Finite Element Analysis On Wafer Micro-crack Damage Layer Depth And Warp In Wire Saw Slicing SiC Crystal

SiC single crystal is the third generation of semiconductor materials after the Si and GaAs semiconductor materials, with a wide bandgap, high heat conductivity, high saturated electron drift velocity, high breakdown electric field strength and stable chemical properties. However single-crystal SiC with high hardness, high brittleness etc, makes the process of wafer slicing is, low efficiency, poor surface quality and the wafer is easy breakage. The micro-crack damage layer depth and warp of wafer is the important indicators of evaluating the wafer slicing quality, can directly affect the workloads of next grinding and polishing, the mechanical strength and qualified rate of the wafer. The warp of the wafer is body defect once formed is difficult to improve in the next working procedures. The wafer slicing process is the key to control the micro-crack damage and warp of wafer. Based on finite element method, sawing stress field, the micro-crack damage layer depth and warp of the SiC wafer during the electroplated diamond wire saw slicing process is researched in this paper,and the main work is as follows:To consider the brittle fracture removal mechanism of the process of slicing SiC single crystal, the brittle cracking constitutive model introduced in this paper, the finite element analysis model of SiC single crystal by wire saw slicing and the wafer micro-crack damage layer depth analysis model was founded. Selecting the Si crystal with the similar machining mechanism and material characteristics of SiC object single crystal as sawing objects to demonstrated the analysis model, the relations between cutting parameters and the micro-crack depth of wafer has been further researched. Because of the micro-crack damage layer depth analysis model ignored the effects of defects,wiresaw vibration and cutting heat,so the simulation values were less than experimental results, The simulation values and experimental results have the same change tendency, meanwhile the relative errors in 20%～30% at different cutting parameters, so the analysis model can be used for analyzing and calculating the micro-crack damage layer depth rapidly. The micro-crack damage layer depth of wafer decrease along with the decrease of feed speed and the increase of wire saw speed, with respect to wire saw speed the feed speed has obvious influence on the wafer micro-crack damage layer depth.According to several types of defects existed in the SiC single crystal by the PVT growth, the SiC single crystal with hole defect cut by wire saw has been numerical simulated. Combining the stress concentration related theory, the stress field during the SiC single crystal with different position and size hole defects wire sawing has been discussed in this paper.Base on the analysis of the stress concentration of the defects, the wafer breakage probabilities has been further discussed. The results show that the hole defect will cause local stress concentration, so the micro-crack initiation and propagation will increase,the hole defects with different position and size have a significant difference effects on the stress concentration level, the most serious stress concentration and the biggest wafer breakage probabilities when the hole defect very close to wafer inner surface, while the defects size change had little influence on the stress field when the hole defect is lacated on the center of sawing removal layer so the wafer breakage probabilities dropped to the lowest, it is a good cutting position.Based on the thermoelastic theory,the thermal analysis model of wire saw slicing on SiC single crystal have been constructed, the change rule and internal relations of wafer analyzed and explored between the temperature gradient, the thermal stress and the node thermal deformation displacement field during wire saw slicing. According to the node displacement of thermal deformation on the wafer surface, the analysis and calculation model for wafer warp founded, analyzed the relationship between wafer thickness,cutting parameters(wire saw speed,feed speed,wire saw diameter and wire saw tension)and warpage. The results show that the highest temperature lacated on the kerf of the crystal, both the temperature gradient,the thermal stress field and the the thermal deformation displacement have a similar increased rule, the thinner the wafer the smaller the wafer stiffness made the less warpage, the wafers warp values decrease along with the increase of wire saw diameter and tension and the decrease of wire saws and feed speed, with respect to wire saw speed the feed speed decreased has obvious improvement on the wafer warpage.