Our purpose is to realize a multi-pixel sub-millimeter/terahertz camera with the superconductor - insulator -
superconductor photon detectors. These detectors must be cooled below 1 K. Since these detectors have high impedance,
signal amplifiers of each pixel must be setting aside of them for precise signal readout. Therefore, it is desirable that the
readout system work well even in cryogenic temperature.
We selected the n-type GaAs JFETs as cryogenic circuit elements. From our previous studies, the n-type GaAs JFETs
have good cryogenic properties even when those power dissipations are low. We have designed several kinds of
integration circuits (ICs) and demonstrated their performance at cryogenic temperature. Contents of ICs are following;
AC coupled trans-impedance amplifiers, voltage distributors for suppressing input offset voltage of AC coupled CTIAs,
multiplexers with sample-and holds, and shift-registers for controlling multiplex timing. The power dissipation of each
circuit is 0.5 to 3 micro watts per channel.
We also have designed and manufactured 32-channel multi-chip-modules with these ICs. These modules can make 32-
channel input photo current signals into one or two serial output voltage signal(s). Size of these is 40mm x 30mm x 2mm
and estimated total power dissipation is around 400 micro watts.
We discuss on the development of a 32-ch cryogenic readout module for a superconductive imaging submillimeter-wave
camera (SISCAM). The module is composed of GaAs-JFET integrated circuits such as capacitive trans-impedance
amplifiers (CTIAs), multiplexer with sample-and-holds and shift-registers. Performances of these integrated circuits are
evaluated; 1) amplifier gain of 5000 and gain-bandwidth more than 500 kHz, 2) operation of 32-ch multiplexers
addressed by shift-registers, and 3) operation of voltage distributors. Using these GaAs-JFET cryo-ASICs, a 32-ch
cryogenic readout module was fabricated for an imaging array of SIS photon detectors at 650 GHz.
We have been developing cryogenic readout integrated circuits (ROICs) for sensitive detectors at far-infrared and
submillimeter wavelengths: The ROICs are constructed from SONY GaAs-JFETs, which have excellent performance
even at less than 1 K. In addition, it is suitable device for ultra low background applications because of the extremely
low gate leakage current. In the spring of 2008, we have designed and fabricated 4-ch AC-coupled capacitive
transimpedance amplifiers and several basic digital circuits giving multiplex function for 32-element SIS photon detector
array. The expected performance of the amplifier is as follows; open loop gain of >2000, power consumption <1.5 μW,
and input referred noise ~ 1 μV/√Hz@1Hz. A summary of this 2008's experimental production and initial test results are
presented in this paper.
Developments on large format array of superconducting tunnel junction detectors are discussed and recent activities in
readout electronics developments and focal plane optics designs are presented. We have been working on submillimeter-wave
SIS photon detectors at 650 GHz using niobium tunnel junctions, which have high sensitivity, large dynamic range
and fast response. Here we discuss on an implementation plan of large format array with cryogenic readout electronics
and compact focal plane optics design. GaAs-JFETs operate at less than 1 K with low noise, low power dissipation and
fast response. We have demonstrated operation of cryogenic integrating amplifiers and digital electronics for SIS photon
detectors with multiplexed readout. Combined with compact focal plane optics, we now have a conceptual design of
large format array of SIS photon detectors in submillimeter-wave. Further development to realize higher sensitivity
superconducting tunnel junction detectors with extremely low leakage current are foreseen.
High sensitivity submillimeter-wave focal plane array using SIS photon detector is being developed, which we call SISCAM, the superconductive imaging submillimeter-wave camera. In the course of the detector evaluations, we have measured performance of the SIS photon detectors under various operating conditions. Advantages of the SIS photon detectors are explained by the nature of antenna coupled quantum detectors. Their input coupling can be designed to have band-pass characteristics owing to the distributed junction design. This reduces requirements for infrared blocking filters and enhances optical efficiency. The detector performance is evaluated under background loading and they show background limited performance. Measurement at 4 K shows the SIS photon detector operates under shot noise limit of thermal leakage current and its NEP is 1x10<sup>-14</sup> W/Hz<sup>0.5</sup>, that is better than bolometers at 4.2 K, whereas the same detector has NEP of 10<sup>-16</sup> W/Hz<sup>0.5</sup> at 0.3 K. Dynamic range of SIS photon detectors is estimated to be higher than 10<sup>9</sup>, which surpass the dynamic range achievable with TES bolometers. Nine-element array of SIS photon detector, SISCAM-9, is developed and their performance is evaluated in a submillimeter-wave telescope. With a development of integrated electronics with GaAs-JFET charge integrating readout circuit, the SIS photon detector will be an ideal imaging array in submillimeter-wave region. Due to its large dynamic range and shot noise limited performance under various operating condition, SIS photon detectors can be used for various astronomical instrumentations as well as for other fields of terahertz technologies.
