A Study of Copper Corrosion for a Cleaning Process with Quartz ...

International Conference on Planarizaiton/CMP Technology ・ November 19 – 21, 2009 Fukuoka
A Study of Copper Corrosion for a Cleaning Process with Quartz Crystal
Microbalance and Comb Electrodes
Shohei Shima, Yutaka Wada, Katsuhiko Tokushige, Akira Fukunaga, and
Manabu Tsujimura
Precision Machinery Company, Ebara Corporation
4-2-1 Honfujisawa, Fujisawa-shi 251-8502, Japan
E-mail: shima.shohei@ebara.com
Keywords: copper, corrosion, quartz crystal microbalance, comb electrode, dissolved oxygen,
conductivity
Abstract. We used a 10 MHz quartz crystal microbalance (QCM) to detect small mass changes
during the copper surface cleaning process. The geometrical effect of galvanic corrosion, such as
the cathode-to anode area ratio was quantitatively determined and we found that the etching rate
was proportional to the square root of the area ratio. Corrosion phenomena in de-ionized water
(DIW) were clarified in terms of dissolved oxygen (DO) and conductivity (S) of DIW. It was not
sufficient to suppress the DO concentration in DIW alone to decrease the copper corrosion rate.
Suppression of DIW conductivity also becomes necessary when a potential difference exists on Cu
films locally. The time constant for the diffusion of oxygen into a DIW layer was 0.6 sec in a 10
um layer and 0.06 sec in a 1 um layer. This shows that the oxygen diffusion into DIW is
significantly fast. We were able to estimate that in an actual spin-rinse-dry (SRD) process, the
DIW layer was immediately saturated in oxygen in atmospheric conditions.
Introduction
During chemical mechanical planarization (CMP), post-CMP cleaning has been one of the critical
processes influencing the yield of devices [1]. It is required for etching off particles and residues
from the surface of copper without roughening the surface during the cleaning process. On the
other hand, during the polishing process, we can add the chelate agents into slurries to form a thick
passivation layer on the polishing surface. During the cleaning process, the thickness of the
paasivated surface layer needs to be as thin as possible to maintain good adhesion to the insulating
layer above it and to have a low contact resistance in the via hole. For this reason, Cu corrosion
susceptibly is remarkably low during the cleaning process. We conducted a Cu corrosion study
with a quartz crystal microbalance (QCM) [2-4] to detect small changes in mass, and used comb
electrodes to detect the conductivity of electrolyte. We examined the effect of both dissolved
oxygen (DO) and conductivity on Cu corrosion in de-ionized water (DIW) and citric acid. The
comb electrodes were used to detect the DO change in the spin-rinse-dry (SRD) process.
Experimental
Copper corrosion phenomena were evaluated using a potentiostat system with three electrodes and
the etching rate was measured with a QCM used as a working electrode as shown in Fig.1. The
frequency of the QCM was 10 MHz and the electrode area was 3 mm in diameter, so we were able
to detect mass changes as small as 0.31 ng/Hz. Copper films were sputtered on an Au electrode of
the QCM using a metal mask. The area of the counter electrode is typically 3 cm 2 and we used
Ag/AgCl as a reference electrode.
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International Conference on Planarizaiton/CMP Technology ・ November 19 – 21, 2009 Fukuoka
We used DIW and citric acid as electrolytes for our corrosion study and examined the
dependence of the etching rate on concentration of DO and on the conductivity (S) of the
electrolytes. DO was adjusted by N 2 bubbling in DIW and S was controlled using Na 2 SO 4 as a
supporting electrolytes. We measured DO and S by commercial meters in bulk electrolyte. In the
CMP process, many SRD systems were used in CMP equipments. The thickness of the electrolyte
layer on the wafer may actually be less than several hundred micro meters in the SRD process, so
we tried to estimate DO and S in the thin layer of electrolyte with a comb electrode as shown in
Fig.2. The comb electrode was made of gold and patterned with lines and spaces of 2 um with 65
fins, and the total space length was 130 mm. We dispensed drops of DIW into the window of a
comb electrode using a pipet and monitored the resistivity change between two electrodes (W1
and W2) .
