Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
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Journal of the Taiwan Institute of Chemical Engineers
journal homepage: www.journals.elsevier.com/journal-of-the-taiwan-institute-of-chemical-engineers
Combined fluid flow simulation with electrochemical measurement for
mechanism investigation of high-rate Cu pattern electroplating
Qing Wang a, Zhe Wang a, Yongtong Wang a, Yao Tong a, Mingxiang Chen *, a, b
a
b
School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fluid flow simulation
Cu pattern
High-rate electroplating
Additive adsorption
Background: Cu pattern electroplating plays an essential role in the interconnection and microstructure fabri­
cation of high-power device packaging. However, improving efficiency while ensuring the preparation quality is
an urgent breakthrough direction.
Methods: A new electrolyte formula suitable for the high-rate Cu pattern electroplating was put forward,
including 300 mg/L polyethylene glycol (PEG, molecular weight=8000) and 6 mg/L thiazolinyl polydipropyl
sulfonate (SH110). The mechanism of high-rate Cu pattern electroplating was investigated by the combination of
fluid flow simulation and the electrochemical measurements including galvanostatic measurements (GMs) and
cyclic voltametric (CV) tests. The Cu pattern was electroplated in the Haring-Cell with air bubble as the forced
convection method.
Significant findings: The simulation results indicate that the flow rate on the ring-shaped cathodic pattern surface
gradually increases with the Cu growing up, which also increase from the pattern surface to the bulk electrolyte.
The GMs results suggest that the cathodic polarization of the convection-dependent additives raises with the
current density, which is strengthened by strong convection with current density lower than 6 ASD but have the
opposite effect with current density larger than 8 ASD. The CV tests indicating that the aggregation of weak
convection and strong polarization promote the antagonistic effect between thiolate of SH110 and PEG, the
nucleation and the electroplating rate. Nevertheless, the synergistic effect of thiazoline of SH110 and PEG are
enhanced with strong convection and weak polarization. The adsorption model during the Cu pattern electro­
plating process is established and verified by electroplating experiment with the optimum current density of 8
ASD. The Cu pattern is mirror bright with the high electroplating rate of over 90 μm/h.
1. Introduction
There is increasing demand for high current density and ultra­
stability power devices for 5 G communication [1], material processing
[2], and lighting industrial sectors [3]. The power devices are required
in a hostile environment, including high temperature, humidity, and
electromagnetic interference, so they face specific reliability challenges
[4]. Excellent property materials are promoted, such as Al2O3 ceramic
substrate to achieve high thermal conductivity [5], Cu microstructure to
enhance the electromigration tolerance [6], and to implement special
functions. Direct plated Cu (DPC) ceramic substrate [7] brings signifi­
cant advantages by combining the two materials through the Cu pattern
electroplating process [8]. The pattern electroplating process contains
two successive parts of patter transfer [9] and Cu electroplating [10].
The negative dry film has been widely used in the patten transfer process
with excellent adhesion, outstanding thickness uniformity, low cost, and
short processing time [11]. It is laminated to sputtered Cu surface,
covered by the film with the microcircuit pattern, then exposed to ul­
traviolet, and developed to define areas for pattern Cu electroplating
[12]. However, variations in pattern size and dry film thickness change
the mass transfer condition [13], further increasing the difficulty of Cu
electroplating and reducing the electroplating rate [14].
The Cu electroplating rate is controlled by the current density [15]
stated by faraday’s law [16]. Still, the electrolyte of a specific formu­
lation has its limiting current density (Ilim) [17], which is influenced by
the main salt concentration of the virgin make-up solution (VMS) [18],
the type and concentration of additives [19], and mass transfer condi­
tion (convection intensity) [20]. An excessive increase in the current
* Corresponding author.
E-mail address: chimish@hust.edu.cn (M. Chen).
https://doi.org/10.1016/j.jtice.2022.104528
Received 18 July 2022; Received in revised form 17 August 2022; Accepted 14 September 2022
Available online 25 September 2022
1876-1070/© 2022 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Q. Wang et al.
Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
density to achieve high-rate electroplating will reduce the Cu layer
properties, resulting in rough or even nodulate the Cu layer surface and
a decline in physicochemical properties such as hydrogen embrittlement
or corrosion resistance reduction [21,22]. However, raising the main
salt concentration and adjusting the electrolyte formula is verified to be
an effective method to raise the Ilim [23]. In general, three additives,
classified by their functions, are applied to the Cu electroplating process.
The accelerator contains sulfur in the molecular structure, which can
increase the electroplating rate and promote nucleation [24–26]. The
suppressor is a high molecular organic substance that effectively im­
proves the wettability of the surface and the cathodic polarization
[27–29]. The leveler’s functional group, including nitrogen, can
improve the current distribution uniformity and flat the Cu layer [30].
Besides, there is also a class of additives that contains two functional
groups with opposite effects simultaneously, which varies with process
parameters such as concentration, convection, or current density [28,
31–34]. The additives produce marked effects by adsorbing on the
cathodic surface [35,36]. Most additives have complex interactions
[37–39], and only a tiny amount of the additives can significantly alter
the electroplating process. Several electrochemical measurements based
on the rotation disk electrode (RDE) method are implemented to char­
acterize the additives’ function and interaction mechanism effectively
[40,41].
Moreover, most additives are sensitive to convection, and their
adsorption characters and interactions are also affected [42,43]. Suit­
able convection is conducive for the additives to take effect and
ameliorate mass transfer conditions, which is vital to improving the Cu
layer quality [44]. Actually, the convection at the pattern bottom cannot
be obtained by direct measurement, but the flow rate simulation is an
effective method [33]. The impact of pattern size on the flow rate can be
systematically explored by establishing the simulation model according
to the actual electroplating process parameters, which have been
applied to micro-features like micro through holes and have achieved
good results. Researches have been carried out to raise the electroplating
rate by changing additives [45] and adding complex auxiliary processes
such as ultrasound [46]. However, the high-rate pattern electroplating
technique (> 90 μm/h) by a simple auxiliary process like an air bubble is
still lacking. No study has been reported to directly observe the flow rate
distribution in a pattern. Accordingly, urgent breakthroughs are needed
in the technology to electroplate Cu patterns with excellent properties at
a high rate and in the according mechanism of additives.
Herein, we proposed an electrolyte formula suitable for high current
density that can achieve ultra-high electroplating rates over 90 μm/h. By
combining the fluid flow simulation with the electrochemical mea­
surement, the variation of the flow field with the pattern size and the
corresponding convection-dependent mechanism of additives during
high-rate Cu pattern electroplating were investigated. The additives’
adsorption model was established and was further verified by the Cu
pattern electroplating experiments. The Cu pattern electroplating model
was based on actual parameters of the Haring-Cell and the patterns with
air bubbles as the forced convection method to explore the flow field
distributions that vary with the pattern sizes. Multiple rotation speeds
and cathodic potential range were applied to the RDE methods to
examine the influence of convection and current density on the addi­
tives’ electrochemical reaction and the electroplating amount. The
electroplating rate and the Cu layer properties were analyzed and
compared with the fluid flow simulation and electrochemical results.
The optimum current density was proposed for the electrolyte formula,
and the mechanism of high current density to improve the Cu layer
properties was pointed out.
Fig. 1. The Haring-Cell Cu pattern electroplating system: (a) structure diagram,
(b) fluid flow simulation model.
with the dimension of 250 mm × 60 mm × 100 mm and an electro­
lyte volume of 1.5 L (Fig. 1(a)). The cathode contained ring-shaped
patterns, the difference between inner and outer diameter was 1 mm,
and the dry film thickness was 300 μm. The phosphor copper anodes
were attached to the two sides of the Haring-Cell, which size were 150
mm × 60 mm × 3 mm. The air bubbles came into the electrolyte through
the air vents at the bottom with a flow of 0.6 L/min. The simulation
model was established based on the above parameters, as shown in Fig. 1
(b). The air inlets included 20 vents at the bottom and were described by
wall condition. In contrast, the air outlet was the electrolyte’s upper
surface, approximated as a slip condition, and the air exit freely at this
boundary. Other boundaries were presented by wall conditions for the
solution with no air flux. The fluid flow simulation was conducted to
investigate the convection change with the pattern size to guide the
following electrochemical experiments.
