High resolution 3D microstructures made by localized electrodeposition of nickel

Article abstract of DOI:10.1149/1.1393439

Techniques to create three-dimensional microstructures are still a matter of research, and localized electrodeposition is a promising path to inexpensive free-form microfabrication in metal. The principle is to put the tip of a needle very close to a conducting substrate submerged in a plating bath, apply typically 4-5 V, and move the tip as the metal deposits. In this paper we report improved deposition control and the benefits of adding ammonia and ammonium formate to nickel plating solutions for enhanced resolution and surface smoothness. A mirror-smooth 600 μm tall nickel column 3 μm in diameter was built at 4 μm/s. Still higher axial deposition rates could be attained, as well as structure diameters down to 2 μm. The broadening of previously built structures near the one being grown was studied, and the usefulness of an auxilliary electrode was confirmed. The possibility of branching and joining structures was also investigated in order to realize the vision of building structures of arbitrary shapes.

Full text of DOI:10.1149/1.1393439

1810
Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
High Resolution 3D Microstructures Made by Localized
Electrodeposition of Nickel
Anders Jansson, Greger Thornell, and Stefan Johansson
Department of Materials Science, Uppsala University, S-751 21 Uppsala, Sweden
Techniques to create three-dimensional microstructures are still a matter of research, and localized electrodeposition is a promis-
ing path to inexpensive free-form microfabrication in metal. The principle is to put the tip of a needle very close to a conducting
substrate submerged in a plating bath, apply typically 4-5 V, and move the tip as the metal deposits. In this paper we report
improved deposition control and the benefits of adding ammonia and ammonium formate to nickel plating solutions for enhanced
resolution and surface smoothness. A mirror-smooth 600 ␮m tall nickel column 3 ␮m in diameter was built at 4 ␮m/s. Still high-
er axial deposition rates could be attained, as well as structure diameters down to 2 ␮m. The broadening of previously built struc-
tures near the one being grown was studied, and the usefulness of an auxilliary electrode was confirmed. The possibility of branch-
ing and joining structures was also investigated in order to realize the vision of building structures of arbitrary shapes.
© 2000 The Electrochemical Society. S0013-4651(99)05-011-9. All rights reserved.
Manuscript submitted May 3, 1999; revised manuscript received January 24, 2000.
surface can be a very efficient form of mass transport because fresh
electrolyte is brought directly to the surface, breaking up the diffu-
sion layer.8 It was shown6 that the deposition profile from nickel
sulfamate at 4.2 V, using a 500 ␮m diam insulated platinum wire
positioned 500 ␮m from the substrate, resembled the shape of the
Laplacian field at the substrate surface, as calculated by numerical
methods, ignoring double-layer charging. It may serve a pedagogic
purpose to note that the calculated field is essentially the same as
that given by the analytical treatment of a point charge with the
method of an imaginary mirror charge,9 Fig. 1.
While some of the more conventional techniques for fabricating
microstructures have reached a fairly developed stage, techniques to
create three-dimensional microstructures are still a matter of re-
search. In order to realize the vision of building structures of an arbi-
trary geometrical shape, methods giving localized processing are of
great interest.1 Using a modified scanning tunneling microscope, a
very small freestanding column 25 ϫ 280 nm has been produced by
deposition from a metal carbonyl gas,2 indicating that submicrome-
ter resolution can be attained. Two flexible methods in the microm-
eter range are laser-assisted etching3 or deposition4 from a gas. It has
also been demonstrated5 that electrodeposition with a scanning elec-
trochemical microscope can produce two-dimensional submicrome-
ter structures. The extension of localized electroplating to three
dimensions has been presented,6 and we believe that it is a promis-
ing path to inexpensive free-form microfabrication in metal.
When the plating current is high, the slow transport of species to
the electrode causes amplification of existing surface irregularities,
The principle of localized electroplating is to put the tip of a nee-
dle very close to a conducting substrate submerged in a plating bath,
apply typically 4-5 V between tip and substrate, and move the tip as
the metal deposits from the electrolyte. The substrate is a metal sur-
face and the electrolyte contains the metal ions to be reduced and de-
posited in order to build the structure. When the tip of the needle is
close to the structure, a highly localized deposition can be achieved
and the rate of axial growth may be several micrometers per second.
This technique has just recently received attention6,7 and there are
still questions regarding mechanisms and reproducibility of structure
size and quality. Some of these questions are dealt with in this paper,
and a recipe to make nickel structures 2-3 ␮m in diam are presented.
