ieee leos optical mems

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All Optically Driven MEMS Deformable Mirrors via Direct Cascading with Wafer Bonded GaAs/GaP PIN Photodetectors Vaibhav Mathur, Shiva R. Vangala, Xifeng Qian, William D. Goodhue Department of Physics and Applied Physics,University of Massachusetts,Lowell Bahareh Haji-Saeed, Jed Khoury Air Force Research Laboratory/RYHC,Hanscom Air Force Base,MA-01731

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A novel all optically driven MEMS mirror for adaptive optics

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Page 1: IEEE LEOS Optical MEMS

All Optically Driven MEMS Deformable Mirrors via Direct Cascading with

Wafer Bonded GaAs/GaP PIN Photodetectors

Vaibhav Mathur, Shiva R. Vangala, Xifeng Qian, William D. GoodhueDepartment of Physics and Applied Physics,University of Massachusetts,Lowell

Bahareh Haji-Saeed, Jed KhouryAir Force Research Laboratory/RYHC,Hanscom Air Force Base,MA-01731

Page 2: IEEE LEOS Optical MEMS

Outline

•Background & Motivation

•Device Design

•Fabrication

•Characterization

•Conclusion and Future work

Page 3: IEEE LEOS Optical MEMS

Adaptive optics

• Wave front aberration correction

• Spatial light modulators

• Moving MEMS mirrors for dynamic correction

Image Credit: Canada-France-Hawaii Telescope. Starburst galaxy NGC7469

With AO Without AO

Medical Imaging (Human Retina)

Image courtesy Center for Adaptive Optics.

With AO Without AO

image credit: Center for Adaptive Optics

Page 4: IEEE LEOS Optical MEMS

Spring Plate Mirrors

Dynamic correction using MEMS mirror

Page 5: IEEE LEOS Optical MEMS

Motivation

Electrical actuation

Dense array of micro mirrors

• Dense arrays with electrical actuation not practical

• Optically driven actuation can solve this problem

• InGaAs detectors at 1550nm wavelength (earlier work)

• New waferfusion approach using GaAs on GaP

• Spring plate mirror

“All optically driven MEMS deformable device via a photodetector array” J. Khoury et.al. Proc. Of SPIE Vol 6368 636804

Page 6: IEEE LEOS Optical MEMS

Earlier Work

Mylar on GaAs Mylar on InGaAs PIN

Aluminized mylar membranes

• 1mm × 1mm • Slow response

InGaAs based PIN back substrates

• Not suitable for large arrays • Require passivation

“All optically driven MEMS deformable device via a photodetector array” J.Khoury et. al. Proceedings of SPIE Vol. 6368. 636804. (2006)

Page 7: IEEE LEOS Optical MEMS

Device• Optical actuation through a transparent back substrate

• Low stress silicon nitride spring plate mirror

• GaAs PIN detectors

• TaN high value load resistor

V Mirror

V Total

V PIN

V R

2-D Schematic of a single pixelEquivalent circuit

diagram

Page 8: IEEE LEOS Optical MEMS

Working Principle

VTotal = V PIN + V RV Mirror

V Total

V PIN

V R

I-V curve of the load resistor

Equivalent circuit

I-V characteristics of the PIN photodiode

Page 9: IEEE LEOS Optical MEMS

Working Principle

V Total

No light

V R= A

V Total

V R= B

Operation points of the MEMS device Equivalent circuit

• Low actuation voltage desirable for mirrors

• High value load resistance

• High current contrast (ILight - IDark)

Page 10: IEEE LEOS Optical MEMS

Spring plate fabrication

Page 11: IEEE LEOS Optical MEMS

Spring plate fabrication

Page 12: IEEE LEOS Optical MEMS

Spring Plate• PECVD low stress SiN films* (23MPa residual stresses)

• Mechanical characterization

* Indenter Studies

* FEM simulations

• Optical characterization

* Interferometer studies

* FEM simulations

COMSOL snapshot showing voltage actuation

“Stress investigation of PECVD dielectric layers for advanced optical MEMS” A. Tarraf et.al. J.Micromech. Microeng. Vol.14 pg 317-323

