Download - 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
Outline
•Background & Motivation
•Device Design
•Fabrication
•Characterization
•Conclusion and Future work
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
Spring Plate Mirrors
Dynamic correction using MEMS mirror
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
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)
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
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
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)
Spring plate fabrication
Spring plate fabrication
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
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
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
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
Photoresponse Testing
Photo response test setup
GaAs PIN (before bonding)
Measurement probes
830nm LASER
GaAs PIN sample
Laser
NP
Probes
GaAs PIN Photoresponse
Wafer Fusion Schematic
3-D schematic of sandwiched wafers before fusion
Wafer Fusion
3-D schematic of fixture components
Quartz tube
Graphite fixtures
Graphite shims
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
GaAs PINs on GaP
Schematic of polished and patterned sample
Schematic of polished and etched sample
GaAs PINs on GaP
Wafer bonded interface
GaAs
GaP
After polish & wet etch
GaAs
GaP
GaAs PINs on GaP
Close up
Cross section
GaAs PINs on GaP
Top view
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
Characterization
SU8
PIN
SU8
RV
• External load resistor 200K ohms -------> 2 M ohms
• 2 micron spacing between spring plate and SU-8
Results
A (Dark) B (Light) Difference
• Cascading via external load resistor
• 200KΩ with GaAs PINs
• 1MΩ resistor with bonded PINs
Final Device
Spring plate and thin film resistor
PIN with SU-8 pillars
Device Schematic
Final Testing
On PIN diode
On spring plateTo Interferometer
Work in progress!
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
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