Improvement of Back-Side Cosmetic Defects And Wafer Strength

Abstract

The continued challenge to keep up with Moore's law with aggressive device scaling, and shrinking wiring dimensions established perpetual need for novel materials and dictates ever tighter semiconductor process fidelity. Despite large progress with reducing defect densities on the device side of Si wafers, considerably less attention is being paid to the wafer back-side. Back-side wafer defects have been shown to reduce yield by leading to die breakage during packaging processes. Scratches and voids formed on the back-side of the wafer during various manufacturing processes have been shown to create weak spots on the wafer, which can act as initiation points for die crack formation and propagation. In this paper, we present the technical details of a back-side wet etch clean process which helps reducing back-side defect densities significantly. The process is set up on a single wafer wet etch tool at IBM's East Fishkill 300 mm wafer fabrication facility. The process removes the outer back-side layer of the silicon wafer which has become defective and damaged as a result of previous processing steps. This new defect removal process increases die strength, which is measured as wafer mechanical strength, and removes surface defects from the back-side of the wafer. Our results show that removal of 15–30 microns from the wafer back-side reduces the amount of cosmetic defects on the back-side of the wafer by 90%, while increasing die strength by 60%. The effects of the back-side wet etch clean process on the wafer optical appearance and mechanical properties were characterized, and supporting data such as atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), defect inspection, thermal interface material adhesion test, and die strength is included. Results from package level modules indicate an increase in reliability compared to the uncleaned Si back-side. It can be concluded that wet Si removal process described in this paper is a viable method to reduce the back-side defect density with an associated increase in final module product yield.