Mechanical/Structural Properties of the Key Thin Film Materials Ag, Cu, & Ni for Electronics Applications

Abstract

Thin metallic films and coatings of Ag, Cu, and Ni for interconnect and contact in next generation nanoelectronic are expected to have high conductivity, high temperature stability, good ohmic contacts to both p- and n- type semiconductors, and also superb mechanical properties. These metallic thin film are used in a variety of electronic applications due to their low cost of production, non-toxicity and excellent electrical conductivity. However, their applications in nanoelectronics are still limited due to diffusion into the commonly used silicon substrates and the tendency of developing oxides under atmospheric conditions. Thin oxide layers form immediately on metallic films surfaces’ upon contact with air even at room temperature. Thin metallic oxide films `tend to exhibit brittle behavior with a sharp jump in the mechanical properties such as hardness and modulus and results in high resistivity. Although the formation of oxides is still perceived as the primary impediment in using thin metallic films in nanoelectronics, thin metallic films exhibit promising applications in large-area electronics such as memories, MEMS, and microprocerssors. Metallic films of Ag, Cu, and Ni each of 150, 300, 600 and 1000 nm thick were deposited on Si using E-beam evaporation and deposition technique. A shutter was used to successively cover increments of 1² of the wafer at a time, starting with the wafer fully uncovered. A thin titanium layer of 10 nm is first deposited over the entire Si wafers followed by 150, 300, 600, and 1000 nm of the 2² target. Vacuum conditions at the start were ~1x10-7 torr and reached as high as 2x10-6 torr during the copper deposition due to outgassing. Similar vacuum conditions were maintained for the other films. The Ag, Cu, and Ni films thickness were verified using field emission scanning electron microscopy. A sample cross sectional FE-SEM of the 150 nm thick Ag film is shown in Figure1. Nanocrstalline grain structure formation is observed to dominate the crystal film growth. The nanomechanical properties were measured using nanoindentation to determine the modulus and hardness of the Ag, Cu, and Ni films. Indentations of 1/3 and 2/3 of the film thickness in addition to 2 µm deep indents were performed on each film to study the film properties irrespective of the Si substrate influence as well as the substrate influence on the mechanical properties. Figures 2 and 3 depict hardness versus normalized depth of indentation to film thickness for Ag and Cu. The hardness increases as the film thickness decreases and remains nearly flat (no size effects) for the same film with depth of indentation except for the very thin films as they suffer what is known as tapping effect [1].