  Experiments seeking to detect rare event interactions such as dark matter or
coherent elastic neutrino nucleus scattering are striving for large mass
detectors with very low detection threshold. Using Neganov-Luke phonon
amplification effect, the Cryogenic Dark Matter Search (CDMS) experiment is
reaching unprecedented RMS resolutions of ~ 14 eV$_{ee}$ . CDMSlite is
currently the most sensitive experiment to WIMPs of mass $\sim$5 GeV/c$^{2}$
but is limited in achieving higher phonon gains due to an early onset of
leakage current into Ge crystals. The contact interface geometry is
particularly weak for blocking hole injection from the metal, and thus a new
design is demonstrated that allows high voltage bias via vacuum separated
electrode. With an increased bias voltage and a $\times$ 2 Luke phonon gain,
world best RMS resolution of sigma $\sim$7 eV$_{ee}$ for 0.25 kg (d=75 mm, h=1
cm) Ge detectors was achieved. Since the leakage current is a function of the
field and the phonon gain is a function of the applied voltage, appropriately
robust interface blocking material combined with thicker substrate (25 mm) will
reach a resolution of $\sim$2.8 eV$_{ee}$. In order to achieve better
resolution of $\sim$ eV, we are investigating a layer of insulator between the
phonon readout surface and the semiconductor crystals.
