Dielectric properties of chemically vapour-deposited Si3N4

The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si₃N₄ were measured in the temperature range from room temperature to 800° C. The a.c. conductivity ( _a.c.) of the amorphous CVD-Si₃N₄ was found to be less than that of the crystalline CVD-Si₃N₄ below 500° C, but became greater than that of the crystalline CVD-Si₃N₄ over 500° C due to the contribution of d.c. conductivity ( _d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si₃N₄ are smaller than those of the crystalline CVD-Si₃N₄ in all of the temperature and frequency ranges examined. The relationships of ^n-1, (- ) ^n-1 and/(- ) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si₃N₄ were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si₃N₄ was smaller than that of the crystalline CVD-Si₃N₄.     

Fractography of chemical vapour-deposited Si3 N4

The fractography of massive amorphous and crystalline chemical vapour-deposited silicon nitride (Pyrolytic-Si₃ N₄) prepared under various deposition conditions using SiCl₄, NH₃ and H₂ as the source gases has been carried out at room temperature in order to clarify the relation between fracture surfaces and structural features. For amorphous Py-Si₃ N₄, three types of fracture surfaces are observed; i.e. (a) a clean contour-like fracture surface, (b) a contour-like fracture surface including black spots and (c) a cone boundary fracture surface. The fracture mode of the crystalline Py-Si₃ N₄ depends greatly on the microstructure. The fracture of fine-grained and low-density Py-Si₃ N₄ occurs intergranularly, probably due to the presence of the undetectable amorphous Py-Si₃ N₄ between grain boundaries, while the coarse-grained and high-density Py-Si₃ N₄ with preferred orientations shows transgranular fracture. The fracture surfaces of massive Py-Si₃ N₄ are made in comparison with those of the varieties of Si₃ N₄ and SiC.     

Microstructure development of Si3N4-TiN composite prepared byin situ compositing

Si₃N₄-TiN nano-composite has been prepared by anin situ compositing method with silicon nitride powder, aluminium iso-propoxide and titanium butoxide as the raw materials. The densification mechanism during hot-pressing and the microstructure of the densified materials were characterized from the point of view of phase composition, TiN inclusion distribution and the interface properties. The dense materials were mainly composed of -sialons, residual -Si₃N₄, silicon oxynitride, and TiN. The materials were featured by the very fine size of the precursor-derived particulate TiN which was homogeneously distributed in the matrix.     

Preparation and dielectric properties of Si3N4/SiCw composite ceramic

Gelcasting was employed to fabricate Si₃N₄/SiC whisker (SiCw) composite ceramics, and the effects of heat-treatment temperature on the length-to-diameter ratio of the whiskers and SiCw content on microwave dielectric properties were studied. Compared with pure SiCw of spherical structure obtained at temperature of 1,750 °C(Ar), pure SiCw treated at 1,600 °C(Ar) showed rod-like structure, higher dielectric properties and more evenly distribution in Si₃N₄/SiCw composite ceramics. Both the real (ε′) and imaginary (ε″) permittivity of Si₃N₄/SiC whisker (SiCw) composite ceramics decreased with increasing frequency and increased as the whisker content raised owing to the interface and SiCw playing a role of dipole in the frequency range of 8.2–12.4 GHz. In addition, comparing the ceramics with lower content of SiCw, the reflectivity of the composite ceramics moved to a lower frequency; the maximum absorption peak reached ?22.4 dB at the whisker content of 15 wt%.     

Microstructure of Si3N4-TiN composites prepared by chemical-vapour deposition

The shapes and the distribution of TiN within Si₃N₄-TiN composites prepared by the chemical-vapour deposition of a SiCl₄-TiCl₄-NH₃-H₂ system have been examined using an electron microscope. The TiN dispersion in the amorphous Si₃N₄ matrix was granular and its maximum size was 3 nm. The TiN dispersions in- and-Si₃N₄ matrices were contained in their respective crystal grains; however, the shape of the TiN dispersions in the-Si₃N₄ matrix was markedly different from that in the-Si₃N₄ matrix. Granular TiN dispersions with an average size of 10 nm were observed in the-Si₃N₄ matrix. On the other hand, the TiN dispersions in the-Si₃N₄ matrix were columnar with a diameter of several nm having its axis extended to the direction parallel to thec-axis of the-Si₃N₄ crystal.     

Preparation and dielectric properties of Si3N4 thin films

Thin Si₃N₄ films of thickness 50–1500 nm were prepared by a low pressure, room temperature chemical vapour deposition process. The dielectric properties of the layers were studied in the frequency range from 10 Hz to 1 MHz and at temperatures between 77 and 400 K. We observed a σ'(ω, T) = A(T)ω^s(T) dependence with s?1 (where σ' is the real part of the a.c. conductivity). For samples prepared in various ways, A varied between a constant value and a superlinear dependence on temperature. At 77 K, however, σ' for all the samples was found to be approximately the same (about 8 × 10^?12 Ω^?1 cm^?1 at 1 kHz) irrespective of the preparation parameters.     

Fracture toughness of Si3N4 and its Si3N4 whisker composite without sintering aids

Si₃N₄ without sintering aids is studied with special interest to the fracture behaviour and its relation to microstructure. Cracks propagated almost transgranularly and no rising R-curve behaviour was found, because crack-wake region gave no contribution on toughening due to very high grain-boundary bonding strength. Microstructure with highly elongated grains was obtained by addition of 20%Si₃N₄ whisker, but fracture toughness was found to be similar to that of the monolithic Si₃IM₄ with equiaxed grains. It is recognized that fracture toughness is not determined simply by apparent microstructural parameters such as mean aspect ratio of grains when grain-boundary bonding is sufficiently strong. Detailed examination of microfracture behaviour is, therefore, necessary for the analysis of toughening in this kind of composites.     

