Structure Of Silicon Nitride Research Articles

Nanomechanical resonant structures in silicon nitride: fabrication, operation and dissipation issues

We report the fabrication of silicon nitride mechanical devices with lateral dimensions as small as 50 nm. The measured resonant frequencies of these devices (8.5–171 MHz) are the highest reported for silicon nitride structures. We have employed both electrostatic and piezoelectric excitation of these structures. We have also studied the effects of thin metal films on dissipation in these structures and found that the absence of these metal coatings results in a three to four times higher quality factor of the structures.

Imaging of the crystal structure of silicon nitride at 0.8 Ångström resolution

High-resolution transmission electron microscopy is utilized to examine the crystal structure of a silicon nitride ceramic using focus variation methods to achieve sub-ångström resolution at the absolute theoretical information limit of the transmission electron microscope. Specifically, crucial requirements of high instrumental stability, a coherent electron source and optimum imaging conditions have been met by the one-Ångstrom microscope (OÅM) at the National Center for Electron Microscopy in order to obtain a resolution of 0.8 Å. The resulting high-resolution images reveal the individual atom positions of the in-plane projected crystal structure of silicon nitride and permit detailed structural information. The images correspond closely to computed and simulated images of this crystal structure.

Structure and reactivity of amorphous silicon nitride investigated with density-functional methods

We present the first results of our investigation of amorphous silicon nitride using ab initio total energy and molecular dynamics methods. The structural models of a-Si 3N 4 and a-Si 3N 4:H are based on continuous random alternating networks. The strong chemical order and the absence of like-atom bonds such as Si–Si and N–N bonds is maintained even after ab initio molecular dynamic simulations at elevated temperatures. The modeling results in two structures for a-Si 3N 4 with densities of 3.2 and 2.6 g/cm 3 , each consisting of 112 atoms. Both structures have very few topological defects, in total only one three-connected Si is present. Although the low-density structure contains a pore, there is no detectable trend of total energy or average connectivity with density. By discarding selected atoms and terminating dangling bonds with hydrogen, two models of a-Si 3N 4:H were constructed free of under-coordinated atoms and with densities of 2.8 and 2.5 g/cm 3 . The hydrogenated models show less distortion of bond lengths and bond angles in comparison to the hydrogen-free models. The density of states (DOS) of the band gap region indicates the presence of gap states that determine the reactivity of the phase.

Influence of the experimental conditions and the grinding of the starting materials on the structure of silicon nitride synthesised by carbothermal reduction

Silicon nitride has been obtained by carbothermal nitridation of silica using Constant Rate Thermal Analysis (CRTA) method. This method permits to maintain the CO concentration generated in the reaction in a constant value previously selected by the user. The results obtained have shown that the synthesis of α-Si3N4 is improved by increasing the partial pressure of CO in the vicinity of the sample. The previous grinding of the mixture of carbon/silica to be used as raw material in the synthesis of silicon nitride leads to an important reduction of the temperature, at which the carbothermal reduction occurs at the time that an important modification of the particle morphology takes place.

Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: I, Mechanical Characterization

In this Part I of a two‐part study of Hertzian indentation in silicon nitride we characterize irreversible contact damage as a function of microstructure. Three controlled silicon nitride microstructures are examined, representing a progression toward greater long‐crack toughness: fine (F), bimodal with predominantly equiaxed α grains; medium (M), bimodal with mostly β grains of intermediate size; and coarse (C), with almost exclusively elongated β grains. An effect of increasing the microstructural heterogeneity in this sequence is to suppress ring cracking around the indenter, ultimately to a degree beyond that expected from increased toughness alone. Along with the crack suppression is a parallel tendency to enhanced damage accumulation beneath the indenter, such that the contact in the coarsest microstructure is predominantly quasi‐plastic. The characterization of damage includes the following: determination of indentation stress‐strain curves, to measure the level of quasi‐plasticity; measurement of threshold loads for the initiation of ring cracking and subsurface yield, to quantify the competing damage processes; and measurement of characteristic dimensions of the ensuing cracks and deformation zones in their well‐developed stages. These quantitative results are considered in terms of formal contact mechanics, along with finite element modeling to generate the essentially elastic‐plastic fields in the different silicon nitride structures. This contact mechanics description serves also as the basis for subsequent analysis of strength degradation in Part II. Implications concerning microstructural design of silicon nitride ceramics for specific applications, notably bearings, are considered.

MINDO/3 calculation of the electronic structure of silicon nitride

The electronic structure of silicon nitride has been calculated by the semiempirical quantumchemical method MINDO/3 in the cluster approximation. The effect of cluster size and of boundary conditions on the partial density of one-electron states is analyzed. The results of the calculation are compared with experimental data on amorphous silicon nitride. The origin of a peak in the upper part of the valence band, which is seen in the SiL2,3 spectrum but not reproduced in the calculations is discussed.