Results and Discussion
Galvanic effects
Figure 3 shows the galvanic corrosion rate in three different systems of Cu/Cu, Cu/Ta and Cu/Ru
in 0.5 mole/l citric acid. The etching rate in the Cu/Cu system is nearly zero, because both of the
anode (QCM) and the cathode (CE) are composed of the same material and the potential difference
between the two electrodes is zero. We measured the oxidation reduction potential for Cu and Ta,
and they were – 20 mV and -79 mV respectively. In the Cu/Ta system, Cu was nobler than Ta in
citric acid and, as a result Cu could not be etched. On the other hand, the oxidation reduction
potential of Ru was 651 mV and Ru is significantly nobler than Cu. The potential difference
between Cu and Ru was 671 mV, so that the etching rate of Cu was markedly large due to the
galvanic effect.
It is said that galvanic corrosion is influenced by geometrical effects such as the anode-tocathode
area ratio; an increasing in the etching rate occurs when the ratio of anode to cathode
becomes large. The QCM area is constant and we can not change the anode area, so we varied the
cathode area of Ru. Figure 4 shows the etching rate and corrosion current dependency on the area
ratio of the cathode (Ru ) by the anode (Cu-QCM) in the Cu/Ru system. Both etching rate and
corrosion current increased as the area ratio increased and it was quantitatively found that the
proportionality factor was equal to the square root of the area ratio.
Conductivity and dissolved oxygen effects
Figure 5 shows the relation between the Cu etching rate and the anodic potential for DIW and
citric acid with varying the conductivity. DO was saturated and nearly 8 ppm in all conditions of
DIW and citric acid. In the case of DIW without Na 2 SO 4 , there was no corrosion even at oxidation
potential of 210 mV. However, with the addition of Na 2 SO 4 , Cu is easily corroded and this shows
that corrosion in DIW is very sensitive to the conductivity of DIW. On the other hand, the effect of
conductivity on the etching rate is smaller in citric acid. The effects of both conductivity and DO
will be discussed more precisely, later (see Fig.7).
We examined the effect of the supporting electrolyte on the electrode reaction of electrochemicals.
All data in Fig.5 is plotted in relation to the corrosion current in Fig.6. All experimental data are
aligned in a straight line. This result shows that Na 2 SO 4 works as a conductivity controlling agent
alone and does not affect the electrode reaction. We found that Cu was ionized as cupric ions, Cu
++, both in DIW and in citric acid, by calculating and comparing the amount of total coulomb
consumed in the reaction and the etched mass of Cu.
Figure 7 shows the dependence of the etching rate on the conductivity of both in DIW and
citric acid with the addition of Na 2 SO 4 of 0.01, 0.1 and 1 mole/l. The etching rate is very low at 0
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International Conference on Planarizaiton/CMP Technology ・ November 19 – 21, 2009 Fukuoka
mV potential both in DIW and in citric acid, even in high conductivity and high DO conditions.
The etching rate increases at a higher potential where oxidation initiates at the Cu QCM electrode.
We examined the effect of DO on the etching rate by controlling the DO concentration with N 2
bubbling in the electrolyte. In the case of citric acid, the etching rate was extremely reduced by
decreasing the DO concentration to 0.6 ppm by N 2 bubbling, even at 100 mV potential. However,
the amount of reduction in the etching rate in DIW is smaller than that of citric acid. A further
decrease in the etching rate can be accomplished by decreasing the conductivity in DIW. These
results suggest that the concentrations of both DO and conductivity should be suppressed to avoid
corrosion in the DIW cleaning process. In citric acid, the surface of Cu is more effectively
passivated with a complex layer of citrate in a lower DO concentration, independent of
conductivity.
DO change of DIW due to the dissolution of oxygen from air
We examined the DO concentration decrease with N 2 bubbling at first, followed by the
measurement of the DO increase due to the dissolution of oxygen from air using a DO meter. A
decrease in DO concentration is rapid at the time constant of about 50 sec in 300 ml DIW with 1
litter/min N 2 bubbling. However, a DO increase due to the dissolution of oxygen from air is
markedly slow at the time constant of about 660 sec, as shown in Fig.8. We monitored the
conductivity change of DIW with a conductivity meter and the conductivity also increased with
nearly the same time constant. This result shows that we can monitor the conductivity of DIW with
a direct relation to the DO change.
