Comparative study of depth and lateral distributions of electron excitation between scanning ion and scanning electron microscopes

In order to study the contrast difference between scanning ion microscopes (SIM) and scanning electron microscopes (SEM), the depth and lateral distributions of secondary electrons escaped from surfaces of 17 metals with atomic numbers, Z2, of 4-79 were calculated for bombardment with 30 keV Ga ions and for 10 keV electrons. For both projectiles, the excitation depth generally decreased with increasing Z2, while showing the same periodic change as the secondary-electron yield. However, an opposite trend in Z2 dependence between the Ga ion and electron bombardments was calculated with the lateral distribution of secondary electrons escaped from the surface. Except for low Z2 metals, the lateral distribution, which is much narrower for 30 keV Ga ions than for 10 keV electrons, indicates that the spatial resolution of the secondary-electron images is better for SIM than for SEM, if zero-sized probe beams are assumed. Furthermore, the present calculation reveals important effects of electron excitation by recoiled material atoms and reflected electrons on the lateral distribution, as well as the secondary-electron yield, for the Ga ion and electron bombardments, respectively.     

Monte Carlo simulation of topographic contrast in scanning ion microscope

Topographic contrast of secondary-electron (SE) images in a scanning ion microscope (SIM) using a focused gallium (Ga) ion beam is investigated by Monte Carlo simulation. The SE yield of heavy materials, in particular, due to the impact of 30 keV Ga ions increases much faster than for the impact of electrons at < or =10 keV as a function of the angle of incidence of the primary beam. This indicates the topographic contrast for heavy materials is clearer in a SIM image than in a scanning electron microscope (SEM) image; for light materials both contrasts are similar to each other. Semicircular rods with different radii and steps with large heights and a small wall angle, made of Si and Au, are modeled for comparison with SE images in SEM. Line profiles of the SE intensity and pseudo-images constructed from the profiles reveal some differences of the topographic contrast between SIM and SEM. We discuss not only the incident-angle effect on the contrast, but also the effects of re-entrances of primary particles and SEs to the neighboring surface, the effect of a sharp edge on the sample surface, and the effects of pattern size and beam size.     

Origins of material contrast in scanning ion microscope images

A Monte Carlo simulation of ion-induced kinetic electron emission (KE) was carried out to study the material contrast in scanning ion microscope (SIM) images, i.e. secondary electron (SE) yields decreasing with atomic number Z2 of the target, which is opposite to that for scanning electron microscope (SEM) images. The simulations show that SE yields decrease with increasing Z2 for the targets of Al (Z2 = 13), Cu (Z2 = 29) and Au (Z2 = 79) bombarded by 10 approximately 40 keV gallium (Ga) ions. Details of the SE yield according to the collision partners (i.e. Ga ion, recoiled target-atom and excited electron) clarify the origins of material (or Z2) contrast in the Ga-SIM images. Cause and effect on the material contrast are as follows: the heavier (or slower) collision partner transfers less energy to the excited electrons and leads to a poorer multiplication of other excited electrons in the cascade process. The simulation also predicts that the Ga-SIM images are more sensitive to the outermost target surface than the SEM images. material contrast, atomic number contrast, secondary electrons, secondary electron yield, scanning ion microscope, scanning electron microscope     

Monte Carlo simulation for the sputtering yield of Si3N4 thin film milled by focused ion beams

The sputtering yield of the Si3N4 thin film is calculated by Monte Carlo method with different parameters. The dependences of the sputtering yield on the incident ion energy, the incident angle and the number of Gallium (Ga) and Arsenic (As) ions are predicted. The abnormal sputtering yield for As at 90 keV occurs when the incident angle reaches the range between 82°and 84°.     

Monte Carlo simulation for the sputtering yield of Si3N4 thin film milled by focused ion beams

The sputtering yield of the Si3N4 thin film is calculated by Monte Carlo method with different parameters. The dependences of the sputtering yield on the incident ion energy, the incident angle and the number of Gallium （Ga） and Arsenic （As） ions are predicted. The abnormal sputtering yield for As at 90 keV occurs when the incident angle reaches the range between 82° and 84°.     

