Femtosecond Laser Micromachining - Deepak Rajput

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

1 of xx

Femtosecond Laser Micromachining

02/03/2010

Spring 2010 MSE503 Seminar

Deepak RajputCenter for Laser Applications

University of Tennessee Space InstituteTullahoma, Tennessee 37388-9700

Email: [email protected]: http://drajput.com

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

2 of xx

Outline

IntroductionLaser micromachiningFemtosecond laser micromachining (FLM)UTSI researchSummary

2

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

3 of xx

Introduction

Laser: Theodore Maiman (1960)

Laser micromachining: cutting, drilling, welding, or other modification in order to achieve small features.

Laser micromachining of materials:Automotive and machine toolsAerospace MicroelectronicsBiological devices

3

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

4 of xx

Introduction

Laser micromachining:Direct writingMask projectionInterference

Direct writing: desired pattern fabricated by translatingeither the sample or the substrate.Mask projection: A given feature on a mask isilluminated, which is projected on the substrate.Interference: Split the primary beam into two beams,which are superimposed in order to create a pattern. Theinterference pattern is projected on the substrate and themicromachined pattern corresponds with the intensityprofile of the pattern. 4

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

5 of xx

Direct Writing

Reference: Journal of Materials Processing Technology, Volume 127, Issue 2, Pages 206-210 5

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

6 of xx

Mask Projection

Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008) 6

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

7 of xx

Interference

Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)

( )2/sin2

12cos2)(

θλ

π

=

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ +=

l

lxIxI o

Intensity distribution: 0 to 4Io

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

8 of xx

Combined Techniques

Scanning Near-field Optical Microscopy (SNOM) +Atomic Force Microscopy (AFM) = ablation + etching

The setup involves the coupling of the laser light into the tip ofsolid or hollow fiber.

Laser Induced Nano Patterning = interferencesubpatterns generated by microspheres.

A regular two-dimensional array of microspheres acts as anarray of microlenses.

8

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

9 of xx

Combined Techniques

SNOM arrangement for nanopatterning

Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008) 9

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

10 of xx

Combined Techniques

Reference: Appl. Phys. A. 76, 1-3 (2003)

Laser-induced surface patterning by means of microspheres

10

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

11 of xx

Laser Micromachining

Laser beam:Continuous wave mode (CW)Pulsed mode

CW: output constant with timePulsed: output is concentrated in small pulsesLaser micromachining requirement: minimize the heattransport to the region immediately adjacent to themicromachined region.Laser micromachining is often carried out by usingpulsed laser, which delivers high energy at short timescales and minimizes heat flow to surrounding material.

11

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

12 of xx


Types of lasers used: Infrared to UltravioletExcimer lasers: 157, 193, 248, 308, or 351 nm wavelengthdepending on the composition of the gas in the cavity.Most materials absorb UV wavelengths. Hence, theyprovide both low machining rates and high machiningprecision.Diode-pumped solid state (DPSS) lasers – Nd:YAG

DPSS: 355 nm (3rd harmonic) and 266 nm (4th harmonic)Ti:sapphire solid state lasers (700 nm – 1100 nm)CO2 gas lasers (10,600 nm): limited roles (low operatingcosts and high throughput) because of spot sizelimitation (50-75 micrometers).

12

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

13 of xx


Laser-material interaction leading to ablation.Material removal occurs when the absorbed energy ismore than the binding energy of the substrate material.Energy transfer mechanism depends on materialproperties and laser properties.Absorption: Thermal or/and Photochemical processes

13

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

14 of xx

Absorption Mechanism

Thermal AblationCommonly observed with long wavelength and continuouswave (CW) lasers e.g., CO2 lasers.Absorption of laser energy causes rapid heating, which resultsin melting and/or vaporization of the material.May be associated with a large heat-affected zone.

Photochemical AblationCommonly observed with short wavelength and pulsed lasers.Occurs when the laser photon energy is greater than the bondenergy of the substrate material.Vaporization occurs due to bond-dissociation due to photonabsorption.Thermal effects do not play a significant role.

