# Study on the Influence of Sapphire Crystal Orientation on Its Chemical Mechanical Polishing
## 1 Introduction  
Due to the excellent material properties such as high hardness, good light transmittance, high chemical and thermal stability, sapphire has been widely used in high-end technical fields such as communications, electronics, aerospace, optics, and biological engineering [1; 2; 3; 4; 5; 6]. As a typical anisotropic material, sapphire (\(\alpha\)-Al\({}_{2}\)O\({}_{3}\)) with different crystal planes has different mechanical and optical properties [7]. At present, sapphire is among the most important substrate materials in the LED field [2; 8; 9]. The C-plane sapphire wafer is currently the most widely used sapphire substrate for GaN growth by major companies [4; 10]. The main reason is that the sapphire crystal growth process along the C axis is mature and the finished product is prepared. The cost is relatively low and the physical and chemical properties are stable. However, GaN films grown on some non-C-planes, such as R-plane and M-plane sapphire substrates, are non-polar and semi-polar [11]. Compared with traditional C-plane sapphire barrier crystals, GaN films grown on R-plane or M-plane sapphire could greatly improve the luminous efficiency of the device. Therefore, the demand for high-quality R-plane and M-plane sapphire substrates is increasing. The sapphire crystal planes such as C-, A-, M- and R-plane have attracted more and more attention from researchers and become current research hotspots [12].  
Nanoindentation technology is currently the most useful method to explore the mechanical properties at sub-micro and nano-scales [13]. [PERSON] et al. [14] evaluated the elastic-plastic behavior of the C-plane of single-crystal sapphire by using ultra-low nanoindentation loads with a Berkovich indenter. [PERSON] et al. [15] studied the crystallographic orientation effect on the incipient plasticity and stochastic behavior of a sapphire single crystal by spherical nanoindentation. [PERSON] et al. [16] studied the deformation and cracking behavior of sapphire under spherical nanoindentation for suppling andenriching the existing studies. [PERSON] et al. [17] did the research to investigate the interaction of nanoscale damages with static and dynamic contact and induced damages in alumina by using nanoindentation.  
Nowadays, lots of researchers have focused on studying the processing performance of sapphire with different crystal orientations. [PERSON] et al. [18] analyzed the material removal mode of three crystal planes in mechanical planarization and revealed the removal mechanism of sapphire substrates. [PERSON] et al. [19] did the research and reported that the grinding force of the A-plane is higher than that of the C-plane of sapphire in micro slot-grinding experiment. [PERSON] et al. [20] did the experiment by double-sided planetary grinding and found that the crystal orientations with higher Young's modulus and fracture toughness were more difficult to realize the material removal. [PERSON] et al. [21] did the research on the machinability of the A-plane sapphire under diamond wire cutting in different cutting directions.  
However, there are few studies which combine the micro-scale material properties with macro-level processing although a lot of research has been performed to reveal the micromechanical behavior of sapphire by nanoindentation. Therefore, this paper will correlate the nanoindentation experiment with the results of sapphire chemical mechanical polishing and then reveal the nature of the anisotropy characteristics on the sapphire processing results. Apart from that, it will lay a theoretical basis and a significant guidance for the selection of parameters in processing with different crystal orientations.