Noise in secondary electron emission: the low yield case

Studies concerning assessment of the image quality in scanning electron microscopes and studies evaluating the detective efficiency of the secondary electron (SE) detectors in these microscopes must be based on statistics of SE emission. The vast majority of previous studies have applied Poisson statistics, although their prerequisites have not been satisfied in most cases. This paper is concerned with the limits to the applicability of Poisson statistics to SE emission. Adequate definition of a non-Poisson factor in the variance of the number of SEs emitted is discussed, and a simple formula for this factor is derived for the low yield case in which both the primary and the backscattered electron are assumed not to release more than one SE. These conditions are met with conductive specimens composed of light elements at primary electron (PE) energies of tens of keV. For the lightest specimens, such as carbon, the non-Poisson factor can even be neglected for PEs >10 keV.     

Monte Carlo Simulation of Damage Depth in Focused Ion Beam Milling Si3N4 Thin Film

The damage properties of Focused Ion Beam（FIB） milling Si3N4 thin film are investigated by the detailed analyzing images of nanoholes and simulation of Monte Carlo. The damage depth in the Si3N4 thin film for two different ion species（Gallium and Arsenic） under various parameters（ion energy, angle of incidence） are investigated by Monte Carlo method. The simulations show the damage depth increases with the increasing ion energy, the damage depth is dependent on the angle of incident ion, the curves of the damage depth for Ga ion and As ion at 30 keV nearly superpose, while the damage depth for Ga with 90 keV ion is more than that for As ion with the same energy.     

Glancing-angle ion bombardment for modification and monitoring of semiconductor surfaces

Using glancing-angle ion bombardment for surface modification rather than conventional near-normal incidence ions has the advantages of reducing damage and implantation projected ranges, reducing channeling, reducing sputtering, and preferentially removing surface asperities leading to flat surfaces. The effects of bombardment conditions on the surface morphology and perfection of GaAs (001), InP (001), and Si (001) surfaces are reported. Air-exposed surfaces were cleaned and smoothened to near atomic flatness without damage under optimal conditions. Sputtering yield, measured using film thicknesses and changes in reflection high-energy electron diffraction oscillations, decreased with decreasing incidence angle. The low sputtering yield and minimal damage make a glancing-angle geometry ideal for real-time characterization by ion scattering spectroscopy. Surface composition measurements on single monolayers of InAs on GaAs showed that the glancing-angle Ar beam did not measurably change the In coverage over relatively long times. A new ion beam monitoring technique was also developed that utilizes the advantages of glancing-angle ions. Specular scattering of 3 keV He ions was observed for incidence angles of 2–6° from GaAs (001). Oscillation in the specularly scattered ion current during GaAs growth were observed with periods corresponding to monolayer growth times. The oscillations allow a simple quantitative interpretation based on scattering by adatoms and step edges.     

Low energy ion bombardment effects in gold/aluminum nitride/silicon junctions

Aluminum nitride was grown on n-type silicon substrates utilizing chemical vapor deposition. Low energy (5 keV) ion bombardment of methane, argon, and nitrogen was investigated as a method for surface modification prior to metal deposition. Gold contacts were deposited on ion bombarded and as-grown material and a back ohmic contact was formed to the silicon. Current-voltage measurements indicate that bombarded regions exhibit rectifying behavior. Interpretation of these results is presented with comparison of measurements on areas with and without implantation. Samples were also characterized with Raman scattering, x-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoelectric measurements.     

