Burn-in Failure Analysis of 0.5μm 1MB SRAM: Barrier Glue Layer Cracks and Tungsten Plug “Worm Holes”

A new 0.5 um 1 Megabit SRAM which employed a double metal, triple poly CMOS process with Tungsten plug metal to poly /silicon contacts was introduced. During burn-in of this product, high currents, apparently due to electrical overstress, were experienced. Electrical analysis showed abnormal supply current characteristics at high voltages. Failure analysis identified the sites of the high currents of the bum-in rejects and discovered cracks in the glue layer prior to Tungsten deposition as the root cause of the failure. The glue layer cracks allowed a reaction with the poly/silicon, causing opens at the bottom of contacts. These floating nodes caused high currents and often latch-up during burn-in. Designed experiments in the wafer fab identified an improved glue layer process, which has been implemented. The new process shows improvement in burn in performance as well as outgoing product quality.