Research On Novel Structure And Radiation Effect Of Super Junction IGBT

Insulated gate bipolar transistor(IGBT)has become the core device of power electronic system at medium and high-power level because it integrates the advantages of high input resistance,fast switching speed and easy driving of MOS devices and low forward conduction voltage of bipolar devices.IGBT technology has developed to the seventh generation.New structures and processes such as micro pattern trench(MPT),field stop(FS)technology,carrier stored(CS)technology,dummy gate(DG),floating P-body region and thin wafer process are applied which significantly improve the static,dynamic and reliability performances of IGBTs.However,from the perspective of device structure of the current seventh generation IGBT,the pitch of trench has been reduced to 1.6?m,it will cause more serious bow problem of wafer to continue reduce the trench pitch,and narrower trench titch will limit the size of contact which may affect the over-current and short-circuit capacity of IGBTs.It seems that the performance of IGBT is close to the silicon limit.Therefore,whether super junction(SJ)technology can become an effective method to improve the performance of IGBT is the research hotspot at present.This dissertation focuses on the design,new structure and new process of SJ-IGBT with floating P-pillar structure so as to verify the applicability of SJ technology.Moreover,the FP-SJ-IGBT suffers neutron and?-ray radiation are studied so as to understand its application potential in radiation environment.The main research contents and innovative work of this dissertation are as follows:1.Design of SJ-IGBT with floating P-pillar structureThe equivalent circuit of the IGBT with floating P-pillar structure(FP-SJ-IGBT)based on trench gate is established.Its conduction stage experiences three modes:unipolar conduction mode under low current,mixed conduction mode under medium injection and conductivity modulation mode under high current.FP-SJ-IGBT will overcome the problem that the SJ-IGBT designed and manufactured based on conventional SJ-MOS structure(connected P-pillar and P-body,CPP)suffers weak conductivity modulation effect at high current density.Based on the theoretical analysis,the structural parameters and process design of an FP-SJ-IGBT are carried out,and it is manufactured on an 8-inch platform in China.After packaging and testing,it can be seen that the forward conduction voltage of the developed 650 V device is 1.7 V when the forward conduction current density is 449 A/cm~2,which is12.8%and 5.56%lower than the reported CPP-SJ-IGBT under the same current density.The turn-off loss at this current density is 0.23 m J,which is much lower than the 3.3 m J of CPP-SJ-IGBT reported by experiments in the literature.In addition,when the junction temperature is 150?and the DC bus voltage is 400 V,the short-circuit time of the FP-SJ-IGBT is not less than 10?s.2.Design of ultra-thin FP-SJ-IGBTA 45?m 650 V ultra-thin FP-SJ-IGBT is proposed and realized for the first time.Thin wafer process is an effective method to improve the electrical parameters of IGBTs,which can reduce the forward conduction voltage,turn-off loss and thermal resistance of IGBTs that improve the current conduction capacity of IGBTs.For 600-650 V IGBTs,due to the limitation of the breakdown voltage of the device and the manufacturing capacity of the equipments in the foundry,the silicon wafer thickness of IGBT is generally not less than 60?m.The manufacturing process of the ultra-thin FP-SJ-IGBT is designed.Through equipment transformation and monitoring ability improvement,process researches such as back grinding and N-type isolation layer injection are carried out to reduce the device thickness to about 45?m.The results after packaging and testing are as follows:the breakdown voltage is distributed between 660-790 V,when the current density is 650 A/cm~2,the forward conduction voltage of the ultra-thin FP-SJ-IGBT is 1.86 V and the turn-off loss is 0.34 m J,which is 7.92%and 22.7%lower than that of FP-SJ-IGBT respectively.Under room temperature and 400 V DC bus voltage,the short-circuit time is not less than 10?s,and it is not less than 8?s when the junction temperature up to 150?.With the decrease of forward conduction voltage,turn-off loss and thermal resistance,the operation current density of the ultra-thin FP-SJ-IGBT can reach 650 A/cm~2.3.Design of Multi-Trench FP-SJ-IGBTA special feature of FP-SJ-IGBT is that P-pillar and P-body is not connected,so that the SJ structure and MOS structure can be designed separately and independently in device design,which can integrate the technology used in conventional IGBTs with floating SJ structure.The pitch of the trench of top MOS structure will not be