APPLICATIONS OF EXCIMER LASERS

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APPLICATIONS OF EXCIMER LASERS

• Priyanka Milinda Rupasinghe

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Eximer lasers produce wavelengths from 351 nm to
as short as 157 nm in UV region.
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Importance of UV laser light in Industry

• Invention of short wavelength lasers like UV
  lasers started a new era of laser technology
  which offers best performance in industrial micro
  applications.
• Since short wavelength can be focused to a
  smaller spot size than any other source like
  visible or IR light, UV lasers produce enormous
  advantage in micro structuring, marking and
  drilling applications.
• Since diffraction increases linearly with
  wavelength, the minimum imaged feature size that
  can be achieved by any optical system is limited.
  Therefore UV light is the best suitable source in
  achieving the minimum image size than any other
  source.
• The photoablation the physical interaction
  between high energy UV photons and many solid
  materials especially like organics. Therefore UV
  photons can directly break the molecular bonds
  without any damage of surrounding materials.

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This 100-µm gear, micro machined with an Excimer
laser, is shown against the hair on the leg
of a fly.
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• Applications of Excimers
• Cutting and drilling

When the optical wavelength decreases most of the
materials absorb light more strongly, This
special quality of ultraviolet lasers can be used
in cutting applications while the longer
wavelength sources dont.
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Even within the ultraviolet regime, some
materials show remarkable changes in absorption
depth. Following figure presents the absorption
depth of PMMA (polymethylmethacrylate) at three
excimer laser wavelengths.

• The absorption depth of PMMA changes by a factor
  of 100 between the ArF and XeF.
• At longer wavelength 351 nm, PMMA requires a
  high laser power for cutting and the quality of
  cut is poor.
• It requires low power at 193 nm and produces a
  high quality cut.

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The essential processes involved in machining
with pulsed UV laser light and the effect
produced on a work surface by a single pulse.
(a) During the ablation process, a pulse of UV
laser light is focused onto a highly-absorbing
substrate. Vaporization of material in the
illuminated region produces a plume of
ejected molecules and particles. (b) After the
optical pulse, a thin layer of material has been
removed with little damage to unilluminated
adjacent areas.
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Single pulse ablation depth and associated Energy
density for three types of materials
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Ablation (etch) depth as a function of energy
density for ablation of Kapton (polyimide) by a
248 nm KrF excimer laser.
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• 100µm holes drilled in high-density polyethylene
  with
• a twist drill bit
• a KrF laser

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Blind hole in aluminum oxide drilled with a KrF
excimer laser beam with uniform energy
distribution. Diameter of the hole is
approximately 30 microns.
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Electronic packaging

• To produce compact devices such as cellular
  phones, it may be required to have thousands of
  vias (small holes) with diameters as small as 10
  µm in a single circuit board.
• The most suitable method to create these vias is
  using excimer lasers with shorter wavelengths
  like 248 nm or 308 nm.
• However from direct laser ablation method we can
  produce very clean holes in the range of 1- µm in
  few seconds.

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Wire stripping

• Excimer lasers are used to stripe insulation from
  very thin wires which is having a diameter as
  small as 100µm or less.
• As an example very thin wires which used to
  connect the read write heads of computer hard
  disk can be striped using this method.

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Laser stripped insulation from 100µm diameter
pH-sensor wire
Stripping of insulation from miniature cable of
50 micron diameter copper wires using a KrF
excimer laser
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Photomask Technique

• In this technique, a mask (called a reticle)
  containing the pattern to be machined is
  illuminated by a laser light and then images this
  pattern on to the work surface using a optical
  system as shown in figure
• Since individual pulse ablates only a very thin
  layer of material, it must be exposed to numerous
  laser pulses to achieve the desired depth on the
  work surface

• Advantages
• Submicron depth control by controlling the number
  of pulses used.
• High process repeatability.

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Eye Surgery
The excimer laser has brought more accuracy to
corneal surgery and vision correction than ever
before. One pulse of the excimer laser removes
0.25 microns of tissue. To put this into
perspective, a typical human hair is 70 microns
in thickness. Surgeons have never had a device as
accurate as the excimer laser in eye surgery.
Electron Micrograph of a Human Hair Ablated by
the Excimer Laser
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• Human eye with excimer laser radial pattern
• Incision length 3mm
• width 70 microns
• depth 300 microns

Same eye after one week. Diopter change 2D
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THANK YOU