Direct Copper Pattern Plating on Glass and Ceramic Substrates Using an Al-Doped ZnO as an Adhesive and Conducting Layer

Abstract

Three substrates, Al2O3, AlN and glass, were directly metallized by copper electroplating using Al-doped ZnO (AZO) as an adhesive and conducting layer between these substrates and the electroplated copper layer. The AZO was synthesized in sol-gel solution, where the Al content in the AZO sol-gel was 2 at.%. Because the AZO is a conductor that can be etched by an acidic solution, copper patterns can be directly electroplated on these substrates coated with the patterned AZO. The AZO patterns went through dry film coating, pattern formation and dry film removal process steps. Two direct copper pattern electroplating processes were proposed in this work, where one process could protect the patterned AZO from being undercut because AZO layer was patterned first and then copper was directly electroplated on the patterned AZO; the other process could not do so because copper was electroplated on the AZO layer and then both the electroplated copper layer and the AZO layer were simultaneously patterned. The copper electroplating solution must be alkaline; otherwise the AZO coated on the substrates would be etched in an acidic copper electroplating solution.

Full text

Journal of The Electrochemical Society,164 (12) D687-D693 (2017) D687
Direct Copper Pattern Plating on Glass and Ceramic Substrates
Using an Al-Doped ZnO as an Adhesive and Conducting Layer
Chia-Wen Cheng,=Po-Fan Chan,=and Wei-Ping Dow∗,z
Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan
Three substrates, Al2O3, AlN and glass, were directly metallized by copper electroplating using Al-doped ZnO (AZO) as an adhesive
and conducting layer between these substrates and the electroplated copper layer. The AZO was synthesized in sol-gel solution,
where the Al content in the AZO sol-gel was 2 at.%. Because the AZO is a conductor that can be etched by an acidic solution, copper
patterns can be directly electroplated on these substrates coated with the patterned AZO. The AZO patterns went through dry film
coating, pattern formation and dry film removal process steps. Two direct copper pattern electroplating processes were proposed in
this work, where one process could protect the patterned AZO from being undercut because AZO layer was patterned first and then
copper was directly electroplated on the patterned AZO; the other process could not do so because copper was electroplated on the
AZO layer and then both the electroplated copper layer and the AZO layer were simultaneously patterned. The copper electroplating
solution must be alkaline; otherwise the AZO coated on the substrates would be etched in an acidic copper electroplating solution.
© The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0721712jes] All rights reserved.
Manuscript submitted June 14, 2017; revised manuscript received July 24, 2017. Published August 12, 2017.
Three-dimensional (3D) chip stacking packaging technology is
dominating the state-of-the-art electronic products, especially portable
electronic products.1–7To stack up several chips, direct interconnec-
tion is too difficult to be carried out. Instead, stacking up chips with
interposers8will be easily implemented through an architectural de-
sign of re-distribution lines (RDLs).
For 2.5D chip packaging using an interposer, the architectural ma-
terials may be a silicon wafer, a glass wafer, an epoxy with glass
fibers, depending on process integration, cost, and reliability, etc.
Therefore, through-silicon vias (TSVs), through-glass vias (TGVs),
and through holes (THs) are formed in these substrates for verti-
cal I/O interconnection among multi-chip stacking.2,9–12 To stack For
RDL fabrication, metallic lines have to be formed on these interposers
with good adhesion. However, glass surface is extremely smooth and
has few chemical function groups. Hence, glass surface is difficult
to be metallized without surface pretreatment. Roughening glass sur-
face by using chemical etching solution is a common approach to
enhance the adhesion of a deposited metal layer due to an anchor-
ing effect.13,14 A typical chemical etching solution is HF solution,
which is too toxic to be widely accepted by industry. Alternatively,
glass surface modification using self-assembly of silane molecules
combining with Pd catalyst adsorption can metallize the glass surface
through copper electroless deposition, where microcontact printing is
a common example.15–25 However, the deposited metal layer usually
exhibited weak adhesion on the glass surface.
For LED packaging fabrication, both Al2O3and AlN are good
substrates for heat dissipation.26–32 The heat dissipation performance
determines the luminance and lifetime of the LED. Therefore, the
ceramic substrate is not only in charge of heat dissipation of LED but
also shoulders the RDL of LED packaging. Unfortunately, the met-
allization of Al2O3and AlN substrates with copper is hard because
both Al2O3and AlN are not conductors. Roughening their surfaces
and then using electroless deposition was a common approach.27,33–41
Surface metallization, mask design, photoresistant coating, exposure,
development, and then pattern etching are the standard operation pro-
cess (SOP) for RDL formation on the ceramic substrates. The SOP
takes a long time and is expensive, even high risk, because the process
flow is long, the palladium used as the catalyst of the electroless depo-
sition is expensive, and after baking the copper electroless deposition
layer, copper blister may be formed on the substrates.26 Therefore,
another approach, adhesive layer (i.e., chemical bonding) instead of
anchor effect (physical bonding), was proposed. The promising ad-
hesive layer that links the ceramic material and copper layer together
=These authors contributed equally to this work.
∗Electrochemical Society Member.
zE-mail: dowwp@dragon.nchu.edu.tw
is ZnO.42–47 However, although ZnO is a good adhesive layer, it is
not a conductor. The copper RDL has to be formed on the ZnO layer
by electroless deposition and subsequently copper electroplating is
employed to increase the RDL thickness.
An adhesive layer (AdL) between the metal layer and the glass
surface is necessary.25,48 The AdL plays a bridging material because
it is capable of bonding the metal layer to the glass surface, even
a ceramic surface. A basic bonding mechanism was explained by
Fujishima et al.,42,43,45,46 who employed a ZnO layer as an AdL to
enhance the adhesion of a deposited copper layer on a glass surface.
ZnO reacted with glass to form an intermediate oxide and provided
anchoring effect to bond the deposited copper layer. However, ZnO
is not a conductor, so the ZnO-coated glass substrate must be further
metallized with copper electroless deposition and then electroplated
with copper to increase the copper thickness on the glass substrate.
Herein, the physical property of ZnO is modified by doping alu-
mina into the ZnO to form a conductive Al-doped ZnO (AZO) layer.
The conducting mechanism of AZO is similar to that of indium tin
oxide (ITO).49–60 Because an acidic solution can dissolve AZO, AZO
layer can be patterned before and after copper metallization using an
acidic etching solution. We demonstrate two metallization processes
that can form copper patterns on the glass and ceramic substrates with
good copper layer adhesion by means of the AZO coating process. One
process is to deposit an AZO layer onto the glass or the ceramic sub-
strate by dip coating and then copper film was directly electroplated
onto the baked AZO layer. Finally, copper patterns were formed after
an etching process. The other process is to deposit an AZO layer onto
the glass or the ceramic substrate through a baking process and then
an AZO patterns were formed first by an etching process. Following
that, copper was directly electroplated on the patterned AZO.
Experimental
The dimensions of the glass and ceramic substrates were 20 mm ×
20 mm. Before the AZO coating process, these substrates were cleaned
in a DI water bath and then transferred into an alcohol bath with
ultrasonic auxiliary for 30 minutes. Following that, these substrates
were transferred into a bath containing 1M NaOH solution for 5
minutes, and then transferred into a bath containing H2SO4/H2O2
solution with a volume ratio of 1:1 in order to remove any organic
contamination. Finally, these substrates were cleaned with DI water
again to remove any chemicals.
The AZO sol-gel was prepared by mixing Zn(CH3COO)2·2H2O
with AlCl3·6H2O (Al/Zn =2 at. %) in dehydrated alcohol at 70◦C, and
slowly adding 2-aminoethanol (C2H7NO) in the solution. The molar
ratio of Zn(CH3COO)2·2H2O to 2-aminoethanol was 10:1. Following