Background
The choice of needle is an important question. In our earlier
experiments, thin platinum wires were sealed with glass and exposed
at the tip by polishing. This technique gave a satisfactory microelec-
trode, but problems occurred during experiments due to consump-
tion of platinum, resulting in a hole in the tip that became filled with
gas, effectively stopping further processing. Later, Madden and
Hunter6 reported the successful use of a commercially available nee-
dle, and although a more recent contribution7 presents a structure
that was made with a glass-coated platinum wire, we have taken ad-
vantage of the commercially available tips. Interestingly, the column
made with the glass-coated wire tip7 has a very uniform width, yet
an open texture with a surface of canals, as if the deposition had
occurred inside a tube.
There is still much to investigate in localized electroplating. It is
believed6 that a localized negative surface charge on the substrate
(due to electrostatic attraction) and the convection coming from bub-
bles formed at the tip are two important factors regarding the local-
ization of the deposition. Convection from bubbles detaching from a
Figure 1. Static electric field from a point charge outside a metal surface.
Sketches picturing the driving force setting up a localized surface charge, and
the resulting surface charge and field at equilibrium. The mirror charge and
field lines inside the metal are merely a mathematical model to calculate the
Laplacian field at the surface interface.
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
1811
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
eventually forming dendritic-type deposits.10,11 For a given current,
a low exchange current density (i.e., the rate of electron transfer at
zero net current) favors this outward growth,11 and at a certain over-
voltage dendritic growth may arise.10 In this context, it feels appro-
priate to mention that a strong electric field may even transform iso-
In our experiments, we allowed the structures to grow into con-
tact with the tip, and a current limiter was therefore needed. The
plating current at 4-5 V was on the order of 1 mA, and it was noted
that using the inherent 6 mA current limiter of the power supply unit
made the tip and structure stick together. An active current limiter
was designed (shown in Fig. 2) to cut the current to less than 0.1 mA
as soon as the voltage dropped due to a contact event. It was ob-
served that vibrations from nearby equipment made the needle go in
contact with the structure about 50 times a second, and the circuit
was modified to set the voltage high only when the contact was lost
for more than half a second, in order to ensure deposition only when
the tip was moved intentionally. During this delay time, the voltage
applied to the needle was 1.7-3 V (setable) and the resulting “OFF-
current” was about 0.02-0.05 mA. The circuit was also modified to
cut the current already on a sudden voltage drop of about 0.2 V in
order to respond to a contact event as early as possible. When relat-
ing the rate of growth to the tip speed, note that the OFF-to-ON
delay time was the same at all tip speeds.
Prior to deposition, the polished nickel substrate was washed
with acetone and ethanol and put in position. Some 0.4 mL elec-
trolyte solution was added to cover the surface to a depth of about
3 mm, and the tip was brought in electrical contact with the sub-
strate. This positioning was made with the OFF-current (through the
47 kΩ resistor, Fig. 2). The output from the operational amplifier
controlling the ON-voltage was then connected, and the tip was
withdrawn from the substrate. The progress of fabrication was mon-
itored by the voltage drop when the structure grew into electrical
contact with the tip, and also by visual inspection of bubble genera-
tion and the structure itself (if not too thin), using a light optical
stereomicroscope (70 times magnification). Tip speeds resulting in
ON-periods of about 0.5 s were found to be reliable speeds of oper-
ation for most plating baths. Afterward, the electrolyte was removed
with a Pasteur pipette and the substrate was released and repeatedly
immersed in fresh water. The structures were investigated in a scan-
ning electron microscope (Zeiss DSM960A) equipped with an ener-
gy-dispersive X-ray spectroscope (EDS) and photographed at an
angle of 45Њ unless otherwise specified.
lated micrometer-sized copper particles into submicron wires.12
A
qualitative statement which may well be applicable to our system is
that the overall shape of the deposit is controlled by the Laplacian
field, whereas the microscopic morphology depends on the diffu-
sional field.13
The plating bath used by Madden and Hunter,6 consisting of 2 M
nickel sulfamate in 0.5 M boric acid, is known to give easily repro-
ducible coatings of high purity and with low residual stress.14,15 We
found no particular reason to assume this solution to be optimal for
localized electroplating and set out to try other electrolytes as well.
We report in this paper the benefits of adding ammonia and ammo-
nium formate to various nickel-based electrolytes, mainly nickel sul-
fate, for enhancing the local deposition rate and the structural quali-
ty of the deposit.
