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

Micro-Laser Assisted Machining (µ-LAM)

Solution ?

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

Experimental Setup of µ-LAM System

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

)

µ-LAM System

Laser Head

UMT Tribometer

Laser Cable and BDO

Diamond Cutting Tool

UMT Computer

Diamond Tools In μ-LAM

Chardon Diamond Tool

K&Y Diamond Tool

WMU Diamond Tool

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

Questions

THANK YOU

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)

Hardness - Temperature

RELATIVE HARDNESS OF THE 90 nm AND 95 nm DEEP SCRATCHES w/LASER AND w/o LASER

C U T T I N G S P E E D = 1 µ m / s e c

Scratch Depth(nm)

Machining ConditionThrust Force

(mN)Relative Hardness

(GPa)

90 w/laser 25 18

95 w/o laser 70 62