  Small collection-electrode monolithic CMOS sensors profit from a high
signal-to-noise ratio and a small power consumption, but have a limited active
sensor volume due to the fabrication process based on thin high-resistivity
epitaxial layers. In this paper, the active sensor depth is investigated in the
monolithic small collection-electrode technology demonstrator CLICTD. Charged
particle beams are used to study the charge-collection properties and the
performance of devices with different thicknesses both for perpendicular and
inclined particle incidence. In CMOS sensors with a high-resistivity
Czochralski substrate, the depth of the sensitive volume is found to increase
by a factor two in comparison with standard epitaxial material and leads to
significant improvements in the hit-detection efficiency and the spatial and
time resolution.
