SEM characterization of silicon nanostructures: can we meet the challenge?

The current semiconductor technology road map for device scaling champions a 4.5 nm gate length in production by 2022. The scanning electron microscope (SEM) as applied to critical dimensions (CD) metrology and associated characterization modes such as electron beam-induced current and cathodoluminescence (CL) has proved to be a workhorse for the semiconductor industry during the microelectronics era. We review some of the challenges facing these techniques in light of the silicon nanotechnology road map. We present some new results using voltage contrast imaging and CL spectroscopy of top-down fabricated silicon nanopillar/nanowires (<100 nm diameter), which highlight the visualization challenge. However, both techniques offer the promise of providing process characterization on the 10-20 nm scale with existing technology. Visualization at the 1 nm scale with these techniques may have to wait for aberration-corrected SEM to become more widely available. Basic secondary electron imaging and CD applications may be separately addressed by the He-ion microscope.     

Imaging and analysis of nanowires

We used vapor-liquid-solid (VLS) methods to synthesize discrete single-element semiconductor nanowires and multicomposition nanowire heterostructures, and then characterized their structure and composition using high-resolution electron microscopy (HRTEM) and analytical electron microscopy techniques. Imaging nanowires requires the modification of the established HRTEM imaging procedures for bulk material to take into consideration the effects of finite nanowire width and thickness. We show that high-resolution atomic structure images of nanowires less than 6 nm in thickness have lattice "streaking" due to the finite crystal lattice in two dimensions of the nanowire structure. Diffraction pattern analysis of nanowires must also consider the effects of a finite structure producing a large reciprocal space function, and we demonstrate that the classically forbidden 1/3 [422] reflections are present in the [111] zone axis orientation of silicon nanowires due to the finite thickness and lattice plane edge effects that allow incomplete diffracted beam cancellation. If the operating conditions are not carefully considered, we found that HRTEM image delocalization becomes apparent when employing a field emission transmission electron microscope (TEM) to image nanowires and such effects have been shown to produce images of the silicon lattice structure outside of the nanowire itself. We show that pseudo low-dose imaging methods are effective in reducing nanowire structure degradation caused by electron beam irradiation. We also show that scanning TEM (STEM) with energy dispersive X-ray microanalysis (EDS) is critical in the examination of multicomponent nanowire heterostructures.     

用湿化学法在石英玻璃衬底上制备ZnO纳米线阵列

运用湿化学法在石英玻璃衬底上制备了ZnO纳米线阵列,用SEM、XRD和分光光度计对其形貌、晶体结构和发光性能进行了表征。结果表明:所制备的ZnO纳米线为六角纤锌矿结构,其直径为60～200nm,长度为0.1～3μm;ZnO纳米线的光致发光(PL)峰值为380nm,在波段为340～380nm时有很强的吸收峰,具有优越的紫外光响应特性;ZnO纳米线阵列具有高度取向性。     

Micro-Raman analysis of stress in machined silicon and germanium

Residual strain in single point diamond machined crystalline silicon and germanium has been measured with high spatial resolution (≈ 2 μm) using Raman microprobe spectroscopy. Raman spectroscopy is a direct, non-destructive technique which provides a spatial resolution down to the excitation wavelength and which may be applied to a wide range of non-conducting materials. Raman scattering was used to measure local strain at various points across single point plunge and feed cuts in crystalline silicon and germanium. Spectra were obtained using various excitation wavelengths (514.5 and 488.0 nm), which, due to their differing penetration lengths in the various materials, can provide depth profiles of the residual stress down to approximately 1 μm. In single point plunge cuts little evidence of surface damage was seen and the residual stresses are compressive. Using a 514.5 nm excitation wavelength, we measure a compressive stress of 250 MPa (2.5 kbar) near the outer edge of a single point plunge cut in silicon. At this wavelength, the penetration depth of the laser is 1.0 × 10⁻⁴ cm. This compressive stress was observed to increase to 600 MPa (6.0 kbar) at a depth of 0.6 × 10⁻⁴ cm which was measured using a 488.0 nm excitation wavelength. In single point feed cuts, regions of heavy fracturing were observed as well as regions of little visible damage. In damaged areas tensile stresses of 200–300 MPa (2.0–3.0 kbar) were measured in silicon while in germanium the tensile stress in such regions is 50–100 MPa (0.5–1.0 kbar). In undamaged areas the stresses are compressive with measured values of 50 and 30 MPa (0.5 and 0.3 kbar) for silicon and germanium respectively.     

