Objective Silicon carbide (SiC) is a high-performance third- generation semiconductor material that is poised to meet the growing demand in the optoelectronic field. Further exploration of SiC processing at atomic and close- to- atomic scales is of great importance for both fundamental science and industrial applications. The hard and brittle nature of SiC renders traditional methods, such as ultraprecision machining, impractical for achieving high- quality surface treatments. Although some approaches, such as ion implantation or laser heating, can improve the machinability of hard and brittle materials, achieving high- quality processing remains elusive. Laser direct writing emerges as a potential solution for atomic- and close- to- atomic- scale manufacturing (ACSM). However, there is a lack of understanding regarding the laser- material interactions at the atomic and close- to- atomic scales. A hybrid two- temperature model- based molecular dynamics (TTM-MD) method can reveal the interactions from an atomic perspective; however, the one-dimensional (1D) TTM-MD model ignores the lateral propagation of heat and stress, which limits the understanding of surface modification and ablation. In this study, a three-dimensional (3D) TTM-MD model is developed to study the femtosecond laser ablation of 4H-SiC at varying fluences. The atomic trajectories, temperature fields, structural morphologies, and ablation products are analyzed. We hope that the 3D TTM-MD model will be helpful in the material processing of femtosecond lasers at the atomic and close- to- atomic scales. Methods A 3D profile of the absorption of laser energy by materials is obtained by analyzing a Gaussian beam. A 3D TTM-MD code is developed using the open- source software LAMMPS. The coupling of the TTM and MD is achieved by using a coarse- grained electron temperature grid. The dimensions of the simulation box are 1000 angstrom x502 angstrom x500 angstrom. The size of the laser beam is 200 angstrom, and the absorption depth is set to 50 angstrom. The femtosecond laser ablation of 4H-SiC is simulated based on this model, and the influence of the laser fluence on laser ablation is also discussed. Periodic boundary conditions are employed in all three directions. The interactions of the atoms in the simulation are described using the Tersoff potential. The time step is set to 0.5 fs. The phase- explosion threshold temperature of 4H-SiC is calculated through a constant- pressure MD simulation in which a system containing 2880 atoms is heated at a rate of 2 K/ps in an isothermal- isobaric (NPT) ensemble. After the simulations, the atomic configurations are extracted and analyzed for discussions on the material removal depth, modification layer thickness, and ablation plumes. Results and Discussions Owing to the rapid deposition of laser energy, an initial conical elevation structure forms in the irradiated zone. This is similar to the Gaussian energy distribution of a laser beam (Fig. 2). The ejection path of the ablation plume exhibits a hemispherical shape, and clusters or small droplets are deposited around the laser- affected zone in subsequent processes. This is a major factor in the contamination of clean surfaces during laser processing. At the end of the simulation, a volcano- shaped ablation crater forms on the material surface. The temperature field distribution during the ejection of the ablation plumes (Fig. 3) indicates intense laser absorption and thermal diffusion. The dimensions of the ablation crater, including the diameter, depth, and thickness of the modification layer, exhibit an upward trend concurrently with the enhancement of the laser fluence (Fig. 4). In addition, a constant- pressure MD simulation provides the threshold temperature of the phase explosion and the size distribution of the clusters after explosive boiling (Fig. 5). At higher fluences, the ablation products show higher proportions of clusters with smaller sizes, and the vaporization process is more pronounced. The laser- affected zone satisfies the temperature condition of the phase explosion, and the cluster size distributions in the ablation plumes are consistent with those observed after explosive boiling (Fig. 6). These results provide evidence that supports the phase- explosion mechanism. Conclusions A 3D TTM-MD model is developed to study the interaction between lasers and matter. This model is employed to investigate the modification and ablation of the surface of 4H-SiC by femtosecond laser irradiation at the atomic and close- to- atomic scales. At laser fluences of less than 50 mJ/cm(2), both laser- induced removal and modification are observed on the surface. At a laser fluence of 20 mJ/cm(2), the expansion of the modification layer in the beam center compensates for the structural height change that is induced by the removal owing to vaporization, and a bowl- shaped structure forms, owing to the expansion of the edge caused by phase transition. At a laser fluence of 10 mJ/cm(2), the laser- material interaction is dominated by a crystalline- to- amorphous phase transition with no observable material removal and a modification layer thickness of only 2 nm. Furthermore, the analysis of the temperature field and ablation plumes in the 3D TTM-MD simulations indicates that the laser ablation process is primarily attributable to the phase explosion mechanism. These simulation results prove the surface processing capabilities of the femtosecond laser on 4H-SiC at the atomic and close- to- atomic scales and thereby provide theoretical guidance for material processing in the field of ACSM using a femtosecond laser. It is expected that femtosecond-laser-induced phase transformation combined with acid etching or polishing technologies can be used to remove materials at the atomic and close- to- atomic scales, which makes it an important technological approach in ACSM.
