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ELECTRODEPOSITION OF AG ALLOYS WITH NI AND W FROM A THIOUREA-CITRATE ELECTROLYTE

A Dissertation Prospectus Presented

By

Avinash Kola

to

The Department of Chemical Engineering

In partial fulfillment of the requirements

For the degree of

Doctor of Philosophy

In the field of

Chemical Engineering

Northeastern University

Boston, Massachusetts

June 6th, 2014

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ABSTRACT

Tungsten alloys, such as Ni-W are well recognized for their outstanding corrosion

resistance, wear resistance and catalytic properties towards hydrogen generation. Nickel-

tungsten alloys also have the potential to act as a barrier layer in the semiconductor

industry, to prevent diffusion of conducting metals (e.g.,Cu, Au) into the substrate.

Tungsten, cannot be reduced alone, and requires the presence of certain inducing

elements (e.g., Ni, Co, Fe) exhibiting induced codeposition to form alloys of W. This

mechanism is not well understood. Electrodeposition of binary alloys of silver, such as

tin-silver, silver-nickel and electroless deposition of silver-tungsten alloys are potential

alternatives for lead-free materials in electronic packaging, printed circuit boards and

other electronic components. However, the electrodeposition mechanism of Ag-Ni-W

alloys has never been examined. Interest to electrodeposit all three elements, Ag-Ni-W

comes from the motivation to tailor desired properties, including the increase of Ag

hardness, corrosion resistance at high temperatures, while maintaining the favorable

electrical properties of pure Ag. The examination and fundamental understanding of such

a system will contribute towards developing a superior alloy with combined properties

(Ni-W and Ag)

The first report of the electrodeposition of a ternary Ag-Ni-W alloy is presented.

The addition of Ag was found to lower the deposition rate of Ni and W. Chemical

equilibria calculations were used to estimate the concentration of possible complexed

species present in the electrolyte within a pH range 2 to 8. The most dominant species of

Ag present were [AgTu4]+ and [AgTu3]+ irrespective of the pH, while NiHCit2 was the

dominant at lower pH, while NiCit2 was negligible below pH 3 and was found to increase

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with pH and was highest at pH 8. The concentration of Ni-Tu was found to be present

only at low pH and above pH 4 was negligible. Only one W-cit species was found to

present in the pH range 2-4 and was negligible over pH 4.

Ag-Ni-W nanowire deposition was attempted in polycarbonate templates with 50

nm diameter and length of 6 um. The nanowires we released by dissolution of the

membrane in dichloromethane. Transmission electron microscopy (TEM) showed that

the Ag-Ni-W nanowires had smooth morphology, however they were non-uniform in

length. The non-uniformity and weak strength of the nanowires is a strong function of the

deposition conditions and the electrolytic hydrogen production, due to the low pH which

is high in H+ ion concentrations. Understanding the reaction mechanism of the ternary

alloys deposition and increased pH can help improve the current efficiency during

deposition thereby resulting in alloys with different compositions suitable for a variety of

technological applications.

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1.0 INTRODUCTION

Tungsten alloys are well known for their outstanding properties, for example, Ni-

W alloys are useful catalysts for electrolytic hydrogen generation, W imparts superior

hardness and wear resistance to its alloys with Ni and Co, and W alloys have improved

corrosion resistance compared to their codeposited counterparts [1-20]. The

electrodeposition behavior of W alloys was coined by Brenner as “induced

codeposition”, as W ions cannot be electrodeposited by itself, but can be fully reduced if

codeposited with iron group elements, such as Ni, Co and Fe [21].

The recent advancement in multilevel interconnects technology is key for signal

routing in ultra-large scale integrated (ULSI) circuits. Typical metallic conductors

include an adhesion layer, a barrier layer, a conducting layer, a capping layer, and

possibly an antireflective coating. Currently, aluminum and copper are used for on-chip

interconnects. The integration of electroless technology in integrated circuit production

was reviewed by Shacham-Diamand et al. [22]. Barrier layers of cobalt-tungsten-

phosphorus (Co-W-P) and nickel-tungsten-phosphorus (Ni-W-P) layers were deposited

on silicon and silicon dioxide. The Co-W-P and Ni-W-P layers have higher hardness and

melting point than similarly deposited Co-P and Ni-P [23]. Therefore, films with tungsten

are expected to have better reliability and to act as better diffusion barriers for Cu

interconnects when compared to similar films without tungsten.

