Doped ceria abrasives with controlled morphology and preparation thereof
The present invention relates to doped ceria (CeO2) abrasive particles, having an essentially
octahedral morphology. The abrasives are brought into a water-based slurry, for use in a
Chemical Mechanical Polishing or Chemical Mechanical Penalization (CMP) process. CMP is
a process to planarize structures on silicon wafers during integrated circuit manufacturing after
thin film deposition steps, for example in Shallow Trench Isolation (STI) polishing.
Today, about 50 % of all STI polishing is performed using ceria (CeO2) based slurries. Even
though the mechanical abrasivity of ceria is low compared to conventional abrasive particles
like silica or alumina, it is particularly interesting for polishing oxide layers due to its chemical
affinity for silica. Because of this high chemical affinity, removal rate and selectivity towards
Si3N4 are high, even with a reduced ceria content in the slurry. Indeed, ceria slurries typically
contain only 1 wt % of the abrasive material, whereas silica based slurries are characterized
by an abrasive content of at least 12 wt % and in most cases even 20 to 30 wt %.
Another important characteristic of abrasive slurries concerns the level of detectivity they
induce in the substrate. The currently available CeO2 materials generate a too high detectivity
level in CMP, certainly in view of the coming technology nodes in semiconductor
manufacturing (45, 32 and 23 nm nodes), which have increasingly stringent detectivity
requirements. The detectivity is essentially determined by the abrasive, and therefore it is
obvious to focus developments on providing modified ceria abrasives.
As generally known, the overall polishing efficiency essentially depends on the intrinsic
properties of the ceria abrasive itself (e.g. morphology, crystallographic structure, particle size
distribution, purity). It is generally assumed that abrasives with a spherical morphology lead to
a lower detectivity than sharp or angular particles, as is the case when polishing STI with
colloidal silica against fumed silica. However, as the chemical component of the CMP process
is much more important with ceria abrasives, and mechanical removal is limited to separating
reaction products from the wafer under pure shear forces, it is not straightforward that
spherical ceria abrasives will also result in a lower detectivity. Feng et al., in Science, 312,
1504, 2006, have prepared a spherical Ti-containing CeO2 particle by flame synthesis,
resulting in an improved CMP behavior. However, as shown by Transmission Electron
Microscopy (TEM), the abrasive particle consists of an inner CeO2 core completely
encapsulated in a molten shell of titania. Since this shell results in a different surface chemistry
compared to CeO2 based particle, it is not obvious whether the improved CMP behavior can
effectively be attributed to the spherical shape.
It would be highly beneficial if the synthesis of the abrasive particle could be tailored in such a
way that the desired optimal morphology is obtained. Almost all state of the art ceria abrasives
used in STI slurries today are produced by a precipitation and calcination process, often

followed by grinding down to smaller particle size. This synthesis method leads to poly-
crystalline particles. D.-H. Kim etal., Japanese Journal of Applied Physics, 45, 6A, 4893-4897,
2006, synthesized poly-crystalline particles having a typical size of a few hundred nanometers
with an irregular morphology, which moreover fragment easily during application in a CMP
process.
Several authors mention alloying, doping or mixing with other oxides of ceria, without referring
to a specific morphology, and yielding poly-crystalline material. JP-2007-31261 discloses ceria
abrasive particles which reduce scratches on silicon oxide films during polishing. These ceria
particles contain one or more elements having an ionic radius larger than the ionic radius of
tetravalent cerium (e.g. yttrium) and are characterized by a high crystallinity, being defined
here as having a low amount of defects such as dislocations in the crystal. The particles are
produced by precipitation followed by an adequate heat treatment. There is also a need for
grinding the material after the calcination process.
EP-126675 describes a cerium based polishing composition obtained by mixing a solution of
cerium salt, a solution of a base, such as sodium hydroxide, and a solution of at least one salt
of a trivalent rare earth, which is chosen from the group consisting of the lanthanides and
yttrium; filtering off the precipitate; drying and calcining it. US-2006/032836 discloses a method
to prepare a polishing slurry of doped cerium oxide abrasive particles. Doping with Y is one of
the numerous options. The synthesis method used is precipitation and calcination. JP-
3793802 provides a method of synthesizing a ceria powder or a metal oxide-added ceria
powder. However, the technology used to synthesize the particles is again a classical
precipitation and calcination route, not yielding mono-crystalline particles with uniform
morphology.
According to Biswas et al., Materials research Bulletin, vol. 42, no 4, 2007, pp. 609-617, doped
CeO2 is prepared using a wet chemical synthesis route. More specifically a urea-formaldehyde
polymer gel combustion method is applied. Y-doping is aimed at enhancing the ionic
conductivity. There is no information about the influence of Y-doping on the particle
morphology. The gel combustion process in general allows limited control over process
conditions and is not expected to produce a well defined particle size or morphology.
In general, ceria based slurries prepared with such standard calcined abrasives give rise to
higher detectivity than equivalent silica formulated slurries. In addition, the production process
of the ceria abrasives leads to broad variations in quality of the powder, which in turn leads to
important batch-to-batch variations of the slurries formulated with those particles.
In principle, the above mentioned problems can be solved by applying a bottom-up gas phase
synthesis route for the preparation of the CeO-2 particles. Such a method enables to control
particle properties to a certain extent, by varying the process parameters such as the
quenching rate, the residence time, and the temperature. In US-7264787 it is shown that such