D688 Journal of The Electrochemical Society,164 (12) D687-D693 (2017)
Figure 1. Process schemes of copper metallization of substrates. (a) Electroless deposition process, (b) AZO dip coating process.
that, the temperature of the solution was cooled down to 25◦Cand
the solution was continuously agitated for 2 hours and, subsequently,
stewingitat25
◦C for 24 hours. Regarding the sol-gel synthesis method
of AZO, many papers have reported in detail.49,50,54,61,62
After surface pretreatment of the substrate as mentioned above, the
substrate was put in the AZO sol-gel to do dip coating for 5 minutes.
Following that, the AZO-coated substrate was dried at 25◦Candthen
transferred in a furnace whose temperature had been maintained at
350◦C for 10 minutes. The dip coating process was repeated three
times. After the dip coating process, the AZO-coated substrate was
heated in the furnace at 500◦C for one hour to form a layer of hybrid
oxide between the AZO and the substrate and also made the substrate
be conductive due to the AZO layer.
The cross-section and element distribution of the AZO-coated sub-
strate was imaged and analyzed using scanning electron microscopy
(SEM, JEOL, JSM-6010LA), backscattered electron imaging (BEI)
and energy-dispersive X-ray spectroscopy (EDS), respectively. The
resistivity of the AZO layer was measured by using a four-point
probe instrument. The “E&ESC” logotype was adopted as the pat-
tern of copper electroplating. The pattern of the “E&ESC” logo-
type on the substrates was formed through dry film coating, pattern
formation of the dry film, acidic etching, and then copper electro-
plating. An alkaline copper electroplating solution was used for the
direct copper electroplating on the patterned AZO. Its composition
and formula was reported in previous works.63,64 Two organic ad-
ditives, bis-(3-sulfopropyl) disulfide (SPS) and polyethyleneimine
(PEI), were employed in the alkaline copper electroplating
solution.63
Results and Discussion
Figure 1illustrates two copper metallization processes of the glass
and the ceramic substrates. Process (a) is a typical copper electro-
less deposition process, which Pd is employed as the catalyst to be
physically adsorbed on the substrate surface. Following the catalyst
adsorption, the Pd-coated substrate is transferred into a copper elec-
Figure 2. Resistivity measurements after AZO coating and was baked on
glass substrates. The coating times were 1, 3 and 6. After each AZO coating,
the AZO-coated glass substrate was baked at 350◦C for 10 minutes in order
to remove the solvent of the sol-gel and to bond the AZO layer to the glass
substrate.
troless deposition solution to metallize the substrate surface. After the
copper electroless deposition, the substrate is electroplated in an acidic
copper plating bath to increase the copper layer thickness. Process (a)
is called panel plating because the substrate is wholly metallized with
a layer of copper. Hence, the subsequent process is an etching step in
order to make circuit patterns (i.e., copper RDL). In the meanwhile,
the Pd catalyst adsorbed on the substrate between two copper lines
has to be removed completely; otherwise the copper lines will have
risk of short.