The excellent conductivity of silver and the relative simple procedure of its

electroless deposition is a motivation to produce stable and corrosion-resistant silver

films. Electroless silver plating baths are very unstable and thus short lived. Not at all

surprising is the fact that the electrolyte stability and plating rate are markedly affected by

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the pH [24]. Electrodeposition has the ability to better control the rate and achieve a

higher rate than electroless deposition, and very stable electrolytes with cyanide have

been the traditional norm. Over the years, many other electrolytes have been proposed,

such as the ones involving nitrate, iodide, thiourea, thiocyanate, sulfamate, and

thiosulfate. Thiourea is considered to be an effective chelating agent for Ag, and various

groups have investigated the deposition of Ag with thiourea [25-29].

Silver being a noble metal, there exists a large difference in the reduction

potential between Ni (-0.29 VSHE) and Ag (+0.799 VSHE), and a complexing agent is one

way to achieve deposits with different alloy compositions, as it lowers the difference in

reduction potentials. One advantage of thiourea, for deposition involving Ag and Ni from

the same electrolyte, is the specific complexation of thiourea with Ag compared to Ni

ions.

The present work is devoted towards the replacement of cyanide electrolytes and

to develop a single ternary stable electrolyte for the electrodeposition of Ag-Ni-W alloys

that has good properties and a non-polluting nature. This proposal will address the

electrodeposition parameters for Ag-Ni-W electrodeposition. As a first step in developing

a single electrolyte for the ternary alloy, Ag-Ni-W alloys have been fabricated to probe

the reaction mechanism and gain a better understanding for a variety of technological

applications.

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2.0 CRITICAL REVIEW

Ni-W alloys and Ag alloys with Ni or W can be fabricated by a variety of

methods such as sputtering, electron beam evaporation and electrodeposition. The

electrodeposition route can be cost effective compared to other techniques. In this chapter

a review of the literature relevant to the electrodeposition of Ni-W and Ag alloys, its

applications and the various advances towards tailoring its properties will be presented.

The electrodeposition of tungsten from aqueous solutions has been attempted since the

early 1930s. However, the electrodeposition of tungsten was found to be hindered by the

formation of an oxide layer, which could not be further reduced further [30]. Holt et al.

[31] showed that tungsten electrodeposition could be achieved in the presence of certain

metals (Co, Ni , Fe) with tungsten content as high as 50 wt %. This phenomenon is

known as “induced codeposition” and was first coined by Brenner in 1963 [21].

2.2 Electrodeposition of Ternary Ag-Ni-W Alloys

2.2.2 Interest in Ag-Ni Alloys

Ag-Ni alloys also exhibit remarkable catalytic properties [32], and are promising

candidates for electrical contacts and switches [33, 34]. However, the Ag- Ni deposition

system remains one of the least examined systems to date via electrodeposition synthesis

[28, 35-37]. According to the phase diagram [38], Ag and Ni are immiscible in bulk form

and even intermetallic phases were not observed at high temperatures. However, the

alloying effect at the nano-scale level is quite different from bulk, since the heat of

formation reduces with decreasing particle size and hence alloying these metals becomes

a possibility. Another aspect of the Ag-Ni system is the large difference in reduction

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potential Ag(+0.799 VSHE) and Ni (-0.25 VSHE) [28], an important feature of thiourea

especially while considering Ag-Ni deposition from the same electrolyte is that, while

thiourea binds strongly to Ag+ ions, it form a weak complex with Ni2+ ions. This selective

complexation becomes crucial to lower the large difference in redox potentials between.

Liang et al. [28] investigated the formation of metastable Ag-Ni solid solution

and their phase separation to elemental form upon thermal annealing. Excessive thiourea

(0.2 M) was used to stabilize the electrolyte, and act as a complexing agent to selectively

complex Ag (10 mM) and lower the reduction potential between Ag and Ni. The addition

of 0.2 M thiourea polarized the reduction potential of Ag from 0.03 V MSE to -0.650 V,

which indicates a strong complexing effect between thiourea and Ag, while the addition

of 0.2 M thiourea shifted the potential of Ni (0.15 M) from -1.04 V to -1.08 V due to a

weak complexing effect.

