News On Rare Earth Elements

Cerium Oxide in Glass Polishing, Semiconductor CMP, and Silicon Carbide

Release time：2026-10-16 Views：11

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A smartphone cover glass looks smooth and transparent, but how does it reach that optical-grade surface finish?

The answer is not simply abrasive grinding. The real process is a carefully controlled chemical–mechanical reaction. In a cerium oxide polishing slurry, cerium oxide particles interact with the glass surface at the nanoscale. They remove microscopic high points while helping protect the low points.

The factor that ultimately determines whether a screen can deliver a clear image is not only how hard cerium oxide is. It is how chemically active its surface can be.

Cerium is the element that gives cerium oxide this activity. Its value extends far beyond polishing. From grinding wheels and semiconductor wafers to high-temperature structural ceramics, cerium uses changes in oxidation state to influence the behavior and performance of other materials.

Cerium Oxide Polishing: A Temporary Chemical Link with Silicon Dioxide

Chemical mechanical polishing, or CMP, is a core process in semiconductor wafer manufacturing and advanced optical-glass processing. Traditional polishing relies mainly on abrasive particles to remove material mechanically. Cerium oxide slurry takes a different approach: it combines chemical bonding with mechanical shear.

At the interface, the cerium oxide particle surface can form temporary Ce–O–Si bridge bonds with silicon dioxide. Mechanical shear then removes the chemically softened silicon–oxygen layer. Water subsequently breaks the Ce–O–Si bonds from below, regenerating the particle surface so that it can continue working.

The key to this mechanism is the flexible conversion between Ce³⁺ and Ce⁴⁺. A higher proportion of Ce³⁺ on the particle surface generally means stronger chemical activity toward SiO₂ and a higher polishing rate.

According to the source article, cerium oxide can provide a SiO₂ removal efficiency 3–5 times higher than conventional colloidal silica abrasives. In other words, for the same abrasive mass, cerium oxide can remove material at several times the rate of silica slurry. This is why cerium oxide-based polishing slurries are widely associated with large-diameter wafers, smartphone cover glass, and high-end optical lenses.

Selectivity is equally important. In shallow trench isolation (STI), the process must selectively remove SiO₂ without damaging the Si₃N₄ stop layer. The intrinsic selectivity ratio of pure cerium oxide is reported as approximately 10:1 to 30:1. With polymer inhibitors such as polyacrylic acid, or PAA, the selectivity window can reach 50:1 to 200:1, which is suitable for advanced process requirements.

For upstream rare-earth suppliers, the difference between CMP-grade cerium oxide and ordinary glass-polishing powder is the difference between rough grinding and precision finishing. Critical quality factors include:

• Particle-size distribution

• The tail of abnormally large particles

• The Ce³⁺/Ce⁴⁺ ratio

• Specific surface area

• Clean filling and packaging capability

These parameters directly affect polishing rate, surface quality, defect control, and process stability.

Cerium-Modified Silicon Carbide: A Tougher Crystal with a Small Addition of Cerium

Polishing solves the problem of surface flatness. However, some materials must be sufficiently durable before they can be polished or used as abrasives.

Silicon carbide abrasive is one example. Ordinary green silicon carbide has high hardness but relatively high brittleness. Under load, abrasive grains can fracture easily, causing grinding efficiency to decline as the particles break down.

Adding a small amount of cerium oxide to the smelting charge produces a modified silicon carbide abrasive known as cerium-modified silicon carbide. Its main phase remains α-type silicon carbide, but its performance can improve significantly.

The source article reports a microhardness of 36.29 GPa. For a single 46# particle, compressive strength increased from 4,420 kg/cm² for green silicon carbide to 7,070 kg/cm², an improvement of 60%. Toughness increased from 50% to 75%.

The proposed mechanism is related to high-temperature crystal and grain-boundary control. During smelting, cerium can enter the SiC lattice or concentrate at grain boundaries, modifying crystal defects. The crystal remains silicon carbide, but crack propagation becomes more difficult.

When used for grinding titanium alloys, cast iron, and difficult-to-machine high-speed steel, cerium-modified silicon carbide can offer better cutting efficiency and service life than ordinary green silicon carbide.

4H-SiC Single-Crystal Growth: Cerium Helps Select the Crystal Polytype

Silicon carbide is not limited to one crystal structure. It has more than 250 polytypes, but the semiconductor industry primarily requires one of them: 4H-SiC.

During physical vapor transport, or PVT, growth, other polytypes such as 6H and 15R can appear. These unwanted inclusions may become polytype defects in the crystal and directly affect wafer electrical performance.