14

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

15 of xx

Factors Affecting Laser Ablation

Laser ablation demonstrates “threshold” behavior inthat ablation takes above certain “fluence” level.The “threshold” is a function of laser properties andsubstrate material properties.Laser properties: laser fluence, wavelength, peak power.Material properties: optical (absorption) and thermal(diffusivity) properties.Pulse duration affects the heat-affected zone.

15

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

16 of xx

Femtosecond Laser Machining (FLM)

Exhibit extremely large peak power values.Laser material interaction in femtosecond lasers isfundamentally different than that in long wavelengthlasers.Induces nonlinear effects (e.g., multiphoton absorption).MPA: The simultaneous absorption of two or morephotons can provide sufficient energy to cleave strongbonds.As a result, relatively long wavelength lasers withfemtosecond pulse widths can be used to machinematerials that are otherwise difficult to machine.

16

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

17 of xx

Femtosecond Laser Micromachining

First demonstrated in 1994 by Du et al followed byPronko et al in 1995 to ablate micrometer sized features.The resolution since then has improved to machinenanometer sized features.Advantages of femtosecond laser micromachining(FLM):

The nonlinear absorption induces changes to the focal volume.The absorption process is independent of the material.Fabrication of an optical motherboard by bonding severalphotonic devices to a single transparent substrate.

17

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

18 of xx

FLM: Physical Mechanisms

Results from laser-induced optical breakdown.Laser-induced optical breakdown:

Transfer of optical energy to the material by ionizing a largenumber of electrons that, in turn, transfer energy to the lattice.As a result of the irradiation, the material can undergo a phaseor structural modification, leaving behind a localized permanentchange in the refractive index or even a void.

Absorption: the absorption of light in a transparentmaterial must be nonlinear because there are no allowedelectronic transitions at the energy of the incidentphoton.

18

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

19 of xx


For such nonlinear absorption to occur, the electric-fieldstrength in the laser pulse must be approximately equal tothe electric field that binds the valence electrons in theatoms – of the order of 109 V/m, corresponding to alaser intensity of 5 x 1020 W/m2.To achieve such electric-field strengths with a laser pulse,high intensities and tight focusing are required.Example: a 1-microJoule, 100 femtosecond pulse focusedto a spot size of 16 micrometers.

19

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

20 of xx


20

Laser-induced optical breakdown

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

21 of xx


The laser pulse transfers energy to the electrons throughnonlinear ionization.For pulse durations greater than 10 femtoseconds, thenonlinearly excited electrons are further excited throughphonon-mediated linear absorption.When they acquire enough kinetic energy, they can exciteother bound electrons – Avalanche ionization.When the density of excited electrons reaches about 1029

/m3, the electrons behave as a plasma with a naturalfrequency that is resonant with the laser – leading toreflection and absorption of the remaining pulse energy.

21

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

22 of xx


22

Sub-picosecond: absorption, ionization, and scattering eventsNanosecond: pressure or shock wave propagationMicrosecond: thermal energy propagation

Reference: Gattass RR and Mazur E, Nature Photonics, Vol 2, 219 – 225, 2008

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

23 of xx


For pulses of subpicosecond duration, the timescale overwhich the electrons are excited is smaller than theelectron-phonon scattering time (about 1 picosecond).Thus, a femtosecond laser pulse ends before theelectrons thermally excite any ions.

Reduces heat affected regionIncreases the precision of the method.

FLM: deterministic process because no defect electronsare needed to seed the absorption process.The confinement and repeatability of the nonlinearexcitation make it possible for practical purposes.

23

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

24 of xx

Bulk Damage

If the absorption is purely nonlinear, the laser intensityrequired to induce a permanent change will dependnonlinearly on the bandgap of the substrate material.Because the bandgap energy varies from material tomaterial, the nonlinear absorption would vary a lot.However, the threshold intensity required to damage a

material is found to vary only very slightly with thebandgap energy, indicating the importance of avalancheionization, which depends linearly on I.Because of this low dependence on the bandgap energy,femtosecond laser micromachining can be used in abroad range of materials.

