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.

Please visit http://drajput.com.

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Femtosecond Machining

  1. 1. 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
  2. 2. Outline <ul><li>Introduction </li></ul><ul><li>Laser micromachining </li></ul><ul><li>Femtosecond laser micromachining (FLM) </li></ul><ul><li>UTSI research </li></ul><ul><li>Summary </li></ul>
  3. 3. Introduction <ul><li>Laser: Theodore Maiman (1960) </li></ul><ul><li>Laser micromachining: cutting, drilling, welding, or other modification in order to achieve small features. </li></ul><ul><li>Laser micromachining of materials: </li></ul><ul><ul><li>Automotive and machine tools </li></ul></ul><ul><ul><li>Aerospace </li></ul></ul><ul><ul><li>Microelectronics </li></ul></ul><ul><ul><li>Biological devices </li></ul></ul>
  4. 4. Introduction <ul><li>Laser micromachining: </li></ul><ul><ul><li>Direct writing </li></ul></ul><ul><ul><li>Mask projection </li></ul></ul><ul><ul><li>Interference </li></ul></ul><ul><li>Direct writing: desired pattern fabricated by translating either the sample or the substrate. </li></ul><ul><li>Mask projection: A given feature on a mask is illuminated, which is projected on the substrate. </li></ul><ul><li>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. </li></ul>
  5. 5. Direct Writing Reference: Journal of Materials Processing Technology, Volume 127, Issue 2, Pages 206-210
  6. 6. Mask Projection Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
  7. 7. Interference Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008) Intensity distribution: 0 to 4I o
  8. 8. Combined Techniques <ul><li>Scanning Near-field Optical Microscopy (SNOM) + Atomic Force Microscopy (AFM) = ablation + etching </li></ul><ul><ul><li>The setup involves the coupling of the laser light into the tip of solid or hollow fiber. </li></ul></ul><ul><li>Laser Induced Nano Patterning = interference subpatterns generated by microspheres. </li></ul><ul><ul><li>A regular two-dimensional array of microspheres acts as an array of microlenses. </li></ul></ul>
  9. 9. Combined Techniques SNOM arrangement for nanopatterning Reference: Dahotre and Harimkar, Laser Fabrication and Machining of Materials (New York: Springer 2008)
  10. 10. Combined Techniques Reference: Appl. Phys. A. 76, 1-3 (2003) Laser-induced surface patterning by means of microspheres
  11. 11. Laser Micromachining <ul><li>Laser beam: </li></ul><ul><ul><li>Continuous wave mode (CW) </li></ul></ul><ul><ul><li>Pulsed mode </li></ul></ul><ul><li>CW: output constant with time </li></ul><ul><li>Pulsed: output is concentrated in small pulses </li></ul><ul><li>Laser micromachining requirement: minimize the heat transport to the region immediately adjacent to the micromachined region. </li></ul><ul><li>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. </li></ul>
  12. 12. Laser Micromachining <ul><li>Types of lasers used: Infrared to Ultraviolet </li></ul><ul><li>Excimer lasers: 157, 193, 248, 308, or 351 nm wavelength depending on the composition of the gas in the cavity. </li></ul><ul><li>Most materials absorb UV wavelengths. Hence, they provide both low machining rates and high machining precision. </li></ul><ul><li>Diode-pumped solid state (DPSS) lasers – Nd:YAG </li></ul><ul><ul><li>DPSS: 355 nm (3 rd harmonic) and 266 nm (4 th harmonic) </li></ul></ul><ul><li>Ti:sapphire solid state lasers (700 nm – 1100 nm) </li></ul><ul><li>CO 2 gas lasers (10,600 nm): limited roles (low operating costs and high throughput) because of spot size limitation (50-75 micrometers). </li></ul>
  13. 13. Laser Micromachining <ul><li>Laser-material interaction leading to ablation. </li></ul><ul><li>Material removal occurs when the absorbed energy is more than the binding energy of the substrate material. </li></ul><ul><li>Energy transfer mechanism depends on material properties and laser properties. </li></ul><ul><li>Absorption: Thermal or/and Photochemical processes </li></ul>
