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LEKIDs effort in Italy

LEKIDs effort in Italy. Martino Calvo. B-Pol workshop, IAP Paris, 28 - 30 July. X s =  L int =  (L m,int +L k ). Microwave Kinetic Inductance Detectors: working principle. Superconductors below a critical temperature T c have electrons divided in two different populations:

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LEKIDs effort in Italy

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  1. LEKIDs effort in Italy Martino Calvo B-Pol workshop, IAP Paris, 28 - 30 July

  2. Xs=Lint=(Lm,int+Lk) Microwave Kinetic Inductance Detectors: working principle • Superconductors below a critical temperature Tc have electrons divided in two different populations: • theCooperPairs, electrons bound together with an energy E=2D3.528*kbTc by the electron-phonon interaction. They act as superconducting carriers. • theQuasi-Particles, single electrons which act as carriers in a normal metal. In this two fluids model the total conductivity of the material is: = 1(nQP)- j2(nCP) and the complex surface impedance is: Zs = Rs(1,2) + iXs (1,2) Quasi-Particles Cooper Pairs

  3. T<Tc QP n′CP< nCP CP hn>2E The values of Rs and Xs depend on the densities of QPs and CPs. By measuring them, we can get information on nQP . Which are the effects of incoming radiation on a superconducting strip? nQP (m-3) temperature (K) Lx (pH/square) • Zs changes because: • nCP increases • nQP decreases • both Rs and Xs increase, in particular Lkin film thickness (nm) How can we measure the small variation in Lk?

  4. The superconductor can be inserted in a resonating circuit with extremely high Q. Two different possibilities: Cc Lmag Cl Lkin RQP Feedline Inductive Coupling Inductive section Capacitive section 1) Distributed l=bias/4 resonators response depends on where the photon hits the sensor equivalent circuit: RLC series needs some sort of antenna 2) Lumped resonators l<<bias no current variation along its length, acts as free absorber equivalent circuit: RLC series

  5. RF carrier (f 1 + f 2 + f 3 + ... + f N) C1 C2 CN L1mag L2mag LNmag L2kin L1kin R1QP R2QP LNkin RNQP Pixel 1, f1 Pixel2, f2 Pixel N, fN The fact that each resonator has no effect even few MHz away from its resonant frequency makes these detectors ideal for frequency domain multiplexing: • order of 103-104 pixels read with a single coax low thermal load! • Extremely simple cold electronics: one single amplifier can be used for 103-104 pixels. The rest of the readout is warm. • Very flexible: different materials and geometries can be chosen to tune detectors to specific needs. • Very resistant: materials are all suitable for satellite and space missions, like CMB mission. Architecture of typical multipixel readout system

  6. Lumped resonators for millimetric wavelengths: design process 2mm 280m 2mm 4m Sonnet simulation pixel size: needs to be of order of at least one wavelength meander section: optimization of the matching with the free space impedance If >>s 3) Capacitive section: choice of the resonance frequency Very low C!

  7. Our first LEKID mask: Design Fabrication

  8. Lumped resonators for millimetric wavelengths: materials and thicknesses dT/dNQP (K) temperature (K) Si 400m Si 389m Fractional absorption Fractional absorption frequency (GHz) frequency (GHz) • Superconducting metal:Aluminum • ok for mm waves: ngap= 90 GHz • Tc = 1.27 K Aluminum thickness t: higher responsivity lower t higher resistivity = better free space matching t=20nm, 40nm lower t Substrate material:Silicon and Sapphire free space substrate resonator back short Si 400m, Si 170m, Sa 300m

  9. Power sweep S21,norm (dB) frequency (GHz) S21,norm (dB) frequency (GHz) Measurements: resonances S21 (dB) frequency (GHz) Typical Q factors of 10000-20000, limited in these first chips by the strong coupling to the feedline Qi as high as 40000 already at 305mK

  10. Higher T Higher nqp Higher losses Higher T Lower ncp Lower f0 Effect of temperature sweep on: phase amplitude

  11. the red crosses correspond to the base temperature resonant frequency Phase shift (degree) nQP(m-3) Temperature sweeps (deg/m-3) Volume≈3100m3 Phase shift (degree) All responsivities are in the interval: nQP(m-3)

  12. System modified for optical measurements: 300mK 300K 30K 2K KID Ain Fluorogold (400GHz lowpass) Fluorogold + 145GHz bandpass filter Polyethilene window d 300mK chopper BB(77K)

  13. Signal ≈ 19deg

  14. Si 400m Fractional absorption frequency (GHz) Quasi-particles lifetime To measure QP , we can use the signal due given by incoming cosmic rays: QP=55.6±3.6s Absorption efficiency

  15. The optical Noise Equivalent Power: Noise level ≈ Typical photonic NEP from ground ≈

  16. Cosmic rays issue • We have seen that CR can be useful to determine QP ,but... • too many of them! Rate of approximately 1 per minute! The use of membranes could help solving this issue! Membranes: The choice of the materials and thicknesses of layers has to be done in order to have a tensile structure with eq~ 50MPa 1, h1 Equivalent stress 2, h2 3, h3

  17. Different solutions tested: 1) Trilayer (SiO2/Si3N4/TEOS) Wet etching in TMAH a) To membrane eq= 30MPa Wet chemical etching provides an high degree of selectivity to thermal oxide htot= 1m p-type HR 500m DSP Si b) Leaving 15m Si field oxide deposition (SiO2) 400nm LPCVD nitride deposition (Si3N4) 150nm 54.74° Anisotropic etchant LPCVD thermal oxide deposition (TEOS) 450nm a) 2) Quadrilayer (SiO2/Si3N4/TEOS/ Si3N4) eq= 50MPa b) htot= 1m LPCVD nitride deposition (Si3N4) 20nm underetch

  18. Fabrication at FBK “Fondazione Bruno Kessler”, Trento Results: SiO2/Si3N4/TEOS/ Si3N4: 98% success SiO2/Si3N4/TEOS: 91% success Hopefully, membranes will: decrease the number of CR observed decrease the noise contribution due to the substrate

  19. Conclusions • The Microwave Kinetic Inductance Detectors have many characteristics that make them ideal for CMB experiments which require large arrays of detectors. • We have developed distributed detectors but with a lumped geometry in order to optimize their coupling to the millimetric radiation. • We have observed a light signal finding absorption efficiencies up to 40%, in good agreement with the theoretical predictions. The model assumed is therefore sound and can be used for further development • The measured NEP is • The next steps: • Further optimization of the single pixel (a new mask is already under test) • Development of KIDs on membranes to check the possibility of using them on balloon-borne and space missions

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