Experimental
Figure 2 is a schematic drawing of the equipment used. The posi-
tion of the substrate can be changed by dc motors operated via a joy-
stick, and the actual position is provided by Heidenhain optical en-
coders with a resolution of 5 nm. The speed in the z direction was
regulated in discrete steps: 140, 70, 35, 18, 9, 4.4, 2.2, 1.1, 0.54,
0.27, or 0.14 ␮m/s. We refer to this as the tip speed, and maximum
tip speed refers to the highest speed level that could be used reliably
to produce structures. The substrates, 12 mm in diameter, were lathe-
machined from 99.0% nickel metal (Harald Pihl AB, Sweden), and
polished on soft cloth using diamond abrasives down to 1 ␮m size.
The glass or epoxy insulated platinum/iridium needles used (avail-
able from FHC, Bowdoinham, USA) had chemically etched tips of
approximately 0.5 ␮m radius. Unless specifically noted, the needle
used was epoxy-coated (no. UEP8GCCEXN1N/17 MΩ). To reduce
the likelhood of an instability during plating due to partial stripping
of the epoxy coatings, the tips were conditioned at 4 V for about
5 min in 1 M CuSO4 to give a current of 0.5 mA. This caused the
epoxy to withdraw somewhat, leaving a relatively stable conical tip
with a length of approximately 50 ␮m and an area of 4000 ␮m2.
The auxilliary electrode, introduced for ability to control the sub-
strate potential, was a coil made of a 125 ␮m thick nickel (99.98%)
wire (GoodFellow, England). The original surface area of the coil
was about the same as that of the substrate, but the coil was soon
roughened by deposition. Preliminary tests connecting the substrate
to ground and the auxilliary electrode to Ϫ0.1 V seemed to produce
a local etch, and therefore we decided to keep the auxilliary elec-
trode grounded when used. The reference electrode used to measure
the electrochemical potentials was a 2 mm Ag/AgCl electrode with
a glass-coated wire attached (Science Products GmbH, Hofheim,
Germany). It was put in a Pasteur pipette together with saturated KCl
and a KCl-saturated agar plug in the outlet. Accuracy was confirmed
by tests against a commercial Ag/AgCl reference electrode.
Aqueous solutions of nickel sulfamate, nickel chloride, nickel
perchlorate, and nickel sulfate were tested at concentrations of 0.5-
2 M. Additives tested were boric acid, ethylenediamine, ammonia,
and ammonium formate. Most of the plating solutions tested are
described and named in Table I. For instance, T1-sulfamate refers to
entry 1 in the table. Plating conditions and notes on solution charac-
teristics are also included. The temperature in the laboratory was
22ЊC. For T1-sulfamate and T9-sulfate, the current and potential
were measured as a function of applied voltage and tip distance.
The amount of unintended deposition on nearby structures was
studied, that is to say the broadening of previously built structures
near the one being built. First two structures were built, separated by
a distance of 160 ␮m, and then a third structure was built in between.
Differences in structure widths within the triplet were then investi-
gated in the SEM. The structures were built 140 ␮m high, and the
distance between the structures was about the closest possible with
respect to the taper of the tips used. This study was made with the
T1-sulfamate and the T9-sulfate, both without using the auxilliary
electrode and having it connected to ground. Details are given in
Figure 2. A drawing of the experimental setup. The voltage applied during
ON-periods is set with V2, whereas the OFF-period voltage is governed by V1.
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

1812
Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
Table I. An overview of some electrolyte solutions investigated (plating conditions and remarks included).
T no.
Plating bath composition
Applied voltage (V)
Remarks
1 ␮m/sa at 4.5 V
1
2
3
2 M Ni(NH2SO3)2, 0.5 M H3BO3
0.5 M Ni(NH2SO3)2, 0.12 M H3BO3
1 M Ni(NH2SO3)2, 0.25 M H3BO3, 2 M HCOONH4, 2 M NH3
4.2,b,c 4.5,b,c 4.8,b,c 5.0b
4.3,b 4.8b
Hollow structure at 4.8 V
pH 8
4.3,b 5.0b
8 ␮m/sa at 5 V
4
5
6
7
2 M NiCl2, 0.25 M H3BO3
1 M NiCl2, 2.5 M NH2CH2CH2NH2
1 M Ni(ClO4)2, 1 M HCOONH4
4.2b
No OFF-to-ON delay
9 ␮m/sa at 4.8 V, pH 9
Precipitate forming in days
pH around 7.3
4.3,b 4.8b
4.3,b 5.0b
4.2,b 4.8b
1.2 M NiSO4, 1.7 M NH3, 0/0.4/1.2/3 M HCOONH4
Crystals forming next day
pH 7.3
8
9
0.5 M NiSO4, 1 M NH3, 3.5 M HCOONH4
1 M NiSO4, 2 M NH3, 2 M HCOONH4
4.0,c 4.4,c 5.5c
2/4/8 µm/sa at 4.0/4.4/5.5 V
pH 7.6, 4 ␮m/sa at 4.5 V
Precipitate forming in days
4.2,b,c 4.5,b,c 4.8b,c
a Maximum tip speed (discrete levels).
b Without auxilliary electrode.
c With auxilliary electrode grounded.
