Polycrystalline Silicon Nitride Research Articles

Summary of research in NEDO Cat-CVD project in Japan

This paper reviews the recent progress in the catalytic chemical vapor deposition (Cat-CVD) research project, supported by the New Energy and Industrial Technology Development Organization (NEDO). Based on the results obtained in the project, firstly, the deposition mechanism of Cat-CVD is discussed, together with fundamental issues required for making a deposition apparatus. Secondly, the properties of Cat-CVD Si-based thin films such as amorphous silicon (a-Si), polycrystalline silicon (poly-Si), and silicon nitride (SiNx) films are demonstrated. Finally, the feasibility of such films for device application is discussed.

High-density silicon and silicon nitride cones

High-density cone-shaped silicon and silicon nitride have been synthesized on Si(1 0 0) substrates via plasma-assisted hot-filament chemical vapor deposition using a gas mixture of nitrogen, hydrogen and methane. Aligned silicon cones containing 3–10 at% C and N have been formed with less than 1 h growth. Further growth can lead to the increase of cone size and density, as well as to the formation of polycrystalline silicon nitride films on the tip and surface. The formation of these materials is thought to be due to the remodification of Si substrates under the effect of plasma and active C and N species. Different nucleation and growth styles were obtained under different growth conditions and reactive gas mixtures.

Microstructure of Swift Heavy Ion Irradiated SiC, Si3N4 and AlN

ABSTRACTCross-section transmission electron microscopy was used to investigate the microstructure of single crystal silicon carbide and polycrystalline silicon nitride and aluminum nitride following room temperature irradiation with either 245 MeV Kr or 710 MeV Bi ions. The fluences ranged from 1×1012/cm2 (single track regime) to 1×1013/cm2. Ion track formation was observed in the Bi ion-irradiated Si3N4 specimen in regions where the electronic stopping power exceeded a critical value of ∼15 keV/nm (depths <24 μm). Ion track formation was not observed at any depth in 245 MeV Kr ion-irradiated Si3N4, in which the maximum electronic stopping power was 14.5 keV/nm. There was no evidence for track formation in either SiC or AlN irradiated with 710 MeV Bi ions, which indicates that the threshold electronic stopping power for track formation in these two ceramics is >34 keV/nm. The high resistance of SiC and AlN to track formation may be due to their high thermal conductivity, but further study is needed to quantitatively evaluate the suitability of the various track formation models.

Fabrication and electron transport in multilayer silicon-insulator-silicon nanopillars

We report the fabrication of sub-0.1-μm-diam silicon nanopillars with a polycrystalline silicon and silicon nitride multilayer structure. The polycrystalline silicon layers, which are 20 nm thick and are separated by 2–3 nm-thick silicon nitride tunnel barriers, make potentially useful structures for the observation of single-electron charging effects. Measurements of electron transport at 4.2 K between contacts at the top and bottom of the pillars show a zero current region and current steps attributed to Coulomb blockade.

CAT-CVD Process and its Application to Preparation of Si-Based Thin Films

AbstractThis is to review the present understanding on Cat-CVD (catalytic chemical vapor deposition) or hot wire CVD. Firstly, the deposition mechanism in Cat-CVD process is briefly mentioned along with key issues such as the effect of heat radiation and a method to avoid contamination from the catalyzer. Secondly, the properties of Cat-CVD Si-based thin films such as amorphous silicon (a-Si), polycrystalline silicon (poly-Si) and silicon nitride (SiNx) films are demonstrated, and finally, the feasibility of such films for industrial application is discussed.

Microhardness Load Size Effect in Individual Grains of a Gas Pressure Sintered Silicon Nitride

Vickers microhardness as a function of the indentation load and of grain orientation was studied in individual grains of a gas pressure sintered (GPS) polycrystalline silicon nitride using indentation loads in the range from 1 to 50 g. The microhardness values are lower than the microhardness of ß‐Si3N4single crystals, which is probably caused by the softening of the silicon nitride lattice by the presence of oxygen and aluminum atoms. The indentation load size effect (ISE) was more evident in the prismatic planes of the grains. The threshold load for indent formation is below 1 g for both basal and prismatic planes of Si3N4grains in the studied GPS silicon nitride.

