A hybrid method of ultrafast laser dicing and high density plasma etching with water soluble mask for thin silicon wafer cutting

Abstract

Future silicon wafers are getting thinner in semiconductor industry to satisfy the requirement of miniaturizing package size in mobile device applications. Development of thin wafers is the mutual goal of current semiconductor researches. In this paper, a novel method incorporating the convenience of laser ablation and speed of plasma etching is proposed to replace traditional mechanical dicing saw for thin wafer cutting. The main idea is to use laser to create patterns on the silicon wafer coated with water soluble protection material and the unprotected area is exposed to the high-density plasma which is later etched through. This study includes development of the water-soluble protecting mask material, design of inductively coupled plasma (ICP) and microwave plasma sources, pattern scribing utilizing ultrafast laser, and trial of an etching (Bosch) process associated with the high-density plasma chamber to create high aspect ratio trench. The polyvinyl alcohol (PVA) based protecting material has excellent solubility and is extremely easy to be rinsed by water. With appropriately added chemicals, optical absorption is improved for laser ablation. A 532nm 10 pico-second laser is used to perform surface scribing with minimal recast and heat affected zone (HAZ) at the edges. Good plasma homogeneity is demonstrated in the designed 12″ microwave and ICP chambers. With the protection coating the silicon wafer can be etched at 7.2µm/min rate and 10.11 etch selectivity by the microwave and ICP sources in the chamber. Finally, etching process trials are performed, and the results show that deep and high aspect ratio trenches (~15µm wide, ~95µm deep) can be achieved which is fully acceptable for wafers under 100µm thickness.