FT2224 Applied Optics

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FT222/4 Applied Optics

• Module 6
• Micro-optics
• Refractive Lenses

M Kufner, S Kufner,Micro-Optics and Lithography
(VUB Press 1997)
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Basics of Lithography
Spin coating Form even coating of known
thickness
Mask Fabrication Design and reduction. Usually
metal (eg Cr) on clear substrate. Alternative is
direct e-beam writing for smallest feature size.
Exposure Short wavelength light for small
feature size.
Develop and remove resist.
Etching Direct or Lift-off Wet etch Simple but
difficult to control Dry etch e.g. Reactive Ion
Etch. good vertical.
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Etching
Wet etch Cheap, fast, difficult to control,
limited accuracy, isotropic
Dry etch Accurate, slower, expensive equipment
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Lift-off
When resist on its own is not strong enough to
resist the etch process e.g. RIE or salt
diffusion, a metal mask is geerated.
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Lenses. Bulk -gt Micro-optical.
For an optical system scaled down by M
lt1 Lens diameter D M.D Focal Length
f M.f f number f/ f/D invariant Resolution
l. invariant Depth of focus 4 l.
f/2 invariant Length M.Length Volume M
3 Vol Weight a Volume M3 Weight
Example Spherical Lens 1/f (n-1)1/r1 1/
r2 gt f r/2(n-1) r1 r2 r
Diam/2 f/ f/D 1/4(n-1) for n1.5,
f/ .5
Lower limits Diffraction (aperture gt l )
fabrication ( gt l )
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Micro-optics in Photosensitive Glass

• Special glass sensitive to UV irradiation.
• SiO2 (80) Li2O, Na2O, K2O,
• Al2O3, ZnO, Sb2O3, Ag .
• Lens Formation
• Irradiation causes densificaton of exposed
  regions
• Thermal treatment (gt600 C)
• Unexposed regions shrink in all three dimensions
  (and become opaque)
• Exposed regions crystalise and densify.
• For dimensions lt1mm, exposed region swells
  upwards as surrounding areas contract
  isotropically, producing a spherical surface.

Additional effect Irradiated regions are x20 to
x40 times easier to etch Can use to produce
holes or other structures.
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Monomer Diffusion Deep Proton Lith or Excimer
Laser
PMMA Polymer chain sensitive to breakage by deep
UV, X-Ray, E-beam, or Proton beam.
Irradiated regions show decrease in molecular
weight.
When exposed to hot monomer vapour (styrene),
monomer diffuses in and causes local swelling.
(Typically 85-95 C, 45 min)
Swelling fixed (hardened) by polymerisation -
expose to (lower levels) of UV.
Irradiated regions may be preferentially etched
to create other features
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Melting and Surface Tension
Cylinders of photoresist of specific proportion
are created by lithography, typically 5 60 mm
thick on a transparent substrate.
The resist is then melted (140 C) and surface
tension pulls the molten photoresist into a lens
shape.
Lens may be used directly, but resist is soft,
sensitive to moisture, and difficult to coat.
More robust lenses can be made by transferring
the pattern into the substrate by etching (but
with a loss in profile which must be allowed for).
Typical diameters 20-500 mm. Typical F-number
1-2.5
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Gradient Index Lenses
Local radial variation of refractive index due to
in-diffusion of a dopant into the host.
Protective metal mask is laid on the substrate.
(Usually glass).Circular openings are created in
the metal mask.
The substrate is placed in a bath of molten salt
(typically 10s of hours)
Ion exchange in the exposed regions diffuse
isotropically into the substrate, giving a
complex (often exponential) dopant profile.
This leads to a small varying refractive index
profile, (in an still flat structure.)
In glass usually Na ions are replaced by Ti (max
Dn.1), Ag (max Dn.08), or K (max Dn.02). May
also be done in LiNbO3, or some polymers.
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The profile may be altered by the addition of an
electric field to drive the diffusing ions deeper
into the substrate (so called field assisted
diffusion). These techniques are also used for
forming planar wave-guide structures.
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GRIN RODs (SELFOC)
Long rod shape having radial profile. Possibly
formed by ion diffusion inside a hollow glass
tube which is then melted and drawn into a solid
rod.?? Diameter lt 4 mm.
All points across the surface are imaged
Pioneered by Nippon Sheet Glass http//www.nsgame
rica.com/
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