2.2. Electrochemical measurements
The electrochemical measurements were carried out in the threeelectrodes system (CS2350H, Corrtest) with the test electrolyte 100
mL at 25 ◦ C. The electrodes contained the platinum rotating disk elec­
trode (Pt-RDE) as the working electrode (WE), the platinum sheet
electrode with an area of 20 mm × 20 mm as the counter electrode (CE),
and the mercurous sulfate electrode (MSE) as the reference electrode
(RE), respectively. The VMS herein included 120 g/L CuSO4 (AR,
Sinopharm Chemical Reagent Co), 60 g/L H2SO4 (AR, Sinopharm
Chemical Reagent Co), and 50 mg/L chloride ion provided by NaCl (AR,
Sinopharm Chemical Reagent Co). A higher concentration of the main
salt was beneficial to increase the Ilim. The chosen additives were 6 mg/L
thiazolinyl polydipropyl sulfonate (SH110, AR, WengJiang reagent) and
2. Modeling and experiments
2.1. Modeling
The electroplating experiments were carried out in the Haring-Cell
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Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
Fig. 2. The process of Cu pattern electroplating on the ceramic substrate.
300 mg/L polyethylene glycol (PEG, molecular weight=8000, AR,
Macklin regent).
The galvanostatic measurements (GMs) were implemented to
explore the cathodic polarization of additives at various cathodic cur­
rent densities and convection. The current densities had a wide range,
including 2 A/dm2 (ASD), 4 ASD, 6 ASD, 8 ASD, 12 ASD, and 16 ASD.
The convection was controlled by the Pt-RDE rotation speed. 1000 rpm
(high speed) stood for strong convection, while 100 rpm stood for weak.
After that, the cathodic polarization potentials were recorded and
applied in the cyclic voltammetry (CV) measurements, which were
carried out within five cycles from 1 V vs. MSE to the minimum cathodic
polarization potential, followed by the scan in the opposite direction
within the same range. Each CV measurement was cycled with a scan
rate of 0.1 V/s. The purpose was to explore the influence of cathodic
polarization on the Cu electroplating processes and amount, the
adsorption characteristics of additives, and the corresponding
mechanism.
sputtered Cu surface was put into the electroplating electrolyte after
pickling (5% H2SO4) to remove the oil and oxide layer on the surface,
micro etching (5% H2SO4 + 50 g/L Na2S2O8) to increase the surface
roughness, and activation (12% H2SO4) to keep surface fresh. The same
electrolyte and current densities as the electrochemical measurements
were used for the electroplating experiments. The Cu layer properties
under multiple electroplating parameters were examined by the scan­
ning electron microscope (SEM, GeminiSEM300, Zeiss) and the atomic
force microscope (AFM, SPM9700, Shimadzu) in normal mode at 25 ◦ C.
The electroplated Cu layer thickness and the outer profile were
admeasured by a step profiler (DektakXT, Bruker). The corresponding
electroplating rate was calculated by dividing the thickness by the
electroplating time.
2.3. Pattern electroplating
The fluid flow in Fig. 3(a) is symmetrically distributed at y = 125
mm, and the high flow rate zones alternate with the low flow rate ones.
The high flow rate zones are near the air bubble vents (near both sides of
the cathode and two anodes). The maximum rate appears at a certain
height from the bubble vents due to the decreasing liquid pressure
received by the air bubbles during ascending, resulting in increased air
bubble volume and electrolyte disturbance. Significantly, in the
enlarged view of the pattern cross-section on the cathode (Fig. 3(b)), the
pattern influences the flow rate distribution and forms a symmetrically
distributed semicircular-shaped affected zone with the flow rate
decreasing sharply.