an approach allows optimizing the particle size and the particle size distribution, but not the
particle morphology.
US-2007/048205 describes the synthesis of -2 using a hydrogen/oxygen flame. It discloses
that the surface chemistry of the particles can be influenced by varying specific process
conditions. The influence on the particle's morphology or the use of Y as a doping element is
not mentioned.
A particle growing in a gas phase process will tend to minimize its surface energy. This will
result in a particle shape where specific index planes are preponderant. Additionally, growth
kinetics can also play an important role in determining the particle shape, as planes with high
growth rates tend to disappear. It is observed that the powder prepared using a gas phase
method is typically characterized by a truncated morphology.
It is an object of the present invention to provide a novel doped -2 abrasive, containing
particles having an optimized morphology for use as abrasive in CMP, resulting in a low
detectivity level and a high removal rate.
To this end, and according to this invention, an yttrium-doped ceria powder is proposed, with
particles having a specific surface area of 10 to 120 m2/g, and characterized in that at least 95
wt%, preferably at least 99 wt%, of the particles are mono-crystalline. The particles are
additionally characterized in that their surfaces consist of more than 70%, preferably of more
than 80%, of planes parallel to {111} planes.
Advantageously, the particles comprise from 0.1 to 15 at% of the doping element versus the
total metal content. The particles may advantageously further consist of so-called unavoidable
impurities only. Cerium is indeed typically accompanied by up to about 0.5 wt% of other
lanthanides, which are considered as unavoidable impurities.
In another embodiment, this invention concerns the use of the above-mentioned particles for
the preparation of a fluid mixture consisting of either one of a dispersion, a suspension, and a
slurry. In a further embodiment, the above fluid mixture is defined.
The invention also concerns a gas phase process for synthesizing the yttrium-doped ceria
powder described above, comprising the steps of: providing a hot gas stream; and, introducing
into said gas stream a cerium-bearing reactant, an yttrium-bearing reactant, and an oxygen-
bearing reactant; the temperature of said gas stream being chosen so as to atomize said
reactant, the reactant being selected so as to form, upon cooling, doped ceria particles.
Preferably, the cerium-bearing reactant comprises either one or more of cerium chloride,
oxide, carbonate, sulphate, nitrate, acetate, and an organo-metallic cerium compound.
Moreover, the yttrium-bearing reactant could advantageously comprises either one or more of
a metal chloride, oxide, carbonate, sulphate, nitrate, acetate, and an organo-metallic metal
compound.

