（自然科学版）
第 20 卷 第 6 期
2014年12月
JOURNAL OF SHANGHAI UNIVERSITY (NATURAL SCIENCE)
Vol. 20 No. 6
Dec. 2014
DOI: 10.3969/j.issn.1007-2861.2014.04.007
Effect of AIBI as Free Radical Initiator on Abrasive-Free
Polishing of Hard Disk Substrate
REN Xiao-yan,
LEI Hong,
CHEN Ru-ling, CHEN Yi
(Nano-Science and Technology Research Center, Shanghai University, Shanghai 200444, China)
Abstract: To optimize the existing slurry for abrasive-free polishing (AFP) of hard disk
substrate, a water-soluble free radical initiator, 2, 20 -azobis [2-(2-imidazolin-2-yl) propane]
dihydrochloride (AIBI) is introduced to the H2 O2 -based slurry. The polishing results show
that, the material removal rate (MRR) of hard disk substrate polished with H2 O2 -based
slurry containing AIBI is obviously higher than that without AIBI. The acting mechanism
of the improved MRR is investigated. Electron paramagnetic resonances tests show that, by
comparison with H2 O2 slurry, H2 O2 -AIBI slurry provides higher concentration of hydroxyl
radicals. Auger electron spectrometer analyses further demonstrate that the oxidation ability of H2 O2 -AIBI slurry is much greater than H2 O2 slurry. In addition, potentiodynamic
polarization tests show that the corrosion dissolution rate of hard disk substrate in H2 O2 AIBI slurry is increased. Therefore that stronger oxidation ability and a higher corrosion
dissolution rate of H2 O2 -AIBI slurry lead to higher MRR can be concluded.
Key words: abrasive-free polishing (AFP); hard disk substrate; material removal rate;
initiator
Chinese library classification: TH 117.3
Document code: A
Article ID: 1007-2861(2014)06-0680-09
With increase of hard disk rotational speed and decrease of distance between the magnetic
head and substrate, minimizing roughness and defects of the polished surface are important
for hard disk substrate[1-2] . Chemical mechanical polishing (CMP), as a global planarization
technique, has been widely used in the manufacturing of ultra-precision surfaces such as in
integrated circuits (ICs) and computer hard disk substrates[3-5] . During the CMP process, the
material removal rate (MRR) is subject to mechanical effect by abrasive wear coupled with
simultaneous chemical effects via slurries at the wafer/pad interface. At the same time, there
are many scratches, pits, and other micro defects on the surface due to abrasive particles in
CMP slurry[6-7] . To avoid breakage of the devices caused by the polishing process, abrasivefree polishing (AFP) has been proposed[8] .
To a large extent, the chemical components and the properties of slurry determine the
polishing performances. At present, the effect of different additives or catalysts on polishing
hard disk substrate with H2 O2 -based slurry has been studied. The introduction of copper (II)
ion[9] and the oxidation-redox system initiator (K2 S2 O8 -NaHSO3 , C4 H10 O2 -Na2 SO5 )[10-11]
Received: Nov. 5, 2014
Project supported by the National Natural Science Foundation of China (51175317), the Doctoral Program
of Higher Education of China (20123108110016)
Corresponding author: Lei Hong, Professor, Ph. D., E-mail: hong [email protected]
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can obviously improve MRR, possibly due to the fact that both of them can accelerate H2 O2
decomposition to yield hydroxyl radicals. These are stronger oxidizers than H2 O2 and can
enhance oxidation of the hard disk surface, therefore improving AFP performances.
Azo initiator is an effective initiator[12-14] . The lower initiator temperature of azo initiator
may be effective in accelerating the production of hydroxyl radicals. Up to now, no report
on the use of azo initiator in AFP process has been published. In this paper, a water-soluble
azo initiator, 2, 20 -azobis [2-(2-imidazolin-2-yl) propane] dihydrochloride (AIBI, see Fig.1) is
introduced in H2 O2 -based slurry, and the polishing performance is investigated.
