Si-N Bonding at the SiO 2 /Si Interfaces During Deposition of SiO 2 by the Remote Pecvd Process
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SI-N BONDING AT THE SI02/Si INTERFACES DURING DEPOSITION OF S102 BY THE REMOTE PECVD PROCESS
Y. Ma, T. Yasuda, S. Habermehl and G. Lucovsky Departments of Physics, and Material Science and Engineering, North Carolina State University, Raleigh, NC 27695-8202
Thin films of Si0 2 were deposited on Si substrates by remote PECVD following an in-situ cleaning/surface passivation with atomic-H. Si-N bonds were found in the immediate vicinity of the Si0 2/Si interface when nitrous oxide, N20, was used as 0atom source gas in the remote PECVD process. Si-N bonds, as well as Si-surface roughening produced by H-atom etching, contribute to the formation of high densities of midgap trapping states, Dit -1011 cm-2eV"1, at the Si0 2/Si interface. Eliminating the H-atom processing step, and exposing the Si surface to plasma-generated O-atoms prior to the Si0 2 deposition: i) eliminated Si-N bonding at the Si/Si0 2 interface; ii) reduced midgap Dit to -1-3x10° cm-2eVl; iii) eliminated surface roughening; and iv) improved process latitude and reproducibility.
INTRODUCTION
In the low-temperature (200-3001C) deposition of Si0 2 onto crystalline Si substrates by remote PECVD, N20 is commonly used as the O-atom source gas. Even though N is a constituent of this reactant gas, N-atoms are not detected in the bulk oxide films deposited by remote PECVD [1]. In this paper, we report N-atom concentrations in the immediate vicinity of the Si0 2/Si interface, mostly in Si-N bonds at the Si substrate. We have correlated this Si-N bond formation with relatively high densities of traps at the Si0 2/Si interface, and with poor process reproducibility. Moslehi et al. [2], using NH3 in the rapid thermal annealing of Si0 2 thin films, found a N-atom build up at the Si0 2/Si interface, and improved device breakdown and reliability, while the fixed charge in the dielectric was increased. Ahn et al. [3] annealed LPCVD Si0 2 in an N20 ambient, and also found a N-atom build up at the Si0 2/Si interface. They observed that resistance to constant current stress was better in these annealed LPCVD oxides than in thermally grown oxides. Hwang et al. (4] and Chu et al. [5] formed Si0 2 thin film by Rapid Thermal Processing CVD (RTPCVD) using N20 as the O-atoms source gas. They detected about 3 at.% N at the Si0 2/Si interface region, observing that the resistance to current stress, and the charge to breakdown character were improved. Recently, He et al. [6] found also a N-atom build up at an SiO 2/Si interface when a remote PECVD oxide was subject to a post deposition NH3 plasma. Fountain et al. [7] found an increase in Dit when a nitride was deposited by remote PECVD on to a remote PECVD oxide layer in an NO or ONO structure, and attributed this to the formation of Si-N bonds at the Si0 2/Si interface. This paper describes a new two-step, plasma-assisted surface-passivation/thin film deposition process sequence that can eliminate Si-N bond formation at the Si0 2/Si interface when N20 is used as the O-atom source gas in the deposition of Si0 2/Si on to Si sub
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