Some properties of annealed Si3N4 layers on GaAs

We have used Rutherford backscattering and electrical measurements to asses the efficiency of pyrolytically deposited Si₃N₄ as an encapsulant for ion-implanted GaAs.Outdiffusion of gallium and arsenic into the Si₃N₄ layers occurred in all cases, the quantity being dependent on the source of GaAs. However, we conclude that Si₃N₄ layers afford better protection than did the SiO₂ coatings that we previously investigated.     

Effect of the microstructure of Si3N4 on the adhesion strength of TiN film on Si3N4

The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si₃N₄ was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase ( or β) of starting Si₃N₄ powder, and sintering temperature. The microstructure of Si₃N₄ was characterized in terms of grain size, aspect ratio of the elongated grain, and β-to- phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si₃N₄ were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si₃N_4, such as critical loads for yield (P_y) and for ring cracking (P_c). The effect of the microstructure of Si₃N₄ on adhesion strength evaluated by scratch test is discussed. TiN films on Si₃N₄ showed high adhesion strengths in the range of 80–140 N. Hardness and the P_y of Si₃N₄ substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si₃N₄, substrates with finer grain sizes and higher phase ratios, which show high hardness and high P_y, were suitable for higher adhesion strength of TiN film.     

颗粒级配对Si_(3)N_(4)w/Si_(3)N_(4)复合材料弯曲强度与介电性能的影响EI北大核心CSCD

为了获得一种弯曲和介电性能良好的氮化物陶瓷材料,本工作首先以氮化硅晶须(Si_(3)N_(4w))为原料,采用喷雾造粒工艺制备3种具有不同粒径分布的Si_(3)N_(4w)球形颗粒粉体,研究雾化盘转速对Si_(3)N_(4w)球形颗粒粉体粒径分布的影响。然后以喷雾造粒得到的Si_(3)N_(4w)球形颗粒为原料,采用干压法制备3种颗粒级配的Si_(3)N_(4w)预制体,研究颗粒级配Si_(3)N_(4w)预制体的孔径分布。采用化学气相渗透(CVI)和先驱体浸渍裂解(PIP)工艺在3种颗粒级配的Si_(3)N_(4w)预制体中进一步制备Si_(3)N_(4)基体,研究Si_(3)N_(4w)/Si_(3)N_(4)复合材料制备过程中的物相和微结构演变以及颗粒级配对Si_(3)N_(4w)/Si_(3)N_(4)复合材料的微结构、密度、弯曲强度和介电性能的影响。结果表明:3种颗粒级配的Si_(3)N_(4w)预制体均具有二级孔隙特征,其中小孔孔径均约为0.7μm,大孔孔径分别为45.2,30.1μm和21.3μm。在制备的3种颗粒级配的Si_(3)N_(4w)/Si_(3)N_(4)复合材料中,S13样品的颗粒级配效果最好,复合材料的弯曲强度达到81.59 MPa。此外,该样品的介电常数和介电损耗分别为5.08和0.018。良好的弯曲强度和介电性能表明制备的Si_(3)N_(4w)/Si_(3)N_(4)复合材料有望应用于导弹天线罩领域。     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

Hydrogen etching of Si3N4 layers with plasma assisted hot wire CVD

In order to develop sustainable processes for clean manufacturing environment for thin film or other solar cell production, we studied the hydrogen etching of silicon nitride (Si₃N₄) films on flat crystalline silicon (c-Silicon) substrates. With an arrangement primarily constructed for hot wire CVD (HWCVD) deposition of thin silicon films also cleaning processes with atomic hydrogen were studied with a simplified three wire assembly. The three filaments could be biased independently by different potential. A variation of hydrogen pressure and flow was performed to find out conditions of high etching rates for the Si₃N₄ layers. The etching rate was simply determined by measuring the time for total removal of the film, since this could be easily detected by the change of the anti-reflection property. Etching rates of 0.1 nm/s have been obtained under 15 Pa and a flow of 50 sccm. An intensive study was carried out of the direct current (DC) plasma hot wire CVD conditions.     

