Upcoming experiments aim to produce high fidelity polarization maps of the cosmic microwave background. To achieve the required sensitivity, we are developing monolithic, feedhorn-coupled transition edge sensor polarimeter arrays operating at 150 GHz. We describe this focal plane architecture and the current status of this technology, focusing on single-pixel polarimeters being deployed on the Atacama B-mode Search (ABS) and an 84-pixel demonstration feedhorn array backed by four 10-pixel polarimeter arrays. The feedhorn array exhibits symmetric beams, cross-polar response less than -23 dB and excellent uniformity across the array. Monolithic polarimeter arrays, including arrays of silicon feedhorns, will be used in the Atacama Cosmology Telescope Polarimeter (ACTPol) and the South Pole Telescope Polarimeter (SPTpol) and have been proposed for upcoming balloon-borne instruments

Appel, J. W.

Austermann, J. E.

Beall, J. A.

Becker, D.

Benson, B. A.

Bleem, L. E.

Carlstrom, J. E.

Chang, C. L.

Cho, H. M.

Crites, A. T.

Essinger-Hileman, T.

Fox, A.

George, E. M.

Halverson, N. W.

Harrington, N. L.

Henning, J. W.

Hilton, G. C.

Hubmayr, J.

Irwin, K. D.

Li, D.

Newburgh, L. B.

Nibarger, J. P.

Niemack, M. D.

Parker, L. P.

VanLanen, J.

