The 4H-SiC Thick Homoepitaxial Key Technologies Research And Device Verification

Silicon carbide(SiC),as the representative of the third generation semiconductor materials,has superior physical properties such as wide bandgap,high breakdown electric field,high electron saturation mobility and high thermal conductivity.It is one of the ideal semiconductor materials for preparing high voltage,high temperature and high power electronic devices,especially for high voltage applications in grid systems,which could be said as"Born for the grid".After about 30 years of development,SiC devices have been widely used in low and middle voltage field,with the strong growth of application market demand,there will be explosive growth in the next few years.But,because of grid features,such as super high voltage,large current,high power,high quality SiC epilayers with ultra-thickness,low doping concentration and low defect are required.There still have many problems unsolved in this field,which restrict SiC device application in grid system.In this work,we studied the key technologies of thick SiC homoepitaxial layers,then made 6500V diodes for testing SiC epitaxial materials.The main contents and highlights are as follows:1)The growth mechanism of SiC homogeneous epitaxy was analyzed theoretically.For thick epitaxial film materials,the technical route of rapid epitaxy growth is analyzed,and the main equipment of rapid epitaxy growth in the market is mainly introduced.In order to judge the quality of materials accurately,the characterization methods of thickness,doping concentration,defects and minority carrier lifetime were studied.2)Based on the on-line etching technique,a periodic epitaxy technique for the growth of thick film materials with low defects was developed.By decomposing the traditional epitaxy growth into several"growth-etching-purge-regrowth"processes,stopping the growth during the epitaxy growth process and carrying out multiple"etching-purge"processes,the particles falling from the chamber can be cleaned in time,and other defects induced by them can be suppressed.Thick epitaxial film experiments with 70?m show that this technology can reduce the triangular defects about 30%,the defect control level reaches domestic leading level,and promote the transformation of BPD to TED and reduce the density of BPD.By optimizing the gas flow distribution,1.21%thickness non-uniformity and 2.5%doping concentration non-uniformity are achieved on the 6 inch substrate,reaching the domestic leading level.3)Based on the PL and?-PCD testing techniques,the factors affecting minority carrier lifetime of ultra-thick film(180?m)SiC epitaxy materials were analyzed.By comparing PL with?-PCD mapping,it was found that in the defect-free region,the minority carrier lifetime is 3.02?s,the minority carrier lifetime in the triangular surface region is 0.77?s,and the minority carrier lifetime near the triangular edge(a large number of stacking faults)is 1.34?s.It was found that in thick epitaxial materias,the effect of triangular defects on minority carrier lifetime is greater than stacking faults.4)The technologes on enhancing minority carrier lifetime have been studied,and three menthods of improving minority lifetime are compared.The effects of high temperature oxidation,high temperature annealing,chemical mechanical polishing and epitaxial process on minority carrier lifetime were systematically studied.It was found that the minority carrier lifetime was increased to 5?s by long-time high temperature oxidation and annealing,which can meet the needs of high voltage bipolar devices.The mechanism of high temperature oxidation is revealed:the C near the oxide layer interface diffuses to the epitaxial layers,and the C-vacancy defects affecting minority carrier lifetime are repaired.After surface treatment by CMP,it is found that the minority carrier lifetime of the whole wafer is more uniform,because the CMP reduces the surface recombination rate.Increasing C/Si ratio(C rich condition)and CMP have certain effect on enhancing minority carrier lifetime,but they are far from the needs of devices.5)The growth technology of p-type epitaxy and the it's mechanism were studied.The linear relationship between p-type doping concentration and TMA flow rate was obtained in the range of 1E16-1E19 cm^-3.In order to control the defects in p-type epitaxy,the special"etching-purging"process of periodic epitaxy was introduced before p-type epitaxy growth.Because of the etching and cleaning effect of the"etching-purge"process,foreign particles on the surface could be etched in time and cleaned up,which provided a good surface condition for p-type epitaxy growth,inhibited the generation of defects in p-type epitaxy,and reduced the influence of p-type epitaxy on the yield of device manufacturing.It is also found that the internal stress caused by Al doping enhances the stacking fault.The formation mechanism is analyzed by PL and molten KOH.The lattice stress caused by Al doping releases the dislocation at the interface,and then it is transformed into stacking fault in the epitaxial growth process.6)The 6 inch,70?m thick film 4H-SiC epitaxial material was verified by the 6500V diode device with the active region of 67mm².The results show that the JBS diode shows good forward characteristics,when the forward bias voltage is from 0 to 5 V.When the forward current reaches 25A,the diode voltage drop is less than 2.89V.When the reverse leakage current of the diode reaches 10?A,the reverse breakdown voltage of the diode reaches 7800V,and the breakdown efficiency reaches 82%;the yeild with 60%was got.The improvement of the positive characteristic of Pi N diode by improving the minority carrier lifetime was verified.At the same time,the effect of low defect density(especially low BPD)on the improvement of the positive voltage degradation and the improvement of the device manufacturing yield were verified also.The influence of epitaxial defects on the performance of devices were studied.It was found that the triangle and drop defects have great influence on the breakdown voltage and leakage current of devices,which will greatly reduce the reverse breakdown voltage,and the influence is more serious than that of thin epitaxial defects.In the process of thick film epitaxy,we must focus on the control for defects.