Eom et al. [35], examined the deposition of the alloy in sodium citrate

electrolytes. A shift in the Ag ion deposition potential to more negative values was

observed in the presence of citrate. They identified pH to be a dominant factor to control

the composition of the samples, as it affected the complexing of the metal ions with

citrate. At low current densities (0.5 mA/cm2), dendrite formation was observed with a

film composition of Ag60Ni40, and the microstructure transformed to a granular deposit

with increasing current density, which correspondingly increased the Ni content. The

dendrite formation was attributed to the low concentration of Ag ions in the electrolyte.

2.2.3 Interest in Ag-W Alloys

The application of silver for ultra large scale integration is promising due to its low bulk

resistivity (1.59 µΩcm), relatively high melting point and higher electromigration

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resistance compared to Cu, which is widely used in the down-scaling of interconnects.

Suitable conductivities were reported for Ag fabricated by sputtering [39, 40] and

electroplated thin films [41], while some drawbacks of Ag, such as corrosion in air and

diffusion in SiO2 , could be avoided by using a suitable binary alloy such as Ag-W

instead of pure Ag [42].

Electroless deposition was intensively adapted to examine Ag-W thin films [23,

42-50] . Shacham-Diamand et al. [23], examined and compared the effect of tungsten

concentration in the electrolyte, microstructure and morphology of Ag and Ag -W thin

films, and related their influence on the electrical properties. Thin films containing

tungsten with improved reliability and as effective diffusion barriers was first

demonstrated in Co-W-P and Ni-W-P thin films, which exhibited higher hardness and

melting points than films deposited without tungsten. Inberg et al. [48-50] examined the

Ag-W electroless deposition system extensively. They found that increasing the tungsten

concentration in the electrolyte decreased the deposition rate of Ag, resulting in smooth

and high quality Ag-W films. A maximum of 3.2 at % W was achieved when the [WO42-

]/ [Ag+] molar ratio was unity. Higher concentrations of tungsten in the electrolyte did

not increase the tungsten at % in the deposit.

Glickman et al. [45] examined the factors contributing to the resistivity of Ag-W

films and was able to achieve a considerable decrease in the resistivity for 100 and 50 nm

Ag-W films. They achieved a resistivity of 4.5 µohm-cm and 6.5 µohm-cm for 100 nm

and 50 nm films respectively for very low W at % (0.6 and 0.9). They reported grain

boundary scattering to be a dominant factor in controlling the electrical resistivity of sub-

100 nm films. Using post vacuum annealing, at low temperatures ~ 150 ºC for 1 hr, a

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considerable drop in resistivity could be achieved, while 75-80 % of the resistivity drop

was attained at annealing temperatures of 200-250 ºC for 1 hr.

In spite of the recent efforts in exploring Ag-W alloys as potential candidates for

electronic applications, information on the mechanism of Ag-W electrodeposition is very

scarce in the literature. A detailed understanding of the mechanism and the effect of one

element on the other is crucial.

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3.0 EXPERIMENTAL

A thiourea-citrate electrolyte was used for the electrodeposition of Ag-Ni-W

alloys. The electrolyte conditions and procedure, deposit characterization, and cell

designs are presented in this section.

3.1 Electrolyte, Conditions and Procedure

The composition of the electrolyte for the deposition of Ag-Ni-W alloys is listed

in Table 1. All electrolytes were prepared using de-ionized ultra-filtered (D.U.I.F) water

from Fischer Scientific. The pH of the electrolytes was measured maintained at a value of

2. Copper plates from ESPI metals were used as working electrodes and a rectangle piece

of Pt was used as the anode. All substrates were cleaned in dilute H2SO4 (10 vol %) in

order to remove any copper oxides and cleaned in D.I.U.F water before deposition. The

deposition was galvanostatically controlled using a Solartron potentiostat/function

generator model 1287A. For partial current density measurements, the current density

applied were 8 mA/cm2, 20 mA/cm2, 40 mA/cm2 and 80 mA/cm2 .The area in this case

was 0.39 cm2.

The Ag-Ni-W nanowires were electrodeposited, under the same deposition

conditions mentioned above, in a polycarbonate membrane having the smallest region of

the pore 50 nm in diameter and 6 µm in length. The nanowires were then dissolved in

dichloromethane to remove the supporting template and release the nanowires. The

dissolved nanowires were then subjected to centrifuge, and then dichloromethane was

replaced with fresh solution. This procedure was repeated 3 times. The deposit thickness

and composition was analyzed with a KEVEX Omicron energy dispersive X-ray

fluorescence analyzer (XRF), at 40 keV, 2 mA in air with an acquisition time of 60 sec.

