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Femtosecond Machining
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Femtosecond Machining

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This presentation was made as a seminar requirement by Deepak Rajput at the University of Tennessee Space Institute, Tullahoma, Tennessee, USA in spring 2010. ...

This presentation was made as a seminar requirement by Deepak Rajput at the University of Tennessee Space Institute, Tullahoma, Tennessee, USA in spring 2010.

Please visit http://drajput.com.

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Femtosecond Machining Femtosecond Machining Presentation Transcript

  • 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_address] Web: http://drajput.com
  • Outline
    • Introduction
    • Laser micromachining
    • Femtosecond laser micromachining (FLM)
    • UTSI research
    • Summary
  • 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
  • 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.
  • Direct Writing Reference: Journal of Materials Processing Technology, Volume 127, Issue 2, Pages 206-210
  • Mask Projection Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
  • Interference Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008) Intensity distribution: 0 to 4I o
  • 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.
  • Combined Techniques SNOM arrangement for nanopatterning Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
  • Combined Techniques Reference: Appl. Phys. A. 76, 1-3 (2003) Laser-induced surface patterning by means of microspheres
  • 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.
  • 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 (3 rd harmonic) and 266 nm (4 th harmonic)
    • Ti:sapphire solid state lasers (700 nm – 1100 nm)
    • CO 2 gas lasers (10,600 nm): limited roles (low operating costs and high throughput) because of spot size limitation (50-75 micrometers).
  • 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
  • Absorption Mechanism
    • Thermal Ablation
      • Commonly observed with long wavelength and continuous wave (CW) lasers e.g., CO 2 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 10 9 V/m, corresponding to a laser intensity of 5 x 10 20 W/m 2 .
    • 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.
  • FLM: Physical Mechanisms Laser-induced optical breakdown
  • 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 10 29 /m 3 , 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.
  • 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
  • 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.
  • 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.
  • Applications
    • Waveguides
    • Active devices
    • Filters and resonators
    • Polymerization
    • Nanosurgery
    • Material processing
    • Microfluidic devices
    • Rapid prototyping
  • FLM at the UT Space Institute
    • Single-pulse ultrafast-laser machining of high aspect nano-holes at the surface of SiO 2
    • Volume 16, No. 19, Optics Express , PP 14411
    • White Y., Li X., Sikorski Z., Davis L.M., Hofmeister W.
  • 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
  • Single Pulse Nano-holes Nano-holes machined by single laser pulses at different energies 1.2 μ J 1.6 μ J 2.4 μ J 1.2 μ J
  • Single Pulse Nano-holes Dependence of nano-hole diameter at the surface on the pulse energy
  • 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
      • DualBeam TM SEM/FIB (CNMS, ORNL)
    • Replication method: fast, non-destructive, and inexpensive.
    • Used a cellulose-based acetate films (35 micrometer).
  • Single Pulse Nano-holes Nano-holes machined with laser pulse energy of 1.6 μ J Replication method
  • Single Pulse Nano-holes Nano-holes machined with laser pulse energy of 2 μ J Replication method
  • Single Pulse Nano-holes Dependence of hole depth (by replication) on the pulse energy
  • Single Pulse Nano-holes Dependence of aspect ratio (by replication) on the pulse energy
  • Single Pulse Nano-holes DualBeam TM SEM/FIB Schematics of the DualBeam TM SEM/FIB tool
  • Single Pulse Nano-holes DualBeam TM SEM/FIB Scope image inside the chamber of the tool
  • Single Pulse Nano-holes DualBeam TM SEM/FIB SEM image of the sectioned nano-holes in the trench at zero degree
  • Single Pulse Nano-holes DualBeam TM SEM/FIB View of the trench after 90 o rotation and 25 o tilt AB = AC/tan52 o = 0.78 AC
  • Single Pulse Nano-holes
    • 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.
    11.7 8.3 3.9 0.6 AB ( μ m) 15 10.7 5 0.7 AC ( μ m) #4 #3 #2 #1 Nano-hole
  • Single Pulse Nano-holes DualBeam TM SEM/FIB SEM image at 52-degree tilt of FIB cross-sectioned nano-hole
  • 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.
    • Thanks !