Remote Plasma-Enhanced CVD of Fluorinated Silicon Nitride Films

An investigation has been made of the effect of dilution of nitrogen with NF3 on the properties of silicon nitride deposited by remote plasma-enhanced CVD (PECVD). For NF3/N2 mixtures with < 1.5% NF3, fluorinated films with the structure of silicon nitride are formed but the total concentration of bonded hydrogen in the films decreases with increasing NF3 flow rate, which leads to an improvement of the electrical properties of the films compared with unfluorinated films. Such films could be of interest for device applications. For NF3/N2 mixtures with NF3 > 1.5 %, films with the structure of silicon dioxide are formed, but they do not contain enough oxygen to give typical SiO2 IR spectra. It is suggested that the silicon dioxide structure arises because of isoelectronic substitution of oxygen by NF.

Electronic structure of amorphous Si 3N 4: experiment and numerical simulation

The electronic structure of amorphous silicon nitride was studied by XPS, X-ray emission, and ELS. The partial densities of states for silicon (3s, 3p, 3d) and nitrogen (2s, 2p) atoms were determined. The results of experiments were compared with the calculated electronic structure of Si 3N 4. The calculations were made using quantum-chemical semiempirical method MINDO/3 in cluster approximation. The top of the Si 3N 4 valence band in terms of band structure is two-degenerated. There are Si3s,3p-N2s,2p bonding states and N2p π nonbonding states at the top of valence band. The electronic structures of Si 3N 4 and SiO 2 are similar. The experimentally energy of plasmon oscillations determined for Si 3N 4 is 23.5 eV. It was found that not only Si3s,3p, N2p orbitals of upper valence band take part into plasmon oscillations but N2s states of lower valence band, too.

Preparation and crystal structure of a new barium silicon nitride, Ba5Si2N6

Single crystals of a new ternary nitride, Ba5Si2N6, were synthesized by slow cooling from 750°C using a starting mixture of Ba, Si, Na and NaN3, where Na and NaN3 were a flux and a nitrogen source respectively. It crystallizes with orthorhombic symmetry: space group P212121 (No. 19), a = 6.159, b = 10.305, c = 15.292 Å, and Z = 4. The crystal structure was determined from single-crystal data and refined to R1 = 0.0495 for all 1637 observed reflections and 89 variables. A pair of SiN4 tetrahedra contained in the structure forms a nitridometallate anion of [Si2N6]10 by edge sharing.

Silicon nitride crystal structure and observations of lattice defects

In view of the considerable progress that has been made over the last 40 years on the microstructural design of silicon nitride and related materials of tailored properties for specific applications, a clear review of the current understanding of the crystal structure and crystal chemistry of silicon nitride is timely. The crystal structures, crystal chemistry, and lattice defect nature of silicon nitride are critically reviewed and discussed, with emphasis placed firstly on the structural nature of α-silicon nitride (whether it is a pure silicon nitride, or should better be regarded as an oxynitride); and secondly on the space group of β-silicon nitride (whether it is P63/m or P63). In conjunction with recent observations of vacancy clusters in α-silicon nitride, a comprehensive view compatible with all the experimental facts with respect to the structural nature of α-silicon nitride is tentatively presented.

Optimizing the Structure and Properties of Silicon Nitride for Rolling Contact Bearing Performance

The total potential for the application of ceramic materials to rolling-element bearings has yet to be defined. There seems to be no doubt, however, that a commercially important segment of technology will benefit from the unique operating capabilities of ceramic and hybrid ceramic/steel bearings. In order to assure reliability and cost effectiveness in these bearings, it is essential that bearing manufacturers have a means to discriminate among the many materials and suppliers that are competing to supply the market with “bearing-quality” silicon nitride. In this program, 11 candidate monolithic and two silicon carbide whisker-reinforced, silicon nitride materials were evaluated with respect to their rolling-contact fatigue performance. When they were tested at a maximum Hertzian contact pressure of 5.930 GPa (860 ksi), with steel loading balls and SAE-20 mineral oil lubricant, the materials exhibited life differences of several orders of magnitude. Attempts to correlate the fatigue performance with such properties as density, hardness and elastic moduli were generally unsuccessful. However, the microstructural features such as porosity, second-phase distribution and, in particular, the crystal size and morphology, showed a good qualitative relationship to the fatigue life and failure mode. Under these relatively high-stress conditions, fatigue durability increased dramatically as the microstructure tended toward finer, more equiaxed grains and a uniform, minimum distribution of second phases.

Atomic force microscopy study of FG-annealed and PECVD silicon nitride AR-coated silicon solar cells

Atomic force microscopy has been used to study the surface structure and roughness of PECVD silicon nitride coated silicon solar cells. The surface roughness increases in some areas of the solar cells after forming gas annealing. It may be one of the reasons for better light absorption on the surface of the solar cells resulting in better solar cell performance.

