Download Investigation on surface roughness in chemical mechanical

Vol. 35, No. 6
Journal of Semiconductors
June 2014
Investigation on surface roughness in chemical mechanical polishing of TiO2
thin film
Duan Bo(段波)Ž , Zhou Jianwei(周建伟), Liu Yuling(刘玉岭), Wang Chenwei(王辰伟),
and Zhang Yufeng(张玉峰)
Institute of Microelectronics, Hebei University of Technology, Tianjin 300130, China
Abstract: Surface roughness by peaks and depressions on the surface of titanium dioxide (TiO2 / thin film, which
was widely used for an antireflection coating of optical systems, caused the extinction coefficient increase and
affected the properties of optical system. Chemical mechanical polishing (CMP) is a very important method for
surface smoothing. In this polishing experiment, we used self-formulated weakly alkaline slurry. Other process
parameters were working pressure, slurry flow rate, head speed, and platen speed. In order to get the best surface
roughness (1.16 Å, the scanned area was 10 10 m2 / and a higher polishing rate (60.8 nm/min), the optimal
parameters were: pressure, 1 psi; slurry flow rate, 250 mL/min; polishing head speed, 80 rpm; platen speed, 87 rpm.
Key words: TiO2 thin film; surface roughness; CMP; process parameters
DOI: 10.1088/1674-4926/35/6/063003
EEACC: 2570
1. Introduction
Titanium dioxide (TiO2 / has found many industrial applications including pigments (Balfour, 1994 and Winkler,
2003), gas sensors (GRel et al., 1991 and Sberveglieri, 1995),
electrochemical and catalytic systems (Fujishima and Honda,
1972 and Henderson, 2011)Œ1 . In addition, TiO2 films have
several advantages for applications in coatings on eye-glasses,
glass sheets, flat panel displays and different optical systemsŒ2 4 , because the TiO2 host material could provide a
higher optical refractive index (n D 2.70) and a large dielectric constant (" D 114) that possesses an electronic structure
of a semiconductor. Its thin film (depth < 1 m) has a wider
band of optical transparencyŒ5 . Hence, the TiO2 thin films as
an antireflection layer is applied to the telescope optical lens,
mirror stealth radar exploration, high speed aircraft windows,
laser emitting apparatus of the optical lensesŒ3 , prisms, solar
cellsŒ6 and other equipment.
With the prompt development of optical systems in recent years, the TiO2 films processing quality requirements are
tighter, especially in the manufacture of telescope optical lens
and laser emitting apparatus of the optical lenses. On one hand,
the surface roughness will affect the internal defects in the multilayer structure of the antireflection layer; on the other hand,
the smaller internal defects of TiO2 thin films, the smaller the
extinction coefficient is. However, limited research has been
conducted in the TiO2 thin films surface processing. Furthermore, there are some disadvantages to the traditional processing method, such as grinding. The challenges with traditional
grinding with diamond wheels include the grinding marks and
subsurface damage generated on the ground wafer surfaceŒ7; 8 .
The chemical mechanical grinding (CMG) method is also proposed, but CMG process pressure and the mechanical strength
required for the polished section are relatively high and therefore it is easy to damage the thin film structure. In addition,
other methods, such as electrolytic in-process dressing (ELID),
laser processing and ultrasonic grinding, could not meet the
surface roughness requirement.
In order to solve the above problems, chemical mechanical polishing (CMP)Œ9;10 is first used in TiO2 films processing.
CMP technology combines mechanical polishing and chemical
etching, by means of the abrasion and chemical effect. CMP
can be applied in thin TiO2 films processing. The relationship between the polishing parameters and surface roughness
in CMP was investigated to analyze the polishing characteristics and mechanism during the CMP processŒ11 .
2. Experimental
In this researchall test wafers were processed using an
E460E CMP polisher, made in Alpsitec, a French Company.
An IC 1000TM pad was used, and the ex-situ pad conditioning
was conducted with a TBW diamond conditionerŒ12 . The surface roughness was measured using an Agilent 5600LS AFM.
The TiO2 thin film, supplied by the Institute of Optics and
Electronics, Chinese Academy of Sciences, was deposited on
quartz glass substrates in a RF magnetron sputtering system
with a base pressure of 6 10 4 Pa at room temperature. The
polish rate was calculated from the weight difference before
and after polishing, which was weighed in an analytical balance
(Mettler Toledo AB204-N). The one-layer structure of TiO2
(purity 99.99 %) thin film is shown in Fig. 1, and the diameter
of the sample was 0 mm.
The removal rateŒ13 was calculated from the following
Eq. (1):
* Project supported by the Natural Science Foundation of Hebei Province (No. E2013202247), the Science and Technology Plan Project of
Hebei Province (Nos. Z2010112, 10213936), and the Hebei Provincal Department of Education Fund (No. 2011128).
† Corresponding author. Email: [email protected]
Received 8 December 2013, revised manuscript received 20 January 2014
© 2014 Chinese Institute of Electronics
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Duan Bo et al.
Fig. 1. Schematic of TiO2 thin film structure.
Fig. 3. The relationship between TiO2 surface roughness and polishing pressure.
Fig. 2. Calculation schematic of surface roughness.
m
;
R2 t
RR D
(1)
where RR: the removal rate; m: quality differential value; :
material density; R: radius; t : polishing time.
Surface roughness is illustrated in Fig. 2. The smaller the
surface roughness is, the smoother the surface will be. Surface
roughness Ra can be calculated from Eq. (2).
Ra D
1
l
Z
l
Z.x/dx;
(2)
0
where l and Z.x/ are the sample length and the distance of
from reference points to the x-axis, respectively.
