Development of High-Temperature-Resistant Seed Layer for Electrodeposition of Copper for Microelectronic Applications

Copper is the most commonly used material for interconnects within microelectronics. However, electromigration and high resistance within the material limit the use of copper in nanoscale applications. New, two-dimensional (2D) materials such as graphene, carbon nanotubes (CNTs), and transition-metal dichalcogenide (TMDs) are being developed for the next generation of interconnects. A composite consisting of a forest of vertically aligned carbon nanotubes within a matrix of copper is a proposed solution to the issues which present themselves in nanoscale copper interconnects. The fabrication of CNTs and graphene often requires high temperatures that exceed the material limitations of copper in microelectronic processing. If these 2D materials are used in conjunction with copper, it is desirable to delay copper deposition to late stages of microfabrication in order to avoid exposing copper to high temperatures. It is proposed that a seed layer be deposited before deposition/growth of CNTs/graphene or other TMD materials, such that later deposition of copper via electroplating is feasible. A conductive seed layer must be introduced underneath the forest of CNTs in order to allow for electroplating to take place. Copper itself may not be used as a seed layer since it migrates at elevated temperatures, which prevents growth of carbon nanotubes. Thus, the seed layer must be able to withstand high temperatures, must not migrate or diffuse into silicon or barrier layers, and must also be a suitable seed layer for copper electroplating. This research focuses on evaluating different metals as potential seed layers for copper electroplating for nanoscale applications. Layers of various metals of 100 Å, 275 Å, and 1000 Å thickness were deposited onto silicon wafers via physical vapor deposition. They were then electroplated with copper and examined visually for quality and consistency. A digital microscope was used for microscopic examination, then a scanning electron microscope was used to examine the development of copper over the seed layers and for elemental mapping to verify copper deposition. Seeds consisting of 1000 Å platinum, 1000 Å nichrome, and 100 Å silver showed strong promise for use as seed layers for nanoscale applications. Platinum and nichrome were found to develop uniform copper coatings and strongly adhere to the silicon wafer. To protect platinum and nichrome from diffusion into silicon during chemical vapor deposition (CVD) CNT growth, a barrier layer may be required. Copper developed uniformly over the 100-Å silver seed. Silver is unlikely to diffuse into silicon at the temperatures required during CVD CNT growth. Due to poor adhesion between the silver seed and silicon wafer, an adhesion layer may be used to improve reliability.