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Mehdi Fardmanesh Supercondutor Electronics Research Laboratory (SERL)

Mehdi Fardmanesh Supercondutor Electronics Research Laboratory (SERL) Electrical Engineering Department Sharif university of technology. 1. Superconductor Electronics Research Laboratory(SERL). Out lines. Part I : Theoretical optimization Operation principle Theoretical Limit

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Mehdi Fardmanesh Supercondutor Electronics Research Laboratory (SERL)

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  1. MehdiFardmanesh Supercondutor Electronics Research Laboratory (SERL) Electrical Engineering Department Sharif university of technology 1 Superconductor Electronics Research Laboratory(SERL)

  2. Out lines Part I : Theoretical optimization • Operationprinciple • TheoreticalLimit • Detectivityparameters • Substratethickness • Modulationfrequency • Thermalconductivity Part II :Experimental Results • Substrate thinning • Absorber effect • Detectivity calculation • Conclusion and Summary 2 Superconductor Electronics Research Laboratory(SERL)

  3. Part I Theoretical Detectivity Optimization 3 Superconductor Electronics Research Laboratory(SERL)

  4. Bolometer - Principles • A device which transforms a change in the incident input power into a change of electrical resistance. • A bolometer in general consists of: • 1- Absorber • 2- Thermometer • 3- A thermal link to the temperature reservoir Superconductor Electronics Research Laboratory(SERL)

  5. Operation Mechanism Cut off frequency for semiconductor detectors: Transition curve and bolometric response Comparison of different types of infrared detectors [SERL] Superconductor Electronics Research Laboratory(SERL)

  6. Detectivity • A criterion to determine device performance • Different noise sources 6 Superconductor Electronics Research Laboratory(SERL)

  7. Theoretical limit • Considering only background noise • It can be enhanced by reducing field of view of detector 7 Superconductor Electronics Research Laboratory(SERL)

  8. Important terms in total NEP • Phonon NEP • Johnson NEP • 1/f NEP 8 Superconductor Electronics Research Laboratory(SERL)

  9. Responsivity • Optical Response • Maximum bias current

  10. Maximum Response & Total NEP • Maximum Optical Response • Considering linear transition  • Total NEP 10 Superconductor Electronics Research Laboratory(SERL)

  11. Detectivityvs. thermal conductance & Normal resistance(1mm thickness) • Almost independent of device resistance • Strong variation versus thermal conductance • Decreases at low and high value of thermal conductance Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 1mm at 80Hz modulation frequency. Superconductor Electronics Research Laboratory(SERL)

  12. Detectivityvs. thermal conductance & Normal resistance(10µm thickness) • Lower values of optimum thermal conductance • Higher detectivity • Decreases at low and high value of thermal conductance Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 10µm at 80Hz modulation frequency. Superconductor Electronics Research Laboratory(SERL)

  13. Detectivity Vs. thermal conductance & Normal resistance(10µm thickness) • Ignoring 1/f noise • Detectivity as high as 2×1010 • Very strong dependence on thermal conductance • Decreases at low and high value of thermal conductance Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 10µm at 2Hz modulation frequency. Superconductor Electronics Research Laboratory(SERL)

  14. Substrate thickness effects • Thinning the substrate would increase device detectivity • Thickness independent at high thermal conductance • There is an optimum value for thermal conductance Detectivity versus thermal conductance of a bolometer over SrTiO3 substrate with device area of 9 mm2, surface absorption of 20% and modulation frequency of 30Hz. 14 Superconductor Electronics Research Laboratory(SERL)

  15. Modulation frequency effect • Very low detectivity at low value of modulation frequency • Frequency independent at high value of thermal conductance • Increasing frequency decreases the detectivity • There is an optimum value of thermal conductance Detectivity versus thermal conductance of a bolometer over SrTiO3 substrate with device area of 9 mm2, surface absorption of 20% and substrate thickness of 100µm. 15 Superconductor Electronics Research Laboratory(SERL)

