  We have been developing monolithic active pixel sensors, X-ray Astronomy SOI
pixel sensors, XRPIXs, based on a Silicon-On-Insulator (SOI) CMOS technology as
soft X-ray sensors for a future Japanese mission, FORCE (Focusing On
Relativistic universe and Cosmic Evolution). The mission is characterized by
broadband (1-80 keV) X-ray imaging spectroscopy with high angular resolution
($<15$~arcsec), with which we can achieve about ten times higher sensitivity in
comparison to the previous missions above 10~keV. Immediate readout of only
those pixels hit by an X-ray is available by an event trigger output function
implemented in each pixel with the time resolution higher than $10~{\rm \mu
sec}$ (Event-Driven readout mode). It allows us to do fast timing observation
and also reduces non-X-ray background dominating at a high X-ray energy band
above 5--10~keV by adopting an anti-coincidence technique. In this paper, we
introduce our latest results from the developments of the XRPIXs. (1) We
successfully developed a 3-side buttable back-side illumination device with an
imaging area size of 21.9~mm$\times$13.8~mm and an pixel size of $36~{\rm \mu
m} \times 36~{\rm \mu m}$. The X-ray throughput with the device reaches higher
than 0.57~kHz in the Event-Driven readout mode. (2) We developed a device using
the double SOI structure and found that the structure improves the spectral
performance in the Event-Driven readout mode by suppressing the capacitive
coupling interference between the sensor and circuit layers. (3) We also
developed a new device equipped with the Pinned Depleted Diode structure and
confirmed that the structure reduces the dark current generated at the
interface region between the sensor and the SiO$_2$ insulator layers. The
device shows an energy resolution of 216~eV in FWHM at 6.4~keV in the
Event-Driven readout mode.