Preparation of the rod-like β-Si3N4 particles favoring the self-reinforcing Si3N4 ceramics

The β-Si₃N₄ particles were prepared by heating original α-Si₃N₄ powder with rare earth oxide Nd₂O₃ or Yb₂O₃ additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si₃N₄ was also investigated by XRD. The results showed that Yb₂O₃ could accelerate the transformation of Si₃N₄ more effectively than Nd₂O₃ and the powder heated at 1700 °C with over 4 wt.% Yb₂O₃ has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb₂O₃ had ideal β-Si₃N₄ rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si₃N₄ powder to prepare self-reinforced Si₃N₄ ceramic by hot-pressed sintering. The fracture toughness of the seeded Si₃N₄ ceramics increased to 9.1 MPa m^1/2 from 7.6 MPa m^1/2 of the unseeded Si₃N₄ ceramics, while the high value of strength was still kept at 1200 °C.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

Preparation and properties of pressureless-sintered porous Si3N4

Wave-transparent materials used at high temperature environment generated by high supersonic and hypersonic speeds must possess excellent mechanical property. In this paper, porous Si₃N₄ ceramics with high strength were fabricated by low molding pressure (10 MPa) and pressureless sintering process, without any other pore forming agents. The sintering behavior and the effect of porosity on the mechanical strength and dielectric properties were investigated. The flexural strength of porous Si₃N₄ ceramics was up to 57–176 MPa with porosity of 45–60%, dielectric constant of 2.35–3.39, and dielectric loss of 1.6–3.5 × 10⁻³ in the frequency range of 8–18 GHz, at room temperature. With the increase of porosity, the flexural strength, dielectric constant, and dielectric loss all decreased.     

Dense and near-net-shape fabrication of Si3N4 ceramics

With silicon nitride significant progress has been made in order to search for fully dense, strong, reliable structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more ceramic components based on Si₃N₄ are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect-free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs. On the other side, the high hardness of Si₃N₄ ceramics is almost always cost prohibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication processes that can produce complex-shaped components with a minimum of machining as well as to minimize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si₃N₄ ceramics is given in detail. All kinds of these techniques (injection molding, gelcasting, robocasting, mold shape deposition, rapid prototyping) and their advantages and disadvantages are explained.     

Synthesis and photoluminescence of belt-shaped Si3N4 whiskers

Belt-shaped Si₃N₄ whiskers have been synthesized by a carbothermal reduction and nitridation method. The whiskers had an average width of 800 nm, width-to-thickness ratios of 4-6, and a length in the range of several tens of microns to hundreds of microns. Photoluminescence (PL) spectrum of the whiskers showed a strong blue emission peak at 410 nm, and PL lifetime measurement exhibited a rapid decay within a few nanoseconds. The growth of the whiskers was supposed to be dominated by a vapor-solid (VS) mechanism.     

凝胶注模成型制备纳米复合多孔氮化硅陶瓷

采用凝胶注模成型两步法烧结工艺,利用纳米碳粉增强,成功地制备出了具有高强度、结构比较均匀并有较高气孔率的氮化硅多孔陶瓷。借助X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能谱(EDS)、Archimedes法和三点弯曲法等方法对多孔氮化硅陶瓷的微观结构和基本力学性能进行了研究。结果表明:在适当工艺条件下可制成平均强度>100 MPa、气孔率>60%的多孔氮化硅陶瓷。SEM照片显示气孔是由长柱状β-Si₃N₄晶搭接而成的,气孔分布均匀。XRD图谱显示有SiC生成。发育良好的柱晶结构、均匀的气孔分布以及反应生成的SiC微晶是获得高性能的主要原因。      

Comment on the dielectric breakdown properties,current density-voltage and capacitance-voltage characteristics of ion-beam-synthesized Si3N4 layers

Some recent work by Yadav and Joshi on ion-beam-synthesized Si₃N₄ layers has suggested that below the nitride layer an amorphous silicon layer is formed. Electrical conduction through the layers was ascribed to space-charge-limited conduction.It is shown that a fuller analysis of the published capacitance-voltage characteristics indicates that the nitride layer is of approximate thickness 1000 Å, while the amorphous silicon layer may either be absent or of thickness up to 2.3 μm, depending on the annealing conditions. It is confirmed that the current density-voltage characteristics indicate that space-charge-limited conduction takes place, and it is also shown that for a single discrete trapping level the trapping concentration is in the range from 3.3 x 10¹⁹ to 3.6 x 10²⁴ m^-3 for the amorphous silicon and from 3.4 x 10²² to 3.7 x 10²⁷ m^-3 for the nitride; if trapping is via traps exponentially distributed in energy below the conduction band edge the trapping concentrations are 8 x 10²⁷ m^-3 for the amorphous silicon and 1.4 x 10³⁰ m^-3 for the nitride. Although the latter type of trapping distribution is often observed in amorphous insulating layers, it is concluded that the high trapping concentrations calculated in the present case are unlikely to be realistic and that a more complex type of trapping distribution is probably responsible for the effects observed.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

气体流量比对反应溅射Si3N4薄膜的影响

利用射频磁控反应溅射法,以高纯Si为靶材,高纯N2气为反应气体,在Si衬底上制备出了Si3N4薄膜,研究了气体流量比对薄膜质量的影响.结果表明,薄膜的沉积速率主要与气体的流量比有关,随着气体流量比的增加,沉积速率下降,靶面的溅射由金属模式过渡到氮化物模式;薄膜中N/Si的原子比增加;红外吸收谱的Si-N键的振动峰向标准峰逼近.     

