Research on High Efficiency Polishing of 4H-SiC Substrate by Friction Induced Chemical Reaction of Pure Iron –

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 1. Abstract

This work proposes a processing technique based on pure iron friction induced chemical reactions for efficient and low damage processing of single crystal 4H-SiC wafers. By systematically exploring the removal rate and surface quality of materials under process conditions, this method achieved high material removal rate (375 nm/min) while achieving excellent surface roughness (Ra value of 2.2 nm). The study further revealed the key role of friction velocity in the material removal mechanism: friction can trigger solid-state chemical reactions at the interface between metal and SiC, and effectively remove the products through subsequent mechanical action, thereby forming a processing effect with high surface integrity. On this basis, a material removal model for friction processing was constructed, indicating that the final removal rate is significantly affected by both the solid-state reaction rate and the reactant removal efficiency.

2. Experiment

The experiment used an nitrogen doped single crystal 4H-SiC substrate as the workpiece, with dimensions of 5 mm × 5 mm × 4 mm in the friction experiment. The polishing tool was a pure iron polishing plate with a square grid or a Φ 50.8 mm × 10 mm iron plate, and the polishing equipment was a UNIPOL-1200S pressure grinding and polishing machine. The auxiliary equipment includes a diamond dresser for maintaining the flatness of the polishing plate, a high-precision electronic balance (accuracy 0.1 mg) for measuring SiC mass loss, an Rtec MFT-5000 multifunctional friction and wear testing machine, as well as a high-speed infrared thermal imager and thin film thermocouple for friction temperature monitoring.

During the polishing process, SiC wafers are mounted on the worktable clamp, fixed on a pure iron polishing plate, and no coolant is used. The key parameters include: the width/spacing of the waste bin is 2 mm/11 mm, the rotation speed of the workpiece and polishing plate is 60 rpm and 100 rpm respectively, and the pressure is 637 Pa. The wafer height is measured every 10 minutes, and the material removal rate (MRR) is calculated according to the formula MRR=Δ l × 10 ³/t (nm/min) based on the thickness loss Δ l and time t. The friction experiment was conducted at room temperature of 20 ± 1 ° C, with a pressure of 0.2 MPa, a speed of 2-8 m/s, and a time of 10-25 min. The MRR was calculated based on the mass loss Δ m, SiC density ρ, friction area S, and time t using the formula MRR=Δ m × 10 ⁶/(ρ× S × t). The friction temperature was monitored by an infrared thermal imager and a thin film thermocouple.

The samples were subjected to ultrasonic cleaning with anhydrous ethanol for 10 minutes before the experiment, and then air dried. The surface roughness Ra was measured using a ZYGO three-dimensional optical profilometer; The surface morphology was observed by scanning electron microscopy (SEM), and the chemical composition was analyzed by XPS; The distribution of elements on the surface of the iron plate was characterized by EDS mapping. Subsurface damage was observed using transmission electron microscopy (TEM) and high-resolution TEM, and the crystal structure was analyzed by selected area electron diffraction (SAED). TEM samples were prepared using focused ion beam (FIB) and coated with a platinum layer on the surface to prevent ion damage.

Fig. 1 Schematic diagram of removal mechanism

Fig. 2 Surface morphology of 4H-SiC

3. Conclusion

This article proposes a new method for SiC wafer processing based on iron mediated friction induced chemical reactions. This method utilizes friction to promote a solid-state chemical reaction between Fe and SiC, generating a reaction layer that is easy to remove; By continuous friction, the reaction layer is peeled off, resulting in SiC wafers with extremely low surface/subsurface damage. The article also explores the corresponding material removal mechanisms, and the main conclusions are as follows:

In terms of processing effectiveness, this study successfully developed and validated a new SiC processing technology, which efficiently removes materials through frictional chemical reactions between iron and SiC surfaces under low temperature, low pressure, and low speed conditions, while achieving high material removal rates and almost non-destructive processing surfaces.

The frictional chemical reaction process is driven by two key factors: frictional heat that catalyses the reaction between the metal and SiC, and shear forces that continually clear away the resulting products. This cycle exposes fresh material surfaces, enabling sustained and efficient removal.

The behavior of the reaction layer has a significant impact on the processing: if the reaction product adheres to the metal or SiC surface, it will hinder direct contact between the two, resulting in a gradual decrease in material removal rate as the processing progresses.

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Reference：

Wu, M. ,  Huang, H. ,  Wu, Y. ,  Xu, Z. ,  Li, T. , &  Macleod, I. , et al. (2024). Mechanism of friction-induced chemical reaction high-efficient polishing single crystal 4h-sic wafer using pure iron. Tribology International,193(000).