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IEEE LEOS Optical MEMS

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A novel all optically driven MEMS mirror for adaptive optics

A novel all optically driven MEMS mirror for adaptive optics


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  • Mention name, photonics center, umass lowell advisor William Goodhue Speak the title
  • Our group active in adaptive optics MEMS, device evolved New design, working priciple, optical Fabrication process, particularly interesting is the bonding Optical setup for I-V characteristics Finally, summarize
  • Adaptive optics very important in fields like astronomy , LASER communication systems to remove atmospheric aberrations working principle : measures wave front distortion, and compensating for them using spatial phase modulators since these aberrations are constantly changing , dynamic correction using MEMS mirrors
  • Adaptive optics very important in fields like astronomy , LASER communication systems to remove atmospheric aberrations working principle : measures wave front distortion, and compensating for them using spatial phase modulators since these aberrations are constantly changing , dynamic correction using MEMS mirrors
  • Earlier work used mylar sheets, then moved to spring plates, electrical actuation All optical technique is much better for such devices as the arrays become smaller in dimensions and denser Earlier work with InGaAs detectors , 1550nm Now trying GaAs PINs
  • Adaptive optics very important in fields like astronomy , LASER communication systems to remove atmospheric aberrations working principle : measures wave front distortion, and compensating for them using spatial phase modulators since these aberrations are constantly changing , dynamic correction using MEMS mirrors
  • Incoming beam incident on the MEMS mirror Moving spring plate mirror made of SiN for correction Parallel plate capacitor Back illumination PIN diode Causes drop across thin film resistor made from TaN
  • Idea is to get a voltage contrast across the spring plate Use a voltage source, or a fixed load resistor with current contrast The current contrast obtained from PIN detector (Light minus dark) Total voltage is sum of two, so easier to visualize on the same graph
  • When no light is shining, its on point A When light shines, moves to point B So the following are desirable for optimum operation: High load resistance Acutation voltage as low as possible Lowest amount to dark current, and maximum current when light shines
  • Simple interferometer setup Have a good understanding of the spring plate movement
  • Simple interferometer setup Have a good understanding of the spring plate movement
  • Mixed frequency results in lower stresses, which in turn means low actuation voltages InP etched with HCl Mechanical characterization done using comsol , and hysitron nanoindenter Optical characterization by monitoring fringes on Michelson interferometer
  • Simple interferometer setup Have a good understanding of the spring plate movement
  • We investigated various materials, but TaN is easier to fabricate and pattern High sheet resistance Explain figures
  • Molecular beam epitaxy for controlled high quality growth Precise control of the growth rate and doping levels
  • Molecular beam epitaxy for controlled high quality growth Precise control of the growth rate and doping levels
  • Molecular beam epitaxy for controlled high quality growth Precise control of the growth rate and doping levels
  • Molecular beam epitaxy for controlled high quality growth Precise control of the growth rate and doping levels
  • Explain figure, and how we sandwich the samples face to face 2-D stress analysis used to optimize the fixture, for max stress on the samples, and least on the tubes
  • Explain figure, and how we sandwich the samples face to face 2-D stress analysis used to optimize the fixture, for max stress on the samples, and least on the tubes
  • Br-IBAE used for forming these pillars Though in final device we use wet etching to reach upto the P region
  • Br-IBAE used for forming these pillars Though in final device we use wet etching to reach upto the P region
  • Br-IBAE used for forming these pillars Though in final device we use wet etching to reach upto the P region
  • Br-IBAE used for forming these pillars Though in final device we use wet etching to reach upto the P region
  • Inverted microscope modified to test the samples Probes, and HeNe or 830nm
  • Inverted microscope modified to test the samples Probes, and HeNe or 830nm
  • After bonding the photo contrast drops to 20 micro amps, so requires higher load resistor to cause same drop across spring plate
  • Transcript