In a particularly advantageous embodiment, the oxygen-bearing reactant is embodied by either
one or both of the cerium-bearing reactant and the yttrium-bearing reactant.
The hot gas stream can be generated by means of either one of a gas burner, a hot-wall
reactor, and a radio frequency or direct current plasma. The gas stream can be quenched
immediately after the formation of doped ceria particles. This could avoid unwanted particle
growth during a relatively slow cooling cycle.
A still further embodiment of the invention concerns the process of polishing a substrate,
comprising the steps of: providing a CMP apparatus comprising a substrate carrier, a rotating
polishing pad, and means for feeding an abrasive slurry onto the polishing pad; placing the
substrate to be polished on the substrate carrier; pressing the substrate against the rotating
polishing pad; and, feeding an adequate amount of abrasive slurry onto the polishing pad;
characterized in that said abrasive slurry is the above-defined fluid mixture.
This process is particularly suitable for polishing substrates comprising a coating of either one
or more of silicon dioxide, silicon nitride, copper, copper barrier and tungsten, or consists of a
glass-like surface.
Excellent results were thus achieved by applying a gas phase synthesis process, combined
with the addition of a doping element. 'Doping' in this context means incorporating a doping
element in the fiuorite lattice of the CeO2, by substitution of a small part of the Ce4+ ions with
the doping element's ions. This may cause oxygen deficiency, increase lattice strain and
change the zeta-potential, and as a consequence it may also affect the different surface
energies and as such bring the energy of high index planes closer to those of low index
planes.
When used to polish thin films (e.g. SiO2) in a CMP process during the manufacturing of
semiconductor integrated circuits, the obtained particles give rise to a lower detectivity
compared to state-of-the-art ceria abrasives and with a comparable removal rate.
The crystal structure of ceria (CeO2) is cubic, according to the Fm-3m space group. The unit
cell is made up of a face-centered cubic (fcc) cerium lattice and a cubic oxygen cage within
this fee cerium lattice. Due to this fee structure, the shape of small-sized ceria particles is
dominated by the truncated octahedron, defined by {100} and {111} facets. Some high-index
facets like the {113} facet can also be present, but in much smaller amounts. This is due to the
larger surface energy of these high index planes. A few higher-order surfaces are observed,
leading sometimes to rounded corners or shapes.
To acquire a statistical shape distribution, the powders are dispersed by adding methanol to
the powder in a mortar and agitating gently. Drops of the dispersion are deposited on carbon-
film TEM support grids. High Resolution Transmission Electron Micrographs (HR-TEM) are
recorded. Thirty images at sufficiently high magnification are taken for indexing and visual

confirmation of the statistical distribution. For particle analysis, 100 particles in clear view on
the TEM images are selected.
Of these particles, the {111} planes and {100} planes are indexed and counted.
In Figure 1, the predominant particle shapes, which are the octahedron (Figure 1A) and the
truncated octahedron, are shown (Figure 1B). The truncated octahedron is also shown in [011]
zone axis, the zone axis in which the particles are mostly imaged (Figure 1C). It is clear from
this Figure that almost all ceria nano-particles have surfaces dominated by {111} and {100}
type facets. Figures 2 A-E show different examples of (truncated) octahedron type doped ceria
particles.
Examples
1. The starting material is prepared by mixing an aqueous Ce-nitrate solution with an
aqueous Y-nitrate solution in such a way that the Y-content amounts to 5 at% compared to the
total metal content A100 kW radio frequency inductively coupled plasma is generated, using
an argon/oxygen plasma with 12 Nm'/h argon and 3 Nm3/h oxygen gas. The mixed Y- and Ce-
nitrate solution is injected in the plasma at a rate of 500 mL/h, resulting in a prevalent (i.e. in
the reaction zone) temperature above 2000 K. In this first process step the Y/Ce-nitrate is
totally vaporized followed by a nucleation into Y-doped CeO2. An air flow of 10 Nm'/h is used
as quench gas immediately downstream of the reaction zone in order to lower the temperature
of the gas below 2000 K. In this way the metal oxide nuclei will be formed. After filtering a
nano-sized Y-doped CeO2 powder is obtained, characterized by the fact that the doping
element is fully incorporated into the CeO2 lattice. The specific surface area of the resulting
powder is 40 ± 2 m2/g (BET), which corresponds to a mean primary particle size of about 20
nm.
2. The apparatus according to Example 1 is operated in similar conditions. However, the
starting solution is prepared in such a way that it contains 2.5 at% Y compared to the total
metal content. After filtering a nano-sized Y-doped CeO2 powder is obtained, characterized by
the fact that the doping element is fully incorporated into the CeO2 lattice. The specific surface
area of the resulting powder is 40 ± 2 m2/g (BET), which corresponds to a mean primary
particle size of about 20 nm.
3. (Comparative) The apparatus according to Example 1 is operated in similar conditions.
However, the starting solution is a pure Ce-nitrate solution without any added Y. After filtering
a nano-sized pure CeO2 powder is obtained, with a specific surface area of 40 ± 2 m2/g (BET).
This corresponds to a mean primary particle size of about 20 nm.