Moreover, the relative height of the pattern and the dry film also
3. Discussion
3.1. Fluid flow simulation
The typical pretreated flow for the ceramic substrate before the
pattern electroplating experiment was as follows (Fig. 2). The ceramic
substrate was first cleaned by the deionized water and then sputtered by
titanium (Ti) as the intermediate layer to improve the adhesion and Cu
as a seed layer to conduct current. The negative dry film with a certain
thickness was laminated on the sputtered Cu surface and was covered by
the film. Afterward, the substrate was exposed to ultraviolet and
developed by 1% Na2CO3 (AR, Sinopharm Chemical Reagent Co) to
transfer the pattern from the film to the dry film. In this way, the surfaces
to be plated were exposed, while the dry film protected other parts. The
Fig. 3. Flow field distribution: (a) of the Haring-Cell, (b) enlarged view around the Cu pattern.
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Fig. 4. Fluid flow rate distribution with 300 μm thick dry film, the Cu pattern thicknesses are: (a) 0 μm, (b) 100 μm, (c) 200 μm, and (d) 400 μm. The highest and
lowest flow rates on the pattern surface with different thickness are summarized in Fig. 4(e).
other is mirror-symmetrical. Firstly, the pattern shape leads to the un­
even distribution of the flow field on the pattern surface. As shown in
Fig. 4(a-d), the flow rate is lower on the left side of the pattern while
larger on the right side. The detailly explored liquid is surrounded by the
dry film and the Cu pattern upper surface. The liquid flow lines are
exhibited in Fig. 5. The flow lines’ characteristics are close, which
appear as the concave arc of lines that point from the inner ring to the
outer ring. The minimum and maximum flow rates are located in the
lower-left and upper-right corners of the liquid. Secondly, the pattern
thickness is lower than the dry film in Fig. 4(a-c), and the flow rate on
the Cu pattern upper surface increases (more evident in Fig. 5) while the
affected zone area reduces with the thickness. Thirdly, the Cu pattern
height in Fig. 4(d) exceeds the dry film thickness, the affected area
moves inside the inner ring of the pattern, and the flow rate at the left
corner decrease. In addition, the corresponding flow rate is recorded in
Fig. 4(e), with the lowest flow rate and the highest flow rate on the
pattern surface located at the left and right corners, respectively. The
highest and lowest flow rates keep increasing with the Cu pattern
thickness lower than the dry film, while the lowest flow rate slightly
decreases with the pattern height exceeds the dry film. The pattern
protruding from the dry film leads to a sharp decrease in the liquid flow
Fig. 5. Flow line distribution in the pattern with the liquid height of: (a) 300
μm, (b) 200 μm, and (c) 100 μm.
affects the flow rate distribution. Fig. 4 simulates the flow rate change
during the Cu pattern electroplating process with four typical pattern
thicknesses, which are 0 μm, 100 μm, 200 μm, and 400 μm, respectively.
Since two cross-sections with symmetrical distribution can be obtained
in Fig. 3(b), only one side is selected for detailed exploration, and the
Fig. 6. GMs measurements of the electrolyte with multiple current density: (a) 2 ASD ~ 6 ASD, and (b) 8 ASD ~ 16ASD.
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Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
Fig. 7. CV measurements of 300 mg/L PEG under multiple RDE rotation speed with the lowest polarization potential of: (a) − 0.65 V vs. MSE, (b) − 0.7 V vs. MSE, and
(c) − 0.75 V vs. MSE. The values of Q are summarized in (d).
rate within the inner ring of the ring-shaped pattern. Therefore, the Cu
pattern electroplating time should generally be controlled so that the Cu
layer thickness does not exceed the dry film thickness.
deterioration of the electroplating environment under high current,
which can be manifested by the curve vibrating violently with 16 ASD
and the RDE rotation speed of 100 rpm (weak convection). Therefore,
the electroplating current density should not be too large, and the cur­
rent density of less than 8 ASD is a suitable range.
The CV curves are used to further explore the effects of cathodic
potential on the additives’ action and the electroplating processes. Based
on the GMs results, the polarization potential ranges from − 0.64 V vs.
