Theoretical And Technological Research On SiC Super-Junction Schottky Diodes

In recent years,with the rapid development of new energy vehicles,energy Internet,rail transit and national defense,silicon carbide(SiC)is more and more popular in industry and academia due to its superior material properties.As the third generation wide band gap semiconductor material,SiC has excellent material properties such as wide band gap,high critical electric field,high electron saturation velocity and high thermal conductivity.Therefore,SiC power devices are promising candidates for high voltage,high frequency and high temperature power electronics applications.SiC unipolar devices have the advantages of high breakdown voltage,low specific on-resistance,high switching speed and low switching loss.After long-term research,tremendous progresses have been made and the device performance is gradually close to the one-dimensional theoretical limit of SiC unipolar devices.However,it is a great challenge to break the one-dimensional theoretical limit of SiC unipolar devices and further reduce the specific on-resistance and conduction loss of conventional SiC unipolar devices.Super-Junction technology is a quite effective technical means to achieve low on-resistance and high breakdown voltage at the same time.Super-Junction technology is a mature technology that successfully breaks the one-dimensional theoretical limit of silicon-based unipolar devices,and breaks the original shackle between specific on-resistance and breakdown voltage.It is successful to improve the device performance and realize the industrialization and application.It is foreseeable that applying Super-Junction structures to SiC devices could also reduce the specific on-resistance and improve the conduction performance.Nowadays there are two typical processes to realize the Super-Junction structure,one is based on multi-epitaxy process and the other is based on trench process.Multi-epitaxy process requires multiple epitaxial growth and ultra-high energy ion implantation process,which greatly increases the production cost and difficulty.There are two ways to realize the Super-Junction structure with the trench process.One is to fill the trench by p-type epitaxial growth to form the Super-Junction P-region,which requires a complex epitaxial growth process.The other is to utilize sidewall ion implantation to form the Super-Jjunction P-region and fill the trench with dielectrics.Compared with the first way,this process is simplified and cost-saving.Therefore,"trench-etching and sidewall-implantation"method is selected to fabricate SiC Super-Junction devices in this thesis.Meanwhile,systematic theoretical and experimental investigations of the SiC Super-Junction Schottky diode(SiC SJ SBD)are carried out.The main research contents and innovations of this thesis include:(1)Comprehensive mechanism investigations and design optimizations of SiC SJ SBD devices are carried out in this thesis.The influence of structural parameters on the electrical performance of the device is also analyzed.Based on the actual fabrication capability,the multi-epi structure is proposed,which improves the conduction and blocking abbilities of devices with non-90°trenches significantly at the same time.It is found that the trench angle has an important influence on the on-resistance,breakdown voltage and charge imbalance window of devices.The 90°device is always achieving the highest comprehensive device performance.However,the trench angle of the SiC Super-Junction device relies on the actual fabrication capcabilty,and it is difficult to obtain a 90°trench.The multi-epi structure is proposed to improve the performance of the device with non-90°trench significantly.(2)The impact of the transition region on the blocking performance of the device is studied in detail.And this thesis proposes a simple and efficient novel termination structure,which is suitable for“trench-etching and sidewall-implantation”method.The structure achieves an efficient electric field modulation in termination region and a blocking voltage close to 100%of the theoretical value.With the P-region formed by sidewall implantaion,the electric field is successfully confined within the termination trench.Compared with conventional wide-trench termination,the proposed termination has a stronger reliability during high d V/dt conditions.Through finite element numerical simulation,it is found that the outermost mesa width(OMW)and wide-trench termination width(WTW)affect the blocking ability of the device seriously.Through the design optimization of OMW and WTW,the blocking performance of the termination and the reliability of the dielectric in the termination trench are greatly improved.(3)In this thesis,the SiC etching technology is systematically studied,and the influence of process conditions on etching rate and morphology is analyzed in detail.After that,the"Two-Phase Etching"process for improving SiC deep trench etching is proposed.The trench with high aspect ratio and smooth bottom can be achieved,which is used for the Super-Junction device.The impacts of process conditions on SiC etching rate,micro-trench formation and etched surface roughness are studied.Further,the"Two-Phase Etching"process is developed,which is more flexible to control the trench shape,and a 15?m-deep micro-trench-free smooth SiC trench with the aspect ratio of 5is obtained.The filling effect,breakdown electric field and mechanical strength of the dielectric are the key factors which affect the blocking performance of the device.The combination of"Si O₂+PI+Si O₂"method proposed in this paper realizes the composite dielectric filling process with high breakdown electric field and no void.(4)Based on the research into the theoretical mechanism,structural design and process technology of SiC Super-Junction devices,a 2k V class SiC SJ SBD has been fabricated.Through the electrical performance test of the device,the optimization of the theoretical model and device structure is demonstrated,and the fesibility of the“trench etching and sidewall implantation”method,the"Two-Phase Etching"process and trench filling process with composite dielectrics is also confirmed.This thesis reports a 1920V SiC SJ SBD with the specific on-resistance of 1.6m?·cm²,which successfully breaks the one-dimensional theoretical limit of SiC unipolar devices.The research method of device structure design,process development and test analysis proposed in this thesis provide a useful theoretical basis and experimental reference for the development of SiC Super-Junction devices based on the“trench-etching and sidewall-implatation”method.It is foreseeable that this thesis and follow-up research will comprehensively promote the rapid development of SiC power devices,achieving the innovative breakthrough of the device performance and further promoting the wide application of SiC devices in many important fields related to national security,such as new energy power generation,high-voltage transmission,smart grid,electric vehicles and national defense industry.