Ion beam polishing for three‐dimensional electron backscattered diffraction

Serial sectioning by focused ion beam milling for three‐dimensional electron backscatter diffraction (3D‐EBSD) can create surface damage and amorphization in certain materials and consequently reduce the EBSD signal quality. Poor EBSD signal causes longer data acquisition time due to signal averaging and/or poor 3D‐EBSD data quality. In this work a low kV focused ion beam was successfully implemented to automatically polish surfaces during 3D‐EBSD of La‐ and Nb‐doped strontium titanate of volume 12.6 × 12.6 × 3.0 μm. The key to achieving this technique is the combination of a defocused low kV high current ion beam and line scan milling. The line scan was used to restrict polishing to the sample surface and the ion beam was defocused to ensure the beam contacted the complete sample surface. In this study 1 min polishing time per slice increases total acquisition time by approximately 3.3% of normal 3D‐EBSD mapping compared to a significant increase of indexing percentage and pattern quality. The polishing performance in this investigation is discussed, and two potential methods for further improvement are presented.     

Low damage lamella preparation of metallic materials by FIB processing with low acceleration voltage and a low incident angle Ar ion milling finish

Metallic materials are known to be very sensitive to Gallium (Ga) focused ion beam (FIB) processing. Crystal defects formed by FIB irradiation degrade the transmission electron microscope image quality, and it is difficult to distinguish original defects from FIB process-induced damage. A solution to this problem is the low acceleration voltage and low incident angle (LVLA) Argon ion milling, which can be incorporated as an extensional countermeasure for FIB damage removal and eventually for preparation of high-quality lamellae. The transmission electron microscope image quality of iron single crystal could be improved by removing crystal defects using the low acceleration voltage and low incident angle Argon ion milling finish. Lamella quality of the processing result was almost similar with that of the conventional electrolytic polishing. As a practical application of the process, low damage lamella of stainless cast steel could be prepared. Effectiveness of the FIB system equipped with the low acceleration voltage and low incident angle Argon ion milling function as a tool to make high-quality metallic material lamellae is illustrated.     

Analysis of FIB‐induced damage by electron channelling contrast imaging in the SEM

We have investigated the Ga⁺ ion‐damage effect induced by focused ion beam (FIB) milling in a [001] single crystal of a 316 L stainless steel by the electron channelling contrast imaging (ECCI) technique. The influence of FIB milling on the characteristic electron channelling contrast of surface dislocations was analysed. The ECCI approach provides sound estimation of the damage depth produced by FIB milling. For comparison purposes, we have also studied the same milled surface by a conventional electron backscatter diffraction (EBSD) approach. We observe that the ECCI approach provides further insight into the Ga⁺ ion‐damage phenomenon than the EBSD technique by direct imaging of FIB artefacts in the scanning electron microscope. We envisage that the ECCI technique may be a convenient tool to optimize the FIB milling settings in applications where the surface crystal defect content is relevant.     

Reduction of electrical damage in specimens prepared using focused ion beam milling for dopant profiling using off-axis electron holography

GaAs specimens containing p-n junctions have been prepared using focused ion beam (FIB) milling for examination using off-axis electron holography. By lowering the FIB operating voltage from 30 to 8 kV, we have shown a systematic reduction of the electrically 'inactive' thickness from 220 to 100 nm, resulting in a significant increase in the step in phase measured across the junctions as well as an improvement in the signal-to-noise ratio. We also show that the step in phase measured across the junctions can be influenced by the intensity of the electron beam.     

Optimization of EBSD parameters for ultra-fast characterization

Ultra-fast pattern acquisition of electron backscatter diffraction and offline indexing could become a dominant technique over online electron backscatter diffraction to investigate the microstructures of a wide range of materials, especially for in situ experiments or very large scans. However, less attention has been paid to optimize the parameters related to ultra-fast electron backscatter diffraction. The present results show that contamination on a clean and unmounted specimen is not a problem even at step sizes as small as 1 nm at a vacuum degree of 6.1 × 10(-5) Pa. There exists an optimum step size at about 50 data acquisition board units. A new and easy method to calculate the effective spatial resolution is proposed. Effective spatial resolution tends to increase slightly as the probe current increases from 10 to 100 nA. The fraction of indexed points decreases slightly as the frame rate increases from 128 patterns per second (pps) to 835 pps by compensating the probe current at the same ratio. The value 96 × 96 is found to be the optimum pattern resolution to obtain optimum speed and image quality. For a fixed position of electron backscatter diffraction detector, the fraction of indexed points as a function of working distance has a maximum value and drops sharply by shortening the working distance and it decreases slowly with increasing the working distance.     