NASA Technical Reports Server

J Low Temp Phys (201 2) 167,904-910 
DOl 10.1 007/sl 0909-0 11-0420-7 
An All Silicon Feedbom-Coupled Focal Plane 
for Cosmic Microwave Background Polarimetry 
J. Hubmayr . J. W. Appel· J.E. Austermann . J.A. Beall . D. Becker· 
B.A. Benson· L.E. Bleem • J.E. Carlstrom· C.L. Chang. H.M. Cho • 
A.T. Crites· T. Essinger-HiIeman· A. Fox· E.M. George· N.W. Halverson · 
N;L. Harrington. J. W. Henning· G.C. Hilton· W.L. Holzapfel· K.D. Irwin· 
A.T. Lee . D. Li . J. McMahon· J. Mehl . T. Natoli· M.D. Niemack • 
L.B. Newburgh· J.P. Nibarger . L.P. Parker· B.L. Schmitt· S. T. Staggs· 
J. Van Lanen . E..J. Wollack • K.W. Yoon 
Received: 25 July 20111 Accepted: 14 November 20ll! Published online: I December 2011 
4;) Springer Science+Business Media. LLC (outside the USA) 2011 
Contribution of l\"1ST. not subject to copyright. 
1. Huhmayr(I8l)· J.A. Beall · D. Becker· H.M. Cho· A. Fox· G.C. Hilton · KD. Irwin· D. Li· 
M.D. Niemack . J.P. Nibarger . J. Van Lanen 
National Institute of Standards and Technology, Boulder, CO, USA 
e-mail: hubmarr@ nist.gov 
l.W. Appel· T. &singer-Hileman. L.B. Newburgh· L.P. Parker· S.T. Staggs 
Princeton University, Princeton, NJ, USA 
J,E. Austennann· N.W. Halverson· l .W. Henning 
University of Colorado, Boulder, CO, USA 
B.A. Benson· L.B. Bleem · J.E. Carlstrom· Col.. Chang A.T. Crites· J. MehJ· T. Natoli 
University of Chicago, Chicago, IL, USA 
J.E. Carlstrom· C.L Chang 
Argonne National Laboratory-HEP Division, Argonne, Il, USA 
E.M. George. N.L. Harrington· W.L. Holzapfel · A.T. Lee 
University of California, Berkeley, Berkeley, CA. USA 
1. McMahon 
University of Michigan. Ann Arbor, MI. USA 
B.L. Schmitt 
University of Pennsylvania, Philadelphia, PA, USA 
EJ. Wollack 
National Aeronautics and Space Administnttion Goddard Space Flight Center. Goddard. MD, USA 
K.W. Yoon 
Stanford University, Palo Alto, CA. USA 
~ Springer 
https://ntrs.nasa.gov/search.jsp?R=20120016544 2019-08-30T23:12:00+00:00Z
JLow Temp Phy. (2012) 167:904-910 905 
Abstract Upcoming experiments aim to produce high fidelity polarization maps of 
the cosmic microwave background. To achieve the required sensitivity, we are de-
veloping monolithic, feedhom-coupled transition edge sensor polarimeter arrays op-
erating at 150 GHz. We describe this focal plane architecture and the current status 
of this technology, focusing on single-pixel polarimeters being deployed on the At-
acama B-mode Search (ABS) and an 84-pixel demonstration feedhom array backed 
by four IO-pixel polarimeter arrays. The feedhom array exhibits symmetric beams, 
cross-polar response <-23 dB and excellent uniformity across the array. Monolithic 
polarimeter arrays, including arrays of silicon feedhoms, will be used in the Atacama 
Cosmology Telescope Polarimeter (ACTPol) and the South Pole Telescope Polarime-
ter (SPTpol) and have been proposed for upcoming balloon-home instruments. 
Keywords Cosmic microwave background· Polarimeter · Transition edge sensor 
1 Introduction 
Cosmic microwave background (CMB) polarimetry sheds light on the inflationary 
epoch, neutrino physics and the nature of dark energy. These measurements pose an 
incredible experimental challenge due to the weak signal strength in the presence 
of a large background and astrophysical foregrounds. Advances in our understand-
ing of this fundamental physics, from an experimental side, require highly sensitive 
instruments with strong control over polarization systematic errors. 
To meet these technical challenges, we are developing an all silicon, feedhom-
coupled focal plane architecture using silicon platelet feedhoms and microfabricated 
planar orthomode transducers (OMTs) that couple the incident radiation to an ar-
ray of multiplexed transition edge sensors (TESs) [1]. This monolithic plarform is 
scalable to large arrays, which increases focal plane sensitivity, and minimizes po-
larization systematics due to the beam properties provided by a single-moded, corru-
gated feedhom. The tolerances provided by silicon micromachining yield feedhoms 
with symmetric beam patterns and excellent array uniformity. Additionally, fabricat-
ing from silicon substantially reduces physical and thermal mass, ideal for satellite 
and balloon-borne applications. Variants of this technology will be used by ABS [2], 
SPTpol [3] and ACTPol [4] and is proposed for deLITE [5], a balloon-home CMB 
polarimeter. 
Here we describe this focal plane architecture through a 150 GHz demonstration 
array composed of a complete 84-pixel monolithic feedhom array coupled to four, 
IO-pixel polarimeter arrays. We highlight measurements which characterize the beam 
properties of the feedhom array and cryogenic measurements of the polarimeter array. 
2 Technology Overview 
An element in the focal plane is a planar ortho-mode transducer (OMT) coupled to 
a corrugated feedhom. Power from the OMT is sensed with transition edge sensors. 
We implement this concept by fabricating a silicon, corrugated feedhom array and, 
~ Springer 
906 I Low Temp Phy. (2012) 167:9{)4-910 
~,~ ~ 
-
gfl1t11 Feedhorn 
\l~' " § , I Cu 0 
I I . / mter a e I 0: i .~;I - i .~ j i ~ i ~ ~ ~ ~ ~ ! , Ii ~ :.1°) ..... -
I ri'edft.:J'~, H\!c..f.Kr· 
-, : 
. 
: . 
C-\ fT :mln '~ :1 , 
~.w·. 
. • L , 
'f ~.h""" i =: feed horn array 
Fig. 1 (Color online) Silicon feedhom coupled polarimeter may design. Left: This cross-section, ex-
