Formation of ultrasmooth and highly stable copper surfaces through annealing and self-assembly of organic monolayers

Copper (Cu) has been extensively used as an interconnect material for microelectronic devices because of its high electrical and thermal conductivity and excellent electromigration resistance. However, the formation of relatively rough Cu surfaces (∼5 nm roughness) and Cu-oxide layers upon exposure to air still hinders their reliable application in a wide range of fields. In this article, we show the potential values of highly stable and ultrasmooth polycrystalline bare Cu obtained by simple annealing and chemical modification for a wide range of Cu-based electronic devices. The morphological properties and oxidation behavior of annealed Cu surfaces, before and after coating by self-assembled monolayers of terephthalic acid (TPA), were examined upon exposure to ambient air conditions (∼110 days). Thin films of polycrystalline Cu, deposited on top of an adhesion layer of tantalum nitride (TaN) and annealed for 8 h at 580 °C under 2 × 10 ^-7 Torr, provided ultrasmooth Cu surfaces (R _rms = 0.15-1.1 nm for fresh samples) and had a stable Cuoxide layer after 65 days (∼3.5 nm). These observations were perceived to be superior to nonannealed polycrystalline Cu samples. Coating fresh (oxide-free) samples of ultrasmooth Cu with TPA molecules created a closely packed monolayer with a standing-up phase configuration and molecular coverage of ∼90%. The TPA-coated Cu surface has not shown any detectable oxidation during the first 2 weeks of exposure. The protection efficiency of this layer was found to be superior to those reported earlier on polycrystalline Cu surfaces. The oxidation mechanisms of both annealed and nonannealed Cu surfaces are presented and discussed.