How can manufacturers encourage the growth of 4H-SiC?

Research has shown that adding CeO₂ as a cerium source to SiC powder can help. Cerium vapor becomes enriched near the growth interface and promotes nucleation of the 4H polytype.

In this application, cerium does not primarily enter the crystal lattice as a dopant. Instead, it acts more like an interface regulator. By lowering the growth-interface energy and changing the competition among polytypes, cerium can make 4H-SiC more thermodynamically favorable.

This illustrates an important feature of cerium chemistry: cerium can influence a material not only through bulk composition, but also through interfaces, surface energy, defect chemistry, and nucleation behavior.

Cerium Oxide as a Sintering Additive for SiC Ceramics

Sintering is another major challenge in silicon carbide ceramics. Because SiC has strong covalent bonds and a low self-diffusion coefficient, it is difficult to densify without a sintering aid.

Cerium oxide can react with SiO₂ on the surface of SiC particles and form a eutectic liquid phase below 1,700°C. This liquid phase promotes particle rearrangement and densification during sintering.

The rare-earth silicate liquid phase formed at the grain boundaries does more than fill gaps. The Ce³⁺/Ce⁴⁺ redox cycle can also create oxygen vacancies and influence the composition of the grain-boundary phase. This may improve high-temperature strength and oxidation resistance.

That is why high-purity cerium oxide is receiving increasing attention as a sintering additive for SiC ceramics. The addition level is usually only a few percentage points, but it can strongly influence the composition and distribution of the grain-boundary phase.

Catalysis, SOFC Electrolytes, and UV-Shielding Materials

In automotive three-way catalytic converters, cerium oxide acts as an oxygen buffer. Under lean conditions, it absorbs oxygen into its lattice. Under rich conditions, it releases oxygen. This behavior again depends on the reversible Ce³⁺ ↔ Ce⁴⁺ redox cycle.

This property is known as oxygen storage capacity (OSC). The source article gives an OSC of approximately 25 μmol-O/g for pure CeO₂. Introducing zirconium to form a cerium–zirconium solid solution can increase the value by one order of magnitude. Further introduction of elements such as Pr, Nd, and La can raise the reported capacity to more than 1,000 μmol-O/g.

The Ce³⁺/Ce⁴⁺ cycle and oxygen vacancies are also relevant to energy materials and transparent functional materials.

For solid oxide fuel cells (SOFCs), gadolinium-doped ceria, or GDC, and samarium-doped ceria, or SDC, are candidate electrolyte materials for intermediate-temperature operation.

For UV shielding, nano-CeO₂ can absorb ultraviolet radiation and may be used in transparent coatings, glass, and polymer systems.

Cerium Metal and Master Alloys

Cerium metal has a silver-gray appearance and a melting point of 795°C, which is relatively low among rare-earth metals. It oxidizes readily in air and can even ignite spontaneously. This reactivity led to its early use in lighter flints.

Today, more important applications for cerium metal include alloy additives and hydrogen-storage materials. In hydrogen-storage alloys, cerium can improve hydrogen absorption and desorption kinetics. In aluminum and magnesium alloys, cerium can act as a modifier, refine grains, and improve mechanical performance.

Cerium master alloys—such as aluminum–cerium and magnesium–cerium alloys—serve as practical carriers for introducing cerium into molten metal. Adding pure cerium directly to a melt can lead to problems related to melting behavior, density differences, and burn-off. In a master alloy, cerium enters the melt in a form that is easier to melt and disperse.

The source article cites a study of an Al/Mg bimetallic composite. When 1.0% cerium was introduced through a magnesium–cerium master alloy, interfacial bonding strength increased from 37.22 MPa to 53.29 MPa, an improvement of 43%.

Matching Cerium Products to Applications

Moving from 3N to 4N and then to 5N purity represents a major step in separation and purification technology. Each additional “9” can require tighter control of impurities and a more demanding production process.

Shanghai Shinian Metal Materials Co., Ltd. provides high-purity cerium oxide products for polishing, catalysis, UV shielding, and related applications. Key impurities can be controlled according to the requirements of the target application.

Product	Typical specification	Downstream applications
Nano cerium oxide	99.95%	Glass and CMP polishing, exhaust-gas catalysis, UV-shielding coatings
Cerium oxide	99.999%	Precision optical polishing, semiconductor CMP, specialty glass
Cerium metal and master alloys	99.9% / 99.99%	Mischmetal alloys, Al–Ce high-temperature alloys, Mg–Ce alloys