Fig. 1 Structure of water-soluble azo initiator
1
1.1
Experimental
Preparation of slurries
In the preparation, 5% hydrogen peroxide, 6% polymethylacrylic acid as dispersing agent
and 0∼0.06% AIBI were added into deionized water in a container under continuous stirring
at room temperature.
1.2
Free radical trapping
The effect of AIBI on the concentration of free radical in H2 O2 was measured by EMXelectron paramagnetic resonance (EPR) (center field: 3 518.07 G, scan width: 100 G, microwave power: 10 mW, scan time: 5.24 s, scanning frequency: 10 times), and dimethyl
pyridine N-oxide (DMPO) was added as trapping agent. The H2 O2 -AIBI slurry (40 µL) sample was mixed with DMPO (10 µL, 0.5 mol/L). The EPR spectra were subsequently acquired
at ambient temperature. The H2 O2 slurry sample for EPR test was prepared with the same
procedure except adding AIBI.
1.3
Polishing tests
Polishing tests were performed with UNIPOL-1502 polishing equipment (Shenyang Kejing Instrument, Co. Ltd., China), using as work pieces hard disks with Φ 95 mm×
1.25 mm (a center hole with Φ=25 mm) aluminum alloy substrates plated with NiP consisting of about 85% nickel and 15% phosphorus elements with an average roughness (Ra) of
about 34.68 nm. The polishing pad used is a Rodel porous polyurethane pad. The polishing
conditions are as follows: the down force was 4 kg, the rotation speed was 80 r/min, the
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polishing time was 30 min, and the slurry supplying rate was 160 mL/min. After polishing,
the hard disk substrates were washed with ultrasonic in a cleaning solution containing 0.5%
surfactant in deionized water and dried by a multifunctional drying system.
The mass of the hard disk substrate before and after polishing was measured with an
analytical balance to calculate the material removal rate (MRR) according to Eq. (1):
MRR =
4 × 109 m
,
πd2 tρ
(1)
where MRR is the material removal rate (nm/min), m the mass of material removed (g), d the
diameter of work piece (mm), t the polishing time (min), and ρ the density of hard disk (hard
disk is plated with a NiP layer consisting of about 85% nickel and 15% phosphorus elements,
ρ=7.9 g/cm3 ). The Ra value and the surface topography after polishing were measured with
an Ambios XI-100 surface profiler (Ambios Technology Corp., USA, ZYGO) with a vertical
resolution of 1.0×10−11 in texture modes and a measuring area of 500 µm×500 µm.
1.4
Static soaking experiment and surface analysis
Static soaking experiments were carried out under static conditions. The hard disk
substrates were immersed in slurries at 25 ◦ C for 48 h. The substrates were then washed and
dried before auger electron spectroscopy (AES) tests.
The contents of elements on the substrate surfaces were analyzed using a PHI 680-Auger
electron spectroscopy under the following conditions: ion beam current of 1 µA, ion beam
voltage of 2 kV, electron beam current of 10 nA, electron beam voltage of 10 kV, scan area
of 20 µm×20 µm, and sputtering speed as SiO2 of 17 nm/min.
1.5
Electrochemical measurements
All electrochemical experiments were performed with a Solartron electrochemical work
station in a three-electrode cell. The liquids of the cell were prepared with chemicals and
deionized water, 6% dispersing agent + 5% H2 O2 + x% AIBI (x = 0 or 0.02) according to
the slurries used in AFP process. The round hard disk sheet sealed with epoxy resin was
used as a working electrode having a diameter of 10 mm. A saturated calomel electrode
(SCE) and a platinum electrode were used as the reference electrode and the counter electrode, respectively. The potentiodynamic polarization curves were obtained at a scan rate of
1 mV/s.
