The objective of the proposed research is to address two major challenges that 3D Heterogeneous Integration (3D HI) architectures face: (1) difficulties in the scaling down of interconnect technologies and (2) the impact of 3D HI on the power delivery network (PDN). The scaling down of interconnect technologies is critical in the realization of high-performance systems, but bonding processes are becoming increasingly more expensive and difficult with pitch scaling. In this work, a 3D integration technology that enables ultra-dense input/output (I/O) bonding to approach on-chip via densities using cobalt selective atomic layer deposition (Co ALD) is demonstrated. A thermal study is presented to investigate the benefits of the Co ALD bonding technology relative to current state of the art u-bumps and hybrid bonding. Following the thermal analysis, experimental results of Cu/Gap/Cu lateral test structures with gap sizes ranging from 50 nm– 200 nm and pitches ranging from 1 um – 10  um will be presented. The results show the feasibility of using selective Co ALD for high-density Cu-Cu interconnect bonding. SEM and XPS are used to characterize the testbed before and after Co deposition and to measure the selectivity of Co ALD on the Cu and Si3N4 substrate. Compared to other thermocompression bonding technologies such as Cu-Cu direct bonding or hybrid bonding, the Co ALD chip-to-wafer bonding process deposits an intermediate layer between the Cu pads and therefore, does not require metal diffusion to create the bond. This process also does not require an external mechanical force, extreme chemical mechanical polishing, and surface cleanliness. In addition, the Co ALD process has exceptional film thickness controllability at the angstrom range with I/O pads that have various diameters and pitches. Future work will focus on demonstrating this bonding technology with two stacked dies to form a realistic, 3D vertical gap test bed. The power delivery network of 3D architectures will also be evaluated and technologies such as backside-power delivery networks (BS-PDN) will be explored.