We have successfully fabricated a superconducting transition edge sensor (TES), bolometer that centers on the use of electron-phonon decoupling (EPD) for thermal isolation. We have selected a design approach that separates the two functions of far-infrared and THz radiative power absorption and temperature measurement, allowing separate optimization of the performance of each element. We have integrated molybdenum/gold (Mo/Au) bilayer TES and ion assisted thermally evaporated (IAE) bismuth (Bi) films as radiation absorber coupled to a low-loss microstripline from niobium (Nb) ground plane to a twin-slot antenna structure. The thermal conductance (G) and the time constant for the different geometry device have been measured. For one such device, the measured G is 1.16 x 10(exp -10) W/K (plus or minus 0.61 x 10(exp- 10) W/K) at 60 mK, which corresponds to noise equivalent power (NEP) = 1.65 X 10(exp -18)W/vHz and time constant of approximately 5 microseconds

Benford, Dominic

Brown, Ari-David

Chervenak, James

Jethava, Nikhil

Kletetschka, Gunther

Mikula, Vilem

U-yen, Kongpop

NASA Technical Reports Server

Development of superconducting transition edge sensors based on
electron-phonon decoupling
Nikhil Jethava*'b , James Chervenak', Ari-David Brown', Dominic Benford', Gunther Kletetschkaa,
Vilem Mikulaad, Kongpop U-yens
'NASA Goddard Space Flight Ctr, Greenbelt, MD, USA 20771;
bGlobal Science and Technology, 7855 Walker Drive, Greenbelt, MD 20770;
'MEI Technologies, Inc., 7404 Executive P1 #400, Lanham, MD 20706;
dDepartment of Physics, Catholic University of America, 200 Hannan Hall, Washington, DC 20064
ABSTRACT
We have successfully fabricated a superconducting transition edge sensor (TES), bolometer that centers on the use of
electron-phonon decoupling (EPD) for thermal isolation. We have selected a design approach that separates the two
functions of far-infrared and THz radiative power absorption and temperature measurement, allowing separate
optimization of the performance of each element. We have integrated molybdenum/gold (Mo/Au) bilayer TES and ion
assisted thermally evaporated (IAE) bismuth (Bi) films as radiation absorber coupled to a low-loss microstripline from
niobium (Nb) ground plane to a twin-slot antenna structure. The thermal conductance (G) and the time constant for the
different geometry device have been measured. For one such device, the measured G is 1.16x10 -10 W/K (± 0.61x10-
10 W/K) at 60 mK, which corresponds to noise equivalent power (NEP) = 1.65 X 10-18W/ MHz and time constant of —5 gs.
Keywords: Bolometer, Thermal Conductance, Electron-Phonon decoupling, Transition Edge Sensors
1. INTRODUCTION
A TES bolometer has a higher sensitivity-response time product than a semiconducting bolometer with the same
geometry due to the strong negative electrothermal feedback intrinsic in typical a voltage-biased TES 1'z'3. TES
bolometers are inherently low impedance devices, so they are well matched to being read out by DC SQUID amplifiers.
SQUID amplifiers are mature and have been used extensively for TES bolometer arrays in formats of up to 1000 pixels.
Because SQUIDS operate at the base temperature of the bolometer, they can be coupled very closely, removing the
complex interfaces necessary with semiconducting bolometers and FET amplifiers.
Most TES employ fragile mechanical isolation in order to decouple the detector temperature from a cold stage
temperature in order to achieve high sensitivity. Since, the electronic and phononic temperatures of the sensor can be
significantly different, one can eliminate the need for mechanical isolation if T«1K and sensors volume is small (-10s
pm). The weak interaction (thermal coupling) between the electrons and the phonons in metals allows a large
temperature rise of the electrons for small input power. The main advantage of using this effect for thermal isolation
rather than fragile mechanical isolation is that the bolometer does not require micromachining or assembly. Instead, the
radiative power is coupled directly into the electrons in the metal, and the temperature of these electrons is measured
