SiC Laser Surface Modification: Mechanisms, Effects, and Machining Performance Optimization +

PAM-XIAMEN, as one of leading advanced semiconductor material suppliers in China, can supply conductive and semi-insulating SiC substrates and epitaxial wafes for research purposes. For additional information, please send inquiries to [email protected].

Silicon Carbide (SiC) is a core material for third-generation semiconductors, yet its extreme hardness and brittleness pose significant challenges for achieving efficient and low-damage machining. Traditional chemical mechanical polishing is inefficient and struggles to meet industrial demands. Ultrashort-pulse laser surface modification, by precisely controlling the surface and subsurface structure of the material, offers an innovative pathway to enhance the machinability of SiC.

1. Interaction Mechanisms between Laser and Wide-Bandgap Semiconductor Materials

The interaction mechanism between laser and wide-bandgap materials such as SiC is complex, depending on laser parameters (wavelength, pulse width, energy density) and the material’s electronic state distribution, bandgap characteristics, etc. For transparent brittle materials, when the laser photon energy is lower than the material bandgap, nonlinear absorption (e.g., multi-photon absorption, avalanche ionization) becomes the dominant mechanism. Ultrashort-pulse lasers (e.g., femtosecond lasers), with their extremely high peak power (up to 10¹⁸W/m²), can excite valence electrons to the conduction band via multi-photon absorption within an ultrashort duration (<10⁻¹²s), generating free electrons and subsequently triggering avalanche ionization, thereby achieving localized energy deposition in the material.

In contrast, the mechanism of long-pulse lasers (e.g., nanosecond lasers) is predominantly thermal. Their pulse width is much longer than the electron-phonon coupling time of the material, causing energy transfer via thermal diffusion, which easily forms a large heat-affected zone (HAZ) and may induce thermal defects such as microcracks and recast layers. This thermally dominant mechanism manifests macroscopically as the formation of “blast holes,” propagation of microcracks, and material melting–resolidification processes.

Fig. 1 Schematic of interaction mechanisms between laser and wide-bandgap semiconductor materials and internal structural evolution: (a) laser–material interaction process; (b) nonlinear ionization mechanisms; (c) laser-induced internal structural changes, such as waveguides, nanogratings, nanopores, and crack propagation.

2. Modification Mechanisms and Surface/Subsurface Structural Evolution for Different Pulse Widths

2.1 Femtosecond Laser Modification: Cold Processing Advantages and Subsurface Damage

Femtosecond lasers (fs, 10⁻¹⁵s) have an extremely short pulse width, and processing is completed before heat transfers to the lattice after energy deposition in the electron system, thus often regarded as “cold processing.” However, studies by Huang et al. indicate that thermal effects remain non-negligible even in femtosecond laser processing. Femtosecond laser modification of 4H-SiC surfaces primarily generates two typical structures:

(1) Laser-Induced Periodic Surface Structures (LIPSS): At energy densities slightly above the ablation threshold, regularly arranged LIPSS forms on the surface. The orientation of LIPSS is perpendicular to the laser polarization direction, and the period is related to the wavelength, categorized into low-spatial-frequency LIPSS (LSFL, period >λ/2) and high-spatial-frequency LIPSS (HSFL, period <λ/2).

(2) Surface Decomposition and Phase Transformation: At high energy densities, laser irradiation causes decomposition of the SiC surface, transforming it into amorphous silicon (a-Si) and amorphous carbon (a-C), a process confirmed by EDS and GIXRD analyses.

Fig. 2 (a-c) SEM morphology and (f) GIXRD spectrum of 4H-SiC surface after femtosecond laser modification; red dots in the figure mark EDS sampling locations.

While femtosecond laser modification enhances machining performance, it also introduces subsurface damage. TTM-MD (Two-Temperature Model – Molecular Dynamics) simulations show that after laser energy injection into the electron system, approximately 26% of the energy is transferred to the ion system, causing a sharp rise in near-surface ion temperature (peak exceeding 8000K). Ions gain high kinetic energy (>2×10⁵eV) and escape, resulting in a rough surface and subsurface pores. The density of these pores decays with increasing depth, becoming sources of internal defects.

2.2 Picosecond Laser Modification: Coupling of Phase Explosion and Thermal Effects

Liu et al. reported that picosecond lasers (ps, 10⁻¹²s), with pulse widths between femtosecond and nanosecond, exhibit a modification mechanism coupling phase explosion and thermal effects. At high laser energy densities, the material overheats rapidly beyond its thermodynamic critical point, causing superheated liquid to transform into a liquid–gas mixture and undergo explosive boiling, i.e., phase explosion. This results in the deposition of numerous resolidified spherical SiO₂ particles on the surface.

Fig. 3 (a1-d1) SEM morphology and corresponding EDS elemental content maps (a2-d2) of 4H-SiC modified by picosecond lasers at different energy densities

Subsurface damage from picosecond laser modification primarily manifests as microcracks. Temperature field simulations indicate that the focal center temperature can reach 3200 K, exceeding the evaporation temperature of SiC. Rapid cooling after laser cessation generates significant thermal stress gradients, and when the stress exceeds the fracture strength of SiC, cracks initiate. Cracks mainly occur in the recast zone, originating from tensile stresses induced by thermal effects and volume changes of molten material due to overlapping adjacent spots (thermal–cold cycles).

