We report first results of laboratory tests of Si:As
blocked-impurity-band (BIB) mid-infrared (4 to 28 μm) detectors developed
by IMEC. These prototypes feature 88 pixels hybridized on an integrated cryogenic readout electronics (CRE). They
were developed as part of a technology demonstration program for the future Darwin mission. In order to be able to separate
detector and readout effects, a custom build TIA circuitry was used to characterize additional single pixel detectors.
We used a newly designed test setup at the MPIA to determine the relative spectral response, the quantum efficiency, and
the dark current. All these properties were measured as a function of operating temperature and detector bias. In addition
the effects of ionizing radiation on the detector were studied. For determining the relative spectral response we used a dualgrating
monochromator and a bolometer with known response that was operated in parallel to the Si:As detectors. The
quantum efficiency was measured by using a custom-build high-precision vacuum black body together with cold (T ~ 4K)
filters of known (measured) transmission.
The Photodetector Array Camera and Spectrometer (Pacs) instrument aboard the Herschel space observatory
contains an integral field spectrometer with two camera channels which consist of 25 linear arrays of 16
stressed Gallium doped Germanium crystals (Ge:Ga) each. The space radiation environment induces changes in
the detector performance. Therefore, testing the Ge:Ga detectors under space radiation environment during the
commissioning phase (CP) is important for optimization of later detector operation in orbit. The test program
for Ge:Ga detector tuning during this phase has been designed according to findings obtained in laboratory
experiments: Protons as well as a 137Cs-γ-source have been used to simulate the space radiation environment
and to induce the radiation impacts on the photoconductor arrays. From comparison of the performance of
the detectors during CP versus laboratory tests the best strategy for operating the detectors during scientific
observations will be derived. This includes annealing, proposals for on-board data reduction algorithms and the
best estimated strategy for well-calibrated scientific measurements.
The photoconductors used in the integral field spectrometers of the PACS instrument onboard the Herschel
space observatory consist of stressed gallium doped germanium crystals featuring cut-off wavelengths of 127μm
and 205μm. The measured transient responses of these Ge:Ga photoconductors to a step change in the incident
photon flux level as well as a test setup that allows creation of transients by different methods are presented in
this paper. The transient response of extrinsic photoconductors is caused by charge carriers drifting or diffusing
to a contact region and recombining. This limits the initial gain of the device. Because of potentially long time
constants, the transient behavior presents a serious challenge to high-sensitivity, low-temperature extrinsic semiconductors.
In particular at low IR photon fluxes it usually is impossible for the detector to reach steady-state
behavior during a reasonable observation time.
However, since the time constants depend on the inverse photon flux, theory suggests the transient times for the
high thermal background levels anticipated for PACS to be of the order of tens of milliseconds. Experimentally
we find the response time to be limited by the transition time between the different infrared fluxes. The experimental
studies on the transients are accompanied by numerical calculations. The results support the prediction
that transients are not expected to play a major role for the low signal regime in PACS.
This paper reports on the fabrication and characterization of a linear array of Blocked Impurity Band (BIB) far infrared detectors and of the related Cryogenic Readout Electronics (CRE). It is part of the ESA DARWIN project which aims at the study of exoplanets by means of null interferometry and requires high performance infrared detector arrays in the 6 18μm range. Si:As BIB detectors have been fabricated on an infrared transparent Silicon substrate enabling backside illumination. The buried contact, the active and the blocking layers are deposited by epitaxy; the doping profile is controlled by adjusting the growth parameters. Access to the buried contact is provided by anisotropic silicon etch of V-grooves in the epi layers. Spray coating of photoresist is used for the lithography of the wafers with high topography. The CRE is composed of an input stage based on an integrating amplifier in AC coupled feedback with selectable integrator capacitors, of a sample and hold stage which provides isolation between input and sampling capacitance, and of an output buffer with multiplexing switch. The readout is optimized for low noise with minimum operating temperature of 4K. Linear arrays made of 42 and 88 detectors and having 30μm pixel pitch with various active areas are fabricated. Detector arrays are coupled to the CRE by Indium bumps using flip-chip technology. Measurements on the readout show reduced noise, good linearity and dynamic range. First detector characterization results are presented.
The ESA Herschel space observatory will be launched in 2008 into the Earth-Sun L2 orbit and the three instruments onboard
will be exposed to cosmic radiation during the 4 years lifetime of the satellite. To study the impact of ionizing
radiation on the Ge:Ga photoconductors of the PACS instrument (Photodetector Array Camera and Spectrometer), we
performed a series of irradiation measurements at the cyclotron of the University of Louvain la Neuve, Belgium
simulating the in-flight predicted proton fluxes including solar flare events. The PACS integral field spectrometer
contains two 25×16 pixel arrays of Ge:Ga crystals: a low stressed configuration is used in the wavelength range from 55
to 105 μm, and a high stressed device covers the range 105 to 210 μm. Calibration of the detector modules under
realistic IR background fluxes is done at MPE Garching and MPIA Heidelberg. 70 MeV protons were generated at the
cyclotron test site. They were attenuated on their way to the detectors by beam conditioning elements and the metal
shields of the cryostat before they reached the Ge:Ga crystals with a mean energy of 17 MeV and a standard deviation
of 1.5 MeV. According to predictions the expected proton fluxes were set to nominally 10 ps-1cm-2 and to 400 ps-1cm-2
simulating solar flares. We observed radiation-induced glitches in the detector signal, changes in responsivity, increase
in noise and transient behavior. The ongoing data evaluation indicates optimal operating parameters, the best curing
method and frequency, calibration procedures and data processing algorithms aiming for a high photometric accuracy.
