Development of an 8×8 CPW microwave kinetic inductance detectors (MKIDs) Array at 0.35 THz

Abstract

Microwave Kinetic Inductance Detectors (MKIDs) are rather promising for THz direct detector arrays of large size, particularly with simple frequency-division multiplexing. Purple Mountain Observatory is developing a terahertz superconducting imaging array (TeSIA) for the DATE5 telescope to be constructed at Dome A, Antarctica. Here we report on the development of a prototype array for the TeSIA project, namely an 8×8 CPW MKIDs array at 0.35THz. The array consists of 64- pixel superconducting resonators in the frequency range of 4- 5.575 GHz with an interval of 0.025 GHz and 64-pixel twin-slot antennas at 0.35 THz for coupling THz radiation to the detector array. Based on our design, we fabricate the MKIDs array using TiN superconducting film with Tc about 4.5 K. And the performance is characterized at different temperatures. Detailed results and analysis will be presented.

Full text

J Low Temp Phys (2016) 184:103–107
DOI 10.1007/s10909-015-1382-y
Development of an 8 ×8 CPW Microwave Kinetic
Inductance Detector (MKID) Array at 0.35 THz
Jing Li1,2·Jin-Ping Yang1,2·Zhen-Hui Lin1,2·
Dong Liu1,2·Sheng-Cai Shi1,2·S. Mima3·
N. Furukawa3·C. Otani3
Received: 15 September 2015 / Accepted: 16 November 2015 / Published online: 29 December 2015
© The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract Microwave kinetic inductance detectors (MKIDs) are promising for THz
direct detector arrays of large size, particularly with simple frequency-division mul-
tiplexing. Purple Mountain Observatory is developing a terahertz superconducting
imaging array (TeSIA) for the DATE5 telescope to be constructed at Dome A, Antarc-
tica. Here we report on the development of a prototype array for the TeSIA, namely an
8×8 CPW MKID array at 0.35 THz. The resonance frequencies of the MKIDs span
the 4–5.575 GHz band with an interval of 25 MHz. Each detector is integrated with a
twin-slot antenna centered at 0.5 THz and with a relative bandwidth of 10%, while the
whole MKID array with a micro-lens array. Detailed design and measurement results
will be presented.
Keywords Microwave kinetic inductance detectors (MKIDs) ·CPW ·Supercon-
ducting resonator ·TiN ·TeSIA
1 Introduction
China is planning to construct an observatory at Dome A, Antarctica, which has been
found to be an excellent site (with low precipitable water vapor and low atmospheric
boundary layer) on the earth for THz and Optical/IR astronomy. The 5-m THz telescope
(DATE5 [1]), mainly targeting at the 350 and 200 µm atmospheric windows, is one of
BJing Li
lijing@pmo.ac.cn
1Purple Mountain Observatory, CAS, Nanjing, China
2Key Laboratory of Radio Astronomy, CAS, Nanjing, China
3RIKEN Center for Advanced Photonics, Wako, Japan
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104 J Low Temp Phys (2016) 184:103–107
the two telescopes to be built there. A science case for the DATE5 is to observe extreme
starburst galaxies at different redshifts to better understand the nature and evolution of
these enigmatic and important objects. To meet this requirement, a superconducting
imaging camera named TeSIA is proposed [2]. TeSIA operating at 350 µm has an
arraysizeof32×32 pixels and requires a background-limited sensitivity as high as
10−16 W/Hz0.5. A long-wavelength (850 µm or 0.35 THz) prototype with a smaller
size of 8 ×8 pixels is being developed with both transition edge sensor (TES) and
microwave kinetic inductance detectors (MKIDs) [3]. Here we mainly report on the
development of an 8 ×8 CPW MKID array at 0.35 THz.
As is well known, microwave kinetic inductance detectors (MKIDs) use frequency-
domain multiplexing (FDM) that allows thousands of pixels to be read out over a single
microwave transmission line followed by a cryogenically cooled low-noise amplifier
[4]. In addition, a large number of MKIDs can be integrated with a filter bank to
realize on-chip spectrometers such as DESHIMA and SuperSpec [5,6]. The MKID
array we are developing makes use of TiN superconducting films [7] with a critical
temperature of approximately 4.5 K. Such a superconducting film is just suitable for
both the operating temperature of 0.3 K and the frequency of 0.35 THz for our 8 ×8
CPW MKID array. The 8 ×8 TiN MKID array will be integrated with an 8 ×8micro-
lens array of 0.95-mm-diameter hyper-spherical Si lens. The FDM readout for this
MKID array is similar to those used by other groups, but makes use of a commercial
arbitrary wave-function generator to generate 64-tone input signal with 45-dB SNR.
2 MKID Design and Fabrication
We adopted the coplanar-waveguide (CPW) type resonator to design our 8 ×8TiN
MKID array because it has a relatively simple architecture of only one thin-film layer
on the substrate. This kind of resonator has a quarter-wavelength transmission line
with one end capacitively coupled to a microwave feed line and the other matched to
a planar antenna (twin-slot antenna, for example) [8]. Figure 1shows the overall view
of a fabricated MKIDs that consist of a coupler, a resonator, and a planar antenna, as
well as the enlarged parts of the coupler and the antenna.
Fig. 1 Overall view of an MKIDs as well as its enlarged parts of the coupler and the antenna
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J Low Temp Phys (2016) 184:103–107 105
Tab l e 1 Process parameters for
TiN MKIDs Parameter Value
Si substrate thickness ∼380 µm
Si substrate resistivity ≥1kcm
Si substrate size 3-inch
Ti target size 6-inch
Target-substrate distance 130 mm
Gas flow Ar:50 sccm & N2:10 sccm
Deposition Temperature Room temperature
TiN film thickness 100nm
Etching gas CF4
We first designed the coupler, which is an elbow structure at the end of the resonator
near the microwave feed line, just as shown in Fig. 1. With this layout, the feed line
and the coupler share the same metal section as the ground. The CPW feed line
was designed to have a 10µm center strip width and 6µm gaps, corresponding to