    • 1. 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
    • 2. Outline
      • Background & Motivation
      • Device Design
      • Fabrication
      • Characterization
      • Conclusion and Future work
    • 3. 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
    • 4. Spring Plate Mirrors Dynamic correction using MEMS mirror
    • 5. Motivation Dense array of micro mirrors “ All optically driven MEMS deformable device via a photodetector array” J. Khoury et.al. Proc. Of SPIE Vol 6368 636804 Electrical actuation
      • 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
    • 6. Earlier Work Mylar on GaAs Mylar on InGaAs PIN “ All optically driven MEMS deformable device via a photodetector array” J.Khoury et. al. Proceedings of SPIE Vol. 6368. 636804. (2006)
      • Aluminized mylar membranes
      • 1mm × 1mm
      • Slow response
      • InGaAs based PIN back substrates
      • Not suitable for large arrays
      • Require passivation
    • 7. Device 2-D Schematic of a single pixel Equivalent circuit diagram
      • 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
    • 8. Working Principle I-V curve of the load resistor Equivalent circuit I-V characteristics of the PIN photodiode V Total = V PIN + V R V Mirror V Total V PIN V R
    • 9. Working Principle Operation points of the MEMS device Equivalent circuit V Total No light V R = A V Total V R = B
      • Low actuation voltage desirable for mirrors
      • High value load resistance
      • High current contrast (I Light - I Dark )
    • 10. Spring plate fabrication
    • 11. Spring plate fabrication
    • 12. Spring Plate 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
      • PECVD low stress SiN films* (23MPa residual stresses)
      • Mechanical characterization
      • * Indenter Studies
      • * FEM simulations
      • Optical characterization
      • * Interferometer studies
      • * FEM simulations
    • 13. Interferometry Michelson interferometer setup Dark Light Difference Interference fringe patterns “ 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 5-15 volts required for Mirror actuation
    • 14. 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
    • 15. GaAs PIN Diodes 0.6 µm 2 µm 1 µm P I 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, 20 th April 2006 Mesa etch Schematic of ohmic contacts
      • HIgh breakdown voltage 32 volts
      • Photo response of upto 80-90µA
      • Photo current increases linearly with laser power
      • Peak efficiency at 800-890 nm*
    • 16. Photoresponse Testing Photo response test setup GaAs PIN (before bonding) Measurement probes 830nm LASER GaAs PIN sample Laser N P Probes
    • 17. GaAs PIN Photoresponse
    • 18. Wafer Fusion Schematic 3-D schematic of sandwiched wafers before fusion
    • 19. Wafer Fusion 3-D schematic of fixture components Quartz tube Graphite fixtures Graphite shims
    • 20. Wafer Fusion FEM simulation of the thermal stresses during bonding Wafer bonding fixture Wafer fusion furnace
      • Custom made fixture and furnace
      • 100-200 MPa force on sample
      • 650-700°C bonding temperature
    • 21. GaAs PINs on GaP Schematic of polished and patterned sample Schematic of polished and etched sample
    • 22. GaAs PINs on GaP Wafer bonded interface GaAs GaP After polish & wet etch GaAs GaP
    • 23. GaAs PINs on GaP Close up Cross section
    • 24. GaAs PINs on GaP Top view
    • 25. Characterization 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 Probe
    • 26. Characterization V
      • External load resistor 200K ohms -------> 2 M ohms
      • 2 micron spacing between spring plate and SU-8
      SU8 P I N SU8 R
    • 27. Results A (Dark) B (Light) Difference
      • Cascading via external load resistor
      • 200KΩ with GaAs PINs
      • 1MΩ resistor with bonded PINs
    • 28. Final Device Spring plate and thin film resistor PIN with SU-8 pillars
    • 29. Device Schematic
    • 30. Final Testing On PIN diode On spring plate To Interferometer Work in progress!
    • 31. Conclusion
      • GaAs PINs wafer bonded on GaP successfully
      • Si 3 N 4 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
    • 32. 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