|M.Sc Student||Fraenkel Nethanel|
|Subject||Structural and Electrical Characterization of Cobalt|
Thin Films for Usage as Gate Metal in Advanced
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Moshe Eizenberg|
|Full Thesis text|
Integrated circuits (ICs) are now at the core of the majority of instruments and devices which serve us for nearly every function and activity. Technological advancement is therefore usually dependent upon, and intertwined with, improvements and breakthroughs in the semiconductor industry. Improvements in device technology stem from the ability to gain more functions out of each IC, which translates to more transistors per chip. Since ICs have not substantially grown in size over the years, this requires that the transistor shrinks in size. As device performance requirements increase and the node size has scaled to below 45nm, poly-silicon gates have been replaced by metal gates.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 Corporation Ltd, Cobalt thin films (ranging in thickness from 12nm to 200nm) 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 417oC, 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. All but the PVD Co remained HCP after short heat treatments up to a temperature of 500oC. Even under a lengthier heat treatment only the PVD films transformed to the FCC phase and remained in the FCC phase when cooled back to room temperature. We attribute this discrepancy to the difference in purity between layers deposited by different methods. The films deposited by ALD and CVD are contaminated by a few atomic percent of carbon and oxygen. Since the phase transformation propagates through the movement of dislocations, any contamination might hinder it. Capacitors of ALD-Co/SiO2/Si structure were measured for finding the EWF of ALD Co. Adhesion of ALD Co to SiO2 proved to be unreliable. To solve this, we attempted capacitors with a WCN film as a nucleation layer for the ALD Co on top of the SiO2. Unfortunately, the presence of the WCN layer prevented the ALD Co from affecting the EWF.