In the copper metallization adopted for better speed and reliability in microelectronic devices, the choice of underlayer is one of the major concerns. Its requirements include low electrical resistance, adhesion to Cu and adjacent dielectric materials, confinement of Cu from drifting, and nucleation and texture enhancement of the overlying Cu film. Metallic tantalum is a very promising candidate. In this paper, the thermal stability of Cu/Ta interfaces is investigated to estimate the reliability of such a Ta underlayer.

Thin Cu and Ta films were consecutively deposited without breaking vacuum using a dc sputtering method to simulate the seed layer and underlayer in a Cu metallization structure. All the synthesized Ta films were the metastable tetragonal structure rather than the stable body-centered cubic (bcc) phase.

We have shown that (111) Cu grows epitaxially on top of (002) tetragonal Ta even though the Cu and Ta have hexagonal and tetragonal atomic arrangement on those planes, respectively. Using electron diffraction in cross-sectional and plan-view samples, an unexpected atomic matching at the interface was discovered as shown in Figure 1(b).