TES Bolometer Arrays for CMB Polarization

and SZ Measurements

# Helmuth Spieler

Lawrence Berkeley National Laboratory

# Berkeley, CA 94720

Presentation to Interagency CMB Task Force July 29, 2004

Participants

## Senior Staff

J. Clarke (UCB)

W. Holzapfel (UCB)

A.T. Lee (UCB)

P.L. Richards (UCB)

H. Spieler (LBNL)

## Postdocs

H.M. (Sherry) Cho (UCB)

Matt Dobbs (LBNL)

Nils Halverson (UCB)

Huan Tran (UCB)

## Engineers

John Joseph (LBNL)

Greg Engargiola (UCB RAL)

Chinh Vu (LBNL)

## Students

M. Fogel (undergrad)

Z. Kermish

T. Lanting

M. Lueker

### J. Mehl

M. Moore

M. Myers

R. O'Brient

M. Scherzer

D. Schwan

Detector development driven by 3 experiments

1\. APEX-SZ

Galaxy cluster search using Sunyaev-Zel'dovich Effect

UC Berkeley, LBNL, and MPIfR Bonn

12m telescope on Atacama Plateau

320 bolometer array

2\. South Pole Telescope

Univ. Chicago, UC Berkeley, Case Western, Harvard CfA, Univ. Illinois,
LBNL

10m telescope at South Pole

1000 bolometer array

3\. PolarBear

UC Berkeley, LBNL, UC San Diego

3m telescope on White Mountain, CA

Specifically designed for CMB polarization

Test bed for new technologies

"Multi-color" pixels with dual polarization

Phase 1: 320 detectors

Phase 2: 1200 detectors

Key technologies

Wafer-scale bolometer arrays using photolithographic techniques

Voltage-biased superconducting transition edge sensors (TES)

stable operating point

controlled responsivity

Antenna-coupled devices

Integrated dual-polarization arrays with bandpass filtering

Future: wideband antennas with multiple frequency bands

Frequency-domain multiplexing

Two wires per multiplexed bolometer set

One SQUID per multiplexed bolometer set

Utilize experience from large HEP detectors

Focal Plane Design for APEX-SZ and SPT

![](media/image1.jpeg){width="5.4222222222222225in" height="4.325in"}

TES Spiderweb Arrays (APEX-SZ, SPT)

![](media/image2.png){width="3.3333333333333335in"
height="2.252083333333333in"}

fabricated 55-bolometer array

(Jared Mehl)

![](media/image3.png){width="4.333333333333333in"
height="3.5395833333333333in"}

![](media/image4.jpeg){width="3.0833333333333335in"
height="2.9652777777777777in"}

5"

photomontage

of focal plane:

![](media/image5.png){width="4.279861111111111in" height="2.2in"}

Cross-Section of a Pixel

![](media/image6.png){width="5.6in" height="3.4625in"}

Calculated efficiency vs.

backshort spacing

Backshort at a+b= 150 μm

gives good efficiency

at 150 and 217 GHz.

#### Bolometer Design Parameters for SPT

-   Geometry: Spider Absorber with TES sensor

-   Frequencies: 150 GHz, 217 GHz

-   Absorber Diameter: 3 mm

-   Suspension: (8) 1 mm thick, 0.5mm long legs

-   Thermal Conductance: G = 2 x 10^-10^ W/K

P~sat~ \~ 40 pW

-   Expected Loading \~ 11 pW (from ACBAR)

-   Sheet resistance of absorber: \~ 400 ohms/square

> 90 and 270 GHz bands considered: different arrays

Antenna-Coupled Arrays

#####  Highly integrated design

#####  

-   Antenna defines beam

-   On chip filters define band

-   Microstrip transmission lines to load resistor coupled to bolometer

-   Dual polarization in same pixel

-   Multiple frequencies possible

#####  

##### 

#####  TES multiplexable

Extensible to wideband polarization-sensitive antennas

with multiband readout

##### Antenna-Coupled Prototype Pixels (Mike Myers)

![](media/image7.jpeg){width="4.35625in" height="4.629861111111111in"}

Antenna coupling to optics by dielectric lenses

![](media/image9.jpeg){width="4.552083333333333in" height="3.8625in"}

Test Apparatus (M. Myers)

