SiAl Alloy Feedhorn Arrays: Material Properties, Feedhorn Design, and Astrophysical Applications

TL;DR

This paper explores the use of SiAl alloy CE7 for cryogenic detector packaging, demonstrating its suitable thermal and machining properties for astrophysical millimeter wave detectors.

Contribution

It introduces CE7 as a promising material for feedhorn arrays and baseplates, combining the advantages of silicon and metals for astrophysical applications.

Findings

CE7 is superconducting below ~1.2 K.

CE7's thermal contraction closely matches silicon.

CE7 can be machined with high precision.

Abstract

We present here a study of the use of the SiAl alloy CE7 for the packaging of silicon devices at cryogenic temperatures. We report on the development of baseplates and feedhorn arrays for millimeter wave bolometric detectors for astrophysics. Existing interfaces to such detectors are typically made either of metals, which are easy to machine but mismatched to the thermal contraction profile of Si devices, or of silicon, which avoids the mismatch but is difficult to directly machine. CE7 exhibits properties of both Si and Al, which makes it uniquely well suited for this application. We measure CE7 to a) superconduct below a critical transition temperature, $T_c$, $\sim$ 1.2 K b) have a thermal contraction profile much closer to Si than metals, which enables simple mating, and c) have a low thermal conductivity which can be improved by Au-plating. Our investigations also demonstrate that CE7 can be machined well enough to fabricate small structures, such as \#0-80 threaded holes, to tight tolerances ($\sim$ 25 $\mu$m) in contrast with pure silicon and similar substrates. We have fabricated CE7 baseplates being deployed in the 93 GHz polarimeteric focal planes used in the Cosmology Large Angular Scale Surveyor (CLASS). We also report on the development of smooth-walled feedhorn arrays made of CE7 that will be used in a focal plane of dichroic 150/220 GHz detectors for the CLASS High-Frequency Camera.