1. University of California, Berkeley
TES Bolometer Array
For the APEX-SZ Camera
Hello, my name is Jared Mehl, and I’m a graduate student in physics at the University of California-Berkeley. I’m going to be presenting a description of the APEX-SZ Galaxy cluster survey, focusing on the TES bolometer array.
Jared Mehl
University of California, Berkeley
LTD 12, Paris, 2007

2. Collaborators U. C. Berkeley McGill University MPIfR
Bill Holzapfel
Zigmund Kermish
Adrian Lee
Martin Lueker
Jared Mehl
Tom Plagge
Paul Richards
Dan Schwan
Martin White
McGill University
Matt Dobbs
Trevor Lanting
James Kennedy
MPIfR
Rolf Guesten
Ruediger Kneissl
Ernst Kreysa
Karl Menten
Dirk Muders
Peter Schilke
Axel Weiss
University of Colorado
Nils Halverson
Dan Becker
Amy Bender
Cardiff University
Peter Ade
University of Bonn
Kaustuv Basu
Frank Bertoldi
Martin Nord
Florian Pacaud
Here’s a list of all of the APEX-SZ collaborators. The US and Canadian team members were primarily responsible for the receiver, while our German collaborators were primarily responsible for the APEX telescope. Both teams are collaborating on data analysis.
LBNL
Helmuth Spieler
MPE
Hans Boehringer
NIST
Hsiao-Mei Cho

3. APEX-SZ 12 m antenna, ALMA prototype 5100 m Atacama Plateau, Chile
320 element TES bolometer array
Frequency domain SQUID
multiplexer readout
Pulse-Tube Cooler (PTC)
Use Sunyaev-Zel’dovich effect to
study galaxy clusters
Operating and taking data
The APEX-SZ camera is on of the instruments installed on the APEX Telescope. APEX has a 12 m primary designed for sub-mm observations, and is an ALMA prototype. It is located at 5100m altitude on the Atacama Plateau in Chile, which is an excellent site for mm-wave astronomical observations. The APEX-SZ camera consists of a 320 element TES bolometer array, instrumented with a frequency domain SQUID multiplexer readout, and cooled to 250 mK with a combination of mechanical Pulse-tube cooler and 3 stage He sorption fridge. We use the sunyaev-zeldovich effect to discover and study galaxy clusters. The full configuaration receiver was deployed in April 2007.

4. TES Array Horn-coupled 55 Pixels Per Wedge 6 Wedges Per Array
Here we see the APEX-SZ detector array. A feedhorn structure couples each detector to the receiver and telescope optics. Underneath, six identical wedges with 55 pixels per wedge form the array.
Horn-coupled
55 Pixels Per Wedge
6 Wedges Per Array

5. TES Array Horn-coupled 55 Pixels Per Wedge 6 Wedges Per Array
Each pixel has a 3 mm diameter gold spiderweb absorber, seen here (*POINT*). The spiderweb shape is chosen to reduce the heat capacity of the absorber, which reduced the optical time constant of the bolometer, as well as to reduce x-section to cosmic ray hits.
Horn-coupled
55 Pixels Per Wedge
6 Wedges Per Array

6. TES Bolometer t Au spiderweb absorber Al/Ti Bilayer TES
R ~ 200  / sq
~ 9 ms t
optical
Al/Ti Bilayer TES
R ~ 1.2 
normal
T ~ 450 mK C
The spiderweb film’s thickness gives a sheet resistance of 200 ohm/sq, which is not the ideal for absorption, but is a compromise with thermal evaporation constraints.
Optical power absorbed in the web heats up the Aluminum / Titanium bilayer transition edge sensor. The TES is located at the center of the web to reduce the optical time constant contributions from thermal spreading time in the web. The TES metal thicknesses are chosen to set the Tc to 450 mK and normal resistance to roughly 1.2 ohms.
A gold thermal link runs parallel to the bias leads from the TES to the thermal bath, and is used to tune the thermal conductivity of the bolometer to 200 pW/K.
The entire bolometer sits on a 1um thick low-stress silicon nitride suspension, with is thermally isolated from the bath temperature by a xenon difluoride gas etch, with removes the Si under the suspension.
I fabricated these bolometers at the Berkeley Microlab cleanroom facility.
Au Thermal Link
G ~ 200 pW / K
low-stress Si N substrate, released with XeF etch 3 4 2

7. Bolometer Cavity Design
Band-defining filters
Waveguide
Feed Horn
Spiderweb Absorber
TES
Here we see a cross-section schematic of the bolometer cavity design. The APEX-SZ detectors observe the sky in a band centered at 150 GHz. The upper edge of the band is defined by metal mesh filters, and the lower edge of the band is defined by the waveguide frequency cutoff. The silicon wafer thickness is chosen to create a resonant cavity with the field maximum located at the spiderweb absorber.
Silicon
Wafer
Airgap
Invar Backshort

8. Bolometer Cavity Simulation
150 GHz Simulation, 200 Ohm/sq Abs, λ/4 Backshort
Absorbed Power
Reflected Power
Radiated Power
The bolometer cavity design was optimized using the 3d finite element analysis e&m simulation package HFSS. Shown here is the results of one of these simulations for a quarter-lambda in silicon backshort distance and the non-optimal 200 ohm/sq absorber. The green curve is the power absorbed in the spiderweb, the blue is the power reflected back out of the horn, and the red is the power radiated out of the bolometer cavity to nearby pixels. The red curve divided by 6 (for 6 neighboring pixels) represents the upper limit on pixeltopixel crosstalk. We are currently running simulations of more exotic geometries to further improve bolometer performance.
Optimize bolometer cavity
HFSS - Ansoft Corporation - 3D finite-element electromagnetic sims

