Short Pulse Laser Micro-Machining

A. Weck, T.H.R. Crawford, A. Borowiec, D.S. Wilkinson, H.K. Haugen, J.S. Preston


Motivations and Research outline


Compared to continuous-wave and nanosecond pulse irradiation, ultrashort pulse laser machining of materials typically produces a smaller heat affected zone. Knowledge and understanding of the wall morphology of holes drilled with such lasers is useful in various applications. The research focuses on the systematic examination of holes drilled by ultrafast lasers in copper sheets looking at the following aspects:
  • Effect of number of pulses per hole (up to 30000 pulses)
  • Effect of pulse duration (from 150 fs to 7 ns)
  • Effect of laser wavelength and polarization (400 nm and 800 nm)
  • Effect of drilling evironment (air and rough vacuum)
  • Sub wave-length ripples observation
  • Laser transmission measurements (using a photodiode)
Hole surface and inside wall morphologies were observed in a scanning electron microscope and the intensity going through a hole is recorded on a photodiode during laser drilling.

Experimental procedure


A commercial regeneratively amplified Ti:Sapphire laser system produces pulses which are focussed onto 100 microns thick copper foil, typically with a 5x microscope objective. A spinning half-wave plate (HWP) placed in the beam, rotating ~15 degrees per pulse, can be used to 'scramble' the polarization.

Laser set-up

Close up showing the photodiode under the sample
To observe the inside walls of the holes, samples were cut along the length of the holes using a microtome either parallel or perpendicular to the electric field of the incoming laser beam.

Microtome cut

Laser hole cross section

Results


Effect of number of pulses per hole:


N=100

N=1000

N=10000

N=30000
As the number of pulses N is increased, the wall morphology evolves from a rough 'splattered' structure to periodic ripples. A spinning half-wave plate (~15 degrees between pulses) was placed in the beam to 'scramble' the polarization. The ripple period is approximately 300 nm. With the spinning polarization, the ripples form rings along the length of the hole.

Effect of pulse duration:

Effect of pulse duration of the morphologies observed inside and on the surface of the holes.
Irradiation in air with <35 ps pulses produced ripples on the walls. Irradiation in vacuum produced a droplet-covered surface, with small droplets for <35 ps pulses. Significant debris was produced by the shorter pulses, in the form of a fine foam. Between 10 and 35 ps the nature of the material removal process appears to change. This is time frame is close to the electron-phonon relaxation time.

Effect of laser wavelength and polarization:

Each laser hole drilled with 30000 pulses per hole, a pulse length of ~150 fs, and an energy per pulse of approximately 10 microJ.
Effect of laser wavelength on ripples spacing
  • The period for a wavelength of 800 nm irradiation is ~300 nm.
  • The period for a wavelength of 400 nm irradiation is ~160 nm.
A laser hole drilled using 30000 pulses per hole in air, nu=800 nm, and pulse length of ~10 ps.
Effect of laser polarization on ripples orientation
  • In both cases the ripples run perpendicular to the polarization.
  • When the spinning half-wave plate was used, circular rings were formed along the hole.
Thus, we argue that the ripples are due to interference effects, similar to those predicted by Sipe et. al. (Phys. Rev. B 27, 1141 (1983)).

Effect of drilling environment:


Surface of a hole drilled in vacuum

Surface of a hole drilled in air

Close-up on debris found next to a hole drilled in air
The pictures above show the entrance of a laser hole drilled with 1000 pulses, a pulse duration of 150fs and a pulse energy of 0.89uJ. Significantly more debris remain on the surface for irradiation in air atmosphere, compared to irradiation in a rough vacuum.

Significantly sub-wavelength ripples


Fine ripples at the entrance of a laser hole
Fine ripples observed at the entrance of a laser hole drilled with 30000 pulses, a pulse length of ~30ps, and with a spinning half wave plate. The ripple spacing is approximately 75nm. While substancially sub-wavelength ripple periods have been reported on a variety of materials, we have found only one report of periodic nanostructures on metals in the literature (Yasumaryu et al., Appl. Phys. A81, 933 (2005)).

Laser Transmission Measurements

A large-area photodiode (~1cm2) is placed below the copper sheet to record the transmitted laser signal as a function of time:

  • Up to a pulse length of 1ps, the curves are really similar
  • For 10 and 35ps, the curves are very noisy
  • For 220ps and 7ns, the curves are less noisy
The technique shows gradual widening of the hole with number of pulses and allows the determination of the number of pulses to penetrate the sheet.
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