Laser Dicing and Subsequent Die Strength Enhancement Technologies for Ultra-thin Wafer

Abstract

Current mechanical wafer dicing process adopting diamond grit shows advantages of low cost and high productivity. However, mechanical process for ultra-thin wafers would induce residual stress or mechanical damage, which can lead to wafer broken and die cracking. With the development of laser technology, laser precision micromachining has been employed for thin semiconductor wafer singulation, which shows advantages of no chipping, small kerf width, and high throughput over mechanical blade dicing. However, thermal damage to the chip induced by laser ablation results in die strength degradation. For ultra thin chip, low die strength tends to induce die crack in packaging process. Thus, thermal damage to the chip needs to be studied. In this study, first we made a comparison between mechanical blade sawing and laser ablation processes. Die strength and microstructure changes were studied by means of bending test and transmission electron microscope (TEM) analysis, respectively. Die strength results showed that the die strength obtained by laser dicing was far lower than that obtained by blade sawing. TEM analysis demonstrated that formation of microcracks and porosities in laser diced face, caused the die strength degradation. In addition, significant deviation between frontside and backside die strength was found in the laser micromachinned dies. The reason for this deviation was clarified as the defects density difference existing in top and bottom layer of the chip sidewalk Experiments results showed that the die strength obtained by laser dicing can not meet the demand of the packaging process. It tends to crack or fracture in the die attach or wire bonding process. Thus, it is essential to improve the die strength. Thus, in this investigation, etching processes including wet-etch and dry-etch were attempted to recover the die strength by removing the chip side wall damage. SEM and TEM images indicated that, before etching, the laser diced side walls were with rough surfaces, voids and microcracks. After etching, the surfaces got smooth and most of the voids and microcracks were removed. Chip strength measurement also verified the partial die strength recovery after etching process.