Vol. 35, No. 11
Journal of Semiconductors
November 2014
Evaluation of planarization performance for a novel alkaline copper slurry under a
low abrasive concentration
Jiang Mengting(蒋勐婷)Ž , Liu Yuling(刘玉岭), Yuan Haobo(袁浩博),
Chen Guodong(陈国栋), and Liu Weijuan(刘伟娟)
Institute of Microelectronics, Hebei University of Technology, Tianjin 300130, China
Abstract: A novel alkaline copper slurry that possesses a relatively high planarization performance is investigated under a low abrasive concentration. Based on the action mechanism of CMP, the feasibility of using one
type of slurry in copper bulk elimination process and residual copper elimination process, with different process
parameters, was analyzed. In addition, we investigated the regular change of abrasive concentration effect on copper and tantalum removal rate and within wafer non-uniformity (WIWNU) in CMP process. When the abrasive
concentration is 3 wt%, in bulk elimination process, the copper removal rate achieves 6125 Å/min, while WIWNU
is 3.5%, simultaneously. In residual copper elimination process, the copper removal rate is approximately 2700
Å/min, while WIWNU is 2.8%. Nevertheless, the tantalum removal rate is 0 Å/min, which indicates that barrier
layer isn’t eliminated in residual copper elimination process. The planarization experimental results show that an excellent planarization performance is obtained with a relatively high copper removal rate in bulk elimination process.
Meanwhile, after residual copper elimination process, the dishing value increased inconspicuously, in a controllable range, and the wafer surface roughness is only 0.326 nm (sq < 1 nm) after polishing. By comparison, the
planarization performance and surface quality of alkaline slurry show almost no major differences with two kinds
of commercial acid slurries after polishing. All experimental results are conducive to research and improvement of
alkaline slurry in the future.
Key words: alkaline slurry; abrasive concentration; planarization performance; acid slurry
DOI: 10.1088/1674-4926/35/11/116002
EEACC: 2520
1. Introduction
With the feature size of integrated circuit device shrinking
constantly and the metal wiring layers increasing continuously,
the wafer surface must undergo global planarization in order to
improve the reliability performance and extend the life of the
device. Chemical mechanical planarization (CMP) has become
a key step in Damascene processing to achieve global planarization for copper interconnectsŒ1 3 . Conventional chemical mechanical planarization is divided into three steps: copper
bulk elimination (P1), residual copper elimination (P2) which
is a soft landing between copper polishing and barrier polishing (P3)Œ4; 5 . Figure 1 shows the schematic illustration of copper CMP. After three-step polishing, the barrier film is eliminated and a desired topography obtained. During the CMP
process, the mechanical grinding action of the abrasive particles is usually adopted to improve the polishing rate. Traditional copper slurry is often a high concentration of abrasive,
addition of BTA, and different kinds of slurries used in copper bulk elimination and residual elimination process. However, depression, microscopic scratches, residual particles and
other surface damage will generate in the polishing process,
affecting the reliability of the device, while high concentration
of abrasive will also cause damage to the substrateŒ6 10 . So a
low abrasive concentration of slurry urgently needs to be developed.
In order to reduce process order complexity and reduce
slurry cost, a kind of alkaline copper slurry (pH: 9–10) that
can be used in P1 and P2 process concurrently under different
process parameters, was exploited by the Institute of Microelectronics of Hebei University of Technology. It is composed
of colloidal silica (particle size: 50–60 nm), 0.9 vol% FA/O
VI chelating agent (a kind of more hydroxyl amine organics),
0.5 vol% nonionic surfactant and 1.5 vol% hydrogen peroxide
(30 wt.%) as oxidizer, with no addition of BTA. Copper removal rate and WIWNU were investigated with the increasing
Fig. 1. (Color online) The schematic illustration of copper CMP process.
* Project supported by the 02 Major Program of the National Medium–Long Term Science and Technology Development Project of China
(No. 2009ZX02308).
