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Electrical characterization of a superconducting hot spot microbolometer

Electrical characterization of a superconducting hot spot microbolometer. S.Cibella , R. Leoni, G. Torrioli, M. G. Castellano, A. Coppa, F. Mattioli. IFN-CNR, Roma, Italy. Outline. THz technology Antenna-coupled superconducting microbolometers Basic principles. Detector fabrication

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Electrical characterization of a superconducting hot spot microbolometer

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  1. Electrical characterization of a superconducting hot spot microbolometer S.Cibella, R. Leoni, G. Torrioli, M. G. Castellano, A. Coppa, F. Mattioli IFN-CNR, Roma, Italy

  2. Outline • THz technology • Antenna-coupled superconducting microbolometers • Basic principles • Detector fabrication • Electronic readout • I-V characteristic measurements • NEP measurements • Conclusions

  3. Microwave (millimeter to RF) X Ray Ultraviolet Visible Infrared THz gap 14 11 12 15 13 16 16 16 10 10 10 10 10 10 10 10 10 10 Hz Hz Hz Hz Hz Hz Hz Hz Hz THz technology THz radiation is a potentially powerful technique in security screening application : • Penetration • High-resolution 3-D imaging • Spectroscopy • Safe THz frequency domain 1mm (300 GHz) – 100 μm (3 THz)

  4. Antenna-coupled superconducting microbolometers: how do they work? • Lithographic antenna electrically coupled to a temperature sensor, the bolometer (suspended Nb bridge). • Formation of a Normal-state hot spot in the middle of the suspended superconducting bridge per T>TC • Input power modulates the • current trough the bridge TC N Modulation the volume of the hot spot LH L Modulation of R Antenna

  5. Microbolometer fabrication 3 step process which use electron beam lithography (EBL) • 100kV FEG • beam spot: 8 nm • Mask fabrication (up to 5”) • direct writing (up to 5”) reactive ions etching (RIE) in CHF 3 /SF6 gas mixture inductive coupled plasma (ICP) etching in an SF6/Ar gas mixture

  6. Si substrate 100 nm Si3N4 40 nm Nb • First fabrication step: • exposure by EBL • deposition of a 70 nm Ti/Au and lift off to define pads, antennas and alignment markers • Second fabrication step: • Define the temperature sensor on the HSQ electronic resist • etching with reactive ions (RIE) in CHF 3 /SF6 gas mixture HSQ strip Nb/Si3N4 bridge • Third fabrication step: • Expose another HSQ strip layer , 3 um wide, to encapsulate the Nb strip • Etching by ICP (inductive coupled plasma). Detector fabrication

  7. Bolometer technologies: detector fabrication 22x1x0.040 (μm)3 suspended Nb bridge Logarithmic spiral antenna with a nominal band from 300 GHz to 1 THz

  8. To≈5K Rfb - Vout Rx=1Ω AD797 Vb Rx Cx=100nF + Z Cx Electronic Readout A current sensitive transimpedance amplifier provides: • a constant Voltage bias • an output related to the bolometer current 4He Rfb=1kΩ Bolometer Vacuum can I

  9. Linear part: ohmic behavior of the bridge in its normal state I-V characteristics I V0 V V0

  10. Measured I-V characteristics

  11. Current responsivity (SI)

  12. Rfb - + Vb √(NEPPh)2+(int/SI)2 AD 797 in=2 pA/√Hz Vn=0.9 nV/√Hz NEP=inT/SI Measured electrical NEP

  13. Conclusions • An antenna coupled hot spot microbolometer has been fabricated • A simple room-temperature readout based on a transimpedance amplifier has been developed • Noise equivalent power of 40 fW/Hz1/2 has been measured • Hot spot microbolometers are a good choice for a THz-camera with a simplified electronic readout

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