Nano indentation results

Page 13: IEEE LEOS Optical MEMS

Interferometry

Michelson interferometer setup

Dark Light

Difference

Interference fringe patterns

5-15 volts required for Mirror actuation

“Patterned multipixel membrane mirror MEMS optically addressed spatial light modulator with megahertz response” G.Griffith IEEE Photonics Technology Letters Vol.19 No.3 Feb 1, 2007

Page 14: IEEE LEOS Optical MEMS

Thin Film Resistor

Thin film resistor testing

• Common materials TaN, Ni-Cr etc

• Ta (Tantalum) sputtered in presence of Nitrogen

• Sheet resistance upto 1KOhm/□

• Upto 2 MOhms resistors patterned

Patterned resistors

Page 15: IEEE LEOS Optical MEMS

GaAs PIN Diodes

• HIgh breakdown voltage 32 volts

• Photo response of upto 80-90µA

• Photo current increases linearly with laser power

• Peak efficiency at 800-890 nm*

0.6 µm

2 µm

1 µm

PI

N

GaAs

≈ E+18

≈ E+15

≈ E+18

300

MBE growth structure of GaAs PINs

“Photoconductive optically driven deformable membrane for spatial light modulator applications utilizing GaAs substrates” B.Haji-Saeed et. al. App. Opt. Vol.45, No. 12, 20th April 2006

Mesa etchSchematic of ohmic contacts

Page 16: IEEE LEOS Optical MEMS

Photoresponse Testing

Photo response test setup

GaAs PIN (before bonding)

Measurement probes

830nm LASER

GaAs PIN sample

Laser

NP

Probes

Page 17: IEEE LEOS Optical MEMS

GaAs PIN Photoresponse

Page 18: IEEE LEOS Optical MEMS

Wafer Fusion Schematic

3-D schematic of sandwiched wafers before fusion

Page 19: IEEE LEOS Optical MEMS

Wafer Fusion

3-D schematic of fixture components

Quartz tube

Graphite fixtures

Graphite shims

Page 20: IEEE LEOS Optical MEMS

Wafer Fusion

FEM simulation of the thermal stresses during bonding

Wafer bonding fixture

• Custom made fixture and furnace

• 100-200 MPa force on sample

• 650-700°C bonding temperature

Wafer fusion furnace

Page 21: IEEE LEOS Optical MEMS

GaAs PINs on GaP

Schematic of polished and patterned sample

Schematic of polished and etched sample

Page 22: IEEE LEOS Optical MEMS

GaAs PINs on GaP

Wafer bonded interface

GaAs

GaP

After polish & wet etch

GaAs

GaP

Page 23: IEEE LEOS Optical MEMS

GaAs PINs on GaP

Close up

Cross section

Page 24: IEEE LEOS Optical MEMS

GaAs PINs on GaP

Top view

Page 25: IEEE LEOS Optical MEMS

Characterization

Probe

Photo response test setup

• Back illumination through GaP bonded samples

• Photo response goes down to 10-20 μA

CCD image of single PIN back illumination

Page 26: IEEE LEOS Optical MEMS

Characterization

SU8

PIN

SU8

RV

• External load resistor 200K ohms -------> 2 M ohms

• 2 micron spacing between spring plate and SU-8

Page 27: IEEE LEOS Optical MEMS

Results

A (Dark) B (Light) Difference

• Cascading via external load resistor

• 200KΩ with GaAs PINs

• 1MΩ resistor with bonded PINs

Page 28: IEEE LEOS Optical MEMS

Final Device

Spring plate and thin film resistor

PIN with SU-8 pillars

Page 29: IEEE LEOS Optical MEMS

Device Schematic

Page 30: IEEE LEOS Optical MEMS

Final Testing

On PIN diode

On spring plateTo Interferometer

Work in progress!

Page 31: IEEE LEOS Optical MEMS

Conclusion

• GaAs PINs wafer bonded on GaP successfully

• Si3N4 micro mirrors actuated via external load resistor to bonded PINs sample

• Final MEMS device fabricated and currently under testing

Future work :

• Reduce thin film resistor feature size

•Large array device using conductive SU-8

• Incorporate microlens array

Page 32: IEEE LEOS Optical MEMS

Acknowledgement• Funded by United states Air Force under

contract # FA8718-05-C0081

• Fellow grad students, Vikram Singh Prasher and Kevin Anglin

www. uml.edu/photonics