Contacts in advanced CMOS: History and emerging challenges
Suicide materials used as contacts in CMOS devices have evolved over many technology nodes. This article traces the often forgotten defectivity related reasons that were the primary drivers for a change in materials or process flow - evolving from Ti to Co and Ni suicides, and the more recent return to Ti-based liner suicides. The criteria used for the selection of these metal suicides have undergone a dramatic change with the advent of 3-D transistors and trench suicide contacts, and is now primarily guided by the value of interfacial contact resistivity (ρc). Furthermore, using results from synchrotron X-ray diffraction and pole-figure analysis, we present how phase formation and microstructure of contacts vary with Ti thickness, alternative annealing treatments, and substrate composition and orientation. We show that microstructure in very thin films can change from amorphous to epitaxial, a factor likely to become important for the contacts in upcoming generations of devices.