Program and Abstract Book - SRON

19 th International Symposium on Space Terahertz Technology
2-5
Temperature Dependence of HEB Mixer Performance
Shoichi Shiba a , Ken Shimbo a , Ling Jiang a , Nami Sakai a , Mika Sugimura a ,
Hiroyuki Maezawa b , and Satoshi Yamamoto a
a Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
b STE Laboratory, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
We are developing a hot-electron bolometer (HEB) mixer receiver for astronomical
observations in the terahertz region, which will be used for the ground-based
observations from high altitude sites. We have fabricated the two types of the HEB
mixers, a diffusion cooled mixer using Nb and a phonon cooled mixer using NbTiN,
in our laboratory, and have carried out their critical evaluations.
For the NbTiN HEB mixer, we optimized the fabrication condition of the NbTiN
film on the Z-cut crystalline quartz substrate. The film was deposited at the room
temperature by using the RF plasma assisted sputtering of the NbTi target under the
N 2 /Ar buffer gas. In this case, the T c value is very sensitive to the N 2 concentration in
the buffer gas. Therefore the N 2 flow rate was carefully optimized for the thin film,
and finally the best T c value of 9.8 K was obtained for the 5 nm thick film. This T c
value is fairly good for the room temperature deposition.
We fabricated the HEB mixers with a combination of lift-off and etching processes.
First, the HEB structure is formed with a bilayer of Au/Nb or Au/NbTiN on the quartz
substrate without breaking vacuum. This ensures a good electric contact between the
two layers without suffering from natural oxidation of the superconductor surface.
Then the Au layer on the microbridge part is removed by the Ar plasma etching.
The evaluation of the HEB mixer is carried out at 810 GHz using a waveguide-type
mixer block. The mixer is cooled down to 4 K by a two-stage GM refrigerator. We
measured the noise temperatures by using the Y-factor method. The highest Y-factor
obtained for the NbTiN mixer is 0.65 dB at the IF frequency of 1.1 GHz, which
corresponds to the receiver noise temperature of 1300 K. As for the Nb device, the
highest Y-factor of 0.3 dB is obtained.
The bath temperature dependence of the Y-factor was also measured for the NbTiN
mixer by using an electric heater attached to the cold head of the refrigerator. The Y-
factor becomes lower for the higher bath temperature. However, it is almost constant
below 6 K, if the bias current is adjusted to the optimum point by changing the LO
input power. This behavior means that the bath temperature is not a very critical issue
for practical operation of the mixer for astronomical observations. Further analyses
on the temperature dependence of the performance are now in progress. In addition,
we will discuss a possible difference in the temperature dependence between the
NbTiN and Nb based mixers.
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