Surface oxidation kinetics of Si3N4-4%Y2O3 powders studied by Bremsstrahlung-excited Auger spectroscopy

Samples of silicon nitride powder containing 4.0% Y₂O₃ in weight were heated in air at temperatures between 900 and 1000 °C. The average SiO₂ layer thickness on the Si₃N₄ powder particles, as a function of time at a particular temperature, was measured by Bremsstrahlung-excited Auger electron spectroscopy. Oxidation was found to follow a linear rate law with an activation energy of 56±1.5 kcal mol^–1. The yttrium level measured by X-ray photoelectron spectroscopy was also found to decrease as a function of the oxide layer thickness. This suggests that there is a reaction between the Si₃N₄ and Y₂O₃ particles which results in the formation of an yttrium-rich phase at the interface between the surface SiO₂ layer and the underlying Si₃N₄ particle.     

Profiling of deep traps in silicon oxide–nitride–oxide structures

Thermally stimulated exoelectron emission has been applied for high-resolution depth profiling of traps in amorphous SiO₂/Si₃N₄/SiO₂ (ONO) dielectric stacks used in silicon–oxide–nitride–oxide–silicon (SONOS) memory devices. It is shown that maximum density of traps responsible for charge storage in ONO structures is at the interface between top silicon oxide and silicon nitride in ONO.     

Characterization of low pressure chemically vapour- deposited thin silicon nitride films

Silicon nitride films prepared on silicon by low pressure chemical vapour deposition (LPCVD) were characterized by electrical measurements (current-voltage and capacitance-voltage), Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). The contact current versus contact field characteristics were interpreted in terms of the Fowler-Nordheim tunnelling of holes from silicon into silicon nitride, with the field of charged traps and the effect of changing the triangular shape of the barrier under steady state conditions taken into account and on the assumption of a Poole-Frenkel detrapping mechanism. AES data show that the LPCVD process yields stoichiometric Si₃N₄ films. RHEED data shows that films of thickness more than 10 nm are amorphous. Some crystalline structures of Si₃N₄ and SiO₂ are observed for thicknesses of less than 10 nm.     

Dielectric properties in GHz range of porous Si3N4–BN–SiO2 ceramics with considerable flexural strength prepared by low temperature sintering in air

Abstract

Porous Si₃N₄–BN–SiO₂ ceramics with ultimate apparent porosities between 0·140 and 0·799 were fabricated in air at 1100°C by partial sintering using core starch as both consolidator and pore former in the green bodies. The pores were derived from burning off the starch, the partial oxidation of silicon nitride and the stack of particles of the start materials. Effect of retaining time on the microstructure of sintering bodies was analysed by SEM analysis. Reference intensity ratio (RIR) technique based on the X-ray diffractometry results demonstrated the phase components content of sintered bodies. Influence of porosity on the flexural strength of porous Si₃N₄–BN–SiO₂ ceramics was investigated. The ceramic with a porosity of 0·140 attained a maximal flexural strength of 60±4·11 MPa. In addition, the dielectric constants and loss tangents were presented for porous Si₃N₄–BN–SiO₂ triphase ceramics in the frequency range of 18–40 GHz, and the real part of dielectric constant of the materials reached as low as 2·67 at the porosity of 0·732 at a frequency of 20 GHz.     

Liquid-phase bonding of silicon nitride ceramics using Y2O3–Al2O3–SiO2–TiO2 mixtures

Hot-pressure sintered β-Si₃N₄ ceramic was bonded to itself using Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures. Reactive behavior at interface between Si₃N₄ and Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures during silicon nitride ceramic joining was studied by means of scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray diffraction (XRD) and auger electron spectroscopy (AES). The joint strength under different bonding conditions was measured by four-point bending tests. The results of EPMA, AES and XRD analyses show that the liquid glass solder reacts with silicon nitride at interface, forming the Si₃N₄/Y–Si–Al–Ti–O–N glass/TiN/Y–Si–Al–O glass gradient interface. From the results of four-point bending tests, it is known that with increase of bonding temperature and holding time, the joint strength increased reaching a peak, and then decreased. The maximum joint strength of 200 MPa measured by the four-point bending tests is obtained for silicon nitride bonded at 1823 K for 30 min.     

