1. 1. Finishing of Individual Sapphire Wafers University of Florida Center for Manufacturing Innovation A.J Garcia November 24, 2014
2. 2. Sapphire (Al203) Background Hexagonal structure 9 mohs scale hardness Chemically and biologically inert Non-thrombogenic 2040 C melting point Low thermal expansion coeff. Wide transmission range 0.18 m 5.5 m Stretches from IR to UV Birefringent Electrical insulator Anisotropic 2 (http://www.cyberphysics.co.uk/topics/light/emspec t.htm)
3. 3. Applications Electronics Epitaxial growth of semiconductors Gallium nitride LED manufacturing Silicon-on-sapphire integrated circuits Radiation hardened devices Scratch resistant screens Corrosion resistant components Nozzles, crucibles Optical windows and lenses in extreme environments
4. 4. Motivation Applications demand precision surfaces Electrical industry Uniform semiconductor growth Precision form requirements Optical industry Image distortion Incomplete transmission
5. 5. Effect of magnet arrangements Guide magnet arrangement investigation Workpiece 10 mm x 10 mm x 1 mm rectangular sapphire Slurry 50-70 m diamond abrasive mixed with lubricant Abrasive surface P120 grit abrasive paper Guide magnet arrangement 1, 2, 3 Set guide magnet rotation speed 350 rpm Finishing time 20 min 0.85 0.9 1.75 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 1 2 3 Ave.ThicknessReductionm/min Guide magnet arrangement Aim to improve tool motion Eliminating sticking Arrangement 3 enabled ideal motion Motion produced greater thickness reduction across surface
6. 6. Surface roughness 6 measurements along diagonal 2.02 mm spacing Abrasive: 0-0.5 m diamond abrasive slurry Average initial roughness: 937 nm Sa d 0 y x 10 mm 10 mm Measurement locations (equally spaced)
7. 7. WOT roughness reduction: Results Average roughness after finishing: 700 nm Sa Partial finishing of surface Smooth plateaus amid rough valleys 0-0.5 m abrasive does not penetrate valleys 0 200 400 600 800 1000 1200 2.02 4.04 6.06 8.08 10.1 12.12 SurfaceRoughnessRa[nm] Diagonal Distance From Corner [mm] Before After
8. 8. Results
9. 9. Surface Roughness with Flooded Basin 0 0.5 1 1.5 2 2.5 3 3.5 4 0 6.3 12.6 18.9 25.2 31.5 RoughnessSa[nm] Radial distance from center [mm] Before finishing After finishing
10. 10. Radial Distance [mm] 0 6.3 12.6 Unpolished surface Polished surface Radial Distance [mm] 18.9 25.2 31.5 Unpolished surface Polished surface Sa = 1.77 nm Sa = 3.24 nm Sa = 1.40 nm Sa = 1.55 nm Sa = 1.68 nm Sa = 1.68 nm Sa = 0.67 nm Sa = 0.77 nm Sa = 1.52 nm Sa = 1.67 nm Sa = 1.35 nm Sa = 1.99 nm
11. 11. Tool magnet sticking Evidence of excessive magnetic flux High magnetic force High normal reaction force High friction Exclusion of lubricant and diamond particles N S N S Magnetic force Guide magnet Tool magnet Workpiece Iron particle Abrasive particle Normal force Jig
12. 12. Magnetic field density -10 10 30 50 70 90 110 130 0 5 10 15 20 25 30 Magneticfluxdensity[mT] Radial distance from center, r [mm] 3mm 6mm 9mm 12mm 15mm 12.7 mm r Tool magnet Guide magnet 3 mm jig height Steep drop in magnetic flux density Maximum of 121 mT 9 mm jig height 71 mT Consistent across tool magnet Jig height 0 16 mm + jig height
13. 13. Surface roughness with 9 mm jig height Average before: 6.0 nm Sa Average after: 0.9 nm Sa 0 1 2 3 4 5 6 7 8 9 10 0 6.3 12.6 18.9 25.2 31.5 SurfaceRoughnessSa[nm] Radial Distance from Center of Polishing [mm] Before Finishing After finishing
14. 14. Abrasive path simulation End goal: Develop method for predicting material removal Plan: Design mathematical model of ideal particle motion Observe correlation between surface changes and number of particle passes Observe changes caused by parameter variation Introduce corrective terms
15. 15. Simulation Plot Example parameters: R = 3, r = 1, h = 0.5, = 6.28, t = 1, res = 2 xmin = -5, xmax = -4, ymin = -1, ymax = 1
16. 16. Simulation inaccuracies Inaccuracies noticed when h = 0 = + cos() cos + = + sin() sin + = + cos() = + sin() Becomes parametric equations of a circle Simulation generates an annulus
17. 17. Simulation inaccuracies Generate circle using parametric circle equations = cos() = sin() A = 10, = 21 rad/s t = 0 s : 900 s Also generates an annulus Thickness increases with t res too low
18. 18. High res circle Same parameters to generate circle res increased from 4 to 6 Correctly represents a circle Lower res limit set to 6