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Optimization of Ultrasonic-Assisted Polishing SiC Through CFD Simulation

  • Wenjie Zhai
  • Bo GaoEmail author
  • Jingzhong Chang
  • Hongxiang Wang
Original Articles
  • 27 Downloads

Abstract

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.

Keywords

Polishing CFD simulation Ultrasonic vibration Cavitation 

1 Introduction

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 [18]. 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. [19] 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. [20] 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

The schematic diagram of ultrasonic-assisted polishing is shown in Fig. 1. In detail, the main module of the ultrasonic-assisted polishing is present in Fig. 1a, which contains a rotating polishing pad, a vibrating and rotating polishing head, etc. And the generation of cavitation bubbles is revealed in Fig. 1b, which indicates that propagation of sound wave in polishing slurry has created pressure drop, so the cavitation bubbles can generate (when the amplitude of pressure drop is larger than ambient pressure). After generating of cavitation bubbles, bubbles will undergo a collapse process, which can promote the mass transportation and generate heat to improve the MRR and polishing quality. The collapse process and effect of cavitation bubbles are demonstrated in Fig. 1c.
Fig. 1

Schematic diagram of ultrasonic-assisted CMP: a the main module, b generation of cavitation bubbles, c collapse of cavitation bubbles, (1) reaction-modified layer, (2) cavitation bubbles, (3) abrasives, (4) polishing slurry

Aiming at investigating the distribution and intensity of flow flied properties (e.g., absolute pressure, vapor volume fraction and velocity), a two-dimensional multiphase (pure electrolyte, electrolyte vapor, abrasives and non-condensable dissolved gas) model was established using GAMBIT. In view of the varied film, the non-structural triangle and dynamic grid was selected through the unsteady dynamic grid turbulence module of FLUENT. In addition, a mixture cavitation model was selected to investigate the effect of ultrasonic cavitation on workpiece. Figure 2 shows the geometry and boundary conditions of the simulation model.
Fig. 2

Model of the simulation: a diameter of polishing pad (200 mm), b depth of polishing slurry (20 mm), c film thickness (variable), d diameter of workpiece (50 mm)

Considering the instruction of transducer, the displacement and velocity equation of moving boundary were obtained as shown in Eqs. (1) and (2), respectively.
$$ y = - A\cos (2\uppi ft - \varphi ) $$
(1)
$$ v = 2\uppi fA\sin (2\uppi ft) $$
(2)

Here “A” is amplitude (μm); “f” is frequency (Hz); “t” is time (s); and “φ” is initial phase.

It was assumed that the entire computational area is adiabatic so that the effect of heat conduction was neglected. The acceleration of gravity (g) was taken as − 9.8 m/s2, and the mass fraction of incompressible gas in the polishing slurry was taken as 1.5 × 10−5 at the room temperature. The main parameters of the simulation are given in Table 1.
Table 1

Main parameters of the simulation

Model

Dimension

Turbulent

Multiphase

Flow

Time step

Iterative steps

Unsteady

Two

K epsilon

Mixture

Incompressible

0.5 μs

2000

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

The periodic variance of absolute pressure and vapor volume fraction under workpiece at different time steps is shown in Fig. 3, in which the periodic consistency of two curves and an inverse relationship between absolute pressure and vapor volume fraction are observed. It is due to the fact that vapors grow up when absolute pressure is smaller (less than saturated vapor pressure) and collapse when the absolute pressure is larger, which is the so-called cavitation phenomena. It must be pointed out that the collapse of cavitation bubbles can both strengthen the mechanical impact of abrasives on workpiece and improve the velocity of chemical reaction due to the acceleration of mass transportation and appearance of local high temperature, which are beneficial to MRR. In the simulation, the saturated vapor pressure (2300 Pa) was set as the lowest pressure.
Fig. 3

Relationship between absolute pressure and vapor volume fraction at different time steps

The objective of Figs. 4 and 5 is to investigate the distributions of absolute pressure and vapor volume fraction under workpiece surface at different positions. Therefore, we select the most obvious time 3T/4 of absolute pressure and T/2 of vapor volume fraction. However, at other time, it is too weak to illustrate the distributions of absolute pressure and vapor volume fraction clearly. Obviously, an inverse trend between absolute pressure and vapor volume fraction is observed. In addition, a uniform vapor volume fraction under workpiece (dia. 12 mm) can be observed at different times; however, it will fluctuate violently in other regions. Therefore, ultrasonic vibration can assure a uniform cavitation under the center of workpiece surface in polishing process, which is beneficial to the polishing quality.
Fig. 4

