(Electrodeposition Division Research Award Address) Superconformal Deposition: Where We Are and Where We Might Go
Authors: Josell Daniel , Moffat Thomas 
Published: December 2020

Superconformal deposition processes for void-free filling of features have developed remarkably over the first two decades of this century. Superconformal copper deposition, aka “superfill”, enabled void-free copper filling of features with conductive surfaces having aspect ratios unimaginable when implemented in on-chip interconnects in 1997. The only known mechanism, already more than 40 years old, provided only more modest superconformal feature filling through a gradient of deposition rate that scaled with a gradient of suppressor concentration arising from incorporation in the depositing metal. A new mechanism, for the copper “superfill”, was determined in 2001: local coverage of a strongly bound but surfactant accelerator arose through area change that accompanied deposition on nonplanar surfaces. Copper “superfill” through this Curvature Enhanced Accelerator Coverage, or CEAC, mechanism accompanied and accounted for the decrease of feature size in on-chip interconnects over the next ten years. Along the way the CEAC mechanism was shown to be generic to the coinage metals of silver and gold. In all cases, feature filling was shown to be predicted a priori by CEAC-based models using only kinetics derived from studies on planar substrates.

In the meantime, the microelectronics industry began to pursue wafer stacking that hinged upon void-free copper filling of much larger Through Silicon Vias, TSV, for which the CEAC mechanism was poorly suited. Demonstrations of bottom-up copper filling incompatible in both geometry and additive-chemistry with the CEAC mechanism required a new mechanistic understanding. In 2012 the old, linear model for suppressor containing electrolytes was replaced by a model reflecting suppressors whose deactivation was so non-linear in potential and concentration that suppression breakdown yielded increasing negative current with increasing positive potential during cyclic voltammetry. Models based on this S-shaped negative differential resistance (S-NDR) predicted the extreme bottom-up nature of the copper filling. Subsequent years brought demonstrations of generality with gold, nickel, cobalt, zinc and copper, albeit manifesting in two very different filling geometries: activation of only the bottom-up surface and activation of the bottom surface with a portion of the adjacent sidewalls. Filling of features having much larger dimensions as well as geometries such as through holes was also demonstrated. In all cases, the filling evolution was shown to be a-priori predictable by S-NDR based models using only kinetics derived from studies on planar substrates.

Today we find ourselves with the next superconformal filling process requiring new mechanistic understanding. Gold filling exhibits the bottom-up geometry of suppressor-derived S-NDR filling but with a bismuth additive that accelerates as in CEAC filling. It is thereby explained by neither. Void-free, bottom-up filling has been achieved in trenches whose depths range from a few micrometers to a quarter of a millimeter. Trenches with aspect ratios as low as unity and as high as fifty-six (!) have been successfully bottom-up filled, and surface profiles indicate the limit has not been reached. Adsorption and deposition exhibit a unique nonlinear dependence on transport, and bottom-up filling halts automatically at a depth within patterned features defined by potential that, equivalently, yields a tell-tale potential signature upon filling under galvanodynamic control. Uniformity of filling is excellent across patterned substrates. The new process satisfies a need outside the area of microelectronics: gold filling of trenches in x-ray diffraction gratings. As in the past, understanding will enable superconformal electrodeposition to meet the evolving needs of a new technology.

Figure 1