24

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

25 of xx

Applications

WaveguidesActive devicesFilters and resonatorsPolymerizationNanosurgeryMaterial processingMicrofluidic devicesRapid prototyping

25

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

26 of xx

FLM at the UT Space Institute

Single-pulse ultrafast-laser machining of high aspect nano-holes at the surface of SiO2

Volume 16, No. 19, Optics Express, PP 14411

White Y., Li X., Sikorski Z., Davis L.M., Hofmeister W.

26

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

27 of xx

FLM at the UT Space Institute

Experimental Set-up

Ti-sapphire laser:Center wavelength: 800 nmRepetition rate: 250 kHzPulse width: 200 femtosecond (FWHM)Average power of 1 W.

Objective lens (dry):Numerical Aperture: 0.85Working distance: 0.41 - 0.45 mmCorrection collar to adjust for spherical aberration

Fused silica (200 micrometers) of refractive index 1.453 at 800nm

Piezoelectric nanostage with 200 micrometers range of motion27

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

28 of xx

Single Pulse Nano-holes

28Nano-holes machined by single laser pulses at different energies

1.2 μJ 1.6 μJ

2.4 μJ 1.2 μJ

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

29 of xx


29

Dependence of nano-hole diameter at the surface on the pulse energy

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

30 of xx


Depth analysis

Conventional technique: Atomic Force MicroscopyProblems in obtain signal from the bottom of a nanometer sized,high-aspect ratio feature.Techniques used:

Replication methodDualBeamTM SEM/FIB (CNMS, ORNL)

Replication method: fast, non-destructive, and inexpensive.Used a cellulose-based acetate films (35 micrometer).

30

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

31 of xx


31Nano-holes machined with laser pulse energy of 1.6 μJ

Replication method

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

32 of xx


32Nano-holes machined with laser pulse energy of 2 μJ

Replication method

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

33 of xx


33

Dependence of hole depth (by replication) on the pulse energy

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

34 of xx


34

Dependence of aspect ratio (by replication) on the pulse energy

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

35 of xx


35

DualBeamTM SEM/FIB

Schematics of the DualBeamTM SEM/FIB tool

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

36 of xx


36

DualBeamTM SEM/FIB

Scope image inside the chamber of the tool

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

37 of xx


37

DualBeamTM SEM/FIB

SEM image of the sectioned nano-holes in the trench at zero degree

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

38 of xx


38

DualBeamTM SEM/FIB

View of the trench after 90o rotation and 25o tilt

AB = AC/tan52o

= 0.78 AC

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

39 of xx


39

Nano-hole #1 #2 #3 #4AC (μm) 0.7 5 10.7 15AB (μm) 0.6 3.9 8.3 11.7

The FIB sectioning confirmed that the replication technique doesnot overestimate the depth of the holes.In fact, the replication technique most probably underestimatesthe depths.It might be due to the difficulty of the polymer to reach thebottom of the nano-hole and/or distortion of the acetate nano-wires during gold coating.

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

40 of xx


40

DualBeamTM SEM/FIB

SEM image at 52-degree tilt of FIB cross-sectioned nano-hole

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

41 of xx

Summary

41

Femtosecond lasers enable direct writing of nanoscale features.FLM can be used to fabricate fluidic and photonic componentsFocusing the femtosecond laser pulse with a high numericalaperture with spherical aberration is the key to produce highaspect ratio features.Self-focusing due to Kerr nonlinearity is also expected.The fabrication of high aspect ratio nano-holes demonstrated.

MSE

503

M

ater

ials

Sci

ence

Sem

inar

Sprin

g 20

10

42 of xx

Thanks !

Femtosecond Laser Micromachining - Deepak Rajput

Documents

Transcript of Femtosecond Laser Micromachining - Deepak Rajput