Nethanel Fraenkel, MSc Candidate
Department of Material Science and Engineering, Technion
As device performance requirements increase and the node size has scaled to below 45nm, poly-silicon gates have been replaced by metal gates. The single most challenging requirement for these gates is the different work function needed for nMOS and pMOS devices. The solution is not as easy as simply choosing two metals with documented different work functions, since the vacuum work function of a metal reported in literature generally differs from the work function measured when it is in contact with a semiconductor or with a dielectric, denoted as the effective work function (EWF). This is due to metal/dielectric or dielectric/semiconductor interactions. Different deposition methods and subsequent thermal treatments may lead to different structural properties of the metal and possibly to different interactions with the dielectric, thus different EWF values.
In this research, done in collaboration with Lam Research Ltd, Cobalt thin films (typical thickness of 30nm) deposited by Atomic Layer Deposition (ALD), Chemical Vapor Deposition (CVD) or Physical Layer Deposition (PVD), were investigated for the potential use as gate metals. Sheet resistance measurements performed in vacuum at various temperatures combined with x-ray photoelectron spectroscopy (XPS) measurements showed that PVD films were the purest and had the lowest resistance. For bulk Cobalt, a phase transition from the hexagonal closest packed (HCP) phase to the face centered cubic (FCC) phase is expected to occur at 4150C, and this is reversible upon cooling. X-ray diffraction (XRD) measurements of our films taken at room temperature and at elevated temperatures showed that all Co films (regardless of their deposition method) were in the HCP phase as deposited as well as after short heat treatments up to a temperature of 5000C. Lengthier heat treatments showed that only the PVD films transformed to the FCC phase and stayed in the FCC phase when cooled back to room temperature. Capacitors of Co/SiO2/Si structures, with the Co electrode prepared by the various deposition methods discussed above, were fabricated for measurements of the EWF and correlation with the films' microstructural properties.
Supervisor: Prof. Moshe Eizenberg