Step Coverage and Conformity

The ability to coat a surface with a smooth film of constant thickness (conformity), and to fill a contact opening (interconnection) 

The interconnecting holes are narrow and deep (at times less than 0.25 im wide and up to 2 im or more in depth) and, after a diffusion-barrier layer is applied, it must be filled completely with a high-conductivity metal (usually aluminum or tungsten) to provide the low-resi stance plug for inter-layer connections. Typically, CVD provides better step coverage and conformity than sputtering and other physical-vapor deposition processes. 

A typical example of the complexity of interconnection assembly consists of a MOCVD-TiN diffusion barrier combined with a low-temperature (260°C) CVD aluminum with low resistivity ( 3 lQ-cm). A copper doping forms an overlying sputtered Al-Cu film. This aluminum plug provides a large reduction in resistance compared to the classical tungsten process. k  

To mitigate the problem, a diffusion barrier is incorporated between the aluminum and the silicon (see Sec. 5 below). It is also possible to replace aluminum by alloys of aluminum and copper or aluminum and silicon, which have less tendency to electromigration. These alloys are usually deposited by bias sputtering. However, they offer only a temporary solution as electromigration will still occur as greater densities of circuit elements are introduced. It was recently determined that improvements in the deposition of aluminum by MOCVD at low temperature with a dimethyl aluminum hydride precursor may reduce the problem.bl 

The electromigration problem has led to the investigation of other electrical-conductor materials, such as tungsten (presently used in contact and via fills) and more recently copper. 

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