Effect of MEMS-compatible thin film hard coatings on the erosion resistance of silicon micromachined atomizers

A comparison of five different MEMS-compatible, thin film, hard coatings has been performed using silicon (Si) micromachined atomizers as test devices. Single-crystal and polycrystalline silicon carbide, silicon nitride, silicon dioxide, and diamond-like carbon were either thermally grown or chemically vapor deposited onto the non-planar topography of the device. Eighteen-hour long, industry-standard erosion tests were performed to qualitatively evaluate the coatings for erosion resistance when exposed to an aggressively contaminated test fluid. No wear was measured on the majority of the swirl chamber floor for the five coatings, though varying degrees of erosive wear were observed at the edge of the exit orifice. The single-crystal silicon carbide demonstrated the best wear performance of all the coatings, exhibiting no noticeable changes of its surface morphology as a function of the erosion tests. The silicon nitride and diamond-like carbon coatings showed the most wear, exhibiting micro-cracking, chipping and flaking at the exit orifice edge.

Inorganic Solid-State Chemistry with Main Group Element Carbodiimides

Organosilicon carbodiimides have been successfully applied as single-source precursor compounds for the synthesis of novel ternary Si-, C-, and N-containing solid phases. Their thermally induced decomposition gives either amorphous silicon carbonitrides or polycrystalline silicon nitride and silicon carbide mixtures, materials that are presently of technological interest for their exceptional hardness, strength, toughness, and high temperature resistance even in corrosive environments. This review is concerned with the synthesis, characterization, and thermal stability of element carbodiimides. The main part of this paper is focused on polymeric silicon-based carbodiimides obtained by the reaction of chloro(organo)silanes with bis(trimethylsilyl)carbodiimide. In the case of RSiCl3, novel poly(silylcarbodiimide) gels are formed. Starting from silicon tetrachloride, new crystalline SiCN phases (namely, SiC2N4 and Si2CN4), have been isolated. Their crystal structures as well as their thermal behavior in the range between room temperature and 1600 °C are discussed. Moreover, preliminary results on the synthesis of germanium- and boron-containing carbodiimides are reported. It is also shown that carbon-based carbodiimides can be obtained by the reaction of cyanuric halides with bis(trimethylsilyl)carbodiimide. These materials are investigated as precursors for the synthesis of new carbon nitrides with high hardness.

Effect of neutron radiation on the dielectric, mechanical and thermal properties of ceramics for rf transmission windows

The behavior of electrically insulating ceramics was investigated before and after exposure to neutron radiation. Mechanical, thermal and dielectric specimens were studied after exposure to a fast neutron dose of 0.1 displacements per atom (dpa) at Oak Ridge National Laboratory (ORNL). Four materials were compared to alumina: polycrystalline spinel, aluminum nitride, sialon and silicon nitride. Mechanical bend tests were performed before and after irradiation. Thermal diffusivity was measured using a room temperature laser flash technique. Dielectric loss factor was measured at 105 MHz with a special high resolution resonance cavity. The materials exhibited a significant degradation of thermal diffusivity and an increase in dielectric loss tangent. The flexural strength and physical dimensions were not significantly affected by the 0.1 dpa level of neutron radiation. The aluminum nitride and S silicon nitride showed superior rf window performance over the sialon and the alumina. The results are compared to radiation studies on similar materials.

Surface micromachining for microelectromechanical systems

Surface micromachining is characterized by the fabrication of micromechanical structures from deposited thin films. Originally employed for integrated circuits, films composed of materials such as low-pressure chemical-vapor-deposition polycrystalline silicon, silicon nitride, and silicon dioxides can be sequentially deposited and selectively removed to build or machine three-dimensional structures whose functionality typically requires that they be freed from the planar substrate. Although the process to accomplish this fabrication dates from the 1960's, its rapid extension over the past few years and its application to batch fabrication of micromechanisms and of monolithic microelectromechanical systems (MEMS) make a thorough review of surface micromachining appropriate at this time. Four central issues of consequence to the MEMS technologist are: (i) the understanding and control of the material properties of microstructural films, such as polycrystalline silicon, (ii) the release of the microstructure, for example, by wet etching silicon dioxide sacrificial films, followed by its drying and surface passivation, (iii) the constraints defined by the combination of micromachining and integrated-circuit technologies when fabricating monolithic sensor devices, and (iv) the methods, materials, and practices used when packaging the completed device. Last, recent developments of hinged structures for postrelease assembly, high-aspect-ratio fabrication of molded parts from deposited thin films, and the advent of deep anisotropic silicon etching hold promise to extend markedly the capabilities of surface-micromachining technologies.

