SiO2 / Si Interface Structures and Reliability Characteristics

Abstract

This paper shows that a structural transition layer of SiO2 exists at an SiO~/Si interface prepared by thermal oxidation of St. Using a newly developed grazing-incidence x-ray diffraction of synchrotron radiation, the transition layer density (2.4 g/cm 3) is found to be lower than the immediate bulk SiO2 density (2.6 g/cm3), and its thickness is approximately 7 nm. Electrical properties of the SiO2 films are examined by using Fowler-Nordheim tunneling electrons which are injected from the polycrystalline silicon gate electrode into the SiO2 film. The injected charge-to-breakdown (Qbd) rapidly degrades when the SiO2 film thickness decreases below approximately 7 nm due to dielectric breakdown in the transition layer. Based on theoretical analysis, the mechanism of the dielectric breakdown in the transition layer is proposed to be St-St bond formations via hypervalent Si atoms and a replacement reaction of an oxygen atom with an electron. Introduction of nitrogen atoms into the transition layer improves the Qbd degradation of thin SiQ films, because the St-St bond formation is suppressed by stress relaxation in the transition layer.With the down-scaling of gate dielectric films to less than 10 nm in thickness, there are new reliability concerns for the dielectric films in metal oxide semiconductor (MOS) integrated circuits. Time-dependent dielectric breakdown (TDDB) is one major reliability issue. When Fowler-Nordhelm (F-N) tunneling electrons are injected from a polycrystalline silicon gate electrode into an SiO2 film (gate injection), the TDDB characteristics of the MOS structure rapidly degrade with decreasing SiO2 film thickness. 1-3 When the F-N tunneling electrons are injected in the Si substrate (substrate injection), the TDDB characteristics are much better than for the gate injection. 3-5 It is also reported that the TDDB characteristics for gate injection are significantly improved by the introduction of nitrogen (N) atoms into the SiQ film. 6-8 These reports suggest that the TDDB characteristics strongly depend on the SiO2 film structure. However, the asymmetric properties and improvement of the TDDB characteristics have not been fully discussed on the basis of structural analyses. The purpose of this work is to study how and why the TDDB characteristics depend on the SiO2 film structure.The dielectric breakdown properties of SiO2 films have been extensively studied in the last few decades. According to those studies, the constant-current TDDB measurement is one of the most suitable means of evaluating the reliability of SiO2 films. 9 The Qbd, which is defined as the injected charge-to-breakdown in the constant current TDDB measurement, quantitatively indicates the reliability of SiQ films. Many samples are examined in the TDDB measurements, and the cumulative failures of TDDB are statistically investigated as a function of Qbd. The cumulative failures are classified into two groups. One is time-dependent breakdowns in the short time range of the TDDB plots, which indicate metallic impurities. The other ...

Comprehensive Understanding on the Role of Tunnel Oxide Top Nitridation for the Reliability of Nanoscale Flash Memory

“…5 (modified from [9]), which shows two possible inelastic TAT modes. The model assumes available defects for as-grown and stressed SiO 2 , which is quite well established and generally accepted, particularly for near bottom interface [10]. Process (a) represents FN tunneling at the top interface and subsequent TAT or direct tunneling, which would have Φ dependence.…”

The impact of nitrogen profile engineering on ultrathin nitrided oxide films for dual-gate CMOS ULSI

“…This Qw degradation, which is due to boron incorporation in the dielectric film, is a serious problem for a surface channel PMOS transistor with a thinner oxide film. The boron diffusion and incorporation enhance creation of the strained and broken Si-0 bonds in a structural transition layer (STL) near the dielectric/substrate interface [7]. In a thinner film, the STL occupies a larger part of the film (Fig.…”

Plasma-induced charging damage in ultrathin (3-nm) gate oxides

“…While for nMOS devices, apparently the superior oxide robustness under substrate injection polarity (i.e., highly positive potential) protects the devices from charging damage at the wafer edge. The strong polarity dependence of was believed to be due to the weakness of structure transition layer (STL) located in the SiO /Si interface [15], [16]. The injected energetic electrons may release energy at the STL interface and cause bond breaking, a precursor of dielectric breakdown, under gate injection.…”

“…The formation of strong Si-N bonds in place of strained Si-O bonds and weak Si-H bonds enhances the interface hardness, resulting in improved gate oxide integrity [3]. Since it has been speculated that trap creation mechanism responsible for SILC is hydrogen-related, the incorporation of nitrogen by N O-nitridation is expected to terminate Si dangling bonds at the SiO /Si interface as well as to reduce the stress/strain in the structure transition layer (STL) [15], [24]. As a result, gate leakage current after plasma charging can be reduced.…”