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Laser-induced copper deposition from aqueous and aqueous–organic solutions: state of the
art and prospects of research
View the table of contents for this issue, or go to the journal homepage for more
2015 Russ. Chem. Rev. 84 1059
(http://iopscience.iop.org/0036-021X/84/10/1059)
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I. Introduction
Laser metal deposition is one of promising methods of
metallization of dielectric surfaces. The state of the art of
research in this field can be characterized as primary
accumulation of basic knowledge. An advantage of laser
deposition is that even the surface of wide band-gap
(43 eV) dielectric materials can be metallized without any
pretreatment, which is difficult to achieve by other methods.
One of the laser deposition techniques is laser-induced
chemical liquid-phase deposition (LCLD). It is based on
localized deposition o f me tal from a sol ution via chemical
reduction of the metal or decomposition of its salt (com-
plex) induced by laser radiation. Laser metal deposition
from electrolyte solutions can be used to fabricate micro-
and nanosized metal structures on the surface of dielectric s
and semiconductors of different types.
1
V A Kochemirovsky Candidate of Chemical Sciences, Associate Professor
of the Chair of Laser Chemistry and Laser Material Science at the SPbSU.
Telephone: +7(812)327 ± 1504, e-mail: v.kochemirovsky@spbu.ru
Current research interests: inorganic chemistry, laser chemistry, materials
science.
M Yu Skripkin Candidate of Chemical Sciences, Associate Professor of the
Chair of General and Inorganic Chemistry at the SPbSU.
Telephone: +7(812)428 ± 4069, e-mail: m.skripkin@spbu.ru
Current research interests: inorganic chemistry, solution chemistry.
Yu S Tveryanovich Doctor of Chemical Sciences, Professor, Head of the
Chair of Laser Chemistry and Laser Material Science at the SPbSU.
Telephone: +7(812)428 ± 7479, e-mail: yu.tveryanovich@spbu.ru
Current research interests: laser chemistry, nanodispersed materials, thin
films.
A S Mereshchenko Candidate of Chemical Sciences, Associate Professor
of the SPbSU.
Telephone: +7(812)428 ± 7479, e-mail: a.mereshchenko@spbu.ru
Current research interests: photochemistry, laser chemistry, inorganic
chemistry.
DOI 10.1070/RCR4535
Laser-induced copper deposition from aqueous and aqueous ± organic
solutions: state of the art and prospects of research
V A Kochemirovsky, M Yu Skripkin, Yu S Tveryanovich, A S Mereshchenko, A O Gorbunov,
M S Panov, I I Tumkin, S V Safonov
Saint Petersburg State University
Universitetskaya nab. 7-9, 199034 Saint Petersburg, Russian Federation
Contents
I. Introduction 1059
II. Chemical factors influencing chemical liquid phase deposition of copper 1060
III. Effect of dielectric surface properties 1064
IV. Prospects and key trends of development of laser-induced liquid phase deposition technique 1065
Information about the factors influencing laser-induced deposition of metals, primarily copper, from aqueous and
aqueous ± organic solutions are generalized and described systematically. Laser-induced deposition techniques and
mechanisms of chemical and laser-induced deposition of local copper and other metal structures onto the dielectric surface
are considered. The effects of the solution composition, the nature of the reducing agent and the properties of the dielectric
surface on the deposition process are discussed. Possible photochemical reactions induced by laser radiation and the role of
these reactions in the photoreduction of metals are considered. The key trends and prospects in the development of laser-
induced chemical liquid phase deposition are mentioned.
The bibliography includes 132 references.
A O Gorbunov Post-graduate Student of the Chair of General and
Inorganic Chemistry at the SPbSU.
Telephone: +7(812)428 ± 4069, e-mail: a.o.gorbunov@spbu.ru
Current research interests: inorganic chemistry, solution chemistry.
M S Panov Candidate of Chemical Sciences, Postdoc of the Chair of
Laser Chemistry and Laser Material Science at the SPbSU.
Telephone: +7(812)428 ± 7479, e-mail: m.s.panov@spbu.ru
Current research interests: photochemistry, laser chemistry.
I I Tumkin Research Engineer of the same Chair.
