Lecture 22 Optical MEMS (4) · Optical MEMS (4) Agenda: ÊRefractive Optical Elements –...

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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