2.3 Continuous-Wave and Nanosecond Laser Modification: Thermal-Dominated Planarization and Defect Repair

Modification by continuous-wave (CW) and nanosecond (ns) lasers is thermally dominated, suitable for surface defect repair and planarization. Han et al. (2025) compared the repair effects of CW lasers and femtosecond lasers on surface defects (e.g., triangular defects, carrot defects) in 4H-SiC epitaxial layers. The study shows that CW lasers achieve surface planarization via melting–resolidification. Laser heating melts the material, and under the influence of surface tension, Marangoni effect, and gravity, molten material flows from convex to concave areas, thereby repairing the morphology. Surface roughness can be reduced from sub-micrometer levels in the original defect zone to below 0.071μm.

Fig. 4 Comparison of 3D topography of 4H-SiC surface defects before and after laser repair: (a) original defective surface; (b) surface after CW laser repair; (c) surface after femtosecond laser repair.

Nanosecond lasers are more suitable for high-efficiency material removal processes such as wafer dicing. Their thermal effects can effectively induce the formation of high-density dislocation layers and microcrack networks inside the material, thereby reducing the binding strength of the crystal and facilitating subsequent detachment. However, the larger HAZ of nanosecond lasers can easily lead to surface ablation and excessive crack propagation, requiring precise process parameter control.

3. Impact of Laser Surface Modification on SiC Machining Performance

3.1 Tribological Behavior and Improvement in Material Removal Mechanism

The tribological behavior of laser-modified surfaces changes significantly. Scratch tests show that the fluctuation frequency of tangential force increases on modified surfaces, but the coefficient of friction (COF) stabilizes, fluctuating around 0.2, unlike the strong dependence on axial load observed on pristine surfaces. This indicates that the modified layer alters the mechanical state of the contact interface.

Regarding material removal, modified surfaces exhibit superior performance. CLSM (Confocal Laser Scanning Microscopy) observations show that lateral cracks generated during scratching on modified surfaces are suppressed, scratch profiles better conform to the indenter shape, and scratch depths are greater. Acoustic emission (AE) signals also indicate that energy release peaks during material removal weaken after modification, meaning less energy is required for crack propagation, and material is removed in a “softer” manner. This is mainly attributed to:

(1) Micro-nano structures like LIPSS reducing local stiffness;

(2) Lower brittleness of the C-rich and Si-rich phases produced by surface decomposition;

(3) Subsurface pores providing stress concentration points, promoting localized material failure.

Hình 5 So sánh (a) bản đồ chiều cao CLSM sau khi cào trên bề mặt nguyên sơ và bề mặt được sửa đổi bằng laser femto giây (tải 16,5 W, 8 N) và cấu hình chiều cao (bd) dọc theo các đường khác nhau

3.2 Tính chất cơ học: Giảm độ cứng và nâng cao khả năng gia công

Các thử nghiệm thụt nano trực tiếp chứng minh tác động làm suy yếu của việc biến đổi tia laser đối với các tính chất cơ học của SiC. Nghiên cứu của Liu và cộng sự. (2024) cho thấy độ cứng vết lõm nano của 4H-SiC nguyên sơ là khoảng 36,9GPa, trong khi sau khi điều chỉnh bằng laser pico giây (mật độ năng lượng 4–16J/cm²), độ cứng bề mặt giảm đáng kể xuống còn 2,4–20,7GPa. Giảm độ cứng có tương quan nghịch với mật độ năng lượng laser. Các đường cong chuyển vị tải thể hiện hiện tượng “bật vào” sau khi sửa đổi, cho thấy biến dạng dẻo trong quá trình thụt đầu dòng và độ giòn giảm. Việc giảm độ cứng trực tiếp cải thiện khả năng gia công vật liệu, mang lại lợi ích cho các quá trình hoàn thiện tiếp theo như CMP trong việc đạt được tốc độ loại bỏ vật liệu cao hơn và chất lượng bề mặt tốt hơn.

Laser surface modification is an effective means to enhance the machining performance of hard and brittle 4H-SiC materials. With continuous advancements in laser technology and process optimization, laser surface modification is expected to play a more critical role in SiC wafer manufacturing and functional surface construction for devices.

Whether you need SiC wafer for research or for industrial applications, please contact us email at [email protected] và [email protected].

 

References:

1. Huang, Y., Zhou, Y., Li, J., & Zhu, F. (2023). Femtosecond laser surface modification of 4H-SiC improves machinability. Applied Surface Science, 615, 156436.
2. Li, H., Wang, H., Li, Y., Lu, X., Li, L., Yan, Y., & Guo, W. (2025). Micro-nanoscale laser subsurface vertical modification of 4H-SiC semiconductor materials: mechanisms, processes, and challenges. Discover Nano, 20(1), 116.
3. Liu, H., Li, Z., Zhang, P., Zuo, D., & Xie, W. (2024). Study of damage mechanism on single crystal 4H-SiC surface layer by picosecond laser modification (PLM). Applied Surface Science, 672, 160722.
4. Han, X., Zhou, J., Li, R., Wang, S., Dong, F., Sun, C., & Liu, S. (2025). Surface Modification and Crystal Quality Improvement of 4H-SiC Film via Laser Treatment: Comparison of Continuous Wave and Femtosecond Pulse Laser. Materials, 18(8), 1781.