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Metal microdistribution in electroless copper plating
Surface
and Coatings
METAL
Technology,
MICRODISTRIBUTION
29 (1986)
73
- 76
IN ELECTROLESS
R. G. GOLOVTSHANSKAYA, REKUS
S. S. KRUGLIKOV,
Mendeleyev
Technology,
(Received
Institute August
of Chemical
73
Moscow
N. A.
COPPER
PLATING
MOROZOVA
and
N.
G.
(U.S.S.R.)
20,1985)
Summary It is shown that a copper electroless plating solution without addition agents produces either positive or negative levelling depending on the pH. The positive levelling power increases with the temperature, solution agitation and pH value and decreases with an increase in bath loading. A mechanism of levelling action based on the electrochemical nature of an electroless copper plating reaction is proposed.
1. Introduction It nickel effects plating
has been shown recently [l, 21 that true levelling may take place in and copper electroless plating processes. This paper will discuss the of plating conditions on the levelling power of a copper electroless solution based on formaldehyde as the reducing agent.
2. Experimental
details
Flat 2.5 cm X 1.6 cm nickel plates with a series of parallel ridges of triangular cross-section were used in the plating experiments. The surface milcroprofile is shown in Fig. 1. The levelling power P was calculated using the equation [ 31
p=: -
2.3a
(1)
2x&
where a is the distance between the peaks, i.e. the period of the microprofile (195 pm), h,,, is the average plate thickness and Ho and Ht are the peak heights before and after copper plating. Equation (1) was derived for a sinewave microprofile and, therefore, the microprofile shown in Fig. 1 can be used only for a comparative evaluation of the levelling power.
@ Elsevier Sequoia/Printed
in The Netherlands
74
Fig. 1. Initial
microprofile
of the cathode
surface.
In each plating experiment a copper deposit 12 - 14 pm thick was obtained, and the deposition rate was expressed as the average value for the whole plating time. At the start of each experiment the electroless copper plating solution contained 0.08 M CuS04*5H20, 0.16 M ethylenediaminetetraacetic acid (EDTA), 0.1 M Na&Os and 0.13 M formaldehyde; the pH was adjusted to a prescribed value by adding concentrated NaOH solution. During the course of each experiment the solution was agitated by a magnetic stirrer and NaOH solution was added continuously to maintain a constant pH.
3. Results
and discussion
The plots in Figs. 2 and 3 show that at a relatively low bath loading the electroless copper plating solution produces appreciable true levelling, especially at higher temperatures and in stirred solutions. The effect of the formaldehyde concentration on the average deposition rate (see Fig. 4) suggests possible transport control of the overall reaction even in stirred solutions. At higher temperatures the overall deposition rate increases considerably (at 40 “C the rate is double that at 25 “C). Consequently, at 40 “C the role of diffusion may be even more important. The microdistribution of copper deposits may be considered in terms of an electrochemical mechanism of the electroless deposition process. According to both Donahue and Meerakker [4, 51, anodic oxidation of formaldehyde involves dissociative chemisorption of the methylene glycol anion as
P OL
P 414
0, 3
0.12
0,2
o.ro
0.1
i
Fig. 2. Levelling power US. bath curve 2, 32 “C; curve 3, 40 “C.
loading
Fig. 3. Levelling power us. bath loading 1, 25 “C; curve 2, 32 T;curve 3, 40 “C.
in unstirred in stirred
2
solutions
solutions
(400
(pH 12.5):
curve
1, 25 “C;
r.p.m.)
(pH 12.5):
curve
75
20
40
60
80
ff.5
C,MtyL
jZ.0
Fig. 4. Effect of initial formaldehyde concentration on the rate (curve 1) and levelling power (curve 2) (25 “C; pH 12.5; 400 r.p.m.; 1-l). Fig. 5. Effect of pH on the rate of copper deposition (curve 2) (25 “C; 400 r.p.m.; bath loading, 1.8 dm’l-I).
