Burn-in problem on 0.5 μm 1 MB SRAM-the role of barrier glue layer on the W plug

During the introduction phase of the new 0.5 μm 1 MB SRAM, a severe problem was encountered during the burn-in process of this product. This product would draw high current which caused severe package blistering and even a melted socket. Failure analysis performed on the rejects discovered that cracks on the barrier layer prior to tungsten deposition are the root cause of the failure. In some rejects, abnormal contacts were observed underneath the cracks. The findings led us to believe that during tungsten deposition, chemical reaction takes place between WF₆ and exposed silicon underneath the glue crack. The abnormal contacts will in turn cause floating nodes in the circuitry that draw high current and eventually result in latch-up. A design of experiment on the glue process has been conducted in the wafer fab and an improved glue process has been implemented. The new process shows improvement in burn-in performance as well as product out-going quality