Electrochemical Anodization of 2-nm-Thick Tantalum Films for Self-Assembling of Molecularly Thick Layer as a Barrier for Copper

TaN in conjunction with adhesive Ta is a key barrier component of Cu-Damascene structure. However, the limited scalability and stability of TaN pose a dire need of an alternative molecularly thick (≤1 nm) barrier. To this end, the feasibility of anodizing 2-nm-thick Ta films with a mild ethanol based solution, rather than the commonly used strongly acidic (aggressive) chemical electrolytes, to enhance the formation of (3-aminopropyl) trimethoxysilane (APTMS) monolayer barrier is studied. Performing the anodization in an H2O-containing (1%) ethanol solution results in a prompt oxidization of the Ta film surface into fully passive Ta2O5 layer with extremely smooth topography, subsequently facilitating the growth of a fully oriented monolayer serving triply as a seed adsorber (for electroless Cu plating), barrier and adhesion promoter. Using Ta (pristine)/Cu [and Ta (pristine)/SAM/Cu] as a control, Ta (anodized)/SAM/Cu exhibits markedly higher adhesion strength (25.2 vs <1 MPa) and resistance against thermal failure (>500 vs 425 °C) of Cu. H2O is heavily involved in the anodization efficiency of the Ta films and is the key underlying factor deciding orientation ordering and barrier capacity of the monolayer fabricated. This is related to H2O-enhanced surface hydroxylation and Ta2O5 passivation of Ta.