directly.
Detectors that achieve the required NEP will be designed taking into account this effect, called electron phonon
decoupling (EPD). This method can produce low NEPs at achievable temperatures while enabling large-format arrays
with reasonable fill factor and rapid thermal response time.
https://ntrs.nasa.gov/search.jsp?R=20100024510 2019-08-30T10:10:15+00:00Z
2. FABRICATION
Figure 1. (a) SEM image of 25 µm2
 device. (b) SEM image of 50 µm 2
 device. (c) SEM image of 1225 µm 2
 device.
(d) Image shows the Nb ground plane, an insulator and Mo/Au TES sensor. (e), (f) and (g) are images of 300 GHz,
1 THz, and 3 THz devices, respectively. For (e), (f) and (g) , a thin layer of Bi is deposited to act as an radiation
absorber.
2.1 Electron-Phonon decoupling
Figure 2. The thermal block diagram of the proposed EPD.
2.2 Detector design
We have integrated a small volume resistor, a similarly small volume TES, and a low-loss microstripline to an antenna
structure to fabricate EPD TES receiver. We have chosen to implement a twin-slot antenna because of its symmetric
beam pattern, high efficiency at short wavelengths, good rejection of out-of-band radiation, and the convenient ground
plane that can be used to shield DC wiring. Short sections of microstrip with low doss at THz frequencies (high quality
Au, which modeling shows will have —O.1dB loss) will allow manipulation of the placement of circuit components for
optical coupling purposes. Nb bias line chokes allow on-chip filtering of the TES bias lines and improved shielding.
Figure 2 shows the schematic layout of complete EPD detector.
Niobium (Nb) will be employed as both the ground plane and stripline material in the lower frequency range of interest
(300 - 500 GHz) because of its superconductivity. However, at frequency above its energy gap Nb acts like a normal
metal with relatively high impedance. Consequently, gold, which is a low impedance normal metal film will be used,
and retains low radiation loss properties over a wide frequency range. In order to minimize both radiation loss and high
reactance, we aimed to maximize the gold microstrip sheet conductance so as to permit a variety of microstrip
geometries. We have recently these devices and plans for testing are underway70.
We use ion assisted thermally evaporated (IAE) Bismuth (Bi) as radiation absorber' o. It is found that the residual
resistance ratio of the IAE Bi is an order of magnitude higher than that of evaporated (TE) Bi. IAE Bi can be useful for
antenna-coupled sub-millimeter bolometers, because it is resistive (i.e., it has a low reactance) to high frequencies. This
contrasts with TE Bi, which is reactive at frequencies as low as 1 THz. The improved performance of IAE Bi over that
of TE Bi might be attributed to its smaller grain structure and/or lower impurity concentration. Because the IAE Bi
resistivity r is a thickness-independent quantity, we can easily estimate the noise power P of a termination resistor
fabricated with this material10
EPD-TES
ground plane	 Absorber Superconducting plug
Microstrip leads
Si substrate	 Dielectric
Antenna slats	 layer
Figure 3. This schematic side view of the circuit fabrication concept for producing the EPD TES devices.
2.3 Detector fabrication
Electron-phonon decoupled TES were fabricated on 4" Si(001) wafers. One notable aspect of our sensor design is the
absence of membranes, which greatly simplifies the fabrication process. An Nb ground plane was DC magnetron sputter
deposited and etched, via a CF4/02
 plasma, in regions designated for the slot antennas. A silicon dioxide dielectric was
next applied via electron cyclotron resonance plasma enhanced chemical vapor deposition. This was a low temperature
process, in which the wafer temperature never exceeded 180 C, and enabled a conformal dielectric coating. Niobium
microstrip lines were then deposited and etched in a manner similar to that used to fabricate the ground plane. The TES
consisted of a Mo/Au bilayer film and was deposited on top of the silicon dioxide using electron beam deposition and