Table II. In this study, the overall (unlocalized) deposit on the sub-
strate was measured with the help of an interferometer (WYKO NT-
2000). Using dots of nail lacquer (Estée Lauder no. 95) which were
removed afterward, the original substrate surface was masked and
could be used as a height reference.
(Fig. 3). We have also noted that T1-sulfamate gives a higher plating
speed when using relatively poorly glass-insulated needles, probably
due to the increased and less downward bubble stream (using this
kind of tip, a column 1400 ␮m tall and 30 ␮m diam was erected).
This is in accordance with the high deposition rate of 6 ␮m/s report-
ed for T1-sulfamate using an uninsulated electrode.6
Chemicals.—NiCl2 (98%), Ni(ClO4)2, Ni(H2NSO3)2и4H2O
(98%), NiSO4и6H2O, HCOONH4 (97%), H3BO3 (99%), and
NH2CH2CH2NH2 (99%) were bought from AB Labkemi Sigma-
Aldrich, Sweden, and used without further purification. The deion-
ized water used was of 18 M⍀ cm quality, and the ammonia (25%)
was of VLSI selectipure grade (Merck, Germany).
For nickel chloride (2 M) without additives the substrate was
blackened in the vicinity of the tip and no structures could be pro-
duced. Addition of boric acid (T4) produced a hollow structure which,
compared with the structure in Fig. 3, was very uniform (50 ␮m diam,
wall thickness 5 ␮m). Addition of NaCl resulted in hollow structures
with larger diameter. Nickel chloride with addition of ethylenedi-
amine (T5) produced solid but rough structures 10-20 ␮m diam at
high speed (maximum tip speed 9 ␮m/s at 4.8 V). The bluish elec-
trolyte turned yellow or red in the vicinity of the tip, depending on the
amine concentration. The same color changes were obtained when
bubbling chlorine (made from HCl and KMnO4) into the solution. In
this case it appears that one advantage of the amine is to engage the
chlorine in reactions other than etching that do not interfere with the
deposition of nickel. Ethylenediamine greatly enhanced local deposi-
tion rates in other solutions as well, but as it is very corrosive, pits
were sometimes produced in the substrate.
Results and Discussion
An overview of some solutions and plating conditions tested is
given in Table I. T1-sulfamate has a relatively narrow voltage range
around 4.5 V at which structures can be built. Deposition at lower
voltages results in significant deposition all over the substrate,
whereas higher voltages give dark “burnt” structures containing an
increased amount of sulfur, as detected by EDS. Anodic decomposi-
tion products from sulfamate ions can exert significant effects in
nickel plating baths, regarding the sulfur content and quality of the
deposit. Sulfite, azodisulfonate, and a compound suggested to be
diimide S-sulfonate have been considered to be of importance.16 By
increasing the voltage to 5.5 V or more, it is quite possible to oxidize
an already built structure until it falls apart. When diluting the T1-
sulfamate 1:4 (T2), the resulting structure turned hollow at 4.8 V
Even without additives, nickel perchlorate and nickel sulfate pro-
duced structures, although the surface quality was not too high. For
nickel sulfate the effect upon addition of ammonia/ammonium for-
mate was investigated at 4.2 and 4.8 V, and the result can be viewed
Table II. Regarding broadening of nearby structures for T1-sulfamate and T9-sulfate.
Substrate vs.
reference
(V)
Applied voltage
(V)
Auxilliary
electrode
Tip speed
(␮m/s)
Tip current
(mA)
Diameter (␮m)
T no.
(1-3-2)a
Fig.
4.2
4.5
4.5
4.5
4.5
4.8
Yes
No
Yes
No
Yes
No
0.14
0.27
0.27
0.27
0.27
0.54
0.9
0.7
1.0
0.9
1.1
1.1
Ϫ0.51
Ϫ0.80
Ϫ0.55
Ϫ0.78
—
34, 28,b 28b
19, 11, 15
13, 11, 12
18, 14, 19
14, 14,b 14
19, 17, 18
10a
10b
—
—
10c
—
1
—
4.2
4.2
4.2
4.8
Yes
Yes
No
0.27
0.27
0.27
4.47
0.7
0.8
0.8
2.1
Ϫ0.56
Ϫ0.61
Ϫ0.77
Ϫ0.61
5.5, 5.5, 5.5
6, 8, 8.5
11, 9.5, 9.5
4.5, 4, 4.5
—
—
—
—
9
Yes
a Built in this order (Fig. 10).
b In the lower part.
