Download Electroless Deposition of Copper

17
ELECTROLESS DEPOSITION OF COPPER
MILAN PAUNOVIC
In Chapter 1 (Part A), we discuss the overall reaction and the
mixed-potential theory of electroless metal deposition in
general. In this chapter we discuss the specific case of copper:
the overall reaction, fundamental, and technological aspects
of its electroless deposition.
The overall reaction for electroless copper deposition,
with formaldehyde (HCHO) as the reducing agent, is
Cu2 þ þ 2HCHO þ 4OH ! Cu þ 2HCOO þ 2H2 O þ H2
ð17:1Þ
where HCOO (formic acid) is the oxidation product of the
reducing agent. The fundamental aspects of this reaction are
presented in the following five sections: (17.1) electrochemical model, (17.2) anodic partial reaction, (17.3) cathodic
partial reaction, (17.4) kinetics of deposition, and (17.5)
modeling. Some special cases of electroless copper deposition solutions are given in an Appendix to this chapter.
17.1
17.1.1
ELECTROCHEMICAL MODEL
Mixed-Potential Theory
The mixed-potential theory was developed by Wagner and
Traud [1] for the purpose of interpreting metal corrosion
processes. Paunovic [2] and Saito [3] applied the theory to the
interpretation of electroless deposition of copper.
According to the mixed-potential theory, the overall
reaction, given by Eq. (17.1), can be decomposed into a
simple reduction reaction, the cathodic partial reaction
catalytic surface
þ
Cu2solution
þ 2e ƒƒƒƒƒƒƒƒ! Culattice
ð17:2Þ
and one oxidation reaction, the anodic partial reaction
HCHO þ 2OH ! HCOO þ H2 O þ 12H2 O þ e
ð17:3Þ
Thus the overall reaction (17.1) is the result of the
combination of two different partial reactions, Eqs. (17.2)
and (17.3). These two partial reactions, however, occur at one
and the same electrode, namely the metal–solution interphase. Each of these reactions strives to establish its own
equilibrium potential, Eeq. The result of this process is the
creation of a steady state with the compromised potential
called the steady-state mixed potential, Emp.
17.1.2
Evans Diagram
According to the mixed-potential theory the overall reaction
of the electroless copper deposition can be described electrochemically in terms of two current–potential (i–E) curves,
as shown in Figure 17.1. Figure 17.1 was constructed in the
following way: First, the current–potential curve of the
reduction of cupric ions in the solution containing H2O,
0.1 M CuSO4, 0.175 M EDTA (ethylenediaminetetraacetic
acid) and NaOH to pH 12.50 (no CH2O present) at 24 C
(0.5) is determined using a galvanostatic technique. At this
electrode only one reaction occurs, the reduction of Cu2 þ .
An electrode with only one electrode process is called a
single electrode. The result is shown as i(Cu2 þ ) ¼ f(E)
in Figure 17.1. The current–potential curve was recorded
starting from the equilibrium potential, Eeq(Cu/Cu2 þ ) ¼
0.47 V versus SCE. Second, the current–potential curve
for the oxidation of formaldehyde at the single electrode was
determined using the galvanostatic technique. The solution
in this case contained H2O, 0.05 M CH2O, 0.075 M EDTA
(excess of EDTA used in the solution for the single cathodic
reaction), and NaOH to pH 12.50 (no CuSO4 was present in
this solution). The temperature was 24 C (0.5). The result
Modern Electroplating, Fifth Edition Edited by Mordechay Schlesinger and Milan Paunovic
Copyright Ó 2010 John Wiley & Sons, Inc.
433
434
ELECTROLESS DEPOSITION OF COPPER
FIGURE 17.1 Current–potential curves for reduction of Cu2 þ
ions and for oxidation of reducing agent Red, formaldehyde,
combined into one graph (Evans diagram). Solution for the Tafel
line for the reduction of Cu2 þ ions: 0.1 M CuSO4, 0.175 M EDTA,
pH 12.50, Eeq (Cu/Cu2 þ ) ¼ 0.47 V versus a saturated calomel
electrode (SCE); for the oxidation of formaldehyde: 0.05 M HCHO
and 0.075 M EDTA, pH 12.50, Eeq (HCHO) ¼ 1.0 V versus SCE;
temperature 25 C (0.50 C). (From Paunovic [2] with permission
from the American Electroplaters and Surface Finishers Society.)
is shown as i(CH2O) ¼ f(E) in Figure 17.1. The current–
potential curve was recorded starting from the equilibrium
potential, Eeq(CH2O) ¼ 1.0 V versus SCE. It is seen from
Figure 17.1 that these two polarization curves, i(Cu2 þ ) ¼
f(E) and i(CH2O) ¼ f(E), intersect. The coordinates of intersection are (1) abscisa, i ¼ 1.9 103 A cm2 and (2)
ordinate, E ¼ 0.65 V versus SCE. According to the mixedpotential theory, current density i ¼ 1.9 103 A cm2 is the
rate of the electroless deposition of copper expressed in terms
of amperes per square centimeter. The potential E ¼ 0.65 V
versus SCE is the mixed potential (Emp) of the electroless
copper system under study. The rate of deposition expressed
in milligrams per hour per square centimeter is calculated on
the basis of Faraday’s law using the equation
w ¼ i 1:18 mg h1 cm2
where i is given in milliamperes per square centimeter. For
i ¼ 1.9 103 A cm2 it is 2.2 mg h1 cm1.
