硅纳米线拉伸与熔化行为的原子模拟

Molecular dynamics simulations with Stillinger-Weber potential are used to study the tensile and melting behavior of single-crystalline silicon nanowires（SiNWs）.The tensile tests show that the tensile behavior of the SiNWs is strongly dependent on the simulation temperature,the strain rate,and the diameter of the nanowires.For a given diameter,the critical load significantly decreases as the temperature increases and also as the strain rate decreases.Additionally,the critical load increases as the diameter increases.Moreover,the melting tests demonstrate that both melting temperature and melting heat of the SiNWs decrease with decreasing diameter and length,due to the increase in surface energy.The melting process of SiNWs with increasing temperature is also investigated.     

Effects of dopant,temperature, and strain rate on the mechanical properties of micrometer gold-bonding wire

An experimental study on the thermomechanical properties of microelectronic gold-bonding wires was conducted using high-precision, microforce tensile tests. The load-displacement behavior of three types of gold wire (GL-2 type, FA type, and SR type) was carefully recorded for determining the elastic modulus and stress-strain curves. Tests were conducted with miniature specimens at temperatures ranging from room temperature to 250°C and load rates on the order of 1 mm/min and 10 mm/min. The testing results indicated that the tensile strength of all three types of wire decreased with temperature, especially the SR type. The load strain rate has a significant effect on the SR type but little effect on the GL-2 and FA wires. The stress-strain curves from these tests were analyzed to fit into empirical constitutive models that account for the strain, temperature, and strain-rate effects.     

A New Route to Large‐Scale Synthesis of Silicon Nanowires in Ultrahigh Vacuum

An approach for the large‐scale synthesis of high‐purity silicon nanowires (SiNWs) in ultrahigh vacuum is presented. A mixture of Si and SiO₂ is evaporated by an electron beam, and the growth temperature is 700 °C, which is much lower than those used for other oxide‐assisted growths. A new type of single‐crystal SiNWs, with [221] orientation, is thus synthesized. Moreover, it is experimentally demonstrated that SiO intermediates are formed in the process, and the nanowires are obtained via a disproportionation reaction of 2SiO → Si + SiO₂. A growth mechanism is proposed and the critical factors for the formation of 1D nanowires are also determined. The approach is particularly compatible with the mature Si‐based technology, and is favorable for device integration and practical applications.     

A systematic study of the impact of etching time to the sensitivity of SiNW sensor fabricated by MACEtch process

In this paper, silicon nanowires (SiNWs) was fabricated by a combination of metal-assisted chemical etching (MACEtch) and nanosphere lithography. We get the silicon nanowires with different specific surface area by changing the etching time. The microscopic structure of the silicon nanowires is observed by field emission scanning electron microscope (FESEM). The gas sensing performances of the SiNWs with different specific surface area have been systematically examined by measuring the resistance change towards the concentrations of NO₂ in the range of 1–5 ppm at room temperature (RT, 300 K), the gas sensor composed of SiNWs showed perfect gas sensitive property and possessed a short response–recovery time. The main reason of these excellent attributes is quite likely that high specific surface area of the SiNWs, and NO₂ sensing mechanism of the SiNWs was also further explained, which can be attributed to the oxygen in the air and detected NO₂ extract electrons from the surface of the SiNWs, and the resistivity of SiNWs changed with the changing of space-charge layer under the of SiNWs surface.     

Structural and optical properties of silicon nanowires synthesized by Ag-assisted chemical etching

Metal-assisted chemical etching (MACE) of silicon in an aqueous solution of hydrofluoric acid and hydrogen peroxide is established for the fabrication of large-area uniform silicon nanowire (SiNW) arrays. The effect of the silver catalyst layer thickness on the morphology of the synthesized nanostructures and nanowires is investigated. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) reveal that the morphology of the fabricated silicon nanostructures remarkably depends on the catalyst layer thickness, and an optimum layer thickness is necessary for the fabrication of SiNWs. Also the effect of different etching times on the structural and optical properties of the fabricated SiNWs is investigated. FESEM showed a linear increment of the nanowire length and slight diameter changes through different etching times. The ultralow reflectance of SiNWs in the absorption region through the measurement of specular and diffuse reflectance showed that with increase in the etching time, the total reflectance remarkably decreases. A broadband visible photoluminescence (PL) emission from these wires was observed, and it could be stated that the silicon nanocrystals (SiNCs) are mostly responsible for the PL emission. The SiNC sizes were determined by an analytical model through a frequency shift in the Raman spectrum. The synthesized optically-active SiNWs could, therefore, be considered as a promising candidate for a new generation of nanoscale opto-electronic devices.     

