Excimer-laser ablation and micro-patterning of ceramic Si3N4

3 N₄ has been investigated. The ablation threshold in air, Φ_th, is around 0.3±0.1 J/cm² with ArF- and 0.9±0.2 J/cm² with KrF-laser radiation. With fluences Φ_th<Φ<4 J/cm² the irradiated surface is either very flat or it exhibits a cone-type structure, depending on the number of laser pulses employed. With fluences of 5 to 10 J/cm², the sample surface becomes very smooth, much smoother than the original mechanically polished surface. Pores, scratches, and cracks observed on the non-irradiated surface are absent within the illuminated area. In this regime, the ablation rates are typically 0.1 to 0.2 μm/pulse. Received: 10 April 1997/Accepted: 11 April 1997     

Microstructure and oxidation behaviour of SiC fibre-reinforced Si3N4-AlN-Al2O3-Y2O3 matrix composites

A series of SiC fibre-reinforced Si₃N₄-AlN-Al₂O₃-Y₂O₃ matrix composites with different matrix compositions are fabricated by slurry infiltration followed by hot pressing at 1600°C for 30 min. The diffusion of yttrium and aluminium into fibres is apparent during the high temperature processing. All the as-processed composites show fracture with fibre pull-out. After heat treatment in air at 1000°C for 60 min, composites with minimal Y₂O₃ and Al₂O₃ in the matrix composition demonstrate the fracture behaviour with most extensive fibre pull-out. Composites with the highest aluminium and yttrium oxide content form an yttrium–aluminium–garnet phase and an aluminosilicate glassy phase. The latter phase provides an oxygen diffusion path, resulting in the removal of the carbon-rich interphase by oxidation. This results in catastrophic fracture without fibre pull-out after heat treatment of the composite in air.     

Si3N4的晶体化和ZrN/Si3N4纳米多层膜的超硬效应

研究了Si3N4层在ZrN/Si3N4纳米多层膜中的晶化现象及其对多层膜微结构与力学性能的影响.一系列不同Si3N4层厚度的ZrN/Si3N4纳米多层膜通过反应磁控溅射法制备.利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.结果表明,由于受到ZrN调制层晶体结构的模板作用,溅射条件下以非晶态存在的Si3N4层在其厚度小于0.9 nm时被强制晶化为NaCl结构的赝晶体,ZrN/Si3N4纳米多层膜形成共格外延生长的柱状晶,并相应地产生硬度升高的超硬效应.Si3N4随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.     

Fluoride-doped MWCNT/Si3N4 composite with improved mechanical and structural properties

The addition of carbon nanotubes (CNT) in ceramic composites has stimulated a substantial interest due to their high mechanical, thermal and electrical properties. This approach used fluoride additives (AlF₃ and MgF₂) to prepare multi-walled carbon nanotubes/silicon nitride (MWCNT/Si₃N₄) composite densified at 1700 °C for 1 h by hot press (HP) sintering. The microstructural analyses of MWCNT/Si₃N₄ composites indicate that the fluoride additives have substantially improved densification and the transformation of α-Si₃N₄ to β-Si₃N₄. As observed, the mechanical properties, i.e. flexural strength, fracture toughness, Young's modulus and hardness of MWCNT/Si₃N₄ composites are improved with an increasing concentration of MWCNT. These results attributed to the highly dense composites, strong interfacial interaction and the pull-out mechanism of MWCNT and β-Si₃N₄. The maximum values of fracture toughness flexural strength, Young's modulus, and hardness were 12.76 ± 1.15 MPa.m^0.5, 883 ±46 MPa, 260 ±9 GPa, and 26.4 ± 1.3 GPa, respectively. The improved mechanical properties also ascribed to the synergistic strengthening and toughening influence of MWCNT and β-Si₃N₄.     

Si3N4在h-AlN上的晶体化与AlN/Si3N4纳米多层膜的超硬效应

采用反应磁控溅射法制备了一系列具有不同Si3N4层厚度的AlN/Si3N4纳米多层膜,利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.研究了Si3N4层在AlN/Si3N4纳米多层膜中的晶化现象及其对多层膜生长结构与力学性能的影响.结果表明,在六方纤锌矿结构的晶体AlN调制层的模板作用下,通常溅射条件下以非晶态存在的Si3N4层在其厚度小于约1nm时被强制晶化为结构与AlN相同的赝形晶体,AlN/Si3N4纳米多层膜形成共格外延生长的结构,相应地,多层膜产生硬度升高的超硬效应.Si3N4随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.分析认为,AlN/Si3N4纳米多层膜超硬效应的产生与多层膜共格外延生长所形成的拉压交变应力场导致的两调制层模量差的增大有关.     

Dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me₃Si)₂NH) (Me:CH₃) and SiH₄–C₂H₂, respectively, by a laser-induced gas-phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured at a frequency range of 8.2–12.4 GHz. The real part (′) and imaginary part (″) of the complex permittivity, and dissipation factor (tg δ=″/′) of nano Si/C/N composite powder are much higher than those of nano SiC powder and bulk SiC, Si₃N₄, SiO₂, and Si, especially the tg δ. The promising features of nano Si/C/N composite powder would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si₃N₄ phase. The charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. Because there exists graphite in the nano Si/C/N composite powder, some charge carries are related to the sp³ dangling bonds (of silicon and carbon) and unsaturated sp² carbons. The high ″ and tg δ of nano Si/C/N composite powder were due to the dielectric relaxation. The nano Si/C/N composite powder would be a good candidate for electromagnetic interface shielding material.     

Linear expansion coefficient of the SiC/Si biomorphic composite

In the temperature range 100–650 K, the linear expansion coefficient β was measured for the SiC/Si biomorphic composite, a new cellular ecoceramic fabricated from a porous cellular carbon matrix prepared through pyrolysis of wood (white eucalyptus) in an argon ambient with subsequent infiltration of molten Si into the channels of the matrix and the formation of 3C-SiC. The SiC/Si samples studied had an “excess” ～30% volume concentration of Si and a porosity of ～13–15%. The measurements were conducted on samples cut along (β_∥) and across (β_⊥) the tree growth direction. The measured values of β(T) of SiC/Si are compared with literature data available for the linear expansion coefficients of Si and 3C-SiC.     

Electrical properties of bio-SiC and Si components of the SiC/Si biomorphic composite

The electrical resistivity ρ of bio-SiC, a highly porous cellular material prepared from a biomorphic composite SiC/Si based on white eucalyptus wood through the chemical removal of silicon, was measured in the temperature range 5–100 K. The electrical resistivity of bio-SiC was found to be anisotropic along and across the cellular pores. The activation energy of charge transfer in bio-SiC was estimated. The measured values of ρ for the SiC/Si biomorphic composite and bio-SiC were used to determine the electrical resistivity ρ and the carrier concentration in silicon, which is one of the constituents of the composite.     

Two-band conduction of Si3N4

The kinetics of charge accumulation in a metal-silicon nitride-silicon oxide-semiconductor structure has been investigated theoretically. The results of the performed calculation have been compared with experimental data. For agreement between theory and experiment, the calculation should take into account both the electron transfer and the hole transfer simultaneously. The calculations have predicted that the charge carrier capture cross section should be less than 10^?14 cm².     

Micropipes and voids at β¨SiC/Si(100) interfaces: an electron microscopy study

 The microstructure of β-SiC/Si(100) interfaces generated by carbonization and subsequent growth in a chemical vapor deposition (CVD) reactor was investigated by transmission electron microscopy (TEM). Differently prepared cross section and planar specimens allowed a detailed characterization of interface defects. Besides pyramidal voids, which were frequently reported to appear at SiC/Si interfaces within the substrate, recently discovered micropipes are of special interest. Both kinds of defects form by outdiffusion of silicon during the carbonization process. In contrast to voids. which initially remain empty, micropipes develop by simultaneous ingrowth of SiC. The area densities of micropipes were found to be orders of magnitude higher than those of voids. Micropipe formation may be due to a high density of SiC nuclei preexisting on the substrate surfaces after pretreatments. The simultaneous development of voids and micropipes is discussed on the basis of results obtained from a short-time carbonization experiment. Received: 26 August 1996/Accepted: 20 September 1996     

IR Transmittance of Si/SiO2/Si3N4/Si Insular Structures Formed by Selective Laser Annealing

Journal of Applied Spectroscopy - Periodic Si/SiO2/Si3N4/Si structures with an insular surface layer were formed by selective laser annealing. A study by Fourier infrared spectrometry in the range...     

氮化硅粉末的傅里叶变换红外光谱研究

利用透射法和漫反射法,研究了纯氮化硅粉末和KTBr烯释后样品的红外光谱特征,比较了不同方法获得得的红外光谱的异同,实验发现,纯氮化硅粉末的曼反射光谱在约1200cm^-1处出现一尖锐的强峰,而KBr稀释样品的透光谱和温反射光谱不出现此峰,研究认为,该峰对应氮化硅颗粒表面Si-O键的振动。     

Laser machining of Si3N4 ceramics

Laser machining of ceramics is a promising alternative to diamond-based techniques. Technological problems and influencing factors are exemplified for the structural shaping of silicon nitride ceramics using Nd:YAG pulses of several hundred microseconds. Ablation rate and ablation geometry are discussed in relation to irradiation conditions and microstructure.     

