Otto Frenzl

FIRST-PACS: design and performance of the sensor engineering models

The Photoconductor Array Camera and Spectrometer (PACS) will be equipped with two sensor arrays consisting of 16 X 25 pixels each. Arranged in linear arrays of 16 detectors the sensitivity of the sensors is tuned to the wavelength ranges 60 micrometers to 130 micrometers and 130 micrometers to 210 micrometers , by applying different levels of stress to the Ge:Ga crystal utilizing a special leaf spring which is part of each of the 25 modules. The electronics of the sensors are mounted on the same module but thermally isolated from the sensor level which is at a lower temperature of about 2 K. The sensors are read out by a specially developed integrating and multiplexing cryogenic read-out electronics. With a fore optics made of light cones in front of the detector cavities a 100% filling factor is achieved and a high quantum efficiency close to 0.5 is expected. In order to achieve extremely good stress uniformity in all detectors and therefore equal cutoff wavelengths, a high degree of the quality of the Ge:Ga detectors and of the assembling components used for this dedicated stress mechanism is required. The first 6 engineering modules have been successfully manufactured and tested afterwards. The relative responsivity of a set of pixels has been determined and a good performance has been demonstrated for the sensors, which are very close to fulfilling the requirements for PACS aboard the infrared spectra telescope FIRST.

32-pixel FIRGA demonstrator: testing of a gallium arsenide photoconductor array for far-infrared astronomy

A gallium arsenide photoconductive detector, which is sensitive in the far-infrared wavelength range from approximately 60 micrometers to 300 micrometers , offers the advantage of extending considerably the long wavelength cut-off of presently available photodetectors. FIRGA is an ESA sponsored GaAs detector development program which is approaching completion. The FIRGA study is intended to prepare the technology for large 2D GaAs detector arrays for far-infrared astronomy. The primary goal of the development is the preparation of a monolithic 32 element demonstrator array module with associated cryogenic read-out electronics. Continuous progress in material research has led to the production of pure and doped n-type GaAs layers using liquid phase epitaxy. We prepared sample detectors from those materials and investigated their electrical and infrared characteristics. Finally, a multi-layer structured detector device was manufactured. The 4 X 8 element array configuration is defined by sawing a split pattern into the layers with pixel size 1 mm X 1 mm. The device is back illuminated. The 32 pixels are connected to two cryogenic read-out electronics chips mounted close-by. Results of the initial detector performance tests are reported. We determined dark current, responsivity and response transients. Ongoing development activities will concentrate on material research, i.e. the production of n-GaAs layers of ultra-high purity and those with improved FIR characteristics using new centrifugal techniques for material growth.

4x32 FIRGA array: a pacesetter for a 52x32-element gallium arsenide focal plane array

FIRST and SOFIA are both future IR observatories with 3m class main mirrors having sophisticated instrumentation aboard. The present design of the FIRST imaging spectrometer PACS requires two large far-IR photoconductor arrays of 25 X 16 pixels each, the baseline material is stressed and unstressed Ge:Ga. A gallium arsenide photoconductive detector which is sensitive in the far IR (FIR) wavelength range from about 60 micrometers to 300 micrometers might offer the advantage of extending considerably the long wavelength cut- off of presently available photodetectors. FIRGA is an ESA sponsored detector development program on this matter involving international partners. The aim is a monolithic 4 X 32 demonstrator array module with associated cryogenic read-out electronics. Recent progress in material research has led to the production of Te-doped n-type GaAs layers using liquid phase epitaxy. We prepared sample detectors from those material and investigated their electrical and IR characteristics. First measurements indicate that GaAs has in principle considerable potential as a FIR photon detector. Theoretical modeling of GaAs detectors can help with the detector design and allows the prediction of response transients as a function of detector parameters. Present development activities are mainly concentration on material research, i.e. the production of GaAs:Te with improved FIR characteristics. Results of the current test and measurements are reported. The FIRGA study is intended to prepare the technology for large 2D GaAs detector arrays for far IR astronomy.

Performance of the PACS photoconductor arrays aboard FIRST

For the instrument PACS (Photoconductor Array Camera and Spectrometer) aboard FIRST two photoconductor arrays dedicated for different wavelengths bands are foreseen in the spectrometer section. Each camera consists of 25 linear arrays of 16 stressed Ge:Ga crystals. The arrays with 2 X 400 pixels will be used for imaging line spectroscopy in the wavelengths ranges 60 micrometers to 110 micrometers and 110 micrometers to 210 micrometers . The detectors are read out by a new generation of integrating and multiplexing cryogenic read-out electronics, which is currently under redesign and further development. The development of the arrays has passed the engineering phase and entered the qualification status. We summarize the results obtained with the engineering arrays and the first 6 qualification models with respect to their performance such as uniformity of current sensitivity and cutoff wavelength. The impacts of the result of the first 6 linear arrays on the qualification model are described together with the final design which is supposed to fulfill the specifications and requirements of the instrument PACS.

Modular 25x16 pixel stressed array for PACS aboard FIRST

For the Photoconductive Array Camera and Spectrometer (PACS) 2 sensor arrays consisting of each 16 X 25 pixels are foreseen. The sensors arranged in linear arrays with 16 detectors are tuned to the wavelength ranges 60 micrometer to 130 micrometer and 130 micrometer to 210 micrometer by applying different levels of stress to the Ge:Ga crystals utilizing a special leaf spring which is part of each of the 25 modules. The electronics of the sensors are mounted on the same module but thermally isolated from the sensor level which is linked to a 1.7 K stage. The sensors are read out by a new generation of the integrating and multiplexing cryogenic readout electronics (CRE). With the optical design a 100% filling factor is achieved and with a fore optics made of light cones in front of the detector cavities a high detection efficiency close to 1 is expected. In order to achieve extreme high stress uniformity among all detectors and therefore equal cutoff wavelengths, a high degree of the quality of the Ge:Ga detectors and of the assembling components used for this dedicated stress mechanism is required. The first two engineering modules have been successfully manufactured and tested afterwards. The relative responsivity of a set of pixels has been determined and a good performance demonstrated for the sensors which are very close to fulfill the requirements for PACS aboard the infrared telescope FIRST.

FIRSA - the demonstrator array of the FIR camera for the photoconductor instrument PACS on ESA's FIRST satellite

PACS covers the wavelength range 80-210 micrometers in spectrometric and photometric imaging modes. The long wavelength camera is a 16 X 25 pixel array of stressed Ge:Ga detectors. In order to demonstrate the feasibility of such a large array, one of the 25 linear arrays was manufactured. It consists of 16 elements of 1.5 mm3 each separated by ceramic plungers and stressed by one single mechanism. As preamplifier a dedicated CMOS circuit was developed, based on similar circuits successfully operating in ISOs photometer. In particular, it was intended to increase the gain of the CRE in order to minimize the debiasing effects on the low bias operated detectors. Two complete linear demonstrator arrays were manufactured and independently tested under various low background conditions in a 1.7 K environment at MPIA and MPE. The feasibility of the concept chosen was demonstrated in several functional tests. Valuable experience was gained to guide the development of the next generation of CREs and arrays.