The experimentally determined rate of electroless Cu
deposition under the conditions above, using the weight gain
method, is 1.8 mg h1 cm2. This rate is obtained when the
time of deposition is counted from the instant of immersion of
the copper plate (substrate) into the solution. If the time of
deposition is counted from the instant the mixed potential is
reached (about 4 min after immersion of the Cu substrate),
the deposition rate is 1.9 mg h1 cm2. The experimentally
determined mixed potential Emp for the same conditions is
0.65 V versus SCE.
Examination of Figure 17.1 and the results of direct
experimental measurements show that there is a relatively
good agreement between the direct experimental and the
theoretical values (Evans diagram). Thus we can conclude
that the mixed-potential theory is essentially verified for this
case of electroless copper deposition. These conclusions are
confirmed by Donahue [4], Molenaar et al. [5], and El-Raghy
and Abo-Salama [6]. The significance of this conclusion is
that on the basis of the mixed-potential theory one can use the
kinetic parameters for the partial anodic and cathodic reactions to deduce a variety of predictions and characteristics of
the overall reaction of electroless copper deposition. For
example, the effect of additives on the overall reaction can be
resolved into separate effects on the partial reactions and use
these results to select the best conditions for electroless
deposition.
17.1.3
Interaction between Partial Reaction
The original mixed-potential theory assumes that the two
partial reactions are independent of each other [1, 2]. In some
cases this is a valid assumption. However, it was shown later
that the partial reactions are not always independent of each
other [7, 8]. For example, Schoenberg [9] has shown that the
methylene glycol anion (the formaldehyde in an alkaline
solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate
complex and thus influences the rate of the cathodic partial
reaction. Ohno and Haruyama [10] showed the presence of
interference in partial reactions in terms of current–potential
curves.
17.1.4
Presence of Interfering Reactions
In the presence of interfering (or side) reactions, partial
reactions ia and/or ic may be composed of two or more
components. One example is the electroless deposition of
copper from solutions containing oxygen [11, 12]. In this
case the interfering reaction is the reduction of the oxygen,
and the cathodic partial current density ic is the sum of two
components,
ic ¼ ic ðCu2 þ Þ þ ic ðO2 Þ
where ic(Cu2 þ ) is the cathodic partial current density for
reduction of copper ions Cu2 þ and ic(O2) is that for reduction
of the oxygen.
17.2
17.2.1
ANODIC PARTIAL REACTION
Overall Reaction
Most electroless copper solutions employ formaldehyde as
the reducing agent. The overall reaction of the electrochemical
ANODIC PARTIAL REACTION
oxidation of formaldehyde at the Cu electrode in an alkaline
solution proceeds according to Eq. (17.3).
17.2.2
Mechanism
17.2.3
Formation of electroactive species proceeds in three
steps:
1. Hydrolysis of H2CO,
H2 COþH2 O!H2 CðOHÞ2 ðmethylene glycolÞ
ð17:4Þ
Cannizzaro Reaction
One important side reaction in electroless copper deposition
is disproportionation of formaldehyde (Cannizzaro reaction):
2HCHO þ OH ! HCOO þ CH3 OH
The overall anodic partial reaction (17.3) proceeds in at least
two elementary steps:
1. Formation of electroactive species
2. Charge transfer from electroactive species to the
catalytic surface (electron injection)
435
ð17:10Þ
In this reaction, between two molecules of formaldehyde,
one molecule is oxidized into formic acid and the other is
reduced into methanol. The rate of this reaction increases
with increasing pH and temperature [13].
17.2.4
Kinetics
The major factors determining the rate of the anodic partial
reaction are pH and additives. Since OH ions are reactants
in the charge-transfer step, Eq. (17.7), the effect of pH is
direct and significant [14, 15].
The reduction potential (the rest potential) of formaldehyde increases linearly with pH according to Nernst’s
equation
2. Dissociation of methylene glycol,
H2 CðOHÞ2 þ OH ! H2 CðOHÞO þ H2 O ð17:5Þ
3. Dissociative adsorption of the intermediate
H2C(OH)O involving breaking of C–H bond,
H2 CðOHÞO ! ½HCðOHÞO ads þ Hads
ð17:6Þ
0
E ¼ Ecsp
0:118 pH
ð17:11Þ
0
combines the standard electrode potential E0 and
where Ecsp
the concentration term in Nernst’s equation [14]. The rest
potential of a copper single electrode (absence of copper
ions) in the solution of formaldehyde as a function of pH is
shown in Figure 17.2. The average slope qE/q(pH) of experimental functions in Figure 17.2 is 0.096 V decade1. A
where the subscript ads denotes adsorption of species and
[HC(OH)O]ads is electroactive species.
Charge transfer, the electrochemical oxidation (desorption)
of electroactive species, proceeds according to the reaction
½HCðOHÞO ads þ OH ! HCOO þ H2 O þ e
ð17:7Þ
where HCOO (formic acid) is the oxidation product.
The adsorbed hydrogen, Hads, may be desorbed in the
chemical reaction
Hads ! 12 H2
ð17:8Þ
or in the electrochemical reaction
Hads ! H þ þ e
ð17:9Þ
In electroless deposition of copper, when the reducing
agent is formaldehyde and the substrate is Cu, Hads desorbs
in the chemical reaction (17.8). If the substrate is Pd or
Pt, hydrogen desorbes according to the electrochemical
reaction (17.9).
FIGURE 17.2 Rest potential of a copper electrode in 0.13 M
formaldehyde and 1.0 M KCl as a function of pH. Curve 1: the
absence of ligand; curve 2: the presence of 0.05 M EDTP (ethylenedinitrilo-tetra-2-propanol).