Journal of The Electrochemical Society,164 (12) D687-D693 (2017) D689
Figure 3. Copper metallization processes on various substrates. (a, d, g) glass, (b, e, h) Al2O3, (c, f, i) AlN. The AZO coating time was three, including AZO
baking at 350◦C for 10 minutes. After the AZO coating process, the AZO-coated substrates were directly plated in a copper plating solution to deposit copper
films on the AZO-coated substrates.
Alternatively, a process (b) is proposed and illustrated in Fig. 1b.
The cleaned substrate is dipped in the AZO sol-gel bath for 5 minutes
and then slowly pulled out from the AZO sol-gel bath. Following the
AZO sol-gel dip coating, the AZO-coated substrate is transferred in a
furnace to be baked at 350◦C for 10 minutes. The AZO coating process
is repeated for three times in order to increase the AZO thickness and
the roughness thereof. The AZO-coated substrate becomes conductive
due to the AZO layer, so copper can be electrodeposited directly on
the substrate in an alkaline electroplating solution.
The coating times of the AZO sol-gel on the substrate, such as
a glass plate, either influence AZO resistivity or AZO transparency,
as shown in Fig. 2. Obviously, when the AZO coating times were
increased, the AZO resistivity was correspondingly decreased and its
transparency was also lowered. Evidently, the resistivity of one-time
AZO coating was high (i.e., 5.65 ×10−2·cm) because the AZO
layer was too thin to be a good conducting layer.65 The AZO resistivity
was decreased after three-time coating, which was very close to that
of six-time coating (i.e., 3.5 ×10−2·cm). This result implies that
there is a minimum coating times to increase the AZO thickness with
the minimum resistivity, as shown in Fig. 2. To decrease the resistivity
of AZO to 1.54 ∼1.66 ×10−2·cm, even 2.0 ×10−3·cm, its
thickness has to be increased.59,65,66 Herein, the thickness of the AZO
coated on the substrate was 0.45 μm (see Fig. 4), which is much
thinner than that reported in previous works.59,65,66 Therefore, all the
results subsequently appearing in this work went through three-time
AZO coating rather than six-time AZO coating in order to shorten the
overall process time.
Figure 3shows that AZO films not only could be coated on the
glass substrate (see Figs. 3a,3d) but also on ceramic substrates, in-
cluding Al2O3(see Figs. 3b,3e) and AlN (see Figs. 3c,3f) substrates.
Surface etching process for roughening the substrate surface was not
necessary. AZO films were tightly coated on these substrates. Because
the AZO films were conductive, these AZO-coated substrates could be
directly electroplated with copper in an alkaline electroplating solu-
tion, as shown in Figs. 3g,3h,3i. A Scotch tape (3 M, #610) could not
peel off the electroplated copper film that had been made a cross-cut
pattern on it. Because the AZO is a sol-gel solution rather than powder
before drying, its adhesion strength depends on its dry thickness and
coverage, that is, the sol-gel concentration coated on these substrates,
especially the ZnO concentration in the sol-gel solution. The relation-
ship between the ZnO concentration and the adhesion strength of the
AZO layer is described in the supplemental material.
To understand the root cause of good adhesion, a typical SEM
cross-section image of Cu/AZO/glass is shown in Fig. 4. Besides,
element mapping detected by EDS was also carried out and shown in
Fig. 4. Figures 4a and 4b show that the mean thickness of the AZO film
is around 0.45 μm and the interface between the glass surface and the
AZO surface is very flat, almost no roughness. This result indicates
that the root cause of strong adhesion between the glass surface and
the AZO surface does not rely on roughness (i.e., anchoring effect) but
chemical bonding.42,43,46,47,50,67,68 Previous articles45,69 have confirmed
that ZnO can react with glass at a high temperature to form a composite
oxide (i.e., ZnSiO3), such that the two materials, glass and ZnO, merge
together within a short range after being baked.
On the other hand, the mean copper thickness plated on the AZO
was around 0.55 μm and the interface between the AZO surface and
the electroplated copper surface was rough, as confirmed by Figs.
4a and 4b. Because copper atom easily diffuses into oxides, such as
Al2O3or ZnO,39,42,45,46,70,71 to form hybrid oxides, the electroplated
copper film can be tightly attached on the AZO surface. The copper
and zinc elements distribution was characterized by EDS mapping
and shown in Figs. 4d and 4e, respectively, to make sure that the
gray and white color images shown in Fig. 4c are copper and AZO,
respectively. Both Al2O3and AlN can react with ZnO to form ZnAl2O4
spinel72 and (AlN)1-x(ZnO)xcompound.73 AlN also can be co-doped
into ZnO to form covalent bond.74–76 Actually, another approach for
spinel formation is also possible in the presence of H2O, that is, 2AlN
+3H2O→Al2O3+2NH3and then Al2O3+ZnO →ZnAl2O4
(spinel).
Since AZO can be dissolved in an acidic solution, two processes
can be employed to make copper patterns. Figure 5illustrates one
metallization process that the substrate is entirely coated with AZO

