Oxidation removal mechanism based on ReaxFF-MD in 4H-SiC CMP

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ObjectivesSilicon carbide has the advantages of high temperature stability, high electron mobility, and wide bandgap, and has been widely used in fields such as new energy vehicles, photovoltaics, and integrated circuits. Nowadays, the industry has put forward strict requirements for atomic level surface quality in the processing of 4H-SiC wafers, but 4H-SiC materials have extremely high hardness, brittleness, and chemical inertness, making their processing extremely difficult. Therefore, the molecular dynamics (MD) method based on reactive force field (ReaxFF) is adopted to reveal the oxidation removal mechanism of 4H-SiC in H2O2 aqueous solution during chemical mechanical polishing at the atomic/molecular level.MethodsThe friction curves, the number of removed atoms, the temperature and the potential energy changes of 4H-SiC workpieces under different scratch parameters during the polishing process are studied through MD simulation. The bond breaking and formation during the oxidation process are investigated through the ReaxFF, and the oxidation process and the mechanism of 4H-SiC in H2O2 aqueous solution are deeply explored. Meanwhile, through nano-scratch experiments, the influence mechanism of H2O2 oxidation on the removal behavior of 4H-SiC materials is revealed, and the accuracy of MD simulation results is verified. During the experiment, loads of 300, 400, and 500 mN are selected for constant load nano-scratch testing, with a scratching speed of 10 µm/s, a scratching direction from right to left, and a scratching distance of 100 µm. The 4H-SiC experimental wafers are divided into three groups of untreated, chemically treated, and electrochemically treated. The effects of oxidation condition and polishing pressure on the surface morphology and penetration depth of 4H-SiC in the nano-scratch experiment are compared and analyzed.Results(1) When the polishing pressures are 0.1, 0.2 and 0.3 nN, the trend of the friction force curves is similar, that is, the friction force in the initial stage of abrasive particle movement rapidly increases and then tends to stabilize. Meanwhile, the number of removed atoms from 4H-SiC workpieces increases with the increase of polishing pressure. The main reason is that as the polishing pressure increases, the contact between the abrasive particles and the workpiece becomes tighter, and the interaction force between atoms increases, thereby increasing the friction force. (2) Three different scratch speeds of 100, 250 and 500 m/s are set for simulation. As the scratch speed increases, the number of removed atoms from the workpiece slightly increases, and the temperature and the potential energy of 4H-SiC are significantly increase. Under the condition of a certain scratch distance, the higher the scratch speed, the shorter the scratch time, that is, the elastic recovery time of the 4H-SiC surface is shortened, resulting in a higher potential energy. In addition, the scratching of abrasive particles also promotes the flow of polishing solution, thereby intensifying the movement and even direct detachment of Si atoms connected to atoms in the polishing solution, resulting in an increase in the atomic removal rate of the workpiece. (3) In the nano-scratch experiments, compared with the chemically oxidized 4H-SiC sample, the electrochemically oxidized 4H-SiC sample is the first to form cracks at a load of 400 mN, and the cracks formed in the electrochemically oxidized 4H-SiC sample are more obvious at a load of 500 mN. When the load increases to 400 and 500 mN, the penetration depths of the two groups of 4H-SiC treated by chemical oxidation are greater, and when there is an electric field accelerating the oxidation reaction, the penetration depth of the 4H-SiC surface is greater.ConclusionThe increase of polishing pressure and scratching speed can improve the atomic removal rate of the 4H-SiC surface. This is attributed to the fact that as the polishing pressure and the scratching speed increase, the contact between the abrasive particles and the workpiece becomes tighter, and the flow of polishing solution enhances the binding of H2O2/H2O molecules to Si, leading to an increase in atomic removal rate. In the nano-scratching experiment, as the load increases, the penetration depth of 4H-SiC also increases. This is attributed to the transformation of 4H-SiC to SiO2 caused by chemical oxidation, resulting in a decrease in the hardness and penetration resistance of 4H-SiC material, and the U-shaped grinding morphology on its surface is similar to that in MD simulation. In addition, during the CMP reaction, the H2O2 aqueous solution reacts with 4H-SiC to form Si—OH, Si—H2O, Si—O—C, Si—O—Si, and Si—H chemical bonds. The continuous formation and breaking of chemical bonds result in the formation of an oxide layer on the surface of 4H-SiC, significantly reducing its surface hardness and facilitating the removal of its surface materials.

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