DC magnetron sputtering for the Mo and Au, respectively. The Mo/Au deposition was conducted inside a custom set of
vacuum chambers which are connected via a common loadlock. This configuration allowed for a high degree of T,
uniformity and reproducibility of the TES bilayer film across the wafer, because the Mo surface was never exposed to
atmosphere. The TES were delineated using a positive photoresist mask, and ion milling and reactive ion etching, with a
CF4IO2 plasma, were used to etch the Au and Mo, respectively. Niobium bias leads were then deposited and etched in a
manner similar to that used to fabricate the ground plane and were aligned so as to overlap the TES by 0.5 pm and 1.0
pm for 25, 50 lum and 100, 300, 1225 pm devices, respectively (see Figure 1). Aluminum pads were deposited through
a liftoff mask and made contact to the bias leads.
The final step in the fabrication process involved deposition of a bismuth termination resistor. Bismuth is a highly
reactive material, which makes it susceptible to subsequent fabrication processes, e.g., photoresist application and
developing. Consequently, we first patterned a PMGI lift-off mask prior to IAE deposition of 99.999% bismuth. The
bismuth was then lifted-off in acetone. Fabrication of the TES diagnostic devices, whose purpose was to solely
characterize the TES, was a much more simple process. This process only consisted of TES deposition on a Si02-coated
Si(OOI) wafer and deposition of the Nb bias leads and Al pads.
3. EXPERIMENTAL RESULTS
We have fabricated devices in different geometries. Table 1 presents details of these devices and their characteristic
parameters. We used Resistance-Temperature curves: G(T) = J2 R, /(VT) (Figure 4) to determine the thermal conductance
for each device 6,1,9. Here, G(T)is the thermal conductance at the temperature T, R„ is the normal state resistance of
sensor, and VT is the hysteresis in the Resistance-Temperature curve while increasing and decreasing the fridge
temperature.
Table 1. The surface area, superconducting transition temperature (Tc) and normal state resistance (Rn) for four different detector
geometries are listed. The TES was fabricated using bilayer of molybdenum (Mo) and gold (Au) material. The Mo thickness = 50 nm
and the Au thickness = 20 nm for all the devices. Tc and Rn values are calculated at lowest excitation current, i.e, 0.5 µA.
Device Surface area
(w)
On Nb Ground
plane
Rn
(.Q/El)
Tc
(mK)
A 25 No 0.28 410
B 50 No 0.45 559
C 300 Yes 0.36 232
D 1225 Yes 0.36 341
Temperature (K)
Figure 4. Superconducting transition curves of Device B at different bias current. The "Up" and "Down" in the plot
corresponds to increasing and decreasing the bath temperature of cryostat, respectively. We used Adiabatic
Demagnetization Refrigerator (ADR) with dry pulse tube cryostat to reach to the base temperature of 50 mK. Thin Nb
foils and Lead tape magnetically shield the device package.
The proposed EPD approach will employ a thermal diagram (Figure 1) and electrical connection (Figure 2). An antenna
couples power into an absorbing resistor, which thermalizes the power into the electron bath in a time given by its
internal thermal conductance, GwF. This power is communicated through G,iceiron_elec,ron (Ged to the TES. The power then
dissipates into the lattice phonons through a weak thermal link Geleciron phonon (Gep). Provided that Gep<<G WF and Gep«Gee,
which we calculate to be true for temperatures below — 400mK, the sensitivity and time constant of the detector are then
dominated by Gep. This approach is very similar to that used by other research groups 8 '9, who have tested EPD devices
fabricated by our group, yielding accurate knowledge of materials parameters related to our processes.
The power conducted between electrons at a temperature Te and phonons at a temperature of Tp is given by:
Pep =EV( e —TP^	 (1)
where V is the volume of the metal and S is a material-dependent constant. A typical value of E for gold at modest
