Femtosecond Laser Micromachining

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Femtosecond, Laser, Machining, Micromachining, Device fabrication, Biological devices, Single molecule detection, DNA detection

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Femtosecond Laser Micromachining


Spring 2010 MSE503 Seminar

Deepak Rajput
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700
Email: drajput@utsi.edu
Web: http://drajput.com

> Introduction
> Laser micromachining
> Femtosecond laser micromachining (FLM)
> UTSI research
> Summary
* 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

* 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
Mask Projection
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
Combined Techniques
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 (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).
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., 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.

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 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.

FLM: Physical Mechanisms

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.
FLM: Physical Mechanisms
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.
* 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 SiO2

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

Single Pulse Nano-holes

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
* Replication method: fast, non-destructive, and inexpensive.
* Used a cellulose-based acetate films (35 micrometer).