![](media/image10.png){width="7.219291338582678in"
height="3.033464566929134in"}

Test Apparatus (Mike Myers, Dan Schwan)

![](media/image11.jpeg){width="7.091666666666667in"
height="4.727777777777778in"}

Typical parameters:

-   G \~ 9x10^-10^ W/K

-   Expected white noise from \< 1 Hz to 100 Hz (thermal fluctuation
    limit)

-   T~C~ \~ 450mK

-   τ \~ 0.2 - 0.3 ms

-   R~load~ \~ 25 W (10 W design value)

##### Preliminary results

-   Polarization sensitivity

#####  Chopped LN load and wire grid polarizer

Cross polarization 3% upper limit

-   Rough beam map

> \~ 4° symmetric beam, matches expectation

Frequency Response (optical transmission)

![](media/image12.png){width="5.25in" height="4.072916666666667in"}

Performance adequate, but not optimized

x2 improvement expected (no anti-reflective coating yet)

(for comparison: Planck spec is 30% efficiency)

Readout

Heat leaks through connecting wires from 0.25K bolometer stage to 4K
SQUID stage is prohibitive for large arrays

Solution: Multiplexing

constraints:

low impedance required to maintain constant voltage bias

power budget at 0.25K stage \<1 μW

Two options:

time domain multiplexing (K. Irwin et al.)

1 SQUID per bolometer

frequency domain multiplexing

LBNL design: 1 SQUID reads out \~30 bolometers

More bolometers possible: discussion later

####  Principle of Time-Domain Multiplexing

1\. AC bias bolometers (\~100 kHz -- 1 MHz)

each bolometer biased at different frequency

2\. signals change sensor resistance

**⇒** modulate current

**⇒** transfer signal spectrum to sidebands adjacent to bias frequency

**⇒** each sensor signal translated to unique frequency band

3\. combine all signals in common readout line

4\. retrieve individual signals in bank of frequency-selective
demodulators

MUX circuit on cold stage

![](media/image13.wmf){width="8.6in" height="4.314583333333333in"}

"Comb" of all bias frequencies fed through single wire

Tuned circuits "steer" appropriate frequencies to bolometers

Current return through shunt-fedback SQUID amplifier (low input
impedance)

![](media/image14.wmf){width="9.9in" height="4.73125in"}System Block
Diagram

Intermodulation

SQUID output voltage approx. sinusoidal function of flux ⇒ non-linear:
![](media/image15.wmf)

Non-linear terms lead to mixing products.

For two input frequencies *f*~1~ and *f*~2~ : 3^rd^ order distortion ⇒ 3
*f*~1~

3 *f*~2~

2 *f*~1~ ± *f*~2~

2 *f*~2~ ± *f*~1~

What levels are of concern? Bolometer noise current: 10 pA/Hz^1/2^

Bandwidth: 1 kHz

Total noise current: 320 pA

Bolometer bias current: 10 μA

*i~noise\ ~*/ *i~bias~* = 3.2 ⋅10^-5^ ( -90 dBC )

Depends on ratio of signal to max optical loading

System must be designed for very low distortion -- choose appropriate
technology

Similar constraint applies to all frequency multiplexing schemes

Carrier Nulling

![](media/image16.wmf){width="8.9in" height="4.631944444444445in"}All of
the information is in the sidebands, so the carrier can be suppressed to
reduce dynamic range requirements.

How many bolometers can be MUXed?

1\. Frequency spacing of bias carriers depends on selectivity of tuned
circuits.

2\. Minimum *LC* bandwidth (*Q*) set by bolometer time constant.

3\. Channel spacing set by allowable cross-talk and noise leakage from
other channels.

4\. Minimum frequency set by bolometer thermal time constant

(typ. min. 100 kHz)

5\. Maximum frequency set by large-signal bandwidth of SQUID feedback
loop.

Loop gain-bandwidth product: set by a) required dynamic range

b\) distortion in SQUID

limited by total wiring length of feedback loop

Example: round trip wiring length of 20 cm limits loop gain-bandwidth
product

to \~100 MHz (at 1 MHz extend dynamic range x100)

H. Spieler, Frequency Domain Multiplexing for Large-Scale Bolometer
Arrays, in Proceedings Far-IR, Sub-mm & mm Detector Technology Workshop,
Wolf J., Farhoomand J. and McCreight C.R. (eds.), NASA/CP-211408, 2002
and LBNL-49993, www-physics.LBL.gov/\~spieler.

