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Abstract
Reaction-bonded silicon carbide is considered to be one of the most excellent and feasible materials for lightweight large telescope optics due to its high specific stiffness and strength. Unfortunately, it is a material with high hardness and a complex two-phase structure, which cause the conventional polishing process to be inefficient. In this paper, a femtosecond laser selective polishing technique based on the difference in ablation thresholds between SiC and Si phases was proposed to reduce the surface roughness and improve polishing efficiency. The multi-pulse ablation thresholds of SiC and Si at an effective pulse number of 49 used for the polishing process were calculated as 0.168 and 0.066 J/cm2, respectively. In the experiment, 0.08 J/cm2 was selected to selectively remove the Si layer to optimize the roughness of RB-SiC, and the surface evolution under different scanning times was analyzed. An optical surface with a roughness of 11.21 ± 0.26 nm was obtained by selective polishing with 3 scans on the initial surface roughness of 33.72 ± 0.83 nm. The change in surface morphology showed that the Si layer with uneven distribution of protrusions on the initial surface was effectively removed and the SiC did not change by 3 scans. Raman spectrum indicated that this selective polishing did not change the surface structure, and XPS spectrum showed that selective polishing effectively removed the Si layer on the surface, and part of the SiC was also decomposed into C and Si. Furthermore, femtosecond laser selective polishing can further improve the polishing efficiency through parameter optimization, which has a potential application in improving the polishing efficiency of RB-SiC.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Reaction-bonded silicon carbide (RB-SiC) as a type of silicon carbide (SiC) ceramic is expected to be one of the most excellent and feasible materials for lightweight large telescope optics (especially for large-size and complex shapes mirror [1]) due to its excellent property [2]. Unfortunately, RB-SiC is recognized as a difficult-to-machine material [3] because of its high hardness [4]. Meanwhile, RB-SiC is a typical complex material [5], 15%-30% of remnant Si is left in the green body during the chemical reaction between the liquid Si with carbon in sintering process [6,7]. The existence of the remnant Si further increases the difficulty of polishing RB-SiC, because the polishing properties of the two materials are very different.
The abrasive polishing technique is a commonly used method of RB-SiC polishing to obtain the high surface quality [8–11]. H. Y. Tam et al [12]. put forward a two-stage manufacturing process by solid abrasives to fabricate RB-SiC optical components efficiently. The initial surface roughness was reduced from Ra 110.8 nm to 21.6 nm after 5 minutes by continuously adjusting the size of the abrasive from 30 to 10 µm; then a roughness of 10.7 nm surface was obtained after 15 minutes with diamond pastes of different sizes as polishing abrasives. H. Cheng et al. obtained a surface with a surface roughness of 1.14 nm by Magnetorheological finishing(MRF) of RB-SiC [13]. The initial surface roughness of 34.39 nm reached 26.74 nm after 20 hours of pre-polishing by MRF fluids carrying oil, and then reached 1.14 nm after 50 hours with the polishing of MRF fluid with diamond. B. Gao’s team [14] proposed a new core/shell synthesize abrasive for RB-SiC polishing and a quality surface of 0.497 nm was obtained under the effect of ultraviolet catalysis. High-precision polishing of RB-SiC with lower roughness has achieved by existing machining techniques, but these methods are usually complicated or time-consuming.
In recent years, laser has become an attractive new technique used as polishing tool due to its unique advantage(Non-contact, high controllability, high quality and high efficiency) [15], especially for pulse laser micro-polishing [16], which could be successfully applied to reduce the surface roughness for hard-to-machine materials [17]. Y. X. Cui et al [18]. reported a study of femtosecond laser polishing (fs-LP) for CVD Nano polycrystalline diamond film, and the proper fs-LP process was set to reduce the surface roughness from 73.84 nm to 31.88 nm. Minghui Hong’s team [19] used 355 nanosecond laser to polish the chemical vapor deposition diamond and the optical quality surface with the average roughness of 8.02 nm was obtained. Furthermore, pulse laser polishing has received more and more attention in the field of SiC polishing, such as X. Z. Xie et al [20]. studied the effect of femtosecond laser modification on the CMP process of SiC substrates and the polishing process of SiC by fs-LP was explored and investigated by Q. Z. Zheng’ team [21,22], SiC ceramic polished with the underwater fs-LP, and the effects of laser frequency and pulse energy on the surface quality were experimental investigated to obtained a smooth polished surface with surface roughness of 0.76 µm. However, their results of the surface quality of SiC by fs-LP were far away from the optical quality, and RB-SiC is a typical complex material, there are few reports on the interaction between femtosecond laser processing and complex material and further exploration is warranted.
