In local interconnection of advanced LSI, copper alternative materials have been sought to mitigate the increase of Cu line resistivity with decreasing line width. The required properties for the alternatives are small value of r0 x l for low resistivity in narrow lines, slow diffusivity for good electro-migration resistance under high current density, low melting temperature for easy grain growth at low process temperature, small Young’s modulus for easy CMP. Proposed materials to date include pure metals, alloys, carbides, nitrides, and silicides. In addition, low and stable material cost is an important factor to be considered.
To provide feasible alternatives which satisfy these properties, we focus intermetallic compounds. For example, NiAl is an ordered bcc compound having a bulk resistivity of 9 mW-cm. It is a well-known heat resistant material having a high melting point of 1638 °C, thus a good electromigration reliability may be expected. Meanwhile, NiAl has a large cohesion energy of -568 kJ/mol, hence the interdiffusion and detrimental reaction of individual Ni or Al ions with Si and SiO2 may be suppressed. Furthermore, Al oxide has heat of formation of -1678 kJ/mol, much higher than -911 kJ/mol of SiO2, so there is a tendency for Al atoms to bond with O atoms at the NiAl/SiO2 interface, possibly leading to good adhesion.
Experimental results showed good adhesion and no interdiffusion between NiAl film and SiO2/Si substrate. Similar results were obtained in other selected intermetallic compounds. The possibility of intermetallic compounds will be discussed in detail as the Cu alternative.