Research On The Characteristics And Technology Of 4H-SiC BJT Power Devices

Silicon carbide?SiC?is considered as a promising material to build the next generation power switching devices due to its superior properties such as higher critical field,wider bandgap,and higher thermal conductivity compared to silicon.In a series of power switches,bipolar junction transistor?BJT?has a very low forward voltage drop,high current capability and fast switching speed.However,in order to meet the needs of the market,it needs to have very high current gain and breakdown voltage close to the ideal value.Furthermore,it is necessary to solve the problem of device performance degradation such as current gain???and forward saturation voltage drop?VCESAT?during long-term operation.Based on ICAC?Integrated Circuit Advanced Process Center?technology platform of the Institute of Microelectronics of Chinese Academy of Sciences,1200 V 4H-SiC BJT was designed and fabricated.In this thesis,the fabrication processes of the emitter,base mesa etching,and mesa termination etching were carefully studied.The annealing processes of emitter and base metal in device manufacturing were explored also,in which,the encountered problems were analyzed.In process aspect,the dry etching and ohmic contact were studied.Firstly,the dry etching process of SiC was studied and a single-step process of dry etching was developed.The influence of metal mask and SiO₂ mask on the surface morphology was analyzed by experiment and SiO₂ was comprehensively chosen as a mask for SiC etching.Then,the SiC was etched with chlorine-based and fluorine-based etching gas,respectively.It was found that the fluorine-based etching gas was better.Secondly,the single-step process of N/P-type SiC ohmic contact was developed.Ni and Ni/Ti/Al/Ni were selected as N-type and P-type ohmic contact metal,respectively.After constantly optimizing the metal thickness and annealing conditions by experiment,the specific contact resistances of the emitter and the base could well meet the design requirements of the device.Meanwhile,the metal lift-off techniques of BJT was developed.By adjusting spin coating speed,flood exposure time and other process conditions,the inverted trapezoidal structure was achieved.Then,based on significant efforts on the development of SiC key processes,the device layout of 4H-SiC BJTs was designed and the 1200 V 4H-SiC BJTs were fabricated.After fulfillment of the fabricated 4H-SiC BJTs,their static characteristics were tested.The forward-open voltage of the base-emitter diode was 2.8V,and the reverse leakage current was 58 p A@-4V.The maximum current gain???of a conventional BJT was 33.75@JC=90A/cm²,the forward saturation voltage drop?VCESAT?was 0.5V@JC?100A/cm²,and the specific on-resistance was 3.7m?·cm².Therefore,conventional BJT performance reached the design target.