Transport phenomena that control electroplated copper filling of submicron vias and trenches

Scaling analysis and finite element modeling of the governing transport equations are used to show that in the submicron features of damascene ultralarge scale integrated interconnect structures, diffusion of cupric ion and trace additives is the only transport process that affects plating uniformity. Potential variations in solution or in the metal film, while important on wafer (dm) length scales, are completely negligible on feature (mu m) length scales. Convection is also unimportant in submicron features. As a result, changes in process parameters such as fluid flow rate, barrier film conductivity or thickness, or solution conductivity are unlikely to result in significant changes in plating uniformity or void formation inside trenches and vias. On the other hand, changes in plating current density, feature geometry, or additive concentration in the plating bath will have a large impact on filling profiles. The impact of diffusive limitations on feature filling is described by a dimensionless parameter xi(D). Low values of xi(D), indicate that for typical damascene structures and plating conditions, cupric ion depletion is only on the order of a few percent, indicating that copper plating is essentially conformal. Only very deep or high-aspect features will pinch off as a result of cupric ion depletion. For additives that are depleted at the metal surface, values of xi(D) can be large, indicating the potential for substantial additive depletion inside trenches and vias. xi(D) scales with L-2/W. As a result, deeper features (larger L) plate less uniformly, as do more narrow features (smaller w). However, if aspect ratios (L/w) are held constant as critical dimensions decrease, beneficial effects of additive depletion will diminish in future device generations. (C) 1999 The Electrochemical Society. S0013-4651(99)02-092-3. All rights reserved.