A low noise, low power readout electronics circuit at 4 K in standard CMOS technology for PACS/Herschel

IMEC has designed, in the framework of the PACS project (for the European Herschel Space Observatory) the Cold Readout Electronics (CRE) for the Ge:Ga far-infrared detector array. Key specifications for the CRE were high linearity (3%), low power consumption (80 μW for an 18 channel array), and very low noise (200 e^-) at an operating temperature of 4.2 K (LHT - Liquid Helium Temperature). IMEC has implemented this circuit in a standard CMOS technology (AMIS 0.7 μm), which guarantees high production yield and uniformity, relatively easy availability of the technology and portability of the design. However, the drawback of this approach is the anomalous behavior of CMOS transistors at temperatures below 30-40K, known as kink and hysteresis effects and under certain conditions the presence of excess noise. These cryogenic phenomena disturb the normal functionality of commonly used circuits or building blocks like buffer amplifiers and opamps. We were able to overcome these problems and developed a library of digital and analog building blocks based on the modeling of cryogenic behavior, and on adapted design and layout techniques. These techniques have been validated in an automated cryogenic test set-ups developed at IMEC. We will present here in detail the full design of the 18 channel CRE circuit, its interface with the Ge:Ga sensor, and its electrical performance and demonstrate that all major specifications at 4.2 K were met. Future designs and implementations will be equally presented.