Collimated sputtering is a physical vapor deposition (PVD) method where a collimator is inserted between a conventional “full-face-erosion” sputtering target and a substrate (Figure 1). The collimator is a plate of hexagonal cells that acts as a filter to remove obliquely incident atoms before they arrive at the substrate. Only material with a nearly normal incidence trajectory may pass through the collimator and deposit on the substrate. Collimated sputtering was initially evaluated for conductor-level depositions in order to improve the filling of recessed features. Although the method has been successfully used to fill damascene structures, depositing thick conductor films is inefficient because most of the sputtered material is captured by the collimator, causing the collimator to clog quickly, necessitating frequent replacement.
A more common use of collimated sputtering is associated with the deposition of thin “liner” films. For example, thin, collimated aluminum alloy films have been used as underlayers for aluminum reflow processes. Also, collimated Ti/TiN films are used as contact/adhesion layers for chemically vapor-deposited (CVD) W metallization. Collimation provides better bottom and sidewall coverage for small, high aspect-ratio features than conventionally sputtered films do.
Coliimated sputtered films often exhibit unique properties because the angle of incidence of depositing atoms is controlled. Collimated AlMg alloys have superior electromigration resistance compared to noncollimated AlMg films. Collimated TiN films appear to exhibit denser grain structures when compared to films deposited with higher amounts of obliquely incident flux (Figure 2).