Distribution of absolute pressure at different horizontal positions

Fig. 5

Distribution of vapor volume fraction at different horizontal positions

In order to investigate the distributions of the flow field parameters under workpiece at different depths, we selected five parallel planes (0 mm, 1 mm, 2 mm, 6 mm and 10 mm beneath the workpiece surface), respectively. The strongest flow field parameters—absolute pressure at 3T/4 and vapor volume fraction at T/2—were researched, respectively. Figures 6 and 7 show the absolute pressure and vapor volume fraction are stronger on the workpiece surface (0 mm) than other depths; however, they drop sharply at larger depth from the workpiece surface, even near zero. In addition, there is a raise of absolute pressure and a drop of vapor volume fraction at the 10 mm plane (surface of polishing pad), which may be due to the junction of sound waves at two sides, and its detailed reasons will be researched in further study.
Fig. 6

Distribution of absolute pressure at different vertical positions (0 mm workpiece surface, 10 mm away from workpiece surface)

Fig. 7

Distribution of vapor volume fraction at different vertical positions (0 mm workpiece surface, 10 mm away from workpiece surface)

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

During ultrasonic-assisted polishing, propagation of mechanical wave resulted from ultrasonic vibration in the slurry affects the distribution of slurry velocity. The direction and magnitude of velocity vector in the flow field can affect the MRR and polishing quality directly, so the simulation of velocity vector clouds was conducted at some special time. Figure 8 shows that the velocity vector is layered at each time, in addition the direction and magnitude of velocity at each layer are various. Results in Fig. 8 indicate that the existence of ultrasonic vibration can generate a sound wave in polishing slurry, pushing the slurry move upward and downward, which can improve the impact of abrasives on workpiece, and prevent abrasives from agglomerating. Therefore, both the MRR and surface quality can be improved.
Fig. 8

Velocity vector cloud at different positions: a 0, b T/4, c T/2, d 3T/4

Figure 8 shows that the velocity vectors of slurry are mainly in vertical direction at T/2, which means that abrasives in slurry impact workpiece vertically right now. In detail, the vertical impact of velocity distribution under workpiece (dia. 50 mm) at T/2 is shown in Fig. 9, from which we can see that velocity is as large as 3.68 m/s with little variance, which is helpful to the MRR and polishing quality.
Fig. 9

Distribution of vertical velocity of slurry at different horizontal positions (at T/2)

3.2 Influence of Ultrasonic Vibration on the Intensity of Flow Field Characteristics

3.2.1 Influence of Frequency

In order to develop the effects of varied frequencies on flow field parameters, the intensity equations of absolute pressure, velocity and vapor volume fraction were defined by Eqs. (3), (4) and (5), respectively.
$$ F_{\text{t}} = \int_{0}^{t} {p{\text{d}}t} $$
(3)
$$ F_{\text{v}} = \int_{0}^{t} {v{\text{d}}t} $$
(4)
$$ F_{\text{air}} = \int_{0}^{t} {a{\text{d}}t} $$
(5)

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).

Figure 10a, b shows the varieties of intensity of absolute pressure and velocity with varied frequencies, respectively. It is obvious that absolute pressure and velocity increase first and then decrease in the form of parabola with the increase in frequency. The variety of intensity of vapor volume fraction with varied frequencies is shown in Fig. 10c. The intensity of vapor volume fraction increases with frequencies almost proportionally; the higher the ultrasonic frequency, the higher the frequency of the formation and destruction of cavitation bubbles, and thus the stronger cavitation phenomena. There is no doubt that the appropriate increase in ultrasonic frequency can improve the MRR; however, in practice, the characteristics and cost of ultrasonic vibrator should also be considered. According to Fig. 10, an optimal value of frequency near 25 Hz was obtained to acquire the highest MRR.
Fig. 10

Intensity of flow field parameters at different frequencies: a absolute pressure, b velocity, c vapor volume fraction

3.2.2 Influence of Amplitude

Similar to frequency, ultrasonic amplitude has a direct effect on flow field parameters. Figure 11a–c shows the intensity curves of absolute pressure, velocity and vapor volume fraction with the varied amplitudes, respectively. According to Fig. 11a, b, the intensity of absolute pressure increases first and then decreases in the form of parabola and the intensity of velocity increases first and then maintains stability with the increase in amplitude. However, there is an almost positive correlation between intensity of vapor volume fraction and amplitude, as shown in Fig. 11c. It is obvious that the appropriate increase in amplitude can improve the MRR; in practice, the characteristics and cost of ultrasonic vibrator should also be considered. In the simulation, as indicated in Fig. 11, an optimal value of amplitude near 42 μm was obtained to acquire the highest MRR.
Fig. 11