  16. Analytical optimization • Optimum thermal conductance • Optimum NEP Superconductor Electronics Research Laboratory(SERL)

  17. Optimum thermal conductance • Increases by increasing modulation frequency • Decreases by decreasing substrate thickness Optimum thermal conductance versus modulation frequency and substrate thickness Superconductor Electronics Research Laboratory(SERL)

  18. Maximum detectivity • Decrease by increasing modulation frequency • increases by decreasing substrate thickness Maximum Detectivity versus modulation frequency and substrate thickness Superconductor Electronics Research Laboratory(SERL)

  19. Optimum detectivity For detectivity in the range of 1010, substrate thickness should be as low as 30µm whereas modulation frequency should be as low as possible . Surface Conductance Limitation “Effect of Substrate Thickness on Responsivity of Free-Membrane Bolometric Detectors,”� To appear in IEEE Journal of Sensors Maximum detectivity versus substrate thickness of a bolometer over SrTiO3 substrate with device area of 9 mm2 and surface absorption of 20%. Superconductor Electronics Research Laboratory(SERL)

  20. Part II Experimental Optimization Superconductor Electronics Research Laboratory(SERL)

  21. Device fabrication Sample ‘A’ 200nm thin YBCO film deposited on 1mm thick SrTiO3 substrate using PLD YBCO film was patterned using standard photolithography process Au coated pads Silver paste for contacts Sample ‘B’ Superconductor Electronics Research Laboratory(SERL)

  22. New design of cold head Reducing thermal conductance is essential in order to achieve high detectivity Reducing thermal contact between substrate and cold head Free standing bolometer 22 Superconductor Electronics Research Laboratory(SERL)

  23. Thinning the substrate Thinning the substrate using our customized grinding machine Initial thickness  1000µm Final thickness  200µm 23 Superconductor Electronics Research Laboratory(SERL)

  24. Thermal diffusion length • Depends on modulation frequency and substrate materials • Decreases at high value of modulation frequency • Determines the knee frequency of device Thermal diffusion length of common substrate materials versus modulation frequency Superconductor Electronics Research Laboratory(SERL)

  25. Thinning effect on the responsivity and noise • No major effect on the device noise at high frequencies • Increasing device response in the frequency range lower than the knee frequency • Slight increase of 1/f noise • Slight destructive effect on the film quality f -1 slope Knee frequencies f -0.5 slope Measured values of optical response and voltage noise of sample A before and after thinning the substrate. Superconductor Electronics Research Laboratory(SERL)

  26. Absorber effect • Increasing surface absorption of device  increases device response • Applying thin layer of black paint as an absorber on the backside of device • Backside absorber • no destructive effect on film quality • Wider range of absorber materials • Only enhances responsivity in the frequency range in which Lf>Ls Measured values of optical response and voltage noise of sample “A_200” before and after applying absorber layer on the back side of device. 26 Superconductor Electronics Research Laboratory(SERL)

  27. Detectivity calculation • About one order of magnitude gain in device detectivity by thinning and using absorber • Thinner substrate thickness  Higher device detectivity • SrTiO3 is brittle at low thicknesses • Using silicon substrate as a support Calculated detectivity versus modulation frequency of sample “B” 27 Superconductor Electronics Research Laboratory(SERL)

  28. Results conclusion 28

  29. New device structure • Structural optimization of bolometer 29

  30. Extrapolation of Responsivity • 20 times increase in device responsivity • Expected value of detectivity is • 7.2×1010 Planned response 10µm thickness Extrapolated value of responsivity for 10µm substrate thickness 30 Superconductor Electronics Research Laboratory(SERL)

  31. Summary • Formulation of Detectivity Optimization • Thinning substrate • Reducing thermal conductance • Using absorber layer • One order of magnitude gain in detectivity by model based device engineering • A bolometer with detectivity of 3.6×109 Obtained • Successful use of Absorber on the backside • No destructive effect on film quality • Possibility of wider range of absorber materials • Target: Wide Band D* over 1010 31

  32. Thanks for you attention 32

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