Introduction

With the rapid growth of the third-generation semiconductor industry, 4H-SiC (4H Silicon Carbide) wafers have become essential in applications such as electric vehicles, photovoltaic energy storage, high-voltage inverters, smart grid systems, and 5G communications.
However, due to the intrinsic material characteristics of SiC, including:

1. Ultra-high hardness
2. Strong covalent bonding structure
3. Exceptional chemical inertness

the processing difficulty of SiC is significantly higher than that of conventional silicon wafers.
Particularly during the final stage of wafer fabrication, CMP (Chemical Mechanical Polishing) plays a critical role in determining wafer surface quality, defect control, and overall device yield.
Although conventional CMP can achieve atomically smooth surfaces, it still faces several major challenges:

1. Low material removal rate (MRR)
2. Long processing cycles
3. High manufacturing cost
4. Difficulty in controlling uniformity for large-diameter wafers

As a result, extensive research efforts worldwide have focused on improving the efficiency of SiC CMP processes.
This article systematically reviews the most advanced optimization technologies currently used in 4H-SiC CMP, including:

1. Chemical enhancement approaches
2. Photocatalytic CMP
3. Special-atmosphere CMP
4. Mixed abrasive technologies
5. Fixed abrasive polishing
6. Electrochemical CMP
7. Plasma-assisted CMP
8. Chemical–mechanical synergistic enhancement technologies

providing a comprehensive overview of future trends in ultra-precision SiC wafer processing.
 

1. Chemical-Improvement Approaches

Because 4H-SiC exhibits extremely high chemical stability, CMP efficiency is often limited by the oxidation reaction rate of the SiC surface.
Therefore, a large body of research has focused on improving the oxidation efficiency of SiC during CMP.

1.1 Fenton-Like Reaction Enhanced CMP

The Fenton reaction is a typical advanced oxidation process (AOP).
Compared with conventional H₂O₂ oxidation systems, Fenton reactions generate highly reactive hydroxyl radicals (•OH), significantly enhancing the oxidation rate of the SiC surface.
Since hydroxyl radicals possess a higher oxidation potential than H₂O₂, they can oxidize 4H-SiC surfaces much more efficiently.
Studies have shown that:

1. Fe₃O₄ exhibits the best catalytic performance
2. The Fe²⁺ concentration and H₂O₂ ratio directly affect radical generation
3. Excess Fe²⁺ leads to flocculent precipitation
4. Excess H₂O₂ can suppress •OH generation

After implementing Fenton-like CMP:

1. Material removal volume increases significantly
2. SiC surface oxidation rates are substantially enhanced

However, several important challenges remain.

Technical Challenges

(1) Strict Acidic Environment Requirement

Fenton systems typically require pH < 3; otherwise, Fe²⁺ tends to convert into Fe(OH)₃ precipitates.

(2) Severe Abrasive Agglomeration

Strong acidic conditions can induce aggregation of colloidal SiO₂ particles, resulting in:

1. Reduced mechanical removal capability
2. Poor surface uniformity
3. Increased scratch risk

Although this issue may be less pronounced at laboratory scale, it becomes much more severe in industrial mass production, where:

1. A single CMP cycle may last 3–5 hours
2. Slurry circulation times can exceed 10 hours

thereby further amplifying particle agglomeration effects.
 

1.2 Special-Gas Atmosphere CMP

Researchers have found that modifying the CMP atmospheric environment can effectively improve oxidation efficiency.
Typical gas environments include:

3. Oxygen (O₂) atmosphere
4. Ozone (O₃) atmosphere

Experimental studies demonstrate that:

5. MRR increases significantly under O₂ atmosphere
6. Introducing ozone bubbles into the slurry can increase MRR from 25 nm/h to 200 nm/h

The fundamental mechanism is the enhancement of oxidizing species concentration, thereby accelerating SiC surface oxidation.

Technical Challenges

This approach requires:

1. A sealed CMP chamber
2. Gas delivery systems
3. Dedicated safety protection systems

In particular, ozone possesses strong oxidative activity and safety risks, which still limit large-scale industrial implementation.