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SEM analysis was done on a Hitachi S4800 at 3.0 KV and 4.5 K and 15.0 K

magnification. TEM images were taken on a JEOL, JEM 1010 at 80 KV.

Table 1. Electrolyte composition for Ag-Ni-W alloys.

Chemical Concentration (M)

Nickel Sulfate(M) 0.05

Sodium Tungstate (M) 0.015

Silver Sulfate 0.005

Sodium Citrate (M) 0.285

Thiourea (M) 0.650

Agitation of the electrolyte in the Hull cell is done by using air, bubbling close to

the surface of the cathode to ensure uniform mixing. A flow meter is used to monitor the

entering feed rate of air into the Hull cell. Five different flow rates, 1-5 L/min, were

examined. For the conventional Hull cell experiments, two agitation conditions were

examined, no agitation and 5 L/min. The primary purpose of any form of agitation to the

electrolyte is to eliminate or minimize concentration gradients near the electrode surface

to avoid mass transport limitations. In the case of Ag-Ni-W electrodeposition there is an

order of magnitude lower amount of Ag ions so that mass transport effects may be

considerable in that case. Air agitation can help then to control the boundary layer

thickness during deposition, which is a crucial factor when depositing alloys involving

diffusion limited species.

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4.0 RESULTS AND DISCUSSION

4.5 Electrodeposition of Ni-W with/without Ag

To examine the effect of Ag on the codeposition mechanism of Ni-W, partial

current density measurements were done on a Cu substrate in a parallel configuration.

Due to the instability of the electrolyte at pH 8, it was not examined further for this

electrolyte concentration and deposition parameters. Polarization curves of the Ni-W

electrolytes with and without Ag at an agitation rate of 5 L/min are shown in Figure 1.

The deposition potential of Ni-W begins at -0.77 V and rises rapidly with increasing

potential. The addition of Ag shows a clear variation in the deposition characteristics,

with a start in deposition at -0.55 V, which is an indication of Ag deposition, reaches it’s

limiting current density and then rises in current density around -0.88 V. We can also

observe a slight shift in the Ni-W reaction.

E Vs Ag/AgCl

0.4 0.5 0.6 0.7 0.8 0.9 1.0

i (m

A/c

m2 )

0

5

10

15

20

Figure 1. Polarization curves for Ag-Ni-W electrolyte with and without Ag (I).

NiW+ 5mMAg+

NiW only

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Galvanostatic deposition was done on a Cu substrate in a parallel configuration

for both the electrolytes at applied current densities of 8 mA/cm2, 20 mA/cm2, 40

mA/cm2 and 80 mA/cm2. Upon addition of Ag (I) into the electrolyte, as expected, Ag

rich deposits are obtained at current densities, 8 mA/cm2 and 20 mA/cm2. As the current

density increases, Ni composition increases in the deposit, due to the onset of Ni

deposition at higher current densities, which is also observed from the polarization

curves. However, the tungsten composition remains constant irrespective of the applied

current density. With the addition of Ag in the electrolyte, nodules formation can be seen

on the substrate and is uniform across the surface of the deposit (Figure 2 (a,b)), these

nodules tend to get bigger with an increase in current density (Figure 2 (c,d)). Table 2

shows the composition variation with Ag (I) ions in the electrolyte. At 80 mA/cm2, the

Ag composition drops drastically, indicating the onset of kinetic reduction of Ni.

Table 2. Composition of Ni, W and Ag.

Current density (mA/cm2) Ni wt % W wt % Ag wt %

8 0.7 0.2 99.0 20 4.1 0.3 95.4 40 7.3 1.0 91.6 80 35.7 0.5 63.7

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Figure 2. Optical images of Ag-Ni-W deposits at 8 mA/cm2, 20 mA/cm2, 40 mA/cm2 and 80 mA/cm2: (a), (b), (c), (d) respectively.