† Corresponding author. Email: jmtlyh@163.com
Received 1 April 2014, revised manuscript received 23 May 2014
© 2014 Chinese Institute of Electronics
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Fig. 2. Cross sectional view of the 300 mm copper wafer. (a) Copper blanket wafer. (b) Copper pattern wafer.
of abrasive concentration in P1 and P2 process, respectively.
Meanwhile, planarization performance and surface roughness
characterization of the peak-to-valley ratio (P/V) and the rootmean-square surface roughness (Sq) were also evaluated. In
addition, we compare the planarization performance and surface quality of alkaline slurry with two kinds of commercial
acid slurries.
Table 1. Process parameters in experiments.
Recipe
Value
P1
P2
Head speed (rpm)
97
57
Plate speed (rpm)
103
63
Working pressure (mdaN/cm2 /
137
34
Back side pressure (mdaN/cm2 /
137
34
Flow rate (mL/min)
300
300
2. Experiment
The experiments were conducted on an Alpsitec E460E
polisher using a Rohm & Haas IC 1000TM polishing pad. For
measuring copper removal rate and WIWNU, a 300 mm blanket wafer (the structure of the wafer is shown in Fig. 2(a)) with
1.2 m copper thickness was applied. Meanwhile, 300 mm patterned wafers whose section structure is shown in Fig. 2(b)
were also used for evaluating the planarization performance
of slurry. A 3 inch tantalum disk (99.99% purity) was used
to measure tantalum removal rate, which was calculated by
weighing the weight loss of the tantalum disk using an analytical balance (Mettle Toledo AB204-N, 0.1 mg accuracy). The
weighing method was according to the following equation:
MRR D m= R2 t ;
(1)
where MRR denotes the Ta removal rate, R is the radius of Ta
wafer, is the density of Ta, and t denotes polishing time, respectively.
Copper removal rate was determined by the variation of
copper film thickness before and after CMP. The film thickness of copper blanket wafer and step height of copper pattern
wafer, before and after polishing, were measured by an XP-300
profiler produced by the AMBIOS Company. The dishing is a
width of 70 70 m pad. Meanwhile, WIWNU is calculated
in the light of the following equationsŒ11 :
WIWNU D MRR = M ;
MRR D
v
uP
u n
u
.M
t i D1 i
n
(2)
M /2
1
:
(3)
Twenty-one points, which are etched by using a certain
concentration of nitric acid solution on the wafer diameter, are
selected to calculate the WIWNU and each point is scanned
three times. M is the average removal rate of 21 points. MRR
is the standard deviation of Mi . Mi .i D 1, 2, 3, , 21) is copper removal rate of each corrosion point. The smaller WIWNU,
the better the consistency of the wafer surface after polishing
was acquired. Before and after polishing, the surface roughness
that characterized the peak-to-valley ratio (P/V) and the Sq, is
measured by Agilent 5600LS atomic force microscopy (AFM).
Process parameters applied in the polishing experiments are
presented in Table 1. To evaluate the property of slurry, a series of experiments were carried out.
3. Results and discussion
3.1. Optimization of the removal rates and WIWNU
Figure 3 describes the variation of copper removal rate and
WIWNU as a function of abrasive concentration in P1 and P2
process. In bulk copper elimination process, copper removal
rate increases promptly in the inception phase as the abrasive
concentration increases from 0 to 3 wt%, while it increases
slowly after the abrasive concentration achieves 3 wt%. However, the WIWNU decreases with abrasive concentration increasing, and then almost approaches a constant. The WIWNU
decreases from 5.3% to 3.4% quickly. As the abrasive concentration increased, the change tendency of removal rate and WIWNU also exist in residual copper elimination process. When
the abrasive concentration is 3 wt%, the copper removal rate
and WIWNU in bulk copper elimination and residual copper
elimination process are 6125 Å/min, 3.5% and 2706 Å/min,
2.8%, respectively. This could be explained by the fact that the
increasing amount of abrasive particles can effectively promote
mechanical grinding action in CMP, accelerating the oxidation
layer which generates through copper reacting with oxidizer
and overlaps on the surface of the wafer eliminated. Meanwhile, the abrasive particles are also conducive to facilitate the
reactants and reaction products mass transfer in the wafer surface, avoiding the uneven distribution of reactants and reaction products locally. Therefore, in a low abrasive concentration range, the increase of abrasive concentration is in favor of
obtaining a high removal rate and a low WIWNU.
