Development of 109-pixel MKID arrays for the 100-GHz band continuum camera

概要

Observations of millimeter and submillimeter bands are very important in astronomy and astrophysics to understand
the formation and evolution of stars and galaxies in the universe. The 109-pixel MKID (Microwave Kinetic
Inductance Detector) camera installed on Nobeyama 45-m radio telescope has been developed for continuum
observations through the atmospheric window at 100-GHz band. The scientific targets of this camera are to explore
the character of the candidates of distant galaxies found by the AKARI deep survey in the North Ecliptic Pole
Region (NEP-Deep survey) and to investigate the distribution and properties of H II regions in the Milky Way by a
Galactic plane survey.
The sensitivity of radio continuum camera is evaluated by Noise Equivalent Power (NEP) or Noise Equivalent
Flux Density (NEFD) and is related to observation time needed to detect a target source. An important factor of the
sensitivity is the optical efficiency that is the fraction of the radiation power absorbed by a detector in incident
radiation into a camera. The optical efficiency of the Nobeyama MKID camera with a conventional MKID array
made of a single layer of aluminium (Al) was 0.03±0.02. One of the leading factors for this low efficiency is the
dissipation of energy in the focal plane array. This loss is caused by the Cooper-pair breaking at the ground plane
made of Al, which responds to the photons above 90 GHz, during transmission of 100-GHz photons from the planar
antenna to the MKID.
In order to reduce the dissipation loss on the ground plane, we introduced the hybrid MKID array employing a
niobium titanium nitride (NbTiN) as the ground plane material. In NbTiN with a transition temperature 13 K, the
Cooper-pair is only broken by photons above 950 GHz. Therefore, NbTiN can be used as a ground plane material
for 100-GHz band MKID and planar-antenna with lower loss.
We fabricated the 109-pixel hybrid MKID array in the clean room at the Advanced Technology Center (ATC) of
the National Astronomical Observatory of Japan (NAOJ). Based on the DC properties of NbTiN and Al at low
temperatures, we designed a 100-GHz antenna-coupled hybrid MKID array. We optimized the fabrication
parameters for the hybrid MKID array. These parameters provide good MKID patterns over the entire 3-inch wafer
as designed. ...