MSE503 Materials Science Seminar Spring 2010 1 of xx Femtosecond Laser Micromachining 02/03/2010...

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

1 of xx

Femtosecond Laser Micromachining

02/03/2010

Spring 2010 MSE503 Seminar

Deepak RajputCenter for Laser ApplicationsUniversity of Tennessee Space

InstituteTullahoma, Tennessee 37388-

9700Email: [email protected] Web: http://drajput.com

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

2 of xx

Outline

Introduction Laser micromachining Femtosecond laser micromachining (FLM) UTSI research Summary

2

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

4 of xx

Introduction

Laser micromachining:Direct writingMask projectionInterference

Direct writing: desired pattern fabricated by translating either the sample or the substrate.Mask projection: A given feature on a mask is illuminated, which is projected on the substrate. Interference: Split the primary beam into two beams, which are superimposed in order to create a pattern. The interference pattern is projected on the substrate and the micromachined pattern corresponds with the intensity profile of the pattern.

4

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

5 of xx

Direct Writing

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

6 of xx

Mask Projection

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

7 of xx

Interference

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

2/sin2

12

cos2)(

l

l

xIxI o

Intensity distribution: 0 to 4Io

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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 of solid or hollow fiber.

Laser Induced Nano Patterning = interference subpatterns generated by microspheres.

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

8

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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 heat transport to the region immediately adjacent to the micromachined region. Laser micromachining is often carried out by using pulsed laser, which delivers high energy at short time scales and minimizes heat flow to surrounding material.

11

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

12 of xx


Types of lasers used: Infrared to UltravioletExcimer lasers: 157, 193, 248, 308, or 351 nm wavelength depending on the composition of the gas in the cavity.Most materials absorb UV wavelengths. Hence, they provide both low machining rates and high machining precision. 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 operating costs and high throughput) because of spot size limitation (50-75 micrometers).

12

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

13 of xx


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

13

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

14 of xx

Absorption Mechanism

Thermal AblationCommonly observed with long wavelength and continuous wave (CW) lasers e.g., CO2 lasers.Absorption of laser energy causes rapid heating, which results in 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 bond energy of the substrate material.Vaporization occurs due to bond-dissociation due to photon absorption.Thermal effects do not play a significant role.

14

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

15 of xx

Factors Affecting Laser Ablation

Laser ablation demonstrates “threshold” behavior in that ablation takes above certain “fluence” level.The “threshold” is a function of laser properties and substrate 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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

16 of xx

Femtosecond Laser Machining (FLM)

Exhibit extremely large peak power values.Laser material interaction in femtosecond lasers is fundamentally different than that in long wavelength lasers.Induces nonlinear effects (e.g., multiphoton absorption).MPA: The simultaneous absorption of two or more photons can provide sufficient energy to cleave strong bonds. As a result, relatively long wavelength lasers with femtosecond pulse widths can be used to machine materials that are otherwise difficult to machine.

16

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

17 of xx

Femtosecond Laser Micromachining

First demonstrated in 1994 by Du et al followed by Pronko et al in 1995 to ablate micrometer sized features.The resolution since then has improved to machine nanometer 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 several photonic devices to a single transparent substrate.

17

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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 large number of electrons that, in turn, transfer energy to the lattice.As a result of the irradiation, the material can undergo a phase or structural modification, leaving behind a localized permanent change in the refractive index or even a void.

Absorption: the absorption of light in a transparent material must be nonlinear because there are no allowed electronic transitions at the energy of the incident photon.

18

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

19 of xx


For such nonlinear absorption to occur, the electric-field strength in the laser pulse must be approximately equal to the electric field that binds the valence electrons in the atoms – of the order of 109 V/m, corresponding to a laser 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 focused to a spot size of 16 micrometers.

19

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

20 of xx


20

Laser-induced optical breakdown

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

21 of xx


The laser pulse transfers energy to the electrons through nonlinear ionization. For pulse durations greater than 10 femtoseconds, the nonlinearly excited electrons are further excited through phonon-mediated linear absorption.When they acquire enough kinetic energy, they can excite other bound electrons – Avalanche ionization. When the density of excited electrons reaches about 1029 /m3, the electrons behave as a plasma with a natural frequency that is resonant with the laser – leading to reflection and absorption of the remaining pulse energy.

21

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

23 of xx


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

Reduces heat affected regionIncreases the precision of the method.

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

23

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

24 of xx

Bulk Damage

If the absorption is purely nonlinear, the laser intensity required to induce a permanent change will depend nonlinearly on the bandgap of the substrate material. Because the bandgap energy varies from material to material, the nonlinear absorption would vary a lot. However, the threshold intensity required to damage a material is found to vary only very slightly with the bandgap energy, indicating the importance of avalanche ionization, which depends linearly on I. Because of this low dependence on the bandgap energy, femtosecond laser micromachining can be used in a broad range of materials.

24

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

25 of xx

Applications

WaveguidesActive devicesFilters and resonatorsPolymerizationNanosurgeryMaterial processingMicrofluidic devicesRapid prototyping

25

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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 800 nm

Piezoelectric nanostage with 200 micrometers range of motion

27

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

29 of xx


29

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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 method DualBeamTM SEM/FIB (CNMS, ORNL)

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

30

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

31 of xx


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

Replication method

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

32 of xx


32

Nano-holes machined with laser pulse energy of 2 μJ

Replication method

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

33 of xx


33

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

34 of xx


34

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

35 of xx


35

DualBeamTM SEM/FIB

Schematics of the DualBeamTM SEM/FIB tool

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

36 of xx


36

DualBeamTM SEM/FIB

Scope image inside the chamber of the tool

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

37 of xx


37

DualBeamTM SEM/FIB

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

39 of xx


39

Nano-hole

#1 #2 #3 #4

AC (μm) 0.7 5 10.7 15

AB (μm) 0.6 3.9 8.3 11.7

The FIB sectioning confirmed that the replication technique does not overestimate the depth of the holes.In fact, the replication technique most probably underestimates the depths. It might be due to the difficulty of the polymer to reach the bottom of the nano-hole and/or distortion of the acetate nano-wires during gold coating.

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

40 of xx


40

DualBeamTM SEM/FIB

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

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

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 numerical aperture with spherical aberration is the key to produce high aspect ratio features.Self-focusing due to Kerr nonlinearity is also expected.The fabrication of high aspect ratio nano-holes demonstrated.

MS

E50

3

Mat

eria

ls S

cien

ce S

emin

ar

S

prin

g 20

10

42 of xx

Thanks !

MSE503 Materials Science Seminar Spring 2010 1 of xx Femtosecond Laser Micromachining 02/03/2010...

Documents

Transcript of MSE503 Materials Science Seminar Spring 2010 1 of xx Femtosecond Laser Micromachining 02/03/2010...