Study On Homoepitaxy Technology Of 4H-SiC For IGBT Devices

Higher voltage,higher power density and higher efficiency is the eternal development trend of power electronic device technology.Compared with the conventional silicon(Si)material,4H crystalline silicon carbide(4H-Si C)material has superior physical properties such as wide bandgap,high critical breakdown electric field and high thermal conductivity,which makes it ideal for developing large high voltage Si C power electronic devices.When the breakdown voltage of unipolar device reaches 10k V or higher,high conduction resistance becomes an obstacle to hinder its further application.The bipolar device,represented by the insulated gate bipolar junction transistor,can achieve lower on resistance through conductance modulation effect which makes it more suitable for high voltage and high power applications such as smart grids.The structure of IGBT device demands higher quality of epitaxial material.This paper focuses on the key technologies of high-speed epitaxy and epitaxial defect control.The research contents and achievements are as follows:(1)Obtain the process window of inlet C/Si ratio at different epitaxial rates and analyze the corresponding mechanismThrough a series of epitaxial experiments,the ranges of inlet C/Si ratio to avoid the formation of giant step-bunching are obtained at different growth rates.The process window of C/Si ratio can be divided into three sections,corresponding to silicon-rich atmosphere section,carbon-silicon equilibrium atmosphere and carbon-rich atmosphere respectively.Based on the experimental data,the equilibrium point for silicon and carbon atmosphere in the reaction chamber was obtained with a C/Si ratio of 0.84 in the epitaxial system.The upper and lower limits of C/Si ratio process window at different epitaxial rates are also obtained.It is found that the upper and lower limits of the C/Si ratio process window converge to the equilibrium C/Si ratio with the increase of the epitaxial rate.(2)Master the technology of precise control of low concentration n-type doping and regulate the uniformity wellAiming at the demand of high-volt IGBT for precise low concentration(2E14cm^-3)n-type doping,the variation of n-type doping efficiency with C/Si ratio and other parameters is analyzed.When the C/Si ratio varies in the silicon-rich atmosphere section,the n-type doping efficiency decreases rapidly with the increase of C/Si ratio under the effect of both C/Si ratio and growth rate.When the C/Si ratio varies in the carbon-silicon equilibrium or carbon-rich atmosphere section,the decreasing trend of n-type doping efficiency with the increase of C/Si ratio becomes less steep.To achieve precise low concentration n-type doping,the C/Si ratio needs to be restrained in the carbon-silicon equilibrium or carbon-rich atmosphere section.At the growth rate of 60?m/h,the background doping concentration can be lower than1E14 cm^-3 using a C/Si ratio above 0.84,and the n-type doping efficiency can be controlled as low as 9.69E13 cm^-3/sccm.Excellent thickness and doping concentration uniformity were achieved by adjusting the flow ratio of main hydrogen and n-type doping source in the central and side gas paths.(3)Master the technology of precise control of high-concentration p-type dopingIn order to meet the demand of bipolar devices for high concentration(1E19 cm^-3)p-type doping,the variation of p-type doping efficiency with the flux of dopant(Trimethylaluminium,TMA)was studied using trichlorosilane and silane as silicon source respectively.It can be found that the introduction of chlorine-based gas source can significantly reduce the Al doping efficiency.In order to achieve Al doping concentration of 1E19 cm^-3 level in chlorine-atmosphere,the flow rate of TMA should be extra high and the carbon source introduced by TMA can directly affect the actual C/Si ratio.The silane process is optimized for high concentration p-type doping.The6-inch p-type epitaxial wafers grown by silane process with Al doping concentration?1E19 cm^-3 has a doping nonuniformity(5 points in the radial direction)less than 10%which meets the requirements of the device design.(4)Obtain the nucleation and extension models of the epitaxial defects,and achieve thick epitaxial films with low defect densityBy studying the relationship between the distribution of surface morphology defects and the failure distribution of SBD devices judged by reverse break down voltage(V_BR),the hazard level of surface morphology defects was distinguished.The triangle defect is a kind of fatal defect.The device which has a triangle defect in the active region will inevitably be fail.The epitaxial layer is removed step by step through polishing to the substrate interface or below the substrate interface,and the corresponding relationship between the surface morphology defects and the substrate dislocations is traced,the nucleation models of triangle defect,carrot defect and the extension models of TSD,TED,BPD and SF were obtained.Through process optimization and substrate screening,the usable area(2mm×2mm cell)of the epitaxial wafers was effectively increased to more than 95%,and the triangular defect density was reduced to less than 0.3cm^-2.To reduce BPD which causes the performance degradation of bipolar devices,a composite buffer layer was designed to promote the conversion from BPD to TED;the epitaxial wafer with nearly Zero BPD was realized.(5)Increase the epitaxial layer's minority carrier lifetime by reducing the concentration of Z_1/2 deep level defectsThe concentration of Z_1/2 deep level defects in the samples prepared under different conditions is measured by the deep level transient spectrum,and the influence of growth parameters such as epitaxial rate,effect of C/Si ratio and growth temperature on Z_1/2 concentration is studied.In the experimental conditions,the Z_1/2deep level defects in the epitaxial layer can be reduced by decreasing the growth temperature and adopting the C-Si equilibrium condition.The minority carrier lifetime of the epitaxial layer was increased from 1?s to 7?s by optimizing the process conditions and the subsequent oxidation treatment at 1450°C.(6)Developed the 20k V 4H-Si C n-channel IGBT device based on the domestic epitaxial wafer.The epitaxial structure of 20 k V 4H-Si C n-channel IGBT was designed,and the special technology of back thinning and laser annealing for IGBT was optimized.Finally,the n-channel IGBT was successfully developed,the blocking voltage was up to 20 k V and the differential specific on-resistance was only 36 m??cm².The performance advantage of high current density of high voltage and high power bipolar device is fully exhibited.