Investigation on critical thickness dependence of ALD TiN diffusion barrier in MOL

Titanium nitride (TiN), a refractory material is actively been used as a diffusion barrier in Middle-of-the-Line (MOL) contacts. In the typical MOL stack (titanium (Ti)/TiN/tungsten(W)), it acts as a fluorine (F) diffusion barrier and also as an adhesion layer to W. During W deposition, F from W precursor chemistry can react with Ti to form a highly resistive titanium fluoride (TiFx) compound. The formation of TiFx creates >200% volume expansion which can result in cracks on the TiN layer or W delamination. In order to be an effective diffusion barrier layer, TiN has to be dense which demands a critical thickness. In this work, we study the materials properties of ALD TiN as a function of thickness in the range 11–35 Â. We found a linear increase in resistivity of TiN on Ti with thickness. Columnar structured microstructure was observed for TiN films with thickness 30.4 and 34.7 Â. These films were also found to be slightly denser. TEM electron diffraction indicated that the 34.7 Â film is preferentially oriented along [111]. The roughness of the film increases with thickness. Three roughness zones with different crystallization modes have been identified which indicates thickness dependent crystallization. From our study, it is clear that in addition to density, the formation of preferential orientation of TiN might play a role for critical thickness requirement.