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

2. Microwave Kinetic Inductance Detectors: working principle Superconductors below a critical temperature T c have electrons divided in two different populations: - the Cooper Pairs, electrons bound together with an energy E=2  3.528*k b T c by the electron-phonon interaction. They act as superconducting carriers. - the Quasi-Particles, single electrons which act as carriers in a normal metal. In this two fluids model the total conductivity of the material is:  =  1 (n QP ) - j  2 (n CP ) Quasi-Particles Z s = R s (  1,  2 ) + i X s (  1,  2 ) Cooper Pairs and the complex surface impedance is: X s =  L int =  (L m,int +L k )

3. n QP (  m -3 ) temperature (K) The values of R s and X s depend on the densities of QPs and CPs. By measuring them, we can get information on n QP. Which are the effects of incoming radiation on a superconducting strip? n′ CP < n CP QP CP T<T c h  >2  Z s changes because: n CP increases n QP decreases both R s and X s increase, in particular L kin How can we measure the small variation in L k ? film thickness (nm) L x (pH/square)

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

5. C1C1 R 1 QP L 1 kin L 1 mag C2C2 R 2 QP L 2 kin L 2 mag CNCN R N QP L N kin L N mag RF carrier (f 1 + f 2 + f 3 +... + f N ) Pixel 1, f 1 Pixel 2, f 2 Pixel N, f N The fact that each resonator has no effect even few MHz away from its resonant frequency makes these detectors ideal for frequency domain multiplexing:  Very resistant: materials are all suitable for satellite and space missions, like CMB mission.  Extremely simple cold electronics: one single amplifier can be used for 10 3 -10 4 pixels. The rest of the readout is warm.  Very flexible: different materials and geometries can be chosen to tune detectors to specific needs.  order of 10 3 -10 4 pixels read with a single coax low thermal load! Architecture of typical multipixel readout system

6. Lumped resonators for millimetric wavelengths: design process 1)pixel size: needs to be of order of at least one wavelength 2)meander section: optimization of the matching with the free space impedance If >>s 3) Capacitive section: choice of the resonance frequency 2mm 4m4m 280  m Sonnet simulation Very low C!

7. Our first LEKID mask: Design Fabrication

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

9. Measurements: resonances S 21 (dB) frequency (GHz) Power sweep frequency (GHz) S 21,norm (dB) frequency (GHz) S 21,norm (dB) Typical Q factors of 10000-20000, limited in these first chips by the strong coupling to the feedline Q i as high as 40000 already at 305mK

10. Effect of temperature sweep on: phase amplitude Higher T Higher n qp Higher losses Higher T Lower n cp Lower f 0

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

12. System modified for optical measurements: 300K30K2K 300mK Polyethilene window Fluorogold (400GHz lowpass) Fluorogold + 145GHz bandpass filter BB(77K) chopper KID d A in 300mK

13. Signal ≈ 19deg

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

15. Noise level ≈ The optical Noise Equivalent Power: 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!  1, h 1  2, h 2  3, h 3 Equivalent stress The choice of the materials and thicknesses of layers has to be done in order to have a tensile structure with  eq ~ 50MPa Membranes:

17. p-type HR 500  m DSP Si field oxide deposition (SiO 2 ) 400nm LPCVD nitride deposition (Si 3 N 4 ) 150nm LPCVD thermal oxide deposition (TEOS) 450nm Trilayer (SiO 2 /Si 3 N 4 /TEOS) Wet chemical etching provides an high degree of selectivity to thermal oxide Wet etching in TMAH a) To membrane b) Anisotropic etchant 54.74° 2) LPCVD nitride deposition (Si 3 N 4 ) 20nm Quadrilayer (SiO 2 /Si 3 N 4 /TEOS/ Si 3 N 4 ) Leaving 15  m Si h tot = 1  m  eq = 50MPa 1) a) b) underetch h tot = 1  m  eq = 30MPa Different solutions tested:

18. Dimension (mm) 1.582.382.783.56 # membranes 30660684 # damaged 4321 # good 30257663 percentage99959775 SiO 2 /Si 3 N 4 /TEOS/ Si 3 N 4 : 98% success # damaged 179112 # good 28951572 percentage94858450 SiO 2 /Si 3 N 4 /TEOS: 91% success Fabrication at FBK “Fondazione Bruno Kessler”, Trento Results: decrease the noise contribution due to the substrate decrease the number of CR observed Hopefully, membranes will:

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