Recent Advances in Understanding of Diamond Wire Sawing of Solar Silicon Shreyes Melkote Morris M. Bryan, Jr. Professor of Advanced Manufacturing Systems Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, Georgia July 8, 2014 c-Si PVMC Workshop on Diamond Wire Sawing San Franciso, CA Outline • Overview of silicon wafering • Slurry vs. diamond wire sawing • Diamond scribing studies • Effect of crystal defects • Short and long term challenges Diamond Wire Sawing Main Features Diamond impregnated wire Higher MRR (> 2x) Better surface quality and low kerf loss Water-based fluid In use for single crystal Si diamond grit steel core Moller.H.J, Phys.Stat.Sol. 203(4): 659-669(2004) Clark.W.J., et al., Int.J..Mach.Tool & Manuf., 43:523-532(2003) Yang.F., et al., J.Elec.Pack.,123:254-259(2001) V Feed Wire Guide Nickel coating Slurry vs. Diamond Wire Sawing Fixed Diamond Wire Sawing Loose Abrasive Slurry Sawing Surface Morphology Yang et al., Adv. Eng. Mat., 2012 Wafer Thickness Comparison 215 Slurry Wafer thckness (µm) 210 DWS 205 200 195 190 185 180 0 50 100 Distance from wire entry (mm) Yang et al., Adv. Eng. Mat., 2012 150 Surface Roughness Comparison Areal surface roughness, Sa (µm) 0.4 MWSS DWS 0.3 0.2 0.1 0 0 50 100 Distance from wire entry (mm) Yang et al., Adv. Eng. Mat., 2012 150 Weibull Plot Yang et al., Adv. Eng. Mat., 2012 Diamond Scribing Studies Ductile mode Brittle mode • Brittle vs. ductile cutting? • How do grit properties and cutting depth affect the cutting mode? Surface Morphology 0.123 µm 0.722 µm 1.225 µm Increasing depth of scribe Scribing of (111) CZ wafer in the [112] direction at 1mm/min Wu and Melkote, Mat. Sci. Eng. A, 2012 XFEM Modeling • • XFEM allows modeling of crack (discontinuity) initiation and propagation Model setup in ABAQUS/Standard – – – – Quasi-static model Linearly increasing scribe depth Frictional contact between scriber and silicon Axisymmetric model CZ (111)<110> Wu and Melkote, ASME J. Eng. Mat. Tech., 2012 Simulation Results Scriber I Scriber II • Critical depth for crack initiation: 387.6 nm (Scriber I ) 34.6nm (Scriber II ) • Implication: higher wire feed rate possible without causing cracks Wu and Melkote, ASME J. Eng. Mat. Tech., 2012 Validation SEM of scribed groove (Scriber I) XFEM model • ‘Chevron’ type crack geometry observed and simulated Measured critical depth = 318±37.2 nm (at location of first observed crack) Model predicted critical depth = 387.6 nm • Hydrostatic pressure (9.45 GPa) predicted prior to cracking is greater than phase transformation pressure (8.8 GPa) • Amorphization confirmed by Raman spectroscopy Wu and Melkote, ASME J. Eng. Mat. Tech., 2012 Effect of Included Angle Conical tip • Scriber with larger included angle yields larger critical depth of cut (more ductile cutting is possible) Wu and Melkote, ASME J. Eng. Mat. Tech., 2012 Effect of Crystal Defects Effect of Dislocation Density: Critical Depth of Cut Region I Region II Region III Region IV Region V 350 Critical depth (nm) 300 250 200 150 100 50 0 (100) grain (311) grain Wu and Melkote, Mat. Sci. Semi. Proc., 2013 Effect of Hard Inclusions • Smooth scratching motion disrupted at the SiNx rods Wu and Melkote, Prec. Eng., 2013 Summary Wafering is a critical step in c-Si PV manufacturing Wafer surface characteristics and mechanical properties depend on the process type and material characteristics Abrasive characteristics play an important role in determining the cutting mode and surface and subsurface quality in diamond wire sawing Demand for high-throughput, better surface quality, and less surface/sub-surface damage calls for improved understanding of wire sawing process science Acknowledgements • NSF IUCRC on Silicon Solar Consortium
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