Telephone: +7(812)428 ± 7479, e-mail: i.i.tumkin@spbu.ru
Current research interests: laser chemistry, inorganic chemistry, materials
science.
S V Safonov Candidate of Chemical Sciences, Assistant Professor of the
same Chair.
Telephone: +7(812)428 ± 7479, e-mail: s.safonov@chem.spbu.ru
Current research interests: laser chemistry, inorganic chemistry, materials
science.
Received 11 November 2014
Uspekhi Khimii 84 (10) 1059 ± 1075 (2015); translated by T N Safonova
Russian Chemical Reviews 84 (10) 1059 ± 1075 (2015) # 2015 Russian Academy of Sciences and Turpion Ltd
At first glance, laser chemical vapour deposition
(LCVD),
2
pulsed laser deposition (PLD),
3
laser-induced
film transfer technique (LIFT)
4
and laser pyrolytic destruc-
tion of solids (LPDS)
5
are alternatives to LCLD. Never-
theless, the LCLD technique has obvious advantages over
the above-listed methods, such as
Ð it is cost-effective because expensive equipment is not
required;
Ð small amounts of toxic wastes are produced;
Ð LCLD provides higher deposition rates compared
with LCVD and LIFT.
Besides, LCLD can be accomplished in one step, if the
surface to be metallized has a composite (two- or multi-
phase) structure or includes areas of substantially different
compositions,
6
or in two steps, if the surface preactivation is
required. A considerable advantage compared to other
methods using laser radiation is that it can be used to
fabricate highly conducting metal structures.
7, 8
The LCLD technique can be employed to deposit the
following metals onto the surface of various di electrics:
copper, silver,
9
palladium,
10
platinum,
11
nickel,
12
gold,
13
chromium and tungsten.
14
The copper deposition is of
most practical interest because copper is widely use d as a
conducting material in microelectronics, as well as due to
the catalytic activity of copper-based nanoparticles.
15, 16
In some cases, the specificity of laser-induced deposition
provides new pathways of chemical reactions.
17, 18
Due to
such features of laser-induced copper deposition as a high
energy density and a considerable temperature gradient
around the laser beam focus, side reactions and the decom-
position of organic components of the solution can
occur.
13, 18
This is why the results of laser-induced deposi-
tion from most of known solutions, in which autocatalytic
reactions take place, substantially differ from the results of
standard chemical copper plating. The causes and mecha-
nisms of these differences have be en little studied. It is
generally assumed that the mechanism of the laser-induced
reaction is analogous to the autocatalytic mechanism.
19
In
some investigations, the influence of the components of the
solution was not considered at all. In these works, the
authors examined only the effects of the laser power and
wavelength,
20
the rate of laser beam scanning of the
dielectric surface and the number of scans of the same area
of the dielectric surface
21
and obtained quite predictable
results, such as an increase or decrease in the geometric sizes
of deposited copper structures.
The methods of laser-induced chemical liquid-phase
deposition can be arbitrarily divided into three groups.
1. One-step processes. The metal deposition is per-
formed in one step. The metal is deposited directly from a
solution onto the substrate using a thermally or photo-
induced redox reaction in solution a t the interface with a
dielectric.
2. Two-step processes involving the laser-induced pre-
deposition of a precious metal. In the first step, the laser-
induced deposition of a precious metal (silver
22
or palla-
dium
23
) template is performed followed by chemical depo-
sition of copper. The precious metal acts as a catalyst for
the redox reaction between copper(
II
) and a reducing agent
in solution. In this case, copper is deposited only onto the
surface of the template thus forming a current-carrying
strip.
3. Two-step processes involving the laser treatment of
the material in air. Initially, the surface of the material
subjected to metallization is treated by laser radiation in air.
Then the material is placed in a hot copper- or nickel-
plating solution. Due to changes in the surface properties of
the laser-irradiated areas, the metal is deposited only onto
these areas.
11
The laser-induced deposition of copper has been studied
most extensively because this metal is of most interest to
researchers as the cheapest highly conducting material for
microelectronics, whic h, in additi on, has catalytic proper-
ties and can be reduced from the ion in solution through
thermochemical and photochemical routes. Almost all stud-
ies on the one-step laser-induced deposition of metal were
performed with copper.