9. Frequency Domain Multiplexer
AC bias bolometers from 200 kHz – 1 MHz
The TES array is instrumented with a frequency domain multiplexer readout system, also built at Berkeley.
A comb of AC carriers, with frequencies ranging from 200 kHz to 1 MHz, bias bolometers in series with an LC resonant circuit that selects the carrier for a given bolometer. Bolometer signals appear as amplitude modulated sidelobes of the carriers. The summed current from the bolometers, with the carriers suppressed by a nulling signal 180 out-of-phase with the carrier comb, is measured with a 100-element SQUID array, operated with shunt feedback SQUID controller. An analog demodulator recovers the observation signal from each sensor.
Note: Point to adrian’s poster

10. Frequency Domain Multiplexer
AC bias bolometers from 200 kHz – 1 MHz
Zero power dissipation
at sub-Kelvin stage
Strong rejection of time-
varying B-fields
This technology is now mature and has some distinct advantages over the time domain multiplexing systems:
-zero power dissipation at sub-Kelvin
-strong rejection of time-varying B-fields, which allows for
- simple shielding requirements.
- in the absence of RF-contamination we see greatly reduced vibration sensitivity.
Simple shielding
requirements
Reduced vibration
sensitivity

11. Frequency Domain Multiplexer
Here are some pictures of the physical implementation of the entire system. Bias carrier and nulling combs originate here in the 16 channel oscillator demodulator board, and pass through the cold Nb inductors and chip capacitors to the bolometers. SQUID arrays, made by NIST, amplify the signal. The SQUID arrays are operated by 8 channel SQUID controller boards. Finally the signal passes back to the oscillator/demodulator board to be digitized.
SQUID array
made by NIST

12. Bolometer Stability t t t t t t Too fast for stable operation > 5.8
sensor
Conservative Stability Criterion:
>
5.8
[ Irwin, JAP,1998 ] t
bias
For current fMUX (AC bias) at 50% bolometer superconducting transition: 2L t =
MUX ~
50 μs
bias R
Early bolometers:
Too fast for stable
operation C
When we first tried to operate our bolometers with the readout system, our TES’ would latch superconducting as soon as they entered the superconducting transition. A conservative stability criterion states that the TES effective time constant must be at least a factor of 5.8 greater than the time constant of the bias circuit. If the bias circuit is too slow, the TES can fluctuate faster than ETF can stabilize it.
For the AC bias fMUX system and a bolometer at 50% rnormal, tau_bias is roughly 50 microseconds. Our early bolometers had an intrinsic time constant (before modification by loopgain) that was only 100 microseconds. This is way too fast for stable operation. We can only tolerate a sensor time constant of 300 microseconds minimum.
*Minimum stability requirement replace 5.8 with 1
*Factor of 2 in t_bias due to ac-bias, capacitor also delays power delivery to TES t
= ~ 100 μs G
With conservative stability criterion we can tolerate: t t
= / (Loopgain + 1) > 300 μs
sensor

13. Increasing Heat Capacity
Current Bolometers:
Add 3 μm thick gold ring
Slow down detectors t
~ 30 ms
To stablilize our detectors, we added a large, 3 um thick gold ring to our TES. This added heat capacity slowed the bolometer intrinsic time constant to 30 milliseconds, which allows stable operation deep into the superconducting transition with large loop gains. It is critical that the added heat capacity be strongly thermally coupled to the TES for this trick to work.
Allows operation deep into transition
Critical for heat capacity to be
strongly coupled to TES

14. Noise Performance 120 aW / Hz 1/2
With the TES’ stabilized, the noise performance is quite good. We get the expected noise perfomance with a flat, white spectrum.
Noise budget for APEX spring deployment:
NEP = sqrt(([NEP_photon]^2 + [NEP_thermal_fluctuation]^2 + [System_Current_Noise*V_bias]^2 )
TOTAL NEP 101 aW/rtHz
SYSTEM NOISE 11 pA/rtHz * 6e-6 = 66 aW/rtHz
THERMAL LINK NOISE 55 aW/rthz
PHOTON NOISE ESTIMATE 44 aW/rtHz

15. Cluster Map Abell 2163 ~1 hour integration Courtesy T. Lanting
Here is a temperature map of the known cluster Abell 2163 made from ~1 hour of integration time during the April 2007 deployment. The SZ effect causes galaxy clusters to appear as cold spots on the sky at 150 GHz. Our map noise is integrating down properly with time and our data pipeline is rapidly becoming mature. The next analysis task is to look at our blank-sky field obseravtions.
Courtesy T. Lanting

16. South Pole Telescope Sub-millimeter Wavelength Telescope:
10 meter telescope (1’ FWHM beam at 150 GHz)
1st Generation Camera:
1 sq. deg FOV
~1000 pixels
Observe in 3+ bands between GHz simultaneously with a modular focal plane
220 GHz
150 GHz
95 GHz
The same technology I’ve described here is also currently deployed on the South Pole Telescope, but scaled up to more channels. SPT has 3 times the pixels of APEX-sz and 3 frequency bands at 95, 150, and 220 GHz. The South Pole Telescope is fully dedicated to SZ camera obseravations and is at the best site for mm-wave astronomy on earth.
150 GHz
Funded by NSF
150 GHz
95 GHz

17. The End.