Design and Fabrication of Argonne/KICP Detectors for
CMB Polarization
V. Yefremenko1, A. Datesman1, G. Wang1, J. Pearson1, V. Novosad1, R. Divan3,
L.E. Bleem2, C.L. Chang2, A.T. Crites2, W. Everett2, J. McMahon2, J. Mehl2,
S.S. Meyer2, J. Ruhl4, J. Sayre4, and J.E. Carlstrom2
1
Materials Science Division, Argonne National Laboratory, 9700 S Cass Ave., Argonne, IL 60439,USA,
Kavli Institute for Cosmological Physics, 5640 South Ellis Ave.,University of Chicago, Chicago, IL 60637,USA
3
Center for Nanoscale Materials, Argonne National Laboratory, 9700 S Cass Ave., Argonne, IL 60439, USA
4
Case Western Reserve University,10900 Euclid Ave., Cleveland, OH 44106,USA
2
Abstract.
We present the design, microfabrication and assembly of dual-polarization absorber-coupled Transition Edge
Sensor (TES) bolometer detectors for cosmic microwave background B-mode polarization studies. The device consists of two
separate dies incorporating suspended silicon nitride membranes within silicon frames, carefully aligned perpendicularly and
fixtured face-to-face. Polarization sensitivity around 95GHz is provided by a single dipole-like absorber element; we briefly
analyze this absorber-in-waveguide configuration in closed form using the EMF method. Proximity effect Mo/Au bilayers
provide control of the TES critical temperature between 400mK and 600mK, with a normal resistance Rn~1. DC magnetron
sputtering, wet etching, and liftoff were employed for TES fabrication. Optimization of the superconducting Mo thin film
utilized independent RF bias applied to the substrate during deposition in a confocal geometry. This technique allows
outstanding thin film uniformity and stress to be achieved simultaneously over a 4” wafer surface, leading to excellent
superconducting properties. Device thermal conductance in the range from 25-250pW/K was achieved using suspended silicon
nitride membranes 1 micron thick. The value of the thermal conductance depends very strongly upon the surface roughness, and
therefore in turn upon the treatment of the silicon nitride material during fabrication.
Keywords: Transition Edge Sensor, CMB polarimetry, low temperature detectors, superconducting bolometer
PACS: 07.57.Kp
INTRODUCTION
Accurate measurement of cosmic microwave
background (CMB) polarization is a critical step in
extending our knowledge both of the early Universe
and of fundamental physics at the highest energies.
Because current sub-mm/mm-wave bolometric arrays
have
already
achieved
background
limited
performance, substantial gains in sensitivity can only
be realized by significantly increasing the number of
detector elements in a focal plane. Thus, design and
technology efforts aim to simultaneously achieve both
the high sensitivity required for state-of-the-art
measurement goals and the reproducibility and
reliability needed for array deployment. Transition
Edge Sensor (TES) bolometers [1] are the most
sensitive detectors for the spectral range of interest,
from 200-1000 m, and are also suitable for
polarization sensitive array applications [2]. Different
absorber-coupled detector architectures utilizing horn
antennas and planar antennas with micro-strip
transmission lines are discussed elsewhere. [3-5]
In this paper, we present the design and
microfabrication of a dual-polarization absorber-
coupled TES detector configuration which consists of
two separate dies incorporating suspended silicon
nitride membranes with silicon frames, carefully
aligned perpendicularly and fixtured face-to-face. This
ANL/KICP design is the result of a collaborative effort
between the Kavli Institute for Cosmological Physics
(KICP) at the University of Chicago and Argonne
National Laboratory (ANL).
DESIGN AND FABRICATION
The general design of the detector is shown in Fig.1.
Each individual frame, corresponding to a single
polarization, consists of a dipole-like absorber for
optical coupling and a superconducting TES for
temperature readout suspended together on a
rectangular silicon nitride membrane via long and
narrow legs. Separation of the absorber and TES
allows flexible component optimization. The gap
between the face-to-face mounted dies, which provides
necessary thermal and electrical isolation, is achieved
using calibrated 50 (or 30) m glass balls fixed to the
surface of the frames. Cross-shaped alignment markers
located at the interior corners of the frame enable
accurate face-to-face die assembly.
A feedhorn and circular waveguide carry
electromagnetic radiation from free space to the
detector, where it is dissipated as heat in the absorber.
The filter stack, feedhorn, waveguide cutoff, backshort
and absorber geometry define the center frequency and
bandwidth of the detector. Each frame includes a cutout which exposes the electrical connections on the
facing die.
Our devices employed amorphous low-stress Si-N
films 1 m thick grown by a commercial vendor on
(100) Si wafers using low pressure chemical vapor
deposition [6].
(b)
(a)
(c)
requirements
of
frequency-domain
SQUID
multiplexing [8], the resistance target is about Rn~1.
Based upon experimental measurements, we found
~30nm Au to be an appropriate thickness for our
design geometry. The proper thickness of the
superconducting Mo layer was then determined based
upon the target operating temperature.