Estimation of DO change of thin layer of DIW with comb electrode
Figure 9 shows the resistivity change of a small droplet of DIW with a low DO concentration on an
Au comb electrode due to the dissolution of oxygen from air. The time constant of the droplet is
about 100 sec in 10 micro litters and in a larger droplet of 50 micro litters shows a larger value of
181 sec. We think this is dependent upon the thickness of the droplet, more specifically, on the
oxygen diffusion from surface to bottom at the comb electrode. We assume that the shape of
droplet could be half of a sphere, and estimate the thickness of the droplet. Figure 10 shows the
relation between the thickness of the DIW layer of a droplet and the time constant. The two results
show a linear relationship, which allows us to estimate the time constant of thinner DIW layers. In
an SRD system, the water layer on a wafer during the DIW cleaning process may be less than 100
um, so we estimate that the time constant is 6sec, 0.6 sec and 0.06 sec at 100 um, 10 um and 1 um,
respectively. During the SRD process, the DIW layer becomes thinner proceeding the drying
process. This result showed that during the SRD process in air, the DO concentration becomes
greater and immediately saturated. If the wafer has a potential non-uniformity locally, the Cu film
on the wafer is apt to corrode easily when DIW has a high conductivity even with a low DO
concentration at first.
Summary
We can evaluate the corrosion phenomena easily and precisely in a quantitative manner using the
QCM technique. In galvanic corrosion, the etching rate depends on the electrode area ratio with
the square root of the anode-to-cathode ratio. It is not sufficient to suppress DO concentration in
DIW to decrease corrosion. The conductivity of DIW must also be suppressed when a potential
difference exists locally on Cu films. The diffusion of oxygen into a DIW layer is markedly fast on
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International Conference on Planarizaiton/CMP Technology ・ November 19 – 21, 2009 Fukuoka
a thin layer on a wafer. In the actual SRD process in air, the DIW layer is immediately saturated in
oxygen when in atmospheric conditions.
Acknowledgement
We would like to acknowledge Jung-whan Lee and Min-soo Kim of Hanyang University for their
help in measuring the etching rate of Cu films depending on DO and conductivity. We also wish to
show our appreciation to Professor Park, who arranged the internship between Ebara and Hanyang
University.
References
[1] M. Imai, Y. Yamashita, T. Futatsuki, M. Shinohara, S. Kondo and S. Saito: Effect of Dissolved
Oxygen on Cu Corrosion in Single Wafer Cleaning Process, Jpn. J. of Appl. Phys., 48 (2009),
pp 04C023 1-4.
[2] S. Zakipour, C. Leygraf and G. Portnoff: Studies of Corrosion Kinetics on Electrical Contact
Materials by Means of Quartz Crystal Microbalance and XPS: J.Electrochm.Soc., 133 (1986),
pp 873-876.
[3] W. Lu, J. Zhang, F. Kaufman and A.C. Hiller: A Combined Triboelectrochemical QCM for
Studies of the CMP of Copper, J.Electrochem.Soc., 152 (2005), pp B17-22.
[4] D.W. Peters: Incorporation of Cu Passivation in Post-CMP Cleaners, ECS Transactions, 11
(2) (2007), pp 447-454.
Figure 1. Etching rate evaluation system
using QCM and potentiostat.
Figure 2. Layout of Au comb electrode.
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International Conference on Planarizaiton/CMP Technology ・ November 19 – 21, 2009 Fukuoka
Figure 3. Galvanic corrosion rate for
three metal system.
Figure 4. Electrode area ratio dependence
on etching rate and corrosion current in
Cu/Ru system.
Figure 5. Comparison of etching rate of
DIW and Citric acid for concentration of
supporting electrolytes.
Figure 6. Relation between etching rate
and corrosion current for all data.
Figure 7. Conductivity effects on etching
rate for DIW and citric acid.
Figure 8. Changes of DO and S during
oxygen dissolution in DIW in air.
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International Conference on Planarizaiton/CMP Technology ・ November 19 – 21, 2009 Fukuoka
Figure 9. Resistance change between Au
comb electrodes during oxygen
dissolution in air.
Figure 10. Time constant of small droplet
dipped on comb electrodes and estimated
value of thin DIW layer.
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