The Effect of Slurry pH and Particle Size on
LiTaO3 Polishing
Kazuki Moriyama, Akira Ozeki, Shinichi Haba, Matsunori Mori
Nitta Haas Incorporated
3-17-1, Kannabidai, Kyotanabe-Shi, Kyoto, 610-0333, Japan
kmoriyama@nittahaas.co.jp

Abstract –The demand of LiTaO3 (LT) wafer mainly used as
substrate of SAW (Surface Acoustic Wave) filter device has
been quite increasing in recent years. From the viewpoint of LT
wafer’s productivity in the polishing process, high removal rate
and good recycle stability are required for slurry to eliminate
generated damages in the previous process. In this study, theeffect of pH and particle size in the slurry on the removal rate
and the polishing stability of LT wafer were investigated. We
found out that acid slurry containing large particle can achieve
high removal rate with good recycle stability. The balance ofpH and particle size would be taken advantage of polishingother hard brittle or chemically stable materials such as
sapphire, silicon carbide and gallium compounds.
INTRODUCTION
LT is one of the ceramics materials mainly used as the substrateof SAW filter device that can distinguish an electric signal ofspecific frequency. In recent years, in addition to the market
growth of smartphones, demand is rapidly expanding due to thelarge increase in the number of devices mounted on eachsmartphone because of high speed and large capacity oftransmission. Under such circumstances, improvement ofproductivity is desired. LT wafer should be polished as well as
typical substrates like silicon, sapphire and silicon carbide inorder to eliminate the damage occurred at previous process.
The features of the LT wafer are different from others, forexample, thinner thickness, brittleness and high chemicalstability.[1] As these features, polishing condition of LT wafer
is generally so mild that removal rate can not be raised easily.
One of the key factors to improve the removal rate effectively
is polishing slurry. Typical one is composed of a normal
colloidal silica with a pH adjuster,[2] but it can’t realize asufficient polishing performance with respect to both a highremoval rate and good stability. In this study, the effect of pHand particle size in the slurry on the removal rate and thepolishing stability of LT wafer were investigated, and we
propose a suitable area that can keep the balance of pH andparticle size with respect to high removal rate with goodstability.
EXPERIMENTS
The effect of slurry pH on removal rate was evaluated by using
slurries containing 20wt% silica abrasives. Mean particle size
was 122nm and slurry pH was controlled by hydrochloric acidand potassium hydroxide from 3 to 9. Removal rate wascalculated by weight loss of 15 minutes polishing. Polishingconditions are shown in Table 1. The effect of particle size onTable 1. Polishing conditions for pH dependence test.
Polisher

12inch single side polisher

LiTaO3 wafer

4inch 42˚ Y-X Black Yellow LiTaO3

PadSlurry

SUBA800TM (XY-Groove)
Colloidal Silica Slurry

Pressure

3psi

Table/Head Speed

120rpm/120rpm

Polishing time

15min

Slurry flow rate

400mL/min

Recycle amount of slurry 1500mL

Table 2. Polishing conditions for slurry recycle test.
Polisher
LiTaO3 wafer
Pad
SlurryPressure
Table/Head Speed
Polishing time
Slurry flow rate
Recycle amount of slurry

12inch single side polisher
4inch 42˚ Y-X Black Yellow LiTaO3
SUBA800TM (XY-Groove)
Colloidal Silica Slurries
3psi
120rpm/120rpm
30min
400mL/min
1500mL

removal rate and recycle stability was evaluated by using threedifferent sizes of silica abrasives. Mean particle sizes were
36nm, 90nm and 122nm respectively. Abrasive concentrationof all slurries was prepared to 20wt% and pH was controlled at
4 by hydrochloric acid and potassium hydroxide. The 30
minutes polishing was repeated 4 times to evaluate recycle
stability for each slurry. Polishing conditions are shown inTable 2.
RESULTS AND DISCUSSIONFig. 1 shows the effect of slurry pH on removal rate. Though
the removal rate in alkali atmosphere was very low, it increased
gradually with the decrease of slurry pH, and became maximal
at pH4. Fig. 2 shows the zeta potential titration curves of LT
wafer and silica abrasives. LT wafer is charged negatively in
pH 6.2 or more and positively in pH 6.2 or less. Silica abrasiveis charged negatively in the range of pH 3 to 9. Fig. 3 shows
the image of relationship between LT wafer and silica abrasivesin acid and alkali atmospheres. LT wafer and silica abrasivesrepulsed each other because both of them are charged
negatively in alkali atmosphere. It is thought that the repulsion

between LT wafer and silica abrasives caused week mechanicalpolishing and lower removal rate. On the other hand, LT wafer
and silica abrasives are attracted each other because LT wafer
is charged positively and silica abrasives are charged
negatively in acid atmosphere. It is thought that the attraction
between LT and silica abrasives caused strong mechanicalpolishing and higher removal rate.