Along with the composition data we are able to calculate the partial current

densities of each species A semi-log plot (i vs EAgCl) is used because kinetic rates are

typically exponential with potential. In Figures 3 (a and b), the reaction rate of Ni and W

drop in the presence of Ag (I), however, also the deposition potential has been shifted to

more positive values. No deposit can be seen at the low current density end in the

absence of Ag (I), by physical observation of the samples. This thermodynamic shift in

potential indicates that even though the reaction rate slows down, the presence of Ag (I)

could in turn induce the deposition of Ni and W due to a more energetically favorable

(a) (b)

(c) (d)

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condition, such as a change in the activity of the solid state. In the case of Ni deposition

we observe a rise in Ni deposition rate at lower potential of -1.0 V, than observed in the

absence of Ag (I), whereas in the case of W we see a very flat profile in the presence of

Ag (I).

E vs. Ag/AgCl (V)0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

log

iW (m

A/c

m2 )

0.0

0.1

0.2

0.3

0.4

0.5

Figure 3. Partial current density, with and without Ag (I) for (a) Ni and (b) W.

E vs. Ag/AgCl (V)0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

log

iNi (

mA

/cm

2 )

0

2

4

6

8

10

12

14

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Figure 4 shows the side reaction partial current density. In the presence of Ag the

side reaction occurs at lower potentials -0.6 V, and consumes most of the current, thereby

indicating a low efficiency of the deposition process. The partial current density increases

drastically compared to the Ag (I). This increase in partial current density could hinder

the deposition of less noble Ni and WO42- ions.

E (V)

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

log

ij (m

A/c

m2 )

0

20

40

60

80

Figure 4. Partial current density of side reactions, with and without Ag (I).

4.3 Electrolyte Stability and Complex Species Distribution

The stability of the electrolyte at pH 2 was much higher compared to the

electrolyte at pH 8. However, the electrolyte at pH 2 would decompose over a period of

time (~ 6 hrs). The electrolyte at pH 8 would precipitate even during deposition, even

though the average current density was low 1.7 mA/cm2. This instability could be related

to different species forming at different pH. Equilibria calculations, help in determining

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the species distribution in an electrolyte, depending on their complex stability constants.

Table 3 shows the mass balance and equilibria calculations used to calculate the

distribution of the complexing species.

Table 3. Mass Balance and Equilibria Equations.

Species Stability constant Log K

Equations

Mass Balance Eq 𝑪𝑵𝒊 𝑪𝑵𝒊 + 𝑪𝑵𝒊𝑪𝒊𝒕 + 𝑪𝑵𝒊𝑯𝑪𝒊𝒕 + 𝑪𝑵𝒊𝑪𝒊𝒕𝟐 + 𝑪𝑵𝒊𝑯𝑪𝒊𝒕𝟐 + 𝑪𝑵𝒊𝑻𝒖 = 𝟎.𝟎𝟓

Mass Balance Eq 𝑪𝑪𝒊𝒕 𝑪𝑪𝒊𝒕 + 𝑪𝑵𝒊𝑪𝒊𝒕 + 𝑪𝑵𝒊𝑯𝑪𝒊𝒕 + 𝑪𝑵𝒊𝑪𝒊𝒕𝟐 + 𝑪𝑵𝒊𝑯𝑪𝒊𝒕𝟐 + 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯+ 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯𝟐 + 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯𝟑 − 𝟎.𝟐𝟖𝟓 = 𝟎

Mass Balance Eq 𝑪𝑾𝑶𝟒 𝑪𝑾𝑶𝟒 + 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯 + 𝑪𝑾𝑶𝟒𝑪𝒊𝒕𝑯𝟐 + 𝑪𝑾𝑶𝟒𝑪𝒊𝒕𝑯𝟑 − 𝟎.𝟎𝟏𝟓 = 𝟎

