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

THz technology X RayUltravioletVisibleInfraredTHz gap Microwave (millimeter to RF) Hz10 15 Hz10 14 Hz10 13 Hz10 12 Hz10 11 Hz10 Hz10 16 Hz10 16 Hz THz frequency domain 1mm (300 GHz) – 100 μm (3 THz) THz radiation is a potentially powerful technique in security screening application : Penetration Penetration High-resolution 3-D imaging High-resolution 3-D imaging Spectroscopy Spectroscopy Safe Safe

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

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

Detector fabrication Si substrate 100 nm Si 3 N 4 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 /SF 6 gas mixture Nb/Si 3 N 4 bridge Third fabrication step: Expose another HSQ strip layer, 3 um wide, to encapsulate the Nb strip Etching by ICP (inductive coupled plasma). HSQ strip

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

Electronic Readout A current sensitive transimpedance amplifier provides: A current sensitive transimpedance amplifier provides: a constant Voltage bias a constant Voltage bias an output related to the bolometer current an output related to the bolometer current Bolometer R fb =1kΩ To≈5K 4 He Vacuum can R fb V out Z VbVb - + AD797 C x =100nF R x =1Ω CxCx RxRx I

I-V characteristics V0V0 I V Linear part: ohmic behavior of the bridge in its normal state V0V0

Measured I-V characteristics

Current responsivity (S I )

Measured electrical NEP R fb VbVb - + NEP=i n T /S I AD 797 i n =2 pA/√Hz V n =0.9 nV/√Hz √ (NEP Ph ) 2 +(i n t /S I ) 2

Conclusions An antenna coupled hot spot microbolometer has been fabricated An antenna coupled hot spot microbolometer has been fabricated A simple room-temperature readout based on a transimpedance amplifier has been developed A simple room-temperature readout based on a transimpedance amplifier has been developed Noise equivalent power of 40 fW/Hz 1/2 has been measured Noise equivalent power of 40 fW/Hz 1/2 has been measured Hot spot microbolometers are a good choice for a THz- camera with a simplified electronic readout Hot spot microbolometers are a good choice for a THz- camera with a simplified electronic readout