An analysis of the stat e of the art of investi gations in the
field of laser-induced deposition of metals shows that the
following three groups of factors have an effect on the
results of this process:
1
Ð physical factors (laser power and wavelength, scan
rate, temperature of the environment and solution);
Ð chemical factors (the composition of the solution,
concentrations of the components, pH, chemical reactions
in solution, photochemical processes i n solution, the char-
acteristic features of complexation in aqueous ± organic
systems);
Ð surface properties of the dielectric substrate (struc-
ture, the presence of activated and catalytic centres, defect
structure, phase composition, chemical properties of the
components of the dielectric materi al).
Depending on the t ype of t he dominant reaction mech-
anism, laser-induced deposition techniques are divided into
photoinduced (photodecomposition of metal salts or com-
plexes in sol ution is a dominant reaction) and thermally
induced (photoprocesses play a minor role, and the metal-
lization results from the high-temperature initiation of
redox reactions involving a metal ion).
The above-listed factors and their influence on the
copper plating process are briefly considered in the follow-
ing sections.
II. Chemical factors influencing chemical liquid
phase deposition of copper
II.1. Composition of the solution and reaction mechanisms
For a long time, t he following reaction was the only one
that was used for the laser-induced reduction of copper
CuL
(n72)7
+ 2 HCHO + 4 HO
7
(1)
Cu
0
+L
n7
+H
2
+ 2 HCOO
7
+2H
2
O
where L is an organic ligand [usually the tartrate or ethyl-
enediaminetetraacetat e anion (EDTA)]; HCHO (formalde-
hyde) was added a s the reducing agent in a 6 ± 7.5-fold
excess.
18
Sulfate or chloride were most often employed a s
copper salts.
Reaction (1) was used long enough for chemical copper
plating of pre-activated surfaces and is known as the
`copper mirror chemical r eaction'.
24
As applied to the
laser-induced reduction of copper, this process was not
efficient in preparing highly conducting finely dispersed
copper s tructures with stable properties. The reaction with
formaldehyde gave a deposit as a layer of copper cubic
crystals fused via vertices and edges (Fig. 1), i.e., the deposit
had a porous structure and low conductivity.
18
V A Kochemirovsky, M Yu Skripkin, Yu S Tveryanovich, A S Mereshchenko, A O Gorbunov,
1060 M S Panov, I I Tumkin, S V Safonov Russ. Chem. Rev. 84 (10) 1059 ± 1075 (2015)
Later on, it was found that m uch better performance can
be achieved with polyols as reducing agents, the lower the
reducing ability of polyol, i.e., the higher its reduction
potential, t he more monolithic and dispersed the copper
deposits, and the electrical conductivity of the deposited
copper (very stable on the time scale) is close to that of pure
copper (Fig. 2).
7, 8, 25
Similar results can be obtained by the addition of some
types of hydrophilic nonionogenic surfactants,
26, 27
which
have a hydrophilic-lipophilic balanc e (HLB) characterize d
by Griffin's HLB values of 11 ± 12.
28
Any types of ionogenic
surfactants interfere with the deposition.
29
It should be noted that the general approach to the
determination of the composition of solutions was the same
as that used for traditional formaldehyde solutions. It was
believed that the solution for laser-induced deposition of
copper should contain, in addition to a copper salt, compo-
nents acti ng as a liga nd, a reduci ng agent an d a pH
regulator. The functions of these components in metalliza-
tion solutions are considered below.
Numerous compositions of nickel-plating solutions were
proposed.
30
The deposition of nickel employing traditional
solutions also involves a number of p roblems. The use of
sodium hypophosphite as the reducing agent results i n the
co-deposition of a m ixture of nickel and phosphorus, as well
as, probably, phosphides; the use of boron-containing
reducing agents leads to the co-deposition of boron.
II.2. Metal salts
In the conventional copper plating technique for the fab-
rication of printed circuit boards, CuSO
4
is used to build-up
a copper layer.
31
The empirical dependence of the deposi-
tion rate (r) of metallic copper (measured in micrometres of
the film thickness per hour) on the copper ion concentration
in solution for the chemical metallization
32
takes the form
r * [Cu]
0.4
where [Cu] is the concentration of the divalent copper
complex with EDTA in solution.