The characterization and calibration of far-infrared (FIR) detectors is a delicate task that requires good knowledge of the
incident flux and its spectral composition. In many test setups the FIR flux to the detectors is provided by means of an
external or internal black body and a set of cold attenuation, band pass, and blocking filters. For scientific instruments (e.g.
PACS aboard ESA's Herschel satellite) band pass and blocking filters are used to achieve the desired spectral throughput
either as order sorting filters in spectrometers or for selecting a wavelength range in imaging cameras. In all cases a detailed
knowledge of the spectral transmittance of the used filters is mandatory for an accurate calibration of the system. We have
build a test platform that allows to measure the transmission of cold (T ~ 4K) filters in the far-infrared. The setup uses
a dual grating monochromator with excellent spectral purity and a resolution up to 800, which is operated under a dry
nitrogen atmosphere to eliminate water vapor absorption bands. An Si-bolometer is used as detector and is read out by a
cryogenic low noise trans-impedance amplifier circuit with common mode rejection and a warm electronics using a lock-in
amplifier and a 22 bit analog-to-digital converter. A cryogenic filter slider in the setup allows for differential measurements
between filters and the use of cold order sorting filters. We present initial results for FIR cut-on and attenuation filters,
demonstrating that our setup is suited to measure transmissions as low as 10-4 over the covered wavelength range.
We present a satellite mission concept to measure the dark energy equation of state parameter ω with percent-level precision. The Very Ambitious Dark Energy Research satellite (VADER) is a multi-wavelength survey mission joining X-ray, optical, and IR instruments for a simultaneous spectral coverage from 4 μm (0.3 eV) to 10 keV over a field of view (FoV) of 1 square degree. VADER combines several clean methods for dark energy studies, the baryonic acoustic oscillations in the galaxy and galaxy cluster power spectrum and weak lensing, for a joint analysis over an unrivalled survey volume.
The payload consists of two XMM-like X-ray telescopes with an effective area of 2,800 cm2 at 1.5 keV and state-of-the-art wide field DEPFET pixel detectors (0.1-10 keV) in a curved focal plane configuration to extend the FoV. The X-ray telescopes are complemented by a 1.5m optical/IR telescope with 8 instruments for simultaneous coverage of the same FoV from 0.3μm to 4μm. The 8 dichroic-separated bands (u,g,r,z,J,H,K,L) provide accurate photometric galaxy redshifts, whereas the diffraction-limited resolution of the central z-band allows precise shape measurements for cosmic shear analysis.
The 5 year VADER survey will cover a contiguous sky area of 3,500 square degrees to a depth of z~2 and will yield accurate photometric redshifts and multi-wavelength object parameters for about 175,000 galaxy clusters, one billion galaxies, and 5 million AGN. VADER will not only provide unprecedented constraints on the nature of dark energy, but will additionally extend and trigger a multitude of cosmic evolution studies to very large (>10 Gyrs) look-back times.
The Photodetector Array Camera and Spectrometer (PACS) is one of three science instruments on board the
Herschel space observatory to be launched in 2008. It will perform imaging photometry and spectroscopy in
the wavelength range from 57 μm to 210 μm. The integral field spectrometer contains two 25 x 16 pixel cameras
of Gallium doped Germanium crystals (Ge:Ga). By stressing these crystals, cutoff wavelengths of 127 μm
(low-stressed, 200 N) and 205 μm (high-stressed, 800 N) are reached. The characterization of these detectors
(responsivity, noise equivalent power (NEP), dark current,...) is carried out at the Max-Planck-Institutes for Astronomy
(MPIA, Heidelberg) and Extraterrestrial Physics (MPE, Garching). Both test facilities allow simulation
of the in-flight operational conditions of the arrays and provide accurate IR fluxes by means of external/internal
black bodies and calibrated cold attenuation filters. A radioactive 137Cs source is used at MPIA to simulate the
steady cosmic radiation impact on the photoconductor arrays in order to study the radiation induced changes
in responsivity, noise, and the transient behavior. The goal is to determine the optimal operating parameters
(temperature, bias, integration time,..) for the operation at the L2 orbit, the best curing method, curing frequency
and calibration procedure for high photometric accuracy. The "lessons learned" on operating, curing,
deglitching and calibrating stressed Ge:Ga detectors during the ISO mission are applied as well as the relevant
reports from IRAS and Spitzer.