Performance Limit Relationship And Key Process Technology Of SiC Superjunction Devices

As a typical representative of third-generation semiconductors,silicon carbide（SiC）has great application prospects in power electronics such as electric vehicles,photovoltaic inverters,rail transportation,and smart grid due to its large bandgap width,high critical breakdown field,high electron saturation drift velocity,and high thermal conductivity.High breakdown voltage（BV）and low specific on-resistance(R_on,sp)are one of the goals pursued by power semiconductor devices.The superjunction（SJ）structure is an effective way to improve R_on,sp and BV performance of SiC power devices from structural level,especially in high-voltage fields.Although some progress has been made in the theoretical optimization and fabrication process research of SiC SJ devices in recent years,most of the theoretical studies are based on silicon（Si）SJ devices.The material properties of SiC have not been analyzed in depth,especially the anisotropy of SiC is more obvious than that of Si,which will cause the structural design,performance evaluation,and subsequent preparation of SiC SJ to be different from the expected ones.This thesis focuses on the key physical phenomenon of avalanche breakdown of SiC devices based on the anisotropic characteristics of SiC materials,and revealed the influence of impact ionization anisotropy on avalanche breakdown path of SiC SJ structures from the physical essence of carrier impact ionization.The optimized design guideline and performance limit relationship for SiC SJ structures are proposed for the first time,and break through the key process technology of"trench etching-epitaxial backfill"for SiC SJ structures.The main research contents and results of the thesis are as follows.1.The avalanche breakdown path of SiC SJ structure is accurately determined.The performance advantages of SiC SJ structure are illustrated based on the coupling relationship between its charge fields and potential fields.The two peak electric field points and the corresponding critical paths where avalanche breakdown may occur were initially identified.By fully considering impact ionization anisotropy of SiC,the impact ionization rates of the above two points are characterized.The integral of impact ionization rate is performed on the corresponding two critical paths,so that the avalanche breakdown path of SiC SJ structure is accurately determined as the curve from the middle bottom point of N pillar to the middle top point of P pillar via the midpoint of PN metallurgical junction,instead of the midline of N pillar as most studies believe.Simulation results based on TCAD Sentaurus also argue for the above conclusion.2.The optimized design guideline and performance limit for SiC SJ structure are proposed.Using the critical depletion of the balanced symmetric SiC SJ structure at avalanche breakdown as an optimization condition for specific on-resistance,the structural design guideline to achieve optimized R_on,sp at a certain BV is proposed.Considering the degradation factors of SiC material properties and SJ structure on conduction capability,the quasi-linear limit relationship between R_on,sp and BV of balanced symmetric SiC SJ structure is further proposed,i.e.,R_on,sp∝BV^1.007.In addition,based on the electric field distribution at different asymmetry degrees（δ）of the width-charge balance（asymmetry）SiC SJ,the variation of avalanche impact ionization on the critical path due to electric field variation is explained.When theδ≤2.5,the avalanche breakdown path is the curve from the middle bottom point of N pillar to the middle top point of P pillar via the midpoint of PN metallurgical junction,and when theδ>2.5,the avalanche breakdown path becomes N-pillar midline instead.The structural design scheme of asymmetric SiC SJ structure under different asymmetry degrees and breakdown voltages is further proposed,and the 0.975th power relationship between R_on,sp and BV of asymmetric SiC SJ structure is proposed,and performance evaluation is realized.3.Completed etching of microgroove-free SiC deep trenches and epitaxial backfill experiments were conducted.The etch morphology of HBr-based and Ar-based gases for 5μm-deep SiC trenches were evaluated and optimized,and the etch morphology of metal mask layer,etch selection ratio,and etch velocity of SiC showed that Ar-based gases are more suitable for etching SiC trenches.The effects of metal mask thickness,RF power and etch gas flow rate on the etch profile of 7μm-deep trenches were also investigated.In addition,epitaxial backfilling experiments were conducted on 3μm-and5.3μm-deep SiC trenches in terms of high-temperature hydrogen etching time,epitaxial velocity,and epitaxial time,respectively.The epitaxial thicknesses on the bottom,side walls,and bench surfaces of trenches were evaluated to lay the foundation for the fabrication of subsequent SiC SJ devices.