Sequence of Mg segregation, grain growth, and interfacial MgO formation in Cu–Mg alloy films on SiO2 during vacuum annealing

Abstract

Cu-alloy films are being explored for integrated circuits, for creating low-resistivity interconnects with stabilized metal/dielectric interfaces via solute segregation, and for interfacial reactions. Here, we describe the pathways of microstructure evolution in supersaturated Cu– 5–12 at. % Mg films, and phase formation at the film/SiO2 interface during annealing. The as-deposited films consist primarily of a Cu–Mg solid solution with trace amounts of orthorhombic CuMg2. Upon annealing to 400 °C, Mg segregates to the surface and the Cu–Mg grains grow from an average size of 20 to 60 nm, resulting in a ∼25%–40% decrease in film resistivity. In the same temperature regime, CuMg2 phase dissolves and fcc Cu2Mg forms. Upon annealing to higher temperatures, Mg segregates to the film/silica interface, reduces SiO2, and forms fcc MgO on the silica side of the interface. The Si released by this interfacial reaction diffuses into the metal film resulting in a ∼40%–190% increase in resistivity, for films with 8–12 at. % Mg, respectively. These results are of relevance for understanding microstructure evolution in alloy films and exploring the use of Cu alloys as interconnects in integrated circuits.

Formation of Ti diffusion barrier layers in Thin Cu(Ti) alloy films

“…[8][9][10][11][12][13] This technique is called "self-formation of the barrier layer" and is attractive to apply to the Cu wires. The self-formation of the barrier layers was observed in bulk alloy when the alloy elements formed solid solution at elevated temperatures and separated into two phases at low temperatures.…”

“…were extensively investigated for the possibility of the self-formation of the passivation (interfacial barrier) layers to improve surface corrosion (oxidation), electrical resistivity, adhesion, reliability, and intermixing. [8][9][10][11][12][13][14][15][16][17] However, high-temperature annealing (Ͼ500°C) for the barrier formation was required, which resulted in deterioration of the device performance and reliability. Therefore, in order to develop a low-temperature annealing (at ϳ400°C) technique for the self-formation of the barrier layer, understanding of the segregation property in the Cu alloy films is important.…”

Characterization of Self-Formed Ti-Rich Interface Layers in Cu(Ti)/Low-k Samples

“…[8][9][10][11][12][13] This technique is conventionally called self-formation of the barrier layer, and application of this technique to the fabrication process of Cu interconnects is attractive. We recently succeeded in forming Ti-rich barrier layers using Cu(Ti) alloy films prepared on SiO 2 /Si substrates after annealing at 400°C.…”

Effect of Annealing Ambient on the Self-Formation Mechanism of Diffusion Barrier Layers Used in Cu(Ti) Interconnects

“…The selfformed thin barrier layers using Cu alloys with various low concentration solutes were extensively studied by various authors. [6][7][8][9][10][11][12] The self-formation of the surface/interfacial layers is one of the coating techniques peculiar to thin films and was applied to prepare the self-passivation (or interfacial barrier) layers to protect surface corrosion and oxidation (or enhance interfacial adhesion). [6][7][8][9][10][11][12][13] Among the dilute Cu alloys, the copper (titanium) [Cu(Ti)] alloy is the best candidate material for the self-formation of the barrier layers, because the solubility limit of titanium (Ti) in Cu is very small (~0.5 at.% at 500°C), as shown in an equilibrium Cu-Ti phase diagram for Cu-rich alloys (Fig.…”