Mass Balance Eq 𝑪𝑻𝒖 𝑪𝑻𝒖 + 𝑪𝑵𝒊𝑻𝒖 − 𝟎.𝟔𝟓𝟎 = 𝟎

Mass Balance Eq 𝑪𝑨𝒈 𝑪𝑨𝒈 + 𝑪𝑨𝒈𝑻𝒖 + 𝑪𝑨𝒈𝑻𝒖𝟐 + 𝑪𝑨𝒈𝑻𝒖𝟑 + 𝑪𝑨𝒈𝑻𝒖𝟒 − 𝟎.𝟎𝟎𝟓 = 𝟎

Mass Balance Eq 𝑪𝑯 𝑪𝑯 − 𝟏𝟎𝒑𝑯 = 𝟎

Equilibria Eq 𝑪𝑵𝒊𝑪𝒊𝒕 5.35 𝟏𝟎𝟓.𝟑𝟓 × (𝑪𝑵𝒊 × 𝑪𝑪𝒊𝒕) − 𝑪𝑵𝒊𝑪𝒊𝒕 = 𝟎

Equilibria Eq 𝑪𝑵𝒊𝑯𝑪𝒊𝒕 9.13 𝟏𝟎𝟗.𝟏𝟑 × (𝑪𝑵𝒊 × 𝑪𝑪𝒊𝒕 × 𝑪𝑯) − 𝑪𝑵𝒊𝑯𝑪𝒊𝒕 = 𝟎

Equilibria Eq 𝑪𝑵𝒊𝑪𝒊𝒕𝟐 8.11 𝟏𝟎𝟖.𝟏𝟏 × �𝑪𝑵𝒊 × 𝑪𝑪𝒊𝒕𝟐� − 𝑪𝑵𝒊𝑪𝒊𝒕𝟐 = 𝟎

Equilibria Eq 𝑪𝑵𝒊𝑯𝑪𝒊𝒕𝟐 13.5 𝟏𝟎𝟏𝟑.𝟓 × �𝑪𝑵𝒊 × 𝑪𝑪𝒊𝒕𝟐 × 𝑪𝑯� − 𝑪𝑵𝒊𝑯𝑪𝒊𝒕𝟐 = 𝟎

Equilibria Eq 𝑪𝑵𝒊𝑻𝒖 1.47 𝟏𝟎𝟏.𝟒𝟕 × (𝑪𝑵𝒊 × 𝑪𝑻𝒖) − 𝑪𝑵𝒊𝑻𝒖𝟎

Equilibria Eq 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯 10.2 𝟏𝟎𝟏𝟎.𝟐 × �𝑪𝑾𝑶𝟒 × 𝑪𝑪𝒊𝒕 × 𝑪𝑯𝟐� − 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯 = 𝟎

Equilibria Eq 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯𝟐 17.03 𝟏𝟎𝟏𝟕.𝟎𝟓 × �𝑪𝑾𝑶𝟒 × 𝑪𝑪𝒊𝒕 × 𝑪𝑯𝟑� − 𝑪𝑾𝑶𝟒𝑪𝒊𝒕𝑯𝟐 = 𝟎

Equilibria Eq 𝑪𝑾𝑶𝟒𝑯𝑪𝒊𝒕𝑯𝟑 21.67 𝟏𝟎𝟐𝟏.𝟔𝟕 × �𝑪𝑾𝑶𝟒 × 𝑪𝑪𝒊𝒕 × 𝑪𝑯𝟒� − 𝑪𝑾𝑶𝟒𝑪𝒊𝒕𝑯𝟑 = 𝟎

Equilibria Eq 𝑪𝑨𝒈𝑻𝒖 7.11 𝟏𝟎𝟕.𝟏𝟏 × �𝑪𝑨𝒈 × 𝑪𝑻𝒖� − 𝑪𝑨𝒈𝑻𝒖 = 𝟎

Equilibria Eq 𝑪𝑨𝒈𝑻𝒖𝟐 10.61 𝟏𝟎𝟏𝟎.𝟔𝟏 × �𝑪𝑨𝒈 × 𝑪𝑻𝒖� − 𝑪𝑨𝒈𝑻𝒖𝟐 = 𝟎

Equilibria Eq 𝑪𝑨𝒈𝑻𝒖𝟑 12.73 𝟏𝟎𝟏𝟐.𝟕𝟑 × �𝑪𝑨𝒈 × 𝑪𝑻𝒖� − 𝑪𝑨𝒈𝑻𝒖𝟑 = 𝟎

Equilibria Eq 𝑪𝑨𝒈𝑻𝒖𝟒 13.57 𝟏𝟎𝟏𝟑.𝟓𝟕 × �𝑪𝑨𝒈 × 𝑪𝑻𝒖� − 𝑪𝑨𝒈𝑻𝒖𝟒 = 𝟎

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A plot of the distribution of the species in an electrolyte mentioned in table 1 is

shown in Figure 5. The entire Ag ions complex with thiourea to form [AgTu4] + and

remains constant irrespective of pH. The NiHCit2 complex (cit –citrate) is dominant at

pH 2 and reduces in concentration with increasing pH. An inverted trend is observed for

NiCit2 species which increases with pH and is dominant at pH 8. Different tungstate

citrate species are present at different pH. The WO4HCit2 is dominant at pH 2 and

reduces after pH 4. The WO4HCitH species increases from pH 3, reaches a peak at pH

5.5 and drops in concentration with increasing pH. The WO4CitH species increases from

pH 6 and is highest at pH 8. The inset of Figure 5 also shows that Ni-Tu complex is very

insignificant, indicating preferential complexation of thiourea with Ag ions. The stability

issue at pH 8 could be due to the different complexed species forming at a higher pH

which are unstable during deposition.

