femtosecond laser micromachining - deepak rajput
TRANSCRIPT
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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
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Outline
IntroductionLaser micromachiningFemtosecond laser micromachining (FLM)UTSI researchSummary
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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 toolsAerospace MicroelectronicsBiological devices
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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
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Direct Writing
Reference: Journal of Materials Processing Technology, Volume 127, Issue 2, Pages 206-210 5
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Mask Projection
Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008) 6
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Interference
Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
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Intensity distribution: 0 to 4Io
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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 ofsolid or hollow fiber.
Laser Induced Nano Patterning = interferencesubpatterns generated by microspheres.
A regular two-dimensional array of microspheres acts as anarray 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) 9
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Combined Techniques
Reference: Appl. Phys. A. 76, 1-3 (2003)
Laser-induced surface patterning by means of microspheres
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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.
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Laser Micromachining
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).
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Laser Micromachining
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
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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.
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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.
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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.
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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.
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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 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.
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FLM: Physical Mechanisms
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.
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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 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.
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FLM: Physical Mechanisms
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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
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FLM: Physical Mechanisms
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.
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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.
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Applications
WaveguidesActive devicesFilters and resonatorsPolymerizationNanosurgeryMaterial processingMicrofluidic devicesRapid 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 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
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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
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Single Pulse Nano-holes
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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 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).
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Single Pulse Nano-holes
31Nano-holes machined with laser pulse energy of 1.6 μJ
Replication method
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Single Pulse Nano-holes
32Nano-holes machined with laser pulse energy of 2 μJ
Replication method
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Single Pulse Nano-holes
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Dependence of hole depth (by replication) on the pulse energy
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Single Pulse Nano-holes
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Dependence of aspect ratio (by replication) on the pulse energy
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Single Pulse Nano-holes
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DualBeamTM SEM/FIB
Schematics of the DualBeamTM SEM/FIB tool
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Single Pulse Nano-holes
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DualBeamTM SEM/FIB
Scope image inside the chamber of the tool
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Single Pulse Nano-holes
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DualBeamTM SEM/FIB
SEM image of the sectioned nano-holes in the trench at zero degree
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Single Pulse Nano-holes
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DualBeamTM SEM/FIB
View of the trench after 90o rotation and 25o tilt
AB = AC/tan52o
= 0.78 AC
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Single Pulse Nano-holes
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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.
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Single Pulse Nano-holes
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DualBeamTM SEM/FIB
SEM image at 52-degree tilt of FIB cross-sectioned nano-hole
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Summary
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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.
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Thanks !