Creep behavior of SiC fiber-reinforced hot-pressed Si3N4 composites

The creep response of SiC fiber-reinforced Si₃N₄ composites has been measured using four-point flexural loading at temperatures of 1200–1450°C and stress levels ranging from 250 to 350 MPa. Parameters characterizing the stress and temperature dependence of flexural creep strain rates were determined. A numerical analysis was also performed to estimate the power-law creep parameters for tensile and compressive creep from the bend test data. The incorpoporation of SiC fiber into Si₃N₄ resulted in substantial improvements in creep resistance even at very high stresses. The steady-state creep deformation mechanism, determined to be subcritical crack growth in the unreinforced matrix, changed to a mechanism in the composites of repeated matrix stress relaxation-fiber rupture-load dispersion by the matrix. Multiple fiber fracture rather than multiple matrix cracking resulted. The tertiary creep in the composite resulted from the rapid growth of the microcracks which initiated from the fiber rupture sites. Fiber strength, matrix cracking stress and interfacial shear strength have been identified as the key microstructural parameters controlling the creep behavior of the composite.     

The dielectric breakdown properties,current density-voltage and capacitance-voltage characteristics of ion-beam-synthesized Si3N4 layers

Si₃N₄ layers were synthesized by 30 keV ¹⁴N₂⁺ ion implantation into silicon at room temperature to a total dose of 10¹⁸ ions cm^?2. Studies of the dielectric breakdown strength and the current density-voltage and capacitance-voltage characteristics were carried out on unannealed samples and on samples annealed in various ambients. It was observed that the dielectric breakdown strength of ion-beam-synthesized Si₃N₄ layers decreases after thermal annealing treatments. The currents through these layers were found to be ohmic for low voltages and space charge limited for higher applied voltages. It was found that the electrically dead layer of damaged silicon underlying the ion-beam-synthesized Si₃N₄ dielectric thin film recovers completely to monocrystalline form after vacuum annealing at 900 °C for 2.5 h.     

The effect of Si3N4 on the thermal expansion behavior of MoSi2

The effect of Si₃N₄ particulates on the thermal expansion coefficient (CTE) of MoSi₂ was investigated. It was observed that as the volume percent of Si₃N₄ increases, the CTE of the MoSi₂-Si₃N₄ composites decreases. In the temperature range 1000–1500 °C, typical of that required for glass melting, about 30–35 vol% Si₃N₄ particulates are needed in the MoSi₂-Si₃N₄ composites such that the CTE of the composite matches the CTE of Mo.     

Plasma-enhanced chemically vapour deposited Si3N4 thin films for optical waveguides

Silicon nitride films for applications in optical waveguides have been deposited by plasma-enhanced chemical vapour deposition (PECVD). Index of refraction, deposition rate, buffered HF etch rate and hydrogen content have been measured for different NH₃-to-SiH₄ ratios of precursor gases in the range 0.6–10. Results show that these magnitudes are nearly constant for 2NH₃:SiH₄4. Thermal annealing of films deposited at high NH₃:SiH₄ ratios yields a large reduction (up to 70%) in the N---H bond concentration as well as a densification of the films. Finally, geometrical parameters necessary for the design of rib-type monomode optical waveguides based on the PECVD silicon nitride films were calculated.     

Fabrication and characterization of porous unidirectional Si2N2O-Si3N4 composite

Porous unidirectional Si₂N₂O-Si₃N₄ composite was fabricated by in-situ nitriding of a porous unidirectional Si substrate. The porous unidirectional Si substrate having a diameter of 450 μm, was prepared by forming ethanol bubbles in a slurry which contained Si, Y₂O₃, Al₂O₃ and methylcellulose powder. After nitridation at 1400 °C, the Si substrate was transformed into Si₂N₂O-Si₃N₄ composite and the pore surface of the unidirectional Si₂N₂O-Si₃N₄ composite was covered throughout with Si₂N₂O fibers, which had a diameter of about 55 nm. The Si₂N₂O fibers were orthorhombic single-crystals with an amorphous layer having a thickness of about 1 nm. The compressive strength of the in-situ synthesized Si₂N₂O-Si₃N₄ composite was about 30 MPa.