 Advances in Technology Innovation , vol. 1, no. 2, 2016, pp. 29 - 32 29 Copyright © TAETI Reduction of Residual Stresses in Sapphire Cover Glass Induced by Mechanical Polishing and Laser Chamfering Through Etching Shih-Jeh Wu 1,* , Hsiang-Chen Hsu 1,2 and Wen-Fei Lin 3 1 Department of Mechanical and Automation Engineering, I-Shou University, Kaohsiung, Taiwan. 2 Department of Industrial management, I-Shou University, Kaohsiung, Taiwan . 3 Department of Material Science and Engineering, I-Shou University, Kaohsiung, Taiwan. Received 02 February 2016; received in revised form 30 March 2016; accept ed 02 April 2016 Abstract Sapphire is a hard and anti-scratch materia l commonly used as cover glass of mobile devices such as watches and mobile phones. A mechanica l polishing using diamond slurry is usually necessary to create mirror surface. Additional cha mfe ring at the edge is sometimes needed by mechanical grinding. These processes induce residual stresses and the mechanica l strength of the sapphire work p iece is impaired. In this study wet etching by phosphate acid process is applied to relief the induced stress in a 1” d ia meter sapphire cover glass. The sapphire is polished before the edge is chamfe red by a picosecond laser. Residual stresses are measured by laser curvature method at different stages of mach ining. The results show that the wet etching process effectively relie f the stress and the laser mach ining does not incur serious residual stress. Ke ywor ds : picosecond laser, laser curvature method, residual stress, stress relief, wet etching, sapphire 1. Introduction Sapphire is the material to cover glasses that are scratch and impact-resistant, yet fle xib le despite the introduction of tempered glass such as Corning ® Gorilla . In addition to the use for watch glasses or as cover or filter glasses for came ra lenses, now it is being tried for cell phone displays as well. Sapphire glass is made of colorless plates of synthetic corundum, i.e . minera ls produced with mo lten alu minu m o xide (Al2O3). In fact, sapphire glass does not have a glass -like a morphous structure, but rather a crystalline structure. With a hardness of 9 on the Mohs scale, sapphire is one of the hardest transparent materials next to diamond [1, 2]. Sapphire has very wide optical trans mission band from UV to near-infrared, (0.15-5.5 µ m)[2]. With its particu lar properties, sapphire glass offers advantages over the chemically hardened glass often used in the display industry. Traditional methods for machining sapphire glass are generally mechanical grinding and polishing. Optica l fabrication processes of these operations lead to the creation of su rface and sub-surface defect and residual stresses [3, 4]. Maybe in mic ro-size, these defects degrade the strength and the performance of functional materia ls. Another new development of sapphire mach ining is laser cutting. Laser mach ining is superior to conventional mechanica l methods in terms of fle xib ility and edge quality when right laser type and proper beam processing is applied. Mechanical saw creates mic ro cracks at edges as the speed is high which in turn deteriorates the bending strength. However, laser thermal e ffect can also induce stress which is not desirable either. A few methods can be applied to e liminate the above consequential effect fro m polishing process, name ly, trad it ional loose-abrasive polishing, wet etching, and dry plas ma etching [5-7]. The first method typically integrates a polishing step into the grinder itself, which offers the advantage of integrating the damage re moval into the grinder tool and builds upon * Corresponding aut hor, Email: wsj007@isu.edu.t w https://en.wikipedia.org/wiki/UV https://en.wikipedia.org/wiki/Near-infrared Advances in Technology Innovation , vol. 1, no. 2, 2016, pp. 29 - 32 30 Copyright © TAETI traditional che mica l mechanica l polishing (CM P) technology. However, it has the disadvantage of low re moval rates and perpetuates the surface profile. The second method uses familiar wet-etching processes to remove surface damage. Wet chemical etching is one of the most common thinning techniques. To etch one side o f the work piece, the workpiece is immersed in etching solution e.g. dilute hydrofluoric ac id (DHF). The other side is protected either by additional layers, or by applying special chucks allo wing the p rocessing of work piece . The th ird method uses atmospheric dry plasma etching to re move surface damage. This method has the advantage such as the surface damage is re moved, the edges are imp roved by rounding the sharp edge, and the surface roughness can be controlled where needed for adhesion [7]. In this paper we de monstrate a production mach ining and compare the residual stresses after diffe rent re liev ing process. The sapphire is polished before the edge is chamfe red by a picosecond laser. Residual stresses are measured by laser curvature method [8] at different stages of mach ining. The results show that the wet etching process effectively relief the stress and the laser machining does not incur serious residual stress. 