cryogenic temperatures (< 1 K) is thought to be around 5 x 10 8 W/m3/K8.
Differentiating Eq. (1) provides the thermal conductance:
Gep = 5EVT 4 .	 (2)
As the film thickness is much smaller than the thermal acoustic wavelength as is certainly the case in these experiments,
the Kapitza conductance is underestimated by acoustic mismatch arguments. [here you seem to be saying that acoustic
mismatch arguments don't accurately model Kapitza conductance; if not, what does? You should clarify this] As has
been determined experiment by many other groups6 ' 7' 8 ° 9, the principal impedance for heat flow is the electron phonon
coupling. The electron phonon conductance depends on T'. Figure 5 plots the measured G values for all the devices at
different base temperature, and T ° fit to the data.
As seen from the Figure 5, Device D does not fit very well to T' fit. The higher order T term (T) could fit Device D data
more precisely. In the cases in which electron scattering mechanisms are governed by impurities, defects, or boundaries
of the film, the shortened mean free path of the electrons should affect the electron-phonon coupling in the disordered
metals. In a disordered film, the electron mean free path le is very short compared to the wave length of phonons; q.le((l
with q as the phonon wave vector. In this dirty limit, M. Yu. Reizer and A. V. Sergeev il
 suggested that the thermal
conductivity should be proportional to T s , not T4 . Since, Device D has larger sensors area, the distortions in the film can
produce imperfections in the film, which can lead to higher G values.
e^
E
Q
101
1 X 
I
0.2 0,25 0.3 0.35 0.4 0.45 0.5 0.55 0.5 0.55 0.7 0.75
Temperature (K)
Figure 5. Thermal conductance of all devices and T4 (straight line) fit to the G values. In order to decouple TES
area from the G values, in this plot, G is divided by device area. G values are plotted with error bars.
EIS
i
X
LIJ
z
The Noise Equivalent Power (NEP) derived from phonon fluctuations is straightforward:
NEP =4k,T2G = 20kb2VT6 .	 (4)
Assuming Device A geometry, and corresponding G value, Figure 6 presents the calculated NEP for the detector. At 60
mK, the achieved NEP is 1.65 x 10 -Y8W/ ^HZ.
The time constant of an EPD detector is given by:
Ce	YT'V	 Y
TeP G P
 52VTe4 5273
where y is the Sommerfeld constant for electronic specific heat. For Aug, y —71 J/K 2m3 . We estimate the time constant,
rep ^ 5 µs, which is compatible with a typical SQUID amplifier or time division SQUID multiplexer. The effects of
thermal conductance and electrothermal feedback are major determinants of the time constant, but the electronic heat
capacity also plays a major role. Using absorber structures contacting the TES, this electronic heat capacity has been
tuned to be about ten times larger (see Table 2) than that of the TES element, slowing the device down from its intrinsic
time constant of — 5ps to be compatible with a multiplexed SQUID.
Table 2. Electronic heat capacity C , for Bi absorber and Au layer in TES. Bi has area of 211.5 µm 2 and thickness of 0.8 µm, while,
Au on TES has area of 25 µm 2
 and thickness of 0.2 nm. y and 2 are obtained from literature s. Heat capacity contribution of Mo is
negligible; hence we have assumed heat capacity of Au as TES heat capacity.
Material V
(m3)
21
(W/m3K4)
1
Y
(J/Km')
C,
(J/K)
Bi (Absorber) 1.68x10 ' 10 0.24 x 109 0.39 2 . 09 x 10-17
Au (TES) 5X10''1 5 x 10 $ 71 1.11 x 10-19
As an aside, we did not observe a correlation between superconducting transition temperature, T,, and the lead-to-lead
distance in our measurements. The lead-to-lead distance for Device A and Device B is 4 gm, and for Device C and
Device D is it 12 gm and 36 µm, respectively.
x`6.02	 0.04	 0.06	 0.08	 0.1	 0.12	 0.14
Temperature (K)
Figure 6. Measured G and calculated NEP for TES with Device A geometry. We aim to operate our detector at
100 mK transition temperature.
(5)
4. CONCLUSIONS AND FUTURE WORK
We have successfully fabricated TES with various geometries which exhibit thermal isolation that is dictated byelectron-
phonon coupling phenomena based upon the relationship between thermal conductance and temperature. The measured