panded model shows the four distinct silicon components tllat create the silicon feedhom-coupled focal 
plane. Right: An image of the 84-pixel hexagonal close-packed silicon corrugated feedhom array inte-
grated with two polarimeter arrays and mounced to a gold-plated copper ring 
separately, a polarimeter array stack composed of a backshort, detector wedge and 
feedhorn interface as shown in Fig. I . 
Following the successful demonstration of an individual silicon feedborn [6], we 
fabricated an 84-pixel silicon platelet corrugated feedhom array shown in the right 
panel of Fig. I . The feedbom array is composed of a stack of thirty-two 500 11m 
thick silicon wafers with features defined by deep reactive ion etch (DRIE) that are 
subsequently stacked, aligned and metalized. The feedborns are designed for single-
moded operation from 125 to 165 GHz. Details on fabrication and 300 K verification 
of the array are discussed in Nibarger et al. [7]. 
Coupled to the feedbom array are 10-pixel polarimeter array stacks. The polarime-
ter array stack consists of four silicon parts: a backshort cap, backshort, detector 
wedge and a feedbom interface as shown in the left panel of Fig. 2. Stacking and 
aligning these parts creates the OMT in circular wavegnide (CWO) of dimensions 
shown in the right panel of Fig. 2. 
Six, lO-pixel hexagonal close-packed detector wedges are fabricated from a 
75 mm diameter, 275 IIffi thick silicon wafer. The 1.6 mm diameter planar OMTs 
lie on 0.5 11m thick silicon nitride membranes. The center-to-center OMT spacing is 
5.26 mm. The backshort cavity is formed by gluing the cap and backshort together. 
In addition to waveguide, the backshort contains stray light absorbing cavities and 
25 IIffi tall radiation blocking structures discussed below. The feedhorn interface con-
tains 250 IIffi tall boss features that are re-entrant with the detector OMT etch to 
extend the waveguide to the OMT. The resulting structure creates a 25 IIffi gap in 
the CWO above and below the planar OMT, which is choked to prevent loss at these 
gaps [8]. The detector wedge outline as well as the features in the support silicon 
parts are defined by DRIE. The backshort, backshort cap and feedhorn interface are 
electroplated with 6 11m thick gold to ensure good waveguide conductance. Simula-
®: Springer 
JLowTernpPhys (2012) 167:904-910 907 
Fig. 2 (Color online) Left: Individual components of a IO-pixel polarimeter array stack. RighI: A CI'OSSM 
section, angled model of the OMT in CWG cavity creatw by the polarimeter anay stack. Dimensions are 
in microns 
tions show the required aligninent between components to be 25 fII1l [8]. Dowel pin 
registration restricts misalignment to 6 fII1l. 
The polarimeter design builds on the previous generation of 150 GHz individ-
ual pixels [1]. Single-pixel devices sbow noise consistent with thermal fluctuation 
noise [9] and optical band-pass measurements that match simulations [10]. However. 
improvements can be made on the 57 ± 9% band average optical efficiency [11] and 
the broad spectrum of absorbed power above the pass-band [10]. 
The lO-pixel polarimeter arrays include several different detector designs in order 
to probe optical efficiency. One detector design has differential lengths of micros trip 
in X-pol versus Y-pol channels to explore dielectric loss in silicon oxide (SiO.). An-
other includes an optimized co-planar waveguide for which we expect a 2% absolute 
efficiency increase due to improved OMT impedance matching. A third design in-
cludes additional gold to niobium microstrip transitions to probe reflections from this 
interface. 
Previous measurements show that out-of-band power is absorbed directly onto 
the TES island. bypassing the filter. We address this out-of-band leak at the array 
level by fabricating radiation blocking structures on the backshort that contact the 
detector wafer around the OMT perimeter. This structure prevents stray radiation 
from reaching the TES island. Additionally. Eccosorb CR-1l7 t filled cavities below 
the TESs reduced out-of-band coupling by a factor of two in single-pixel testing. 
These infrared absorbing cavities are also implemented in the 10-pixel arrays by way 
of etched cavities in the backshort wafer. 
The TES island design varies by experiment 10 optimize sensitivity and match 
the multiplexing readout technique. ABS uses Mo:Cu bilayer TESs with resistance 
suitable for time domain multiplexing (TDM) [12]. SPTpol uses MHz frequency do-
main multiplexing (FDM) [l3] that requires ~ I n sensors. which is achieved by use 
of Al-Mn alloy TESs [14. 15]. ACTPol also uses Mo:Cu bilayer TESs suitable for 
TDM; however with transition temperatures. Te. near 150 mK, a factor of three lower 
1 Emerson and Cuming Microwave Products, http://ww ...... eccosorb.com. Such identification does not imply 
recommendation or endorsement by the National Institllte of Standards and Technology. nOT does it imply 
that the materials OT equipment identified are necessarily the best available for the purpose. 
~ Springer 
908 
Fig.3 (Color online) H-plane, 
E-plane and cross-polar 
response of the central hom of 
the silicon corrugated feedhom 
array at 150 GHz from [7] , The 
solid lines are simulations. The 
cross-polar response deviates 
from simulation due to 
mis-alignment in the 
experimental setup 
-so 
J Low Temp Phys (2012) 167:904-910 
o 
Angle (deg) 
