Optimization of Ultrasonic-Assisted Polishing SiC Through CFD Simulation
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In this paper, a detailed simulation about ultrasonic-assisted polishing was conducted, which is helpful to understand the contribution of the ultrasonic vibration to polishing. The influence of ultrasonic vibration on flow field parameters and optimal ultrasonic parameters was investigated. Results indicate that ultrasonic vibration can produce a cavitation phenomenon, which can contribute to improving the polishing quality and material removal rate (MRR). Optimal ultrasonic frequency, amplitude and film thickness were 42 μm, 25 kHz and 14 mm, respectively. Furthermore, the required minimum film thickness was 1.5 mm, at which cavitation could occur normally. At last, contrast experiment indicated that polishing quality and MRR were much improved when using ultrasonic-assisted polishing. After polishing, there were only a few scratches and MRR was 0.68 μm/h compared with many scratches and 0.32 μm/h MRR of traditional polishing.
KeywordsPolishing CFD simulation Ultrasonic vibration Cavitation
Hard and brittle materials such as silicon carbide (SiC) and gallium nitride (GaN) are promising semiconductors that have been widely used in optoelectronics and high-power high-frequency devices. The applications of these hard and brittle materials require flat and damage-free surface to realize its full potential. Chemical mechanical polishing (CMP) is a conventional process to planarize surfaces of these materials; however, high material remove rate (MRR) and good surface quality are very difficult to be obtained simultaneously due to their high hardness, brittleness and chemical inertness.
In order to improve the MRR and surface quality of these hard and brittle materials, many significant processes based on CMP have been developed. Chen et al. [1, 2, 3] proposed a novel core–shell abrasive, i.e., the cerium oxide (CeO2) abrasive coated on a polystyrene (PS) core, which can optimize the physical contact behavior between the abrasives and workpiece, so the surface quality is much improved. Aiming at improving the MRR of SiC, Kubota et al. add the Fenton reagent into the polishing slurry to promote the chemical reaction between the workpiece and polishing slurry [4, 5, 6]. However, much more needs to be done to realize higher MRR and better surface quality of these hard and brittle materials simultaneously.
Ultrasonic vibration-assisted polishing is a method that removes the material not only by impact of abrasives, revealed by traditional polishing, but also by impact of ultrasonic vibration in the polishing slurry; thus, the MRR can be improved [7, 8, 9, 10, 11]. In detail, the ultrasonic vibration can produce significant mechanical damage by generating and bursting of cavitation bubbles [12, 13, 14], which can improve the MRR obviously [15, 16, 17]. As a novel process, the ultrasonic-assisted polishing has become a research hotspot. However, there is no clear theory to analyze the ultrasonic effects on polishing at present, which prevents the optimization of ultrasonic-assisted polishing. In addition, it is difficult to elucidate the mechanism of ultrasonic-assisted polishing by experiments because of the small polishing gap and short time elapse of cavitation. Instead, the CFD simulation is a good alternative to optimize the polishing parameters in ultrasonic-assisted polishing.
A CFD simulation about ultrasonic-assisted electrochemical machining was conducted by Skoczypiec . The distribution of cavitation bubbles and parameters of flow field in processing gap were developed. Results show that ultrasonic variation can promote mass transportation and reduce polarization of electrode, so the MRR can be improved. Sajjadi et al.  investigated the influence of low ultrasonic frequency (24 kHz) on acoustic streaming and micro-bubble formations using a CFD simulation. Results prove that ultrasonic variation can improve the vibration velocity of micro-bubbles significantly, and when the ultrasonic power is increased by 100 W, the vapors volume fraction will be increased by 4.95% and the vibration velocity of vapors is raised from 29 to 119 cm/s simultaneously. Guo et al.  proposed a novel vibration-assisted polishing machine and developed its influence on the polishing efficiency and surface roughness. After polishing experiment, a smooth surface (0.4 nm Ra) was obtained.
In this paper, aiming at understanding the contribution of ultrasonic vibration to polishing in detail, a CFD simulation about varied ultrasonic parameters was conducted. In order to provide a theoretical guidance for experimental and numerical reference for subsequent mission, influence of ultrasonic parameters on flow field parameters was developed, and optimum ultrasonic parameters were obtained. A simple contrast experiment was conducted, which can prove that the ultrasonic vibration is helpful to improve the MRR and polishing quality.
2 Model Establishment
Here “A” is amplitude (μm); “f” is frequency (Hz); “t” is time (s); and “φ” is initial phase.
Main parameters of the simulation
3 Results and Discussion
3.1 Influence of Ultrasonic Vibration on the Distribution of Flow Properties
3.1.1 Distribution of Absolute Pressure and Vapor Volume Fraction
Results in Figs. 4, 5, 6 and 7 illustrate that the effects of ultrasonic-assisted polishing mainly focus on the near workpiece surface, especially the middle area (dia. 12 mm), which are beneficial to the polishing quality.
3.1.2 Distribution of Velocity Vectors
3.2 Influence of Ultrasonic Vibration on the Intensity of Flow Field Characteristics
3.2.1 Influence of Frequency
Here “p” is absolute pressure (Pa); “Ft” is intensity of absolute pressure (Pa·ms); “v” is velocity of slurry (m/s); “Fv” is intensity of velocity (mm); “a” is vapor volume fraction; “Fair” is intensity of vapor volume fraction (ms); and “t” is time (ms).
3.2.2 Influence of Amplitude
3.2.3 Influence of Film Thickness
What will happen to these properties when the film thickness is smaller?
3.3 Ultrasonic-Assisted Polishing Experiment
Aiming at developing the effects of ultrasonic vibration on polishing quality and MRR of hard and brittle materials (e.g., SiC), a simple contrast experiment was conducted.
MRR of SiC polishing
5%KMnO4 + SiO2
MRR = (m0 −m)/ρSt. Here, m0 (mg) is the initial mass of SiC wafer, m (mg) is the mass of SiC wafer after polishing, ρ (mg/μm3) is the density of SiO2, S (μm2) is the area of SiC wafer and t (h) is the polishing time.
In this paper, we conducted a detailed CFD simulation about the flow flied properties of the polishing slurry and optimized the ultrasonic vibration parameters. A simple contrast experiment was developed, which indicates that the ultrasonic vibration can contribute to realizing a better surface quality and higher MRR compared with the traditional polishing. The detailed conclusions are as follows.
A uniform cavitation area under workpiece was observed, which is beneficial to polishing quality. Flow flied parameters near workpiece surface were stronger and more stable, which can contribute to enhancing polishing quality and efficiency. The optimal ultrasonic parameters (e.g., frequency, amplitude and film thickness) were researched, which were 42 μm, 25 kHz and 14 mm, respectively. Furthermore, the required minimum film thickness was 1.5 mm, at which ultrasonic cavitation could occur normally. Contrast experiment indicated that the polishing quality and efficiency were much improved when using ultrasonic-assisted polishing. After polishing, only a few scratches can be seen and the MRR was 0.68 μm/h compared with traditional polishing (many scratches and 0.32 μm/h).
Furthermore, ultrasonic vibration not only can prevent abrasives from agglomerating but also can affect the chemical reaction process, which will be researched in further study.
This work is financially supported by the National Natural Science Foundation of China (Project No. 51475119).
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