(curve
12.5
ao> pH
of copper deposition bath loading, 1.8 dm’ 1) and levelling
power
a first step: CH,(OH)O-
-
CH(OH)O-,,,
This is succeeded
+ Hads
by oxidation
CH(OH)O-,,,
+OH--HCOO~+H,O+e-
and hydrogen
recombination
=L,,--
H2
(2)
(3)
(4)
The cathodic reaction is the reduction of the Cu-EDTA complex. The electrochemical mechanism suggests that the local rates of cathodic and anodic reactions do not necessarily coincide, i.e. at some sites the highest rate may be that of formaldehyde oxidation and at the others that of copper reduction. However, the reactants, intermediates and final products of both the cathodic and the anodic reactions usually exert an effect on the kinetics of the two reactions. Since the anodic oxidation of formaldehyde is at least partially controlled by diffusion, the rate of this reaction should be greater on micropeaks. Therefore, surface coverage by adsorbed species must also be greater on micropeaks. Possible inhibition of cathodic and anodic reactions by adsorbed reactants and products of the anodic reaction would lead to relative reduction in the rate of the cathodic reaction on micropeaks. Consequently, on microrecesses the rate of copper deposition must be greater than on micropeaks. At pH values below 12.0 the rate of copper reduction decreases considerably and the levelling power drops to negative values (Fig. 5). Hydroxyl ions are consumed in the anodic reaction (3) and as the pH is lowered their rate of consumption may approach the limiting rate of diffusion. Under such conditions surface coverage by adsorbed organic species should depend on the local rate of supply of hydroxyl ions. The greater the rate of supply, the lower must be the local surface coverage by organic
(bJ
(a)
Fig. 6. Photomicrographs of the copper deposits 25 “C; no agitation; bath loading, 1.8 dm2 1-r).
on peak
(a) and recess
(b) (pH 11.5;
species and their inhibiting action on the rate of cathodic reaction. Thus, at a pH of 11.5 or below the microdistribution of the rate of copper deposition changes abruptly compared with more alkaline solutions. The photomicrograph (Fig. 6) of the copper deposit illustrates the strong negative levelling which takes place at pH 11.5.
Acknowledgment The authors helpful discussions
express their on the paper.
thanks
to Professor
K. M. Gorbunova
for
References 1 M. V. Novikova, R. G. Golovtshanskaya, S. S. Kruglikov and N. G. Rekus, EJekfrokhim. (USSR), 19 (1983) 960. 2 R. G. Golovtshanskaya, S. S. Kruglikov, M. V. Novikova, N. G. Rekus and T. G. Anisimova, 28 Internationales Wissenschaffliches KoJJoquium, October 24 - 28, 1983, Technische Hochschule Ilmenau (D.D.R.), Heft 2, S. 265. 3 S. S. Kruglikov and N. Ya. Kovarski, Ifogi Nauki Tekh. Elektrokhim., 10 (1975) 106. 4 J. A. Meerakker, J. AppJ. Elecfrochem., 11 (1981) 387. 5 F. M. Donahue, Oberfltiche-Surf., 13 (1972) 301.
and Coatings
METAL
Technology,
MICRODISTRIBUTION
29 (1986)
73
- 76
IN ELECTROLESS
R. G. GOLOVTSHANSKAYA, REKUS
S. S. KRUGLIKOV,
Mendeleyev
Technology,
(Received
Institute August
of Chemical
73
Moscow
N. A.
COPPER
PLATING
MOROZOVA
and
N.
G.
(U.S.S.R.)
20,1985)
Summary It is shown that a copper electroless plating solution without addition agents produces either positive or negative levelling depending on the pH. The positive levelling power increases with the temperature, solution agitation and pH value and decreases with an increase in bath loading. A mechanism of levelling action based on the electrochemical nature of an electroless copper plating reaction is proposed.
1. Introduction It nickel effects plating
has been shown recently [l, 21 that true levelling may take place in and copper electroless plating processes. This paper will discuss the of plating conditions on the levelling power of a copper electroless solution based on formaldehyde as the reducing agent.
2. Experimental
details
Flat 2.5 cm X 1.6 cm nickel plates with a series of parallel ridges of triangular cross-section were used in the plating experiments. The surface milcroprofile is shown in Fig. 1. The levelling power P was calculated using the equation [ 31
p=: -
2.3a
(1)
2x&
where a is the distance between the peaks, i.e. the period of the microprofile (195 pm), h,,, is the average plate thickness and Ho and Ht are the peak heights before and after copper plating. Equation (1) was derived for a sinewave microprofile and, therefore, the microprofile shown in Fig. 1 can be used only for a comparative evaluation of the levelling power.
@ Elsevier Sequoia/Printed
in The Netherlands
74
Fig. 1. Initial
microprofile
of the cathode
surface.
In each plating experiment a copper deposit 12 - 14 pm thick was obtained, and the deposition rate was expressed as the average value for the whole plating time. At the start of each experiment the electroless copper plating solution contained 0.08 M CuS04*5H20, 0.16 M ethylenediaminetetraacetic acid (EDTA), 0.1 M Na&Os and 0.13 M formaldehyde; the pH was adjusted to a prescribed value by adding concentrated NaOH solution. During the course of each experiment the solution was agitated by a magnetic stirrer and NaOH solution was added continuously to maintain a constant pH.