Intensity of flow field parameters at different amplitudes: a absolute pressure, b velocity, c vapor volume fraction

3.2.3 Influence of Film Thickness

In order to investigate the effect of film thickness on flow field parameters, the thickness of 4–18 mm with 2 mm as an increment was researched, respectively. Figure 12a presents the change in absolute pressure with time when the film thickness is 4 mm. In detail, the varied intensities of absolute pressure, velocity and vapor volume fraction are shown in Fig. 12b, c, d with varied film thicknesses, respectively, in which an optimal film thickness at near 14 mm is obtained, to assure the larger intensities of absolute pressure, velocity and vapor volume fraction simultaneously; thus, the highest MRR can be obtained.
Fig. 12

Intensity of flow field parameters at different film thicknesses: a variation in absolute pressure with varied time when film thickness is 4 mm, b absolute pressure, c velocity, d vapor volume fraction

What will happen to these properties when the film thickness is smaller?

Figure 13a–c shows the varieties of absolute pressure with time when film thicknesses are 200 μm, 1 mm and 1.5 mm, respectively. It is obvious that the absolute pressure spike drops to 2700 Pa quickly when the film thickness is 200 μm, and it decreases to 0.25 MPa with a large fluctuation when the film thickness is 1 mm, indicating that the polishing gap has been almost filled of vapors; thus, the polishing capacity is disappeared. While the film thickness is 1.5 mm, the absolute pressure spike can be maintained stable at 0.4 MPa; this is the so-called minimal film thickness, to obtain the normal polishing capacity.
Fig. 13

Absolute pressure at different film thicknesses: a 200 μm, b 1 mm, c 1.5 mm

The variety of vapor volume fraction with time at film thickness 200 μm is shown in Fig. 14. Vapor volume fraction increases in a fluctuated way from 10% to near 60% in less 1 ms, which means that the small polishing gap is almost filled with vapors, making the absolute pressure decrease sharply (as shown in Fig. 13a) and prevent bubble bursting. In order to apply the cavitation effect to ultrasonic-assisted polishing, sufficient gap is necessary to assure the generating and collapsing of vapors. In this study, a minimum film thickness of 1.5 mm is required so that the benefits of cavitation can be applied to polishing normally.
Fig. 14

Vapor volume fraction when the film thickness is 200 μm

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.

The MRR of SiC via traditional and ultrasonic-assisted polishing is given in Table 2, in which we can discover that the MRR of ultrasonic-assisted polishing (0.68 μm/h) is about two times higher than that of traditional polishing (0.32 μm/h). Therefore, the ultrasonic vibration can contribute to obtaining a higher MRR compared with the traditional polishing.
Table 2

MRR of SiC polishing

Conditions

5%KMnO4 + SiO2

Traditional

Ultrasonic

MRR (μm/h)

0.32

0.68

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.

Figure 15 shows the morphology of SiC before and after polishing in different conditions. Figure 15a, b shows the morphology of SiC before and after polishing via traditional polishing, in which we can observe that the rough surface (exist many peaks and valleys) of initial SiC was modified (peaks and valleys disappeared) after polishing; however, there still remain many scratches. Results of ultrasonic-assisted polishing are shown in Fig. 15c, d; after polishing, the peaks and valleys are removed completely, and just a few scratches can be seen. Therefore, the ultrasonic vibration is helpful to acquire a better polishing quality compared with the traditional polishing.
Fig. 15

Morphology of SiC with 500 × optical microscope: a, b traditional polishing before and after, c, d ultrasonic-assisted polishing before and after, respectively

4 Conclusions

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.

Notes

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Project No. 51475119).

