A polishing tool and a potentiostat were used to simultaneously polish and measure the direct current (dc) open-circuit potential and anodic polarization behavior of chemical vapor deposited tungsten films in the presence of various oxidants. Of the different oxidants tested at pH 1.5 or pH 4.4, (NH,),MoO,, 2 4 formed the most protective passive layer on tungsten. Even in the presence of the most aggressive oxidant, Fe(NO 3 ) 3 , the dissolution rates of chemical vapor deposited tungsten were approximately 3 nm/min during abrasion, which is a very small fraction of typical removal rates reported for chemical mechanical polishing of tungsten. This indicates that electrochemical oxidation followed by abrasive removal of the oxidation product and dissolution may not be the primary mechanism for tungsten removal. Atomic force microscopy scans of polished tungsten films indicate that corrosion assisted fracture may be an important removal mechanism for tungsten during chemical mechanical polishing.

No abstract

No abstract

Multi-strata bonding and inter-strata connectivity are the two key processes to enable 3D chip stacking. Die-to-wafer bonding can be achieved through thermo-compression bonding of micropads composed of Cu and Sn. This paper presents a method of formation and preliminary characterization of micropads consisting of a stack of Sn (top)/Cu/CoWB/Cu. CoWB, which serves as a diffusion barrier for Sn, is formed selectively on copper by electroless plating and Sn is deposited on Cu by immersion plating. The plating rate of Sn was found to strongly depend on temperature and the plating rate decreases with increasing Sn thickness. A plating condition was used which resulted in a Sn rich surface of the micropad which is desired for robust inter-strata bonding. Annealing of the film at 400 ºC resulted in complete conversion of the Cu and Sn film to Cu-Sn intermetallic compounds, which is required for using this metallurgy for bonding applications.