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
1813
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
substrate. This was also noted by Madden and Hunter,6 although in
their case the flattening of the tip was more pronounced. In our case,
the initial positioning was made manually using 2-4 ␮m/s tip speed,
and we expect to improve this procedure in the future. The flatten-
ing of the tip was not found to be critical for the shape of the struc-
tures produced. However, sometimes more than one strand may
build up if tip speed is low, Fig. 6, a phenomenon possibly influ-
enced by a flattened tip. This indicates that the deposition has been
localized to the side of the tip, which is also consistent with the pit
formations at the base of the structures. Furthermore, for the strand
left behind to be able to catch up, the growth rate must probably be
slightly slower very close to the tip surface.
Surface smoothness has a close relation to lateral resolution and
mechanical strength and is therefore of importance in many applica-
tions. We have observed increased smoothness in connection with en-
hanced bubble generation. With the equipment used, it is difficult to
comment on the bubble size distribution, but as suggested in the
schematic figure (Fig. 7), it appeared rather narrow. As reported by
Madden and Hunder,6 the epoxy is stripped away during processing.
This results in a skirt above the tip, and we have observed that the
bubble stream is then effectively directed downward, Fig. 7. Regard-
ing surface roughness, a downward bubble stream should be advanta-
geous if it evens out the mass transfer into recessed areas of the struc-
ture. In this way, the bombardment of bubbles toward the structure
might locally resemble a pulsed plating. A uniform deposition rate in
all areas tends to smooth out original surface roughness, Fig. 8. A
more downward bubble stream decreased the deposition rate for T1-
sulfamate while T9-sulfate still plated fast. Perhaps the mirror-smooth
surfaces often seen for T9-like solutions, for instance in Fig. 4d, is
because of an intense downward bubble stream. Figure 9 offers a clos-
er look at such a surface layer. It appears to be denser than the core
and somewhat thinner close to the tip of the structure.
Figure 3. Diluted T1-sulfamate (T2) produced a hollow structure at 4.8 V.
in the array of Fig. 4. Surface smoothness increased and structure
diameter decreased at higher concentrations of ammonium formate.
On using the higher voltage, both structure width and surface smooth-
ness decreased. Preliminary tests with T6-perchlorate and T3-sulfa-
mate showed comparable results. We attribute the surface smoothen-
ing by ammonium formate to a more intense bubble generation, as
discussed later. The widths of the structures in Fig. 4a-h are approxi-
mately 25, 15, 7, 3, 17, 8, 4, and 2 ␮m, respectively.
Figure 10 shows some results from experiments regarding unin-
tended deposition on nearby structures. Significant broadening of
the nearby structures was observed for T1-sulfamate. The broaden-
ing was less pronounced at higher voltage or when using the auxil-
liary electrode, as seen in Table II. A more detailed quantitative
analysis probably requires needles and coatings that change less dur-
Figure 5 shows the uninsulated area of two conditioned tips, one
unused and the other used in experiments similar to the ones present-
ed in Fig. 4. This tip was later used to grow the structures in Fig. 4.
The used tip in Fig. 5b has a hole at the side. This was otherwise not
observed, but the slight flattening of the tip was common and found
to be caused by the initial positioning of the tip in contact with the
Figure 4. Structural effects upon adding ammonium formate to a plating solution containing 1.2 M NiSO4 and 1.7 M NH3 (two voltages investigated).
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

1814
Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 7. Illustration of how the epoxy coating tends to split open during pro-
cessing, forming a “skirt” which directs the bubble stream more downward.
Figure 5. Two conditioned tips, (a) unused and (b) used in experiments sim-
ilar to the ones presented in Fig. 4. This tip was later used to grow the struc-
tures in Fig. 4.
Figure 8. Illustration of how a deposited layer tends to reduce surface rough-
ness when the rate of deposition is equal in all areas.
ing processing. T9-sulfate showed less or no broadening of nearby
structures, and the deposition profile on the substrate was also nar-
rower than for T1-sulfamate, Fig. 11. The deposition on the substrate
at large distances (>1 mm) was around 5 nm for both electrolytes.