References

  1. 1.
    Chen A, Wang Y, Qin J et al (2015) Chemical mechanical polishing for SiO2 film using polystyrene@ceria (PS@CeO2) core-shell nanocomposites. J Inorg Organomet Polym Mater 25(6):1407–1413CrossRefGoogle Scholar
  2. 2.
    Chen Y, Long R (2011) Polishing behavior of PS/CeO2, hybrid microspheres with controlled shell thickness on silicon dioxide CMP. Appl Surf Sci 257(20):8679–8685CrossRefGoogle Scholar
  3. 3.
    Chen Y, Li Z, Miao N (2014) Synergetic effect of organic cores and inorganic shells for core/shell structured composite abrasives for chemical mechanical planarization. Appl Surf Sci 314(24):180–187CrossRefGoogle Scholar
  4. 4.
    Kubota A, Yoshimura M, Fukuyama S et al (2012) Planarization of C-face 4H-SiC substrate using Fe particles and hydrogen peroxide solution. Precis Eng 36(1):137–140CrossRefGoogle Scholar
  5. 5.
    Zhou Y, Pan GS, Shi XL et al (2014) Chemical mechanical planarization (CMP) of on-axis Si-face SiC wafer using catalyst nanoparticles in slurry. Surf Coat Technol 251(1):48–55CrossRefGoogle Scholar
  6. 6.
    Wang L, Yan QS, Lu JB et al (2014) Comparison of Fe catalyst species in chemical mechanical polishing based on fenton reaction for SiC wafer. Adv Mater Res 1027:171–176CrossRefGoogle Scholar
  7. 7.
    Xu W, Lu X, Pan G et al (2010) Ultrasonic flexural vibration assisted chemical mechanical polishing for sapphire substrate. Appl Surf Sci 256(12):3936–3940CrossRefGoogle Scholar
  8. 8.
    Tsai MY, Yang WZ (2012) Combined ultrasonic vibration and chemical mechanical polishing of copper substrates. Int J Mach Tools Manuf 53(1):69–76CrossRefGoogle Scholar
  9. 9.
    Tso PL, Chang YC (2010) Study on chemical mechanical polishing with ultrasonic vibration. Adv Mater Res 126–128:311–315CrossRefGoogle Scholar
  10. 10.
    Han G, Zhao J, Wang X (2017) Research on unbounded abrasive polishing process with assisted ultrasonic vibration of workpiece. Int J Adv Manuf Technol 88(1–4):209–218CrossRefGoogle Scholar
  11. 11.
    Liu Y, Li SJ, Li Y et al (2012) Process parameters modeling and optimizing for compound machining with ultrasonic vibration on SiC wafer. Appl Mech Mater 217–219:6CrossRefGoogle Scholar
  12. 12.
    Sun B, Zhang HC (2009) Numerical simulation of ultrasonic cavitation based on FLUENT. Lubr Eng 34(04):55–60Google Scholar
  13. 13.
    Wang GG, Sun DB, Zhang XL et al (2007) Dynamic impact behavior during bubble collapsing. J Univ Sci Technol Beijing 29(05):483–485Google Scholar
  14. 14.
    Slimane M, Oualid H, Yacine R et al (2014) Modeling of ultrasonic cavitation as an advanced technique for water treatment. Desalination Water Treat 56(6):1–11Google Scholar
  15. 15.
    Wu YB, Wang LJ (2014) A fundamental investigation on ultrasonic assisted fixed abrasive CMP (UF-CMP) of silicon wafer. Adv Mater Res 983:208–213CrossRefGoogle Scholar
  16. 16.
    Skoczypiec S, Ruszaj A (2007) Application of ultrasonic vibration to improve technological factors in electrochemical machining of titanium alloys. In: Scripts precision and microproduction engineering, pp 143–148Google Scholar
  17. 17.
    Cui XX, Li M, Li DQ (2014) Study on eliminating disfigurement of small-deep holes of the ring laser gyro by grinding with ultrasonic vibration and polishing. Aeronaut Sci Technol 25(02):73–78Google Scholar
  18. 18.
    Skoczypiec S (2011) Research on ultrasonically assisted electrochemical machining process. Int J Adv Manuf Technol 52:565–574CrossRefGoogle Scholar
  19. 19.
    Sajjadi B, Raman AAA, Ibrahim S (2015) Influence of ultrasound power on acoustic streaming and micro-bubbles formations in a low frequency sono-reactor: mathematical and 3D computational simulation. Ultrason Sonochem 24:193–203CrossRefGoogle Scholar
  20. 20.
    Guo J, Morita SY, Hara M et al (2012) Ultra-precision finishing of micro-aspheric mold using a magnetostrictive vibrating polisher. CIRP Ann Manuf Technol 61(1):371–374CrossRefGoogle Scholar

Copyright information

© International Society for Nanomanufacturing and Tianjin University and Springer Nature Singapore Pte Ltd. 2019

Authors and Affiliations

  1. 1.Mechatronics EngineeringHarbin Institute of TechnologyHarbinPeople’s Republic of China

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