Aspect-Ratio-Dependent Copper Electrodeposition Technique for Very High Aspect-Ratio Through-Hole Plating

Copper electrodeposition in high-aspect-ratio through-holes micromachined by deep reactive ion etching is one of the most essential processes for fabricating through-wafer interconnects, which will be used in developing future generation high-speed, compact 3D microelectronic devices. Although copper electrodeposition is a well-established process, completely void-free electroplating in very deep and narrow through-holes remains a challenge, where local current distribution does not remain uniform, resulting in void formation in the via. In this paper, we report the fabrication of very high aspect ratio through-wafer copper interconnects by an innovative copper electroplating technique. Completely void-free electroplating in very deep and narrow through-holes was accomplished by a proposed "aspect-ratio-dependent electroplating technique." In this technique, electroplating parameters were continuously varied along with changing unfilled via depth. Continuously varying current density improves the local distribution of current as per the changing depth and helps in minimizing void formation. The hydrophilic nature of the via surface was also enhanced by wet surface treatment to improve the interaction between the copper electrolyte and via surface. Very fine pitch , through-wafer copper interconnects having an aspect ratio as high as 15 were fabricated by the above innovative technique.