Mechanical property measurements of nanoscale structures using an atomic force microscope

This paper describes nanometer-scale bending tests of fixed single-crystal silicon (Si) and silicon dioxide (SiO2) nanobeams using an atomic force microscope (AFM). The technique is used to evaluate elastic modulus of the beam materials and bending strength of the beams. Nanometer-scale Si beams with widths ranging from 200 to 800 nm were fabricated on a Si diaphragm using field-enhanced anodization using an AFM followed by anisotropic wet etching. Subsequent thermal oxidation of Si beams was carried out to create SiO2 beams. Results from the bending tests indicate that elastic modulus values are comparable to bulk values. However, the bending strength appears to be higher for these nanoscale structures than for large-scale specimens. Observations of the fracture surface and calculations of the crack length from Griffith's theory appear to indicate that the maximum peak-to-valley distance on the beam top surfaces influence the values of the observed bending strengths.     

Characterization of dilute species within CVD‐grown silicon nanowires doped using trimethylboron: protected lift‐out specimen preparation for atom probe tomography

Three‐dimensional quantitative compositional analysis of nanowires is a challenge for standard techniques such as secondary ion mass spectrometry because of specimen size and geometry considerations; however, it is precisely the size and geometry of nanowires that makes them attractive candidates for analysis via atom probe tomography. The resulting boron composition of various trimethylboron vapour–liquid–solid grown silicon nanowires were measured both with time‐of‐flight secondary ion mass spectrometry and pulsed‐laser atom probe tomography. Both characterization techniques yielded similar results for relative composition. Specialized specimen preparation for pulsed‐laser atom probe tomography was utilized and is described in detail whereby individual silicon nanowires are first protected, then lifted out, trimmed, and finally wet etched to remove the protective layer for subsequent three‐dimensional analysis.     

含缺陷铝合金厚板预拉伸断裂研究

由于材料中存在空洞等缺陷,铝合金厚板在预拉伸过程中易发生断裂,对拉伸设备造成损伤。基于GTN损伤本构方程,建立铝合金板预拉伸断裂模型。基于小试件拉伸断裂试验测试结合数值模拟,确定GTN损伤本构参数,对五种典型厚度铝合金板预拉伸断裂进行研究。计算铝合金板断裂过程中的应力应变及损伤变量的变化规律,并将表面位置和断裂截面位置处的各变量进行对比。结果表明,随着拉伸率的增大,铝合金板由平面应力状态变为应变集中,其部位由正中心位置向边缘偏移,空洞体积分数最大值也由正中心位置转移到应变集中处;随厚度增加断裂应变略有降低,且受初始空洞体积分数的影响较大。通过对不同厚度、不同拉伸率的铝合金板预拉伸分析,得到初始空洞体积分数和初始缺陷尺寸随厚度变化规律。     

Use of a near-field optical probe to locally launch surface plasmon polaritons on plasmonic waveguides: a study by the finite difference time domain method

We used the finite difference time domain (FDTD) method to study the use of scanning near field optical microscopy (SNOM) to locally excite the nanometric plasmonic waveguides. In our calculation, the light is funneled through a SNOM probe with a sub-wavelength optical aperture and is irradiated on one end of two types of plasmonic waveguides made of 50 nm Au sphere arrays and Au nanowires. The incident light was well localized at one end of the waveguides and consequently propagated toward the other end, due to the excitation of surface plasmon polaritons. We found that the propagation length of the nanosphere array type waveguide varies from 100 to 130 nm depending on the light wavelength, the size of the probe aperture, and the launching heights. Our result shows that reducing the aperture size and using the light of the plasmon resonance wavelength of the nanosphere array could increase the propagation length and, thus, the efficiency of electromagnetic energy transportation through nanosphere arrays.     

Finite element analysis of the resurfaced femoral head

Failure of the resurfaced femoral head may occur in the short term owing to femoral neck fracture or in the long term owing to aseptic loosening as a result of strain shielding. Resurfacing arthroplasties are not all the same. In particular, there is considerable debate regarding the role of the metaphyseal stem and cementing technique. This study examines the influence of various metaphyseal stem configurations (diameter, percentage length in contact with bone, and bonded versus debonded) and cement mantle thickness on the load transfer within the femoral head. Resurfacing resulted in significant strain shielding in the superior femoral head and elevated strain in the superior femoral neck. Although the increase in strain in the femoral neck was significant, the mean strains were below the yield strain for cancellous bone. Peak strains were observed above the yield strain, but they accounted for less than 1 per cent of the total head-neck bone volume and therefore were unlikely to result in femoral neck fracture. Increasing the stem diameter and increasing the percentage stem length in contact with bone both increased the degree of strain shielding. Bonding the metaphyseal stem produced the most dramatic strain shielding, which also extended into the head-neck junction. In contrast, varying the cement mantle thickness had a negligible effect on the load transfer.     