Effect of Low Temperature ion Irradiation on the Microstructure of Nitride Ceramics

AbstractCross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si3N4) and aluminum nitride (AIN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×1020ions/m2(maximum damage of ∼ 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si3N4in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si3N4specimens irradiated at different fluences, it is concluded that the amorphization is primarily controlled by the implanted Si concentration rather than the displacement damage level. Si3N4 amorphization did not occur in regions well-separated from the implanted ions for doses up to at least 3 dpa at 80 K, whereas amorphization occurred in the ion implanted region (calculated Si concentration >0.01 at.%) for damage levels as low as ∼0.6 dpa. The volumetric swelling associated with the amorphization of Si3N4, is <10%. Amorphization was not observed in any of the irradiated AIN specimens. A moderate density of small (∼3 nm) defect clusters were observed in the crystalline damaged regions of both the Si3N4and AIN specimens at both irradiation temperatures. Aligned network dislocations were also observed in the AIN specimen irradiated to high dose at 80 K.

Microstructure and room-temperature mechanical properties of Si3N4 with various α/β phase ratios

After polycrystalline silicon nitride samples with various α/β phase ratios were fabricated by uniaxial hot-pressing under vacuum, their microstructures, α-to-β transformations, and grain-growth mechanisms were then characterized by electron microscopy. Room-temperature fracture toughness (KIC) increased with increasing β-phase content, and a value of 7.0 ± 0.5 MPa ċ m1/2 was obtained for a sample that contained 60 vol% β phase. The increase in KIC is the result of an increase in elongated-grain fractions with increasing β content, which promotes energy absorption by crack deflection, as well as by grain debonding, pullout, and bridging mechanisms.

Nucleation and early growth of CVD diamond on silicon nitride

The initial growth process of CVD diamond on polycrystalline silicon nitride substrate, a typical cutting tool material, was elucidated in some detail. For the generation of diamond crystallites a gas mixture of hydrogen and methane in a microwave plasma-assisted chemical vapour deposition (MW PACVD) system was used. In order to gain insight into the formation and development of nuclei and the early stages of film growth different series of samples were investigated with deposition times between 5 min and 2 h. Owing to the supposed dependence of the process parameters on diamond nucleation and growth, substrate temperature, reactor pressure and CH 4 concentration were varied. An incubation stage, shortened by high CH 4 percentages, prior to the formation of diamond nuclei was proved. The nucleation density is favoured by high CH 4 concentrations and low process pressures. Our results reveal that, independent of the deposition process parameters investigated, the nucleation step is terminated after ∼25 min. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect intermediate layers, which could be formed during the chemical interaction of reactive plasma species and the substrate surface. A tentative model of the early stages of CVD diamond growth is presented.

Interfacial Fracture Between Boron Nitride And Silicon Nitride And Its Applications To The Failure Behavior Of Fibrous Monolithic Ceramics

AbstractMechanical tests show that fibrous monolithic ceramics can exhibit graceful failure in bending due to marked crack deflection at silicon nitride/boron nitride interfaces. Model sandwich specimens were manufactured from polycrystalline silicon nitride with interphases consisting of mixtures of silicon nitride and boron nitride. The interfacial fracture resistances of the specimens were measured, and the results were correlated to the failure behavior of fibrous monolithic ceramics. It was found that the three modes of failure observed in fibrous monoliths can be correlated with the interfacial fracture resistance. In addition, anisotropy in the properties of the interphases were found to have a profound influence on crack deflection.

Structure and photoconduction studies of densified silicon nitride

Photoconduction measurements have been made on commercially available hot-pressed polycrystalline silicon nitride. Characterization of the material was done using X-ray diffraction and scanning electron microscopy, which indicated poor processing and the presence of an unidentified impurity peak, probably due to the formation of an intermetallic compound. The photoconduction band gap obtained from the extrapolation of the photoresponse curves had a value between 4.9 and 5.1 eV for this material. The bandgap thus obtained shows a slow decrease with increasing applied voltage, which appears to be due to field-assisted photo-emission of charge carriers into the conduction band.

