lekids effort in italy martino calvo b-pol workshop, iap paris, 28 - 30 july
TRANSCRIPT
LEKIDs effort in ItalyLEKIDs effort in Italy
Martino CalvoMartino Calvo B-Pol workshop, IAP Paris, 28 - 30 July
Microwave Kinetic Inductance Detectors: Microwave Kinetic Inductance Detectors: working working principleprinciple
Superconductors below a critical temperature Tc have electrons divided in two different populations:
- the Cooper Pairs, electrons bound together with an energy E=23.528*kbTc 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(nQP) - j2(nCP)
Quasi-Particles
Zs = Rs(1,2) + iXs (1,2)
Cooper Pairs
and the complex surface impedance is:
Xs=Lint=(Lm,int+Lk)
n QP (m
-3)
temperature (K)
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?
n′CP< nCP
QP
CP
T<Tc
h>2
Zs changes because:• nCP increases• nQP decreases
• both Rs and Xs increase, in particular LkinHow can we measure the small variation in Lk?film thickness (nm)
L x (
pH/s
qua
re)
Cc
RQPLkin
Lmag
Cl
The superconductor can be inserted in a resonating circuit with extremely high Q.
Two different possibilities:
Feedline
Inductive Coupling
Inductivesection
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 absorberequivalent circuit: RLC series
C1
R1QPL1
kin
L1mag
C2
R2QP
L2kin
L2mag
CN
RNQPLN
kin
LNmag
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 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.
order of 103-104 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
Zeff Z(w s)w
(Zeff Z0)
(Zeff Z0)
3) Capacitive section: choice of the resonance frequency
2mm
2mm
4m
280m
Sonnet simulation
Very low C!
Our first LEKID mask:Design
Fabrication
Superconducting metal: Aluminum
• ok for mm waves: gap= 90 GHz
• Tc = 1.27 K
ddNQP
Q dT
dNQP
QV
Lint
L
1
t
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 400m, Si 170m, Sa 300m
free space substrate resonator back short
temperature (K)
dT/d
NQ
P (
K)
Si 389m
frequency (GHz)
Fra
ctio
nal a
bsor
ptio
n
Si 400m
Fra
ctio
nal a
bsor
ptio
n
frequency (GHz)
Measurements: resonancesS
21 (
dB)
frequency (GHz)
Power sweep
frequency (GHz)
S2
1,n
orm
(dB
)
frequency (GHz)
S2
1,n
orm
(dB
)
Typical Q factors of 10000-20000,limited in these first chips by thestrong coupling to the feedline
Qi as high as 40000 already at 305mK
Effect of temperature sweep on:
phase
amplitude
Higher T Higher nqp Higher losses
Higher T Lower ncp Lower f0
00800620 ..dn
d
QP
(deg/m-3)
the red crosses correspond to the base temperature resonant frequency
Volume≈3100m3
QPdeg/.dN
d
QP
610320
All responsivities are in the interval:
QPdeg/. 66 10201031
nQP m-3
Pha
se s
hift
(d
egre
e)
nQP m-3
Pha
se s
hift
(d
egre
e)Temperature sweeps
System modified for optical measurements:
300K 30K 2K
300mK
Polyethile
ne wind
ow
Fluorogo
ld(400
GH
z lowpass)
Fluorogo
ld +145
GH
z bandpass filter
BB(77K)
chopper
KID
d
Ain
2dAin
Pin (,T) ( )BB(,T)AKID
300mK
pWPin 4
Signal ≈ 19deg
degd 19
5109QPdN
QPdeg/.dN
d
QP
610320
(0) 1.764kBTc670.
Pabs dNQP(0)
QP
Quasi-particles lifetime
QP=55.6±3.6s
pW.)(dN
PQP
QPabs 750
0
Absorption efficiency %pW
pW.
P
P
in
absabs 20
4
750
Si 400m
Fra
ctio
nal a
bsor
ptio
nfrequency (GHz)
To measure QP , we can use the signal due given by incoming cosmic rays:
Noise level ≈ Hz
deg4107
Hz.N
S 41072
Hz
W.
Hz.
WNEP 16
4
12
104711072
104
The optical Noise Equivalent Power:
degS 19
Typical photonic NEP from
ground ≈
510 16 W
Hz
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, h1
2, h2
3, h3
eq i
i
hi
hii
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 500m DSP Si
field oxide deposition (SiO2) 400nm
LPCVD nitride deposition (Si3N4) 150nm
LPCVD thermal oxide deposition (TEOS) 450nm
Trilayer (SiO2/Si3N4/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 (Si3N4) 20nm
Quadrilayer (SiO2/Si3N4/TEOS/ Si3N4)
Leaving 15m Si
htot= 1m
eq= 50MPa
1)
a)
b)
underetch
htot= 1m
eq= 30MPa
Different solutions tested:
Dimension (mm)Dimension (mm) 1.581.58 2.382.38 2.782.78 3.563.56
# membranes# membranes 306306 6060 6868 44
# damaged# damaged 44 33 22 11
# good# good 302302 5757 6666 33
percentagepercentage 9999 9595 9797 7575
SiO2/Si3N4/TEOS/ Si3N4: 98% success
# damaged# damaged 1717 99 1111 22
# good# good 289289 5151 5757 22
percentagepercentage 9494 8585 8484 5050
SiO2/Si3N4/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
Hz
W. 1610471