From daily use digital cameras, medical X-ray imaging devices to particle tracking detectors in high energy physics (HEP) experiments, silicon pixel sensors efficiently convert photons and charged particles to electric signals. Like all semiconductor devices, the development of pixel sensors follows the cyclic procedure of: design → produce → test → optimize, aiming to achieve high spatial resolution and efficiency as well as low leakage current. Especially in the field of HEP, radiation-tolerant sensors need to be developed to withstand severe radiation damage.