基于应变速率的激光喷丸强化6061-T6铝合金力学性能分析

为研究不同应变速率下激光喷丸对6061-T6铝合金力学性能的影响,对标准拉伸试样进行单面和双面激光喷丸强化处理,随后在0.0001～0.1s-1 4种连续应变速率加载条件下,对未喷丸、单面喷丸、双面喷丸3组试样进行了力学性能测试,同时测试了试样表面残余应力,分析了喷丸前后材料表面粗糙度变化及其对延伸率的影响,探讨了激光喷丸后力学性能变化的微观机理。结果表明,抗拉强度(UTS)及屈服强度随应变速率的增加而增大,延伸率随应变速率的增加略微减小;与未处理试样相比,单面激光喷丸后,铝合金的抗拉强度及屈服强度得到小幅提高,延伸率大约降低了1%;双面激光喷丸后,铝合金抗拉强度最大提高了10.8%,屈服强度最大提高了12.5%,延伸率降低了2%左右。激光喷丸区域晶粒得到细化,位错密度得以增加,6061-T6铝合金的力学性能得到改善。     

TEM characterization of Si nanowires grown by CVD on Si pre-structured by nanosphere lithography

Silicon nanowires (SiNWs) were grown on Si(1 0 0) and Si(1 1 1) substrates by chemical vapour deposition (CVD) via the vapour–liquid–solid (VLS) mechanism with small gold particles used as seeds. In order to control the diameter of nanowires, their density on the substrate and their orientation we controlled the size and the distribution of Au seed particles. This was accomplished using nanosphere lithography (NSL) by which regular arrays of Au nanoparticles can be generated. This allowed us to grow single-crystalline SiNWs perpendicular to the surface of Si(1 1 1) substrates. The SiNWs and their Au caps were studied with respect to their morphology and composition using TEM, HREM and EFTEM methods. Clusters of Au are observed along the surface of SiNWs and the existence of a thin Si film on gold particles capping the SiNWs is demonstrated.     

Vertically-tapered silicon nanowire arrays prepared by plasma enhanced chemical vapor deposition: Synthesis,structural characterization and photoluminescence

Vertically aligned silicon nanowires (SiNWs) have been successfully synthesized using pure silane gas as a precursor by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The effect of the growth temperature on the morphology, structure and photoluminescence properties of SiNWs has been studied. The SiNWs were needle-liked materials with the length of a few microns having the diameters of tens of nanometers near the bottom and a few nanometers at the top. Thinner nanowires have been obtained at the higher growth temperature process. High resolution transmission electron microscopy confirms that the nanowires are composed of a crystalline silicon core with an oxide shell. The PL spectrum of the Si nanoneedles have shown two emission bands around 450 nm and ~750, which originate from the defects related to oxygen fault in the oxide shell and interfaces between the crystalline Si core and the oxide shell, respectively.     

Study of the Piezoelectric Power Generation of ZnO Nanowire Arrays Grown by Different Methods

The piezoelectric power generation from ZnO nanowire arrays grown on different substrates using different methods is investigated. ZnO nanowires were grown on n‐SiC and n‐Si substrates using both the high‐temperature vapor liquid solid (VLS) and the low‐temperature aqueous chemical growth (ACG) methods. A conductive atomic force microscope (AFM) is used in contact mode to deflect the ZnO nanowire arrays. No substrate effect was observed but the growth method, crystal quality, density, length, and diameter (aspect ratio) of the nanowires are found to affect the piezoelectric behavior. During the AFM scanning in contact mode without biasing voltage, the ZnO nanowire arrays grown by the VLS method produced higher and larger output voltage signal of 35 mV compared to those grown by the ACG method, which produce smaller output voltage signal of only 5 mV. The finite element (FE) method was used to investigate the output voltage for different aspect ratio of the ZnO nanowires. From the FE results it was found that the output voltage increases as the aspect ratio increases and starts to decreases above an aspect ratio of 80 for ZnO nanowires.     