Observations of inelastic deformation during high-temperature fatigue and failure of a polycrystalline silicon nitride

Abstract The effects of rate-dependent inelastic deformation were observed during tensile static, dynamic, and cyclic fatigue testing of a hot-isotatically pressed, monolithic, polycrystalline silicon nitride in ambient air at temperatures of 1150, 1260 and 1370°C. Constant stresses in static fatigue ranged from 50 to 300 MPa. Stress rates in dynamic fatigue ranged from 10−4 to 101 MPa/s. Waveforms in cyclic fatigue included trapezoid, triangle and sine at a stress ratio, R, of 0.1, frequencies of 0.1 and 10 Hz, and maximum stresses ranging from 75 to 325 MPa. At 1150°C, all fatigue results showed a similar slow crack growth failure mechanism with no separate cyclic fatigue mechanism. However, at 1260 and 1370°C the failure mechanism was multi-faceted. For both temperatures, the failure in static fatigue was dominated by the accumulation of diffusion-controlled creep cavities. In dynamic fatigue, the inelastic deformation exhibited by non-linear stress-strain curves supported the conclusion of failure by slow crack growth alone at high stress rates and by a mixture of slow crack growth and creep damage at low stress rates. Under cyclic loading, ‘enhanced’ times to failure, attributed to viscous rate effects, occurred regardless of waveform or frequency. The stress rate was related to the stress at the onset to non-linearity, σ0, indicating a quasi-endurance limit below which cyclic fatigue had little ‘enhancing’ effect. Creep compliance relations indicated non-linear viscoelastic (or viscoplastic) behavior, leading to speculation on the role of this behavior in producing the ‘enhanced’ cyclic fatigue resistance.

High Temperature Deformation and Fracture of Polycrystalline Silicon Nitride

High Temperature Deformation and Fracture of Polycrystalline Silicon Nitride

Computer-aided characterization of the elastic properties of thin films

A computerized measuring system is presented which permits determination of the elastic properties of thin films. Membrane theory yields a definite relationship between the deflection of a thin membrane and an external load. From analysis of this function, both Young's modulus and intrinsic stress can be determined independently of each other. The applicability of the measuring method is demonstrated by investigations carried out on rectangular membranes of polycrystalline silicon and silicon nitride of various sizes.

A Comparative Analysis of Brittle Fracture in Amorphous and Polycrystalline Materials

AbstractThe fracture results of an amorphous epoxy resin and polycrystalline silicon nitride tested under monotonically increasing crack mouth opening displacements were compared. A conventional compact tension specimen was used for the epoxy. A new loading system and specimen was designed for the ceramic to permit stable crack growth. Tests conducted on both materials revealed multiple events of subcritical crack growth, dynamic initiation, dynamic growth, and subsequent arrest. The elastic energy release rate at initiation, G11, and arrest, G1a, was evaluated for each event. The value of G11 was found to be higher than G1a for both materials.Also common was the systematic existence of slow growth prior to dynamic initiation. Additionally, both materials showed differences between the fracture surface morphology for slow, 10-6 m/s, and fast, 103 m/s, growth. Although a constant G1a was recorded for the epoxy, large scatter and a possible trend was recorded for G1a in the ceramic.

Scanning electron microscopy of tribochemical films on Si 3N 4 to H 2O at high temperatures

Polycrystalline silicon nitride has been abraded in a ball-on-flat apparatus in a controlled water environment at elevated temperatures up to 1100°C. It was found that the friction coefficient and wear rate were significantly reduced at varying H 2O partial pressures in the temperature range 300–800°C. The high friction and wear values were regained when the temperature was increased; the high temperature transition depended on the water partial pressure. Scanning electron microscopy of those wear tracks, when the friction coefficient was low, showed that the wear groove contained cylindrically shaped debris (rolls) oriented perpendicular to the sliding direction. The “rolls” support the load, lower the friction coefficient and reduce the wear rate.