MSE to − 0.75 V vs. MSE, with the current density rising from 2 ASD to 8
ASD. Therefore, the CV measurements are divided into three groups
with the lowest potential of − 0.65 V vs. MSE, − 0.7 V vs. MSE, and
− 0.75 V vs. MSE, respectively. The high-rate Cu pattern electroplating
electrolyte contains two additives. Their interaction under appropriate
polarization potential and convection intensity are vital to realizing a
high electroplating rate. What’s more, the additives’ complex syner­
gistic or antagonistic mechanisms are affected by the characteristics of
each additive — the cathodic potential and the convection also control
their adsorption characteristics and responses. Accordingly, the effect of
a single additive should be explored first to investigate the interaction
mechanism of the two additives.
The CV curves of 300 mg/L PEG are depicted in Fig. 7. The Cu
electroplating process and Cu deposition amount are affected by the
cathodic potential range and the convection. The integrated values of
3.2. Mechanism of the additives
The pattern height change causes variation in the fluid flow field.
Since the additives are usually sensitive to convection, the shift in the
flow field thus may bring about the alteration in the additives’ effect.
Furthermore, the cathodic polarization of electroplating electrolytes
increases with the current density, which is also affected by convection.
As shown in Fig. 6(a), the strong convection (1000 rpm) enhances the
cathodic polarization with the current density of 2 ASD ~ 6 ASD. The
potential differences (Δη) between the weak and strong convection
decrease with the current density, which are 30 mV, 15 mV, and 12 mV,
respectively. The results suggest that the weak convection is conducive
to Cu electroplating with a current density lower than 6 ASD. Never­
theless, the cathodic potential of the current densities larger than 8 ASD
presents the opposite trend in Fig. 6(b). The weak convection promotes
the cathodic polarization with the negative Δη decreasing with the
current density, which suggests that the electrochemical polarization is
mainly concentration polarization. In addition, the concentration po­
larization increases with the current density, leading to rapid
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Fig. 8. CV measurements of 6 mg/L SH110 under multiple RDE rotation speed with the lowest polarization potential of: (a) − 0.65 V vs. MSE, (b) − 0.7 V vs. MSE, and
(c) − 0.75 V vs. MSE. The values of Q are summarized in (d).
the Cu stripping peak area in VMS with the tested additive without or
with convection are recorded as Q and Q0, respectively. Above all, the
stripping peak area decreases with the RDE rotation speed in Fig. 7(a-c),
suggesting the convection-enhanced suppression effect. After that, the
stripping peak area increases with the potential range, indicating that
more negative potential helps to increase the Cu electroplating amount,
which does not change linearly with the increase of potential in Fig. 7
(d). Besides, the reduction processes are simultaneously affected. There
is no intersection with the minimum cathodic potential of − 0.65 V vs.
MSE (inset in Fig. 7(a)) that highlights the irreversible process, but one
intersection with more negative cathodic potential (insets in Fig. 7(b-c))
shows the promotion in the nucleation [47]. The cathodic potential at
the intersection (E) is the equilibrium potential of the metallic ion/metal
system, Ea ~ Ee shift negatively with the strengthened convection (Fig. 7
(b-c)), showing the enhanced inhibition effect of PEG from the other side
[33]. Moreover, the cathodic current density increases evidently with a
potential over − 0.55 V vs. MSE. This phenomenon is because the oxygen
atoms in the PEG molecule form coordination bonds with cupric ions to
adsorb on the cathode surface, which desorbs with potential more
negative than − 0.55 V vs. MSE so that the inhibition effect on Cu
electroplating is weakened [39]. In a word, high electroplating current
density and weak convection suppress the inhibition effect of PEG and
are conducive to the high-rate Cu electroplating.