436
ELECTROLESS DEPOSITION OF COPPER
detailed discussion of the pH effect on the partial anodic
reaction is given by Duffy et al. [14],
17.3
17.3.1
17.3.2
CATHODIC PARTIAL REACTION
Kinetic Scheme
Examination of the pH dependence of the reduction potential
and the rate of oxidation of formaldehyde shows that the pH
of the electroless copper solution should be above 11.0 in
order to have practical rates of copper deposition [14]. This
pH restriction imposes the use of complexed copper ions in
the electroless solution in order to prevent precipitation of Cu
(II) hydroxide. Cu(OH)2EDTA, EDTP, and tartaric acid are
the most commonly used ligands for copper ions [16].
Thus the electroactive species in the partial cathodic
reaction may be complexed or noncomplexed copper ions.
In the first case the kinetic scheme of the cathodic partial
reaction is one of the simple charge transfer
RDS
Cu2 þ þ e ! Cu þ
ð17:12Þ
Cu þ þ e ! Cu
ð17:13Þ
where RDS stands for rate-determining step (slow step). In
the second case the kinetic scheme is of the charge transfer
preceded by the dissociation of the complex [16]. The
mechanism of the second case involves a sequence of at
least two basic elementary steps:
1. Formation of the electroactive species
2. Charge transfer from the catalytic surface to the
electroactive species
Electroactive species Cu2 þ are formed in the first step by
dissociation of the complex [CuLx]2 þ xp:
½CuLx 2 þ xp ! Cu2 þ þ xLp
ð17:14Þ
where p is the charge state of the ligand L and 2 þ xp is the
charge of the complexed copper ion. The charge transfer
Cu
2þ
þ 2e ! Culattice
rate of dissociation of the complex and the rate of copper
deposition [16].
ð17:15Þ
proceeds in steps, usually with the first charge transfer (one
electron transfer), Eq. (17.12), serving as the rate-determining step [17].
Thus, from the kinetic aspects, the cathodic partial reaction is an electrochemical reaction, Eq. (17.15), that is
preceded by a chemical reaction, Eq. (17.14).
Paunovic [16] has shown that in the electroless
deposition of copper from the Cu(II)EDTA complex
the reduction of the complex is preceded by dissociation
of the same. A correlation has been established between the
Kinetics
The major factors determining the rate of the partial cathodic
reaction are the concentration of the copper ions and the
ligands, pH of the solution, and the type and the concentration of additives. These factors determine the kinetics of the
partial cathodic reaction in a general way, as given by the
fundamental electrochemical kinetic equations discussed in
Chapter 1.
17.3.3
pH Effect
The rest potential of the copper electrode in an alkaline
solution of cupric ions complexed with EDTP shows a
linear pH dependence with a slope qE/q (pH) ¼ 0.066 V
decade1 [14],
E ¼ E0 0:066 pH
ð17:16Þ
This slope is in conformity with the reaction
Cu þ H2 L0 ! CuL0 þ 2H þ þ 2e
ð17:17Þ
where L ¼ EDTP, L0 ¼ EDTP 2H þ [14,18, 19]. The
experimentally observed slope of 0.066 V decade1 [14]
is in good agreement with the theoretical slope of 0.059 V
decade1 [17].
In contrast to the anodic partial reaction, the rate of the
cathodic partial reaction does not depend significantly on pH,
since OH ion is not a reactant in the cathodic reaction.
Moreover the large concentration of OH ions in the metal–
solution interphase (the double layer) can hinder the process
of reduction of complexed copper ions (CuL), especially if
CuL is negatively charged, such as when L is EDTA or
tartrate [3].
17.3.4
Effect of Additives
Schoenberg [9, 20] as well as Paunovic and Arndt [21] have
shown that additives may have two opposing effects: acceleration and inhibition. For example, guanine and adenine
show the accelerating effect on the cathodic reduction of
Cu2 þ ions in the electroless copper solution. The same
additives show an increase in the rate of the electroless
copper deposition. The accelerating and the inhibiting effects
of dipyridyls were examined by Duda [22] as well as by Oita
et al. [23]. In another example, the addition of NaCN to the
electroless copper solution results in the inhibing effect for
reduction of Cu2 þ ions in an electroless solution. This
inhibition increases with an increasing amount of NaCN in
solution [24, 25].
KINETICS OF ELECTROLESS Cu DEPOSITION
17.4 KINETICS OF ELECTROLESS Cu
DEPOSITION
Steady-state electroless copper deposition at mixed potential
Emp is preceded by a non-steady-state period, called the
induction period.
17.4.1
Induction Period
The induction period is defined as the time necessary to reach
the mixed potential Emp at which the steady-state metal
deposition starts to occur. It is determined in a simple
experiment in which a piece of metal is immersed in a
solution for electroless deposition of a metal and the potential
of the metal recorded from the time of immersion (or the time
of addition of the reducing agent), that is, time zero, until the
steady-state mixed potential is established. A typical recorded curve for the electroless deposition of copper on
copper substrate is shown in Figure 17.3. The curve has
been recorded for the system in an argon atmosphere. For a
system in air atmosphere and in the presence of additives in
the solution, the duration of the induction period can be
considerably longer [16].
The problem of the induction period for the overall
process can be resolved into problems of the open-circuit
potentials (OCPs) of the oxidation and reduction partial
reactions, that is, the individual induction period for each
process. Paunovic [16] found that the OCP for the Cu/Cn2 þ
system is reached instantaneously. A typical curve representing the change of the OCP with time for the reducing agent is
presented in Figure 17.4. By comparing these OCP values,
we can conclude that the setting of the OCP of the reducing
agent, CH2O, is the rate-determining partial reaction in the
setting of the steady-state mixed potential in this example of
electroless copper deposition.
The major factors that determine the time required to
reach the rest potential of the reducing agent are the type and
437
the concentration of the ligand present and the pH of the
solution [16].