Degradation behavior of amorphous silicon nitride fibers in low atmospheric pressure

We investigated the degradation behavior of amorphous silicon nitride (Si₃N₄) fibers in low air pressure and presumed the evolution mechanism. The obtained Si₃N₄ fibers were characterized by tensile strength, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and elemental analysis after being annealed (air pressure: 1 Pa–0.1 MPa, temperature: 1000–1600 °C, dwell time: 0–4 h). When air pressure was lower than 100 Pa or higher than 1000 Pa the strength of fibers dropped sharply. Due to the moderate partial pressure of oxygen in 100–1000 Pa, both active oxidation and passive oxidation were restrained resulting in the best mechanical property of fibers in 100–1000 Pa. Besides air pressure, annealing time also affected the thermal behavior of fibers. Firstly Si₃N₄ fibers were passive oxidized to form SiO₂ layer on the surface, and then Si₃N₄ decomposed into free Si and SiO₂ released gaseous SiO. Finally crystallization inside of fibers and formation of nanowires on the outer surface played the leading role in the progress of degradation.

     

A resonant method for determining mechanical properties of Si3N4 and SiO2 thin films

This study investigates the measurement of Poisson's ratio and Young's modulus of silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) thin films using a resonant method. Two thin films, which are SiO₂ and Si₃N₄, are fabricated as the specimens of microcantilever beams and plates using the bulk micromachining. The resonant frequency of the cantilever beams and plates is measured using a laser interferometer. The Young's modulus of thin films can be calculated from the resonant frequency of the cantilever beams, and the Poisson's ratio of thin films is determined by the frequency of the cantilever plates. Experimental results show that the Poisson's ratios of SiO₂ and Si₃N₄ are 0.16 and 0.26, respectively, and the Young's moduli of SiO₂ and Si₃N₄ are, respectively, 55.6 GPa and 131.6 GPa.     

Quantitative analysis of chemical compositions in ultra-thin oxide-nitride-oxide stacked films having wet oxidized blocking layer

2nd-derivative Auger electron spectra analyses and time-of-flight secondary ion mass spectroscopy depth profiles are employed for analyzing quantitatively chemical compositions in ultra-thin oxide-nitride-oxide (ONO) stacked films. Tunnel oxide of 2.3 nm is grown on silicon substrate in nitrogen-diluted oxygen ambient. Nitride of 5.7 nm is immediately deposited on the tunnel oxide by a low-pressure chemical vapor deposition. Blocking oxide is subsequently grown by oxidizing the oxide-nitride structure in a wet O₂ ambient. Some chemical mixing occurs during the ONO formation. That is, the tunnel oxide formed on the silicon substrate changes into SiO_1.11N_0.67 by nitrogen substitution. Oxygen diffuses into the nitride layer, and converts some of the nitride layer into SiO_1.11N_0.67 during wet oxidation. We think the SiON and Si₂NO species exist near the tunnel oxide-nitride and the nitride-blocking oxide interfaces are segments of the Si₂N₂O. These are evidence of dangling bonds as unstable chemical species which may act as charge traps in the oxide-nitride interfaces.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

Preparation and interface modification of Si_3N_(4f)/SiO_2 composites

In order to modify the interface, SiON coating was introduced on the surface of silicon nitride fiber by perhydropolysilazane conversion method. Si₃N_4f/SiO₂ and Si₃N_4f/SiON_c/SiO₂ composites were prepared by sol-gel method to explore the influence of SiON coating on the mechanical properties of composites. The results show that with the protection of SiON coating, Si₃N₄ fiber enjoys a strength increase of up to 24.1% and Si₃N_4f/SiON_c/SiO₂ composites have a tensile strength of 170.5 MPa and a modulus of 26.9 GPa, respectively. After 1000 °C annealing in air for 1 h, Si₃N_4f/SiON_c/SiO₂ composites retain 65.0% of their original strength and show a better toughness than Si₃N_4f/SiO₂ composites. The improvement of mechanical properties is attributing to the healing effect of SiON coating as well as its intermediate coefficient of thermal expansion between Si₃N₄ fiber and SiO₂ matrix.     