Minimization of focused ion beam damage in nanostructured polymer thin films

Focused ion beam (FIB) instruments have proven to be an invaluable tool for transmission electron microscopy (TEM) sample preparation. FIBs enable relatively easy and site-specific cross-sectioning of different classes of materials. However, damage mechanisms due to ion bombardment and possible beam heating effects in materials limit the usefulness of FIBs. Materials with adequate heat conductivity do not suffer from beam heating during FIB preparation, and artifacts in materials such as metals and ceramics are primarily limited to defect generation and Ga implantation. However, in materials such as polymers or biological structures, where heat conductivity is low, beam heating can also be a problem. In order to examine FIB damage in polymers we have undertaken a systematic study by exposing sections of a PS-b-PMMA block copolymer to the ion beam at varying beam currents and sample temperatures. The sections were then examined by TEM and scanning electron microscopy (SEM) and analyzed using electron energy loss spectroscopy (EELS). Our empirical results show beam heating in polymers due to FIB preparation can be limited by maintaining a low beam current (≤100 pA) during milling.     

Shaping the lens of the atom probe: fabrication of site specific, oriented specimens and application to grain boundary analysis

The random sampling provided by classical atom probe sample preparation methods is one of the major factors limiting the types of problems that can be addressed using this powerful technique. A focused ion beam enables not only site-specific preparation, but can also be used to give the specimen, which acts as the lens in an atom probe experiment, a specific shape. In this paper we present a technique that uses low accelerating voltages (10 and 5 kV) in the focused ion beam (FIB) to reproducibly produce specimens with selected grain boundaries <100 nm from the tip at any desired orientation. These tips have a high rate of successfully running in the atom probe and no Ga contamination within the region of interest.This technique is applied to the analysis of grain boundaries in a high purity iron wire and a strip-cast steel. Lattice resolution is achieved around the boundary in certain areas. Reconstruction of these datasets reveals the distribution of light and heavy elements around the boundary. Issues surrounding the uneven distribution of certain solute elements as a result of field-induced diffusion are discussed.     

TIMS（Triton）测定质量范围的扩展及其在溴同位素测定中的应用

稳定溴同位素可用来识别和评价地下水的来源、成因及其形成的水文地球化学与物理过程。由于热电离质谱仪Triton的加速电压是固定10 kV不可调节的,因此,其最大的测定质量数为310 u。为了测定质量数更高的Cs₂Br⁺（m/z 345,347）离子,本研究通过对热电离质谱仪Triton加速电压单元的改造,成功地将加速电压由10 kV降低到8 kV,使可测定的离子质量数从310 u扩展到350 u。在此基础上,建立了在8 kV加速电压下相应的质量校正曲线,进行了基于¹³³Cs₂⁷⁹Br⁺（m/z 345）和¹³³Cs₂⁸¹Br⁺（m/z 347）离子的稳定溴同位素静态多接收测量。结果表明：当溴含量低于10 μg时,测定结果偏低;当溴含量高于10 μg时,测定结果正常。该方法测定溴同位素具有涂样量少（10~20 μg Br）,测定外精度高（0.09‰~0.18‰）和采集数据时间短（8 min可采集100 个数据）等优点,可为地质样品中溴同位素地球化学的研究提供参考。     

Machinability study of high speed steel for focused ion beam (FIB) milling process – An experimental investigation at micron/nano scale

Nowadays the attention is focused on machining of non-silicon materials for miniaturized devices. High speed steel (HSS) is a non-silicon tool material, which is used in metal cutting applications as well as in micro-medical applications. Focused ion beam (FIB) milling process is highly suited for the fabrication of micro tools and other micro devices manufactured from HSS material. This study investigates the machinability aspects of HSS for FIB milling process. Beam current, extraction voltage, angle of incidence, dwell time and percentage overlap between beam diameters are the FIB process parameters, which have been analyzed experimentally to optimize FIB milling process for maximum material removal rate and minimum surface roughness. Beam current is found as the most significant parameter for controlling the material removal rate and surface roughness.     

