  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.