4. (Comparative) A 250 kW direct current plasma torch is used, with nitrogen as plasma
gas. The gasses exit the plasma at a rate of 150 Nm3/h. A Ce-nitrate solution is injected
downstream of the plasma, at a rate of 25 kg/h. In this step, the reactants are vaporized,
resulting in a prevalent gas temperature higher than 2000 K, and nucleate as CeO2 powder.
Further downstream, air is blown at a flow rate of 6000 NrrrVh resulting in a reduction of the
gas temperature. After filtering, a nano-sized CeO2 powder is obtained. The specific surface
area of the resulting powder is 40 ± 2 m2/g (BET), which corresponds to a mean primary
particle size of about 20 nm.
5. The apparatus according to Example 4 is operated in similar conditions. However, the
starting solution is prepared in such a way that it contains 2.5 at% Y compared to the total
metal content. After filtering a nano-sized Y-doped CeO2 powder is obtained, characterized by
the fact that the doping element is fully incorporated into the CeO2 lattice. The specific surface
area of the resulting powder is 40 ± 2 m2/g (BET), which corresponds to a mean primary
particle size of about 20 nm.
6. The apparatus according to Example 4 is operated in similar conditions, however with
a plasma power of 400 kW and an air flow rate of 5000 NrrrVh. In this way a nano-sized Y-
doped CeO2 powder is obtained with a specific surface area of 301 3 rrrVg (BET), which
corresponds to a mean primary particle size of about 30 nm.
7. The apparatus according to Example 4 is operated in similar conditions, however with
a plasma power of 400 kW and an air flow rate of 15000 NmVh. In this way a nano-sized Y-
doped CeO2 powder is obtained with a specific surface area of 80 ± 5 m2/g (BET), which
corresponds to a mean primary particle size of about 11 nm.
8. The method according to Example 7, however with a Ce/Y-acetate solution as starting
material. In this way a nano-sized Y-doped CeO2 powder is obtained with a specific surface
area of 100 ± 10 nWg (BET), which corresponds to a mean primary particle size of about 10
nm.
9. The apparatus according to Example 4 is operated in similar conditions, however with
a plasma power of 400 kW and an air flow rate of 3000 NrrrVh. In this way a nano-sized Y-
doped CeO2 powder is obtained with a specific surface area of 12 ± 2 m2/g (BET), which
corresponds to a mean primary particle size of about 80 nm.

All powder samples contained at least 95 wt% mono-crystalline particles as confirmed by TEM
and XRD analyses. Table 1 gives an overview of the percentage of {111} and {100} planes
present in the powder samples according to the TEM method explained in the previous
paragraphs. It is clear that the yttrium doped samples all have more {111} planes compared
with the undoped ceria powder. Of the planes which are not {111}, Table 1 shows that 50% or
more are {100}, indicating that the shape of the doped ceria particles is also dominated by the
(truncated) octahedron type.
Table 1: Morphology results

10. An yttrium doped ceria powder with 5 at% Y prepared as described in Example 1 is
mixed with water and poly-acrylic acid at a pH of 10 (using KOH), such that the resulting ceria
content is 1 wt% and the weight of the poly-acryl chains is 3.4% of the weight of the ceria, and
the mixture is then sonicated for 10 min. The mixture is then brought on a polishing pad
rotating at 40 rpm, and during 1 min a Si wafer with a deposited SiO2 film rotating at 65 rpm is
pressed against the pad with a pressure of 4 psi. The wafer is then rinsed, cleaned and dried.
The resulting film thickness loss as measured by ellipsometry is 69 nm. The wafer is then
dipped in a 0.2% HF bath until 15 nm of the remaining SiO2 film has dissolved, and then rinsed
and dried such that no water marks remain on the surface. The resulting number of defects on
the film surface larger than 0.15 urn as measured by dark field laser light scattering is 3752.
Both results are considered to be satisfying.
11. An yttrium doped ceria powder with 2.5 at% Y prepared as described in Example 2 is
brought in a mixture which is used for polishing a Si wafer with deposited SiO2 film as
described in Example 10. The resulting film thickness loss before dipping in the HF bath is

75 nm. The resulting number of defects larger than 0.15 urn after dipping in the HF bath is
1750. Both results are considered to be satisfying.
12. (Comparative) A pure ceria powder prepared as described in Comparative Example 3
is brought in a mixture which is used for polishing a Si wafer with deposited SiO2 film as
described in Example 10. The resulting film thickness loss before dipping in the HF bath is only
59 nm, which is too low. The resulting number of defects larger than 0.15 urn after dipping in
the HF bath is 6916.This figure is considered inadequately high.

