Study On Surface Damage And Process Of Ohmic Contact Fabricated By Excimer Laser Irradiation Of 4H-SiC

Silicon carbide(SiC)occupies a pivotal position in semiconductor materials for electronic devices due to its superior physical properties and ultra-high chemical stability.In the process of manufacturing silicon carbide power devices based on silicon surfaces,the preparation of reliable ohmic contacts has always been a research hotspot in SiC-based electronic devices.Traditionally,the preparation of ohmic contact doped layers for silicon carbide devices mainly includes epitaxial growth and Ion Implantation.In recent decades,due to the advantages of excimer lasers with high-energy optical density output,accurate and easy-to-control parameters,selectivity and reproducibility of surface treatment in specific areas,laser irradiation methods have been developed to prepare doped layers to prepare ohmic contacts.Although the doped layer can be obtained,it is easy to cause the appearance of the periodic structure of the substrate.Therefore,this paper studied the reasons for the periodic structure of SiC irradiated by excimer laser,and based on this theory,designed a mask irradiation process based on Si N_xfilm.In this paper,the experimental optical path system of 4H-SiC surface irradiated by KrF excimer laser was established and the experiments were carried out.After excimer laser irradiation,periodic structural damage appears on 4H-SiC surface.In order to analyze the damage of periodic structures,the influences of different energy densities on the spacing,roughness and phase changes of periodic structures were investigated in this paper.The results show that the roughness reaches 135 nm at the minimum energy density of 200 m J.With the increase of energy density,periodic structure spacing increases and roughness increases.In addition,the periodic structure of 4H-SiC surface changes to monocrystalline silicon and graphite carbon.With the increase of laser energy density,the type of object image does not change.In order to further analyze the periodic structural damage of 4H-SiC surface,one-dimensional heat conduction equation was used to simulate the surface temperature of 4H-SiC irradiated by quasi-molecular laser,and the conclusions were drawn as follows:Under different laser energy densities,the surface temperature of 4H-SiC is higher than the peritectic temperature of 4H-SiC(3100 K),the surface of 4H-SiC is transformed into monocrystalline silicon and carbon at this temperature.In order to explore the mechanism of periodic structural damage,this paper calculates the spacing of periodic structures based on the theory of tension wave,and the calculated results are consistent with this experiment.Meanwhile,the reason why the spacing and roughness increase with the increase of energy density is explained by the theory of tension wave.In order to solve the problem of periodic structure roughness,combined with the above experimental analysis,the excimer laser irradiation process using Si N_xfilm as mask was studied.The process flow of mask doping was designed:RCA cleaning,Si N_xfilm preparation,laser irradiation,hot phosphoric acid etching,photolithography,gold-plating degelling and testing.The preparation of Si N_xfilm is particularly important for the preparation of ohm contact doping layer.Therefore,the optimum conditions for the preparation of Si N_xfilm at 75 W power(roughness 1.705 nm,transmittance 63.5%,Si/N ratio 0.45 or so nitrogen-rich structure)were determined through the surface roughness and transmittance test.Through the calculation of one-dimensional heat conduction equation and experimental characterization,it is concluded that 200 nm Si N_xis the best thickness of the mask,and the surface roughness of 4H-SiC is reduced to 4.1 nm,which meets the technical requirements of ohm contact preparation.On this basis,the mask technology experiment using silicon nitride as masking film was carried out.After the experiment using excimer laser to irradiate silicon surface and carbon surface of epitaxial wafer,no ohmic contact was formed on silicon surface,but ohmic contact was formed on carbon surface.By comparing the I-V curves of"L+200"and"L+0"and specific contact resistivity,it is found that"L+200"sample can obtain lower specific contact resistivity ohmic contact than"L+0"sample.In combination with the difference of carrier concentration and electrical properties calculated by Raman fitting,it is concluded that the carrier concentration on silicon surface increases from 9.9×10¹⁴cm^-3to 7.9×10¹⁶cm^-3after excimer laser mask irradiation,which is still at a low concentration and cannot form ohm contact.In contrast,the carrier concentration of SiC carbon surface is 10¹⁸cm^-3.After laser mask irradiation,or high temperature rapid annealing,ohmic contact can be formed.