a characteristic impedance of 50 . The CPW line for the coupler was designed to
have a 3 µm center stripe and 2 µm gaps. The ground space between the feed line and
the coupler was set at 2 µm. Based on this structure, we can have different coupling
strength by changing the coupler length only. Using an electromagnetic microwave
simulator [9], we simulated the dependences of the coupling strength upon the coupling
quality factor (Qc), the resonance frequency ( f0), and the coupler length (Lc). As is
well known, Qccan be calculated directly from S13, i.e., the transmission from the
feed line to the coupler, according to Qc=π/2|S13|2[10]. Note that our simulations
cover a resonance frequency range from 3 to 7GHz and Lcfrom 5 to 1005 µm, and that
the metal film is assumed to be lossless. The designed 8×8 TiN MKIDs have coupling
quality factors ranging from 50 to 1000 k and resonance frequencies between 4 and
5.575 GHz with an interval of 25 MHz. The lengths of the resonators were calculated
straightforwardly according to an effective quarter-wavelength with zero penetration
depth. The twin-slot antenna was designed simply at a center frequency of 0.35 THz
and with a relative bandwidth of 10%. Note that the impact of the high resistivity of
TiN films will be taken into account for further simulation.
The 8 ×8 MKID array was fabricated in the clean room of RIKEN Center for
Advanced Photonics (Japan). As introduced before, we chose TiN superconducting
films for this MKID detector array [11]. Its Tccan be controlled between 0 and 5 K by
the components of Ti and N2[7]. Firstly, a 100-nm-thick TiN film was deposited on
a high resistivity Si wafer in a DC magnetron sputtering system. Secondly, the CPW
lines were defined in contact lithography by a mask aligner. Thirdly, the etching course
was done in an ICP machine. The details of the process are summarized in Table 1.
The fabricated MKIDs, as shown in Fig. 1,haveaTcapproximately equal to 4.5 K.
3 Measured Performances
For the fabricated 8 ×8 TiN MKID array, we mainly measured the dependence of its
transmission characteristic and the Q factors of the resonators upon the bath tempera-
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106 J Low Temp Phys (2016) 184:103–107
Fig. 2 Measured transmission characteristic (left)ofthe8×8 TiN MKID array as a function of frequency
and the distribution of the 64 resonance frequencies (right)
Fig. 3 Resonance-frequency
shift as a function of the bath
temperature normalized to Tc.A
theoretical curve of the
Mattis–Bardeen theory [12]is
plotted for comparison (Color
figure online)
ture. Our measurements were carried out simply by using a scalar network analyzer to
the 8 ×8 TiN MKID array followed by a 0.1–12 GHz cryogenically cooled low-noise
amplifier, which has an equivalent noise temperature of 5 K and a gain of approxi-
mately 35 dB. The 8 ×8 TiN MKID array was cooled by an Oxford 3He/4He dilution
refrigerator down to 22 mK.
The transmission characteristic of the 8 ×8 TiN MKID array was firstly measured
in a frequency range of 0.5–9.5 GHz at different temperatures. Figure 2shows the
measured S21 of the 8 ×8 TiN MKIDs at 22 mK from 3.3 to 4.8 GHz, where 64
resonance dips are located. Also shown in Fig. 2is the distribution of the 64 resonance
frequencies. Obviously the baseline ripple is caused by long microwave cables. It
can be clearly seen that the resonance frequencies are shifted to lower values than the
designed ones, while their intervals are fairly close to the designed value. The frequency
shift is mainly due to considerable kinetic inductance of TiN films. According to the
difference between the designed and measured resonance frequencies, the fractional
ratio of the kinetic inductance is about 0.3. It is worthwhile to point out that we
observed a sharp change of the transmission characteristic at about 4.5 K, which is in
good agreement with the measured superconducting film Tc.
We then fitted the S21 results measured at different temperatures based on nine
parameter model [10]. The internal Q factors of about half the MKIDs are higher than
100k, while the others are above 10k. The measured internal Q factors are obviously
lower than the designed values. We think the TiN film quality needs to be further
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J Low Temp Phys (2016) 184:103–107 107
improved. We plot here the resonance-frequency shift with bath temperature for the
resonator with a resonance frequency of 4.26 GHz at 22 mK. A theoretical curve
of the Mattis–Bardeen theory [12] is plotted for comparison. Obviously, our result
demonstrates a lower roll-off. We think the Dynes density of states (DOS) [13], instead
of the simple BCS DOS, should be taken into account for fitting the result shown in Fig.
3. The difference of superconducting film quality can be distinguished, particularly
for those of high resistivity.
4 Conclusion
We have designed and fabricated a 0.35-THz 8 ×8 CPW MKID array based on TiN
superconducting films of Tcequal to 4.5 K. Its transmission characteristic has been
measured at different bath temperatures. We have found that the Q factors of the
MKIDs measured at 22 mK range from 10 to 100 k. The frequency shift with respect
to the normalized bath temperature (T/Tc)follows a general trend predicted by the
Mattis–Bardeen theory, but with a lower roll-off.
Acknowledgments This work was supported in part by the National Natural Science Foundation of
China under Grant Nos. 11127903 and 11422326, and by Chinese Academy of Sciences under the Strategic
Priority Research Program XDB04010300.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 Interna-
tional License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution,
and reproduction in any medium, provided you give appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons license, and indicate if changes were made.
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