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Yueqin Wu, Dekui Mu, Han Huang
Semiconductor and laser single crystals need to be ground to achieve satisfactory surface integrity and dimensional precision to fulfill functional requirements. Improvement of the surface integrity of a ground crystal will shorten the subsequent processing time, thus reduce manufacturing cost. The development of cost-effective grinding technologies for hard and brittle crystals requires an in-depth understanding of their deformation and removal mechanisms. A great deal of studies were carried out on this important topic in the past several decades. In this paper, Yueqin Wu, Dekui Mu from Huaqiao University, China and Han Huang from The University of Queensland, Australia review the works on the deformation and removal of semiconductor and laser single crystals at extremely small scales, which was published in "International Journal of Extreme Manufacturing" (IJEM), 2 (2020) 012006.
Figure 1 Outline of this review.
2. Research Background
Semiconductor and laser crystals are important materials for modern electronic and photonic devices. Usually, an as-grown single crystal ingot must be shaped into thin substrates with high surface quality and dimensional precision, and more importantly, free of subsurface damage. Given the hard and brittle nature of most semiconductor and laser single crystals, understanding of the deformation and removal behaviors of those crystal materials are crucial for developing effective grinding techniques for them.
3. Recent Advances
To understand the mechanisms of deformation and removal events of hard and brittle single crystals involved in a grinding process, instrumented nanomechanical tests using a well-shaped diamond tip are often employed to mimic the grit - work material interaction involved in material removal. In this review, we focus on the deformation and removal characteristics of commonly used Si, GaAs, β-Ga2O3, YAG and GGG single crystals induced by nanoindentation, nanoscratch and nanogrinding, which are revealed using transmission electron microscopy (TEM) and other advanced materials characterization techniques.
The crystal structures of typical semiconductor and laser crystals are firstly introduced. We then examine the deformation and removal patterns of those crystals under nanoindentation and nanoscratch with the assistance of TEM. A typical example is shown in Figure 2, the nanoindentation-induced deformation in β-Ga2O3 single crystal was examined using TEM. By studying the nanoindentation-induced deformation pattern, the sequence of the formation of crystalline defects and the corresponding critical conditions were revealed for each crystal.
Figure 2 Cross-sectional TEM images of subsurface of indented β-Ga2O3 at different stress levels.
To achieve the best possible surface integrity in the grinding of semiconductor and laser crystals, grinding must be carried out in the so called “ductile” removal regime. So it’s important to link the knowledge of deformation and removal gained from the nanomechanical tests to the design of a pragmatic grinding process. Figure 3 shows the ground subsurface of single crystal β-Ga2O3 using an ultrafine grinding wheel. Figure 4 displays the grinding-induced deformation patterns of single crystal YAG. In both cases, the major patterns of the deformation and removal involved in ductile grinding are similar to what discovered in the nanoscratch and nanoindentation tests. However, we also reveal that the grinding-induced heat can play a significant role in altering the deformation and removal patterns.
Figure 3 Cross-sectional TEM image of the ground β-Ga2O3.
Figure 4 Cross-sectional TEM image of the ground YAG single crystal.
4. Future Prospects
With the advancement of TEM and other materials characterization technologies, it is highly feasible to gain a complete understanding of the deformation and removal mechanisms of hard and brittle single crystals at extreme small scales. This review strongly suggests that the design and development of an effective grinding technology indeed benefits from the fundamental understanding of the deformation and removal mechanisms obtained from nanomechanical testing studies.
5. About the Authors
Han Huang, professor of Mechanical Engineering at The University of Queensland, Australia. Prof Huang has established a leading group working on nanomechanics and nanomanufacturing, and founded a world-class laboratory at UQ that is dedicated to fundamental understanding of material removal mechanisms and mechanical characterization of nanomaterials and nanostructures. Prof Huang has won a number of prestigious research accolades, including ARC Future Fellowship, ARC Australia Research Fellowship, JSPS Invitation Fellowship, Queensland International Fellowship and Singapore National Technology Award. He serves as the associate editor of International Journal of Mechanical Science (Elsevier, IF = 4.6) and has editorial roles in several international journals, including International Journal of Machine Tool and Manufacture (Elsevier, IF = 8.0). He has published over 250 refereed SCI journal papers with citations over 6600 times and an h-index of 44 according to Google Scholar.
Yueqin Wu, professor at Institute of Manufacturing Engineering, Huaqiao University, China. Prof Wu obtained his PhD degree from the University of Queensland, Australia. He is equipped with expert knowledge in both mechanical engineering and materials science. He has been working on the deformation mechanism of semiconductor materials using advanced nanomechanics and electron microscopy. He is“Minjiang Scholar” Distinguished Professor of Fujian Province. He won two ARC projects including the prestigious ARC Discovery Early Career Researcher Award (DECRA). He serves as referee for journals including Materials and Design, Scientific Report, Science of Advanced Materials, Journal of Alloys and Compounds.
Dekui Mu, associate professor at Institute of Manufacturing Engineering, Huaqiao University, China. A/Prof Mu obtained his PhD degree from the University of Queensland, Australia. His research interests focus on the relationship between microstructure and properties in brazing process. He has secured several funds as leading CI including National Natural Science Foundation of China (NSFC) and Scientific Research Project of Science and Technology Department of Fujian province and Xiamen city. He serves as referee for journals including Materials and Design, Diamond and Related Materials, Journal of Alloys and Compounds.