2
2.1
Results and discussions
Polishing performances of substrate in the AFP slurries
To investigate the effect of AIBI on AFP performances, different concentration of AIBI
was added into the H2 O2 slurry. As shown in Fig.2, without AIBI, MRR of hard disk substrate
is only 5.18 nm/min. However, MRR increases to 14.47 nm/min when 0.02% AIBI was added,
indicating that AIBI can effectively improve MRR of hard disk substrate in the AFP.
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Fig. 2 Effect of AIBI concentration on MRR of hard disk substrate in H2 O2 -based slurries
Figure 3 shows images of surface morphology of hard disk substrate before and after
polishing by different kinds of slurries. A rough surface with nano-asperity peaks was observed
on the surface before polishing, and roughness was 34.68 nm (see Fig.3(a)). After being
polished by H2 O2 slurry, surface roughness became 17.79 nm (see Fig.3(b)). However, the
surface polished by H2 O2 slurry containing 0.04% AIBI had an Ra value 11.30 nm (see
Fig.3(c)). A lower Ra value means higher surface planarization. These results indicate that
H2 O2 -based slurry containing AIBI has higher surface planarization performances.
Fig. 3 Surface profiles of disk substrates polished in slurries
2.2
2.2.1
Mechanism analysis
Interaction between AIBI and H2 O2 in AFP slurries
In CMP process, planarization is achieved due to the chemical and mechanical effects
between the wafer, pad and slurry. The key component affecting the disk surface quality is
slurry. Previous researches reveal that H2 O2 solution can be decomposed into hydroxyl radi-
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cals (·OH), which are stronger oxidizers than H2 O2 [15] . It also demonstrated the importance
of ·OH in polishing of Cu[15-17] and Ge[18] .
Fig. 4 Electron spin resonance spectra of spin adducts of free radicals (Trapped by DMPO)
To study the effect of AIBI on ·OH radical concentration in the AFP process, the EPR test
was performed. The electron spin resonance spectra of H2 O2 -AIBI, AIBI, H2 O2 slurries are
shown in Fig.4, respectively. The spectra in Fig.4(b and c) are typical of the ·OR free radical
(R represents the C3 N2 H5 -C(CH3 )2 group) and ·OH free radical, respectively. It is found
that the spectra of H2 O2 -AIBI slurry exhibit both RO· radicals and ·OH radicals. Intensity
of ·OH radical in H2 O2 -AIBI slurry is much larger than that without the AIBI initiator. It can
be inferred that the initiator can strongly catalyze decomposition of H2 O2 into ·OH, which
could accelerate chemical oxidation in the polishing process. The decomposition mechanisms
of H2 O2 in the existence of AIBI may be explained by the following chemical reaction:
R − N = N − R → 2R · +N2 ,
(2)
R · +H2 O2 → RO · +H2 O,
(3)
R · +H2 O2 → ROH + HO·,
(4)
HO − OH → 2HO.
(5)
In addition, when concentration of AIBI is too high, free radicals produced by AIBI can
terminate and decrease ·OH concentration (see Eq.(6)), causing decrease of oxidization ability
and further resulting in the decline of MRR. This is consistent with the behavior as shown in
Fig.2.
R · +R· → R − R.
2.2.2
(6)
Interaction between H2 O2 -AIBI slurry and hard disk substrate
CMP is a surface machining process. The property of surface film on the hard disk
substrate is a key factor in the CMP process. To study the property, the surface of substrate
after immersion was examined with an AES test.
Figure 5 shows element contents in the surface film of a hard disk substrate treated with
H2 O2 -based slurry containing 0.02% AIBI. Before sputtering, elements in the surface consist
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of 65% carbon, 11% nickel, 23.8% oxygen (see Fig.5(a)). After sputtering for 1 min, element
components of the surface consist of 69.8% nickel and 11.4% phosphorus (see Fig.5(b)), corresponding to the elements composition of NiP bulk phase. By comparison with the elements
composition of NiP substrate, oxygen and carbon were introduced to the surface. introduction of oxygen and reduction in the concentration of nickel and phosphorus imply that an
oxidation reaction occurred during in process. Introduction of carbon may be caused by
chemical adsorption of dispersing agent[19] .
