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Flight Qualification of Sensor Front-End Electronics for PACS/Hershel at Liquid Helium Temperature
Keywords: Reliability, LHT, CMOS
IMEC designed for the European Herschel Space Observatory (for PACS) the Cold Readout Electronics (CRE) for the Ge:Ga far-infrared detector array. Key specifications for the CRE were high linearity, low power consumption, high uniformity, and very low noise at an operating temperature of 4.2 K (LHT Liquid Helium Temperature). This circuit has been implemented in a standard CMOS technology which guarantees high production yield and uniformity, relatively easy availability of the technology and portability of the design. The circuits are functional at room temperature, which allows screening prior to integration and qualification, which has an important impact on the production yield. The circuit was mounted on a Al2O3 substrate for optimum electrical performance. On the same substrate bias signal generation, short-circuit protection circuitry and decoupling capacitors for the powerlines were integrated. This lead to a relatively complex substrate containing over thirty passives and one die, integrated by means of conducting and non-conducting glue and nearly eighty wirebonds. Since the detector arrays will be cooled down to 4.2 K prior to launch, the reliability and launch-survivability of the mounted substrate at this temperature and in this harsh environment had to be demonstrated. For this purpose, the quality and associated reliability of every assembly step is verified during substrate mounting. This included the verification of the compatibility of the bond material, optimization of the bond-yield and temperature cycling (between room-temperature and LHT) of the devices. Other tests on qualification models would focus on circuit functionality under proton and gamma irradiation, cryogenic vibration tests to demonstrate launch survivability and exhaustive temperature cycling to qualify the assembly procedure. We will present in detail the complete integration and qualification of the developed circuits, including the assembly and verification during production of the flight models and the qualification of the assembly method on the qualification models.
Patrick Merken, Senior Engineer and Team Leader
IMEC
Leuven, Brabant 3001,
Belgium


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