Electronic structure of silicon nitride studied by both soft x-ray spectroscopy and photoelectron spectroscopy

The valence and conduction band states of crystalline silicon nitride α-Si3N4 have been studied by using two complementary experimental methods. The total valence band distribution has been analyzed by x-ray induced photoelectron spectroscopy. The Si 3p valence states and the Si p conduction states have been probed selectively by x-ray emission and absorption spectroscopies, respectively. The experimental curves have been compared in the same energy scale referred to the Fermi level. Our results show clearly that the N 2pπ states of Si3N4 are located at the top of the valence band while Si 3p states, mixed to N 2p states and, respectively, to N 2s states are located at about EF−8.4 eV and EF−19.6 eV. Our experimental results are in very good agreement with theoretical simulations of the spectra made from recent density of states calculations by Robertson [Philos. Mag. B 63, 47 (1991)] in a tight binding approach.

Near‐field optical microscopy in transmission and reflection modes in combination with force microscopy

SummaryNear‐field optical microscopy is the optical alternative of the various types of scanning probe microscopes. The technique overcomes the classical diffraction limit in conventional optical microscopy. In this paper the concepts of near‐field optics (NFO) are introduced, followed by a short review of current trends in NFO microscopy. Specifically, developments concerning the efficiency and versatility of both aperture and dielectric probe types are discussed. We present our advances in NFO microscopy, using both fibres and integrated silicon nitride (SiN) structures as dielectric probes. The use of an SiN probe as a combined optical and force sensor is shown to be advantageous, as it provides a feedback mechanism and allows direct comparison between topography and dielectric effects. Images of technical and biological samples are presented with a lateral resolution down to 20 nm, depending on the microscopical arrangement used.

Structure and chemistry of silicon nitride and silicon carbonitride thin films deposited from ethylsilazane in ammonia or hydrogen

Structure and chemistry of silicon nitride and silicon carbonitride thin films deposited from ethylsilazane in ammonia or hydrogen

Structure and properties of silicon nitride and [formula omitted] nitride prepared by direct low energy ion beam nitridation

Thin films of silicon nitride, germanium nitride and silicon germanium nitride were formed using direct low energy ion beam nitridation. In this process a monoenergetic nitrogen ion beam directly impinges on the material to be nitrided, in the present work Si(100), Ge/Si(100) and Si 0.89Ge 0.11/Si(100). The energies investigated ranged from 100 eV to 1 keV. Germanium and SiGe alloy were grown on Si(100) using molecular beam epitaxy. The kinetic energy of the ion beam introduces activated nitrogen species athermally into the substrate, and allows the formation of nitrides at low temperatures (20–420°C). Properties of the films such as stress, stoichiometry and microstructure are found to depend strongly on ion energy and substrate temperature. Film stress is highly compressive for samples deposited at room temperature, but decreases with temperature and becomes tensile at 420 °C. Film thicknesses, as measured by cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry, were found to be much greater than the projected range of the ions. The creation of an amorphous layer beneath the amorphous nitride films is observed, and is found to be a strong function of ion energy, temperature and nitrogen ion dose.

Mechanistic Considerations in the Plasma Deposition of Silicon Nitride Films

A stainless steel screen placed directly over the substrate in an RF glow discharge significantly altered plasma deposited (PD) silicon nitride film chemical composition and structure as measured by x‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy. Changes in film composition were more pronounced for PD silicon nitride films formed using as the nitrogen source gas rather than . An increase in the N:Si ratio was observed for all films deposited using the screen. This change in N:Si ratio was also reflected in an increased N‒H and decreased Si‒H bonding structure. These results are similar to those obtained previously in triode and remote plasma reactors, thereby suggesting common deposition mechanisms for films deposited in different reactor configurations.

Effect of dimensional laser machining on the structure and properties of silicon nitride

Effect of dimensional laser machining on the structure and properties of silicon nitride

The effect of composition and exposure to external factors on the electronic structure of amorphous silicon nitride in memory devices

Silicon nitride is widely used in microelectronics as a charge storage layer in MNOS memory devices. It has been studied using soft X-ray emission spectroscopy. To understand fatigue phenomena in memory devices and the nature of localized states (LS) in the band gap, L 2,3 spectra of the valence band (VB) were obtained with attention being paid to the density of LS. a-SiN x and a-SiNx:H samples have been prepared by various methods. They were subjected to an electric field and to ultraviolet irradiation to model the degradation of devices. The investigation of a-SiN x showed that VB and especially LS spectra are very sensitive to composition changes. Increasing the nitrogen content of a-SiN x and a-SiN x:H causes a shift in the LS-spectrum to E c and separation of the maximum into two, as well as changes in the shape of the VB spectrum. It has been found that E field exposure and UV irradiation irradiation the formation of some new SiO bonds in the VB spectrum of SiN x with a shift of the main maximum of the LS spectrum to E c.

The influence of high pressures on structure and properties of silicon nitride

Abstract Silicon nitride as a material with both small coefficient of thermal expansion and high oxidation resistance has long attracted the attention of specialists in various fields of science and technology'. Covalent bonding in this compound is the reason for the use of high temperature sintering.