The results of SH110 with the same CV test parameters are summa­
rized in Fig 8. In Fig. 8 (a-c), the Cu stripping peak areas are much larger
than those of PEG, which also increase with the cathodic potential range
while first increasing and then decreasing with the convection (Fig. 8(bc)). This manifesting SH110 presents the acceleration effect during Cu
electroplating, which can be intensified with weak convection (100
rpm). In addition, the notable difference is the reduction process which
also changes with the test parameters. There are no intersections in the
CV curves during the reduction processes with an RDE rotation speed
lower than 100 rpm in Fig. 8(a-c), and two or more intersections appear
with higher rotation speeds. Indicating the nucleation is promoted with
more negative cathodic potential (Fig. 8(c)) and strength convection (>
500 rpm). In addition, the starting deposition potential keeps decreasing
with the RDE rotation speed, predicting the convection-suppressed ac­
celeration effect. Furthermore, the Q values in Fig. 8(d) reveal the
nonlinearly increased Cu stripping peak area with the negatively moved
minimum cathodic potential. Accordingly, same as PEG, the more
negative cathodic potential and weak convection promote the Cu elec­
troplating rate.
The CV measurements of 300 mg/L PEG + 6 mg/L SH110 are carried
out on the basis of single additive measurements. The cathode and anode
processes in Fig. 9 change significantly compared with those of a single
additive. Above all, the Cu stripping peak area increases firstly and then
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Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
Fig. 9. CV measurements of 300 mg/L PEG + 6 mg/L SH110 under multiple RDE rotation speed with the lowest polarization potential of: (a) − 0.65 V vs. MSE, (b)
− 0.7 V vs. MSE, and (c) − 0.75 V vs. MSE. The values of Q are summarized in (d).
decreases with the rising RDE rotation speed in Fig. 9(a-c), which can be
seen more clearly from the summarized Q values in Fig. 9(d). The ac­
celeration effect of weak convection on Cu electroplating is more sig­
nificant than that of a single additive, which is enhanced by the
interaction of the two additives. Besides, the reduction processes are
more changeable with two additives in the electrolyte. No intersections
exist in the CV curves with lower minimum cathodic potential (Fig. 9
(a)). The change in RDE rotation speed only brings about the variation in
cathodic current density, which has the same trend as the Q value and
confirms that weak convection is helpful to Cu electroplating. In­
tersections begin to appear with the decreasing minimum cathode po­
tential, and the shape of the curve also changes with convection
simultaneously. Therefore, higher cathodic polarization potential pro­
motes nucleation, and the cathode curves can be divided into two cat­
egories according to the approximate shape in Fig. 9(b-c). One is the CV
curves under weak convection (0 rpm, 100 rpm), with more negative
cathodic potentials at the intersections. The other is the ones with strong
convection (500 rpm ~ 2000 rpm) with the intersection potential more
positive. The results suggesting the two kinds of cathode processes are
controlled by different additives, and competitive adsorption is observed
between the two additives. Furthermore, the starting deposition po­
tential dwindled with the convection, which has the same trend as
SH110. Consequently, the two additives have the characteristics of
competitive adsorption, and their interactions change with electro­
plating parameters. An additive may play a major role in dominating the
electroplating process with specific parameters, so it is essential to delve
into the mechanism.
The complex interactions between the two additives are explored
with multiple convection and cathodic potential ranges. As shown in
Fig. 10, the SH110 molecule contains both thiazoline which presents the
inhibition effect, and thiolate showing the acceleration effect. Thus,
there may be a synergistic and antagonistic effect between PEG and
SH110. In Fig. 10(a-c), the CV curve shape of PEG + SH110 at RDE 0 rpm
resembles that of PEG, indicating that the cathodic surface is covered by
a large amount of PEG and a trace amount of SH110 without convection.
The Cu electroplating is thus inhibited, and the process is mainly
controlled by PEG. Nevertheless, the CV curve shape during the reduc­
tion processes changes markedly with strong convection, demonstrating
the process is managed by PEG and SH110. Moreover, the interaction
between SH110 and PEG can be seen obviously from the Cu stripping
peak area ratio between PEG and PEG + SH110 in Fig. 10(d). The value
1 is taken as the dividing point. The two additives show a synergistic
effect with a value less than 1 while an antagonistic effect with a value
more than 1. The lowest values are all located in the middle part with
weak convection (100 rpm ~ 500 rpm), indicating the intensified ac­
celeration effect of SH110.