自适应微反射镜阵列制作与性能测试

为了改善微反射镜存在的驱动电压高、驱动力小的缺点,基于双金属效应,制得可调焦微光学自适应阵列。以硅为基底,先后经过热氧化、光刻显影、HF酸刻蚀、KOH湿法刻蚀,溅射铝膜等微加工工艺,制得的4×4阵列,基底厚50μm,硅表面铝膜厚100nm。利用激光数字波面干涉仪对可调焦微反射镜的动态性能进行了测试。实验表明,在0~11V电压范围内,可产生最大7.91μm的连续变形。     

电化学制备多孔硅的工艺对其形貌的影响

采用电化学腐蚀方法,将不同比例的乙醇和质量分数为40%的氢氟酸混合,并以此混合液为腐蚀液,在光照条件下,制备了N型轻掺杂的多孔硅。讨论了不同电化学腐蚀条件对多孔硅结构的影响。研究表明,电流密度、腐蚀时间和氢氟酸质量分数越大时,制备的多孔硅越深,孔径也越大,当以上三者数值过大时会导致多孔硅机械强度急速减弱。由表面形貌可知,当多孔层孔径小于500nm时其机械强度良好,当孔径超过这一阈值尤其是大于800nm时,多孔层骨架则极易断裂。     

Evaluation on the fracture toughness and strength of fiber reinforced brittle matrix composites

It is well known in the fracture mechanics community that the performance of brittle materials, such as different types of ceramics which have low fracture toughness, improves significantly when fibers are added into the material. This is because the presence of fibers deters the crack propagation. Fibers bridge the gap between two adjacent surfaces of the crack and reduce the crack tip opening displacement, thus make it harder to propagate. Several investigators have experimentally studied how the length, diameter and volume fraction of fibers affect the fracture toughness of fiber reinforced brittle matrix composite materials. However, to this date not much work has been done to develope a micro-mechanics based simplified mathematical model of fiber reinforced composites that can quantitatively explain the increase of the fracture toughness and strength of a composite with volume fraction, length and diameter of fibers, used for strengthening the composite, this is what is attempted in this paper.     

Probing the local temperature by in situ electron microscopy on a heated Si3N4 membrane

We present a method allowing us to obtain localized heating that is compatible with high-temperature operation and real time scanning and transmission electron microscopy. Localized heating is induced by flowing current through tungsten nanowires deposited by focused ion-beam-induced deposition on a 50-nm-thick Si₃N₄ membrane. Based on the heat transport between the nanowire and the substrate, we applied an analytical model to obtain the temperature profile as a function of electrical power. In this model, the key parameter is the thermal resistance between the nanowire and the substrate that we determined experimentally by measuring electrical power and local temperature. The local temperature is measured by observing the evaporation of gold nanoparticle by electron microscopy. These in situ heating and temperature-probing capabilities are used to study the crystallization of the Si₃N₄ membrane and the growth of silicon nanowires.     

Size effects in mechanical properties of boron nitride nanoribbons

Size effects in mechanical properties are investigated through molecular dynamics simulations with Tersoff-like potential for boron nitride nanoribbons (BNNRs) in the armchair and zigzag directions under uniaxial tension. It is found that tensile properties of rectangular BNNRs of fixed lengths are significantly affected by the length-width ratios, while these aspect ratios are less sensitive to tensile properties of rectangular BNNRs of fixed widths. Size effects are minor in square boron nitride nanosheets. For zigzag BNNRs of fixed length, Young’s modulus, fracture stress and fracture strain increase when decreasing the width. For armchair BNNRs of fixed length, Young’s modulus and fracture strain increase, while fracture stress varies slightly when decreasing the width. Young’s modulus of zigzag BNNRs decreases from 299 N/m for a very narrow sheet to 258 N/m for square sheets, while this mechanical property increases from 231 N/m to 250 N/m for armchair BNNRs. Fracture stress of zigzag BNNRs decreases from 44.4 N/m for a very narrow sheet to 35.6 N/m for square sheet.     

