International Conference on Planarizaiton/CMP Technology ･ November 19 – 21, 2009 Fukuoka
Mechano-Electrical Polishing Mechanism of SiO2 by Cerium Oxide Slurry
Yoshio Homma1, a, Tsuyoshi Masuda 1,b , Jayakrishnan Nair 1,c, and James Shen2
1
Asia Pacific Technology Center, Nihon Cabot Microelectronics KK,
1287-19, Kitakoyama, Geino-cho, Tsu-city, Mie 514-2213, Japan
2
Cabot Microelectronics Co., 870 N. Commons Drive, Aurora, IL 60504, USA
a
yoshio_homma@cabotcmp.com, b tsuyoshi_masuda@cabotcmp.com,
c
Jayakrishnan_Nair@cabotcmp.com
Keywords: CMP, Ceria slurry, Adhesion, Pitting, Defects, pH, Valence value
Abstract.
Polishing mechanism of SiO2 films by cerium oxide slurry was analyzed by focusing on
defectivity, adhesion and valence value of cerium oxide abrasive. Using acid-type cerium oxide
slurries, influence of pH on pitting defectivity and adhesion was analyzed. By increasing pH,
removal rate (RR) generally increases in acid region, then saturates in neutral region, then
decreases. Pitting defects and adhesion were generated in weak acid to neutral region, and severe
agglomeration was observed in alkaline region. It was also shown some kinetic energy as well as
down force is necessary to cause adhesion. By adding oxidizers into the slurry, it was observed
that stronger oxidizer enlarges the pitting size, and di-ammonium cerium nitrates (ACN-IV with
Ce4+) eliminated the pitting, while ACN-III (with Ce3+) exhibited no effects. It is presumed that,
ceria (CeO2) abrasive collides onto SiO2 surface, being wrapped with SiO2 layer, and changes to
cerous (Ce2O3). When the cerous abrasive accepts a positive charge from the solution, the cerous
changes to ceria and leaves from the film surface. If the abrasive does not get the charge, the
cerous remains on the surface.
Introduction
Cerium oxide slurry has gathered much attention due to its superior planarization capability to
conventional silica slurry. Usually, cerium oxide is used in an alkaline dispersion in order to attain
larger removal rate by adding alkaline additives and organic surfactants [1][2]. Cerium oxide
abrasive used as such alkaline slurry, however, is known to be quick to settle, sticks on SiO2
surface (adhesion) and needs strong chemical cleaning. In addition, the cost of raw cerium oxide
has become continuously increasing, resulting in high slurry cost. Thus, a cerium oxide slurry is
desired with slow settling, little adhesion/residue generation and low abrasive concentration.
Carter and Johns disclosed that a certain group of acidic additives, such as gultamic, picolinic
and acetic acid. Those slurries containing low concentration cerium abrasive provide high SiO2
removal rate with high selectivity against SiN and with slow settling [3]. Such acidic cerium oxide
slurries are very promising for realizing low-cost, easy to treat SiO2 polishing process. However,
even using such additives, sometimes pitting like defects and/or abrasive residual are observed on
polished wafers.
This paper analyzes the SiO2 polishing mechanism by cerium oxide slurry, through
investigating the relationships between the valence value of cerium oxide abrasive on polishing,
pitting and adhesion characteristics. Acid slurries with hydroxyl benzoic acid (HBA), picolinic
acid (PA) or acetic acid were prepared. The pH value was changed by adding ammonium
hydroxide (NH4OH), potassium hydroxide (KOH) or nitric acid (HNO3). Some oxidizers were
also added to change valence value of cerium oxide. The influences of pH and valence value of the
abrasive of the slurry on polishing performance were disclosed.
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International Conference on Planarizaiton/CMP Technology ･ November 19 – 21, 2009 Fukuoka
Experimental
<Sample Preparation>
A cerium oxide dispersion with abrasive concentration of 5% was used. To investigate low cost
polishing, dispersions with abrasive concentration of 0.1 and 0.3% were prepared. The initial pH
value was kept 4.5, while it was changed from 2.5 to 10 by adding HNO3, KOH or NH4OH.
Slurries with oxidizers were also prepared by adding hydrogen peroxide (H2O2), ammonium
persulfate (APS), diammonium cerium nitrate (DACN-IV with Ce3+), diammonium cerium nitrate
(DACN-IV with Ce4+), in order to see the effect of valence value of cerium oxide on polishing and
adhesion performance. Wafers of blanket plasma CVD TEOS (PTEOS) film wafers of 4 inch in
diameter were prepared.