Fig. 5 AES surface survey of elements in the surface treated with H2 O2 -based slurry containing 0.02% AIBI
Fig. 6 Deep distribution of the elements in the hard disk surface
Figure 6 shows deep distribution of the elements in the hard disk surface, observed with
different sputtering times. The sputtering time for removal of O is 0.17 min (see Fig.6(a)),
and 0.56 min (see Fig.6(b)) for H2 O2 slurry and the H2 O2 -AIBI slurry, respectively. The
sputtering speed relative to SiO2 is 17 nm/min. Therefore, thickness of the oxidation film
formed in H2 O2 slurry and H2 O2 -AIBI slurry can be estimated as being about 2.89 nm and
9.52 nm, respectively. In other words, thicker oxidation film leads to improved oxidation
ability of the slurry. This is consistent with the results of the EPR analysis.
Combining the AES result with the EPR tests, it can be inferred that the existence of
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AIBI can enhance oxidation of the substrate surface. Much more surface area of substrate
can then be oxidized to become nickel oxide. Moreover, compared with the native substrate
surface, binding force between the oxidation film and substrate is weaker, meaning that the
oxidation film can be removed easily under the mechanical action in an AFP process.
2.2.3
Examination of surface reactivity of substrate using electrochemical measurements
The electrochemical test has been widely carried out to elucidate the fundamental mechanisms in CMP[16,20] . It can reflect the corrosion action of metal dissolution and surface
film formation of metal in AFP slurry, which is viable to predict the MRR. Therefore, to
study the acting mechanism of the improved MRR of hard disk substrate in an AFP process,
the potentiodynamic polarization tests were conducted to observe the corrosion behavior of
hard disk substrate in H2 O2 slurry and H2 O2 -AIBI slurry. The potentiodynamic polarization
curves of H2 O2 slurry and H2 O2 -AIBI slurry are shown in Fig.7. The corresponding electrochemical parameters estimated from polarization plots with a Tafel extrapolation method are
presented in Table 1. Compared with H2 O2 slurry, H2 O2 -AIBI slurry has higher corrosion
current density (Icorr ) and polarization resistance (Rp ). This means the AIBI can increase
the chemical dissolution reaction rate between the substrate surface and the slurry in terms
of kinetics[20] . In addition, the anode curve moves to the negative direction more than the
cathode curve, and the corrosion potential (Ecorr ) decreases dramatically when adding 0.02%
AIBI into the H2 O2 -based slurry. This result further indicates that AIBI can increase the
trend of corrosion reaction between the hard disk substrate and slurry from the point of thermodynamics. Combining the kinetic and thermodynamic results, the effects of AIBI on the
corrosive performances of hard disk substrate are consistent. The electrochemical corrosion
effect can help increase MRR by AIBI in AFP of hard disk substrate.
Fig. 7 Potentiodynamic polarization plots curves of hard disk substrates in different kinds of slurries
Table 1 Values of Rp , Icorr , and Ecorr estimated from polarization curves treated with
different slurries
Slurry
H2 O2
H2 O2 -AIBI
Rp /(Ω·cm2 )
107.23
48.041
Icorr /(A·cm−2 )
Ecorr /V
−4
−0.21
−4
−0.23
2.308 2×10
6.689 7×10
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Conclusions
In this paper, MRR of hard disk substrate is improved effectively in H2 O2 -based slurry
containing AIBI. The stronger oxidation ability and higher corrosion rate of H2 O2 -AIBI slurry
accelerate oxidation of substrate and corrosion dissolution of the metal nickel, greatly improving MRR of hard disk substrate. These results imply that H2 O2 -AIBI slurry has good surface
polishing performances in an AFP process.
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