Study On Surface Damage And Wafer Shape Changes During Mechanical Processing Of 4H-SiC Substrates

Silicon carbide(SiC)is a representative of the third generation broadband gap semiconductor materials which has unique combination of advantageous properties such as wide bandgap,high breakdown electric field,high saturated electron drift velocity,high thermal conductivity,excellent chemical stability and thermal stability.All these outstanding characteristics make it the most mature semiconductor material and ideal material for optoelectronic devices,high power and high frequency electronic devices.It can be widely used for the aerospace,radar and communication,automobile power electronic.At present,the growth technology of SiC single crystals has made significant progress.However,the control mechanism of surface morphology and the structural characteristics of mechanical damage during the processing of SiC substrates are still less reported,and relevant industries urgently need to find the mechanism by which the processing process affects the surface morphology of the substrate.This study is based on the surface stress exerted by the surface damage layer on the substrate,and compares the residual stresses on the upper and lower surfaces of the substrate using the Stoney formula stress model;By introducing mechanical damage to the substrate surface through single sided lapping,the influence process of the damaged layer on the substrate surface shape was studied;High resolution XRD was used to record the substrate plane curvature before and after the substrate surface shape change,and the relationship between the substrate macro shape change and the micro substrate plane curvature was analyzed.The main research results include the following:1、Mechanism of surface shape changes during substrate processingThe upper and lower surfaces of 4H-SiC crystals in the<0001>direction are typical polar surfaces composed of C atoms and Si atoms.The surface damage differences on the polar surfaces of SiC substrates were tested and analyzed using atomic force microscopy,high-resolution XRD diffraction,and other characterization methods.The surface shape changes of SiC substrates at different processing stages were tracked using flatness tester test results.The experimental results show that surface damage at different depths can produce differentiated surface stresses,and smaller polar surfaces in the damaged layer tend to form concave surfaces.For conventional machining processes(crystal slicing,lapping,mechanical polishing,and chemical mechanical polishing),different sizes of abrasive particles can have a significant impact on the difference in damage,especially the change in surface shape caused by mechanical polishing.2、Formation Process and Base Plane Bending Variation of SiC Substrate Surface Pattern Affected by Residual StressThe surface shape of SiC substrate undergoes dynamic changes during processing due to the influence of residual stress.Additional damage to a substrate with a smooth surface is caused by a single side lapping,resulting in a rapid change in surface shape that gradually stabilizes.By comparing the curvature of the base plane before and after a significant change in surface type,it is found that the change in surface type cannot change the shape of the curvature of the base plane,but can change the degree of curvature of the base plane.The possible reason is that the surface shape change caused by the damaged layer is an additional stress on the surface,and the internal lattice arrangement is not staggered,but only slightly bent under the action of the surface stress.3、Study on Damage Distribution of lapped SiC Substrate by Etching MethodSetting different temperature parameters determines the optimal conditions for exposing surface abrasive damage by corrosion methods.An external force was applied to the substrate to cause splitting,and the smooth substrate cross-section obtained by splitting was etched with molten KOH.The structure and depth of the damaged layer were observed using an optical microscope.The results show that the damage layer should also include areas that are under the influence of stress but not yet deformed,in addition to conventional forms of cracks and pits.By analyzing the damage thickness of the substrate surface in the radial direction,it is found that the damage degree gradually increases from the center of the substrate to the edge of the substrate,which is consistent with the frequent edge collapse phenomenon of the substrate.