  14. 14. Absorption Mechanism <ul><li>Thermal Ablation </li></ul><ul><ul><li>Commonly observed with long wavelength and continuous wave (CW) lasers e.g., CO 2 lasers. </li></ul></ul><ul><ul><li>Absorption of laser energy causes rapid heating, which results in melting and/or vaporization of the material. </li></ul></ul><ul><ul><li>May be associated with a large heat-affected zone. </li></ul></ul><ul><li>Photochemical Ablation </li></ul><ul><ul><li>Commonly observed with short wavelength and pulsed lasers. </li></ul></ul><ul><ul><li>Occurs when the laser photon energy is greater than the bond energy of the substrate material. </li></ul></ul><ul><ul><li>Vaporization occurs due to bond-dissociation due to photon absorption. </li></ul></ul><ul><ul><li>Thermal effects do not play a significant role. </li></ul></ul>
  15. 15. Factors Affecting Laser Ablation <ul><li>Laser ablation demonstrates “ threshold ” behavior in that ablation takes above certain “fluence” level. </li></ul><ul><li>The “threshold” is a function of laser properties and substrate material properties. </li></ul><ul><li>Laser properties: laser fluence, wavelength, peak power. </li></ul><ul><li>Material properties: optical (absorption) and thermal (diffusivity) properties. </li></ul><ul><li>Pulse duration affects the heat-affected zone. </li></ul>
  16. 16. Femtosecond Laser Machining (FLM) <ul><li>Exhibit extremely large peak power values. </li></ul><ul><li>Laser material interaction in femtosecond lasers is fundamentally different than that in long wavelength lasers. </li></ul><ul><li>Induces nonlinear effects (e.g., multiphoton absorption ). </li></ul><ul><li>MPA: The simultaneous absorption of two or more photons can provide sufficient energy to cleave strong bonds. </li></ul><ul><li>As a result, relatively long wavelength lasers with femtosecond pulse widths can be used to machine materials that are otherwise difficult to machine. </li></ul>
  17. 17. Femtosecond Laser Micromachining <ul><li>First demonstrated in 1994 by Du et al followed by Pronko et al in 1995 to ablate micrometer sized features. </li></ul><ul><li>The resolution since then has improved to machine nanometer sized features. </li></ul><ul><li>Advantages of femtosecond laser micromachining (FLM): </li></ul><ul><ul><li>The nonlinear absorption induces changes to the focal volume. </li></ul></ul><ul><ul><li>The absorption process is independent of the material. </li></ul></ul><ul><ul><li>Fabrication of an optical motherboard by bonding several photonic devices to a single transparent substrate. </li></ul></ul>
  18. 18. FLM: Physical Mechanisms <ul><li>Results from laser-induced optical breakdown . </li></ul><ul><li>Laser-induced optical breakdown: </li></ul><ul><ul><li>Transfer of optical energy to the material by ionizing a large number of electrons that, in turn, transfer energy to the lattice. </li></ul></ul><ul><ul><li>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. </li></ul></ul><ul><li>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. </li></ul>
  19. 19. FLM: Physical Mechanisms <ul><li>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 . </li></ul><ul><li>To achieve such electric-field strengths with a laser pulse, high intensities and tight focusing are required. </li></ul><ul><li>Example: a 1-microJoule, 100 femtosecond pulse focused to a spot size of 16 micrometers. </li></ul>
  20. 20. FLM: Physical Mechanisms Laser-induced optical breakdown
  21. 21. FLM: Physical Mechanisms <ul><li>The laser pulse transfers energy to the electrons through nonlinear ionization. </li></ul><ul><li>For pulse durations greater than 10 femtoseconds, the nonlinearly excited electrons are further excited through phonon-mediated linear absorption . </li></ul><ul><li>When they acquire enough kinetic energy, they can excite other bound electrons – Avalanche ionization . </li></ul><ul><li>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. </li></ul>
  22. 22. 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
  23. 23. FLM: Physical Mechanisms <ul><li>For pulses of subpicosecond duration, the timescale over which the electrons are excited is smaller than the electron-phonon scattering time (about 1 picosecond). </li></ul><ul><li>Thus, a femtosecond laser pulse ends before the electrons thermally excite any ions. </li></ul><ul><ul><li>Reduces heat affected region </li></ul></ul><ul><ul><li>Increases the precision of the method. </li></ul></ul><ul><li>FLM: deterministic process because no defect electrons are needed to seed the absorption process. </li></ul><ul><li>The confinement and repeatability of the nonlinear excitation make it possible for practical purposes. </li></ul>