For T1-sulfamate, the shape of the deposition profiles at the sub-
strate close to the structures was rather independent of the plating
conditions tested. Only the scale of the profile changed, approxi-
mately in proportion to the effective time taken to produce the given
structure. The sharpness of the T9-sulfate profile at about 20-30 ␮m
from the center of deposition seems to be related to rings around the
pillars, particularly visible in Fig. 6a, so that the profile shown for
T9-sulfate is not for the pillar itself. The rings are perhaps connect-
ed with the radius of efficient convection from the bubbles, emitted
as puffs during the ON-periods.
The localization of the deposition is probably connected with the
ohmic drop in the solution, giving faster deposition rates close to the
Figure 6. More than one strand may build up if tip speed is low. Note also
the formation at the base of structures (b) and (c). The structures in (c) and
(d) were made from T9-sulfate and those of (a) and (b) from a similar solu-
tion (1 M NiSO4, 2.5 M HCOONH4, 1.6 M NH3). 4.5 V was applied in all
cases. Tip speed is noted in the figures. (The mirror-smooth structure in (b)
has a diameter of 2 ␮m.)
Figure 9. A column with a dense surface layer (T7-sulfate, 4.8 V, 0.44 mA,
glass-coated tip, low tip speed).
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
1815
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 10. A study of the amount of unintended deposition on nearby structures. The structures shown were made from T1-sulfamate and built in the order left-
right-middle: (a) 4.2 V, 0.9 mA, auxilliary electrode grounded; (b) 4.5 V, 0.7 mA, no auxilliary electrode, (c) 4.5 V, 1.1 mA, auxilliary electrode grounded.
anode, an undesirable situation in conventional electroplating.11
From voltammetry and related topics, the theory of diffusional mass
transfer at microelectrodes is well developed,17 and the electrody-
namic approach would be to start with a calculation of the ohmic
drop at the microelectrode. However, this cannot be done properly
without detailed knowledge about the reactions occurring,18 and fur-
thermore, to our knowledge, it has never been done for a bubble-
forming process at a microelectrode. Looking at the sometimes vig-
orous bubbling from the tip, it seems unlikely that it may be ignored.
Figure 12 shows the voltage-current characteristics of T1-sulfamate
and T9-sulfate with the auxilliary electrode grounded and the tip far
from the substrate. At localized plating conditions (>4 V) the current
appears to be governed by iR-drop at the tip, and we expect the sta-
tic electric field approach to be relevant. When nickel deposition is
accompanied by the simultaneous formation of hydrogen bubbles,
the overall process may also exhibit a linear relationship between
current density and overpotential.19 For T1-sulfamate the effect of
decreasing the tip distance is shown in Fig. 13. As expected, the cur-
rent going to the substrate increases at smaller tip-substrate distance,
and also the potential of the substrate measured farther away increas-
es, indicating a decrease in current density in that part of the sub-
strate. Although the linear correlation between the measured current
and potential data is striking, their quantitative relationship needs
further investigation. Comparable results were acquired for T9-sul-
fate, but substrate geometry and data on tip distance were biased by
the fast buildup of deposit. Even at 50 ␮m tip distance, the deposit
made contact within 1 min, Fig. 14. Upon approaching the substrate
with the tip, the tip current usually decreased with around 30% (with
Figure 12. Voltage-current characteristics for T1-sulfamate and T9-sulfate
(auxilliary electrode grounded, tip far from substrate).
Figure 13. Plots of substrate current and potential vs. tip-substrate distance
for T1-sulfamate at 4 V. Reference electrode was placed relatively far from
the tip, and current was measured using an amperometer between substrate
and ground (cf. Fig. 2). The auxilliary electrode was grounded.
Figure 11. Typical surface deposition profiles measured in the direction
through the midstructures of the triplets in Table II.
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

1816
Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 14. At an initial tip surface distance of 50 ␮m, the buildup from T9-
sulfate made contact in slightly less than 1 min.
less polished substrates and a less downward bubble stream from
glass-coated tips, the opposite effect was noted). Apparently, this is
due to less efficient mass transport to the tip, which is blocked by the
substrate. The percentage of volume occluded by bubbles in the
vicinity of the tip increases as well.
Figure 15. A structure made from T8-sulfate at 5.5 V. Each segment in (b)
corresponds to an ON-period. Note the cracks at the base in (c). The tip speed
was 8 ␮m/s.
Our best recipe for success in localized nickel electroplating is to
use formate anions with the addition of an amine. One important
effect of the amine may be related to pH and hydrogen formation.