However, an increase in the copper salt concentration in
solution during the laser deposition not necessarily leads to
an increase in the amount of deposited metal. As a rule, at
high concentrations (0.1
M
and higher) the topology of the
deposited structures is deteriorated and the amount of
deposited copper decreases with a further increase in the
copper concentration.
33
The investigation of the influence of the anionic compo-
sition of solutions on the results of laser deposition
34
demonstrated that the anion has some effect on this process,
in particular, when using solutions containing an alkali, a
copper salt, a l igand (tartrate) and a reducing agent (form-
aldehyde). A comparison of the results of the liquid phase
deposition of CuSO
4
and CuCl
2
showed that the electrical
resistance of the deposits obtained from a CuCl
2
solution is
3 ± 4 times lower compared with the deposits produced from
aCuSO
4
solution in the power range of 2 50 ± 350 mW. This
may be due to the two-electron character of the reduction of
copper(
II
) to copper(
0
), which apparently proceeds through
the formation of Cu(
I
) in solut ion (Cu
aq
). It is known that
copper(
I
) in aqua complexes is unstable, but it is substan-
tially stabilized upon complexation. The Cu
aq
ion immedi-
ately disproportionates to Cu
0
and Cu
2+
(Ref. 35). In a
CuSO
4
solution, a colloidal metal solution is formed fol-
lowed by the deposition of the metal onto the surface, the
deposit being inhomogeneous. In a CuCl
2
solution, the
reduction of CuCl
ÿ
2
anionic complexes will apparently give
rise to polynuclear heterovalent intermediate s
Cu
+1/0
7Cu
7
7Cu
+1/0
, which will be adsorbed on the
surface, thus improving the topology of the deposited
copper structure.
The effect of the temperature on the deposition process
was noted.
34
Thus, an increase in the temperature of the
solution in a cell from 25 to 45 8C led to an increase in the
amount of deposited metal. This is indirect evidence of the
occurrence of thermochemical processes in solution.
II.3. Reducing agent
The proposed mechanisms of metal reduction based on the
results of investigations of chemical and electrochemical
metallization are described in detail in the literature (see, for
example, Refs 24, 25, 31 and 36).
An electrochemical theory was proposed
37
to explain
the catalytic effect of the metal surface on the chemical
metallization process. According to this theory, t he catho-
dic reduction of metal and the anodic oxidation of the
reducing agent take place in certain areas of t he catalyst
surface. The catalyst promotes the electron transfer from
1 mm
Figure 1. Deposit produced by laser-induced deposition of copper
from the solution 0.01
M CuCl
2
+0.011M KNaC
4
H
4
O
6
.
4H
2
O+
0.05
M NaOH+0.075 M HCHO.
18
2 mm
Figure 2. Deposit produced by laser-induced deposition of
copper from the solution 0.01
M CuCl
2
+0.05 M NaOH+
0.03
M KNaC
4
H
4
O
6
.
4H
2
O+0.075 M xylitol (reducing agent).
7, 8, 25
V A Kochemirovsky, M Yu Skripkin, Yu S Tveryanovich, A S Mereshchenko, A O Gorbunov,
M S Panov, I I Tumkin, S V Safonov Russ. Chem. Rev. 84 (10) 1059 ± 1075 (2015) 1061
the reducing agent directly to the metal ion, which otherwise
proceeds with difficulty.
In the chemical copper plating, hypophosphite, hypo-
sulfite and hydrazine can be used, along with formaldehyde,
to reduce copper (as well as nickel and some other metals).
30
However, these compounds exhibit reducing properties only
at elevated temperature. Besides, these reduci ng agents are
usually not involved in autocatalytic processes, which could
lead to the uncontrolled deposition of metal in the bulk of
the solution as soon as the required temperature is reached.
Hypophosphite a nd hydrosulfite exhibit reducing properties
in an acidic medium; formaldehyde and hydrazine, in an
alkaline medium.
The reduction of copper using formaldehyde as the
reducing agent occurs mainly on grains and surfaces of the
already deposited copper, rather than throughout the bulk
of the solution (autocatalytic reaction). However, the
requirement of autocatalysis is not universal. It might be
supposed that the number of reducing agents that can be
involved in the metallization is much larger than those used
in conventional chemical and elec trochemical reactions.
