Strength of Ta–Si interfaces by molecular dynamics

Abstract

Miniaturization of electronic devices leads to nanoscale structures in the near future. In these length scales the technological choice for metalization and interconnect material seems to be copper mainly because of its low electrical resistance and resistivity against electromigration. In copper metalization a barrier layer between copper and silicon is needed to prevent diffusion. Tantalum seems to be the most common barrier metal. We use a modified embedded atom potential and molecular dynamics to study the energy, structure, and strength of Ta–Si interfaces. The interfacial energy has a negative correlation between the strength of the interface. We propose that mixing on the interface has an important role in interface strength.

Introduction

During last few years copper has received a lot of interest in the electronics community since it has superior mechanical and electrical properties compared to aluminum. These properties include thermal and electrical conductivity and resistivity against electromigration that enable smaller line widths and faster operating frequency. Despite difficulties copper metalization has been demonstrated [1], and for the above reasons it seems that copper metalization will become even more popular in the near future.

One problem in copper metalization is its diffusion into silicon. This is usually prevented by adding a barrier (metal) layer in between silicon and copper. Barrier metals are often polycrystalline and copper diffusion into silicon can occur through barrier grain boundaries even if copper and barrier are immiscible. If the barrier forms silicide with silicon, the silicide layer can prevent also diffusion through grain boundaries [2]. Driving force for the diffusion is the formation of copper silicide at the silicon interface [3]. It seems that tantalum is a very promising barrier material, since it is immiscible with copper, has a high melting point, and forms silicides with silicon [4]. These issues are discussed e.g. in MRS Bulletin, August 1994 and June 1993. The most common silicide observed in silicon metalization is the disilicide, TaSi₂, with the C40 crystal structure [5]. Some studies on tantalum rich systems report also the formation of Ta₅Si₃ [6], which has the tetragonal D8_m structure [7]. Tantalum barrier stability can be slightly improved by adding nitrogen to the barrier layer [8].

Another very important issue is the mechanical reliability of interconnections and their interfaces. Due to differences in coefficients of thermal expansion electronic devices undergo shear deformation. The temperature can change due to varying operating temperature or because operation of the device itself generates heat. Nonetheless, a constant temperature would be extremely hard to maintain. This process induces shear stress in the system and therefore the shear strength of the bulk materials and their interfaces are very important.

The feature size (i.e. line width) in electronics applications is currently around 100 nm and is expected to go down exponentially to 30 nm in 10 years [9]. If the trend goes on further, 10 nm will be reached in 20 years, as depicted in Fig. 1. At these length scales the atomistic nature of materials has to be taken into account and thus the continuum theory fails to describe their behavior. Moreover, the finite element method which can be used to solve the continuum equations is not accurate at very small length scales. One computational method, which describes the motion of the atoms is the molecular dynamics (MD) method [10], in which the trajectories of the atoms are numerically solved.