Figure 1. Schematic illustration of a special-gas atmosphere-assisted CMP system, including auxiliary components such as a pressure chamber and gas tank. A gas tank is integrated into the sealed CMP system to maintain a controlled gas atmosphere during polishing.
 

1.3 Pre-Processing Enhanced CMP

Pre-treatment technologies mainly include:

4. Thermal oxidation pre-treatment
5. Laser pre-treatment

Thermal Oxidation Pre-Treatment

Long-duration thermal oxidation at approximately 1100°C can form a softened oxide layer on the SiC surface.
Advantages include:

1. Reduced CMP difficulty
2. Improved material removal efficiency

Laser Pre-Treatment

Ultrafast lasers (femtosecond, picosecond, and nanosecond lasers) can generate:

1. Micro-pits
2. Micro-grooves

on the SiC surface, thereby:

1. Increasing contact area
2. Improving slurry reaction efficiency

Studies indicate that laser pre-treatment significantly enhances CMP efficiency.

Current Challenges

As the industry moves toward 8-inch SiC wafers, critical challenges include:

1. Laser uniformity control
2. Thermal damage suppression
3. Large-area process consistency

 

1.4 Photocatalysis-Assisted CMP (PCMP)

Photocatalysis-assisted CMP (PCMP) is currently considered one of the most promising CMP enhancement technologies.
Its core principle is the synergistic interaction of:

1. UV irradiation
2. Photocatalysts
3. Oxidizing agents

to generate a large quantity of highly reactive radicals.

Working Mechanism

Typical PCMP systems include:

1. UV light source
2. TiO₂ nanoparticles
3. H₂O₂ oxidant

Under UV irradiation, TiO₂ generates electron-hole pairs, which subsequently produce large amounts of •OH and O₂⁻ radicals, greatly improving SiC oxidation efficiency.

Performance Enhancement

Studies have reported that PCMP can increase:
MRR from 200 nm/h to 352.8 nm/h
while achieving a surface roughness of:
Ra = 0.0586 nm
approaching atomically smooth surfaces.

Sulfate Radical Systems (SR-AOPs)

Compared with •OH radicals, SO₄•⁻ radicals exhibit:
Higher oxidation potential
Longer lifetime
Therefore, K₂S₂O₈-based systems can further improve CMP efficiency.

Current Challenges

PCMP still faces several technical issues:

1. TiO₂ nanoparticle agglomeration
2. UV-induced polishing pad degradation
3. Thermal effects affecting slurry stability

Figure 2. Schematic illustration of the photocatalytic oxidation mechanism of 4H-SiC. The synergistic effect of UV irradiation and TiO₂ increases the generation of •OH radicals in the slurry, thereby enhancing oxidation efficiency during CMP.
 

2. Mechanical-Improvement Approaches

In addition to chemical enhancement, mechanical removal efficiency is also a key factor governing CMP performance.

2.1 Single-Abrasive Systems

Common abrasive materials include:
 

Abrasive	Mohs Hardness
Diamond	10
B₄C	9.4
SiC	9.2
Al₂O₃	9
SiO₂	7
CeO₂	6

Advantages of CeO₂ Abrasives

CeO₂ exhibits unique chemical activity due to:

4. Ce³⁺/Ce⁴⁺ valence transitions
5. Oxygen vacancy structures

which enhance removal of the SiC oxide layer.
Research has demonstrated:

1. MRR exceeding 1 μm/h
2. Surface roughness as low as 0.11 nm

However, CeO₂ abrasives are relatively expensive and more difficult to clean after polishing.

2.2 Mixed Abrasive Slurry (MAS)

Mixed Abrasive Slurry (MAS) utilizes synergistic effects from abrasives with different:

3. Particle sizes
4. Hardness levels
5. Chemical properties

Typical systems include:

1. Diamond + SiO₂
2. Graphene oxide (GO) + Diamond
3. ZrO₂ + Al₂O₃

GO-Based Mixed Abrasive Systems

Graphene oxide not only provides lubrication and reduces scratching, but also promotes radical generation.
As a result, the system achieves:

1. High MRR
2. Low damage
3. Excellent surface quality

Figure 3. Mixed abrasive slurry containing diamond particles and graphene oxide nanosheets:
(a) SEM image of diamond particles;
(b) SEM image of graphene oxide nanosheets with 45% oxidation degree;
(c) Schematic illustration of the enhancement mechanism, where graphene oxide promotes radical generation while reducing scratches caused by diamond abrasives.
 

