Analysis of the performance of CMOS APS imagers after proton damage
CMOS imagers for video cameras and Digital Still Cameras (DSC) have evolved rapidly in the past years, taking advantage from the reduction in the transistor size.
Actually, a wide class of devices optimized for visible light detection is available, characterized by excellent response uniformity, medium sized geometrical surface, small pixel size, good linearity over a wide dynamic range, low noise, low power consumption. Our group has started to investigate the suitability of the adoption of these devices as ionizing radiation detectors for soft X-ray and charged particles, in particular the CMOS imager MT9V011 from Aptina Imaging.
This device features a VGA sensor (640480 pixels) with 5.6 x 5.6 um2 pixel size and 4.0 um thick epitaxial layer and has a built-in 10-bit ADC . The sensor has been extensively characterized in the past, showing excellent performance for particle detection. Among the main ﬁndings, it should be mentioned: single pixel noise from 8 to 40 electrons of Equivalent Noise Charge (depending on the ampliﬁer gain), dynamic range varying from 1–8 keV (highest gain) to 2–150 keV (lowest gain), Signal/Noise (Landau MPV/single pixel noise) 30 for a Minimum Ionizing Particle (MIP), a charged particle detection efﬁciency for a MIP of 99.9% with a fake hit probability of less than 4 10^5 .
Eventually, the measured spatial resolution is about 0.65 mm. To complete the characterization of the MT9V011 sensor as ionizing radiation detector, in this work we have investigated its resistance to radiation damage using the 24 MeV proton beam at the INFN LNS Laboratories (Laboratori Nazionali del Sud, Catania, Italy).
In this work we have irradiated a standard commercial CMOS imager with a 24 MeV proton beam at INFN Laboratori Nazionali del Sud, Catania (Italy) up to a nominal ﬂuence of 10^14 [protons/cm2 ].
The device under test was a standard VGA detector, fabricated with a 130 nm technology without radiation hardening. During the irradiation the detector was operated to monitor the progressive damaging of the sensor and the associated on-pixel electronics. After 18 months from the irradiation damage session, with the detector stored at room temperature, a study on the detection efﬁciency and charge collection capability has been carried out using ﬂuorescent X-ray photons, emitted from copper target. We found that the detector is still working at 10^13 protons/cm2 , with a moderate increase of the noise and a slightly decrease of the detection capabilities.