Hot electron heatsinks for microwave attenuators below 100 mK

We demonstrate improvements to the cooling power of broad bandwidth (10 GHz) microwave attenuators designed for operation at temperatures below 100 mK. By interleaving 9-μm thick conducting copper heatsinks in between 10-μm long, 70-nm thick resistive nichrome elements, the electrical heat generated in the nichrome elements is conducted more readily into the heatsinks, effectively decreasing the thermal resistance between the hot electrons and cold phonons. For a 20 dB attenuator mounted at 20 mK, a minimum noise temperature of Tn ∼ 50 mK was obtained for small dissipated powers ( Pd