应用聚焦离子束/电子束技术的碳纳米管探针制备工艺研究

提出利用电子束诱导铂沉积和聚焦离子束铣削技术,实现碳纳米管原子力显微镜探针针尖的制备和结构优化研究。结合高分辨率扫描电子显微镜观测和纳米操纵仪,利用电子束诱导铂沉积实现碳纳米管固定到普通原子力显微镜探针末端,可实现直径小于10nm的纳米管探针制备。提出基于聚焦离子束铣削和照射技术实现对纳米管针尖的长度、角度的精确调控优化,纳米管探针的角度调控精度优于1°。     

A variation of transmission electron microscope sample preparation for VLSI analysis

Sample preparation for VLSI analysis is often slow due to long ion milling time and because the location of the thin area of the sample is difficult to control. By modifying the standard techniques used with a VCR Group (and perhaps other) mechanical dimpler, the ion milling time can be reduced to less than 30 min. and the location on the thinned area reasonably controlled. These modifications involve the use of a radiused edge on the dimpling tool, a rubber O-ring on the polishing tool, and not rotating the sample platen during polishing. The modifications to the dimpling and polishing tools allow more control of the geometry of the dimple, while not rotating the sample platen allows a thinner sample to be produced and permits the use of the sample translation micrometers to shift the location of the thinned area during polishing. The quality of samples produced using this modified procedure is equivalent to that obtained with the more standard methods.     

Combining Ar ion milling with FIB lift-out techniques to prepare high quality site-specific TEM samples

Focused ion beam (FIB) techniques can prepare site‐specific transmission electron microscopy (TEM) cross‐section samples very quickly but they suffer from beam damage by the high energy Ga⁺ ion beam. An amorphous layer about 20–30 nm thick on each side of the TEM lamella and the supporting carbon film makes FIB‐prepared samples inferior to the traditional Ar⁺ thinned samples for some investigations such as high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). We have developed techniques to combine broad argon ion milling with focused ion beam lift‐out methods to prepare high‐quality site‐specific TEM cross‐section samples. Site‐specific TEM cross‐sections were prepared by FIB and lifted out using a Narishige micromanipulator onto a half copper‐grid coated with carbon film. Pt deposition by FIB was used to bond the lamellae to the Cu grid, then the coating carbon film was removed and the sample on the bare Cu grid was polished by the usual broad beam Ar⁺ milling. By doing so, the thickness of the surface amorphous layers is reduced substantially and the sample quality for TEM observation is as good as the traditional Ar⁺ milled samples.     

Multiscale iterative voting for differential analysis of stress response for 2D and 3D cell culture models

Focused ion beam-scanning electron microscope (FIB-SEM) tomography is a powerful application in obtaining three-dimensional (3D) information. The FIB creates a cross section and subsequently removes thin slices. The SEM takes images using secondary or backscattered electrons, or maps every slice using X-rays and/or electron backscatter diffraction patterns. The objective of this study is to assess the possibilities of combining FIB-SEM tomography with cathodoluminescence (CL) imaging. The intensity of CL emission is related to variations in defect or impurity concentrations. A potential problem with FIB-SEM CL tomography is that ion milling may change the defect state of the material and the CL emission. In addition the conventional tilted sample geometry used in FIB-SEM tomography is not compatible with conventional CL detectors. Here we examine the influence of the FIB on CL emission in natural diamond and the feasibility of FIB-SEM CL tomography. A systematic investigation establishes that the ion beam influences CL emission of diamond, with a dependency on both the ion beam and electron beam acceleration voltage. CL emission in natural diamond is enhanced particularly at low ion beam and electron beam voltages. This enhancement of the CL emission can be partly explained by an increase in surface defects induced by ion milling. CL emission enhancement could be used to improve the CL image quality. To conduct FIB-SEM CL tomography, a recently developed novel specimen geometry is adopted to enable sequential ion milling and CL imaging on an untilted sample. We show that CL imaging can be manually combined with FIB-SEM tomography with a modified protocol for 3D microstructure reconstruction. In principle, automated FIB-SEM CL tomography should be feasible, provided that dedicated CL detectors are developed that allow subsequent milling and CL imaging without manual intervention, as the current CL detector needs to be manually retracted before a slice can be milled. Due to the required high electron beam acceleration voltage for CL emission, the resolution for FIB-SEM CL tomography is currently limited to several hundreds of nm in XY and up to 650 nm in Z for diamonds. Opaque materials are likely to have an improved Z resolution, as CL emission generated deeper in the material is not able to escape from it.     