Claims
1. Yttrium-doped ceria particles having a specific surface area of 10 to 120 m2/g,
characterized in that at least 95 wt%, preferably at least 99 wt%, of the particles are
mono-crystalline, and in that the particles' surfaces consist of more than 70%,
preferably of more than 80%, of planes parallel to {111} planes.
2. Yttrium-doped ceria particles according to claim 1, characterized in that the particles
comprise 0.1-15 at% of doping element versus the total metal content.
3. Yttrium-doped ceria particles according to claims 1 or 2, characterized in that the
particles further consist of unavoidable impurities.
4. Use of the yttrium-doped ceria particles according to any one of claims 1 to 3 for the
preparation of a fluid mixture consisting of either one of a dispersion, a suspension and
a slurry.
5. Fluid mixture comprising the yttrium-doped ceria particles according to any one of
claims 1 to 4.
6. Gas phase process for synthesizing yttrium-doped ceria particles according to any one
of claims 1 to 3, comprising the steps of:

- providing a hot gas stream; and,
- introducing into said gas stream a cerium-bearing reactant, an yttrium-bearing
reactant, and an oxygen-bearing reactant;
the temperature of said gas stream being chosen so as to atomize said reactant, the
reactant being selected so as to form, upon cooling, yttrium-doped ceria particles.
7. Process according to claim 6, characterized in that cerium-bearing reactant comprises
either one or more of cerium chloride, carbonate, oxide, sulphate, nitrate, acetate, and
an organo-metallic cerium compound.
8. Process according to claims 6 or 7, characterized in that the yttrium-bearing reactant
comprises either one or more of an yttrium chloride, carbonate, oxide, sulphate,
nitrate, acetate, and an organo-metallic yttrium compound.
9. Process according to any one of claims 6 to 8, characterized in that the oxygen-
bearing reactant is embodied by either one or both of the of the cerium-bearing
reactant and the yttrium-bearing reactant.
10. Process according to any one of claims 6 to 9, characterized in that the hot gas stream
is generated by means of either one of a gas burner, a hot-wall reactor, a radio
frequency or direct current arc plasma.

11. Process according to any one of claims 6 to 10, characterized in that, after the
formation of yttrium-doped ceria particles in the gas stream, the gas stream is
quenched.
12. Process for polishing a substrate, comprising the steps of:

- providing a CMP apparatus comprising a substrate carrier, a rotating polishing pad,
and means for feeding an abrasive slurry onto the polishing pad;
- placing the substrate to be polished on the substrate carrier;
- pressing the substrate against the rotating polishing pad; and,
- feeding an adequate amount of abrasive slurry onto the polishing pad;
characterized in that said abrasive slurry is a fluid mixture according to claim 5.
13. Process according to claim 12, characterized in that said substrate comprises a
coating of either one or more of silicon dioxide, silicon nitride, copper, copper barrier
and tungsten, or consists of a glass-like surface.

The present invention relates to doped ceria (CeO2) abrasive particles, having an essentially octahedral morphology.
Such abrasives are used in water-based slurries for Chemical Mechanical Polishing (CMP) of subrates such as silicon wafers.
The invention more particularly concerns yttrium-doped ceria particles having a specific surface area of 10 to 120 m2/g, characterized
in that at least 95 wt%, preferably at least 99 wt%, of the particles are mono-crystalline and in that the particles' surfaces consist
of more than 70 %, preferably of more than 80%, of planes parallel to {111} planes. A novel gas phase process for synthesizing
this product is also disclosed, comprising the steps of providing a hot gas stream,- and, introducing into said gas stream a cerium-bearing
reactant, a dopant-bearing reactant, and an oxygen-bearing reactant,- the temperature of said gas stream being chosen
so as to atomize said reactant, the reactant being selected so as to form, upon cooling, doped ceria particles. Abrasive slurries
based on the above ceria offer a low level of induced detectivity in the polished substrate, while ensuring a good removal rate.