  We have been developing a monolithic active pixel sensor, ``XRPIX``, for the
Japan led future X-ray astronomy mission ``FORCE`` observing the X-ray sky in
the energy band of 1-80 keV with angular resolution of better than 15``. XRPIX
is an upper part of a stack of two sensors of an imager system onboard FORCE,
and covers the X-ray energy band lower than 20 keV. The XRPIX device consists
of a fully depleted high-resistivity silicon sensor layer for X-ray detection,
a low resistivity silicon layer for CMOS readout circuit, and a buried oxide
layer in between, which is fabricated with 0.2 $\mu$ m CMOS
silicon-on-insulator (SOI) technology. Each pixel has a trigger circuit with
which we can achieve a 10 $\mu$ s time resolution, a few orders of magnitude
higher than that with X-ray astronomy CCDs. We recently introduced a new type
of a device structure, a pinned depleted diode (PDD), in the XRPIX device, and
succeeded in improving the spectral performance, especially in a readout mode
using the trigger function. In this paper, we apply a mesh experiment to the
XRPIX devices for the first time in order to study the spectral response of the
PDD device at the subpixel resolution. We confirmed that the PDD structure
solves the significant degradation of the charge collection efficiency at the
pixel boundaries and in the region under the pixel circuits, which is found in
the single SOI structure, the conventional type of the device structure. On the
other hand, the spectral line profiles are skewed with low energy tails and the
line peaks slightly shift near the pixel boundaries, which contribute to a
degradation of the energy resolution.