In conventional copper CMP it is often necessary to use
two kinds of slurry: one is used to eliminate excessive cop-
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Fig. 4. Tantalum removal rate as a function of abrasive concentration
in residual copper elimination process.
Fig. 3. Copper removal rate and WIWNU as a function of abrasive
concentration (a) In bulk copper elimination process. (b) In residual
copper elimination process.
per, the other is used to eliminate residual copper. So, if one
kind of alkaline slurry is used in copper CMP, while eliminating residual copper effectively, the tantalum removal rate
must be 0 Å/min, avoiding the barrier layer being eliminated
in the over-polish stage. The abrasive concentration effects on
the tantalum removal rate are presented in Fig. 4. The process
parameters are according to residual copper elimination process. It’s clear that the tantalum removal rate is 0 Å/min when
the abrasive concentration is less than 3 wt%. Subsequently,
the tantalum removal rate begins to increase with abrasive concentration increasing. Taking copper and tantalum removal rate
and WIWNU into account, the optimal abrasive concentration
added in slurry is about 3 wt%.
3.2. Planarization performance
In the study, for evaluating the planarization performance
of alkaline slurry, two kinds of commercial acid slurry were
used to compare with the alkaline slurry. Dishing commonly
occurs where the copper film in the wiring area has been overpolished, because the removal rate of the barrier film is very
low compared to copperŒ12; 13 . The characteristic method of
planarization performance is through the dishing value, which
is equivalent to the residual step height after polishing. The
step height at various locations of patterned wafer as a function
of polishing time is presented in Fig. 5. It’s clear to see that
the initial step height of the patterned wafer is approximately
3750 Å. As the pictures below depict, the step height decreases
Fig. 5. Step height reduction as a function of polishing time. (a) The
alkaline slurry. (b) The commercial acid slurry.
gradually as the polishing time increasing. We can see that after
polishing for 65 s, it reaches the endpoint of bulk elimination
process by using alkaline slurry, while the residual step height
keeps in the range of 410–613 Å, meaning that the reduction of
step height is nearly 3200 Å. However, in contrast, the residual
step height is 407–524 Å by using a commercial acid slurry
after bulk elimination process. But the polishing time is 70 s
longer than alkaline slurry, indicating that the polishing rate of
alkaline slurry is relatively higher than acid slurry.
In residual copper elimination process, the residual step
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Fig. 6. Schematic illustration of chemical reaction model in CMP process.
Fig. 7. The AFM images of the copper wafer surface. (a) Before polishing and (b) After polishing using the alkaline slurry.
height is 737–809 Å, which keeps in a low range of increase
after polishing for 10 s by using the alkaline slurry. Meanwhile,
we can see that the residual step height is approximately 664–
728 Å by using acid slurry after polishing for 15 s. To ensure the
elimination of residual copper completely, an over-polish stage
is necessary. The step height went up to 821–906 Å by using
the alkaline slurry, but increased to 771–832 Å by using acid
slurry. Good planarization performance is obtained, denoting
that it can get global planarization effectively by using one kind
of alkaline slurry.
The planarization process can be concluded as follows.
