|M.Sc Student||Zonensain Oren|
|Subject||Characterization and tuning of the effective work function|
of WCxNy films for advanced MOS device
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Moshe Eizenberg|
|Full Thesis text|
In this research Fluorine Free WCxNy metallization deposited by Plasma Enhanced Atomic Layer Deposition is studied as an intended material for metal gate electrodes in Metal/Oxide/Semiconductor structures. The main aspect addressed is the change in the effective work function of the gate material due to compositional and structural changes induced by small changes in processing. The effective work function is defined as the work function “seen” from the semiconductor in the structure. It is shown than deposition with Hydrogen plasma results in Tungsten carbide films with a low function (~4.2 eV) while introduction of Nitrogen based reactant to the deposition environment, either in thermal or plasma enhanced Atomic Layer Deposition, results in Tungsten Nitride films with a high work function (~4.7 eV). In addition, work function sensitivity to thermal anneals is studied and correlated to chemical/structural changes in the films. It is shown that Tungsten carbide films’ work function remains stable even after 30 sec 700C anneals, but higher temperatures result in severe damage to the film and a rise in effective work function (~4.7 eV). Tungsten nitride’s work function on the other hand shows greater sensitivity to thermal anneals. A decrease in work function of the order of 0.4-0.5 eV is seen after 600C-700C anneals without any visible structural change. This work function change was correlated to Nitrogen segregation to the metal/SiO2 interface. Annealing Tungsten nitride film at higher temperatures results in an effective work function increase by 0.3-0.4 eV; this is probably due to the diffusion of the Nitrogen away from the interface. In addition it was shown that the as deposited EWF remains stable even during several process changes in WCxNy films and that WCxNy films deposited over HfO2 have a higher EWF than when deposited over SiO2. Fluorine diffusion originating from the metal contact above the ALD film trough the ALD and into the dielectric was observed to happen at elevated temperatures. In this aspect WCx films were seen to perform as better fluorine diffusion barriers than WCxNy films.