Electroplating Fundamentals –
Optimizing Cross-wafer Uniformity
Process Application Note #101 –
Download PDF of the full paper
Summary Overview
Two primary factors influencing uniformity
Fundamentally, the uniform deposition of metal within and across a semiconductor wafer is influenced by two primary factors: the definition and control of the electric field profile, and the establishment and maintenance of cation availability.
Current density drives the rate of electrochemical deposition, and the cumulative effect of electric fields allows a well-designed plating reactor to produce exceptional uniformity. Today, an advanced plating reactor can provide uniform distribution of the electric field across the wafer, producing uniform potential and a uniformly distributed current across the wafer. In reactor design, enabling the wafer to spin is seen to be a critically important feature for achieving plating uniformity.
Cation density and availability
For deposition uniformity, two of the most important considerations are cation concentration in the bulk and cation availability at the diffusion layer. Quality electroplating of metals requires that the system operate in an electron-poor condition, such that the reaction rate is limited by the rate of the flux of electrons, i.e., by the current. Mass transfer is the motion of aqueous metal cations to the wafer surface, and several factors influence its rate. Good mixing in the electrolyte bulk is essential, but it is not sufficient to accurately control cation availability because of diffusion differences across the wafer.
As a viscous fluid approaches the wafer surface, its velocity decreases asymptotically; and the layer of fluid slowed by this effect is called the boundary layer. Cations must cross from the region of homogenous bulk concentration, through the stagnant diffusion layer, to the surface of the wafer. The time required for cations to traverse the diffusion layer is significant in achieving uniform deposition. The velocity of the electrolyte must be high enough to produce a sufficiently thin diffusion layer so that cations can diffuse to the surface quickly enough not to impede deposition. The plating reactor should provide a uniform fluid motion profile across the wafer; and this involves two factors: the motion of the fluid as caused by the fluid delivery system, and the motion of the wafer with respect to the fluid in the reactor.
Ohm’s Law
Functioning as an electrolytic cell, the plating reactor is subject to Ohm’s Law, but with the added complexity of unintended electrochemical reaction. As current density is increased, cation consumption rate increases. At some point, if cations are not abundantly available, electrons may initiate side reactions that result in poor-quality deposits, and this problem must be mitigated.
Step-by-step guidance
The final section of this paper provides specific, step-by-step guidance for dialing in cross-wafer (or within-wafer) plating uniformity – on ClassOne plating systems and also on non-ClassOne systems. Note that this paper covers uniform plating on blanket seed wafers. The third paper of this Electroplating Fundamentals series covers the plating of specific feature types.