In this paper, a femtosecond laser selective polishing (fs-LSP) technique based on the different ablation thresholds of Si and SiC phases in RB-SiC was proposed to reduce the surface roughness and improve polishing efficiency by selectively removing the raised Si burrs. Multi-pulse ablation thresholds of femtosecond laser for SiC and Si were analyzed and obtained by Two-temperature model (TTM) and experimental fitting calculations as the basis for laser fluence selecting to fs-LSP. Meanwhile, the laser fluence between the ablation thresholds of two phases for fs-LSP was optimized and the surface morphology and roughness by fs-LSP with different scans were compared. Furthermore, the surface structure and composition changes before and after fs-LSP were analyzed by Raman spectrum and XPS. The results show that the height of the unevenly distributed Si phase on the initial surface was generally higher than that of SiC and the surface roughness could be effectively reduced to obtain a quality surface with the optimization of the number of scans by fs-LSP with a single removal of Si phase.
2. Specimens and experiments
2.1 Specimens and experimental equipment
In this experiment, Si and 6H-SiC crystal polished in advance were prepared for ablation thresholds tests. RB-SiC which was used for fs-LSP and containing about 30% Si was cut into the size with a diameter of 30 mm and a thickness of 4 mm and preprocessed by chemical mechanical polishing (CMP) for about 32 hours with colloidal silica slurry.
The experiments were carried by femtosecond laser direct writing system, which is illustrated in Fig. 1(a). A pulse laser beam with a Gaussian distribution at central wavelength of 1030 nm, pulse duration of 290 fs, and a maximum pulse energy of ∼15 µJ at ∼1 MHz repetition rate was equipped in this system. The laser pulses were separated into two beams by a polarizing beam splitter, and the S-polarized beam was used as the writing laser. A 5x objective with an NA of 0.14 was employed for focusing the beam to a diameter of 12 µm (1/e2). The samples can be translated by air-bearing stages at a resolution of 0.5 nm and the maximum speed of 300 mm/s.
Fig. 1. (a) femtosecond laser direct writing system and (b) fs-LSP schematic. Φ: beam diameter; △x: pulse distance between two successive laser pulses, △y: laser scan spacing.
2.2 Ablation thresholds and fs-LSP
All experiments were carried out in air. Single-pulse ablation thresholds and multi-pulse thresholds at an effective pulse number of 49 were obtained according to the relationship with the ablation line width and the laser energy, as the equation shows [23,24]:
where D is the diameter of ablation pits, B is the width of ablation line, ${\omega _0}$ is the beam radius (1/e2), ${E_0} $ is pulse energy and ${F_{th}}$ is the ablation threshold. In this experiment, D and B of SiC and Si with different laser energy was measured, and the single and multi-ablation threshold was obtained by linear fitting. The scanning speed v and pulse repetition rate ƒ were severally set at 9.8 mm/s and 40 KHz and the effective number of pulses N can be calculated as [24]: The size of polishing area and the laser scanning path at a speed of v and scan spacing of $\triangle y$ are shown in Fig. 1(b), and the overlapping rates in X and Y directions are respectively calculated as [25]: Here $ \mathrm{\Phi \ }$ is beam diameter on the sample surface. In fs-LSP experiments, $ {\psi _x} $ was set as 98% and $ {\psi _{y }}$ was set as 90%, both of which are overing 0.8 can achieve a smooth surface [19]. In addition, Eq. (3) indicates that v can be increased by increasing of the $ f$, thus improving the processing efficiency.2.3 Characterization means
The width of ablation lines and the diameter of ablation pits were measured by Olympus Microscope BX53 and three lines or pits were measured each time to get the average value as the final data and the standard deviation as the experimental error. The surface roughness and topography of RB-SiC before and after fs-LSP were measured by 4D dynamic profiler of NanoCam Sq in an area size of 0.2 mm x 0.2 mm. Furthermore, the surface roughness before and after fs-LSP in the same area were measured three times to obtain the average value as the final data and the standard deviation as the experimental error. In addition, the chemical structure and composition analysis of laser polished surface were carried out by XPS and Raman spectrometer. XPS measurements performed using a Thermo Scientific K-Alpha source emitting at 1000 eV ion energy. The measurement results of XPS were analyzed by the Advantage software. The reference peak of C 1s peak at 284.8 eV was employed for calibrating. Besides, Raman spectroscopy was conducted by an inVia Raman microscope (Renishaw, Wotton-under-Edge, UK) with an excitation source of 488 nm argon-ion laser.