Hydrogen formation competes with nickel deposition at the nucle-
ation sites,19 and hydrogen formation is more pronounced at low pH
and low convection.11 Moreover, since hydrogen ions react at the
cathode to form hydrogen, there is often a depletion of hydrogen
ions in the vicinity of the substrate in conventional electroplating,
and it is desirable to have a buffered solution at slightly acidic pH to
avoid nickel hydroxide from precipitating or being incorporated in
the deposit.14,15 This is why boric acid is added to the sulfamate plat-
ing bath, which then becomes well buffered against high pH but not
against low pH.15 In view of a release of hydrogen ions at an anode
situated very close to the substrate, this may not be optimal in our
case. Alternatively, one may add a complexing agent15 to lower the
concentration of free Ni2ϩ, thus avoiding the formation of nickel
hydroxide even at higher pH. Hydrogen ions formed at the tip might
then even be advantageous if they help to release the nickel ions
from the amine complex so that they can be deposited more easily.
Another important plating characteristic of metal complexes is cer-
tainly the low exchange current density.11,14 As already mentioned,
a lower exchange current density can favor outward rather than lat-
eral growth of existing crystals. Furthermore, there may be a corre-
lation between the exchange current density and the tendency to pro-
duce hollow structures.
Figure 15 focuses on the questions of adhesion and cracking
related to internal stress (see also Fig. 14). In this study we have not
investigated the adhesion to the substrate, but there are methods of
pretreatment14 to improve adhesion. Internal stress usually increases
upon the simultaneous deposition of hydrogen,19,21 and voids filled
with hydrogen may cause cracks. Once a crack is formed it becomes
a locus for further hydrogen formation as well as metal dissolution.21
The addition of ammonium formate increases the current at a
given voltage and enhances the convection from bubbles. Increased
convection increases the plating current density that can be used
without the occurrence of unwanted reactions,14 and we believe that
to produce high-quality structures with the localized electroplating
technique investigated in this paper, the convection from bubbles is
essential. Formate ions form hydrogen ions and carbon dioxide at
the anode,20 and it is advantageous that the gas produced does not
interfere with the deposition or significantly etch the deposit. Fur-
thermore, we have observed that the bubbles from ammonium for-
mate are relatively small and seldom coalesce into big bubbles that
remain attached to the needle. Instead they shrink, supposedly form-
ing carbonic acid. It has happened that columns have broken when a
very big bubble suddenly jumps from its site at the epoxy coating of
the needle, but small bubbles do not seem to be a problem even when
the convection looks vigorous. One complication with the sulfate/
ammonium formate solutions was the formation of a colorless salt
on the tip at low current, for instance when positioning the tip.
Figure 16. A portal made by bridging two separately built structures. At the
apex so formed, a final spire was grown. The different parts of this structure
were built in the order noted in the inset.
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (5) 1810-1817 (2000)
1817
S0013-4651(99)05-011-9 CCC: $7.00 © The Electrochemical Society, Inc.
pected to be optimal for the bridging procedure, and we do not ex-
pect the connection shown to be very strong. All in all we would like
to find a more satisfactory approach to the challenge of joining struc-
tures. Another challenge is to find a more sophisticated use of the
auxilliary electrode.
To test the ability of producing very tall structures, the T9-sulfate
was used at 4.5 V to produce a 600 ␮m tall column, shown in
Fig. 17. At about 300 ␮m the contact events started to come less reg-
ular, possibly because of a slightly swinging motion of the column,
and the tip speed was therefore lowered from 4 to 2 ␮m/s.
Acknowledgment
The Foundation for Strategic Research and the Swedish Nation-
al Board for Industrial and Technical Development (NUTEK) are
gratefully acknowledged for their financial support.
Uppsala University assisted in meeting the publication costs of this
article.
References
1. G. Thornell and S. Johansson, J. Micromech. Microeng., 8, 251 (1998).
2. M. A. McCord, D. P. Kern, and T. H. P. Chang, J. Vac. Sci. Technol., B6, 1877
(1988).
3. T. M. Bloomstein and D. J. Ehrlich, in Proceedings of Transducers, San Fransisco,
CA, pp. 507-511 (1991).
4. K. Williams, J. Maxwell, K. Larsson, and M. Boman, in Proceedings of MEMS 99,
IEEE, Orlando, FL, pp. 232-237(1999).
