LEKIDs effort in Italy Martino Calvo B-Pol workshop, IAP Paris, July
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 )
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)
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
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 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 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 pixels read with a single coax low thermal load! Architecture of typical multipixel readout system
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 4m4m 280 m Sonnet simulation Very low C!
Our first LEKID mask: Design Fabrication
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)
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 , limited in these first chips by the strong coupling to the feedline Q i as high as already at 305mK
Effect of temperature sweep on: phase amplitude Higher T Higher n qp Higher losses Higher T Lower n cp Lower f 0
(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
System modified for optical measurements: 300K30K2K 300mK Polyethilene window Fluorogold (400GHz lowpass) Fluorogold + 145GHz bandpass filter BB(77K) chopper KID d A in 300mK
Signal ≈ 19deg
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:
Noise level ≈ The optical Noise Equivalent Power: Typical photonic NEP from ground ≈
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:
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:
Dimension (mm) # membranes # damaged 4321 # good percentage SiO 2 /Si 3 N 4 /TEOS/ Si 3 N 4 : 98% success # damaged # good percentage 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:
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