Abstract
 
Metallization for next generation IC's
 
Sywert H.Brongersma
 
Inter-university Micro-Electronics Center (IMEC), Kapeldreef 75, B-3001 Leuven, Belgium.
 
 
For more than 30 years the scaling of active areas on a chip has simply lead to reducing sizes of the Aluminum connections and SiO₂ spacers needed for electric contact between these areas. However, when entering the 'sub quarter micron regime', the speed of a microprocessor is no longer solely determined by the switching time of a transistor (gate delay), but also by the RC-delay due to the inter-connects. As a result the processes and materials involved in interconnect technology have changed dramatically over the last five years.
 
The introduction of Copper as the new metal of choice has necessitated several extensive adaptations. Firstly, Copper is an easy diffuser in both Si and SiO₂ and therefore needs to stay confined through the use of diffusion barrier layers. Secondly, instead of the traditional Aluminum deposition and etch a process known as damascene is needed, where the dielectric is deposited and etched followed by a metal fill. As a complication the 'trenches' that are created in this way, and need to be filled with Copper, are deeper than they are wide. Voids during filling can be avoided through the use of several additives in an electroplating chemistry which induce an accelerated growth from the bottom of the trench upward. However, this accelerated 'bottom-up' growth tends to overshoot, producing large hillocks, and the resulting reduction in as-deposited grain size induces a phenomenon known as self-annealing. Here grain growth occurs at room temperature over a period of days or weeks, accompanied by stress and sheet resistance changes.
 
At the same time many low dielectric constant materials, deposited with either CVD or spin-on techniques, are being screened as possible replacements of SiO₂. As the choice of material is now dependent on the size of the structures, issues such as adhesion, etch behavior, and plasma interactions are constantly changing. These are further complicated by the introduction of porosity into these materials in an attempt to approach the dielectric constant of vacuum.