Electron backscatter diffraction applied to lithium sheets prepared by broad ion beam milling

Due to its very low hardness and atomic number, pure lithium cannot be prepared by conventional methods prior to scanning electron microscopy analysis. Here, we report on the characterization of pure lithium metallic sheets used as base electrodes in the lithium‐ion battery technology using electron backscatter diffraction (EBSD) and X‐ray microanalysis using energy dispersive spectroscopy (EDS) after the sheet surface was polished by broad argon ion milling (IM). No grinding and polishing were necessary to achieve the sufficiently damage free necessary for surface analysis. Based on EDS results the impurities could be characterized and EBSD revealed the microsctructure and microtexture of this material with accuracy. The beam damage and oxidation/hydration resulting from the intensive use of IM and the transfer of the sample into the microscope chamber was estimated to be <50 nm. Despite the fact that the IM process generates an increase of temperature at the specimen surface, it was assumed that the milling parameters were sufficient to minimize the heating effect on the surface temperature. However, a cryo‐stage should be used if available during milling to guaranty a heating artefact free surface after the milling process. Microsc. Res. Tech., 78:30?39, 2015. © 2014 Wiley Periodicals, Inc.     

Surface damage induced by FIB milling and imaging of biological samples is controllable

Focused ion beam (FIB) techniques are among the most important tools for the nanostructuring of surfaces. We used the FIB/SEM (scanning electron microscope) for milling and imaging of digestive gland cells. The aim of our study was to document the interactions of FIB with the surface of the biological sample during FIB investigation, to identify the classes of artifacts, and to test procedures that could induce the quality of FIB milled sections by reducing the artifacts. The digestive gland cells were prepared for conventional SEM. During FIB/SEM operation we induced and enhanced artifacts. The results show that FIB operation on biological tissue affected the area of the sample where ion beam was rastering. We describe the FIB-induced surface major artifacts as a melting-like effect, sweating-like effect, morphological deformations, and gallium (Ga(+)) implantation. The FIB induced surface artifacts caused by incident Ga(+) ions were reduced by the application of a protective platinum strip on the surface exposed to the beam and by a suitable selection of operation protocol. We recommend the same sample preparation methods, FIB protocol for milling and imaging to be used also for other biological samples.     

Far‐reaching geometrical artefacts due to thermal decomposition of polymeric coatings around focused ion beam milled pigment particles

Focused ion beam and scanning electron microscope (FIB‐SEM) instruments are extensively used to characterize nanoscale composition of composite materials, however, their application to analysis of organic corrosion barrier coatings has been limited. The primary concern that arises with use of FIB to mill organic materials is the possibility of severe thermal damage that occurs in close proximity to the ion beam impact. Recent research has shown that such localized artefacts can be mitigated for a number of polymers through cryogenic cooling of the sample as well as low current milling and intelligent ion beam control. Here we report unexpected nonlocalized artefacts that occur during FIB milling of composite organic coatings with pigment particles. Specifically, we show that FIB milling of pigmented polysiloxane coating can lead to formation of multiple microscopic voids within the substrate as far as 5 μm away from the ion beam impact. We use further experimentation and modelling to show that void formation occurs via ion beam heating of the pigment particles that leads to decomposition and vaporization of the surrounding polysiloxane. We also identify FIB milling conditions that mitigate this issue.     