affected by the pitch of SJ structure.Based on this,a multi-channel(MT)FP-SJ-IGBT is proposed.The SJ structure with a pitch of 9?m is adopted,the MT1-FP-SJ-IGBT with a trench pitch of 3?m is designed(no emitter regions on both sides of the increased trenches)and MT2-FP-SJ-IGBT(the increased trench is connected to the emitter electrode),and the MPT-FP-SJ-IGBT with a trench pitch of 1.8?m is designed.Through simulation,the trade-off relationship between forward conduction voltage and turn-off loss of these three structures are all better than Infineon's seventh generation IGBT technology.After manufacturing and testing,for MT1-FP-SJ-IGBT,when the current density is 449 A/cm~2,the forward conduction voltage is 1.6 V,which is reduced by 5.88%compared with FP-SJ-IGBT,and the turn-off loss is 0.45 m J,which is increased by 2.27%.Moreover,compared with FP-SJ-IGBT,the short-circuit characteristics have no degradation.When the junction temperature is 150?and the DC bus voltage is 400 V,the short-circuit time is not less than 10?s.In addition,the optimized ratio of miller capacitor to input capacitor relieves the gate voltage oscillation when turning on the device,and the smaller miller capacitor also speeds up the turning off characteristic of the device.4.Neutron induced displacement damage effect of FP-SJ-IGBTThe displacement damage(DD)effect caused by neutron radiation will reduce the minority carrier lifetime in semiconductor materials and the effective doping concentration of devices.After 0-10¹⁴ cm^-2 neutron radiation dose of FP-SJ-IGBT,it is found that,as a bipolar device,FP-SJ-IGBT is mainly affected by the reduction of minority carrier life.DD effect will increase the breakdown voltage and leakage current of FP-SJ-IGBT,but the degradation mechanism is different.The increased leakage current is caused by the reduction of minority carrier lifetime in the depletion region.However,the increase of breakdown voltage is due to the effect of cluster defects,which induces extra electric field.In addition,DD effect eliminates the snapback phenomenon at breakdown point of FP-SJ-IGBT,which is unique to SJ devices.The forward conduction characteristic of FP-SJ-IGBT changes with different neutron radiation dose,which is related to different operation modes.The DD effect leads to the reduction of carrier lifetime,which will make the reconmbination current and diffusion current play different roles in different operation modes.When the neutron radiation dose is greater than 10¹² cm^-2,the output current of the device will decrease.5.Ionizing radiation effect of FP-SJ-IGBT?-ray radiation produces ionizing radiation effect in the oxide layer,which will form positive space charge in the oxide layer and interface state at Si O₂/Si interface.After 200 krad?-ray radiation of FP-SJ-IGBT,it is found that the transfer characteristic curve of the device shifts to the left seriously and produces distortion,indicating that the threshold voltage of the device degrades significantly and also the transconductance.The effect of?-ray radiation on the output characteristics of FP-SJ-IGBT is related to radiation dose and gate voltage.The interface state produced by radiation reduces the minority carrier mobility,thus part of electron current is reduced.Under different gate voltages,the device operates in different modes,and the proportion of the affected electron current determines the influence of radiation on the output characteristics of the device.When the gate voltage is large enough,the affected electron current influenced by radiation can be ignored,and the output characteristic curve of the device is independent of the radiation dose.?-ray radiation has no effect on the breakdown voltage of the device,but it will increase the leakage current.Negative gate voltage can eliminate the influence of fixed charge in the oxide layer and interface state partially,and reduce the increment of leakage current.To sum up,the main work contents in this dissertation is to carry out the research on the operation mechanism and new structure of SJ-IGBT with floating P-pillar structure.At the same time,the process design,device manufacturing and test analysis are carried out.The results show excellent characteristics of SJ-IGBT verifying that SJ technology is a mainstream direction to develop power devices based on silicon materials in the future.Through the research of this subject,the first domestic manufacturing process platform of SJ-IGBT has been established in an 8-inch production line,which has laid a certain foundation for China in this field,including but not limited to dmestic substitution.In view of the confidentiality requirements of the cooperative development with the 8-inch foundry,some refined processes are not described in detail.