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Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
Fig. 10. CV curves of the additives with the minimum polarization potential of: (a) − 0.65 V vs. MSE, (b) − 0.7 V vs. MSE, and (c) − 0.75 V vs. MSE. The Cu stripping
peak area ratio of PEG and PEG + SH110 are summarized in (d).
In Fig. 10(a), the competitive adsorption of PEG and SH110 are
influenced by convection. A small portion of the PEG adsorption sites are
replaced by thiolate of SH110 with weak convection. An antagonistic
effect exists between PEG and SH110 with an RDE rotation speed of 100
rpm ~ 500 rpm (Fig. 10(d)), leading to the accelerated Cu electroplating
process. However, the increasing RDE rotation speed enhances the in­
hibition effect of PEG (1000 rpm ~ 2000 rpm), and PEG and the thia­
zoline of SH110 exhibite a synergistic effect making the inhibition effect
more remarkable than that of PEG alone (Fig. (d)). Unlike the trend in
Fig. 10(a), when the minimum cathodic polarization potential decrease
to − 0.7 V vs. MSE in Fig. 10(b, d), SH110 and PEG first present an
antagonistic effect with no convection (0 rpm) or not too strong con­
vection (100 rpm ~ 1000 rpm) to slightly accelerate Cu electroplating,
and then possessing synergistic effect with strong convection (2000
rpm) to inhibit Cu electroplating. In addition, the acceleration effect of
SH110 is strengthened by the negatively shift cathodic potential to
− 0.75 V vs. MSE in Fig. 10(c) since the Cu stripping peak area ratio in
Fig. 10(d) are all less than 1. Consequently, the weak cathodic polari­
zation and strong convection improve the inhibition effect of PEG and
the synergistic effect between PEG and thiazoline of SH110, while the
strong cathodic polarization and weak convection enhance the accel­
eration effect of SH110 and the antagonistic effect between PEG and
thiolate of SH110.
The adsorption models of the additives during the Cu pattern elec­
troplating process based on the fluid flow simulation results and the
Fig. 11. Adsorption model of the additives during Cu pattern electro­
plating process.
electrochemical measurements are shown in Fig. 11. The convection is
enhanced with the increasing Cu layer thickness, and the convection at
the bottom left corner is slightly weaker than that at the right corner due
to the pattern shape. The thiolate of SH110 tends to be adsorbed in the
weak convection zone, while the thiazoline of SH110 combines with
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experiments is the same as the electrochemical environments, including
2 ASD, 4 ASD, 6 ASD, 8 ASD, 12 ASD, and 16 ASD. The Cu electroplating
rates and the corresponding micromorphologies are classified into three
categories in Fig. 12. The first is the uniform but not bright surface
obtained by a current density range of 2 ASD ~ 4 ASD, and the elec­
troplating rates are relatively low (21.2 μm/h ~ 22.9 μm/h). The weak
cathode polarization (Fig. 6(a)) and no obvious nucleation process
(Fig. 9(a)) shown in the electrochemical measurements rough the Cu
pattern surface. The second is the uniform and bright surface produced
by current density between 6 ASD and 8 ASD. The increase in the
cathodic polarization potential is helpful to the grain refinement and
nucleation (Fig. 9(b-c)), which are the characteristic quantities of the Cu
layer surface brightness and smoothness. Their surface roughness values
are 61.95 nm and 67.04 nm, tested by AFM (Fig. 13). Notably, the
electroplating rate is also raised remarkably to 68.8 μm/h ~ 92.2 μm/h.
The third is the defective Cu layer which nodulates at the edge. The
strong concentration polarization leads to unstable deposition of the Cu
layer (Fig. 6(b)). The edge effect causes a large number of Cu deposits at
the cathode edge, with the electroplating rate at the pattern center
reduced to 42.0 μm/h ~ 51.4 μm/h. In a word, the best current density
applied to the electrolyte containing 300 mg/L PEG and 6 mg/L SH110
is 8 ASD through the observation of the electrochemical measurements,
the electroplating rate, and the micromorphology.
