Electrical Electrical characterization of characterization of a superconducting a superconducting

hot spot hot spot microbolometermicrobolometer

S.CibellaS.Cibella, R. Leoni, G. Torrioli, , R. Leoni, G. Torrioli,

M. G. Castellano, A. Coppa, F. MattioliM. G. Castellano, A. Coppa, F. Mattioli

IFN-CNR, Roma, IFN-CNR, Roma, ItalyItaly

Outline Outline THz technologyTHz technology Antenna-coupled superconducting Antenna-coupled superconducting

microbolometers microbolometers Basic principlesBasic principles

Detector fabricationDetector fabrication Electronic readoutElectronic readout I-V characteristic measurementsI-V characteristic measurements NEP measurementsNEP measurements ConclusionsConclusions

THz technologyTHz technologyX Ray Ultraviol

etVisible

Infrared THz gapMicrowave

(millimeter to RF)

1016

Hz 1015

Hz 1014

Hz 1013

Hz 1012

Hz 1011

Hz 1010

Hz1016

Hz1016

Hz

THz frequency domain

1mm (300 GHz) – 100 μm (3 THz)

THz radiation is a potentially powerful technique THz radiation is a potentially powerful technique in security screening applicationin security screening application : :

Penetration Penetration High-resolution 3-D imagingHigh-resolution 3-D imaging SpectroscopySpectroscopy SafeSafe

Antenna-coupled Antenna-coupled superconducting superconducting

microbolometers: how do microbolometers: how do they work?they work? Lithographic antenna electrically coupled to a temperature Lithographic antenna electrically coupled to a temperature

sensor, the bolometer (suspended Nb bridge).sensor, the bolometer (suspended Nb bridge).

N

LH

Antenna

L

TC

Input power modulates Input power modulates the the

current trough the bridge

Modulation of R

Formation of a Normal-state hot spot in the Formation of a Normal-state hot spot in the middle of the suspended superconducting bridge middle of the suspended superconducting bridge per T>TCper T>TC

Modulation the volume of the hot spotModulation the volume of the hot spot

Microbolometer fabricationMicrobolometer fabrication

•100kV FEG•beam spot: 8 nm•Mask fabrication (up to 5”)•direct writing (up to 5”)

3 step process which use electron beam 3 step process which use electron beam lithography (EBL)lithography (EBL)

reactive ions etching (RIE) in CHF 3 /SF6 gas mixture

inductive coupled plasma (ICP) etching in an SF6/Ar gas

mixture

Detector fabricationDetector fabricationSi substrate

100 nm Si3N440 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 mixtureNb/Si3N4 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: Bolometer technologies: detector fabrication detector fabrication

Logarithmic spiral antenna with a nominal band from 300 GHz to 1 THz

22x1x0.040 (μm)3

suspended Nb

bridge

Electronic ReadoutElectronic Readout A current sensitive transimpedance amplifier provides:A current sensitive transimpedance amplifier provides: a constant Voltage biasa constant Voltage bias an output related to the bolometer currentan output related to the bolometer current

out b

fb

V VI

R

Bolometer

Rfb=1kΩTo≈5

K

4He

Vacuum can

Rfb

Vout

ZVb

-

+

AD797

Cx=100nF

Rx=1Ω

Cx

Rx I

I-V characteristics I-V characteristics

0 0( )n CV GR T T

0 0

0n

V V VI

R V V

G T

V

VI

R

V0

I

V

0CVI G T T

Linear part: ohmic behavior of the bridge in its normal state

V0

Measured I-V Measured I-V characteristics characteristics

124 /G nW K 24 15B bath

fWNEPph k T G

Hz 78.7nR 0 5.14V mV

Current responsivity (Current responsivity (SSII))

dVZ

dI

1

2I

Z RS

V Z

/R V I

I

IS

P

Measured electrical NEPMeasured electrical NEPRfb

Vb

-

+

40

3.8

NEP fW Hz

V mV

NEP=in

T/SI 2

2 204A

T A nn n

eq eq

kTi i

Z Z

AD 797

in=2 pA/√Hz

Vn=0.9 nV/√Hz

√(NEPPh)2+(int/SI)2

ConclusionsConclusions An antenna coupled hot spot microbolometer An antenna coupled hot spot microbolometer

has been fabricated has been fabricated

A simple room-temperature readout based on a A simple room-temperature readout based on a transimpedance amplifier has been developed transimpedance amplifier has been developed

Noise equivalent power of 40 fW/HzNoise equivalent power of 40 fW/Hz1/21/2 has has been measuredbeen measured

Hot spot microbolometers are a good choice Hot spot microbolometers are a good choice for a THz-camera with a simplified electronic for a THz-camera with a simplified electronic readoutreadout