17.4.2
Steady-State Kinetics
There are three electrochemical methods for the determination of the steady-state rate of the electroless deposition of
copper at mixed potential. Paunovic and Vitkavage [26, 27]
used polarization data in the vicinity of the mixed potential to
determine the rate of deposition [26, 27]. Ohno used alternating-current (ac) polarization measurements [28]. The
third electrochemical method is the use of the Evans diagram,
as described in Section 17.1. Ricco and Martin used an
acoustic wave device for in situ determination and monitoring of the rate of deposition [29]. Various empirical rate
equations were determined for electroless deposition of
copper [4, 6].
17.4.3
Effect of pH on the Rate of Deposition
Electroless copper deposition is affected by the pH in two
distinct ways. First, OH ions are reactants in the overall
reaction (17.1) and the partial anodic reaction (17.7) and thus
influence these reactions in a direct way (primary pH effects).
Second, pH affects various phenomena associated with the
structure and composition of the metal–solution interphase [14]. Those phenomena include (1) adsorption, (2)
the structure of the double layer, (3) the structure of the
copper species in the solution, and (4) the ionic strength of
the solution. All these phenomena modulate the rate of
electroless copper deposition in an indirect way (secondary
pH effects).
The primary pH effect is expressed in terms of the reaction
order with respect to OH ions and graphically as rate against
pH. Plots of the experimentally observed plating rates against
pH show an initial increase, a maximum value, and then a
decrease of the rate with increasing pH. An example of the
FIGURE 17.3 Induction period for the solution 0.3 M EDTA, 0.05 M CuSO4, pH 12.50, 2.5 g L1 paraformaldehyde, Cu electrode, 2.2cm2,
25 C, SCE reference electrode, argon atmosphere. (From Paunovic [16], with permission from the Electrochemical Society.)
438
ELECTROLESS DEPOSITION OF COPPER
FIGURE 17.4 Open-circuit potential for the solution 1 g L1 paraformaldehyde, pH 12.50, Cu electrode, SCE reference electrode, EDTA
variable. (From Paunovic [16], with permission from the Electrochemical Society.)
rate of electroless copper deposition as a function of pH is
shown in Figure 17.5. It can be seen from Figure 17.5 that the
maximum rate of deposition, in this specific case (EDTP
solution), is obtained at a pH value of 12.5. The initial
increase of the rate is due to the primary pH effect. The
maximum and decrease of the rate at high pH values were
interpreted in terms of secondary pH effects. The maximum
rate of deposition for the tartrate solution is at pH 12.8 [9].
Two secondary effects were suggested so far: (1) change of
the relative concentration of the methylene glycol and the
hydroxide ions with pH due to the dissociation of methylene
glycol [20] and (2) variation of the transfer coefficient for the
oxidation of formaldehyde with pH [15].
Interpretation of the pH effect in terms of mathematical
models was given by Paunovic [15]. From these it was
concluded that the maximum and the falling off of the rate
at high pH values are caused by the pH dependence of the
kinetic parameters aRed (the transfer coefficient for the
oxidation of the reducing agent; Chapter 1, Part A) and
0
iRed
(the exchange current density for the oxidation of the
reducing agent; Chapter 1, Part A). Dissociation of methylene glycol, as proposed earlier, is an important factor in the
electroless deposition of copper, but it is not sufficient to
explain the pH effect.
17.4.4
Catalysis Phenomena
Catalysis in electroless deposition of copper was studied by
Haruyama and Ohno [30) and by Wiese and Weil [31].
Haruyama and Ohno have shown that the catalytic activity
of metals for the oxidation of the reducing agent in electroless
deposition is mostly determined by the rate constants of the
two reaction steps, that is, the oxidative adsorption and
desorption of an anion radical (see Section 1.3). Wiese and
Weil have shown that copper deposition from EDTAcontaining solutions is catalyzed by chemisorbed methane–
diolatc anion.
FIGURE 17.5 Rate of electroless copper deposition as a function
of pH. The electroless copper solution contained 0.05 M CuSO4,
0.15 M EDTP, 0.07 M paraformaldehyde, and NaOH to give a
desired pH. Oxygen was removed by bubbling argon through the
solution. (From Duffy et al. [14], with permission from the Electrochemical Society.)
17.5
GROWTH MECHANISM
Mechanistically, electroless copper deposition proceeds in
two steps: (1) the thin-film stage (up to 3 mm) and (2) the bulk
stage.
STRUCTURE
17.5.1
Thin-Film Stage
The mechanism of the thin-film formation is characterized
by three simultaneous crystal-building processes [32–34]:
nucleation (formation), growth, and coalescence of threedimensional crystallites (TDCs).
In the initial stages of electroless copper deposition on a
copper single-crystal substrate, (100) plane, the average
density of TDCs increases with time of deposition; in this
stage the nucleation is the predominant process [32, 34].
Later the average density of TDCs reaches a maximum and
then decreases with time. In the stage of decreasing density of
TDCs, the coalescence is the predominant crystal-building
process [34]. A continuous electroless film is formed by
lateral growth and coalescence of TDCs. The process of
coalescence deserves special attention, since many physical
properties of deposit depend on the type of coalescence.
There are two types of coalescence of TDCs. Coalescence
without the proper filling of the space between TDCs results
in incorporation of impurities or additives, generation of
stress, voids, and dislocations (Fig. 17.6b). Coalescence with
filling the space between TDCs, favorably joined crystallites
(Fig. 17.6a), results in copper of better quality than in the first
type of coalescence [35, 36]. The process (type) of coalescence depends to a great extent on the type and concentration
of additives in the solution [34]. The initial stages of electroless copper deposition on Pd-activated nonmetallic (nonconducting) substrates were described by Sard [33] and
Rantell [37].
