  Low noise CCDs fully-depleted up to 675 micrometers have been identified as a
unique tool for Dark Matter searches and low energy neutrino physics. The
charge collection efficiency (CCE) for these detectors is a critical parameter
for the performance of future experiments. We present here a new technique to
characterize CCE in back-illuminated CCDs based on soft X-rays. This technique
is used to characterize two different detector designs. The results demonstrate
the importance of the backside processing for detection near threshold, showing
that a recombination layer of a few microns significantly distorts the low
energy spectrum. The studies demonstrate that the region of partial charge
collection can be reduced to less than 1 micrometer thickness with adequate
backside processing.
