Introduction

As the core cornerstone of the third-generation semiconductor industry, silicon carbide (SiC) substrates are essential base materials for high-end devices such as high-voltage platforms of new energy vehicles, photovoltaic energy storage inverters, and AI server power supplies. They directly determine the withstand voltage performance, energy efficiency loss and service life of terminal products. In the cost structure of SiC power devices, substrates account for 40%–47%, representing the highest technical barrier and the most stringent quality control link in the industrial chain. Commercially mass-produced 6H-SiC substrates feature two intrinsic polar basal planes: Si-terminated (0001) plane and C-terminated (000–1) plane. The two substrate surfaces exhibit essential differences in atomic arrangement, surface free energy, oxidation kinetics and interface state, which are key factors restricting the yield of epitaxial growth and device reliability.
In large-scale mass production, traditional identification and surface state detection methods for SiC substrate polarity rely on high-temperature thermal oxidation and high-precision microscopic characterization, which suffer from low detection efficiency and high costs, failing to adapt to rapid quality inspection of mass substrates. Based on the standardized grinding-polishing gradient process experiments of 6H-SiC substrates, this paper focuses on the internal correlation among intrinsic polarity, surface processing quality and wetting characteristics of SiC substrates. It analyzes the engineering value of the static contact angle technology in incoming quality inspection, process endpoint judgment and polarity sorting of SiC substrates, providing a low-cost and high-efficiency new technical solution for mass production quality control of substrates.

1. Standardized Experimental System for SiC Substrates: Controlling Variables to Restore True Substrate Performance

1.1 Substrate Sample Preparation and Gradient Processing Technology

To accurately explore the influence of different processing technologies on the surface quality and wetting performance of SiC substrates, all experimental samples were taken from the same 6H-SiC single crystal ingot grown by the Physical Vapor Transport (PVT) method. As the mainstream commercial mass production technology for SiC substrates with a market share of over 90%, the PVT method ensures that the sample selection fully conforms to industrial mass production standards. Defective substrate areas with carbon inclusions, dislocation clusters and polytype twins were manually screened out to eliminate the interference of intrinsic substrate defects on experimental data and ensure the research results can directly guide mass production.
Commercially available 2-inch 6H-SiC wafers were adopted in the experiment, and three gradient processes were set to simulate the complete mass production processing flow, accurately reproducing the full-range state of substrates from rough processing to ultra-precision finishing:
1. Rough grinding process: The substrate surface is ground with large-diameter diamond grinding wheels to remove cutting tool marks left by slicing, forming a typical mechanically processed surface with a large number of micro-scratches and sub-surface damage, corresponding to the semi-finished state of rough-processed substrates;
2. 1μm rough polishing process: Wet polishing with 1μm diamond polishing liquid is adopted to quickly eliminate deep surface scratches and significantly reduce surface roughness, serving as the intermediate processing procedure for substrate mass production;
3. 0.1μm fine polishing process: Final polishing with 0.1μm ultra-fine diamond polishing liquid is performed to remove micro-defects and sub-surface residual stress, obtaining device-level ultra-smooth substrates that meet the mass production standards of terminal devices.
After processing, the whole wafer was cut into 10mm×10mm standard samples, strictly divided into Si-faced and C-faced polar substrates. Multiple parallel samples were set for each process and each polarity to eliminate random errors and ensure the guiding value of experimental data for substrate mass production processes.
 

[Figure 1: Optical Image of 6H-SiC Substrate Samples]
(a) Original optical image of 2-inch finished 6H-SiC wafer; (b) 10mm×10mm standard substrate samples after cutting, distinguishing Si-face and C-face, showing sample morphologies under three processing states: grinding, 1μm polishing and 0.1μm polishing.

1.2 Standardized Substrate Preprocessing Flow to Eliminate Surface Contamination Interference

The surface chemical state of SiC substrates is highly sensitive. Native oxide layers spontaneously formed in the air, residual polishing abrasives and organic contaminants will directly change the intrinsic wetting characteristics of substrates and lead to misjudgment in quality detection. To accurately restore the true surface properties of SiC substrates with different polarities and processing states, a mass-production-compatible standardized cleaning process was formulated: BOE buffered etching solution removes native oxide layers, followed by multi-stage ultrasonic cleaning with acetone, methanol and high-resistivity deionized water. Finally, high-purity nitrogen is used for blow-drying to ensure consistent surface cleanliness of all samples, fully conforming to the pre-processing state of device fabrication.

1.3 Standardized Optimization of Substrate Contact Angle Detection Parameters

Based on the stability requirements of mass production detection, gradient droplet tests were carried out to optimize detection parameters and eliminate the interference of gravity and ambient air convection on substrate wetting detection. A fixed volume of 0.5μL ultra-high-purity deionized water micro-droplets was adopted for static contact angle testing. This parameter minimizes external interference, accurately reflects the intrinsic surface wetting performance of SiC substrates, and guarantees the repeatability and stability of detection data for mass quality inspection.
 

[Figure 2: Optimization Curve of Substrate Contact Angle Test Parameters]
Side-view morphologies, contact angle variation curves and data box plots of 0.5μL~6μL gradient deionized water droplets on 0.1μm fine-polished Si-faced 6H-SiC substrates, verifying the rationality of the 0.5μL test parameter.
 