D690 Journal of The Electrochemical Society,164 (12) D687-D693 (2017)
Figure 4. Cross-section of on the AZO-coated glass substrate after copper plating shown in Fig. 3g and the element mapping thereof. (a, b, c) SEM images
enlarged in ×10 K, ×20 K and 5 K, where the white layer is copper and gray layer is AZO, (c, d, e) copper and zinc elements distribution. The thickness of the
copper and the AZO layer is about 0.55 μm and 0.45 μm, respectively.
film first, as illustrated in Fig. 5a and then directly transferred into a
copper electroplating bath to electrodeposit copper on the AZO layer,
as illustrated in Fig. 5b. Following these steps, the entire substrate is
coated with a layer of copper film. Subsequently, a layer of dry film is
attached on the copper surface and goes through a patterning process,
as illustrated in Fig. 5c. The copper and AZO films except for the
substrate can be etched by an acidic solution, and then conductive
patterns are formed on the substrate, as illustrated in Fig. 5d. After the
dry film is removed, the conducting circuits are obtained, as illustrated
in Fig. 5e. The conducting lines are composed of two layers of mate-
rials. The upper layer is copper, the lower layer is AZO. Obviously,
the sidewall of the patterned AZO shown in Fig. 5e is exposed in air,
so an undercut of the AZO sidewall caused by the etching solution
may occur if the etching parameter is not appropriately controlled.
The etching solution has to simultaneously deal with two materials
(i.e., copper and AZO) whose etching rate is different.
Figure 5. Process schemes of copper pattern formation through a panel copper plating on AZO-coated substrates. (a) AZO dip coating and being baked on
substrates at 350◦C for 10 minutes for three times, (b) electrochemical deposition of copper (ECD Cu) on the entire AZO-coated substrates, (c) dry film coating
and the pattern formation thereof, (d) simultaneous etching of copper and AZO layers, (e) dry film removal. The electroplated copper films are bonded to the
patterned AZO. The sidewalls of the patterned AZO are exposed in air.