17.5.2
Bulk Stage
After the formation of the continuous thin film, the deposition
of a thick (1–15-mm) film proceeds, in most cases, through
the following processes [27, 38–41]: (1) preferential growth
of favorably oriented grains, (2) restriction (inhibition) of
vertical growth of nonfavorably oriented grains, (3) lateral
joining of preferentially growing grains, (4) cessation of
growth of initial grains, and (5) nucleation and growth of a
new layer of grains.
In the process of vertical and lateral growth, a preferentially growing grain (TDC) increases its width and subsequently joins laterally with other preferentially growing
grains. Eventually the width of these grains becomes
439
constant, and during further vertical growth, they develop
a columnar shape, Figure 17.7 [38–41]. Then, at a certain
stage, columnar grains no longer grow vertically. This cessation of growth of individual grains is followed by the
nucleation and growth of a new layer of grains. Cessation
of growth perpendicular to the substrate is influenced by the
overpotential and degree of inhibition. This is one of the
fundamental relationships in the correlation between (1)
structure and variables in the plating solution and (2)
structure and electrochemical kinetic parameters of processes composing electroless copper deposition.
The different growth rates on different single-crystal
substrate orientations were observed experimentally. This
experimental observation indicates that certain crystallographic surfaces are more favorable for growth than
others [40, 42]. A shift in the preferred direction of the
growth was observed depending on the solution composition
and concentration of additives [40].
17.6
STRUCTURE
We discuss two different structures: thin-film (up to 1 mm)
and thick-film (1–25 mm) structure. We also discuss microporosity in electroless copper films.
17.6.1
Thin-Film Structure
Nakahara and Okinaka [38] and Paunovic and Zeblisky [39]
have shown that thin films of electroless Cu (up to 1 mm) are
characterized by small, nearly equiaxial grains. The average
grain diameter appears to be about 0.2 mm.
17.6.2
Thick-Film Structure
As mentioned in Section 17.5, a thick film (1–25 mm) of
electroless copper has a columnar structure. Paunovic and
Zeblisky [39] have shown that electroless copper deposited
from EDTA solution containing NaCN, a wetting agent,
formaldehyde, and NaOH exhibits a pH value between
10.8 and 12.5 and that it has a columnar structure with an
average grain diameter, in a plane parallel to the substrate,
between 0.3 and 0.7 mm and an average grain size (height),
and in a plane perpendicular to the substrate, between 6
and 7 mm.
FIGURE 17.6 Two types of coalescence of the TDCs: (a) favorably joined TDC: copper of good quality; (b) improperly joined crystallites:
results in incorporation of impurities or additives, generation of stress, voids, and dislocations.
440
ELECTROLESS DEPOSITION OF COPPER
FIGURE 17.7 Schematic cross section (perpendicular to the
substrate) of the columnar deposit.
17.6.3
Microporosity
Nakahara [35, 36], using transmission electron microscopy
(TEM), has shown that both crystalline and noncystalline
films prepared by evaporation, sputtering, electrodeposition,
and electroless deposition contain a large number of microscopic voids (pores). The presence of vacancies (voids) in
thin films implies that the films contain locally unfilled
regions inside the lattice. Studies of the early stages of film
formation have shown that most microvoids are generated
at the boundaries between faceted TDCs (Section 17.5)
during their coalescence. The mechanism by which these
voids are formed is called ‘‘coalescence-induced void formation.’’ The proposed mechanism is based on the assumption that there is a geometrical misfit large enough to be left
uncovered during the coalescence of TDCs (e.g., Fig. 17.6b).
Voids inside grains could be generated during growth of
multiple-atomic steps.
Voids are important lattice defects that influence the
physical properties of a film, as is shown in the next section.
In one example the number of voids per unit volume
was
1015–10l6 cm3 and the average void size was 25 A [45].
17.6.4
Hydrogen Incorporation
According to Eq. (17.1) the deposition of 1 mol of Cu is
accompanied by the evolution of one equivalent mole of H2.
This results in the incorporation of H2 gas bubbles into the
deposit. As shown in Eqs. (17.6) and (17.8), hydrogen atoms
in H2 originate from the splitting of the C–H bond in the
formaldehyde molecule during dissociative adsorption.
Nakahara and Okinaka [38, 43–45] studied extensively the
incorporation of hydrogen into copper deposit and the
effect of hydrogen bubbles on deposit properties. The content
of hydrogen in electroless copper can be as high as
930 ppm [43].
Nakahara [45] found, using TEM, that small
(20–300-A) gas bubbles are incorporated uniformly through
out the copper films (25–30 mm), whereas large (2000-A)
bubbles are trapped at the grain boundaries. Nakahara and
Okinaka determined that the population distribution is
broad [38]. The size distribution is shown in Figure 17.8.
Thus the density of electroless copper is lower than that of
bulk copper due to the presence of incorporated hydrogen.
FIGURE 17.8 Population distribution of hydrogen gas bubbles as
a function of bubble size.
Grunwald et al. [46] determined that the density of electroless
Cu films is in the range from 8.56 to 8.76 g cm3. The density
of bulk copper is 8.9331 (0.0037) g cm3.
17.7
17.7.1
PROPERTIES
Film Ductility
Okinaka and Nakahara [47] showed that the formation of
small voids and small gas bubbles containing hydrogen are
major factors determining the ductility of electroless copper.
Nakahara and Okinaka [38] showed that brittle films contain
a large number of small as well as large gas bubbles. They
also showed that ductility promoters, such as cyanide ions,
and higher temperatures of deposition facilitate desorption of
hydrogen gas generated in the reaction given by Eq. (17.1).