CdTe纳米线/纳米管薄膜电沉积法制备研究

使用电沉积方法,在碱性体系中Ni箔表面制得直径约为100 nm的CdTe纳米线/纳米管薄膜,并系统研究了电位、镀液浓度及热处理对于该材料微观形貌的影响。XRD结果证明,未经退火处理的薄膜由CdTe相和单质Cd相组成,经300℃N2气氛保护条件下退火4 h后得到的薄膜仅含CdTe相,并有效提高结晶度。SEM结果显示薄膜的微观形貌为纳米线结构,TEM结果进一步证明CdTe纳米线内部为中空结构,形成纳米线/纳米管结构。另外,电沉积实验结果表明,沉积电位对于CdTe纳米线/纳米管结构的形成具有决定作用,表现为阴极电位越正越有利于纳米线结构的形成。     

Formation of Luminescent Silicon Nanowires and Porous Silicon by Metal-Assisted Electroless Etching

Metal-assisted etching of silicon in HF/H₂O₂ aqueous solutions has been used to fabricate luminescent silicon nanowires (SiNWs) and porous silicon. The impact of the gold catalyst layer thickness and the etching solution on the morphology of the synthesized nanostructures and the diameter of the obtained nanowires were systematically investigated. Scanning electron microscopy (SEM) analyses reveal that the morphology of the fabricated structures strongly depends on the composition of the solution and the thickness of the catalyst layer. It has been observed that SiNWs are formed in solutions with H₂O₂ ratios (ξ) below 10 %; increasing the H₂O₂ concentration above this critical value leads to mesoporous (10 % < ξ < 14 %) and macroporous (14 % < ξ < 17 %) structures. Photoluminescence measurements show that SiNWs emit light at about 430 nm. Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analyses were utilized to determine the origin of the emission in the silicon nanostructures. TEM imaging demonstrates that SiNWs are covered by a thin layer of porous silicon, which is assumed to be responsible for their light emission.     

Effects of cooling rate on mechanical properties of near-eutectic tin-lead solder joints

This paper reports the results of a study on the effect of the cooling rate during solidification on the shear creep and low cycle shear fatigue behavior of 60 Sn/40 Pb solder joints, and on bulk solder tensile properties. Solder joints were made with three different initial microstructures by quenching, air-cooling and furnace-cooling. They have similar steady-state strain rates under creep at relatively high shear stresses (i.e. in the matrix creep region) but creep at quite different strain rates at lower shear stresses (i.e. in the grain boundary creep region). These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate. Tensile tests on bulk solders that were cold-worked, quenched and furnace-cooled show that a faster cooling rate decreases the ultimate strength and increases the ductility at low strain rates. The fatigue life of quenched solder joints is shown to be longer than that of the furnace-cooled joints.     

Effect of growth conditions on zinc oxide nanowire array synthesized on Si (100) without catalyst

A uniformly distributed ZnO nanowire array has been grown on silicon (100) substrates by catalyst-free chemical vapor transport and condensation. The effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on growth properties of ZnO nanowire arrays are studied. Scanning electron microscopy, X-ray diffraction, and room temperature photoluminescence are employed to study the structural features and optical properties of the samples. The results show a correlation among experimental growth parameters. There is a zone for substrate temperature, by controlling gas flow rate, that uniformly distributed and well aligned ZnO nanowire arrays can be grown. Also, experiments indicate that ZnO nanowire arrays with different diameter along their length have been formed under various growth conditions in the same distance from source material. It is found that supersaturation is a crucial parameter determining the growth behavior of ZnO nanowire arrays. The growth mechanism of ZnO nanowires is discussed. The room temperature photoluminescence spectrums of ZnO nanowire array show two emission bands. One is the exciton emission band (centered at 380 nm) and the other is a broad visible emission band centered at around 490 nm. As the substrate temperature decreases, the intensity of UV emission increases while the intensity of visible emission peak decreases.     