Journal of The Electrochemical Society,164 (12) D687-D693 (2017) D691
Figure 6. Practical results of Fig. 5. The substrates are (a, d, g) glass, (b, e, h) Al2O3, (c, f, i) AlN. The copper pattern is the “E&ESC” logotype. DF means dry
film. The dimension of these substrates are 50 mm ×50 mm.
The process steps proposed in Fig. 5were carried out using
“E&ESC” logotype as the pattern. Figures 6a–6c show bare glass,
Al2O3and AlN substrates, respectively, after a surface cleaning
step. Following AZO coating process for three times, the surface
color was somewhat different from the original one, as shown in
Figs. 6d–6f. After dry film coating, pattern formation, copper elec-
troplating, etching and dry film removal, the “E&ESC” logotypes
of copper appeared on these substrates, as shown in Figs. 6g–6i.
These logotypes could pass the peeling test by using the Scotch tape
(3 M, #610).
Another process is proposed and illustrated in Fig. 7. Figure 7
illustrates that the substrate is entirely coated with AZO film first,
as illustrated in Fig. 7a, and then followed dry film coating, pattern
formation steps, as illustrated in Fig. 7b. Subsequently, the substrate
is transferred into an etching solution to remove the AZO where is not
covered by the dry film, as illustrated in Fig. 7c. After the patterned
dry film is removed, the AZO film is also correspondingly patterned,
as illustrated in Fig. 7d. The patterned AZO can be directly transferred
into an alkaline copper electroplating bath to electrodeposit copper on
the patterned AZO, as illustrated in Fig. 7e. Obviously, the sidewalls
Figure 7. Process schemes of copper pattern formation through a direct pattern copper plating on AZO-coated substrates. (a) AZO dip coating and being baked
on substrates at 350◦C for 10 minutes for three times, (b) dry film coating and the pattern formation thereof, (c) etching of AZO layers, (d) dry film removal,
(e) electrochemical deposition of copper (ECD Cu) on the patterned AZO. The electroplated copper films are bonded to the patterned AZO. The sidewalls ofthe
patterned AZO are covered by the electroplated copper films.

D692 Journal of The Electrochemical Society,164 (12) D687-D693 (2017)
Figure 8. Practical results of Fig. 7. The substrates are (a, d, g) glass, (b, e, h) Al2O3, (c, f, i) AlN. The copper pattern is the “E&ESC” logotype. DF means dry
film. The dimension of these substrates are 50 mm ×50 mm.
of the patterned AZO lines shown in Fig. 7e are entirely covered by the
electroplated copper because the patterned AZO lines are conductive.
The etching solution used in Fig. 7c only needs to deal with one
materials (i.e., AZO). Therefore, the undercut issue at the sidewalls
of the AZO lines shown in Fig. 7c is easily controlled compared with
the case shown in Fig. 5d.
The process steps proposed in Fig. 7were also carried out using
the “E&ESC” logotype as a pattern. Figures 8a–8c show that the glass,
Al2O3and AlN substrates were coated with AZO films and had the
“E&ESC” pattern of dry films, respectively. Figures 8d–8f show that
the “E&ESC” patterns were formed on these substrates after the AZO
layers where were not covered by the dry film was etched by an acidic
solution and then the patterned dry films were removed. The “E&ESC”
logotype could only be clearly seen on the glass substrate, as shown
in Fig. 8d, because glass is transparent. Subsequently, these substrates
with the “E&ESC” logotypes were directly electroplated with copper
in the alkaline copper electroplating solution to metallize the logotypes
with copper, as shown in Figs. 8g–8i. Figures 8g show that the glass
substrate where was not covered by the patterned AZO and copper was
transparent, so the paper words underneath the glass substrate were
clearly seen. On the other hand, the surfaces of these substrates shown
in Figs. 8e,8f looked nothing on them, but the “E&ESC” logotypes
(i.e., copper patterns) emerged after copper electroplating, as shown
in Figs. 8h,8i. These logotypes also could pass the peeling test by
using the Scotch tape (3 M, #610).
Conclusions
The surface metallization of ceramic and glass substrates with cop-
per patterns are achieved using AZO as a conducting layer. The AZO
layer plays three roles in the copper metallization process. The first one
is to act as an adhesive layer to interlock the substrates and the elec-
troplated copper layer. The second one is to act as a conducting layer
for copper electroplating on it, although the resistivity of the AZO is
3.5 ×10−2·cm. The third one is to provide an approach for direct
copper pattern electroplating because AZO can be etched by an acidic
solution. In this work, the electroless copper deposition process that
needs expensive palladium catalyst and is not environment-friendly is
not employed. Instead, a cheap AZO sol-gel coating process exhibits
a promising approach for direct copper pattern electroplating on these
ceramic and glass substrates.
Acknowledgments
This work is supported by the Ministry of Science and Technology
of Taiwan with a contract number of NSC 103-2622-E-005-002.
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