Oxidation mechanism of porous silicon nitride

The oxidation of reaction-bonded silicon nitride (RBSN) has been studied in air between 800 and 1500° C. The extent of internal oxidation is governed by the radius of the pore channels allowing the oxygen to penetrate the specimen. The velocity of oxygen transport into narrow channels is very low compared to the reaction rate of oxygen with Si₃N₄. Because of these two concurrent processes an oxygen gradient is built up along the channel axis leading to SiO₂ formation mainly at the channel mouth. The typical oxidation isotherm of RBSN is represented by an asymptotic law. The mass gain and the penetration depth of oxidation is calculated, based on reaction-rates of Si₃N₄-powder, oxygen-diffusion-data and the pore-characteristics of the RBSN-materials, and compared with the experimental results. The results clearly indicate, that high quality RBSN may wel be used in oxidizing atmosphere without extensive internal oxidation.     

An Inhomogeneous Grain Boundary Composition in Silicon Nitride Ceramics as Revealed by 300 kV Field Emission Analytical Electron Microscopy

The chemical compositions of the grain boundary phases of silicon nitride (Si₃N₄) ceramics containing additives of 1 mole% and 10 mole% of an equi-molar mixture of Y₂O₃ and Nd₂O₃ have been studied by 300 kV field emission analytical electron microscopy. The energy dispersive x-ray spectra (EDS) are obtained from both two-grains and triple-grain junctions, where an electron beam of about 0.5 nm in diameter is focused. The thickness of the intergranular thin film is found to be about 1 nm, whose value is almost the same between two samples. The sintering additives are highly enriched at the triple-grain junctions, while they are less concentrated at the two-grain junctions. It is also shown that the additives are distributed inhomogeneously within the triple-grain junctions. Based on the composition analysis among the grain boundaries, an inhomogeneous grain boundary composition model for the Si₃N₄ ceramics is proposed.     

Applying RF current harmonics for end-point detection during etching multi-layered substrates and cleaning discharge chambers with NF3 discharge

The present paper reports the results of studying the characteristics of the etching process of multi-layered materials (Si₃N₄/SiO₂/Si and SiO₂/Si) and of cleaning technological chambers covered with silicon nitride films (Si₃N₄) in a NF₃ RF capacitive discharge. The process of chamber cleaning was monitored with a mass spectrometer. The gas pressure, RF voltage amplitude, current-voltage phase shift, ohmic current as well as the second harmonic of the RF current were also recorded. The opportunity of using these parameters for end-point detection of etching and plasma cleaning is discussed. It is found that the second harmonic of the RF current may be successfully used for end-point detection of multi-layered materials etching and to monitor the cleaning process of technological chambers. The cleaning of chambers of complicated design may possess a double-stage pattern.     

Composition and optical properties of silicon nitride films grown on SiO2-glass and R-Al2O3 substrates by reactive RF magnetron sputtering

We have grown silicon nitride (Si₃N₄) films on SiO₂-glass and R-Al₂O₃ substrates by using reactive RF magnetron sputtering deposition methods with N₂ pure gas and N₂ + Ar mixture gas. The film composition, thickness and impurities have been examined by ion beam analysis. It is shown that the films have stoichiometric composition and are free from Ar contamination, when N₂ gas was used for the film deposition. Effects of impurities on the film properties, e.g., optical properties will be discussed.     

A study of insulator materials used in ISFET gates

Experiments with aqueous electrolyte-insulator-semiconductor structures showed that Si₃N₄ is a satisfactory insulator on silicon whereas thermally grown SiO₂ is not. The results can be explained in terms of microcrack formation in SiO₂. The breakdown voltage was found to be relatively independent of the SiO₂ thickness and crack sizes were estimated to be of the order of a few tens of ångströms. No electrically significant bulk hydration effects were found to occur in either insulator in mildly acidic solutions.     

Energy Offset Between Silicon Quantum Structures: Interface Impact of Embedding Dielectrics as Doping Alternative

Ultrasmall silicon (Si) nanoelectronic devices require an energy shift of electronic states for n‐ and p‐conductivity. Nanocrystal self‐purification and out‐diffusion in field effect transistors cause doping to fail. Here, it is shown that silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) create energy offsets of electronic states in embedded Si quantum dots (QDs) in analogy to doping. Density functional theory (DFT), interface charge transfer (ICT), and experimental verifications arrive at the same size of QDs below which the dielectric dominates their electronic properties. Large positive energy offsets of electronic states and an energy gap increase exist for Si QDs in Si₃N₄ versus SiO₂. Using DFT results, the SiO₂/QD interface coverage is estimated with nitrogen (N) to be 0.1 to 0.5 monolayers (ML) for samples annealed in N₂ versus argon (Ar). The interface impact is described as nanoscopic field effect and propose the energy offset as robust and controllable alternative to impurity doping of Si nanostructures.     