The effects of Si3N4 interlayer on the thermal stability and hardness of Ti/TiNx (x = 0.5-1) nanolayered coatings

The polycrystalline Ti/TiN_x multilayer films were deposited by magnetron sputtering, and the as-deposited multilayer coatings were annealed at 500-800 °C for 2-4 h in vacuum. We investigated the effects of annealing temperature and annealing time on the microstructural, interfacial, and mechanical properties of the polycrystalline Ti/TiN_x multilayer films. It was found that the hardness increased with annealing temperature. This hardness enhancement was probably caused by the preferred crystalline orientation TiN(1 1 1). The X-ray reflectivity measurements showed that the layer structure of the coatings could be maintained after annealing at 500 °C and the addition of the Si₃N₄ interlayer to Ti/TiN_x multilayer could improve the thermal stability to 800 °C.     

Heat capacity and velocity of sound in the SiC/Si biomorphic composite

The specific heat at constant pressure and the velocity of sound in the SiC/Si biomorphic composite prepared from white eucalyptus wood are measured in the range 3.5–65 K and at 77 K, respectively. The heat capacity of the SiC/Si sample under investigation is calculated within three proposed models according to the Kopp-Neumann additivity rule.     

Simulation of range profiles for boron implantation into SiO2/Si and Si3N4/SiO2/Si targets

The simple method of profile combination is shown to be applicable to the simulation of boron profiles in SiO₂/Si and Si₃N₄/Si layered targets. This is demonstrated by comparison with range distributions calculated by more sophisticated theoretical methods, i.e. TRIM Monte Carlo simulations and the algorithm of Christel et al., and with experimental data. The method of profile combination can also be applied to layered targets with a crystalline silicon substrate.     

Interface Microstructures of Si3N4/Fe and Fe-Cr Alloys Joints

Interfaces of solid state bonded Si₃N₄ceramics with Fe and Fe-(5, 10, 15 wt%)Cr alloy interlayers inArgon for 1 h at 1100°, 1200° and 1300°C have beencharacterized by electron probe microanalysis (EPMA) and transmissionelectron microscopy (TEM). Smooth interfaces with no evidence ofreactions products resulted when bonding at 1100°C. However, theinteraction between the ceramic and the metal increased at higherbonding temperatures and Cr-contents. In all samples Si and N fromthe ceramic dissolve and diffuse in the metal interlayer, whereas thesintering additives of the ceramic remain inactive.Low Cr-content (5%) interlayers resulted in the formationof an interfacial zone composed of two sublayer structures; adjacentto the ceramic was a thin one containing the sintering aids ofSi₃N₄ and fine precipitates of Fe₃Si and -Fe₄N in a bcc-Fe matrix. The second was thicker and includedfine Fe₃Si,-Fe₄N and-Fe₂N precipitates in abcc-Fe matrix. The bond region with high Cr-content interlayersincluded three sublayer structures. The first one next to the ceramicwas a bcc-Fe matrix containing sintering aids, fine dispersedFe₃Si and-Fe₄N, and CrN. The secondsublayer was similar but without any segregants from the ceramic. Thethird one, finally had a lamellae structure of Cr₂N/bcc-Fe and the Fe matrix contained also-Fe₄N.The interactionbetween the ceramic and the metal interlayer is believed to becontrolled by the solution rate of N in the alloy foils.     

Self-organized structuring of W/C multilayers on Si substrate

After aging at room temperature for several months W/C multilayers (20 periods, single layer thicknesses in the nanometer range) grown on Si-(111) substrates by pulsed laser deposition (PLD) developed homogeneously wrinkled surfaces. Their structures were studied by optical microscopy, atomic force microscopy and X-ray diffractometry. Typical dimensions of debonded areas are some 100 μm in length, about 40 μm in width and 2–3 μm in height. The formation of wrinkles is accompanied by an increase in the free surface by 1–2%. Stress relaxation is considered the driving force of this phenomenon. Received: 26 July 1999 / Accepted: 29 July 1999 / Published online: 16 September 1999     

Robust design of Si/Si3N4 high contrast grating mirror for mid-infrared VCSEL application

A Si/Si₃N₄ high contrast grating mirror has been designed for a VCSEL integration in mid-infrared (λ = 2.65?μm). The use of an optimization algorithm which maximizes a VCSEL mirror quality factor allowed the adjustment of the grating parameters while keeping large and shallow grating pattern. The robustness with respect to fabrication error has been enhanced thanks to a precise study of the grating dimension tolerances. The final mirror exhibits large high reflectivity bandwidth with a polarization selectivity and several percent of tolerance on the grating dimensions.     

Electrical properties of the SiC/Si composite and the biomorphic SiC ceramic fabricated from spanish beech wood

A study has been made of the dependences of the electrical resistivity and the Hall coefficient on the temperature and magnetic field for the SiC/Si composite fabricated from spanish beech wood and bio-SiC, a high-porosity material formed by chemical extraction of silicon from this composite. The main charge transport parameters of these materials have been determined and analyzed. It has been shown that electric transport in bio-SiC is effected by n-type carriers with a high concentration of ～10¹⁹ cm^?3 and a low mobility of ～1 cm² V^?1 s^?1. The relations obtained have been analyzed by invoking the theory of quantum corrections to conductivity.