Study On The Gate-oxide Interface Characteristics Of 4H-SiC MOS Devices

Silicon carbide（SiC）,as an excellent third-generation semiconductor material,has many excellent electrical and physical properties:wide band gap,high breakdown field strength,high thermal conductivity,high carrier saturation drift rate,and small relative permittivity.Therefore,silicon carbide materials are widely used in high-temperature,high-frequency and high-power devices,and have broad application prospects in defense technology and the national economy.SiC MOSFET devices have the advantages of large current density,small size,fast switching speed,high reverse voltage and more.And they are widely used in the field of power electronics.Similar to the Si MOS device process,the gate dielectric Si O₂（silicon dioxide）of the SiC MOSFET device can also be prepared by a thermal oxidation process,so that a high-quality gate dielectric oxide can be obtained.However,due to the excessively high interface state density at the SiC/Si O₂interface,the activation energy required for the breakdown of the oxide layer is reduced,resulting in a decrease in the ability of the gate oxide dielectric to withstand electrical stress and a decrease in the mobility of the SiC channel.MOS capacitor is the basic structure of MOSFET device,and it is also the simplest MOS device.They all contain the SiC/Si O₂ interface structure and gate oxide layer required in this experiment.Therefore,4H SiC MOS capacitor is selected as the experimental sample in this paper.The research contents and results are as follows:1.The influence of high temperature oxidation process of 4H SiC MOS capacitor on the SiC/Si O₂ interface characteristics was studied by experiment and C-V test.First,the4H-SiC MOS capacitor experimental samples were prepared at 1250,1300 and 1350degrees respectively under dry-oxygen oxidation process,and the NO annealing temperatures were 1300 degrees.Secondly,the C-V method was used to analyze.The flat band voltages of the three groups of SiC MOS capacitor samples were 0.684V,1.614V and0.910V respectively;the interface state densities of the three groups of SiC MOS capacitors were 6.24×12cm^-2e V^-1,6.16e×12cm^-2e V^-1 and 5.99×12cm^-2e V^-1.The experimental results show that the flat band voltage of the sample is positively related to the thickness of the oxide layer,but not to the dry oxygen oxidation temperature.With the dry oxygen oxidation temperature rising from 1250℃to 1350℃,the density of SiC/Si O₂interface state gradually decreases,which is consistent with the domestic research report.2.The effects of high temperature NO annealing on the SiC/Si O₂ interface and gate oxygen reliability of 4H SiC MOS capacitor were studied by annealing experiments and C-V and TDDB test methods.Firstly,four groups of SiC MOS capacitor samples were prepared.Sample A was not annealed.Samples B,C and D were annealed at 1300℃,1250℃and 1200℃,respectively.Secondly,the C-V method was used to analyze the experimental samples:the flat band voltage of four groups of SiC MOS capacitors was4.362V,1.267V,2.153V and 2.701V respectively;near the conduction band,the interface state densities of four groups of samples were 9.91×12cm^-2e V^-1,6.39×12cm^-2e V^-1,7.62×12cm^-2e V^-1 and 8.18×12cm^-2e V^-1 respectively.Finally,the breakdown characteristics of gate oxide are analyzed by TDDB test:the breakdown field strength of four groups of SiC MOS samples were 10.01 MV/cm,9.58 MV/cm,9.70 MV/cm,9.60 MV/cm,respectively;the lifetimes were 439 s,329 s,208s,67s.The results show that the density of interface states can be effectively reduced by high temperature NO annealing.The interface density of NO annealed samples is 36%,23%and 17%lower than that of non annealed samples at 1300℃,1250℃and 1200℃,respectively.With the decrease of annealing temperature,although the lifetime of gate oxide layer decreases gradually,the reliability and homogeneity of samples are improved according to the slope of Weibull distribution.3.The influence of ECR microwave N-plasma annealing on the interface state of 4H SiC MOS capacitor was studied by experiments and C-V measurement.Firstly,three groups of SiC MOS capacitor samples（A,B and C）were prepared.The process parameters were the annealing time of ECR microwave N-plasma,corresponding to 5min,10min and 15min,respectively.The annealing temperature was 600℃.Microwave power is 650W,cavity vacuum degree is 10^-4Pa,N₂ flow rate is 70sccm.Secondly,the C-V method was used to analyze the experimental samples.After 5min,10min and 15min,the interface state densities near the conduction band were 5.66×12cm^-2e V^-1,4.30×12cm^-2e V^-1 and6.12×12cm^-2e V^-1,respectively.The experimental results show that the interface state density(D_it)of samples A and b decreases significantly after annealing in N-plasma for 5and 10 minutes.However,after annealing for 15 minutes,the interface state density of sample C increases,which may lead to new interface defects.N-plasma can obviously improve the interface quality of SiC/Si O₂,and the introduced N is evenly distributed in the whole oxide layer and the interface.It can react with the interface defects to reduce the interface state density.And at the same time,it can transform some carbon clusters into deeper level Si-N and C-N bonds.This is the reason for the decrease of the interface state density in the range of 0.2e V<E_C-E<0.6e V.4.The effect of high temperature oxidation process of 4H SiC MOS capacitor on hall mobility of carriers near the interface was studied by experiments and hall measurement.A feasible method for measuring the hall mobility of carriers near the interface was proposed.First of all,three groups of 4H SiC MOS capacitor samples were prepared by dry-oxygen oxidation at 1250℃,1300℃and 1350℃.Secondly,the preparation of PCB board was completed,which was used to connect the hall tester and the sample to be tested.The magnetic field of hall test system was set as 0.3T,and the external grid voltage was 20V.The electron Hall mobility of the samples treated by dry-oxygen oxidation at 1250℃,1300℃and 1350℃were 17.11cm²/（V·s）,23.54cm²/（V·s）and 25.01cm²/（V·s）.The experimental results show that with the increase of dry-oxygen oxidation temperature,the hall mobility of electrons near the interface of SiC MOS samples increases gradually.