values of G are consistent with our initial calculations and published values"'. With a 25 µm 2 sensor area, we estimate
NEP to be — 1.65 X 10-18W/ MHz and time constant of — 5 µs.
We plan on fabricating devices with lower superconducting transition temperature and simultaneously testing different
TES geometries e.g., the zebra normal metal stripes, superconducting imbedded structures in TES devices. The
improvement in the TES designs and geometry will increase the sensitivity of detector by 30% to 50 %, and hence,
estimated NEP is — 2.5 X 10 19 WNHz, which is in the ZeptoWattsNHz sensitivity range. The antennas have been
designed, simulated and produced for frequencies of 300 GHz, 1 THz, and 3 THz. We plan on testing the antenna with
optical input power and integrate them onto TES device. Recently, we have fabricated device with copper (Cu) ground
plane, and testing is planned in near future. We are also in the middle of installing SQUID setup in our cryogenic
systems; hence we should be able to reduce the uncertainties of G values. We are in the process of characterizing
superconducting proximity effect for Mo/Au films, and optismistic about achieveing required transition temperature.
Such devices operating at lower temperature, e.g. 60 mK will be ideal for low background measurements of millimeter
and submillimeter wave astrophysics.
REFERENCES
[1] Irwin, K.D., "An application of electrothermai feedback for high resolution cryogenic particle detection", Applied
Physics Letters 66 (15), pp. 1998-2000 (1995).
[2] Lee, A.T., Richards, P.L., Nam, S.W., Cabrera, B & Irwin, K.D., "A superconducting boiometer with strong
electrothermal feedback", APL, 69, pp. 1801-1803 (1996).
[3] Benford, D.J., Ames, T.A., Chervenak, J.A., Grossman, E.N., Irwin, K.D., Khan, S.A., Maffei, B., Moseley, S.H.,
Pajot, F., Phillips, T.G., Renault, J.-C., Reintsema, C.D., Rioux, C., Shafer, R.A., Staguhn, J.G., Vastel, C. &
Voelimer, G.M., "First Astronomical Use of Multiplexed Transition Edge Bolometers", AIPC, 605, 589 (2002).
[4] Irwin, K.D., Hilton, G.C., Wollman, D.A. & Martinis, J.M. "Thermal-response time of superconducting transition-
edge microcalorimeters", JAP, 83 (8), p.3978-3985 (1998).
[5] Cabrera, B., R. M. Clarke, P. Colling, A. J. Miller, S. Nam, and R. W. Romani, "Detection of single infrared,
optical, and ultraviolet photons using superconducting transition edge sensors", Appl. Phys. Lett., vol. 73, pp. 735—
737 (1998).
[6] Karasik, B.S., W. R. McGrath, M. E. Gershenson, and A. V. Sergeev,"Photon-noise-limited direct detector based on
disorder-controlled electron heating", Journal of Appl. Physics, vol. 87, no. 10, pp. 7586-7588 (2000).
[7] Wei, J., Olaya, D., Karasik, B.S., Pereverzev, S.V., Sergeev, A.V. & Gershenson, M.E., "Ultra-Sensitive Hot-
Electron Nanobolometers for Terahertz Astrophysics", Nature Nanotechnology 3, 496 - 500 (2008).
[8] Ali, S., Timbie, P.T., Siddharth, M., McCammon, D., Nelms, K.L., Pathak, R., van der Weide, D.W., Allen, C.A.,
Abrahams, J. & Chervenak, J.A., "Antenna-Coupled Transition-Edge Hot-Electron Microbolometers", Proc. SPIE,
#5498, 381 (2004).
[9] Barrentine, E. M., Timbie, P. T., Stevenson, T. R., Ali, S., Chervenak, J. A., Wollack, E., Moseley, S. H., Allen, C.
A., Hseih, W. T., Miller, T. M., Benford, D. J., &Brown, A. D., "Sensitivity Measurements of a Transition-Edge
Hot-Electron Microbolometer for Millimeter-Wave Astrophysical Observations", J. Low Temp. Physics, 151, pp.
173-179 (2008).
[10]Brown, Ari-David; Benford, Dominic J.; Chervenak, James A.; Henry, Ross; Kletetschka, Gunther; Mikula, Vilem;
Stevenson, Thomas R.; U.-Yen, Kongpop; Wollack, Edward J. The 13 th International Workshop on Low
Temperature-LTD 13. AIP Conference Proceedings, Volume 1185, pp. 326-329 (2009).
[11] M. Yu. Reizer and A. V. Sergeev, Zh. Eksp., Sov. Phys. JETP 63, 616 (1986); Teor. Fiz. 90, 1056 (1986).


Development of Superconducting Transition Edge Sensors Based on Electron-Phonon Decoupling

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server