Thermal stability of tungsten–titanium diffusion barriers for silver metallization

Tungsten–titanium thin films have been extensively applied as barrier layers for aluminum and copper metallization. The present work investigated the feasibility of tungsten–titanium barrier layers for silver metallization. Reactive sputtered W–Ti was deposited on a Si wafer followed by an Ag thin film on top. The resulting samples were annealed in vacuum at temperatures up to 700 °C. These were then characterized using X-ray diffractometry, Rutherford backscattering spectrometry, secondary ion mass spectroscopy, transmission electron microscopy, scanning electron microscopy and four point probe analysis. The analyses showed that the samples were stable up to 600 °C. Secondary ion mass spectroscopy showed that above 600 °C, agglomeration of silver film started. Si started moving into the tungsten–titanium film above 600 °C. Movement of Si resulted in local Si voiding as indicated by transmission electron microscopy. At Si/W–Ti interface, silicide formation occurred. Silver agglomerated completely at 700 °C. These results showed that W–Ti was an effective barrier layer for silver metallization for process temperatures below 600 °C.