2. Wetting Characteristic Laws of SiC Substrates with Different Polarities and Processing States

2.1 Core Contact Angle Data of Dual-Polarity SiC Substrates Under Different Processes

Detection was performed on three types of SiC substrates (rough grinding, rough polishing and fine polishing) to obtain stable wetting characteristic data of dual-polarity substrates, intuitively reflecting the decisive influence of substrate processing quality and polarity on surface performance:

Substrate Processing Procedure	Average Contact Angle of Si-(0001) Substrate	Average Contact Angle of C-(000–1) Substrate	Contact Angle Difference Between Dual-Polarity Substrates
Rough Grinding Substrate	53.3°	48.5°	4.8°
1μm Rough Polishing Substrate	53.2°	46.2°	7.0°
0.1μm Fine Polishing Substrate	62.0°	55.2°	6.8°

 

[Figure 3: Water Droplet Wetting Morphologies of Substrates Under Different Processes]
Comparative side-view morphologies of deionized water droplets on Si-faced (0001) and C-faced (000–1) SiC substrates under grinding, 1μm rough polishing and 0.1μm fine polishing processes.
 

[Figure 4: Contact Angle Difference Comparison of Dual-Polarity Substrates]
Histogram of static contact angle values of Si-faced and C-faced SiC substrates at different processing stages with error bars showing data deviation.

 

2.2 Core Law: Intrinsic Substrate Properties Dominate Wetting Performance Differences

1. Constant Polarity Characteristics: Whether in rough processing or ultra-precision finishing state, the contact angle of 6H-SiC Si-faced substrates is always higher than that of C-faced substrates. This difference is determined by the intrinsic surface energy of the crystal, which cannot be covered by surface processing scratches and mechanical damage, serving as a stable feature for SiC substrate polarity identification.
2. Higher Processing Accuracy Enhances Polarity Discrimination: With the upgrading of polishing processes, the surface roughness of substrates decreases continuously and defects are gradually eliminated, amplifying the wetting characteristic difference between dual-polarity substrates. Fine-polished finished substrates exhibit the highest polarity identification sensitivity, enabling accurate screening of substrate polarity mixing defects.
3. Superior Surface Stability of C-faced Substrates: Compared with Si-faced substrates, C-faced substrates show smaller contact angle fluctuation from grinding to fine polishing, with more stable surface chemical states and lower sensitivity to subtle processing changes, ensuring better mass production consistency.
 

2.3 Mechanism: SiC Substrate Surface Energy Determines Wetting Characteristic Differences

The performance difference between the two polarities of 6H-SiC substrates originates from the huge gap in intrinsic surface energy: the surface energy of Si-(0001) substrates is only 718 erg·cm⁻², while that of C-(000–1) substrates reaches 1767 erg·cm⁻². C-faced substrates with high surface energy have more surface dangling bonds and higher activity, which bind more easily with water molecules, resulting in stronger hydrophilicity, sufficient droplet spreading and lower contact angles. In contrast, Si-faced substrates have lower surface activity and higher infiltration resistance, leading to larger contact angles. This intrinsic physical property difference provides the core theoretical support for rapid polarity identification and quality grading of SiC substrates.

3. Core Value of Contact Angle Detection in Mass Production Quality Control of SiC Substrates

1. Rapid Polarity Sorting to Avoid Mass Production Scrap: SiC device fabrication has strict requirements on substrate polarity, and polarity mixing directly causes epitaxial layer failure and excessive device leakage current. Contact angle detection can complete polarity judgment of a single substrate within 10 seconds, adapting to mass sorting on production lines, completely eliminating substrate front-back mixing and significantly improving production yield.
2. Rapid Judgment of Substrate Polishing Process Endpoint: The contact angle value of substrates is highly correlated with polishing accuracy — higher surface smoothness corresponds to a larger contact angle. It can be used as a pre-judgment index for qualified polishing procedures, quickly eliminating inferior substrates with insufficient polishing and excessive surface defects and simplifying the quality inspection process.
3. Non-destructive Detection for Full-Process Substrate Inspection: The detection process involves no corrosion, surface modification or sample loss. Substrates after detection can be directly used in subsequent processes such as epitaxy and coating, fully meeting the mass production quality control standards of automotive-grade and industrial-grade SiC substrates.
 

Conclusion

The intrinsic Si/C dual-polarity properties of 6H-SiC substrates determine stable and quantifiable wetting characteristic differences, which become more significant with the improvement of substrate polishing quality. Featuring low cost, high efficiency and non-destructiveness, static contact angle detection technology can accurately realize polarity identification, preliminary quality screening and process state judgment of SiC substrates. It effectively compensates for the shortcomings of rapid quality control in large-scale mass production, and can build a complete substrate quality control system when combined with high-precision characterization equipment, laying a solid foundation for the mass production of high-end SiC power devices.
JXT supplies various high-quality silicon carbide substrates with multiple crystal types including 4H and 6H and full-size specifications, covering products with different polishing grades and polarities. We strictly control the surface roughness, lattice integrity and surface stability of substrates, adapting to the R&D and mass production needs of power devices and optoelectronic devices, and meeting the application scenarios of various precision characterization experiments and industrial batch production.
 

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