Solutions

1\. Maximize dynamic range of SQUID

SQUID is limited by flux, so reducing the mutual input inductance allows
larger input current.

Smaller input mutual inductance

increases input noise current

reduces SQUID transresistance (gain)

Limited by bolometer noise and noise of warm amplifier

⇒ SQUID arrays (many SQUIDs connected in series)

We use 100-SQUID arrays from NIST

2\. Cold feedback loop

Use cryogenic Si MOSFET or GaAs MESFET amplifier at 4K

Reduced wire length increases maximum frequency.

With SQUID array and cold/warm feedback loop \~30 channels per readout
line practical.

Hardware

1^st^ generation prototypes tested end-to-end with bolometers

Now fabricating production prototypes

(APEX-SZ, SPT, PolarBear use similar designs)

Layout of flex interconnect

Focal Plane Assembly from bolometer array to wiring harness to SQUIDs

![](media/image17.png){width="3.620833333333333in"
height="2.8583333333333334in"}![](media/image18.png){width="3.5861111111111112in"
height="3.1215277777777777in"}

(Z. Kermish) G10

#######  

####### 

#######  Kapton flex

####### 

####### 

#######  G10

####### 

####### 

####### 

#######  (Z. Kermish) 

####### MUX chip (0.25K stage)

####### 

####### Superconducting spiral inductors 

####### integrated on a chip ![](media/image19.jpeg){width="4.684027777777778in" height="1.304861111111111in"}

####### (fabbed by Northrop-Grumman)

\_\_\_\_\_\_

5 mm

\_\_\_\_\_\_\_

####### 

####### 

####### ![](media/image20.jpeg){width="4.6930555555555555in" height="2.196527777777778in"}

####### 

####### Capacitors can be integrated 

####### with inductors, 

but external chip capacitors

require less space.

NP0 chip capacitors perform well.

####### SQUIDs mounted as arrays of eight in magnetic shield (4K stage)

![](media/image21.wmf){width="4.188888888888889in"
height="2.676388888888889in"}

SQUID mounting board

SQUIDs mounted on Nb pads

to pin magnetic flux

![](media/image22.jpeg){width="4.016666666666667in" height="3.0125in"}

Magnetic Shield

8-channel SQUID Controller

![](media/image23.jpeg){width="3.9305555555555554in"
height="2.886111111111111in"}

Computer-controlled

SQUID diagnostics

Open/closed loop

Switchable gain

![](media/image24.png)

16-channel Demodulator Board

16 individual demodulator channels

2 DDS freq. generators per channel

on-board A/D

opto-isolated computer interface

(M. Dobbs, J. Joseph, M. Lueker, C. Vu)

MUX measurements using 1^st^ generation boards

(T. Lanting)

Bias comb sent to SQUID (top) and measured at SQUID output (bottom)

![](media/image25.png){width="6.2in" height="5.070138888888889in"}

8-channel MUX

7 bolometers

\+ one dummy resistor

Measured noise agrees with theory

![](media/image26.wmf){width="6.595138888888889in"
height="5.769444444444445in"}

Cross-Talk \<0.8% (in agreement with design simulation)

![](media/image27.wmf){width="6.499305555555556in"
height="5.364583333333333in"}One bolometer illuminated by modulated LED

![](media/image28.wmf){width="7.2in"
height="6.358333333333333in"}Measured Noise Spectrum (preliminary
measurement valid \>0.1Hz)

Summary

Exciting times in cosmology -- New technologies: large leap in
sensitivity

Testable theoretical predictions

Many interesting ideas in the community for next generation CMB
detectors

Concepts must be evaluated as complete system, incl. wiring, readout,
\...

Full system design and operation in experiment is crucial to

assess technology

Example: PolarBear is designed as forefront experiment, but site is

readily accessible for tests and upgrades.

Large array technology requires

Process control

Thorough engineering

Wide range of experience

Sustained funding to bridge "proof-of-principle" to working experiment

Significant costs for professional engineering, infrastructure

Intellectual and technical contributions from different groups

Complementary strengths of University and National Lab.