3. TTM model description
Before experiments, single-pulse and multi-pulse ablation thresholds of SiC and Si were analyzed by TTM, which describes the heat transfer among photons, electrons and lattices [26]. According to the phase explosion theory [27], while the lattice temperature at a material point over 0.9 ${T_{tc}}$ (${T_{tc}}$ is the thermodynamic equilibrium critical temperature), phase explosion is assumed and the laser fluence that produces this lattice temperature is used as the ablation threshold of the material. Through this TTM model, it can be preliminarily determined whether the threshold difference between SiC and Si can achieve fs-LSP.
Two coupled nonlinear differential equations of TTM can be shown as [28]:
The energy absorption rate of the electron system $Q({x,y,z,t} )$ can be calculated as [28]:
Here, $S({x,y,z} )$ is the spatial distribution and $pulse(t )$ is temporal evolution of the absorbed laser energy, which can severally be as Eqs. (8) and (9) [29]:In Eq. (8), the equation of ${\omega _{(z )}}$ is employed as [28]:
The finite element method is employed to resolve the coupled partial differential Eqs. (5) and (6). The initial temperatures of the lattice and electron are assumed to be at 300 K. Boundary conditions are assumed that there is no heat transfer across the top surface, while the lattice and electron temperature on the other five boundaries always equals 300 K. The values of the physical parameters of SiC and Si are listed in Table 1. TTM model was only used to analyze the difference between two thresholds of SiC and Si to ensure selective polishing can be achieved and $ R$, $\; {C_e}$, ${C_l}$, ${K_e}$ and ${K_l}$ were taken at the considered as constant and assuming that they did not change with temperature or plasma formation to simplify the model [28,30–34]. The lattice temperature is calculated by changing the pulse energy and compared with 0.9${T_{tc}}$, and the laser fluence can be expressed as [35]:
Table 1. Values of the physical parameters of SiC and Si
4. Results and discussion
4.1 Ablation thresholds
Before the experiment, a preliminary comparison of the ablation thresholds of SiC and Si was performed by TTM to ensure that the difference of their ablation thresholds was large enough for selective removal. The relationship between the lattice temperature of SiC and Si and laser fluence calculated by TTM is shown in Fig. 2(a) and 2(b). It can be seen that the lattice temperature is proportional to the laser fluence and the single-pulse ablation thresholds of SiC and Si can be calculated from Fig. 2(a) as 0.344 and 0.157 J/cm2, respectively. Similarly, multi-pulse ablation thresholds of SiC and Si are severally 0.284 and 0.006 J/cm2 as shown in Fig. 2(b). Figure 2(c) and 2(d) show the electronic and lattice temperature of SiC and Si changes with time under threshold fluence, and the lattice temperature after the electron and lattice temperature reach equilibrium is exactly around 0.9 ${T_{tc}}$.
Fig. 2. Relationship between the lattice temperature and laser fluence: (a) single-pulse; (b) multi-pulse; the electronic and lattice temperature changes with times under threshold fluence: (c) single-pulse; (d) multi-pulse.