橡胶纯剪试件断裂力学分析

在橡胶材料纯剪试件中,当裂纹的长度和试件的高度尺寸接近时,应变能释放率与裂纹尺寸无关,由于存在这个特点,纯剪试件经常被用来研究橡胶材料的断裂属性。文中对橡胶纯剪试件在试件中心线上分别存在中心裂纹和边缘裂纹,以及粘接界面分别存在中心裂纹和边缘裂纹的情况进行非线性有限元分析,并将有限元计算的应变能释放率与已存在的解进行比较。对于长裂纹,有限元解与已存在的解吻合较好;但是对于小裂纹,有限元解与已存在的解之间仍然存在一些差异。     

Effect of speed on material removal behavior in scribing of monocrystalline silicon

This paper investigates the effect of scribing speed on the surface morphology and material removal behavior in diamond wire sawing of monocrystalline silicon through specially designed high-speed diamond scribing experiments. High-speed scribing tests are performed on a (100) monocrystalline silicon wafer over a wide range of speeds. The results show that a higher scribing speed is prone to inducing more surface defects such as burrs and tearing in the ductile scribing region, and more radial cracks in the brittle scribing region. The critical scribing depth of ductile-to-brittle transition is found to decrease with increasing scribing speed. A strain rate hardening effect is evident in the experimental data, which explains the underlying mechanism for promotion of brittle fracture at higher scribing speeds.     

悬臂梁式硅纳米谐振器的量子压缩效应

分析了厚度分别为12 nm和38.5 nm的悬臂梁式硅纳米谐振器内由海森堡不确定原理所决定的零点位置不确定度,分析结果表明,零点位置不确定度与悬臂梁的厚度和宽度成反比、与悬臂梁的长度成正比,12 nm厚的悬臂梁其零点位置不确定度为4.1×10-3 nm.结合参量泵量子压缩技术,分析了不同厚度的悬臂梁式硅纳米谐振器的量子压缩系数与器件结构尺寸、温度、泵激电压之间的关系,结果显示,量子压缩系数与温度成正比、与泵激电压成反比.当温度为0.01 K、泵激电压为4V时,12 nm厚的悬臂梁式硅纳米谐振器的量子噪声降低了26.56 dB.该项研究有助于提高极薄悬臂梁式硅纳米谐振器在量子噪声影响下的极限检测精度.     

Density standards for a redetermination of the Avogadro constant

Within the IMGC project for the redetermination of the Avogadro constant (N_A) through the measurement of mean molar mass, lattice spacing and density of a silicon single crystal, progress in the latter field has been achieved by establishing Solid Density Standards (SDS). A contactless interferometer was adopted to measure diameters of glass ceramic spheres. This measurement is then associated with roundness profiles to obtain the mean diameter. Mass and volume of the standards are thus traceable to mass and length standards. SDS allow measurements of about 1 ppm uncertainty, confirmed by many comparisons with standards of other laboratories. Work is in progress on absolute mass and volume measurements of silicon artifacts.     

ON THE NATURE OF MACHINING CRACKS IN GROUND CERAMICS: PART II, COMPARISON TO OTHER SILICON NITRIDES AND DAMAGE MAPS

The size and severity of grinding machining cracks in a sintered reaction bonded silicon nitride are compared to data for other silicon nitrides. Crack sizes follow a similar trend with grinding wheel grit size despite differences in microstructures, strengths, and fracture toughnesses. Silicon nitrides with enhanced fracture toughness actually develop deeper machining cracks than in less tough silicon nitrides. Machining damage maps for silicon nitride are presented.     

Convergent beam electron diffraction extinction distance measurements for quantitative analysis of si(1-x),Ge(x)

A new method to determine the concentration of germanium in Si(1-x) Ge(x) single crystals is presented. It is based on extinction distance measurements by means of convergent beam electron diffraction (CBED). The two-beam condition CBED intensity oscillation (the so-called rocking curve) is measured for the 004 diffracted beam and compared with a numerical simulation. Using the two-beam dynamical diffraction approximation theory, this approach yields very precise values for both specimen thickness and effective extinction distance (Ultramicroscopy 87 (2001) 5). First a theoretical extinction distance zetag(x) for strain relaxed Si(1-x)Ge(x) is calculated assuming a solid solution and using tabulated atomic scattering factors of silicon and germanium atoms. It is found that for single crystals zetag(x) decreases from 156 nm in pure silicon to 90 nm in pure germanium. Measurements on calibrated strain relaxed SiGe layers with variable germanium concentrations show an excellent agreement between experimental and calculated extinction distances zetag(x). As a consequence the experimental extinction distance zetag(x) becomes an indirect measure of the germanium concentration with a 1-2 at % sensitivity. The method turns out to be insensitive to strain as experimental zetag(x) values obtained on strained SiGe layers fit the theoretical extinction distance curve calculated for strain relaxed SiGe.