Schematic illustration of the chemical reaction model in CMP
process is described in Fig. 6. In alkaline solution, the copper surface is easily oxidized to Cu2 O, CuO and Cu(OH)2 in
the presence of H2 O2 , and the oxidation products aren’t dissolved as a passivation film overlaps on the wafer surface. Under the polishing process conditions, the protruding regions of
the patterned wafer surface will endure larger kinetic energy
and greater friction energy, which is conducive to breaking oxidation products chemical reaction barrier and leading to copper
film ionization more easily. Besides that, FA/O VI chelating
agent (R[NH2 ]n, R represents a functional group which contains hydroxyl groups) has a strong chelating ability with metal
ions, complexes with Cu2C rapidly to form an easily soluble
reaction product. It can be rapidly taken away from the copper
surface through the mechanical action of the abrasive. Then
a fresh copper film surface is exposed, the oxidation reaction
and complexation reaction keep repeating in circulation. In the
concave regions, the complexation reaction was inhibited to a
large extent, because the passivation film reaction barrier isn’t
broken easily, causing the copper film in concave regions to be
eliminated slowly. Thus, a considerably high removal rate ratio
exists between protruding regions and concave regions in favor
of achieving planarization eventually. The essential chemical
reaction equations can be listed as follows:
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2Cu C H2 O2 D Cu2 O C H2 O;
(4)
Cu2 O C H2 O2 D 2CuO C H2 O;
(5)
CuO C H2 O
OH
! Cu.OH/2 ;
2C
Cu.OH/2 , Cu
C 2OH ;
(6)
(7)
J. Semicond. 2014, 35(11)
Jiang Mengting et al.
Fig. 8. The AFM images of the copper wafer surface. (a) Before polishing and (b) After polishing using the commercial acid slurries.
Cu
2C
C 2NH2
NH2
j
R
j
NH2
NH2
OH
! ŒCuŒNH2
CMP
NH2
j
R
j
NH2 2C
2 :
NH2
(8)
Apart from achieving a high planarization performance,
reducing the surface defects after polishing is also important
for Cu CMP. We used an atomic force microscopy to measure
surface roughness before and after polishing, and the randomly
scanned area is 10 10 m2 for the surface of copper films.
The results of surface morphology represented by the P/V and
the Sq. Figure 7 shows photographs of the polishing results before and after CMP by using the alkaline slurry. It is clear that
the initial P/V and Sq values of the surface before CMP were
measured to be 91 nm and 11.1 nm, respectively. After eliminating the residual copper, the P/V value from 91 dropped to
12.6 nm sharply; at the same time, the Sq value also decreased
from 11.1 to 0.326 nm in the measurement area. Figure 8 shows
photographs of the polishing results before and after CMP by
using the commercial acid slurries. As is shown in the picture,
the initial P/V and Sq values of the surface before CMP were
measured to be 72 nm and 10.8 nm, respectively. After polishing, the P/V value from 72 dropped to 13.6 nm, meanwhile the
Sq value decreased from 10.8 to 0.491 nm. In contrast, the Cu
CMP using the alkaline slurry can obtain a better surface quality, with Sq < 1 nm and considerably low P/V values. It has
advantages for copper polishing in the future.
4. Conclusion
In this paper, we have proposed a kind of alkaline slurry. It
can be applied in the bulk copper elimination and residual copper elimination process simultaneously by adjusting the polishing process parameters. Through the tests of the removal rate
and within wafer non-uniformity, we select the best abrasive
concentration of copper slurry. The results of planarization experiments show that high planarization property is achieved by
using 3 wt% abrasive concentration of alkaline slurry. The step
height is eliminated in a relatively short time. At the end of Cu
CMP, the dishing increase is not obvious. Comparing the pla-
narization performance of the alkaline slurry and two kinds of
acid slurry, it can be seen that the acid slurry has a slightly
higher planarization capability, but there is little difference between this alkaline slurry and the acid slurry. The results obtained from the roughness maps indicate that the copper film
polished by the alkaline slurry has a lower surface roughness
value and good surface quality compared to the acid slurries.
This study may be conducive to simplify the composition and
reduce the abrasive concentration of copper slurry in the future.
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