Post‐thinning using Ar ion‐milling system for transmission electron microscopy specimens prepared by focused ion beam system

We investigate Ar ion‐milling rates and Ga‐ion induced damage on sample surfaces of Si and GaAs single crystals prepared by focused ion beam (FIB) method for transmission electron microscopy observation. The convergent beam electron diffraction technique with Bloch simulation is used to measure the thickness of the Ar‐ion milled samples to calculate the milling rates of Si and GaAs single crystals. The measurement shows that an amorphous layer is formed on the sample surface and can be removed by further Ar‐ion milling. In addition, the local symmetry breaking induced by FIB is investigated using quantitative symmetry measurement. The FIBed‐GaAs sample shows local symmetry breaking after FIB milling, although the FIBed‐Si sample has no considerable symmetry breaking.     

Large-scale process optimization for focused ion beam 3-D nanofabrication

With the ever-decreasing size of manufactured objects, fabrication processes driven by charged particle beams, such as focused ion beam (FIB), become important for a wide spectrum of interdisciplinary applications. A designed three-dimensional (3-D) pattern to fabricate may contain millions of pixels, which will require solving an unprecedented large-scale problem for planning. This paper proposes a general framework of planning FIB milling for fabricating 3-D nanostructures, including model formulations to enable FIB for scalable and automated applications and a corresponding optimization model to support the process planning. The implementation of proposed work does not affect the fabrication quality and yet tremendously reduces the required computational time and data storage during planning. The proposed framework of process planning is further illustrated and verified by simulation and milling experiments of submicron features on Si and Si₃N₄. This research offers an accurate and economical solution to the realization from designs to actual micro/nanoscale models and builds a scientific foundation for immediate development of complex, yet more accurate and cost-effective, beam scanning techniques.     

Dopant profiling of focused ion beam milled semiconductors using off-axis electron holography; reducing artifacts,extending detection limits and reducing the effects of gallium implantation

Focused ion beam (FIB) milling is one of the few specimen preparation techniques that can be used to prepare parallel-sided specimens with nm-scale site specificity for examination using off-axis electron holography in the transmission electron microscope (TEM). However, FIB milling results in the implantation of Ga, the formation of amorphous surface layers and the introduction of defects deep into the specimens. Here we show that these effects can be reduced by lowering the operating voltage of the FIB and by annealing the specimens at low temperature. We also show that the electrically inactive thickness is dependent on both the operating voltage and type of ion used during FIB milling.     

SEM electron channelling patterns as a technique for the characterization of ion-implantation damage

Electron channelling patterns (ECPs) formed in back-scattered images in the scanning electron microscope (SEM) have been used occasionally to confirm surface amorphization during ion implantation. In order to place such observations on a more quantitative basis, the study reported here has explored the variation of ECP appearance with both specimen damage levels (and thus subsurface structures) and SEM accelerating voltage (i.e. sampled depth). Polished and annealed (0001) single crystal sapphire discs were implanted to various damage levels up to both subsurface and full surface amorphization. Damage levels were measured independently by Rutherford back-scattering (RBS). Selected-area ECPs were obtained in a Jeol-840 electron microscope operating over the range 5–40 kV in 5-kV steps. Progressive ECP degradation—in terms of high-order line disappearance—was observed with increasing dose, culminating in total pattern loss when full surface amorphization occurred. However, ECP information could still be obtained from the damaged near-surface material even when a subsurface amorphous layer was present, thus demonstrating the shallow retrieval depth of information from the ECP technique. Indeed, because the spatial distribution of damage from ion implantation is both calculable and measurable, these experiments have also allowed us, for the first time, to explore and demonstrate the shallow sample depths from which the majority of ECP contrast originates (< 150 nm in sapphire at an accelerating voltage of 35 kV), even when the beam penetration is considerable by comparison (～ 5 μm). Furthermore, the way in which this sampled depth varies with SEM accelerating voltage is both demonstrated and shown to be a powerful diagnostic technique for studying the distribution of near-surface structural damage.