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Femtosecond Laser Micromachining 02/03/2010 Spring 2010 MSE503 Seminar

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Femtosecond Laser Micromachining 02/03/2010 ... (1960) Laser micromachining: cutting, drilling, welding, or other modification in order to achieve small features. – PowerPoint PPT presentation

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Title: Femtosecond Laser Micromachining 02/03/2010 Spring 2010 MSE503 Seminar


1
Femtosecond Laser Micromachining02/03/2010
Spring 2010 MSE503 Seminar
  • Deepak Rajput
  • Center for Laser Applications
  • University of Tennessee Space Institute
  • Tullahoma, Tennessee 37388-9700
  • Email drajput_at_utsi.edu
  • Web http//drajput.com

2
Outline
  • Introduction
  • Laser micromachining
  • Femtosecond laser micromachining (FLM)
  • UTSI research
  • Summary

2
3
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 tools
  • Aerospace
  • Microelectronics
  • Biological devices

3
4
Introduction
  • Laser micromachining
  • Direct writing
  • Mask projection
  • Interference
  • 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
5
Direct Writing
Reference Journal of Materials Processing
Technology, Volume 127, Issue 2, Pages 206-210
5
6
Mask Projection
Reference Dahotre and Harimkar, Laser
Fabrication and Machining of Materials (New York
Springer 2008)
6
7
Interference
Intensity distribution 0 to 4Io
Reference Dahotre and Harimkar, Laser
Fabrication and Machining of Materials (New York
Springer 2008)
8
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
9
Combined Techniques
SNOM arrangement for nanopatterning
Reference Dahotre and Harimkar, Laser
Fabrication and Machining of Materials (New York
Springer 2008)
9
10
Combined Techniques
Laser-induced surface patterning by means of
microspheres
Reference Appl. Phys. A. 76, 1-3 (2003)
10
11
Laser Micromachining
  • Laser beam
  • Continuous wave mode (CW)
  • Pulsed mode
  • CW output constant with time
  • Pulsed output is concentrated in small pulses
  • Laser 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
12
Laser Micromachining
  • Types of lasers used Infrared to Ultraviolet
  • Excimer 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 NdYAG
  • DPSS 355 nm (3rd harmonic) and 266 nm (4th
    harmonic)
  • Tisapphire 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
13
Laser Micromachining
  • 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
14
Absorption Mechanism
  • Thermal Ablation
  • Commonly 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 Ablation
  • Commonly 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
15
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
16
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
17
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
18
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
19
FLM Physical Mechanisms
  • 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
20
FLM Physical Mechanisms

Laser-induced optical breakdown
20
21
FLM Physical Mechanisms
  • 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
22
FLM Physical Mechanisms
Sub-picosecond absorption, ionization, and
scattering events Nanosecond pressure or shock
wave propagation Microsecond thermal energy
propagation
22
Reference Gattass RR and Mazur E, Nature
Photonics, Vol 2, 219 225, 2008
23
FLM Physical Mechanisms
  • 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 region
  • Increases 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
24
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
25
Applications
  • Waveguides
  • Active devices
  • Filters and resonators
  • Polymerization
  • Nanosurgery
  • Material processing
  • Microfluidic devices
  • Rapid prototyping

25
26
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
27
FLM at the UT Space Institute
  • Experimental Set-up
  • Ti-sapphire laser
  • Center wavelength 800 nm
  • Repetition rate 250 kHz
  • Pulse width 200 femtosecond (FWHM)
  • Average power of 1 W.
  • Objective lens (dry)
  • Numerical Aperture 0.85
  • Working distance 0.41 - 0.45 mm
  • Correction 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
28
Single Pulse Nano-holes

1.2 µJ
1.6 µJ
2.4 µJ
1.2 µJ
Nano-holes machined by single laser pulses at
different energies
28
29
Single Pulse Nano-holes

Dependence of nano-hole diameter at the surface
on the pulse energy
29
30
Single Pulse Nano-holes
  • Depth analysis
  • Conventional technique Atomic Force Microscopy
  • Problems 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
31
Single Pulse Nano-holes

Replication method
Nano-holes machined with laser pulse energy of
1.6 µJ
31
32
Single Pulse Nano-holes

Replication method
Nano-holes machined with laser pulse energy of 2
µJ
32
33
Single Pulse Nano-holes

Dependence of hole depth (by replication) on the
pulse energy
33
34
Single Pulse Nano-holes

Dependence of aspect ratio (by replication) on
the pulse energy
34
35
Single Pulse Nano-holes

DualBeamTM SEM/FIB
Schematics of the DualBeamTM SEM/FIB tool
35
36
Single Pulse Nano-holes

DualBeamTM SEM/FIB
Scope image inside the chamber of the tool
36
37
Single Pulse Nano-holes

DualBeamTM SEM/FIB
SEM image of the sectioned nano-holes in the
trench at zero degree
37
38
Single Pulse Nano-holes

DualBeamTM SEM/FIB
AB AC/tan52o 0.78 AC
View of the trench after 90o rotation and 25o tilt
38
39
Single Pulse Nano-holes

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.

39
40
Single Pulse Nano-holes

DualBeamTM SEM/FIB
SEM image at 52-degree tilt of FIB
cross-sectioned nano-hole
40
41
Summary
  • Femtosecond lasers enable direct writing of
    nanoscale features.
  • FLM can be used to fabricate fluidic and photonic
    components
  • Focusing 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.

41
42
  • Thanks !
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