so 
than both ABS and SPTpol. In addition, we have developed low Too low saturation 
power devices suitable for balloon-borne and satellite applications [1 6]. The lO-pixel 
demonstration polarimeter arrays have Mo:Cu TESs with Tc ~ 500 mK and satura-
tion powers ~30 times larger than the design value for ABS, SPTpol and ACTPol. 
The large saturation power was chosen to accommodate laboratory optical measure-
ments. 
3 Results and On-going Work 
Measurements of the feedhorn array at 300 K show >25% fractional bandwidth, 
symmetric beams with < -23 dB cross polar response, return loss < -20 dB and 
insertion loss <-0.4 dB, which we expect to be negligible at sub-Kelvin tempera-
tures [7]. Figure 3 shows that the measured beam maps of the central horn are in 
excellent agreement with simulation. The measured cross-polar response deviation 
from simulation is due to alignment accuracy of the experimental setup. The array 
has been thermally cycled to cryogenic temperatures multiple times with no indi-
cation of performance degradation. We are currently expanding the silicon platelet 
array fabrication techniques to produce ring-loaded structures in the feedhorn throat 
that provide octave bandwidth for multi-chroic applications. Ring loaded structures 
have been fabricated as proof of concept, and a 90/ 150 GHz multi-chroic feedhorn 
design exists. 
Optical efficiency measurements at 150 GHz were made by use of an internal cold 
load [II ] on devices with 0, 5 and 10 mm of extra microstrip length from the DMT 
to the TES island. The measured efficiency is consistent with SiD, dielectric loss in 
the microstrip with tan 8 = 0.01. Subsequently, we have developed a lower loss SiD, 
recipe based on measurements of ~6 GHz microstrip resonators that are quick to 
fabricate and measure. Through this development process, we expect to improve the 
optical efficiency to 70% for pixels with filters and 80% for tilterless polarimeters in 
which free-space filters must be used to define the pass-band. 
The detector properties of four lO-pixel polarimeter arrays have been character-
ized. Figure 4 shows the wafer-scale uniformity of Tc and saturation power, Psa,. 
The saturation power is determined at 80% of the normal resistance of the TES 
with a 300 mK bath Icmper.ture. We find Tc = 495 ± 7 mK for 87 devices .nd 
Psa, = 267 ± 29 pW for 51 devices. 
il Springer 
I Low Temp Phys (2012) 167:904--9\0 909 
3 20 
, A¥A 5 0 5 • 
0 
VAT; 4 4 0 4 4 0 
4 
- -
3 
" 
3 00 
, 90 
, eo ;, 
70 
, 60 
, 50 
, 40 
, 30 30 pW 
-, 
3020100 
76 
10 20 30 mK -30 -20 -10 0 10 20 
DistiJnce (mm) Distance (mm) 
Fig. 4 (Color online) Wafer-scale transition temperature (left) and saturation power (right) uniformity of 
lO-pixel arrays. The key in the upper left of the plOIS divides a polarimeter into X-pol, V-pol and dark TES 
islands. Cblorless sections correspond to unmeasured 'lESs, not dead pixels 
4 Conclusions 
The first monolithic-silicon corrugated feedhom array has been fabricated and shows 
performance that matches simulations. The IO-pixel polarimeter arrays show good 
uniformity in Tc and P,at. We plan to measure beam response functions and array 
level optical efficiency of the polarimeter array stacks coupled to the feedhom array 
using a cryogenic optical dewar in the near future. 
The technology will be field tested with ABS, SPTpol and ACfPol, in which both 
TDM and FDM readout techniques are used. TES islands of low saturation power 
have been demonstrated and can be integrated with the existing polarimeter design 
for future balloon-borne or satellite applications. The fabrication techniques are ap-
plicable to multi-chroic polarimeters, for which designs of both polarimeters and 
feedhoms exist. 
Acknowledgements Work at NIST is supported by the NIST hmovations in Measurement Science pro-
gram. The University of Chicago is supported by grants from the NSF (awards ANT-0638937 and PHY-
0114422), !he Kavli Foundation. and the Gordon and Betty Moore Foundation. B.l. Schmidt acknowl· 
edges support from NSP GRFP and NASA NSTRF fellowships, 
References 
I. K.w. Yoon el at .• AlP Conf. 8er.1185, 515 (2009) 
2. T, Essinger·Hileman et aL. AIP Conf, Ser. 1185. 494 (2009) 
3. I . McMahon el at., AlPConf. Ser. l1SS, 511 (2009) 
4 . M.D . Ni. mack el aI .• SPIE Conf. S". 7741, 77411S (2010) 
5. S.T. Staggs et al ., J. Low Temp. Phys. (2011. this issue) 
6. l .W. Britton el at., SPIE Conf. SeT. 7741 , 77410T (2010) 
7. J.P. Nibarger et al., J. Low Temp. Phys. (2011. this issue), doi: 10. 1007/s10909·01 1·0428-z 
8. J. McMahon et aI., AIP Conf. Ser. 1185. 490 (2009) 
9. lEo Austennann et al .• AIP Conf. Ser. 1185. 498. (2009) 
10. L.E. Bleem et al., AlP Conf. Ser. 1185, 479 (2009) 
II. l .W. Henning ' l at .• SPIE Conf. SOT. 7741. 774122 (2010) 
12. W. Doriese et aI. , Appl. Phys. Lett. 85, 4762 (2004) 
910 J Low Temp Phys (2012) 167:904-910 
13. T.M. Lanting e' aI., Appl. Phys. Leu. 86(11), 112511 (2004) 
14. D.R. Schmidt et at., IEEE Trans. Appl. Supercond.ll, 196-198 (2011) 
15. J. Hubmayr et Ill .. IEEE lTans. Appl. Supercond.ll, 203-206 (2011) 
16. M.D. Niemacket aI., J. Low. Temp. Phys. (2011, this issue) 
~ Springer 


An All Silicon Feedhorn-Coupled Focal Plane for Cosmic Microwave Background Polarimetry

An All Silicon Feedhorn-Coupled Focal Plane for Cosmic Microwave Background Polarimetry

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server