2. Method The schematic of the sapphire glass samples under process is as shown in Fig. 1. The sapphire samples are in 1” dia meter. The sapphire glass samples we re po lished by CM P before the edge is chamfered by a picosecond laser and then they under wet etching different processes i.e., wet etching by DHF and dry plasma etching. Residual stress es are measured by laser curvature method at different stages of mach ining. The schematic of the optical setup is shown in Fig. 2. The line deflection (bow) was Fig. 1 Schematic of the sapphire glass samples under process first measured across the center of the sample (initially flat before machining) as shown in Fig. 3. The stress can be then estimated by the curvature. A diagrammatic sketch is shown in Fig. 4. Fig. 2 Optical setup of the laser curvature method Fig. 3 Line deflection (BOW) measured across the center of the sample 3D Stress map Fig. 4 A diagrammatic sketch of the 3D stress map by the laser curvature method Advances in Technology Innovation , vol. 1, no. 2, 2016, pp. 29 - 32 31 Copyright © TAETI 3. Results and Discussion The average surface roughness by different methods of burnish is shown in Fig. 5. Fro m lo w to high the roughness can be listed sequentially : mechanica l polishing, dry etching, wet etching and thermal annealing. However, the difference within acceptable range. The line deflections of the mechanically polished sapphire glass sample (1.44mm thic kness) before and after laser chamfe ring are as shown in Fig. 6. There is a tensile stress residing the measured surface. Interestingly, the laser machining does relief the residual stress induced. This mat be d ue to the instantaneous temperature rise during laser irradiation which leads an annealing effect. Fig. 5 Average roughness by different methods of burnish Fig. 6 Bow of the mechanically polished sapphire glass sample (1.44mm thickness) before and after laser chamfering The bow of the mechanically polished and laser mach ined sapphire glass sample (0.4mm thickness) after dry plasma etching is as shown in Fig. 7. The influence of residual stress on surface deflection is more pro minent in thinner sapphire glass. Befo re the etching the deflection is not symmetrical and irregular while it converts to a symmetrica l shape and the ma xima l deflect ion at the center increases slightly fro m 0.7 to 0.8 μ m a fter 300 seconds as a results of stress rela xation. The deflection at the center then starts to drop to 0.5 μ m until 360 seconds has passed. At this stage the stress has been relieved to the full extent. Fig. 7 Bow of the mechanically polished and laser machined sapphire glass sample (0.4mm thickness) after plasma dry etching Bow of the mechanically polished and laser mach ined sapphire glass sample (0.4mm thickness) after wet etching by DHF is as shown in Fig. 8. The deflect ion is irregular before the wet etching similar to dry etching. After the wet etching starts the bow turns to the other direction i.e., fro m concave to convex. As the etching process progresses the deflection drops until after 180 seconds the ma ximal deflection re mains the same at 0.57 μ m. It is believed the residual stress is relieved to the full e xtent. The reason for the change of concavity is that before the polishing process the sapphire glass is not evenly cut or ground and the work p iece retains its original shape after stress is relieved. Fig. 8 Bow of the mechanically polished and laser machined sapphire glass sample (0.4mm thickness) after wet etching by DHF Advances in Technology Innovation , vol. 1, no. 2, 2016, pp. 29 - 32 32 Copyright © TAETI The optical t ransmission and reflection ratio before and after wet etching are as shown in Fig. 9. There is not much difference in reflection, however, the transmission is improved. Fig. 9 The optical transmission and reflection ratio before and after wet etching 4. Conclusions In this paper, we demonstrate a production mach ining and compare the residual stresses after different re liev ing process. The laser chamfe ring does not incur further stress, on the contrary, it somehow reduces the stress. Both dry and wet etchings are effective in re lief of residual stress induced by mechanical polishing. Also the etching process did not impa ir the transparency of the sapphire glass. Ball-on-ring test is proposed to confirm the effect of residual stresses on the strength of the sapphire work pieces before and after wet etching. Acknowledgement The authors would like to express their appreciation to Ministry of Science and Technology, Taiwan, ROC, for financial supports under project No. MOST103-2221-E-214-018, MOST104-2221-E-214-051 and MOST104-2632- E-214-002. Special appreciation is also extended to E&R Engineering corp. for carrying out all the experiments. References [1] “Gorilla Glass Success, What is Sapphire glass?” http://www.corning.com/news_center/features/ gorillaglasssuccess.aspx, Corning Incorporated. [2] “Everything You Wanted To Know About Sapphire Glass, But Were Afra id To Ask, ” http://www.cultofmac.co m/267068/everyth ing-wanted-know-sapphire-glass -afraid-ask -qa/. [3] D. Wang, J. Lee , K. Ho lland, T. Bibby, S. Beaudoin, and T. Ca le, “ Von mises stress in chemical‐mechanical polishing processes,” J. Electroche m. Soc. , vol. 144, no. 3, pp. 1121-1127, 1997. [4] G. Kermouche, J. Rech, H. Ha md i, and J. M. Bergheau, “On the residual stress field induced by a scratching round abrasive grain,” Wear, vol. 269, no. 1-2, pp. 86-92, May 2010. [5] C. Landesberger, C. Paschke, and K. Bock, “Influence of wafe r grinding and etching techniques on the fracture strength of thin silicon substrates ,” Advanced Materials Research, vol. 325, pp. 659-665, 2011. [6] K. Gu rnett and T. Adams, “Ult ra-thin semiconductor wafe r applications and processes,” III-Vs Rev iew, vo l. 19, pp. 38– 40, 2006. [7] Z. J. Pei, G. R. Fisher, and J. Liu, “ Grinding of silicon wa fers: a rev iew fro m historica l perspectives,” International Journal of Machine Tools and Manufacture, vol. 48, pp. 1297-1307, 2008. [8] J. Wang, P. Shrotriya, and K. S. Kim, “Surface residual stress measure ment using curvature interfero metry,” Expe rimental Mechanics, vol. 46, pp. 39-46, 2006.