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

LAM) System

Amir R. Shayan, H. Bogac Poyraz, Deepak Ravindra, Muralidhar Ghantasala and John A. Patten,

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

IR LASERDIAMONDTOOL

uses a laser as a heating source to thermally soften nominally hard and brittle materials (such as ceramics and semiconductors)

addresses roadblocks in major market areas(such as precision machining of advanced materials and products)

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)

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/ laser 1 350

2 2.5 (25) w/o laser 1 0

3 7.0 (70) w/o laser 1 0

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 #2 NO LASER HEATING

AFM IMAGE OF THE SCRATCH #1

W/ LASER HEATING

Results and Discussion Cont’d

Wyko Optical interferometer profile of the scratch #3 without laser

Results and Discussion Cont’d

Scratch #Machining Condition

Cuttingspeed

(µm/sec)

Average Groove Depth

(nm)1 w/ laser 1 902 w/o laser 1 543 w/o laser 1 95

AVERAGE GROOVE DEPTHS MEASURED WITH AFM

Relative Hardness

2

22

wAd

d

n

AFH

w w: scratch widthAd: pressure areaFn: thrust forceH: relative Hardness

Diamond Tool

Laser Beam

Substrate

Relative Hardness Cont’d

RELATIVE HARDNESS OF THE SCRATCHESC U T T I N G S P E E D = 1 µ m / s e c

Scratch Depth(nm)

Width of scratch(μm)

Machining Condition

Thrust Force(mN)

Relative Hardness

(GPa)

90 1.9 w/laser 25 18

55 1.5 w/o laser 25 28

95 2.1 w/o laser 70 40

Force Analysis at Constant Load

Scratch Depth(nm)

Width of scratch

(μm)

Machining Condition

Thrust Force(mN)

Cutting Force(mN)

Relative Hardness

(GPa)

90 1.9 w/laser 25 8.0 18

55 1.5 w/o laser 25 7.5 28

Force Analysis at Constant Depth of Cut

Scratch Depth(nm)

Width of scratch

(μm)

Machining Condition

Thrust Force(mN)

Cutting Force(mN)

Relative Hardness

(GPa)

90 1.9 w/laser 25 8.0 18

95 2.1 w/o laser 70 27.3 40

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)

Ener

gy (n

W)

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.

The cutting force encountered with laser-heating is ~1/3 of the force seen without while the thrust force with laser-heating is ~1/2 of the force measured without.

Acknowledgement Dr. Valery Bliznyuk and James Atkinson from PCI Department

Mr. Kamlesh Suthar from MAE Department

Support from NSF (CMMI-0757339)

Support from MUCI

Questions

THANK YOU

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

Total Laser 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

µ-LAM System

Laser Head

UMT TribometerLaser Cable

and BDO

Diamond Cutting Tool

UMT Controller

Four diamond tools were designed and

purchased to be used with the above lasers.

A significant improvement to the laser delivery

system was achieved with the addition of a laser

head (from Laser Mechanisms), which provides

precise x, y, and z positioning of the laser beam.

Diamond Tools In μ-LAM

Diamond Tools In μ-LAM

Chardon Diamond

Tool

K&Y Diamond Tool

WMU Diamond Tool

three fiber coupled laser systems (400mW, 10W and 100W) were acquired

and implemented for μ-LAM:

• Furukawa (1480nm wavelength, max 400mW power) 10µm

• VISOTEK (976nm wavelength, max 10W power) 100µm

• IPG (1070nm wavelength, max 100W) 10µm

1480nm-400mW976nm-10W

1070nm-100W

Lasers In μ-LAM

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

Hard

ness

(GPa

)