TTM results showed that the ablation thresholds of SiC and Si were quite different, and selective polishing can be achieved. But the TTM model was simplified by taking the value of R, ${C_e}$, ${C_l}$, ${K_e}$ and ${K_l}$ as constant, which may cause a certain gap between the theoretical calculation threshold and the actual results. Thus, single-pulse and multi-pulse ablation thresholds of SiC and Si were accurately obtained by experiments. The single-pulse ablation areas of SiC and Si under different pulse energy are shown in Fig. 3(a) and 3(b), respectively. From the Eq. (1), a damage whose diameter D scales with a logarithmic law as a function of the laser pulse energy: ${D^2} = 2\omega _0^2\mathrm{\ast ln}({{E_0}} )- \textrm{ln}({\pi \omega_0^2{F_{th}}} )$. Hence, the single-pulse ablation thresholds of SiC and Si (as shown in Fig. 3(c)) were estimated to be 0.474 and 0.108 J/cm2, respectively. In the same way, as shown in Fig. 3(d), the multi-pulse ablation thresholds of SiC and Si can be obtained as 0.168 and 0.066 J/cm2, respectively. In addition, the single-pulse ablation thresholds were compared with the thresholds reported in the literature (as shown in Table 2.), and found that the difference is small, which showed that the threshold test method used is correct.
Fig. 3. . Ablation thresholds tested of SiC and Si: single-pulse ablation areas of SiC (a) and Si (b) with different energy; numerical fit for (c) single-pulse and (d) multi-pulse damage thresholds estimating.
Table 2. Comparisons of single-pulse ablation threshold
Through the threshold experiments, it was found that the ablation thresholds of Si and SiC are quite different and the laser fluence between the two thresholds (from 0.066 to 0.168 J/cm2) enables selective polishing. Before fs-LSP, laser fluence from 0.066 to 0.168 J/cm2 were employed for pre-polishing experiment, and the results showed that 0.08 J/cm2 can obtain a surface with better roughness. Thus, the laser fluence of 0.08 J/cm2 was chosen for fs-LSP.
4.2 Surface roughness and morphology
The surface roughness before and after fs-LSP with the laser fluence at 0.08 J/cm2 under different scanning time was shown in Fig. 4. It can be seen that the surface roughness of the CMP area could reach an average of 37 nm, which serves as the base surface for fs-LSP. For fs-LSP area under the fluence at 0.08 J/cm2, the surface roughness decreased first and then increased with the increase of the number of scans, and reached the minimum value in 3 scans which can reach quality surface with the surface roughness of 11.21 ± 0.26 nm.
Fig. 4. Surface roughness before and after fs-LSP with the laser fluence at 0.08 J/cm2 under different scanning time.
Surface topography of CMP and fs-LSP areas under 3 scans and 6 scans, including 3D and 2D morphology are shown in Fig. 5. Furthermore, the 2D morphology with a length of 200 µm were taken from the line parallel to X direction in 3D and surface morphology. Distribution of SiC and Si is clearly visible in the surface morphology (Fig. 5(a)), where the yellow arrow points to SiC and the green arrow points to Si. In addition, it can be clearly seen from the 2D and 3D topography of the CMP area that SiC has been polished to be relatively flat, and the height of Si is significantly higher than that of SiC. For fs-LSP with 3 scans, it can be seen from Fig. 5(b) that the raised Si layer on the CMP sample was removed, and the SiC did not change, making the sample surface relatively flat; and as the number of scans increased to 6 (Fig. 5(c)), the depth of Si removal increased, making the height of Si lower than that of SiC, resulting in Si depressions. This is why the roughness first decreases and then increases as the number of scans increases.
Fig. 5. Surface topography of the samples with different treatments: (a) CMP, (b) fs-LSP with 3 scans and (c) fs-LSP with 6 scans.
It can also be seen from Fig. 5 that fs-LP removed the Si high peaks, but it also seems to create deep valleys. The laser fluence used for fs-LSP was much smaller than the ablation threshold of SiC and bigger than that of Si. While the Si high peaks were selectively removed by fs-LSP, some of the Si at the valley (as shown in Fig. 5(a)) was also removed, resulting in a more obvious depth of the valley (as shown in Fig. 5(b)). Moreover, it can be seen from Fig. 5(c) that when the number of scans was further increased, the depth of the valley where the Si phase is located was further increased.
