lecture 22 optical mems (4) · optical mems (4) agenda: Êrefractive optical elements –...
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EEL6935 Advanced MEMS 2005 H. Xie 1
Lecture 22Optical MEMS (4)
Agenda:
Refractive Optical Elements– Microlenses– GRIN Lenses– Microprisms
4/4/2005
EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie
Reference: S. Sinzinger and J. Jahns, Chapter 5 in Microoptics, Wiley-VCH, 2003
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Optical Functions and Their Implementation
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Classification of Refractive Optical Elements
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Surface Profile MicrolensesMelted photoresist lenses – reflow lensesMass transportVolume changeLithographically initiated volume growthDispensed or droplet microlensesDirect writing Grey-scale lithography
Gradient-index (GRIN) OpticsGRIN rod lensesPlanar GRIN lenses
Microprisms
Refractive Micro-Optics
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1.1 Melted Photoresist Lenses – Reflow Lenses
1. Surface Profile Microlenses
Fig 5.1
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1.1 Photoresist Reflow Microlenses
Focal length
1crf
n=
−
rc: radius of curvature of the spherical lensn: refractive index of the lens material. n~1.4-1.6 for most polymers.
2
2cylDV tπ =
( )2 / 3sph cV h r hπ= −
Photoresist volumes before and after photoresist reflow:
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( )2
2 2
2c cDr h r − + =
We assume that the photoresist volume does not change during fabrication, i.e., Vcyl = Vsph.. Thus, the thickness is given by
2412 3h ht
D
= +
h
rc
D
rc-h 2 2 / 42c
h Drh
+=
1.1 Photoresist Reflow Microlenses
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Figure 5.2
1.1 Photoresist Reflow Microlenses
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• Surface tension Surface energy: photoresist-air interface; photoresist-substrate interface
• Gravitational energy• Energy balance before and after reflow
Figure 5.3
• Substrate material• Surface treatment• Surface roughness• Facing up or down• Processing temperature• Issue: outer and inner parts
reaches to melting temperature at different times.
1.1 Photoresist Reflow Microlenses
Surface Profile
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•Preshaping
• Long focal length• Aspheric profile
• Melting temperature, processing time• Local heating of just the surface
1.1 Photoresist Reflow Microlenses
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• Pattern Transfer
• Lenses made of substrate such as silicon, fused silica, GaAs, InP
• Anisotropic RIE etching needed
• Equal etching rate for photoresist and substrate
Photoresist
Si
Photoresist
Si
Si
1.1 Photoresist Reflow Microlenses
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1.2 Microlens Formed by Volume Change
• Photosensitive glass (e.g., Fotoform by Corning)• Photocolouration: color change under intense UV illumination• After UV exposure, heated to near melting temperature• Regional crystallization shrinkage local swelling spherical lenses• Typical lens diameters: 400-800µm• Typical numerical aperture: 0.11-0.19
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1.3 Microlens Formed by Volume Growth
• PMMA (polymethyl methacrylate)
• High energy radiation (e.g., UV laser, x-ray, electron or proton beams) breaks polymer chains reduce molecular weight reduced stability
• Exposed to monomer vapor, monomer molecules diffuse into PMMA. The smaller the molecular weight, the more the monomer diffusion
• Different swellings at different regions microlenses
• UV curing
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1.4 Dispensed or Droplet Microlenses
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• AZ4620 photoresist (n=1.62)
• 200ºC for 20 min
• Diameter 300µm
• Focal length: 670 µm
1.5 Microlens Examples -1
C. King, L. Lin and M. Wu, IEEE Photonics Technology Letters, 1996
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• Ring-shape holder
• UV curable polymer droplet
• Manually dispense droplets using micromanipulator
• Surface tension
• Biconvex lens
• Diameter 400µm
• Height: 84 µm
• NA: 0.39
1.5 Microlens Examples -2
Kwon and Lee, MEMS 2002
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• Scratch-drive actuators (SDAs)• 2-D scanning• For 1.55µm wavelength• AZ4620: 11 µm thick• Hotplate: 150ºC for 1min• Diameter 270µm• Focal length: 670 µm
1.5 Microlens Examples -3
Toshiyoshi et al, J. Lightwave Technology, 2003
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1.5 Microlens Examples -4
T.K. Shin et al, IEEE Photonics Technology Letters, 2004
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1.5 Microlens Examples -5
Choo and Muller (UC-Berkeley), Hilton Head Workshop 2004
• 2µm-thick transparent nitride lens holder with 20µm-deep circular well
• Polymer jet printing• Lens diameter: 800µm• Focal length: 2-7mm
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1.5 Microlens Examples -5
Choo and Muller (UC-Berkeley), Hilton Head Workshop 2004
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A. Jain and H. Xie, MEMS 2005
• Maximum displacement of 280 µm achieved
• Actuation Voltage: <10V
1.5 Microlens Examples -6
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• Graded-Index (GRIN) Fiber
2. GRIN Microlenses
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2. GRIN Microlenses
•GRIN Rod Lenses or GRIN Fiber Lenses• SelfocTM
• Input angle may not equal to output angle which depends on the length.
• At half or full cycle, the input and output angles are the same, or focused
• At ¼ or ¾ cycle, the output light rays are parallel, or collimated
• Pitch: The fraction of a full sinusoidal cycle that light goes through before leaving the fiber. For example, a 0.25-pitch lens collimates the input light.
One cycle
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•Planar GRIN Microlenses• PMLTM
2. GRIN Microlenses
• Ion-exchange process: Thermal or field-assisted (electromigration)
• Index change is proportional to the percentage of exchanged ions
• The concentration of exchanged ions changes gradually according to the diffusion process
• Thermal Ion-exchange process• Index change is proportional to the percentage of exchanged ions• The concentration of exchanged ions changes gradually according
to the diffusion process
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3. Microprisms
Challenge:Linear slope with sharp edge
Fabrication Techniques:Deep synchrotron or proton lithographyAnalog lithographyReflow and Mass-transport techniques
Fabricated using analog lithography by E.B. Kley and F. Thoma