<Polishing Conditions and observations>
An experimental polisher (CDP Automatic Mark II A, Logitech Co.) with the platen size of 60
cm is used. The polisher is also equipped with frictional force measurement mechanism [3][4].
The distance between the center of the platen and the wafer is about 15 cm. Rotational speed of the
platen and the wafer carrier is kept 60 rpm, and the down force was varied from 2 to 4 psi (mainly
4 psi, with 60 rpm). A foamed polyurethane, hard polishing pad was used (k-groove). Slurry flow
rate was kept at 150 ml/min. The wafers with blanket Plasma-Enhanced TEOS (P-TEOS) film
were used. After polishing, each wafer is cleaned by scrubbing with PVA brush in DI water, and
spin dried.
Polished wafers are observed by a photomicroscope, and the cross section of pit are ting defects
is observed by a stylus profiler (Dektak 3ST, Veeco/Sloan). The adhered particles were observed
by SIMS.
Resluts and discussion
<Pitting like defects and adhesion of abrasive>
Removal rate (RR) dependence on ceria
abrasive concentration by a dispersion of pH=4.5
under 4psi, is shown in Fig. 1. RR increases by the
increase of abrasive concentration. The RR,
however, is very low when the abrasive
concentration is below 1%. Frictional force (FF)
dependence on pH is shown in Fig. 2. Since FF is
strictly proportional to RR under the same
chemistry, the figure suggests that RR increases
according to pH increase from acid to neutral
region, and then decreases in alkaline region [4].
Slight difference in frictional force for the same pH
is due to the difference in additives. In order to
achieve practical RR in acid region with a very low
abrasive concentration, some additives were
examined for the dispersions with abrasive
concentration of 0.1 and 0.3 wt%. According to
the rezults by Carter, slurries with acids were used
to obtain high RR with low defectivity [3]. The RR
dependence on added hydoroxy benzoic acid
(HBA) is shown in Fig. 3. Significantly improved
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International Conference on Planarizaiton/CMP Technology ･ November 19 – 21, 2009 Fukuoka
RR was realized at around 0.05 wt% both for 0.1
and 0.3 wt% abrasive concentration. Similar results
were obtained using other acids, though the
concentration to give maximum RR changes.
Pitting like defects and/or abrasive adhesion,
however, were frequently generated when pH
value exceeded 5, and quick settling was observed
in neutral to alkaline region.
An example of pitting like defects are shown in
Fig. 4(a), and the cross section of a pitting defect is
shown in Fig. 4(b). Pitting was the depression
having a few millimeters in diameter and about 100 nm
in depth. The pitting has both spotting and striation like
shapes, as if localized etching occured. The RR
dependence on down force was linear using those acid
slurries.
<Effects of kinetic energy>
The causes of the defects and adhesion may be due to
electrostatic adhesion due to a ζ-potential difference
(SiO2 is negative and ceria is positive), a mechanical
adhesion by down force onto SiO2 surface, and/or
influence of CMP process itself. Three experiments are
done, as shown in Fig. 5. The first one is dipping the
SiO2 wafer in the 0.3% ceria dispersion to address
electrostatic effects. The second one is pushing the
SiO2 wafer onto the wet polishing pad (by the
dispersion) under the down force of 4 psi to address
mechanical effects. The last is polishing the wafer
(>2psi) for examining effects of friction and
material removal. These wafers are cleaned and
dried, and the surface was observed by an optical
microscope. The abrasive was found to adhere only
on the polished wafer. This suggests that the cause
of adhesion is not electrostatic, and a certain amount
of kinetic collision energy is necessary.
<Influence of oxidizer>
Since RR and defectivity (pitting-like defects
and abrasive adhesion) are strongly dependent on pH of the slurry, electrical condition of the
slurry may significantly change and influence on defect generation. One possiblity is the change of
valence value, since cerium oxide abrasive is a mixture of ceria (CeO2 with Ce4+) and cerous
(Ce2O3 with Ce3+). In order to examine the influence of valence value of cerium oxide on pitting
and adhesion characteristics, H2O2 or APS are added into the slurry of pH 5 to oxidize cerous to
ceria. The wafers are polished using the slurries, and results are shown in Fig. 6. When polished
using the original slurry (simple dispersion), as shown Fig. 6(a), it is seen that abrasives simply
adhere on p-TEOS surface.
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International Conference on Planarizaiton/CMP Technology ･ November 19 – 21, 2009 Fukuoka
Polishing with the H2O2 added slurry, we see a slight
color change around the abrasive, as shown in Fig. 6(b).