Combined infra-red and X-ray studies of β-silicon nitride and β′-sialons

Previous investigations of the replacement of silicon by aluminium and nitrogen by oxygen in -silicon nitride have been based primarily on X-ray powder diffraction studies. In the present work this technique is coupled with parallel infra-red studies. X-ray analyses of sialons over a wide composition range confirm previous observations that increasing substitution of aluminium for silicon and nitrogen for oxygen in -silicon nitride is accompanied by an increase in cell size, with no evidence of any other structural modification. Parallel infra-red analyses show shifts in certain of the infra-red absorption bands to lower wavenumbers as the degree of substitution increases. Changes in the infrared spectra at the composition Si₂Al₄N₄O₄ indicate structural modifications which are not apparent from the X-ray investigations. It is suggested that these changes are a result of the ordering of the different atom types at this composition.     

Plasmons in double interfaces system of Si3N4/SiO2/Si irradiated by Co

The first level plasmons of Si in the pure Si state, in the SiO₂ state and in the Si₃N₄ state (corresponding to bonding energy 116.95, 122.0 and 127.0 eV) were investigated directly with X-ray photoelectron spectroscopy before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of plasmons of Si in the Si₃N₄ and SiO₂ states, while another was made of plasmons of Si in the pure Si state and in the SiO₂ state. When the Si₃N₄-SiO₂-Si samples were irradiated by ⁶⁰Co, the interface at Si₃N₄/SiO₂ was extended and at the same time the center of this interface moved towards the surface of Si₃N₄. The concentration of plasmon for silicon in the SiO₂ state is decreased at the SiO₂-Si interface, and the effects of radiation bias field on plasmons in the SiO₂-Si interface are observable. Finally, the mechanism of experimental results is analyzed by the quantum effect of plasmon excited by the photoelectron.     

Yb2O3-fluxed sintered silicon nitride

Microstructural development and crystallization behaviour of Yb₂O₃-fluxed sintered silicon nitride materials was investigated using CTEM and HREM. The materials contained 5 and 10 vol% Yb₂O₃ as sintering additives. After densification, both compositions were subsequently heat treated to crystallize the residual amorphous secondary phases present at triple-grain regions. In the material doped with 5 vol% Yb₂O₃, only an amorphous secondary phase was observed after sintering, which was about 80% crystalline (Yb₂Si₂O₇) after the post-sintering heat treatment. A metastable phase was formed in the material with 10 vol% additives after sintering, with about 70% crystallinity in the triple-point pockets. Upon postsintering heat treatment, the material could be completely crystallized. During heat treating, the metastable phase combined with the remaining glass to form Yb₂SiO₅ plus Yb₂Si₂O₇ and a small amount of Si₃N₄ which deposited epitaxially on pre-existing Si₃N₄ grains in areas of low-energy within the triple-point pockets. All materials contained thin amorphous films separating the grains. The amorphous intergranular films along grain boundaries (homophase boundaries) revealed excess ytterbium and oxygen. The thickness of the intergranular films was about 1.0 and 2.5 nm for the grain boundaries and the phase boundaries, respectively, independent of additive content and heat-treatment history.     

Multiple angle of incidence ellipsometric analysis of non-absorbing two-layer and three-layer systems

A method known as multiple angle of incidence (MAI) ellipsometry was used to obtain a complete optical analysis of non-absorbing two-layer and three-layer systems deposited onto silicon single-crystal substrates. It is shown that the use of MAI ellipsometry enables the evaluation of all the optical parameters characterizing the multilayer systems mentioned with a high accuracy if the thin films forming these systems are of sufficient thickness and the values of the optical parameters sought are known with sufficient relative accuracy before the application of this method These conclusions are demonstrated by experimental results obtained for samples of the following systems: SiO₂/Si₃N₄, Si₃N₄/SiO₂ and SiO₂/Si₃N₄/MgF₂. The ellipsometric parameters were measured at a wavelength of 632.8 nm.