We are developing cryogenic readout circuits for the array of superconducting tunneling junctions (STJs) at
submillimeter wavelength SISCAM (Superconductive Imaging Submillimeter-wave CAMera). A current conceptual
design of SISCAM will employ a direct hybrid array system just like CMOS image sensors widely used at optical and
infrared wavelength. Because of relatively large impedance of the STJ fabricated by RIKEN (~10 MΩ in a dark
condition), it requires readout preamplifier with low current noise. Therefore, it is not suitable for the STJ to use a
readout system by Superconductive Quantum Interferences Devices as for Transition Edge Sensor. Instead, we selected
capacitive transimpedance amplifier (CTIA) using a SONY n-type GaAs Junction Field Effect Transistor (JFET).
However, the CTIA has not been used as the readout of the STJ. Therefore, we measured the photocurrent of the STJ by
the CTIA with <i>Silicon JFETs</i> and by transimpedance amplifier (TIA), which is a conventional readout for the STJ, in the
same bias condition, and confirmed both results are in good agreement. Additionally, we report development of readout
integrated circuits with GaAs JFETs. In order to design the CTIA circuit with the GaAs JFETs, we fabricated the
independent GaAs JFETs and matched pairs of them. We measured electrical characteristics of these GaAs JFETs at the
cryogenic temperatures less than 4.2 K. We demonstrated performance of an operational amplifier fabricated with the
GaAs JFETs measuring a differential amplifier with the dual GaAs JFET, and additionally estimate amplifier gain, offset
voltage, and power consumption of the CTIA by the circuit simulation using the PSPICE. In consequence, the expected
performance fulfills the requirements for the readout amplifier of the STJs except for the noise performance.
SIS photon detectors are niobium-based superconducting direct detectors for submillimeter-wave that show superior performance when compared with bolometric detectors for ground-based observations. We present the design and development of the SIS photon detectors together with optical and cryogenic components for wide field continuum observation system on Atacama Submillimeter Telescope Experiment (ASTE). Using antenna coupled distributed junctions, SIS photon detectors give wide band response in a 650-GHz atmospheric window as well as high current sensitivity, shot noise limited operation, fast response and high dynamic range. Optical noise equivalent power (NEP) was measured to be 1.6x10<sup>-16</sup> W/Hz<sup>0.5</sup> that is less than the background photon fluctuation limit for ground based submillimeter-wave observations. Fabrication of focal plane array with 9 detector pixels is underway to install in ASTE.
Readout electronics with Si-JFETs operating at about 100 K will be used for this array. Development of readout electronics for larger array is based on GaAs-JFETs operating at 0.3 K. For the purpose of installing 100 element array of SIS photon detectors, we have developed remotely operable low-vibration cryostat, which now cools bolometers for 350, 450, 850-µm observations down to 0.34 K. GM-type 4-K cooler and He3/He4 sorption cooler is used, which can be
remotely recycled to keep detectors at 0.34 K. Since we have large optical window for this cryostat, sapphire cryogenic window is used to block infrared radiation. The sapphire window is ante-reflection coated with SiO<sub>2</sub> by chemical vapor deposition (CVD). The transmittance of the cryogenic window at 650 GHz is more than 95%.
The Japanese infrared astronomical satellite, ASTRO-F, employs the Far-Infrared Surveyor (FIS) for all sky survey. The FIS has two detector arrays; one covers from 50 to 110 μm wavelength, the other covers from 110 to 200 μm. Each of them uses Ge:Ga operating at 2K. We have developed and evaluated the preamplifiers for these detector arrays. The preamplifiers are required to work at 2K with low noise and low power dissipation. In this paper, we report on the development and evaluation of these cryogenic preamplifiers.
The ASTRO-F project is currently in its final stage of proto-model, which is constructed same as flight-model. Since instrument goals of the Far-Infrared Surveyor (FIS) are unprecedented achievement of high sensitivity and high spatial resolution in far-infrared wavelength, the proto- model stage is important to prove the performance as the flight instrument. We mainly present here the latest optical, thermal, and mechanical properties of the proto- model of the FIS.
Low-noise and low-power cryogenic readout electronics are developed for a focal plane instrument of the IR Imaging Surveyor. We measured the static characteristics and the noise spectra of several types of silicon MOSFETs at the cryogenic temperature where silicon JFETs do not work well due to the carrier freeze-out. The 'kink' behavior of n- channel MOSFETs was observed below the carrier freeze-out temperature, but it was not obvious for the p-channel MOSFET. It was demonstrated the p-channel MOSFETs can be used for the cryogenic readout electronics of the IRIS's far-IR array with an acceptable performance. The amplifier integrated with these MOSFETs showed low-noise at 2K under a low power consumption of 1 (mu) W per MOSFET. We now design and evaluate several circuits that are fabricated by the CMOS process for cryogenic readout.