Mo and Au layers were deposited by DC
magnetron sputtering in an in-line, normal orientation
on 2” Si-N/Si wafers at 300K. To guarantee excellent
interface transparency, deposition occurred under a
single vacuum with only a short pause between Mo
and Au depositions. The base and Ar gas working
pressures were ~10-8Torr and 3mTorr, respectively.
The deposition rates were 0.128nm/s (Mo) and
0.18nm/s (Au). The transition temperature for different
Mo/Au bilayer thicknesses and thickness ratios, for
both in-line and confocal deposition systems, are
presented in Fig. 2. The maximum TC nonrepeatability between depositions for the in-line
system was 38 mK.
FIGURE 1. Design of the dual-polarization absorbercoupled bolometer detector. (a) Mo/Au bilayer TES with Nb
leads 7 m wide. (b) Individual frames rotated 90° and
mounted face-to-face comprise a complete detector. (c)
Absorber and dual TESs on an isolated Si-N structure
suspended on legs 15 and 25 m wide and 1.35 mm long.
Transition Edge Sensor
Device fabrication began with DC magnetron sputter
deposition of superconductor and normal metal thin
films under a single vacuum prior to lithography and
patterning by wet etching. The proximity effect in
normal metal-superconductor bilayers was utilized to
obtain the target operating temperature of 450 mK.
Because Mo (Tc~0.9K) is insoluble with noble metals
and oxidizes only slowly, we employed Mo/Au TES
bilayers. In the case of a clean S/N interface, the
superconducting transition temperature of a bilayer
film should be a function only of the thickness ratio,
ds/dn [7]. Transmission across the S/N interface is
strongly process-dependent, however, so an
experimental approach to TC management was used.
Since the conductivity of gold is much (~5x at RT)
higher than the normal state conductivity of Mo, the
TES normal state resistance Rn is nearly a function of
only the normal film thickness dn. The normal metal
film thickness dn is independent of the thickness ratio
ds/dn which determines the bilayer transition
temperature. Therefore, to a certain extent TC and Rn
may be adjusted independently. Due to the
Transition Temperature (mK)
800
In-line
700
26/28
25/30
Confocal
22/30
29/35
600
25/40
25/35
500
17/30
25/30
26/29
400
17/35
24/28
300
0.4
0.5
0.6
0.7
0.8
0.9
1
Thickness Ratio ds/dn
FIGURE 2. Mo/Au bilayer transition temperatures for
different thickness ratios for both in-line (open) and confocal
(closed) deposition systems. The confocal system was
optimized for deposition of zero-stress thin films.
In addition to the in-line deposition system for 2”
substrates, we have recently brought on-line a new,
confocal deposition system which accommodates
larger sizes. The confocal system features adjustable
tilt magnetron guns and a rotating carriage, which
enable excellent thin film uniformity. [9] Using this
system, we have demonstrated film uniformity better
than 0.5% over 4” of wafer diameter.
Mo thin films deposited in this chamber at room
temperature with a tilted DC magnetron gun exhibit
high tensile stresses, with the tensile peak at a working
pressure near 1.5 mTorr. [10] Because thin films with
a slight compressive stress (near -50 MPa) are required
to obtain ideal superconducting characteristics, this is
not a suitable result. We were able to achieve the
desired film quality at a working pressure of ~4mTorr
via the simultaneous application of 50W RF bias to the
substrate platter during Mo deposition. The quality of
these films is superior to the quality obtained using the
in-line system.
The 110x80m2 TES geometry was defined via
optical lithography and wet etching with KI2 (for Au)
and H3PO4 : HNO3 : CH3COOH : H2O (for Mo). The
edge obtained via wet etching was examined by
atomic force microscopy (see Fig.3).
nm
100
50
1.0
2.0
3.0
m
FIGURE 3. Atomic Force Microscope (AFM) image of the
wet-etched TES edge.
Absorber and Leads
Optical lithography and lift-off processes were used to
fabricate both absorbers and leads. The TES leads
consist of superconducting Nb 7m wide and 120 nm
thick, with a total length between the sensor and the Si
frame of 1400m. The dipole-like absorber consists of
a 10nm thick layer of Au. Its 1162m×18m
geometry was obtained using Ansoft’s HighFrequency Structure Simulator (HFSS) to guarantee
polarization sensitivity in the 95GHz spectral range
[11].
We refer to the absorber geometry as “dipole-like”
because its length is much less than ½. Although
computational tools were employed in its design, an
analytical investigation of the absorber-in-waveguide
configuration is very instructive. Because post-inwaveguide mounting of microwave devices predates
modern FDTD E/M solvers, there is a rich literature
on this topic. [12] The technique is referred to as the
EMF method. [13]
Matching fields and currents at the surface of the
absorber and decomposing the fields in the waveguide
in terms of the waveguide unit modes, it is possible to
express the input impedance Zin for a circular
waveguide supporting only the TE11 mode as
Z in 
1
2
j11TE ( Z11TE  || Z11TE  ),
(1)
I
where I is the peak absorber current, Z11TE± is the
frequency-dependent impedance of the propagating
mode seen looking out from the absorber in either the
positive- or negative-traveling direction, and j11TE is
the projection of the absorber current distribution on
to the known modal basis.