绝缘膜负带电时的表面局部电场与二次电子返回特性

为分析IC多层版图扫描电镜(SEM)对准检测所利用的负带电成像原理,采用Mott弹性散射截面和修正的Bethe非弹性碰撞公式,对点照射入射电子在绝缘膜中的散射过程进行了Monte Carlo模拟,得到负带电绝缘物表面的局部电位分布.在此基础上计算了二次电子从表面出射后在局部场作用下的运动轨迹,获得了SEM像的二次电子信号电流.结果表明,在弱负带电条件下,照射点处表面电位越低,返回表面的二次电子就越多,对应的二次电子信号电流越弱.此结果与SEM实验中图像亮度随照射时间的变化规律相符     

Characterization technique in phase distribution during sample preparation for applications to solder joints and wire-bonding chips

Characterization of solder joint microstructure is essential in evaluation of microelectronic packaging reliability. Analysis techniques and sample preparation play important roles in the material characterization. This study demonstrates several novel approaches in the microstructure evaluation of solder joints. By utilizing precision etching and coating techniques, the interface of the IC packaging chip between the solder and under-bump metallization (UBM) could be revealed much more precisely and the intermetallic compounds were distinctly visible in details by field-emission scanning electron microscopy (FE-SEM). At the Sn-Pb/Ni/Cu interface, the intermetallic compounds defined as Ni₃Sn₄ and Cu₆Sn₅ by electron probe microanalysis (EPMA) were formed between the nickel and the solder. Meanwhile, when the Au-Al wire bonding assembly was etched by the ion beam technique, the interface between gold and aluminum as well as the intermetallic compound could be clearly identified. By adjusting the operating variables, such as etching energy, etching period, and incident angle between the ion beam and sample, the time for sample preparation was significantly reduced. For example, when the cross-section surface of the Sn-Pb solder specimen was perpendicular to the incident ion beam, the highest quality SEM micrographs were obtained at 2.5 keV with an etching time of 7 min. and at 4.5 keV with an etching time of 3 min.     

Electron backscattering diffraction investigation of focused ion beam surfaces

A focused ion beam (FIB) instrument has been used to mill surfaces in single-crystal Si and single-crystal Cu for subsequent electron backscattering diffraction (EBSD) analysis. The FIB cuts were performed using a 30 keV and a 5 keV Ga⁺ ion beam at a stage tilt of 20° to provide a readily obtainable 70° surface for direct EBSD investigation in a scanning electron microscope (SEM). The quality of the patterns is related to the amount of FIB damage induced in the Cu and Si. These or similar methods should be directly transferable to a FIB/SEM dual beam instrument equipped with an EBSD detector.     

Visualization of the interaction between blood vessels and podocytes in the mouse embryonic glomerulus using a barium-infusion method for scanning electron microscopy

The present study revealed three dimensionally the formation of epithelial cells and vascular capillaries in glomeruli of the kidney of the mouse embryo. Barium sulphate was infused through the umbilical vessel of 16.5-17.5-day embryos and the backscattered electron (BSE) image and the secondary electron (SE) image of the identical area of glomeruli were obtained by scanning electron microscopy (SEM). The BSE images directly showed the vascular structure of the glomerulus, while the SE images showed a developmental process of podocytes from epithelial cells. Podocytes were more closely located to the vascular capillary than other epithelial cells. These findings were compared with those obtained in the resin cast model of glomeruli. Thus, the metal infusion-SEM method described here can trace the process of formation of urine filtration barrier in the mouse embryonic glomerulus.     

Cu/Ta/SiO_2/Si多层膜纳米压痕与压痕下微观结构的研究

Cu/Ta/SiO2/Si多层膜结构是目前集成电路制造工艺中的常见结构,其硬度与弹性模量通过纳米压入技术测得。为了表征纳米压痕下的形变微观区域,采用聚焦离子束加工出压痕截面,同时进行扫描电子、扫描离子显微观察,发现样品衬底发生开裂,多层膜结构出现分层现象。TEM分析表明分层出现在Ta/SiO2界面,说明这是该结构的一个薄弱环节。     

Pt/C催化剂颗粒体系SEM成像的计算机模拟

Pt纳米颗粒/C基底体系是典型的纳米催化剂应用体系,本文采用蒙特卡罗方法模拟了该体系的扫描电子显微镜成像。给出了不同尺度的Pt纳米颗粒在C基底中不同深度下二次电子和背散射电子成像的衬度。计算结果显示：(1)在PI／C衬度的形成中,材料的原子序数衬度而不是形貌衬度起了主要作用：(2)只有分布在C基底表面或者表面以下很浅深度内(大约三倍颗粒直径)的Pt颗粒才可以在二次电子信号中被观察到,而背散射信号中则可以观察到更深的Pt颗粒(大约五倍颗粒直径);(3)当颗粒尺度小于几十纳米时。其二次电子信号衬度与通常微米尺度的情形有很大不同,最亮处位于颗粒的中央而不是边缘,且随着颗粒尺度的降低。二次电子产额绝对值也相应降低。     

