Low-noise fully depleted charge-coupled devices have been identified as a unique tool for dark-matter searches, low-energy neutrino physics, and x-ray detection. The charge-collection efficiency (CCE) for these detectors is a critical performance parameter for current and future experiments. We present a technique to characterize the CCE in back-illuminated CCDs based on soft x rays. This technique is used to study two different detector designs. The results demonstrate the importance of the backside processing for the detection of charge packages 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 a thickness of less than $1\mu \mathrm{m}$ with adequate backside processing.

• Received 13 July 2020
• Revised 3 February 2021
• Accepted 17 May 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.064026