2
The design condition for maximum absorption is
Zin=Zo, the waveguide characteristic impedance. The
bandwidth and frequency performance of the
absorber-in-waveguide configuration are described by
the term Z11TE+‖Z11TE-. Because the current distribution
along the length of the x-oriented absorber is known to
follow a cos(x/2ℓ) distribution, Zin may be calculated
straightforwardly for a given set of absorber
dimensions. Unlike HFSS, determination of Zin in this
manner is computationally simple because it does not
require explicit calculation of the fields within the
waveguide.
With a few simplifying assumptions, we predict
~96% absorption at the center frequency for the stated
design via the EMF method, in good agreement both
with HFSS and with measured data. The method
provides valuable physical insight and in the future
should be useful prior to HFSS validation of new
designs.
Thermal Link
Patterning of the thermal link follows fabrication of
the TES, absorber, and leads. Standard KOH wet
etching of silicon (30% in water at 80°C) was used to
obtain a silicon nitride membrane window. Device
thermal conductances G in the range from 25250pW/K were achieved using suspended 1 micron
thick membranes similar to the one shown in Fig. 1(c).
The bridge structure was defined by RIE with CF4,
followed by oxygen plasma cleaning.
FIGURE 4. Thermal conductance at 0.5K as a function of
the width/length ratio for 1m thick Si-N. The leg length is
~1mm in all cases.
To obtain the target value of thermal conductivity
with commercial silicon nitride material, we
investigated thermal transport in bridged membranes
with different width-to-length ratios. As shown in Fig.
4, the target G~200pW/K required for background-
limited performance was achieved with a width-tolength ratio of about 0.08.
Using test samples consisting of a TES together
with a heater on narrow Si-N bridges of various
lengths, we found that the contribution of the
superconducting leads to the thermal conductance G is
negligible (~5%).
When the membrane dimensions are similar to the
phonon mean free path in bulk material, the surface
plays a significant role in phonon scattering. Phonon
reflection from the surface, which dominates the
thermal transport properties of the membrane, is very
sensitive to the conditions under which the Si-N is
grown, and to every step employed during the
microfabrication process. This explains our
observation that the measured thermal conductances
of bridged structures changed dramatically (at least a
factor of two) depending upon the specific processing
steps - especially wet or dry (O2 RIE) cleaning employed. Avoidance of damaging plasma processing
motivates our decision to employ wet etching and
liftoff processing, rather than thin film deposition
followed by RIE patterning.
Characterization of Detector Components
The detector components were characterized using a
3
He cryostat in the temperature range from 0.3 to 0.6K,
using the TES detector as the sole source of membrane
heating. Measuring the TES resistance as a function of
bath temperature for a range of bias currents, we
calculated the thermal conductance G=dP/dT. Typical
measured values of detector component parameters are
presented in Table 1.
array deployment of large numbers of detectors, for
which it will be necessary to achieve good uniformity
of TC across large substrate areas. We discussed EMF
method analysis of the dipole-like absorber geometry,
and met design targets enabling background-limited
performance. Further optical and noise measurements
are ongoing.
ACKNOWLEDGMENTS
We thank the devices group at NIST-Boulder for
providing the SQUID array for TES readout, as well as
Matt Kenyon from JPL for useful discussions about
thermal transport and device microfabrication. The
work at ANL, including the use of the facilities at the
Center for Nanoscale Materials, was supported by
UChicago Argonne, LLC, Operator of Argonne
National Laboratory (“Argonne”). Argonne, a US
Department of Energy Office of Science Laboratory, is
operated under Contract No. DE-AC02-06CH11357.
Work at the University of Chicago is supported by the
National Science Foundation through Grant ANT0638937 and the NSF Physics Frontier Center Grant
PHY-0114422 to the Kavli Institute of Cosmological
Physics at the University of Chicago. KICP also
receives generous support from the Kavli Foundation
and the Gordon and Betty Moore Foundation.
REFERENCES
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2.
3.
4.
TABLE 1. Typical Parameters of Detector Components.
Absorber
- Sheet resistance
TES
- Transition temperature
- Normal resistance
  T / Rn (dR / dT )
Thermal link
- Thermal conductance @ 0.5K
5.
5.6 □
460 mK
~1 
100
6.
7.
212 pW/K
8.
SUMMARY
We discussed the design, microfabrication and
assembly of dual-polarization absorber-coupled
bolometer detectors for cosmic microwave background
B-mode polarization studies. The proposed design is
suitable for array applications. Deposition of uniform,
high-quality superconducting thin films was
demonstrated in a confocal geometry by application of
RF bias during deposition. This technique will enable
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