2.3 Core–Shell Abrasives

Core–shell structured abrasives represent an important direction in advanced CMP technology.
Their structure typically includes:

1. Hard cores (Diamond, PS, etc.)
2. Functional shell layers (SiO₂, TiO₂, CeO₂, etc.)

Advantages include:

3. Simultaneously achieving high MRR and low scratch density
4. Enhanced chemical activity
5. Improved surface quality

Several ECMP systems have already demonstrated high polishing efficiency using core–shell abrasives.
 

2.4 Fixed Abrasive Polishing (FAP)

Fixed Abrasive Polishing (FAP) directly immobilizes abrasive particles onto the polishing pad.
Advantages include:

6. Improved abrasive utilization efficiency
7. Higher MRR
8. Reduced slurry stability issues

However, fractured abrasive particles may cause:

1. Deep scratches
2. Subsurface damage

Therefore, Semi-FAP (semi-fixed abrasive polishing) has emerged as a more favorable alternative.

Figure 4. Schematic diagram of fixed abrasive polishing, in which abrasive particles are fixed onto the polishing pad surface.
 

3. Chemical–Mechanical Synergistic Enhancement Technologies

The most advanced trend in CMP development today is multi-physics coupled CMP technology.

3.1 Electrochemical CMP (ECMP)

ECMP introduces an external electric field to induce anodic oxidation of SiC.

Mechanism

3. External current rapidly forms a softened SiOxCy layer
4. Abrasives mechanically remove the oxide layer

Compared with conventional CMP, ECMP enables much faster oxidation rates.
Some studies have already demonstrated ultra-high polishing efficiency.
 

3.2 Plasma-Assisted Polishing (PAP)

PAP utilizes:

5. Atmospheric-pressure plasma
6. Water-vapor plasma oxidation

to soften the SiC surface.
Research shows that the surface hardness of SiC decreases significantly after plasma treatment, making mechanical removal substantially easier.

Figure 5. Schematic diagram of a PAP system consisting of an independent external plasma generator integrated with a conventional CMP system.
 

4. Future Development Trends

Future 4H-SiC CMP technologies will primarily focus on:

1. AI-driven intelligent CMP control
2. Multi-field coupled CMP
3. Atomic-level defect control
4. Ultra-low-damage CMP
5. Uniform processing of 8-inch wafers
6. Environmentally friendly CMP slurries
7. Highly stable nano-abrasive systems

In the future, the true core competitiveness of CMP technology will no longer rely solely on achieving high MRR, but rather on simultaneously delivering:
“High efficiency + Superior surface quality + Low cost + High process stability”
 

Conclusion

4H-SiC CMP is gradually evolving from traditional mechanical polishing into an advanced ultra-precision manufacturing technology integrating:

8. Chemical processes
9. Mechanical processes
10. Electrochemical techniques
11. Optical methods
12. Plasma technologies
13. Multi-physics synergistic coupling

As photocatalytic technologies, electrochemical methods, AI process control, and novel abrasive designs continue to mature, SiC wafer processing efficiency and surface quality are expected to achieve major breakthroughs, providing stronger support for the rapid development of the third-generation semiconductor industry.
Leveraging extensive expertise in 4H-SiC substrate materials and advanced CMP process control systems, JXT provides conductive and semi-insulating 4H-SiC silicon carbide substrates ranging from 2-inch to 12-inch wafers, with customizable thicknesses and dimensions. Processed using chemically enhanced CMP and synergistic polishing technologies, the products exhibit minimal subsurface damage and excellent within-wafer uniformity, making them ideally suited for epitaxial growth and device fabrication in power and RF applications. The company is committed to helping customers reduce R&D costs, improve mass-production yield, and accelerate the high-quality development of the third-generation semiconductor industry.
 

Related products: 

4H-SiC (4H Silicon Carbide) wafers
4H-N SiC silicon carbide substrates
semi-insulating 4H-SiC silicon