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Silicon carbide (SiC), a third-generation semiconductor material, has widely applied in microelectronics, optoelectronics, aerospace and energy sectors with its advanced specific properties. Ultra-precision polishing, as the final step in the processing of SiC substrates, is a critical process to obtain ultra-smooth, ultra-flat, damage-free substrate surfaces, which has a decisive influence on the subsequent epitaxy. Molecular Dynamics (MD) is an effective simulation method for the investigation of the interaction and motion between atoms in various mechanical machining processes, particularly for the analysis of material removal and material deformation mechanisms in nano- or atomic-level. Recently, Mr. Zige Tian, a joint PhD student of Huaqiao University and Liverpool John Moores University, Prof. Xun Chen of Liverpool John Moores University and Prof. Xipeng Xu of Huaqiao University, China, published a paper "Molecular Dynamics Simulation of the Material Removal in the Scratching of 4H-SiC and 6H-SiC Substrates" on IJEM. In this article, the authors present an investigation of the mechanism of material removal and associated subsurface defects through a set of scratching tests on the C face and Si face of 4H-SiC and 6H-SiC materials by using molecular dynamics simulation. The investigation reveals the material deformation and removal consist of plastic amorphous transformation and dislocation slip that might prone to brittle split. It has been noticed that the material removal at the C face of SiC is more effective with less amorphous deformation than that at the Si face of SiC. Such a phenomenon in scratching relates to the dislocations on the basal plane (0001) of the SiC crystal. It has also revealed that subsurface defects could be reduced by applying scratching depth of cut as integer multiples of a half molecular lattice thickness, which provides a useful foundation to guide the selection of machining control parameters for the best surface quality.
With the development of semiconductor technology, the quality and performance of semiconductor devices are crucial. Obtaining an ultra-smooth, ultra-flat, and surface/sub-surface damage-free substrate surface is of great significance for improving the performance of semiconductor devices. As a third-generation semiconductor material, silicon carbide has many excellent properties, such as wide band gap, chemical inertness, high thermal conductivity and high-temperature stability. The SiC substrate is also considered to be a typical hard-to-machine material because of its high hardness, high brittleness and stable chemical properties. Understanding the mechanism of SiC material removal and defect creation becomes a critical issue in the research and development. In recent years, scholars have used various simulation methods to study the machining process in order to optimize the machining process. Among them, molecular dynamics simulation is usually used for the research of nano-scale or atomic-scale machining. In this paper, molecular dynamics simulation is used to study the material removal mechanism and the sub-surface defects in scratching process on 4H-SiC and 6H-SiC.
3. Recent Advances
In this paper, different material deformation characteristics and removal mechanisms on the C face and Si face of silicon carbide 4H-SiC and 6H-SiC were revealed and analysed by a series of molecular dynamics simulation. Figure 1 shows the MD model and the atomic arrangement of the SiC substrate.
Figure 1. Molecule models of silicon carbide scratching for MD analysis.
Due to the limitation of scratching scales in experiments and MD simulations, it is difficult and unreliable to compare the results directly. Therefore, a specific force defined as the ratio of force over scratching depth is adopted to compare the results from the MD simulations and scratching experiments, as shown in figures 2 (a) and (b). Interestingly, the specific scratching force on C face are lower than that on Si face, and the specific forces on 4H-SiC are greater than those on 6H-SiC in both simulations and experiments. In addition, the C face shows more atom removals than the Si face, which is consistent with the experimental result in previous research. From figure 2 (d) it can be seen that the greater the scratch depth, the greater the number of atom dislocations, and there is no dislocation found when the scratch depth is 0.5 nm. In particular, the number of dislocations occurring on the C face is greater or even twice more than that of the Si face.
Figure 2. (a) Normal specific force of MD simulations; (b) Normal specific force of scratching experiments; (c) Numbers of removed atoms of MD simulations; (d) Number of atom dislocations of MD simulations.