Since 8 ASD is considered the best current density for high-rate Cu
pattern electroplating, the Cu layer profiles, cross-section view, and the
corresponding electroplating rate of the Cu pattern are examined after
electroplated for 10 min, 45 min, and 2 h, respectively. As shown in
Fig. 14, it is certainly a welcome result that the Cu layer shape and the
electroplating rate are consistent with the model in Fig. 11. The Cu layer
thickness after 10 min of pattern electroplating is low in the middle
while high on both ends with the high-rate of 101.4 μm/h. In addition,
the Cu deposition on the edge is inhibited as the layer continues to grow,
the thickness uniformity of the Cu layer is improved with the electro­
plating rate slightly decrease to 92.7 μm/h in 45 min. Since the con­
vection in the lower left corner of the figure is always weaker than that
in the lower right corner (Fig. 4(a-c)), the Cu layer on the left corner is
thicker than that on the right under the cumulative effect of 2 h elec­
troplating process, and the electroplating rate continuously decrease to
91.5 μm/h. In sum, the electrolyte can achieve the high-rate (> 90 μm/
h) electroplating under the current density of 8 ASD, and the Cu layer
has excellent surface quality, including dense, uniform and bright.
Fig. 12. Effect of current density on the electroplating rate and the corre­
sponding micromorphology.
PEG to raise the inhibition effect in the strong convection zone. In
Fig. 11(a), the fluid convection in the Cu pattern is limited by the dry
film, especially at the bottom corners. The thiolates are preferable to
adsorb at the corner, and the combination of PEG and thiazoline are
more likely to adsorb in the middle. And then, the Cu at the bottom
corners of the pattern is electroplated at a relatively high rate, forming a
Cu layer shape with low middle and high sides thickness in Fig. 11(b).
Additionally, the convection is strengthened simultaneously with the
growing Cu pattern. The bulges on both sides are also prone to charge
concentration, leading to more adsorption of thiazoline and PEG and less
thiolate. Therefore, the Cu electroplating rate is probably to decrease,
especially at the corners, to obtain a Cu layer with relatively uniform
thickness (Fig. 11(c)). Under this circumstance, the thickness of the Cu
layer and the dry film is close. More PEG and fewer SH110 are evenly
distributed on the Cu surface with the drop in the Cu electroplating rate.
3.3. Cu pattern electroplating
The current density used in the Cu pattern electroplating
Fig. 13. AFM image of the Cu pattern surface with high current density of: (a) 6 ASD, and (b) 8ASD.
9
Q. Wang et al.
Journal of the Taiwan Institute of Chemical Engineers 139 (2022) 104528
Fig. 14. Cu pattern layer under different electroplating time with the current density of 8 ASD, (a) outer profile, and (b) – (d) cross-section view.
Conclusion
References
The high-rate Cu pattern electroplating mechanism was investigated
by the combination of fluid flow simulation and electrochemical mea­
surements. The simulation results shed light on the influence of the
relative height of the pattern and the dry film on the fluid flow distri­
bution. The electrolyte flow rate on the cathodic pattern surface grad­
ually increased with the copper pattern thickness during the
electroplating process due to the surface approach to the bulk electrolyte
by degrees. On this foundation, the electrochemical measurements
illustrated that the mechanism of high-rate copper electroplating
depended on current density and convection. A larger current density
brought about stronger cathodic polarization and nucleation promotion.
Weak convection and large cathodic polarization were conducive to the
antagonistic effect between the thiolate of SH110 and PEG to promote
Cu electroplating. In contrast, strong convection and small polarization
potential stimulate thiazoline of SH110 and PEG to inhibit copper
deposition synergistically. Notably, the time-dependent additives’
adsorption model during the Cu pattern electroplating process under
high current density was proposed, which predicted the gradually sup­
pressed copper pattern electroplating rate during the process. The model
was verified by electroplating experiments under the optimized current
density of 8 ASD. The results demonstrated that a high-quality surface
and high electroplating rate (> 90 μm/h) were obtained simultaneously,
providing the bases for efficient manufacturing copper patterns for
power devices.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work in this paper.
Acknowledgment
This work was supported by the Aerospace Science and technology
Group Application Innovation Program (No: 09428ADA), Key Research
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China
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