Analysis Of Thermal Damage On Laser Stealth Dicing Of SiC Wafers

SiC is a very promising next-generation semiconductor material.Due to its excellent properties such as large bandgap,high thermal conductivity and high chemical stability,SiC is expected to replace Si for the manufacturing of high-power,high-temperature and high-radiation power devices.Wafer cutting is an important process linked to the manufacturing process of SiC power devices.Laser stealth dicing is widely used in the field of semiconductor wafer dicing due to its advantages of high efficiency,non-contact process and small kerf width.However,thermal damage is inevitably induced in laser stealth dicing,due to the thermal input of the laser.The cutting-induced damage directly affects the fracture strength of the chip and the mechanical-thermal reliability of the chip packaging.Therefore,the low-damage dicing of the wafer is very important for the application of SiC power devices.The main contents of this paper are the thermal damage characterization,the damage mechanism,the influence of cutting process parameters on the thermal damage,and the process regulation of thermal damage on laser stealth dicing of SiC wafers.After laser stealth dicing of SiC wafers,the sidewall and the kerf edge were formed,and the laser-induced thermal damage was distributed on these two surfaces.By observing the surface morphology and color of the sidewall and the kerf edge,analyzing the element composition,element valence,molecular structure,crystal structure and chemical composition on the cutting sidewall,analyzing the element composition and element distribution on the kerf edge,the thermal damage behavior and damage mechanism on the sidewall and the kerf edge was revealed.After laser stealth dicing,a chemically-modified layer(CMP)and a physically modified layer(PML)were formed on the sidewall,and thermal damage only occurs on the CMP,which is manifested as the thermal decomposition of SiC to generate black graphite crystals.No thermal damage was found on the PML.At the atomic scale,the thermal decomposition of SiC was the breaking of Si-C chemical bonds,the sublimation and escape of Si atoms and the combination of C atoms to form a graphitic C layer.Thermal damage on the kerf edge was manifested as thermal cracking,chipping and oxidation caused by heat accumulation.When the heat accumulation was small,the thermal damage on the kerf edge was manifested as thermal cracking and chipping.With the increase of heat accumulation,the temperature of the surface outside the chipping area on the kerf edge rose,accelerating the chemical reaction rate between the SiC material and the oxygen in air.The oxidation of SiC material caused color changes of the surface.The thermal damages during laser stealth dicing of SiC wafers can be classified into chemically thermal damage and physically thermal damage according to their damage mechanism.The thermal decomposition and oxidation of SiC were chemically thermal damage due to the chemical changes in the crystal structure and chemical composition;thermal cracking and chipping did not generate new material components,so the damage process is a physically change,which belongs to physically thermal damage.Laser power and scanning velocity affected the heat accumulation from laser scans.With the increase of the laser power and the decrease of the scanning velocity,the heat accumulation increases,thermal cracks and chipping clustered along the kerf edge until the entire kerf edge was chipped.When the heat accumulation was low,the thermal damage on kerf edge was thermal cracking and edge chipping.With the increase of heat accumulation,the chipping width first increased,and then kept the maximum chipping width unchanged,and the oxidation width of SiC kerf edge increased with the increase of heat accumulation.The thermal damage width of the kerf edge can be used to quantify the thermal damage degree.With the increase of the laser power and the decrease of the scanning velocity,the thermal damage width on the kerf edge increases and the thermal damage degree increases.The regulation of thermal damage can be achieved by adjusting the stealth dicing process parameters.During laser stealth dicing,the energy loss of laser transmission and spherical aberration effect would reduce the laser power density and affect the laser modification process.Based on thermal damage mechanism and the theoretical calculation of the variation of laser power density with the depth of focus,a lowdamage laser stealth dicing process scheme is proposed,which is to ensure the full formation of PML,and to decrease the laser power with the increasing number of laser modified layers.This process scheme effectively suppresses the generation of thermal damage while ensuring the laser modification.A low-damage laser stealth dicing experiment was carried out,and SiC wafer were diced without thermal damage,which verified the effectiveness of process regulation of thermal damage.