Simulation of Microstructure and Surface Profiles of Thin Films for VLSI Metallization

Extract

A crucial step in the manufacture of very large-scale integration (VLSI) integrated circuits is the fabrication of reliable, low-resistance metal interconnects between semiconductor devices. The fabrication of these interconnects is generally performed by depositing a blanket metal film and then patterning it by lithographic and etching techniques. The primary means of depositing thin metal films for VLSI interconnects are sputtering and chemical vapor deposition (CVD).

The creation of reliable interconnects is, however, complicated by a number of issues. In order to obtain low contact resistance, to inhibit reactions with the silicon, and to provide good adhesion to both Si and SiO₂, contact, barrier, and adhesion layers are generally deposited prior to the deposition of the low-resistance metal film that forms the bulk of the interconnect. If these layers are to provide an effective barrier to diffusion of the interconnection metal to the silicon, they must be deposited in a uniform, homogeneous form. It is also necessary that the primary interconnect material have as high step coverage as is possible in order to reduce current crowding, local heating effects, and electromigration. Unfortunately, as VLSI circuit densities have increased, the fabrication of interconnects requires high aspect-ratio contact cuts, and relatively severe local topographies can result. These factors make it difficult to deposit films with good step and bottom coverage.

In addition to these concerns with the film surface profile, another factor is becoming increasingly significant. Both sputtering and CVD produce thin films with characteristic microstructures. This microstructure consists of columns or grains separated by grain boundaries and voids.