This is associated with the i nfluence of lase r radiation on
the potentials of redox reactions in solution.
In experiments performed by von Gutfeld and co-work-
ers,
38 ± 40
the laser beam was focused onto the copper
electrode/copper sulfate solution interface (Fig. 3). The
second electrode was placed in a solution. An external
voltage (U) was applied to a closed circuit. At an insigni f-
icant external voltage, laser irradiation can induce a poten-
tial jump (E) of about 0.4 V at the copper/electrolyte
solution interface and the rate of copper deposition in the
irradiated area increases by a factor of 1000. The authors
attributed this effect to the thermal factor (kinetics of
deposition) and to the stirring of the solution. However,
the simplest calculations by the Nernst equation showed
that the potential change in the solution due to its heating
can reach only 0.1 ± 0.15 V. Besides, a high thermal con-
ductivity of copper should facilitate the deposition of wide
copper structures, which is inconsistent with the experimen-
tal data on the loc alized laser-induced electrode position.
The kinetics of the photochemical component of the
process is weakly related to the temperature factor and
depends to a greater extent on the parameters of laser
radiation (power and wavelength).
41
The improvement of the topology of copper structures
at lower reduction potentials of the polyol used was
explained
25
based on the calculated potential shift of the
electrical circuit and the fact that the calculated value is
consistent with the experimental data obtained by von
Gutfeld and co-workers
38 ± 40
in the s tudy of electrodeposi-
tion. The redox reactions with the participation of reducing
agents with rather low standard reduction potentials (form-
aldehyde, ethanol and so on; Table 1) can already occur in
solution, and the treatment of this solution by laser radia-
tion can give rise to non-localized deposition of copper.
In the reactions using reducing agents with a low
reduction potential (xylitol, sorbitol), the potential differ-
ence sufficient for the redox reaction between copper and an
organic reducing agent to proceed can be achieved only on
the metallized dielectric surface, resulting in the localized
deposition of copper (Fig. 4 ). The formation of copper
nuclei will be determi ned by the defect formation in dielec-
trics under laser irradiation by the mechanism which has
been described in detail by Shafeev and co-workers.
42, 43
This provides additional support to the hypothesis that von
Gutfeld's potential shift is attributed not only to the
thermal e ffect of laser radiation but also to the contribution
of electron photoemission from the deposited metal surface
induced by laser radiation.
44
This interpretation can be extended to the influence of
the stability of a copper complex in solution and the copper
concentration on the laser deposition process,
45
the more so
since polyols can also be coordinated t o copper ions in
solution like classical ligands.
Pt Electrode
Substrate
Continuous-wave
laser beam
Copper layer
(150 nm) serving
as an electrode
Copper salt solution
(without a reducing agent)
for electrodeposition
U
Figure 3. Chart of von Gutfeld's experiment on laser-induced
electrochemical deposition of copper.
38 ± 40
Table 1. Standard reduction potentials of reducing agents used for LCLD (E
0
) and the reduction potential difference between the Cu
2+
/Cu couple
and the reducing agent in solution under standard conditions (DE
0
) and at 80 8C(DE), as well as at the metal/solution interface.
1, 25
Reducing agent 7E
RCH
2
OH=RCOO7
0
/V DE
0
/V DE (calculated in solution) DE (calculated according
/V (+0.1 V) to von Gutfeld at the copper/
solution interface) /V (+0.4 V)
Formaldehyde 0.98 0.78 0.88 1.28
Ethanol 0.56 0.36 0.46 0.86
Ethylene glycol 0.54 0.34 0.44 0.84
Glycerol 0.45 0.24 0.34 0.74
Xylitol 0.10 0.10 0.20 0.60
Sorbitol 0.10 0.10 0.20 0.60
V A Kochemirovsky, M Yu Skripkin, Yu S Tveryanovich, A S Mereshchenko, A O Gorbunov,
1062 M S Panov, I I Tumkin, S V Safonov Russ. Chem. Rev. 84 (10) 1059 ± 1075 (2015)