Figure 17. A 600 ␮m tall and 3 ␮m wide column made from T9-sulfate at
4.5 V (photographed at an angle of 90Њ).
5. O. E. Hüsser, D. H. Craston, and A. J. Bard, J. Electrochem. Soc., 136, 3222 (1989).
6. J. D. Madden and I. W. Hunter, J. Microelectromech. Systems, 5(1), 24 (1996).
7. E. M. El-Giar and D. J. Thomson, WESCANEX´97 Proceedings, Conference on
Communications, Power and Computing, IEEE, pp. 327-332 (1997).
8. L. J. J. Janssen, Electrochim. Acta, 34, 161 (1989).
Since hydrogen ions produced at the tip may be disadvantageous in
this respect, it might be possible to reduce internal stress by choos-
ing another anodic reaction.
9. R. K. Wangsness, Electromagnetic Fields, 2nd ed., John Wiley & Sons, Inc., New
York (1986).
Compared with plating without the auxilliary electrode, having it
connected to ground increased both the substrate potential and the
tip current. The potential changed approximately from Ϫ0.8 to
Ϫ0.6 V vs. the reference, and the tip current increased around 20%,
equivalent to a 0.2 V increase in the applied voltage. Most of the vis-
ible hydrogen bubbles evolved from the auxilliary electrode, and
afterward the auxilliary electrode was positive to the substrate. With
the auxilliary electrode connected to ground it was possible to build
structures from fast-plating solutions like T9-sulfate without con-
necting the substrate at all. (In conformity with the feedback mode
in scanning electrochemical microscopy reduction can still occur
beneath the tip as long as it is accompanied by oxidation at portions
of the substrate farther away.22) However, when reaching a height of
50-100 µm the structures always fell off. Apparently, metal dissolu-
tion at the base of the structure becomes efficient at this distance
from the tip.
We were able to branch structures (Fig. 16, right) simply by posi-
tioning the tip close to the side of a structure and start plating from
there. Joining two structures is a different matter, calling for a re-
fined strategy. Figure 16 shows the result from an attempt to broad-
en two structures to come into contact with each other by position-
ing the tip symmetrically above and relatively far (approx. 50 ␮m)
from the original 15 ␮m gap between the structures. Neither plating
conditions nor the choice of plating solution (T9-sulfate) can be ex-
10. B. E. Conway, J. O’M. Bockris, E. Yeager, S. U. M. Khan, and R. E. White, Com-
prehensive Treatise of Electrochemistry, Vol. 7, Plenum Press, New York (1983).
11. E. Gileadi, E. Kirowa-Eisner, and J. Penciner, Interfacial Electrochemistry-An
Experimental Approach, Addison-Wesley Publishing Company, Inc., England
(1975).
12. J.-C. Bradley, J. Crawford, M. McGee, and S. G. Stephens, J. Electrochem. Soc.,
145, L45 (1998).
13. P. Garik, D. Barkey, E. Ben-Jacob, E. Bochner, N. Broxholm, B. Miller, B. Orr, and
R. Zamir, Phys. Rev. Lett., 62, 2703 (1989).
14. R. F. Bunshah, Handbook of Deposition Technologies for Films and Coatings, Sci-
ence Technology and Applications, 2nd ed., Noyes Publications, Park Ridge, NJ
(1994).
15. Gmelins Handbuch der Anorganischen Chemie, 8th ed., 57 AII, Verlag Chemie,
GmbH, Germany (1968).
16. H. Zhang and S.-M. Park, J. Appl. Electrochem., 24, 1182 (1994).
17. A. J. Bard and L. R. Faulkner, Electrochemical Methods-Fundamentals and Appli-
cations, John Wiley & Sons, Inc., New York (1980); J. M. Davies, F.-R. F. Fan, and
A. J. Bard, J. Electroanal. Chem., 238, 9 (1987); and M. I. Montenegro, M. A.
Queirós, and J. L. Daschbach, Microelectrodes: Theory and Application, (NATO
ASI Series, E197), Kluwer Academic Publishers, Netherlands (1991).
18. K. B. Oldham, J. Electroanal. Chem., 250, 1 (1988).
19. J. Morales, S. Krijer, P. Esparza, S. González, L. Vásquez, R. C. Salvarezza, and
A. J. Arvia, Langmuir, 12, 1068 (1996).
20. C. K. Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems,
Marcel Dekker, Inc., New York (1971).
21. J. O´M. Bockris, Modern Electrochemistry-An Introduction to an Interdisciplinary
Area, Vol. 2, Plenum Press, New York (1970).
22. A. J. Bard, F.-R. F. Fan, J. Kwak, and O. Lev, Anal. Chem., 61, 132 (1989).
Downloaded on 2015-05-04 to IP 134.117.10.200 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).