Thermal Performances of an Improved Package for Cryocooled Josephson Standards

Complex cryogenics still represents a strong limitation to the spread of Josephson voltage standards, and cryogenfree cooling is particularly suitable to simplify their operation. The main downsides of liquid-helium-free systems are related to the chip thermalization: Indeed, at low temperature, the heat transfer between the chip and the coldplate of a cryocooler in vacuum is strongly affected by the quality of the interfaces. In order to increase the thermal performances of cryocooled programmable Josephson standards, we devised and tested a special cryopackage: The chip is embedded into a sandwich structure with high-thermal-conductivity materials subject to a controlled mechanical pressure to reduce the thermal contact resistances. A thin sapphire lamina placed upon the chip allows the heat to be dissipated from the top, thus creating an additional path for the thermal flow. A special bridgelike system with a screw is used as a reproducible torque-to-force converter to exert known pressures to the sandwich. Furthermore, we analyzed the effect of thermal contraction to the actual pressure exerted on the chip, showing a nonnegligible increase when cooled down to cryogenic temperature that can be calculated and corrected for.