碳纳米涂层材料的二次电子发射系数及工艺研究

采用电泳沉积制备薄膜的方法,在铝合金镀银片上均匀的涂覆纳米碳涂层。研究了不同的电泳电压及电泳时间对涂层形貌结构的影响。扫描电子显微镜（SEM）、二次电子发射系数（SEY）测试结果表明,保持阴阳极间距为1cm,在30V的直流电压下电泳15min所获得的样片涂层均匀、连续、致密且具有明显的陷阱结构。实验表明,其SEY最大值σmax达到2.03,对应的入射能量为500eV,E1能量点在60eV。使用Ar离子轰击清洗表面10min后,样片的SEY最大值σmax达到1.76,对应的入射能量Emax为400eV,E1能量点在80eV。     

Precise nanofabrication with multiple ion beams for advanced circuit edit

Gallium focused ion beam (Ga-FIB) systems have been used historically in the semiconductor industry for circuit edit. Significant efforts have been invested to improve the performance of Ga-FIB. However, as the dimensions of integrated circuits continue to shrink, Ga-FIB induced processes are being driven to their physical limits. A helium ion beam offers high spatial resolution imaging as well as precise ion machining and sub-10 nm nanofabrication capabilities because the probe size can be brought to as small as 0.25 nm. However, it is limited by its relatively low material removal rate. Recently, the new Zeiss Orion-NanoFab microscope provides multiple ion beams (He, Ne and Ga as an option) into one platform and promotes the further studies of He and Ne induced deposition and etching processes to compare with a Ga ion beam. Because of the mass difference between He, Ne and Ga ions, the interactions of ions with sample surface and precursor molecules result in different sputtering rates, implantation and deposition yields. This presentation gives an overview of our current studies using this new platform to deposit or mill nanostructures for circuit edit.     

Electron-microscopic imaging of single-walled carbon nanotubes grown on silicon and silicon oxide substrates

Direct imaging of single-walled carbon nanotubes (SWNTs) suspended on pillar-patterned Si or SiO2 substrates is investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The suspended nanotubes are successfully observed by direct TEM imaging and it is seen that they have either individual or bundles of SWNTs. Low energy (< or =2 keV) SEM produces high contrast images of suspended SWNTs. On the contrary, when SWNTs contact a SiO2 substrate, they are imaged using electron-beam induced current. The image brightness depends on the length of SWNTs.     

Monte Carlo simulation of the energy dissipation profiles of 30, 50 and 100 keV incident beams in a layered structure in electron beam lithography

This paper presents a Monte Carlo simulation of the energy dissipation profiles of 30, 50 and 100 keV incident beams in thin film (0.4μm) of electron resist, polymethyl methacrylate (PMMA), on thick silicon substrate in electron beam lithography. The radial scattering and the energy loss of incident electrons (including backscattered electrons from the substrate) are simulated under the illumination of ideal point source and Gaussian round beam spot source, and the histories of 30000–50000 electrons are computed.     

电离效应对EBCMOS中电子倍增层增益的影响研究

采用蒙特卡罗模拟方法研究了电子轰击互补金属氧化物半导体(EBCMOS)成像器件中高能电子轰击半导体时产生的电离效应对电荷收集效率和电子倍增层增益的影响。分析了入射电子能量、p型衬底层掺杂浓度、电子倍增层和钝化层厚度对电荷收集效率和增益的影响。结果表明,增加入射电子能量(小于4 keV)、减小电子倍增层和钝化层厚度、降低掺杂浓度等是提高电荷收集效率和电子倍增层增益的有利途径,可为获得高增益的EBCMOS器件提供理论支撑。