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Spring 2010
Materials Science Seminar
MSE503
Femtosecond Laser Micromachining
02/03/2010
Spring 2010 MSE503 Seminar
Deepak Rajput
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700
Email: [email protected]
Web: http://drajput.com
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Outline
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Introduction
Laser micromachining
Femtosecond laser micromachining (FLM)
UTSI research
Summary
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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
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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.
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Direct Writing
Reference: Journal of Materials Processing Technology, Volume 127, Issue 2, Pages 206-210
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Mask Projection
Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
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Interference
  2x 
I ( x)  2 I o cos
 1

  l

l
2 sin  / 2
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Intensity distribution: 0 to 4Io
Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
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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 =
subpatterns generated by microspheres.
interference
A regular two-dimensional array of microspheres acts as an
array of microlenses.
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Combined Techniques
SNOM arrangement for nanopatterning
Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
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Combined Techniques
Laser-induced surface patterning by means of microspheres
Reference: Appl. Phys. A. 76, 1-3 (2003)
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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.
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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 – 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).
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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
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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.
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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.
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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.
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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.
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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.
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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.
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FLM: Physical Mechanisms
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Laser-induced optical breakdown
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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.
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FLM: Physical Mechanisms
Sub-picosecond: absorption, ionization, and scattering events
Nanosecond: pressure or shock wave propagation
Microsecond: thermal energy propagation
Reference: Gattass RR and Mazur E, Nature Photonics, Vol 2, 219 – 225, 2008
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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.
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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.
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Applications
Waveguides
Active devices
Filters and resonators
Polymerization
Nanosurgery
Material processing
Microfluidic devices
Rapid prototyping
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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.
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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
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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
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Single Pulse Nano-holes
Dependence of nano-hole diameter at the surface on the pulse energy
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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).
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Replication method
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Single Pulse Nano-holes
Nano-holes machined with laser pulse energy of 1.6 μJ
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Replication method
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Single Pulse Nano-holes
Nano-holes machined with laser pulse energy of 2 μJ
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Single Pulse Nano-holes
Dependence of hole depth (by replication) on the pulse energy
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Single Pulse Nano-holes
Dependence of aspect ratio (by replication) on the pulse energy
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DualBeamTM SEM/FIB
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Single Pulse Nano-holes
Schematics of the DualBeamTM SEM/FIB tool
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DualBeamTM SEM/FIB
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Single Pulse Nano-holes
Scope image inside the chamber of the tool
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DualBeamTM SEM/FIB
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Single Pulse Nano-holes
SEM image of the sectioned nano-holes in the trench at zero degree
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DualBeamTM SEM/FIB
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Single Pulse Nano-holes
AB = AC/tan52o
= 0.78 AC
View of the trench after 90o rotation and 25o tilt
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Single Pulse Nano-holes
Nano-hole
AC (μm)
#1
0.7
#2
5
#3
10.7
#4
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 nanowires during gold coating.
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DualBeamTM SEM/FIB
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Single Pulse Nano-holes
SEM image at 52-degree tilt of FIB cross-sectioned nano-hole
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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.
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Thanks !
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