Effects of Nitridation by N 2 O or No on the Electrical Properties of Thin Gate or Tunnel Oxides
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EFFECTS OF NITRIDATION BY N2O OR NO ON THE ELECTRICAL PROPERTIES OF THIN GATE OR TUNNEL OXIDES C.Gerardi, T.Rossetti, M.Melanotte STMicroelectronics, Central Research & Development, Catania Technology Center, Stradale Primosole 50, 95121 Catania, Italy
S.Lombardo, I.Crupi Istituto Nazionale di Metodologie e Tecnologie per la Microelettronica (IMETEM), Consiglio Nazionale delle Ricerche, Stradale Primosole 50, 95121 Catania, Italy
ABSTRACT We have studied the effects of nitridation on the leakage current of thin (7-8 nm) gate or tunnel oxides. A polarity dependence of the tunneling current has been found this behavior is related to the presence of a thin silicon oxynitride layer at the SiO2/Si-substrate interface. The oxynitride layer lowers the tunneling current when electrons are injected from the interface where the oxynitride is located (substrate injection). The current flowing across the oxide when electrons are injected from the opposite interface (gate injection) is not influenced by the oxynitride. The increase of nitrogen concentration leads to a decrease of the tunneling current for substrate electron injection. INTRODUCTION Nitridation of thin oxides obtained by annealing with nitrous oxide (N2O) and nitric oxide (NO) is used in ULSI technology because it improves the dielectric reliability [1-3]. These processes are attractive due to their simplicity and because they are hydrogen-free thus avoiding the problems related to H that generates electron traps. The nitridation processes obtained by using N2O and NO are similar, in fact in both cases the species responsible for the nitridation is NO. However, during N2O nitridation only a small amount of NO is produced by the dissociation of N2O at high temperature, in addition the oxygen by-product of such dissociation continues the oxidation increasing the final oxide thickness [4]. Direct nitridation in NO is more effective since it does not involve N2O dissociation and therefore it can be exploited at lower temperature [5-6]. In this work we have studied the effects of nitridation on the current flowing across the oxide in the Fowler-Nordheim (F-N) regime finding a correlation with the amount of the nitrogen and its position in the oxide. The experiments have been performed on thin oxides grown by wet oxidation and subjected to annealing in NO. Several conditions have been investigated by varying the annealing time and the flux of NO in order to achieve different N concentrations.
1 C7.3.1
EXPERIMENTAL Oxides with a nominal thickness of 7-8 nm were obtained by thermal steam oxidation of p(100) Si substrates. The oxides were designed to be utilized as thin tunnel dielectrics for nonvolatile memory applications. After oxidation the wafers were annealed in NO at a temperature of 850°C with different annealing times and NO fluxes. For comparison one of the oxides (control) was annealed only in N2 with the same thermal budget. Secondary ion mass spectrometry (SIMS) analyses were carried out by using a 2 keV Cs+ primary ion beam and detecting NCs+, OCs+ and S
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