Some ductility-promoting additives, for example, 2,20 -dipyridyl and K2Ni(CN)4, inhibit both the inclusion of hydrogen
and the formation of voids [44]. Table 17.1 shows an example
of the difference in properties between Cu deposited from a
solution in the absence of ductility promoters (solution A)
and a deposit from the solution containing NaCN as a
TABLE 17.1 Properties of Electroless Cu Deposit:
Plexiglas Substrate
Solution A
(Absence of
NaCN)
Solution B
(Presence of
NaCN)
1.2
4.8
0.5–1.0
9 1015
3.6
4.8
0.5–1.0
9 1014
Ductility (elongation %)
As-deposited Cu
After 6 months
Grain size (mm)
Gas bubble densitya
a
N cm3, N ¼ number of gas bubbles.
PROPERTIES
ductility promoter (solution B). It may be seen from
Table 17.1 that the difference in ductility of as-deposited
Cu and in gas bubble density is significant. The table also
shows that there is ductility recovery during room temperature storage. In this example the ductility of the brittle
copper recovered to a value comparable to that of the ductile
copper. The hydrogen content of the brittle films, obtained
from solution A, is in the range from 100 to 200 ppm.
17.7.2 Ductility Recovery during Room
Temperature Storage
The ductility (percent elongation) of electroless copper
generally increases during low-temperature (100–200 C)
annealing. Two mechanisms were proposed to interpret this
ductility recovery process.
According to the first mechanism, proposed by Nakahara
et al. [48], the ductility improvement observed is attributed to
the outdiffusion of hydrogen from the copper lattice. During
electroless copper deposition hydrogen can be codeposited in
atomic (H) as well as molecular (H2) form. Most of the
hydrogen codeposited in electroless copper is molecular. At
room temperature or at low temperature (100–200 C) annealing the molecular hydrogen diffuses out of copper,
interstitially, via a dissociative reaction
H2 ðin the gas-filled void in copperÞ ! 2Hðin copper latticeÞ
ð17:18Þ
The annealing removes all the diffusible hydrogen, leaving in copper-only residual (nondiffussible) hydrogen.
Nakahara et al. [48] distinguish four types of hydrogen
incorporated in electroless copper deposit. Details may be
found in the original literature.
According to the second mechanism, proposed by Honma
and Mizushima [49], ductility improvement is due to structural changes involving recrystallization and grain growth
in electroless copper deposit. They point out that the lowtemperature recrystallization and grain growth are commonly observed in copper films prepared by other growth
techniques such as vapor deposition [50], sputtering [51],
and electrodeposition [52, 53]. The amount of ductility
recovered in electroless copper deposition as a result of
low-temperature annealing, either by the outdiffusion or the
recrystallization mechanism, is determined also by impurity
content [44, 54].
17.7.3
Crack-Free Electroless Copper
The printed circuit (PC) industry requires electroless copper
with properties that allow the copper elements comprising
the PC boards (PCBs) to maintain their integrity during
processing and use. One critical step in processing PCBs
with plated through-holes is by mounting or exchanging
441
components by soldering. In this process the plated copper
is subjected to thermal stress. During soldering, the plated
copper in the through-holes usually expands less than the
substrate. The difference (mismatch) in the thermal expansion depends on the type of substrate and temperature. In the
case of the epoxy-glass substrate the difference at the soldering temperature (260 C) is large and the electroless
copper in this case must be of high quality in order to
maintain its integrity (continuity) during soldering [55].
Depending on the properties of the plating, the copper in
the holes either cracks or resists the stress imposed without
cracking during soldering. Paunovic and Zeblisjy [39] have
shown that when EDTA-based electroless copper solutions
containing NaCN are used the elongation (ductility) of the
25-mm-thick, crack-free copper ranges from 3 to 11%; the
tensile strength of this copper is from 200 to 600 MPa (30,000
to 87,000 psi). These wide ranges may be subdivided into two
smaller ones: class 1 with a high tensile strength and class 2
with a high elongation. A grain diameter in the plane parallel
to the substrate is 0.1–1 mm, and the grain size in the plane
perpendicular to the substrate is 4–10 mm, for the case
Studied of crack-free electroless copper deposit.
17.7.4
Electrical Resistivity
Patterson et al. determined that an electroless Cu layer of
thickness 5000 A deposited on titanium nitride has a resistivity of 2.0–2.7 mV-cm depending on the solution used [56].
Lopatin et al. [57] reported that the electrical resistivity
decreases with the increase of the deposition solution temperature (Fig. 17.9). Dubin et al. reported that the resistivity
decreases down to 1.8–1.9 mV-cm after annealing at 200 C
for 2 h in a H2 ambient [58]. Electrical resistivity of the bulk
copper is 1.7 mV-cm.
17.7.5
Electromigration Resistance
The free-electron theory of metals assumes that the valence
electrons (the conduction electrons) are virtually free to
move everywhere in the metal [59, 60]. In an electric field
the electrons drift toward the positive direction of the field,
producing an electric current in the metal. The high electronic conductivity of metals is explained in terms of the ease
with which the free electrons move [61]. According to
modern quantum electronic theory, the electrical resistivity
of a metal results from the scattering of electrons by the
lattice [61–64]. The scattering does not cause large displacement of the ions in the metal lattice when the current density
is low. However, at a high current density (above 104 A cm2)
the transport of electrons (current) can displace metal ions in
crystal lattice and cause the transport of mass (positive ions)
in the same direction as the electrons (Fig. 17.10). This mass
transport is called electromigration. It occurs in interconnecting conductors (metallic fine lines) in integrated circuits
where the current density is very high [65, 66]. For example,
442
ELECTROLESS DEPOSITION OF COPPER
FIGURE 17.9
Electroless Cu deposition rate (*) and resistivity (&) versus solution temperature.
when a 1.0-mm-wide Al (or Cu) line of 0.2 mm thickness is
subjected to a current I of 1 mA, the current density i is
5 105 A cm2 (line cross-sectional area A in this case
is 0.2 104 1 104 ¼ 0.2 108 cm2; current density
i ¼ (I/A) ¼ 1 103 A/0.2 108cm2 ¼ 5 105 A cm2).
