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