Surface‐Dominated Transport Properties of Silicon Nanowires

Surface effects are widely recognized to significantly influence the properties of nanostructures, although the detailed mechanisms are rarely studied and unclear. Herein we report for the first time a quantitative evaluation of the surface‐related contributions to transport properties in nanostructures by using Si nanowires (NWs) as a paradigm. Critical to this study is the capability of synthesizing SiNWs with predetermined conduction type and carrier concentration from Si wafer of known properties using the recently developed metal‐catalyzed chemical etching method. Strikingly, the conductance of p‐type SiNWs is substantively larger in air than that of the original wafer, is sensitive to humidity and volatile gases, and thinner wires show higher conductivity. Further, SiNW‐based field‐effect transistors (FETs) show NWs to have a hole concentration two orders of magnitude higher than the original wafer. In vacuum, the conductivity of SiNWs dramatically decreases, whereas hole mobility increases. The device performances are further improved by embedding SiNW FETs in 250 nm SiO₂, which insulates the devices from atmosphere and passivates the surface defects of NWs. Owing to the strong surface effects, n‐type SiNWs even change to exhibit p‐type characteristics. The totality of the results provides definitive confirmation that the electrical characteristics of SiNWs are dominated by surface states. A model based on surface band bending and carrier scattering caused by surface states is proposed to interpret experimental results. The phenomenon of surface‐dependent transport properties should be generic to all nanoscale structures, and is significant for nanodevice design for sensor and electronic applications.     

基于扭转载荷的微尺度CSP焊点应力应变分析与优化

建立了微尺度芯片尺寸封装焊点有限元分析模型并对其进行扭转应力应变仿真分析与实验验证.分析了焊点材料、焊点直径、焊盘直径和焊点高度对焊点扭转应力应变的影响;以焊点材料、焊点直径、焊盘直径和焊点高度为设计变量,采用响应面法设计了29组不同水平组合的焊点模型并获取了相应焊点扭转应力,建立了焊点扭转应力与焊点结构参数的回归方程,结合遗传算法对焊点结构参数进行了优化.结果表明：焊点材料为SAC305时扭转应力应变最大,焊点最大扭转应力应变随焊点直径和焊盘直径增加而减小、随焊点高度增大而增大;最优焊点结构参数水平组合为：焊点材料SAC305、焊点直径0.22mm、焊盘直径0.14mm和焊点高度0.14mm;仿真验证表明最优焊点最大扭转应力下降了3.7MPa.     

激光与外载荷联合加载7075铝合金的实验研究

为了研究7075铝合金在激光与外载荷联合加载下的失效行为, 采用最大拉力50 kN的拉伸伺服试验机与工作波长为1070 nm的6 kW连续光纤激光系统对7075铝合金进行了不同预载荷与不同激光功率密度下的联合加载实验, 获得了该材料的拉应力-时间曲线、温度-时间曲线、失效时间-功率密度曲线、失效温度-功率密度曲线等, 分析了功率密度与预载荷对失效过程、失效温度和断裂形貌的影响。结果表明, 在相同的预载荷下, 激光功率密度的增大会导致失效时间非线性下降, 失效温度是否有较大变化取决于预载荷的大小, 当预载荷大(330 MPa, 440 MPa)时, 失效温度随功率密度增加略有升高, 预载荷较小(110 MPa, 220 MPa)时, 失效温度变化规律不单调; 在相同的激光功率密度下, 预载荷增大, 失效时间减少, 功率密度较大、预载荷较小时, 失效行为变得相似; 在一定的功率密度(315 W/cm2, 351 W/cm2)下, 失效温度随预载荷的增大先增大后减小。该结果进一步揭示了7075铝合金的失效机理。     

Impact of the number of chips on the reliability of the solder balls for wire-bonded stacked-chip ball grid array packages