It can be seen from figures 3 (a) and (b) that dislocations and stress concentrations mainly occur on the (0001) basal plane and (10-10) plane under the diamond tip. Dislocations are normally considered to be caused by the stress concentration during the scratching. Figures 3 (d) and (e) shows the changes of Von Mises stress in local area before and after the dislocations occurred on the C face and Si face. The stress peaks shown in the figures may represent the occurrence of the dislocations. The stress peaks of dislocations on the C face are lower than those on the Si face, which means the C face is more prone to dislocations than the Si face in the same scratching conditions. In addition, dislocations on the C face are more likely to occur on the (0001) plane, while dislocations on the Si face are more likely to occur on the (10-10) plane. Dislocations and slips on the (0001) plane would contribute to material removal with good surface quality, while those on the (10-10) plane could cause a severe subsurface layer cracks.
Figure 3. Analysis of subsurface dislocations in MD results. (a) Subsurface morphology of deformation; (b) Von Mises stress distribution in 6H-SiC during scratching; (c) Schematic diagram of the crystal view; (d) Von Mises stresses of in the scratching on C face of 6H-SiC at different time steps; (e) Von Mises stresses of in the scratching on Si face of 6H-SiC at different time steps.
Figure 4 (a) shows scratch deformation of 6H-SiC that the amorphous atom region appears as an inverted triangle together with some distinct steps. The height of these steps is six layers of atoms, which is the height of the 6H-SiC unit cell. In addition, under the right side of the groove, dislocations on the (0001) basal plane are found. These results are highly consistent with the experimental results from the team lead by Professor Feihu Zhange of Harbin Institute of Technology as shown in figure 4 (b). Although there are no obvious cracks, irreversible dislocations have occurred in nanoscale scratching. Due to local temperature and stress changes during processing, these dislocations potentially form micro-cracks. In nanoscale machining, amorphous deformation is considered as a plastic deformation, and atom dislocation is an elastic deformation initiated regular slip or split. Such a slip or split could lead micro crack creation at nanometre level or plastic deformation if the bonds do not break. On the contrary, some dislocations occurred during scratching can recover after tip passes the deformation area.
Figure 4. (a) The cross-sectional subsurface image of 6H-SiC after scratching in MD simulation;
(b) Cross-sectional TEM image of 6H-SiC substrate after scratching test.
Figure 5 shows the atomic layer structures of 4H-SiC and 6H-SiC together with the thickness of the subsurface amorphous deformation versus different scratching depths in terms of atom layer. The trend indicates that the subsurface amorphous deformation does not increase linearly with SiC molecule layers. Decreases or minimum increases in deformation thickness are found when setting the scratching cutting depth as an integer multiple of a half crystal lattice height, which implies better subsurface quality with substrate surface.
Figure 5. The thickness of the subsurface amorphous deformation at different scratching depths. (a) 4H-SiC; (b) 6H-SiC.
Atomic-level precision machining technology is a key technology to obtain an ultra-smooth, ultra-flat, and non-subsurface damage substrate surface, which is bound to play an increasingly important role in promoting the high-quality processing of semiconductor devices. As a nano-scale simulation method, the application of molecular dynamics simulation in the machining field is still in the development stage. Due to the limitation of computation power, the realizable time scale and spatial scale of MD simulations are still quite different from the actual machining. With the rapid development of computer technology and machining technology, this difference will become smaller and smaller. In addition, the methods introduced in this article to analyse the removal mechanism and subsurface deformation mechanism of silicon carbide substrates can also be used in the study of other brittle materials.
5. About the Authors
Dr. Xun Chen is a professor in Liverpool John Moores University and specialises in advanced manufacturing technology including the application of computing science, mechatronics and artificial intelligence to manufacturing process monitoring, control and optimisation, particularly to abrasive machining technology. His researches have been extensively supported by the research councils and industries including EPSRC, TSB, Royal Society, Rolls-Royce, Element Six etc. His major academic contribution is in the area of intelligent high efficiency precision abrasive machining. He has published over 200 research papers widely in top international journals and conferences in the field of Mechanical Manufacturing Engineering.
Professor Xun Chen is a vice chairman of the International Committee for Abrasive Technology, the editor in chief of the International Journal of Abrasive Technology, an associate editor of the International Journal of Automation and Computing, a member of the EPSRC Peer Review College, a member of the Engineering Professors’ Council, a member of the Institution of Mechanical Engineers, a Chartered Engineer, a fellow of Higher Education Academy and a member of International Society of Bionic Engineering.