Thus in microelectronic devices the transport of electrons
(current) can cause the transport of metal ions (mass) in a
metal lattice.
At high current densities (i > 104 A cm2) sufficient
electron momentum is transferred to metal ions in the metal
lattice to physically displace them toward the anode; hence a
net mass transport occurs, as shown in Figure 17.10. This
mass transport, electromigration, results in defect formation
in conductors in microelectronics. Conductor lines undergo
morphological changes due to electromigration where mass
depletion (voids) occurs at the cathode and extrusion (hillocks) occurs at the anode.
Aluminum-based alloys (Al–Cu, Al–Si) are most widely
used as interconnection materials in integrated circuits (ICs).
One of the problems of the Al alloys is their poor resistance to
electromigration (EM)–induced failures. One way to express
resistance to electromigration is in terms of time to failure.
Time to failure is defined as the point at which a 50% increase
of the resistance due to the electromigration stressing has
occurred. The direct-current (dc) and pulse-dc lifetime of
electroless Cu is found to be about two orders of magnitude
longer than that of Al–2% Si at 275 C and about four orders
of magnitude longer than that of Al–2% Si when extrapolated
to room temperature [67, 68].
Another way of expressing resistance to electromigration
is in terms of the activation energy for electrotransport. The
activation energy is about 0.81 eV for electroless Cu, which is
much larger than the typical 0.4–0.5 eV for Al alloys [67].
Thus Cu lines in ICs are expected to have a larger lifetime
than the Al–Si or Al–Cu alloy. For this reason, and because of
the higher conductivity of Cu, electroless Cu and electrodeposited Cu are considered for application as conductors in
IC fabrication [69–72].
FIGURE 17.10 Atomic model of electromigration involving electron momentum transfer to metal ions in the metal lattice during a highcurrent-density flow (i > 104 A cm2).
443
FORMATION OF CU NANOPARTICLES BY ELECTROLESS DEPOSITION OF CU
17.8 DEPOSITION OF ELECTROLESS COPPER
FOR IC ABRICATION
The feasibility of using electroless copper deposition for IC
fabrication has been demonstrated by Ting and co-workers [73–75]. A selective electroless metal deposition process
is a very attractive alternative to the conventional IC fabrication process. At the time of writing, there is renewed intrest
in activity in this area.
Electroless deposition of copper for IC fabrication may be
done on (a) noncatalytic and (b) catalytic surfaces.
17.8.1
Activation of Noncatalytic Surfaces
Two major types of processes have been used to produce
catalytic metallic surfaces: (c) electrochemical and (d)
photochemical.
Electrochemical Activation The catalytic metallic nuclei
of metal M on the noncatalytic surface S can be generated in
an electrochemical oxidation–reduction reaction,
Mn þ þ Red ! M þ Ox
ð17:19Þ
where Mn þ is the metallic ion and M is the metal catalyst. In
many cases the preferred reducing agent Red is Sn. The
preferred nucleating agent Mn þ is Pd. The palladium
catalytic sites on the activated surface are dispersed on the
surface of a substrate in an island-type network [33, 76, 77].
Photochemical Activation Catalytic metallic nuclei of Pd,
Pt, Au, and Cu can be generated in an intramolecular-type
electron transfer resulting from absorption of photon. For
example, catalytic palladium can be formed in the
photochemical reaction
PdAc ! Pd þ Ox
ð17:20Þ
where Ox is the oxidation product of acetate ion, Ac [78].
Other photochemical methods were, for instance, reviewed
by Paunovic [78].
17.8.3 Electroless Deposition of Copper on Catalytic,
Activated Surfaces, and Diffusion Barriers
It is possible to directly deposit electroless Cu on tungsten (W)
barrier layer, as shown by Kim et al. [81]. They used glyoxylic
acid as a reducing agentas proposed by Sacham-Diamand [82].
The use of glyoxylic acid was proposed as a nontoxic,
environmentally friendly alternative to formaldehyde.
Since barrier metals have relatively high electrical resistivity (e.g., Ta is 12.4 mV-cm), it is necessary to cover the
barrier layer with a conductive metal layer. This conductive
metal layer may be a Cu seed layer deposited using PVD or
CVD. When the electroless Cu deposition on a bilayer of
barrier/Cu seed layer is completed, vias and trenches may be,
filled with electroless Cu. The excess Cu is removed using
chemical–mechanical polishing [83].
An activated surface of TiN barrier layer was used for
electroless Cu deposition [84]. The TiN was activated in a
CuSO4–HF solution. For activation, Pd was used, itself
deposited on the barrier layer either by (a) chemical reaction
in solution or by (b) PVD. The Pd catalytic layer can be
formed by an ionized cluster beam (ICB). Electroless Cu film
was successfully deposited on a TaN barrier over a Pd
catalytic layer [85, 86].
17.9 FORMATION OF CU NANOPARTICLES
BY ELECTROLESS DEPOSITION OF CU
Copper nanoparticles are of great interest in microprinting
technology in electronics. Understanding the mechanism of
Cu nanoparticle formation and control of particle size is very
important for this application. Yagi et al. [87] formed Cu
nanoparticles electrochemically (electroless deposition) using
a hydrazine aqueous solution and dispersed CuO particles.