This investigation examines how the number of chips affects the reliability of solder balls for wire-bonded stacked-chip ball grid array packages under thermal cycling tests. The studied objects were packages with one, two, three and four stacked chips. Three-dimensional finite element analysis was utilized to simulate the stress/strain behavior of all studied packages. Two kinds of properties of 63Sn/37Pb eutectic solder were employed individually in the finite element analyses. One property of the solder was assumed to exhibit the elastic–plastic–creep behavior. Temperature-dependent stress/strain curves and Norton’s steady creep equation were used in the analysis. Another property of the solder governed by the Anand’s viscoplastic model was also employed to describe the behavior of solder balls. The simulation results in the elastic–plastic–creep analyses and viscoplastic analyses reveal that the von Mises stress, the non-linear strain, and the inelastic strain energy density of the critical solder balls increase with the number of stacked chips, but the increments become gradually stable as the number of chips increases. Three fatigue life prediction models—Darveaux’s model, the modified Coffin–Manson model and the creep-fatigue model—were applied to evaluate the fatigue life of the studied packages. Prediction results indicate that the fatigue life of the solder balls decreases as the number of stacked chips increases, and the decrease in predicted life shows stable behavior as the number of chips increases. The stable trend is consistent with experimental observation in the thermal cycling tests. By comparing with the experimental data, it is shown that the Darveaux’s model gives better prediction than the other two models.     

Deformation of Top‐Down and Bottom‐Up Silver Nanowires

Atomistic simulations are employed to probe the deformation behavior of experimentally observed top‐down and bottom‐up face‐centered cubic silver nanowires. Stable, <110> oriented nanowires with a rhombic and truncated‐rhombic cross section are considered, representative of top‐down geometries, as well as the multiply twinned pentagonal nanowire that is commonly fabricated in a bottom‐up approach. The tensile deformation of a stable, experimentally observed structure is simulated to failure for each nanowire structure. A detailed, mechanistic explanation of the initial defect nucleation is provided for each nanowire. The three geometries are shown to exhibit different levels of strength and to deform by a range of mechanisms depending on the nanowire structure. In particular, the deformation behavior of top‐down and bottom‐up nanowires is shown to be fundamentally different. The yield strength of nanowires ranging from 1 to 25 nm in diameter is provided and reveals that in addition to cross‐sectional diameter, the strength of the nanowires is strongly tied to the structure. This study demonstrates that nanowire structure and size may be tailored for specific mechanical requirements in nanometer‐scale devices.     

Characterization study of time- and temperature-dependent mechanical behavior of polyimide materials in electronic packaging applications

The thermo-mechanical testing of the type HPP ST polyimide films with high performance, supplied by Dupont, was realized under different strain rates and temperature effects. Therefore, the rate-temperature-dependent stress-strain behavior of materials was investigated and the dependence of the Young’s modulus on temperature and strain rate was reported. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with the unloading-reloading technique to verify the test results. The constant strain rate uniaxial tensile test and long-time creep test at various temperatures were performed to characterize the time-temperature-dependent mechanical property precisely. The cyclic loading test was also implemented on the specimen to investigate cyclic stress-strain behavior. In addition, the nanoindentation test was carried out at room temperature to validate the elastic modulus derived from the uniaxial tensile test. This research is expected to investigate the time-temperature-dependent mechanical behavior of the polyimide materials for different service regimes including tensile and cyclic mechanical loading under elevated temperature in a systematic manner.     

Structural, electrical and optical properties of zinc oxide produced by oxidation of zinc thin films

We have investigated the effects of oxidation temperature on the physical properties of polycrystalline zinc oxide thin films. Zinc thin films are oxidized at different temperatures in air. We have found that increasing the oxidation temperature deteriorates the preferred c-axis orientation. Also, increasing the oxidation temperature enlarges the crystal size and increases the number of needle-shaped crystals on the surface of the ZnO samples. By increasing the oxidation temperature, more than zinc melting point, tensile stresses start to build up in the films. Also by increasing temperature, sheet resistance of the films decreases, while photoluminescence intensity ratio (green to orange) increases. Increasing the oxidation temperature reduces the transparency of the films, too. It is proposed that either an increase in the number of oxygen vacancies or a decrease in the volume of grain boundaries, is responsible for the observed behavior of the films at higher oxidation temperatures.