In this paper, weakly alkaline slurryŒ14; 15 was applied in
polishing TiO2 in this experiment. The CMP slurry is composed of a chelating agent, a surfactant and colloidal silica (average 20 nm in diameter). The pH value of the slurry is 9.0.
3. Results and discussion
Figure 3 shows the surface roughness of TiO2 films under
different pressure conditions. Experimental results revealed
that the TiO2 surface roughness approached a constant at a
lower pressure and then increased with further increasing the
pressure. The reasons are explained as follows: at a low polishing pressure, the CMP mechanical effect would be reduced and
the chemical action of the polishing slurry would be enhanced,
which made the surface roughness better without a scratch or
deformation of the mechanically fragile film. It seems that reducing the polishing pressure would be effective for reducing
scratches. However, if the polishing pressure was too low, it
would deteriorate the planarity, and decrease the polishing rate.
As the pressure was increased to 1 psi, both mechanical and
chemical actions were effective. The polishing resultants were
carried away by abrasives and the polishing pad. In the mean
time, slurry also played the role of a lubricant between the sample and the polishing pad. As a result, the higher removal rate
Fig. 4. The relationship between TiO2 surface roughness and slurry
flow rate.
with good roughness was achieved. However, to further increase the pressure, it would cause the polishing temperature
to be too high, deteriorating the TiO2 surface with defects such
as scratches, orange peel and deformation of the mechanically
fragile film. TiO2 and the chelating agent in the CMP process
can be changed into soluble amine salts. The possible reactions
are shown in the following equations:
C
NH2 –R–NH2 C 2H2 O D NHC
3 –R–NH3 C 2OH ;
TiO2 C 2OH D .TiO3 /2 C H2 O;
(3)
(4)
C
2
2NHC
D
3 –R–NH3 C 2.TiO3 /
NH3 –R–NH3 –TiO3 –NH3 –R–NH3 –TiO3 :
(5)
The optimum pressure of TiO2 -CMP was 1 psi considering
both the removal rate and the surface roughness.
In this paper, the effect of the slurry flow rate in the TiO2 CMP slurry was studied. The result is shown in Fig. 4. Experimental results indicate that the TiO2 surface roughness linearly
decreased up to a critical point at 250 mL/min. The reasons
are explained as follows: if the slurry flow rate was too low,
the polishing resultants could not be removed rapidly from the
sample surface. In addition, the friction force would increase,
which made the temperature non-uniform and increased the
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Fig. 5. The relationship between TiO2 surface roughness and polishing head/platen speed.
Fig. 6. AFM images of TiO2 thin film surface before polishing.
Table 1. The process conditions of TiO2 CMP.
Polishing process
Parameter
Working pressure (psi)
1.0
Polishing head speed (rpm)
80
Polishing platen speed (rpm)
87
Slurry flow rate (mL/min)
250
surface roughness of polished TiO2 . With the flow rate further increased, it could make polishing resultants rapidly separate from the sample surface and reduce the part of the sample surface with the higher temperature resulting from friction.
Hence, a better surface roughness was obtained. As the flow
rate was increased to 350 mL/min, slurry between the pad and
the sample surface had been lost before being completely reacted, resulting in local chemical reaction being too intense.
For all above, the slurry flow rate 250 mL/min was selected,
which saved the cost and ensured that the surface roughness
was better.
Figure 5 shows the surface roughness of TiO2 film under different polishing speed conditions. The surface roughness was slowly decreased and then increased with increasing polishing head/platen speed. The reasons are explained as
follows: at a low speed, such as 40/45 rpm, most of the TiO2
film surface layer was then removed by chemical dissolution
into the slurry and less of it was removed by the mechanical
action of the slurry particles in the low speed polishing process. But the chemical corrosion was serious, which would not
be favorable to an ameliorate degree of finish. With the speed
increased to 80/87 rpm, the mechanical action and polishing
rate improved, which made the slurry uniformly distributed in
the polishing platen with a full chemical reaction. Further increasing the speed made the slurry separate too rapidly from
the TiO2 surface; thereby, the chemical reaction was reduced
and the mechanical action was intense, the surface mechanical
scratch of the TiO2 increased after polishing, and the surface of
the polished TiO2 was worse. Considering the surface roughness and uniformity, the appropriate process parameters were
established as being 80/87 rpm.
The optimal parameters for this experiment are shown in
Table 1. Figure 6 shows AFM images of the surface morphologies of TiO2 thin films before polishing; the surface roughness
Fig. 7. AFM images of TiO2 thin film surface after polishing.
Fig. 8. Surface roughness values and removal rate of TiO2 thin film.
Ra reached 11.7 nm. The scanned area was 10 10 m2 . Figure 7 shows AFM images of the surface morphologies of TiO2
thin film after polishing and surface roughness Ra was 1.16 Å.
The results indicate that the optimized process conditions of
TiO2 have an obvious effect on surface topography correction.
Figure 8 shows surface roughness values and removal rate of
TiO2 thin film, which is 60.8 nm/min.
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Duan Bo et al.
4. Conclusions
In this paper, CMP was applied in TiO2 thin films processing. Furthermore, self-formulated weakly alkaline slurry
was used in the experiment for polishing. TiO2 thin film was
polished by the CMP process with changes to various process
parameters, such as working pressure, slurry flow rate, head
speed and platen speed. The appropriate process parameters in
this experiment are summarized as follows: the working pressure, slurry flow rate, head speed, and platen speed were 1 psi,
250 mL/min, 80 rpm, and 87 rpm, respectively. The removal
rate and the surface roughness were 60.8 nm/min and 1.16 Å
(the scanned area was 10 10 m2 / at this condition.
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