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Device level vacuum packaged micromachined infrared detectors on flexible substrates

Device level vacuum packaged micromachined infrared detectors on flexible substrates. Aamer Mahmood Donald P. Butler Zeynep Çelik-Butler Microsensors Laboratory Department of Electrical Engineering University of Texas at Arlington, Arlington, TX 76019. Outline. Microbolometers

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Device level vacuum packaged micromachined infrared detectors on flexible substrates

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  1. Device level vacuum packaged micromachined infrared detectors on flexible substrates Aamer Mahmood Donald P. Butler Zeynep Çelik-Butler Microsensors Laboratory Department of Electrical Engineering University of Texas at Arlington, Arlington, TX 76019

  2. Outline • Microbolometers • Flexible substrates • Device level vacuum packaging • Design and fabrication • Characterization • Future work

  3. Bolometers • Bolometers are thermal detectors • YBCO is used as the detector material • Change in temperature induces a change in the detector resistance η = absorptivity, β= TCR,  = angular frequency of incident radiation, τ = detector thermal time constant, ΔΦ = the magnitude of the incident flux fluctuation, Geff = thermal conductivity

  4. Sensors on flexible substrates • PI 5878G (liquid Kapton) is used as the flexible substrate • Sensor Arrays on flexible substrates (Smart skins) • Infrared sensors • Pressure/Tactile Sensors • Flow sensors • Humidity sensors • Velocity sensors • Accelerometers • Advantages of flexible substrate micro sensors • Low cost • Lightweight • Conformable to non planar surfaces • High degree of redundancy • Vacuum packaging brings the best out of many MEMS devices

  5. Microbolometer fabrication Trench Geometry (Not to scale)

  6. Fabrication(Silicon wafer)

  7. Fabrication(PI 5878G)

  8. Fabrication(Nitride)

  9. Fabrication(Al)

  10. Fabrication(Sacrificial Polyimide PI 2610)

  11. Fabrication(Support Nitride)

  12. Fabrication(Ti arms)

  13. Fabrication(Au contacts)

  14. Fabrication(YBCO detector pixel)

  15. Fabrication(Photodefinable PI2737 sacrificial mesa)

  16. Fabrication(Al2O3)

  17. Section of vacuum cavity before micromachining Al2O3 Sacrificial PI2737 mesa Nitride Al mirror Sacrificial PI2610 Nitride

  18. Fabrication(Partially micromachined)

  19. Fabrication(Fully micromachined)

  20. Fabrication(Sealed vacuum cavity)

  21. Fabrication(Superstrate PI 5878G)

  22. Single microbolometer

  23. Design considerations • Transmission through optical window • Structural integrity of vacuum element • Lateral dimensions • Cavity resonant wavelength • Axial dimensions

  24. Structural integrity of vacuum element

  25. Al2O3 stress analysis

  26. Thermal analysis Gth ≈ 5x10-6 W/K (Vacuum) ≈10-4 W/K (air)

  27. Fabrication of encapsulated devices Fully micromachined device SEM graph of an unsealed micromachined device Partially micromachined device

  28. Fabrication of encapsulated devices Vacuum cavity SEM graph of sealed device Sealed device SEM graph of cross section of vacuum cavity

  29. VI curve Gth=3.36x10-6 W/K

  30. Temperature Coefficient of Resistance (TCR) R(300K)=53.4 MΩ TCR(300K)=-3.4%/K

  31. Current Responsivity (RI) Current Responsivity (RI) =Output current/Input power RI=6.13x10-5 A/W @ 5Hz

  32. Detectivity (D*) Detectivity = D* = Area normalized signal to noise ratio D* = 1.76x105 cm-Hz1/2/W

  33. Conclusion • Device level vacuum encapsulated microbolometers on flexible substrates have been fabricated • Theoretical thermal conductivity in vacuum is 5x10-6 W/K • Measured thermal conductivity is 3.36x10-6 W/K (Intact Vacuum cavity) • Measured room temperature TCR is -3.4%/K, resistance is 53.4MΩ • Measured RI is 6.13x10-5 A/W, D*=1.76x105cm-Hz1/2/W

  34. Future work • Incorporating more sensors in the smart skins e.g. pressure/tactile sensors, flow sensors, accelerometers • Cavity design to improve/tune optical response • True integrated flexible system

  35. This work is supported by the National Science Foundation • ECS-025612 The End

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