiltered electrolyte. Given that this temperature is midway
between the inhibited and uninhibited growth regimes, this instabil-
ity would not be surprising, since small perturbations in the growth
conditions would translate to major changes in the structure of the
deposit.
The authors thank Dorrance McLean and John Hachman for their
assistance with the fabrication of the test specimens used in this
study as well as Andy Gardia, Michael Rye, and Bonnie McKenzie
for their assistance with metallographic preparation and microscopy.
Sandia is a multiprogram laboratory operated by Sandia Corpora-
tion, a Lockheed Martin Company, for the United States Department
of Energy under contract DE-AC04-94AL85000.
Sandia National Laboratories assisted in meeting the publication costs of
this article.
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