The deposition rate of Cu nanoparticle formation was
examined in situ by an electrochemical quartz crystal microbalance (QCM). Hydrazine acts as a reducing agent:
N2 H4 þ 4OH ! N2 þ 4H2 O þ 4e
17.8.2
Diffusion Barriers
Fabrication of interconnects on chips made of copper introduces new problems, the most important of which is the
diffusion of Cu into Si, SiO2, and other dielectrics [79] and
the reaction of Cu with Si, forming silicides. Diffusion of Cu
through Si results in poisoning of devices (transistors) and
diffusion through SiO2 leads to degradation of dielectrics.
Thus, diffusion barrier layers are an integral part of the
fabrication of copper interconnects. Barrier films isolate
(encapsulate) Cu interconnects from adjacent dielectric material. Diffusion barrier layer is usually formed by physical
vapor deposition (PVD) or chemical vapor deposition
(CVD). However, thin electroless Co or Ni diffusion barriers
were demonstrated by Yoshino et al. [80].
N2 H4 þ OH ! 12 N2 þ NH3 þ H2 O þ e
ð17:21Þ
ð17:22Þ
The reduction of Cu2 þ ions by hydrazine proceeds
through the reactions
2Cu2 þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð17:23Þ
Cu2þ þ2N2 H4 þ2OH !CuþN2 þ2NH3 þ2H2 O ð17:24Þ
Fabrication of Cu nanoparticles was done in the following
steps. First, a CuO colloidal aqueous suspension was prepared by dispersing CuO powder in distilled water. Next,
aqueous solution of gelatin was added as a dispersing agent.
Hydrazine solution was then added to the CuO aqueous
suspension as a reducing agent to deposit Cu nanoparticles.
444
ELECTROLESS DEPOSITION OF COPPER
17.10 ELECTROCHEMICAL CONTROL SYSTEM
FOR ELECTROLESS COPPER DEPOSITION
TABLE 17.A2 Solution Composition and Plating
Condition for Electroless Cu Deposition Using
Glyoxylic Acid as Reducing Agent
Electroless deposition of Cu for IC fabrication demands the
computerized in situ monitoring and control of the deposition
process. An automatic analyzer for the analysis of the
composition and performance of the production of electroless Cu [88–91] is described here. The duration of the
electrochemical analysis is in the range of milliseconds, or
seconds, at most.
This work describes the applications of chronopotentiometry and voltametry in the study and control of electroless
Cu deposition. An EDTA-type electroless Cu solution was
used for these studies, and CH2O (formaldehyde) was the
reducing agent. The anodic partial reaction is described in the
Section 17.2.
CuSO45H2O
EDTA
CHOCOOH
(glyoxylic acid)
2,20 -Bipyridine
Temperature
pH (adjusted with NaOH),
air agitation, continuous
17.10.1
Chronopotentiometry
The potential–time curves were recorded on a Tetronix
storage oscilloscope. Oscilloscope traces were photographed
with a Polaroid camera. Chropotentiograms can be used for
determination of cupric ions and formaldehyde. However, in
this application this is not necessary. It is sufficient to detect
the transition time only. This can be done automatically by
introducing a time interval counter into the electrolysis
circuit [89].
The effect of additives can be determined by recording
potential time curves in the millisecond or microsecond
range [90].
Voltammetry
The voltammograms were obtained with a PAR (Princeton
Applied Research) polarographic analyzer, and the current–
voltage was recorded on an X–Y recorder. Voltammetry, or
cyclic voltammetry [91], can be used for determination of
formaldehyde, cupric ions, additives, and impurities.
TABLE 17.A3 Solution Composition and Plating
Conditions for Electroless Cu Deposition Using
Hypophosphite as Reducing Agent
TABLE 17.A1 Electroless Copper Deposition Solution and
Plating Conditions from an Alkali-Free Solution
Source: Shacham-Diamand [92, p. 136].
0.024 M
0.002 M
0.5 M
0.27 M
0.052 M
0.026 M
9.2
CuSO4
NiSO4
H3BO3
NaH2PO2
Na3C6H5O7 (sodium citrate)
EDTA
pH
Sources: Hung and Chen [95] and Saubestre [96].
TABLE 17.A4 Solution Composition and Plating Conditions
for Electroless Cu Deposition Using Tartrate as Complexing
Agent and Formaldehyde Reducing Agent
CuSO45H2O,g L
KNaC4H4O64H2O, g L1
(sodium potassium tartrate)
NaOH, g L1
MBT, g L1
(mercaptobenzothiazolea)
HCOOH (37%),
(formaldehyde)b mL L1
Temperature, C
Solution A
Solution B
5
25
13
66
7
19.3
—
0.013
10
20
38
25
Sources: Solution A: Goldie [97]; Solution B: Pearlstein [98].
APPENDIX
CuSO45H2O
N(C2H5)4OH
EDTA
CH2O
N(CH3)4CN
GAF RE–610
Temperature
pH, adjusted with N(C2H5)4OH,
tetraethylammonium hydroxide
10 ppm
60 C
12.5
Source: Honma and Kobayashi [93] and Burke, Bruton, and Collins [94].
1
17.10.2
0.03 M
0.24 M
0.20 M
0.05–0.1 M
0.5–l.0 M
0.1 M
0.01 M
0.01 M
0.5–2%
45–55 C
11.9–12.3
a
Added as solution of 10 g L1 MBT in 0.2 M NaOH.
b
Formaldehyde solution with 12.5% methanol as preservative.
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