3. Results
and discussion
The plots in Figs. 2 and 3 show that at a relatively low bath loading the electroless copper plating solution produces appreciable true levelling, especially at higher temperatures and in stirred solutions. The effect of the formaldehyde concentration on the average deposition rate (see Fig. 4) suggests possible transport control of the overall reaction even in stirred solutions. At higher temperatures the overall deposition rate increases considerably (at 40 “C the rate is double that at 25 “C). Consequently, at 40 “C the role of diffusion may be even more important. The microdistribution of copper deposits may be considered in terms of an electrochemical mechanism of the electroless deposition process. According to both Donahue and Meerakker [4, 51, anodic oxidation of formaldehyde involves dissociative chemisorption of the methylene glycol anion as
P OL
P 414
0, 3
0.12
0,2
o.ro
0.1
i
Fig. 2. Levelling power US. bath curve 2, 32 “C; curve 3, 40 “C.
loading
Fig. 3. Levelling power us. bath loading 1, 25 “C; curve 2, 32 T;curve 3, 40 “C.
in unstirred in stirred
2
solutions
solutions
(400
(pH 12.5):
curve
1, 25 “C;
r.p.m.)
(pH 12.5):
curve
75
20
40
60
80
ff.5
C,MtyL
jZ.0
Fig. 4. Effect of initial formaldehyde concentration on the rate (curve 1) and levelling power (curve 2) (25 “C; pH 12.5; 400 r.p.m.; 1-l). Fig. 5. Effect of pH on the rate of copper deposition (curve 2) (25 “C; 400 r.p.m.; bath loading, 1.8 dm’l-I).
(curve
12.5
ao> pH
of copper deposition bath loading, 1.8 dm’ 1) and levelling
power
a first step: CH,(OH)O-
-
CH(OH)O-,,,
This is succeeded
+ Hads
by oxidation
CH(OH)O-,,,
+OH--HCOO~+H,O+e-
and hydrogen
recombination
=L,,--
H2
(2)
(3)
(4)
The cathodic reaction is the reduction of the Cu-EDTA complex. The electrochemical mechanism suggests that the local rates of cathodic and anodic reactions do not necessarily coincide, i.e. at some sites the highest rate may be that of formaldehyde oxidation and at the others that of copper reduction. However, the reactants, intermediates and final products of both the cathodic and the anodic reactions usually exert an effect on the kinetics of the two reactions. Since the anodic oxidation of formaldehyde is at least partially controlled by diffusion, the rate of this reaction should be greater on micropeaks. Therefore, surface coverage by adsorbed species must also be greater on micropeaks. Possible inhibition of cathodic and anodic reactions by adsorbed reactants and products of the anodic reaction would lead to relative reduction in the rate of the cathodic reaction on micropeaks. Consequently, on microrecesses the rate of copper deposition must be greater than on micropeaks. At pH values below 12.0 the rate of copper reduction decreases considerably and the levelling power drops to negative values (Fig. 5). Hydroxyl ions are consumed in the anodic reaction (3) and as the pH is lowered their rate of consumption may approach the limiting rate of diffusion. Under such conditions surface coverage by adsorbed organic species should depend on the local rate of supply of hydroxyl ions. The greater the rate of supply, the lower must be the local surface coverage by organic
(bJ
(a)
Fig. 6. Photomicrographs of the copper deposits 25 “C; no agitation; bath loading, 1.8 dm2 1-r).
on peak
(a) and recess
(b) (pH 11.5;
species and their inhibiting action on the rate of cathodic reaction. Thus, at a pH of 11.5 or below the microdistribution of the rate of copper deposition changes abruptly compared with more alkaline solutions. The photomicrograph (Fig. 6) of the copper deposit illustrates the strong negative levelling which takes place at pH 11.5.
Acknowledgment The authors helpful discussions
express their on the paper.
thanks
to Professor
K. M. Gorbunova
for
References 1 M. V. Novikova, R. G. Golovtshanskaya, S. S. Kruglikov and N. G. Rekus, EJekfrokhim. (USSR), 19 (1983) 960. 2 R. G. Golovtshanskaya, S. S. Kruglikov, M. V. Novikova, N. G. Rekus and T. G. Anisimova, 28 Internationales Wissenschaffliches KoJJoquium, October 24 - 28, 1983, Technische Hochschule Ilmenau (D.D.R.), Heft 2, S. 265. 3 S. S. Kruglikov and N. Ya. Kovarski, Ifogi Nauki Tekh. Elektrokhim., 10 (1975) 106. 4 J. A. Meerakker, J. AppJ. Elecfrochem., 11 (1981) 387. 5 F. M. Donahue, Oberfltiche-Surf., 13 (1972) 301.