Extreme Bottom-up Gold Filling of High Aspect Ratio Features

ConspectusWhere copper interconnects fabricated using superconformal electrodeposition processes have enabled dramatic advances in microelectronics over the past quarter century, gold filled gratings fabricated using superconformal Bi³⁺-mediated bottom-up filling electrodeposition processes promise to enable a new generation of X-ray imaging and microsystem technologies. Indeed, bottom-up Au-filled gratings have demonstrated excellent performance in X-ray phase contrast imaging of biological soft tissue and other low Z element samples even as studies using gratings with inferior Au fill have captured the potential for broader biomedical application. Four years ago, the Bi-stimulated bottom-up Au electrodeposition process was a scientific novelty where gold deposition was localized entirely on the bottoms of metallized trenches 3-μm-deep and 2-μm-wide, an aspect ratio of only 1.5, on centimeter scale fragments of patterned silicon wafers. Today the room-temperature processes routinely yield uniformly void-free filling of metallized trenches 60-μm-deep and 1-μm-wide, an aspect ratio 60, in gratings patterned across 100 mm Si wafers. Four distinctive characteristics of the evolution of void-free filling in the Bi³⁺-containing electrolyte are seen in experimental Au filling of fully metallized recessed features such as trenches and vias: (1) an "incubation period" of conformal deposition, (2) subsequent Bi-activated deposition localized on the bottom surface of features, (3) sustained bottom-up deposition that yields void-free filling, and (4) self-passivation of the active growth front at a distance from the feature opening defined by operating conditions. A recent model captures and explains all four features. The electrolyte solutions are simple and nontoxic, being near-neutral pH and composed of Na₃Au(SO₃)₂ + Na₂SO₃ containing micromolar concentrations of Bi³⁺ additive, the latter generally introduced through electrodissolution from the metal. The influences of additive concentration, metal ion concentration, electrolyte pH, convection, and applied potential have been examined in some depth using both electroanalytical measurements on planar rotating disk electrodes and studies of feature filling, thereby defining and elucidating relatively wide processing windows for defect-free filling. The process control for bottom-up Au filling processes is observed to be quite flexible, with online changes of potential as well as concentration and pH adjustments during the course of filling compatible with processing. Furthermore, monitoring has enabled optimization of the filling evolution, including to shorten the incubation period for accelerated filling and to fill features of ever higher aspect ratio. The results to date indicate that the demonstrated filling of trenches with an aspect ratio of 60 represents a lower bound, a value determined only by the features presently available.