Figure 5. Species distribution of different complexed species in an Ag-Ni-W

electrolyte, inset shows the Ni-Tu complex species.

0

0.01

0.02

0.03

0.04

0.05

1 2 3 4 5 6 7 8 9

Con

cent

ratio

n (M

)

pH

NiCit2

NiHCit2

Wo4HCitH2 [1,1,2]

Wo4HCitH3 [1,1,3]

Wo4

AgTU4

NiCit2

WO4HCitH

2

NiHCit2

WO4HCitH

3

WO4

2-

AgTU

05E-091E-08

1.5E-082E-08

2.5E-083E-08

1 2 3 4 5 6 7 8

Con

cent

ratio

n (M

)

pH

N…

19

4.6 Electrodeposition of Nanowires

The development of nanowires into porous templates is a challenge in systems

where there is a substantial side reaction of gas evolution. If the side reaction is too

voluminous then gas bubbles can block the template pores and prevent deposition.

Guided by the conditions of the thin films fabricated in preceding sections a current

density was selected to deposit Ag-Ni-W nanowires, into the polycarbonate membranes.

Figure 6 shows the resulting Ag-Ni-W nanowires deposited under the same

current density of 1.7 mA/cm2 and then released from the template. From the thin film

results it is expected to have a composition that is Ag rich. What is notable is that the

nanowires have different lengths. Thus, they easily break when released from the

membrane. The longest length achievable after a deposition time of 1800 s was 5.0

microns long, which provides an estimate of the potential deposition rate (~0.0027

microns/s). Nanowires that are more robust are desired.

Figure 6. TEM of Ag-Ni-W alloy nanowires deposited at 1.7mA/cm2 for 1800 s into a PC membrane.

20

6.0 PROPOSAL

The electrodeposition of ternary alloy Ag-Ni-W system has a variety of

technological applications especially in the semiconductor industry. For these alloys to be

viable for large scale industrial production, few key limitations need to be addressed. As

mentioned earlier one goal of this project is to develop an environmentally friendly and

stable electrolyte for silver deposition. Thus far we have achieved this goal and deposited

Ag-rich Ni-W alloys from a stable, non-cyanide, pH 2 electrolyte. However, one

drawback of the low pH is the high concentration of H+ ions in the electrolyte which

increases the side reactions and lowers current efficiency. In order to address the issue of

low current efficiency and to deposit such alloys in deep recessed substrates, for example,

nanowires, a better understanding of the electrodeposition mechanism of Ag-Ni-W alloys

is necessary.

Analysis from the conventional Hull cell, in which agitation by air bubbling is

employed, does give us some information on the effect of Ag (I) addition to Ni and W

deposition rates. But, since Ag (I) is a noble metal species and is mostly under diffusion

limited control, better control over the boundary layer thickness is crucial to overcome

certain limitations such as, poor boundary layer control, the inconsistency due to the

variation in bubble flow pattern and operable limit (maximum flow rate 5 L/min).The

rotating cylinder setup gives a well-defined control over the hydrodynamic conditions

within the electrolyte system. For the next set of experiments, we propose to use the RCE

setup to examine the effect of Ag-Ni-W alloy deposition under different mixing

conditions, and use the complexation model to identify the optimum pH for the

deposition of Ag-Ni-W alloys. The effect of concentration of metal ions and different

additives in the electrolyte on the current efficiency will also be examined.

21

The following tasks will be focused on for future work:

AIM 1: Improve the overall current efficiency for practical applications.

