Research On Radiation Damage And Protection Of Photodiode In CMOS Image Sensor

Abstract:In this study,irradiation damages of the photodiode in complementary metal oxide semiconductor?CMOS?image sensors for gamma rays and electron were analyzed by calculating the deposition energy of two kinds of rays in unit cell sizes.In the meantime,damage degree of three different unit cell sizes was compared.Image dark current of commercial CMOS image sensor with five different unit cell sizes in ⁶⁰Co radiation sources was investigated.Seven various heavy mental oxides?HMO?glasses were applications as transparent shielding materials based on the research.The working life of the commercial CMOS image sensors in complex source terms was prolonged by adding these transparent shielding materials.It can be contributed topopularize the using of transparent shielding materials in complex source terms.The low energy gamma rays are the dominant part of deposition energy by GEANT4 code.The deposition energy increases with increasing of the unit cell sizes in gate oxide SiO₂ for photodiode with different sizes,when the incident gamma energy is below 0.04MeV.The deposition of 5?m unit cell sizes is the biggest when the energy is over 0.04MeV.The contribution of low energy is much greater than high energy in the unit cell.Therefore the low energy gamma rays should be focused more attention when shielding gamma rays.The deposition energy increases with increasing of the unit cell sizes in substrate Si for photodiode with different sizes,the same rule for the whole model.The deposition energy in the unit cell increases with increasing electron energy.The deposition energy in gate oxide SiO₂ is not obvious for a unit cell of three different sizes.The deposition energy in substrate increases with increasing of the unit cell sizes,and the same rule for the whole model.The threshold of the image dark current has appeared in the experiment.The dark current increases with increasing of the dose rate.Average gray ofdark image would appear threshold with the increase of irradiation time.The threshold is different for various CMOS image sensors,and unit cell size is not directly associated with threshold values.Mass attenuation coefficient of the HMO glasses increases with increase in lead weight fraction by MCNP code.The mean free path increases with increasing photon energy and decrease with increasing in lead content.The mean free path would appear threshold value with increasing lead content.The bremsstrahlung photon energy is mainly concentrated in the low energy region when electrons pass through the HMO glasses by GEANT4,and penetration of electrons is faint.The eventual aim of shielding is filtering low energy of input radiation and also reducing the number of coming into the photodiode in CMOS image sensor.