Hot electron heatsinks for microwave attenuators below 100 mK

TL;DR

This paper presents a novel design of microwave attenuators with interleaved copper heatsinks that significantly improve cooling efficiency below 100 mK, reducing noise temperature and thermal resistance.

Contribution

The introduction of interleaved copper heatsinks in microwave attenuators enhances cooling power and reduces noise temperature at millikelvin temperatures, representing a substantial improvement over previous designs.

Findings

Achieved a noise temperature of ~50 mK at 20 mK with low power dissipation.

Demonstrated a power-law relation between noise temperature and dissipated power.

Realized nearly 20 times better cooling power compared to prior designs.

Abstract

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-$\mu$m thick conducting copper heatsinks in between 10-$\mu$m 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 $T_{n} \sim$ 50 mK was obtained for small dissipated powers ($P_d <$ 1 nW) in the attenuator. For higher dissipated powers we find $T_n \propto P_{d}^{(1/4.4)}$, with $P_{d} =$ 100 nW corresponding to a noise temperature of 90 mK. This is in good agreement with thermal modeling of the system and represents nearly a factor of 20 improvement in cooling power, or a factor of 1.8 reduction in $T_n$ for the same dissipated power, when compared to a previous design without interleaved heatsinks.