Linear sweep voltammetry and chronoamperometry methods were used to measure limiting current in an industrial wafer-plating cell. Copper deposition in a dilute solution, under mass-transfer-limited conditions, is used to study the variation of the mass transfer boundary-layer thickness. It is shown that a shear-plate fluid agitation mechanism is capable of generating a thin ͑i.e., Ͻ10 m͒, spatially uniform and nonperiodic boundary layer across the entire wafer. It is anticipated that thin boundary layer deposition will prove to be beneficial in MEMS, flip-chip bumping, and WL-CSP applications.A thin, uniform, and stationary diffusion boundary layer is important in obtaining high-quality electrodeposits. Limiting current depends on thickness of the diffusion boundary layer for a given reactant concentration. Plating current is then usually set below onehalf of limiting current to obtain plated deposits acceptable for their appearance and uniformity. In cases where operating current is set to greater than one-half of limiting current, the effects of mass transfer typically impact the morphology of the deposit, and rough growth is often observed. Moreover, control of limiting current density and diffusion boundary layer thickness becomes even more important when the subject is the plating of metal alloys. In alloy deposition, cell potential is set such that one metal is deposited at its diffusion limiting current. Thus, uniformity of the diffusion boundary layer significantly influences composition of the final plated alloy, and its material properties.The reciprocating paddle cell is a known practical method for depositing alloy films on wafer substrates. It has been shown that the diffusion boundary layer's thickness profile, in a paddle-arm cell, may be periodic in nature due to the wake trailing the paddle. 1 It has also been shown that paddle distance from the cathode has a greater impact on deposition uniformity than does paddle speed. 2 The primary electrochemical technique for measurement of the mass transfer boundary-layer thickness is determination of limiting current. At limiting current, concentration of reacting species is virtually zero over the entire electrode surface. Limiting current measurements are usually carried out in a traditional three-electrode cell, using a potentiostat. Techniques for determining limiting current and mass-transfer coefficient correlation, for a variety of different electrode geometries and fluid flow conditions in laboratory experimental cells, were reviewed by Selman and Tobias. 3 However, only limited methods have been available to characterize the mass transfer boundary layer in industrial wafer plating cells, due to their complicated cell geometries and limited accesses for probes. It is difficult to install a standard reference electrode in an industrial wafer plating cell. Moreover, a potentiostat is not usually available in a typical wafer fabrication facility. These problems are barriers to transferring experimental results obtained from bench-or beaker-scale test...

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