  We are developing an imaging polarimeter by combining a fine-pixel CMOS image
sensor with a coded aperture mask as part of the cipher project, aiming to
achieve X-ray polarimetry in the energy range of
$10$$\unicode{x2013}$$30~\mathrm{keV}$. A successful proof-of-concept
experiment was conducted using a fine-pixel CMOS sensor with a $2.5~\mathrm{\mu
m}$ pixel size. In this study, we conducted beam experiments to assess the
modulation factor (MF) of the CMOS sensor with a $1.5~\mathrm{\mu m}$ pixel
size manufactured by Canon and to determine if there was any improvement in the
MF. The measured MF was $8.32\% \pm 0.34\%$ at $10~\mathrm{keV}$ and $16.10\%
\pm 0.68\%$ at $22~\mathrm{keV}$, exceeding those of the $2.5~\mathrm{\mu m}$
sensor in the $6$$\unicode{x2013}$$22~\mathrm{keV}$ range. We also evaluated
the quantum efficiency of the sensor, inferring a detection layer thickness of
$2.67 \pm 0.48~{\rm \mu m}$. To develop a more sensitive polarimeter, a sensor
with a thicker detection layer, smaller pixel size, and reduced thermal
diffusion effect is desirable.
