Investigation On Irradiation Effects And Hardening Methods Of 4H-SiC VDMOSFETs

With the development of science and technology,especially the development of aerospace technology,nuclear technology and military technology,there is an urgent need for high-performance,low-volume power electronic systems that can be used in various harsh high-temperature and strong-radiation environments.Since SiC power devices have great advantages in terms of high temperature,high power,high efficiency and reduced volume,as the manufacturing process matures,they will be widely used in the above fields.The research of radiation effect and hardening methods of SiC power devices has become very important.At present,the research on the radiation effect of 4H-SiC power MOSFET devices is not systematic,the mechanism of radiation damage needs to be revealed,and the design of hardening methods has not been carried out.In this paper,focusing on the problems faced by the application of 4H-SiC VDMOSFETs in the irradiation environment,the mechanism and hardening methods of the proton and total dose radiation effects of 4H-SiC VDMOSFETs and their key structures are studied,and the main work and achievements are as follows:(1)The research on mechanism and hardening method of the shift of threshold voltageThrough proton and total dose irradiation experimental research,it is clear that the threshold voltage shift caused by ionization effect is a key problem for the application of the device in the irradiation environment,which will cause the device to fail to turn off normally,and also cause the increase of the input,output,and reverse transfer capacitance of the device and the leakage current which will cause the device to not work normally due to insufficient gate drive charge and increase the power consumption of the device.Irradiation experiments on MOS capacitors with different nitridation concentrations at the same irradiation dose revealed that the reason is that the special gate dielectric nitrogen passivation process of SiC MOS devices not only reduces near interface electron traps but also introduces near interface hole traps.As a result,near interface hole traps dominate the traps near the interface.During the irradiation,the ionization effect of protons produces electron-hole pairs along the path of the proton movement.The dominant hole traps will capture more holes to form a net positive charge.Based on this mechanism,we studied the design of gate dielectric passivation process to resist the threshold voltage shift caused by ionizing radiation.We want to reduce the amount of introduced nitrogen by reducing the annealing time of NO while ensuring that the concentration of interface traps and near interface electron traps does not increase significantly,to reduce the threshold voltage shift of the device by reducing near interface hole traps,thereby improving the ability of devices to resist ionizing radiation.We prepared NMOS and PMOS capacitors with NO annealing time of 60 minutes and 90minutes,respectively.The total dose irradiation experiment obtained that the sample with NO annealing time of 60 minutes did not significantly affect the interface state near the conduction band and near interface electron traps.After the irradiation,the effective positive interface charge at the interface is reduced by 6 times,thereby reducing the shift of the threshold voltage of the device under ionization irradiation by 6 times.(2)The research on mechanism,numerical model and hardening method of the decrease of breakdown voltageThrough the experimental research of proton and total dose irradiation,it is clear that the decrease of breakdown voltage caused by the ionization effect is a key issue for the application of the device under the irradiation environment.We conducted irradiation experiments with MOS capacitors and devices prepared in the same process as the terminal,and extracted the corresponding relationship between the irradiation dose and the charges on the terminal interface.And based on this,we established a numerical model of the effect of irradiation on the breakdown voltage of the device using TCAD.Using TCAD simulation,it is revealed that the reason for the decrease of the breakdown voltage of the device is that the positive charge inhibits the expansion of the depletion layer and pushes the peak value of the electric field forward,which leads to the increase of the electric field in the first ring and the uneven electric field distribution.The terminal adopts a nitridation process,and the mechanism of interface charge after irradiation is the same as that of the gate oxide.Then we carried out hardening design from two aspects of terminal structure and terminal process.Based on the established numerical model of the impact of ionization effects on the breakdown voltage of the device,we designed terminal structure that resists the increase of breakdown voltage caused by ionization radiation.The design of planar non-uniform field limiting rings terminal increased the anti-radiation capability of devices to 29 Mrad without increasing the area.And the design of trench terminal makes the anti-ionizing radiation capability of devices further improved to 44 Mrad without increasing the area.Then,we studied the terminal process design for resisting the decrease of breakdown voltage caused by ionizing radiation.The terminal process of first wet oxygen oxidation and then deposition of Si O₂ was adopted.Although the interface prepared by this process will have a large number of traps,the number of electron traps and hole traps are close,which reduces the effective interface charge concentration after irradiation.The device was prepared and the total dose irradiation experiment was performed to verify that the breakdown voltage of the device did not change after 3 Mrad irradiation.(3)The research on mechanism,numerical model and hardening method of the decrease of on-resistanceThrough proton irradiation experimental research,it is clear that the decrease of on-resistance caused by the displacement effect is a key issue for the application of the device under the irradiation environment.The device fails due to the deterioration of on-resistance when the maximum dose is 1×10¹⁴ p/cm²(energy is 5Me V).Through the test and analysis of the key structure of MOS capacitor,lateral MOSFET,and ohmic contact TLM of the device which are irradiated together,the effect of irradiation on the resistance of each part are clear.Combined with the simulation study of the numerical model of the displacement effect on on-resistance of the device established by the TCAD trap model based on the results of the DLTS characterization,it has clarified the mechanism of on-resistance degradation.The defects created by displacement effect capture free carriers to reduce the carrier concentration.When the defect concentration is similar to the epitaxial doping concentration,it will be fully compensated.However,the significant deterioration of the device on-resistance before the complete compensation is due to the narrowing of the JFET region to pinch-off which lead to the sharp increasing of the resistance of the JFET region.Finally,based on the established numerical model of the displacement effect on the on-resistance of the device,we design the width and concentration of the JFET region to suppress the narrowing of the JFET region.The decrease of on-resistance at the dose of sharp deterioration is reduced by 4.5 times without reducing the breakdown voltage.In summary,this article systematically studied the proton radiation effect and total dose radiation effect of 4H-SiC VDMOSFETs,clarified encountered key problems of the device in the radiation environment,revealed the internal mechanism of radiation damage and established the numerical model of the degradation of key electrical characteristics by carrying out the proton and total dose irradiation experiments of 4H-SiC VDMOSFETs and their key structure MOS capacitors(different nitrogen passivation processes),lateral MOSFETs,and ohmic contact TLM,conducting electrical characteristics testing and material-level characterization,combining with SRIM and TCAD simulation.Finally,in view of the key issues of the application of the devices to the radiation environment,based on the radiation damage degradation mechanism and the numerical model,the structure and process of the device have been designed to resist radiation.Some of the hardening methods have been verified by the radiation experiment,which has laid the foundation of the application of 4H-SiC VDMOSFETs under the irradiation environment.