Experiment and Analysis of Damage Effect of Proton Irradiation on Transmission Transistor of CMOS Image Sensor
Objective The image sensor is a device that converts optical signals into electrical signals.It is often employed in digital image information acquisition and plays an important role in daily production and life.In the 1960s,the complementary metal oxide semiconductor(CMOS)image sensor(CIS)was discovered.In 1993,the CMOS active pixel image sensor(APS)with intra-pixel charge transfer was proposed.The development of the photosensitive pixel unit structure has greatly improved the overall performance of CIS.Meanwhile,its superior performance has gradually replaced the charge coupled device(CCD)image sensor and quickly become a research hotspot.However,in some specific application fields,such as aerospace,medicine,and nuclear energy,CMOS image sensors are not immune to the influence of radiation.Radiation particles often have strong penetration ability and can interact with the electronic devices inside the CIS.For a single electronic device,radiation will change the electrical parameters of the device,and for the entire image sensor,radiation will cause a series of problems such as increased dark current and noise,thus greatly reducing the image quality of the image sensor,and bringing great uncertainty to scientific research,medical diagnosis,and industrial production.Thus,we report the experiment and analysis of proton irradiation on the damage effect of the 6T CIS transmission gate.The transmission gate is introduced in the form of transmission transistors and irradiated with a proton with energy of 60 MeV.The effect of proton irradiation on the electrical performance of the transmission transistor can be carried out for the local structure of the pixel circuit.By characterizing the electrical parameters,we reveal the degradation and annealing mechanism of the 6T CIS transmission transistor induced by high-energy proton irradiation at different fluences,providing data support for the simulation and radiation-resistant reinforcement of the CIS transmission transistor.Methods First,in the layout design of the single tube of the transmission gate,the four ends of the transmission gate(TG)are connected to four pads respectively and are introduced to the pins by lead bonding,with the electrical performance of the transmission transistor tested.Then,in Xi'an 200 MeV Proton Application Facility(XiPAF),the proton irradiation energy is 60 MeV,and the fluences are 1×1010,1×1011,and 1×1012 cm-2,respectively.During the irradiation,the sample is fixed on the anti-static sponge without voltage bias.After irradiation,the electrical parameters of the single tube of the transmission gate are tested before and after irradiation on the B1500A semiconductor parameter analyzer to study the changes of the electrical sensitive parameters before and after irradiation and the annealing after irradiation.Results and Discussions The electrical performance of the transmission transistor is tested after irradiation.The experimental results show that,with the increasing proton flux,the forward bias degree of the threshold voltage is increasing[Fig.3(a)],and Fig.3(b)indicates that the decrease in saturation current is also increasing.As shown in Fig.4(a),for the change value of the threshold voltage,the decrease in the threshold voltage change value is not very obvious from 0 to 24 h,and at 72 h,the threshold voltage change value decreases obviously.As shown in Fig.4(b),the decrease in saturation current after irradiation reduces with the increasing annealing time.Combined with TCAD simulation analysis,proton irradiation will generate oxidation layer trap charge and interface state trap charge,degrading the single tube of the CIS transmission gate.As shown in Fig.8(a),the change trend of the threshold voltage change value is similar to the test results,and Fig.8(b)reveals that the decrease in saturation output current increases,which is similar to the test results.Conclusions After proton irradiation,the threshold voltage and saturation current of the single tube of the CIS transmission gate have significant changes,and the changes increase with the rising flux and then affect the normal function of the whole pixel.Then the 72 h annealing experiment is carried out,and the experiment shows that the electrical parameters of the single tube of the transmission gate caused by irradiation damage can be restored to a certain extent by room temperature annealing.The changes in threshold voltage and saturation current of the single tube of the transmission gate of CIS after irradiation are mainly caused by the trap charge of the oxide layer and the interface state.The amount of irradiation-induced trap charge increases with the rising irradiation fluence,among which the trap charge of the interface state increases continuously,and the forward drift of threshold voltage intensifies.This will make it difficult to form an inversion layer in the P-type base region at the Si/SiO2 interface,with decreased saturation current.
remote sensing and sensorsoptical deviceCMOS image sensorproton irradiationirradiation fluenceannealing test
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