The effect of particle size on recycle stability in acid slurry wasevaluated by using three different sizes of silica abrasives asshown in Fig. 4.

Fig. 4. Removal rate reduction with various particle sizes.

Fig. 1. Effect of slurry pH on removal rate of LiTaO3 wafer.

The recycle stability of slurry containing larger particle is better
than smaller one. Fig. 5 shows the particle size increase of theabrasives after 4 runs of 30 minutes polishing to the particle
size before polishing.

Fig. 2. Zeta potential titration curves of LiTaO3 and silica
abrasive.

Fig. 5. Particle size increase after 4 runs of polishing to theparticle size before polishing.

Fig. 3. Relationship between LT wafer and silica abrasives atacid and alkali atmospheres.

It is obvious that the larger particle aggregate hardly comparedto smaller one though all particles are aggregated after
polishing regardless of particle size. The reason causing
aggregation of particles by polishing is thought that theconcentration of positively charged contaminant generated bypolishing of LT wafer increases gradually during polishing. As
a result, positive ions as counter ions surrounding the surface
of abrasive charged negatively are pushed to the surface of theabrasive. In other words, abrasives are attracted each otherwithin intermolecular force working range without repulsion
between counter ions because the electric double layer
thickness becomes thinner with the increase of theconcentration of the positively charged contaminant in theslurry as shown in Fig. 6.[3][4]SUMMARY
The Polishing performance of LT slurry was evaluated from thepoint of view of particle size and slurry pH to achieve highremoval rate and superior recycle stability simultaneously. It isrevealed that acid slurry achieves high removal rate comparedwith alkali slurry and large particle is superior to small one
regarding recycle stability in acid atmosphere. With increasing
demands for improvement of productivity in the polishingprocess, it is important to achieve both high removal rate andsuperior recycle stability. The technique shown in this researchwould be taken advantage of polishing other hard brittle orchemically stable materials such as sapphire, silicon carbide
and gallium compounds.
REFERENCES

Fig. 6. Image of aggregation mechanism by polishing.
Fig. 7 shows slurry pH increase of each slurry with the numberof polishing. As slurry pH increased by polishing, it is thoughtthat positively charged contaminants are increased in the slurry
as described above.

[1]S. Koga, T. Doi, S. Kurokawa, O. Ohnishi, M. Uneda and Y.
Matsukawa “Polishing Mechanism on Oxide OpticalCrystalCMP” The Japan Society For Precision Engineering,Autumn 2011. pp.171-172.
[2]K. Tamai, A. Yasui, M. Serikawa, H. Morinaga, T. Doi, S.
Kurokawa, O. Ohnishi and M. Uneda: “Analysis of Polishing
Friction Factor for Material Removal Rate Improvement inCMP” Journal of the Japan Society of Advanced Production
Technology Vol.29 (1) (2011) pp.53-58.
[3]C. Takai, M. Fuji, M. Takahashi: “Control andcharacterization of nanoparticle dispersability in aqueous andnon-aqueous slurries” Annual report of the Ceramics ResearchLaboratory Nagoya Institute of Technology, 6:32-29[4]J. Brant, H. Lecoanet, M. R. Wiesner: “Aggregation anddeposition characteristics of fullerene nanoparticles in aqueoussystems” Journal of Nanoparticle Research (2005) 7 :545-553.
[5]H. Kamiya, M. IIJIMA “Dispersion Behabior Control ofNanoparticles and its Applications” The MICROMERITICS
No.55 2012 pp.12-18.
AUTHOR BIOGRAPHY

Fig. 7. Slurry pH change with the number of polishing.
On the other hand, the reason smaller particle tends toaggregate easier than larger one is that the distance between
smaller particles is shorter than that of larger particles at thesame particle concentration. Therefore, the collision frequency
between smaller particles is higher than that of larger
particles.[5] As a result, it is considered that a bigger change inparticle size of smaller particle compared to larger particle
during polishing caused bigger reduction of removal rate. Froma series of evaluation results, acid slurry containing large
particle is good for obtaining both high removal rate and good
recycle stability.

Kazuki Moriyama, is in charge of the development ofpolishing slurry at R&D department, who is focusing on new
application fields such as LiNbO3, gallium compounds andresin materials.