  24. 24. Bulk Damage <ul><li>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. </li></ul><ul><li>Because the bandgap energy varies from material to material, the nonlinear absorption would vary a lot. </li></ul><ul><li>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. </li></ul><ul><li>Because of this low dependence on the bandgap energy , femtosecond laser micromachining can be used in a broad range of materials. </li></ul>
  25. 25. Applications <ul><li>Waveguides </li></ul><ul><li>Active devices </li></ul><ul><li>Filters and resonators </li></ul><ul><li>Polymerization </li></ul><ul><li>Nanosurgery </li></ul><ul><li>Material processing </li></ul><ul><li>Microfluidic devices </li></ul><ul><li>Rapid prototyping </li></ul>
  26. 26. FLM at the UT Space Institute <ul><li>Single-pulse ultrafast-laser machining of high aspect nano-holes at the surface of SiO 2 </li></ul><ul><li>Volume 16, No. 19, Optics Express , PP 14411 </li></ul><ul><li>White Y., Li X., Sikorski Z., Davis L.M., Hofmeister W. </li></ul>
  27. 27. FLM at the UT Space Institute <ul><li>Experimental Set-up </li></ul><ul><li>Ti-sapphire laser: </li></ul><ul><ul><li>Center wavelength: 800 nm </li></ul></ul><ul><ul><li>Repetition rate: 250 kHz </li></ul></ul><ul><ul><li>Pulse width: 200 femtosecond (FWHM) </li></ul></ul><ul><ul><li>Average power of 1 W. </li></ul></ul><ul><li>Objective lens (dry): </li></ul><ul><ul><li>Numerical Aperture: 0.85 </li></ul></ul><ul><ul><li>Working distance: 0.41 - 0.45 mm </li></ul></ul><ul><ul><li>Correction collar to adjust for spherical aberration </li></ul></ul><ul><li>Fused silica (200 micrometers) of refractive index 1.453 at 800 nm </li></ul><ul><li>Piezoelectric nanostage with 200 micrometers range of motion </li></ul>
  28. 28. 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
  29. 29. Single Pulse Nano-holes Dependence of nano-hole diameter at the surface on the pulse energy
  30. 30. Single Pulse Nano-holes <ul><li>Depth analysis </li></ul><ul><li>Conventional technique: Atomic Force Microscopy </li></ul><ul><li>Problems in obtain signal from the bottom of a nanometer sized, high-aspect ratio feature. </li></ul><ul><li>Techniques used: </li></ul><ul><ul><li>Replication method </li></ul></ul><ul><ul><li>DualBeam TM SEM/FIB (CNMS, ORNL) </li></ul></ul><ul><li>Replication method: fast, non-destructive, and inexpensive. </li></ul><ul><li>Used a cellulose-based acetate films (35 micrometer). </li></ul>
  31. 31. Single Pulse Nano-holes Nano-holes machined with laser pulse energy of 1.6 μ J Replication method
  32. 32. Single Pulse Nano-holes Nano-holes machined with laser pulse energy of 2 μ J Replication method
  33. 33. Single Pulse Nano-holes Dependence of hole depth (by replication) on the pulse energy
  34. 34. Single Pulse Nano-holes Dependence of aspect ratio (by replication) on the pulse energy
  35. 35. Single Pulse Nano-holes DualBeam TM SEM/FIB Schematics of the DualBeam TM SEM/FIB tool
  36. 36. Single Pulse Nano-holes DualBeam TM SEM/FIB Scope image inside the chamber of the tool
  37. 37. Single Pulse Nano-holes DualBeam TM SEM/FIB SEM image of the sectioned nano-holes in the trench at zero degree
  38. 38. 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
  39. 39. Single Pulse Nano-holes <ul><li>The FIB sectioning confirmed that the replication technique does not overestimate the depth of the holes. </li></ul><ul><li>In fact, the replication technique most probably underestimates the depths. </li></ul><ul><li>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. </li></ul>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
  40. 40. Single Pulse Nano-holes DualBeam TM SEM/FIB SEM image at 52-degree tilt of FIB cross-sectioned nano-hole
  41. 41. Summary <ul><li>Femtosecond lasers enable direct writing of nanoscale features. </li></ul><ul><li>FLM can be used to fabricate fluidic and photonic components </li></ul><ul><li>Focusing the femtosecond laser pulse with a high numerical aperture with spherical aberration is the key to produce high aspect ratio features. </li></ul><ul><li>Self-focusing due to Kerr nonlinearity is also expected. </li></ul><ul><li>The fabrication of high aspect ratio nano-holes demonstrated. </li></ul>
  42. 42. <ul><li>Thanks ! </li></ul>