4.3 Structure and chemical composition analysis
The changes in structure and composition of RB-SiC after fs-LSP at the laser fluence of 0.08 J/cm2 under 3 and 6 scans were analyzed by Raman and XPS spectra. Figure 6 depicts the Raman spectra in the range of 400-1400 cm-1 of RB-SiC before and after fs-LSP, which are utilized to analyze the structural features changes during fs-LSP. It can be seen from Fig. 6 that the Raman spectra displays a strong broad crystalline Si band within 519 cm–1 [42] and two 6H-SiC weak bands corresponding to the folded modes of transverse optic (FTO) at 789 cm−1 and longitudinal optic (FLO) at 956 cm−1 [43]. In addition, as shown in Fig. 6, there was no change in the peak position of Raman spectra after fs-LSP with 3 and 6 scans comparing with CMP.
XPS spectra analyses of the RB-SiC surface before and after fs-LSP were carried out to get a further insight into the chemical reaction during fs-LSP. The results showed that the main compositions of RB-SiC surface before and after fs-LSP were C, Si, O elements (their atomic ratios are shown in Table 3), and part of the O element came from air adsorption. Furthermore, the changes of Si 2p peak and C 1s peak have been specifically analyzed by peak separation, as shown in Fig. 7. It can be seen from the C 1s peak (Fig. 7(a)) and Si 2p peak (Fig. 7(b)) separation that the CMP surface contains a large number of C-Si bonds, a small amount of C-C bond and Si-Si bond, as well as some Si-O bond and C-O bond. After fs-LSP, the Si-Si bond disappeared, and the C-Si bonds decreases slightly; and as scanning numbers increased from 3 to 6, the C-C bonds tended to increase, and the oxidation of Si becomes more intense that leading to the increasing of Si-O bonds which caused an increase in the concentration of O atoms. It can be concluded that the Si layer on the surface of RB-SiC is effectively removed by selective polishing, and part of the SiC is decomposed to Si and C.
Fig. 7. XPS results of RB-SiC surface before and after fs-LSP: (a) C 1s peak separation and (b) Si 2p peak separation.
Table 3. Concentration of Si, O and C atomics /%
4.4 Discussion
Through the above analysis, it can be clearly found that the irregular Si layer on the RB-SiC surface can be effectively removed, thereby reducing the surface roughness of RB-SiC from about 35 nm to 11.21 ± 0.26 nm, which provides a new method for RB-SiC polishing. Furthermore, it can be seen from Eqs. (3) and (4) that fs-LSP can improve the efficiency by adjusting the size of beam diameter on the sample surface and the pulse repetition rate under the same fluence and the same overlap rate. When the beam diameter is set at 60 µm by defocusing and the repetition rate is set at 150 kHz, the scanning speed at this time is 180 mm/s, and the 3 scans time of RB-SiC with a diameter of 60 mm can be calculated to be about 2.8 hours. According to the process data reported by the National University of Defense Technology [44], the diamond abrasive polishing process of RB-SiC with a diameter of 60 mm after grinding is shown in Table 4. Comparing with the conventional diamond abrasive polishing reported in the literature, it can be seen that fs-LSP has the potential to replace polishing step 2 in abrasive polishing to improve polishing efficiency.
Table 4. Diamond abrasive polishing process of RB-SiC [44]
5. Conclusion
Femtosecond laser selective removal is employed for precision polishing on the surface of RB-SiC after traditional mechanical polishing. Through theoretical analysis and experimental calculation, it is finally determined that the multi-pulse ablation thresholds of SiC and Si are 0.168 and 0.066 J/cm2, respectively. Then, a quality surface with a roughness of 11.21 ± 0.26 nm was obtained after selective polishing Si layer at 0.08 J/cm2 with 3 scans. Surface morphology characterization showed that the unevenly distributed Si layer on the surface was effectively removed by selective polishing. In addition, the surface structure was not changed after selective polishing by Raman spectra analyzing, but XPS spectrum analysis showed that the Si layer on the surface has been effectively removed. Moreover, after the parameter adjustment, fs-LSP can effectively improve the polishing efficiency and has the potential to become a new method for RB-SiC polishing.
Funding
Key Project of the Joint Fund for Astronomy of National Natural Science Foundation of China (U1831211); Shanghai Sailing Program (20YF1454800).
Acknowledgements
The authors acknowledge the financial support from the Key Project of the Joint Fund for Astronomy of National Natural Science Foundation of China and Shanghai Sailing Program.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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