Scratch Tests on 4H-SiC Using Micro-Laser Assisted Machining ( μ -LAM) System

1. John A. Patten, Amir R. Shayan, H. Bogac Poyraz, Deepak Ravindra and Muralidhar Ghantasala Western Michigan University Kalamazoo, MI Scratch Tests on 4H-SiC Using Micro-Laser Assisted Machining (μ-LAM) System

2. Motivation • Increasing industrial demand in high quality, mirror-like and optically smooth surfaces • High machining cost and long machining time of semiconductors and ceramics • Reduce the cost in precision machining of hard and brittle materials (semiconductors and ceramics)

3. Potential Applications Grinding Polishing Lapping Diamond Turning • Tool wear • Machining time Machining cost 60-90% Semiconductor wafers Optical lens Ceramic seals

4. Background • Semiconductors and ceramics are highly brittle and difficult to be machined by conventional machining • Lapping, fine grinding and polishing • High tool cost • Rapid tool wear • Long machining time • Low production rate

5. Solution ? Micro-Laser Assisted Machining (µ-LAM)

6. High Pressure Phase Transformation (HPPT) HPPT is one of the process mechanisms involved in ductile machining of semiconductors and ceramics SiC

7. Micro-Laser Assisted Machining (µ-LAM) • addresses roadblocks in major market areas (such as precision machining of advanced materials and products) • uses a laser as a heating source to thermally soften nominally hard and brittle materials (such as ceramics and semiconductors) • represents a new advanced manufacturing technology with applications to the many industries, including • Automotive • Aerospace • Medical Devices • Semiconductors and Optics

8. Objective • The objective of the current study is to determine the effect of temperature and pressure in the micro-laser assisted machining of the single crystal 4H-SiC semiconductors using scratch tests.

9. Scratch Tests • The scratch tests examine the effect of temperature in thermal softening of the high pressure phases formed under the diamond tip, and also evaluate the difference with and without irradiation of the laser beam at a constant loading and cutting speed. • The laser heating effect is verified by atomic force and optical microscopy measurements of the laser heated scratch grooves.

10. Experimental Procedure • Laser Furukawa 1480nm 400mW IR fiber laser with a Gaussian profile and beam diameter of 10μm. • Tool 90 conical single crystal diamond tip with 5μm radius spherical end. • Workpiece single crystal 4H-SiC wafers provided by Cree Inc. NOTE: The primary flat is the {1010} plane with the flat face parallel to the <1120> direction. The primary flat is oriented such that the chord is parallel with a specified low index crystal plane. The cutting direction is along the <1010> direction.

11. Diamond tip (5 m radius) Ferrule (2.5mm diameter) Diamond Tip Attachment (b) (a) • 5 µm RADIUS DIAMOND TIPATTACHED ON THE END OF THEFERRULE USING EPOXY • CLOSE UP ON DIAMOND TIP EMBEDDED IN THE SOLIDIFIED EPOXY.

12. Total Power Calibration Laser output power measurements with and without the diamond tip attached. Total Power coming out of the tip : 43%

13. Laser Beam Profile 2-D 2-D On focus On focus 3-D Out of focus Before attachment of the diamond tip After attachment of the diamond tip The laser driving current is 214mA (~60mW) The laser driving current is 580mA (~75mW)

14. Experimental Setup of µ-LAM System

15. Design of Experiments specifications of the scratches *Experiments performed previously by Dong and Patten (2005).

16. Results and Discussion  AFM measurements have been used to measure the groove size and to study the laser heating effect of the scratches made on 4H-SiC. AFM IMAGE OF THE SCRATCH #3 NO LASER HEATING AFM IMAGE OF THE SCRATCH #4 W/ LASER HEATING

17. Results and Discussion Cont’d AVERAGE GROOVE DEPTHS MEASURED WITH AFM *Experiments performed previously by Dong and Patten (2005). Thrust Force = 25 mN

18. Results and Discussion Cont’d AVERAGE GROOVE DEPTH MEASURED WITH AFM IN (nm) WITH 2 DIFFERENT CUTTING SPEEDS , W/LASER AND W/O LASER

19. Mechanical Energy and Heat

20. µ-LAM System UMT Computer UMT Tribometer Laser Cable and BDO Laser Head Diamond Cutting Tool

21. Diamond Tools In μ-LAM Chardon Diamond Tool K&Y Diamond Tool WMU Diamond Tool

22. Conclusion • Laser heating was successfully demonstrated as evidenced by the significant increase in groove depth (from 54 nm to 90 nm), i.e., reduced relative hardness ~40%, indicative of enhanced thermal softening ~700°C. • AFM measurements of the laser-heat assisted scratch grooves show deeper and wider grooves compared to scratches made without the laser heating assisted methods; which indicates favorable thermal softening effects ~700°C.

23. Acknowledgement • Dr. Valery Bliznyuk and James Atkinson from PCI Department • Kamlesh Suthar from MAE Department • Support from NSF (CMMI-0757339) • Support from MUCI

24. THANK YOU Questions

25. Hardness-Temperature, 6H-SiC

26. Hardness - Temperature relative hardness of the 90 nm and 95 nm deep scratches w/LASER and w/o LASER cutting speed = 1 µm/sec