Effect of the Annealing Atmosphere on the Nanocrystal Spatial Distribution in MOS Structure Gate Dielectric

D. Mateos, M. Curiel, N. Nedev, D. Nesheva, A. Arias, E. Manolov, O. Contreras, B. Valdez, O. Raymond, J. Siqueiros
Autonomous University of Baja California, MX

Keywords: Si nanocrystals, MOS, nonvolatile memories, XTEM, I-V

Summary:

SiOx layers with x = 1.3 were prepared by thermal evaporation of SiO. Films with varying thickness were deposited on n-type c-Si substrates maintained at room temperature. To form the SiO2/SiO2-Si NCs regions, the SiOx films were subjected to a two-step annealing process at 1000°C. First, the samples were annealed in pure N2 atmosphere and then in a 90% N2 + 10% O2 atmosphere (oxidation) with a total annealing time of 60 min. The depth to which the SiOx film was intentionally oxidized was controlled by varying the N2 and N2+O2 annealing times. High-resolution cross-sectional TEM analysis performed with a Jeol 2010 microscope revealed formation of two regions: the first one, far from the top surface exhibits Si nanocrystals with d ~ 3-4 nm, while the second one close to the top surface shows a uniform amorphous phase of SiO2. Larger annealing times in oxidation atmosphere result in formation of thicker SiO2 regions. I-V measurements confirmed the formation of high quality stoichiometric SiO2 on top of the nanocrystal region. It was found that the conduction mechanism limiting the current through the SiO2-Si NCs/SiO2 film is Fowler-Nordheim tunneling.