john a. patten, amir r. shayan, h. bogac poyraz, deepak ravindra and muralidhar ghantasala western...
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Scratch Tests on 4H-SiC Using Micro-Laser Assisted Machining
(μ-LAM) System
John A. Patten, Amir R. Shayan, H. Bogac Poyraz, Deepak Ravindra and Muralidhar Ghantasala
Western Michigan UniversityKalamazoo, MI
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)
Motivation
Tool wear Machining time
Semiconductor wafers Optical lens
Machining cost
60-90%
Ceramic seals
GrindingPolishingLapping
Diamond Turning
Potential Applications
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
Background
High Pressure Phase Transformation (HPPT)
SiC
HPPT is one of the process mechanisms involved in ductile machining of semiconductors and ceramics
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
Micro-Laser Assisted Machining (µ-LAM)
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.
Objective
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.
Scratch Tests
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.
Diamond Tip Attachment
Diamond tip(5 m radius)
Ferrule(2.5mm diameter)
(a) 5 µm RADIUS DIAMOND TIP ATTACHED ON THE END OF THE FERRULE USING EPOXY(b) CLOSE UP ON DIAMOND TIP EMBEDDED IN THE SOLIDIFIED EPOXY.
(b) (a)
Laser output power measurements with and without the diamond tip attached.Total Power coming out of the tip : 43%
Total Power Calibration
Laser Beam Profile2-D 2-D
3-D
Before attachment of the diamond tip After attachment of the diamond tipThe laser driving current is 580mA (~75mW)
The laser driving current is 214mA (~60mW)
Out of focus
On focusOn focus
Design of Experiments
ScratchNo.
Loadingg (mN)
Machining Condition
Cuttingspeed
(µm/sec)
Laser Power (mW)
1* 2.5 (25) w/o laser 305* 0
2* 2.5 (25) w/ laser 305* 350
3 2.5 (25) w/o laser 1 0
4 2.5 (25) w/ laser 1 350
*Experiments performed previously by Dong and Patten (2005).
SPECIFICATIONS OF THE SCRATCHES
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
Results and Discussion Cont’d
Scratch #Machining Condition
Cuttingspeed
(µm/sec)
Average Groove Depth
(nm)
Relative Hardness
(GPa)
1* w/o laser 305* 41 39
2* w/ laser 305* 46 35
3 w/o laser 1 54 30
4 w/ laser 1 90 18
AVERAGE GROOVE DEPTHS MEASURED WITH AFM
Thrus t Force = 25 mN
*Experiments performed previously by Dong and Patten (2005).
Results and Discussion Cont’d
AVERAGE GROOVE DEPTH MEASURED WITH AFM IN (nm) WITH 2 DIFFERENT CUTTING SPEEDS , W/LASER AND W/O LASER
Mechanical Energy and Heat
25 700 32000
5
10
15
20
25
30 27.3
8
000.16000000
0000001 0.9
Mechanical WorkHeat
Temperature (°C)
En
erg
y (
nW
)
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.
Acknowledgement
Dr. Valery Bliznyuk and James Atkinson from PCI Department
Kamlesh Suthar from MAE Department
Support from NSF (CMMI-0757339)
Support from MUCI
Hardness-Temperature, 6H-SiC
20 300 400 500 600 700 800 990 1100 1300 1500 15800
5
10
15
20
25
30
Hardness (GPa)
Temperature ( C)⁰
Ha
rdn
es
s (
GP
a)