  In the context of the CERN EP R&D on monolithic sensors and the ALICE ITS3
upgrade, the Tower Partners Semiconductor Co (TPSCo) 65 nm process has been
qualified for use in high energy physics, and adopted for the ALICE ITS3
upgrade. An Analog Pixel Test Structure (APTS) featuring fast per pixel
operational-amplifier-based buffering for a small matrix of four by four
pixels, with a sensor with a small collection electrode and a very non-uniform
electric field, was designed to allow detailed characterization of the pixel
performance in this technology. Several variants of this chip with different
pixel designs have been characterized with a (120 GeV/$c$) positive hadron
beam. This result indicates that the APTS-OA prototype variants with the best
performance achieve a time resolution of 63 ps with a detection efficiency
exceeding 99% and a spatial resolution of 2 $\mu$m, highlighting the potential
of TPSCo 65nm CMOS imaging technology for high-energy physics and other fields
requiring precise time measurement, high detection efficiency, and excellent
spatial resolution.