Si3N4／SiC纳米复合陶瓷的微观结构

利用ＪＥＭ２０００ＥＸⅡ高分辨电镜和ＨＦ２０００冷场发射枪透射电镜对Ｓｉ３Ｎ４ＳｉＣ纳米笔合陶瓷材料的微观组织，结构和成分进行了研究。结果表明，ＳｉＣ颗粒弥散分布基体相β－Ｓｉ３Ｎ４晶内和晶界，晶内ＳｉＣ颗粒与基体相的界面结构有三种类型；１）直接结合的的界面；２）完全非晶态的界面；３）混合型的界面，晶间ＳｉＣ颗粒与基体相的界面大部分是直接结合的。     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

化学镀镍Si3N4陶瓷与金属钎焊接头的分析

利用扫描电镜,能谱仪及射线衍射仪对化学镀镍Ｓｉ３Ｎ－Ｑ２３５钢钎焊接头进行了微观分析,分析表明,接头为多层复合结构,陶瓷／镀镍层间发生了界面反应,但反应区在界面上不连续,机械结合仍为陶瓷与金属界面的主要结合方式。     

Preparation and properties of Si3N4/PS composites used for electronic packaging

A kind of polymer composite was fabricated using polystyrene as the matrix and Si₃N₄ powder as filler employing the method of heat press molding. Microstructure, thermal conductivity and dielectric constant of the Si₃N₄ filled composite were evaluated. The effect of the volume fraction of Si₃N₄, the particle size of the polystyrene matrix and the silane treatment of Si₃N₄ filler on the thermal conductivity of the composite was investigated; dielectric constant of the composite was evaluated. The main factors that affect the thermal conductivity of the composite were confirmed through theoretical analyzing of the experimental data and the thermal conductivity model. Experimental results show that with the filler content increasing, a thermally conductive network is formed in the composites, thus the thermal conductivity of the composite increases rapidly. The composites experience a highest thermal conductivity of 3.0 W/m K when the volume fraction of the filler reaches 40%. The increasing of thermal conductivity is dominated by the ease of forming a thermal conductive network. A larger polystyrene particle size, a higher Si₃N₄ filler content and the silane treatment of the filler have a beneficial effect on improving the thermal conductivity. The dielectric constant increases with the content of Si₃N₄ filler, however, it remains at a relatively low lever (<4, at 1 MHz).     

Strength,toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol% SiC whisker-reinforced Si₃N₄ material were determined as a function of temperature from 25 to 1400°C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100°C; however, appreciable strength degradation started at 1200°C and reached a maximum at 1400°C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPam^1/2. It was also observed that the composite material exhibited no rising R-curve behaviour at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si₃N₄ matrix did not provide any favourable effects on strength, toughness and R-curve behaviour.     

蓝宝石和硅衬底上氮化硅薄膜的制备和性能研究

采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,在蓝宝石和硅衬底上制备了氮化硅薄膜.并对Si3N4薄膜的沉积速率、成分、结构及红外光学性能等进行了研究.实验结果表明,沉积薄膜中Si和N的比接近3∶4,形成了Si3N4化合物,呈非晶态结构.制备的Si3N4薄膜的硬度明显高于蓝宝石衬底的硬度,且与蓝宝石衬底结合牢固,可提供良好的保护性能.     

Kinetics and properties of chemically vapour-deposited tungsten films on silicon substrates

The kinetic parameters for the deposition of tungsten films on silicon substrates were evaluated. The reactions (hydrogen reduction or silicon reduction of tungsten hexafluoride) were performed in a resistance-heated horizontal reactor. Film structure and composition were examined by X-ray diffraction, scanning electron microscopy and electron microprobe analysis. The physical, optical, chemical and electrical properties of the films were also measured and are discussed.     

Si3N4基复相陶瓷材料的制备及力学性能

基于高马赫数导弹天线罩为应用背景,以Si粉、BN粉、SiO2粉为主要原料,采用反应烧结法制备Si3N4基复相陶瓷材料,探讨了原料组成以及制备工艺对力学材料性能的影响规律,采用SEM观察了材料的断口形貌.结果表明:当原料中Si、BN和SiO2的含量分别为55%、30%和10%时,采用分段升温烧结工艺制备的材料力学性能最好,强度达到94.3MPa,断裂韧性1.69MPa·m1/2,模量45.2GPa.     

pH-dependent adsorption of Au nanoparticles on chemically modified Si3N4 MEMS devices

Microelectromechanical systems (MEMS) are devices that represent the integration of mechanical and electrical components in the micrometer regime. Self-assembled monolayers (SAMs) can be used to functionalise the surface of MEMS resonators in order to fabricate chemically specific mass sensing devices. The work carried out in this article uses atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) data to investigate the pH-dependent adsorption of citrate-passivated Au nanoparticles to amino-terminated Si₃N₄ surfaces. AFM, XPS and mass adsorption experiments, using ‘flap’ type resonators, show that the maximum adsorption of nanoparticles takes place at pH = 5. The mass adsorption data, obtained using amino functionalised ‘flap’ type MEMS resonators, shows maximum adsorption of the Au nanoparticles at pH = 5 which is in agreement with the AFM and XPS data, which demonstrates the potential of such a device as a pH responsive nanoparticle detector.