Using the APS added dispersion, as shown in Fig. 6(c),
large areas of the color change (pitting) are seen, and in
some cases the adhered abrasive is seen in midst of the
changed area. It is presumed that by adding the
oxidizers, the more part of cerous valence of the
abrasive increased from 3 to 4, and polishing capability
increased to result in significant pitting. Since oxidizing
capability of APS is stronger than H2O2, the change
became more significant.
The possibility still remains, however, that the
change of electrical characteristics of SiO2 surface due
to oxidizers rather than valence change influenced the
polishing characteristics.
<Effects of ACN(III) and ACN(IV)>
In order to confirm the effects of valence value of abrasive, ACN(III) or ACN(IV) are added to
the base slurry, since they have similar structures except the valence value of cerium oxide.
P-TEOS wafers are polished and the surface is observed after cleaning. The pH of the dispersions
is adjusted to 4.9. Almost no difference could be seen between the SiO2 surface polished using
ACN(III) added slurry when compared to that by the original dispersion, as shown in Fig. 6(a). On
the other hand, no abrasive adhesion is seen on the
surface polished using ACN(IV) added slurry. Since
ACN(IV) has a very strong oxidizing capability,
cerous in the abrasive is considered to be oxidized to
ceria, and adhesion and pitting were suppressed by
using ceria slurry rather than cerous slurry.
Combination of ACN(IV) and acids mentioned
above was suitable for obtaining high removal rate
and slow settling
<Polishing mechanism model>
The abrasive returned to ceria can be removed
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International Conference on Planarizaiton/CMP Technology ･ November 19 – 21, 2009 Fukuoka
from the surface, while the cerous abrasive
cannot be removed by a simple DIW
scrubbing. We propose the mechano-electrical
model of SiO2 polishing by ceria slurry, as
shown in Fig. 7.
Generally, both cerous and ceria are mixed
in abrasives. Ceria mainly works for polishing
rather than cerous. Abrasives collide onto
while ceria transfers its positive charge to
SiO2 and changes to cerous, since SiO2
surface is negatively charged [5]. In a low pH slurry, cerous easily receives positive charge from
the solution and returns to ceria. The ceria abrasive can leave from SiO2 surface.
On the other hand, in an increased pH region, adhered cerous cannot receive the positive
charge and remains on the SiO2 surface, since supply of positive charge decreases by the increase
of pH. Experiments by ACN(III) and ACN(IV) addition directly support the model, though the
mechanism why ceria adhesion is weaker than cerous is not still clarified yet.
Conclusion
Mechano-electrical mechanism of SiO2 by cerium oxide slurry is proposed. In order that
abrasive reacts with SiO2 for polishing, a certain amount of kinetic energy is needed. Focusing on
valence value of abrasives using slurries with ACN, the electrical behavior is given. By colliding
the ceria abrasive onto the SiO2 surface, the positive charge of the ceria is transferred to negative
SiO2 layer, and the abrasive changes to cerous. The SiO2 layer adheres on the abrasive surface.
When the cerous accepts the positive charge from the solution and returns to ceria, it can be
removed by the cleaning. In an increased pH region, positive ions in the solution decrease and this
makes difficult for the cerous abrasive return to ceria. Thus, the adhesion is tight for removing.
References
[1] Y. Homma, T. Furusawa, K. Kusukawa, M. Nagasawa, Y. Nakamura, M. Saitou, H.
Morishima and H. Sato, “Using Selective CMP with Low Permittivity Organic SOG to Achieve
Low Capacitance Multilevel Interconnection,” Proc. VMIC, pp. 457-463, Santa Clara, 1995.
[2] H. Nojo, M. Kodera and R. Nakata, “Slurry Engineering for Self-Stopping, Dishing-Free
SiO2-CMP,” Proc. IEDM, pp.349-353, 1996.
[3] P.W. Carter and T.P. Johns, “Interfacial Reactivity Between Ceria and Silicon Dioxide and
Silicon Nitrade Surface,“ Electrochem. Solid-State Letters, vol. 8, G218-G221, (2005).
[4]Y. Homma, “Dynamical Mechanism of Chemical Mechanical Polishing Analyzed to Correct
Proston’s Empirical Model,” J. ECS, vol.153(6), G587-590, 2006.
[5] K. Okuda and M. Sato, “Glass Polishing Mechanism with CeO2,” Proc. Fall Meeting JSPE, No.
G23.
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