Although, a pH 2 electrolyte was found to be stable for the electrodeposition of

Ag-Ni-W alloys over the examined concentration range, low pH electrolytes lead to very

low current efficiencies due to the high concentration of H+ ions in the electrolyte leading

to the following reaction below

2𝐻+ + 2𝑒− → 𝐻2

The hydrogen evolution reaction due to high H+ ion concentration can be dealt

with by increasing the pH. A look at the complexation model Figure (5), we can examine

the pH range we can focus on, since we have shown that pH 8 is not stable, and leads to

precipitation during electrodeposition. We notice different species present at pH 2 and pH

8, at pH 2 we know that WO4HCitH3 and NiHCit2 are stable while at pH 8 either NiCit2

or WO42- leads to instability. For this task electrodeposition of the Ag-Ni-W alloy at three

different pH (3, 4, 5) will be done and this will contribute towards answering this

question of which species causes the instability – NiCit2 or Wo42. A constant rotation rate

of 2825 rpm will be used at an applied current density of 1.7 mA/cm2 for 30 min.

Samples will be weighed before and after deposition, XRF analysis will be done to

measure the composition of the alloy, and calculate current efficiencies. Another

approach towards improving current efficiency is to increase the overall concentration of

the metal ions in the electrolyte. For this aim, concentration of each species will be

double to 0.1 M nickel sulfate, 0.01 M silver sulfate, 0.03 M sodium tungstate, 0.570 M

sodium citrate and 1.3 M thiourea. Three experiments at different current densities

1.7mA/cm2, 5 mA/cm2 and 10 mA/cm2 will be applied at a constant rotation rate of 2825

22

rpm. Samples will be weighed before and after deposition and XRF analysis will be used

to analyze the composition of the deposited alloy and calculate current efficiency.

AIM 2: Identify the deposition behavior of Ag induced Ni, W reduction.

Analysis from the partial current density experiments on the Hull cell indicated an

induced effect of Ag ions on Ni and W deposition rate. This is known as under potential

deposition (UPD) behavior of Ni and W species in the presence of Ag ions. The goal in

this aim is to establish if the reduction behavior between Ag ions is coupled with Ni and

W, i.e. if, the rate of Ni or W will increase with Ag deposition rate. Since Ni and W are

under kinetic control in the applied current density range examined, rotation rate should

not affect their deposition rate. If an increase in the deposition rate of Ni or W is

observed, along with an increase in Ag deposition rate, then this is purely due to a

coupled effect between Ag and Ni/W. For this task six current density experiments, 0.5

mA/cm2, 5 mA/cm2, 10 mA/cm2, 20 mA/cm2, 40 mA/cm2 and 80 mA/cm2 for 30 mins

each and at three different rotation rates 706, 1412 and 2824 rpm will be done. XRF will

be used to analyze the composition of the deposited alloys.

AIM 3: Effect of different additives on the deposition of Ag-Ni-W alloys.

The effect of different concentrations of additives, such as thiourea, boric acid,

sodium gluconate and citrate on the deposition of Ag-Ni-W alloys will be examined. One

approach to examine a large set of variables effectively is by using Factorial design

experiments. A 2k design for k =3 factors/variables will be considered. This design has 8

experiments, which enables us to examine the effect of individual variables (each at two

levels), and in addition three binary and one ternary interaction on one single parameter,

for example in our case wt % of W. A comparison of the values of each effect gives us an

23

idea on what parameters play a crucial role in obtaining a desired outcome. Thiourea (0,

0.650 M), boric Acid (0, 0.5M), sodium gluconate (0, 0.5 M) and citrate (0, 285 M) will

be considered for this study, using the optimum electrolyte for Ag-Ni-W after completion

of aim 1 and 2. Each sample will be weighed before and after deposition for calculating

the current efficiency, XRF will be used to analyze the composition of the alloy and SEM

will be done to examine the surface morphology of the deposits.

AIM 4: Investigate the growth mechanism of Ag-Ni-W nanowires.

Template synthesized nanowires have received a great deal of attention over the

past decade because they show great promise in a wide range of applications such as

electronics, sensing, drug delivery and fabrication of solar cells. The electrodeposition of

1D nanostructures such as, Ag nanowires are attractive for their superior electrical and

thermal conductivity.

The electrodeposition of Ag-Ni-W ternary alloy nanowires is a novel aspect to

this work. From preliminary experiments we were able to obtain Ag-Ni-W nanowires

under the deposition conditions examined, however due to the low pH the hydrogen side

reaction causes the nanowires to become brittle and break during release. Results from

aim 1-3 will help towards solving the issue of low current efficiency and develop more

robust Ag-Ni-W nanowires.

24

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