Focused Ion Beam Specimen Preparation Research Articles

Direct measurement of TEM lamella thickness in FIB-SEM.

Transmission electron microscope (TEM) specimen preparation by focused ion beam (FIB) milling requires delicate polishing of a thin window of material during the final stages of the procedure. Over or underpolishing is common and requires extra microscope resources to correct. Despite some methods for lamella thickness measurement being available, the majority of users judge the final polishing step subjectively from scanning electron microscope (SEM) images acquired between milling steps. Here we demonstrate successful thickness determination of thin silicon lamellae using calibrated secondary electron detectors in a FIB-SEM dual-beam chamber. Unlike previous thickness measurement methods it does not require long acquisition times, the use of in-chamber scanning transmission electron microscope (STEM) or energy dispersive x-ray spectroscopy detectors. The calibration aligns a SEM image to an electron energy loss spectroscopy (EELS) map of lamella thickness acquired in a TEM. This calibration reveals the greyscale-thickness dependence of two secondary electron SEM detectors: the through-lens detector (TLD) and the in-chamber electron detector (ICE). It was found that lamella thickness estimation for TLD images is accurate for areas thinner than 0.4 t/λ, whilst ICE images are most accurate for areas thicker than 0.5 t/λ up to 1.1 t/λ. The procedure presented here allows objective lamella thickness determination during the final stages of FIB specimen preparation using conventional imaging modes for common secondary electron detectors. LAY DESCRIPTION: Successful analysis of a material in a transmission electron microscope requires very thin windows of the material to be fabricated. Despite the quality of this analysis relying heavily on the thickness of the window, measuring thickness during window fabrication is not common practice. The authors show that it is possible to measure the thickness of the window directly in a focused-ion-beam chamber with a scanning electron microscope without altering the fabrication procedure, and using electron detectors common to most microscopes.

Plan-view transmission electron microscopy specimen preparation for atomic layer materials using a focused ion beam approach

Using the focused ion beam (FIB) to prepare plan-view transmission electron microscopy (TEM) specimens is beneficial for obtaining structural information of two-dimensional atomic layer materials, such as graphene and molybdenum disulfide (MoS2) nanosheets supported on substrates. The scanning electron microscopy (SEM) image in a dual-beam FIB-SEM can accurately locate an area of interest for specimen preparation. Besides, FIB specimen preparation avoids damages and hydrocarbon contamination that are usually produced in other preparation methods, in which chemical etching and polymer adhesion layers are used. In order to reduce harmful ion-beam bombardment and re-deposition on the thin atomic layers during FIB specimen preparation, we develop a method to protect the atomic layers by making a “microcapsule” to insulate the sample surface. The method is applied respectively to prepare plan-view TEM specimens of a graphene sheet with multiple adlayers and MoS2 atomic layers. Useful electron diffraction results can be obtained from these specimens for understanding the interlayer orientation relationships in the two materials. Auger electron spectroscopy analysis further confirms that the sample surface is free from contamination under the sufficient protection given by the proposed method.

Correlative Raman spectroscopy and focused ion beam for targeted phase boundary analysis of titania polymorphs

Site-specific preparation of specimens using focused ion beam instruments for transmission electron microscopy is at the forefront of targeting regions of interest for nanoscale characterization. Typical methods of pinpointing desired features include electron backscatter diffraction for differentiating crystal structures and energy-dispersive X-Ray spectroscopy for probing compositional variations. Yet there are situations, notably in the titanium dioxide system, where these techniques can fail. Differentiating between the brookite and anatase polymorphs of titania is either excessively laborious or impossible with the aforementioned techniques. However, due to differences in bonding structure, Raman spectroscopy serves as an ideal candidate for polymorph differentiation. In this work, a correlative approach utilizing Raman spectroscopy for targeted focused ion beam specimen preparation was employed. Dark field imaging and diffraction in the transmission electron microscope confirmed the region of interest located via Raman spectroscopy and demonstrated the validity of this new method. Correlative Raman spectroscopy, scanning electron microscopy, and focused ion beam is shown to be a promising new technique for identifying site-specific preparation of nanoscale specimens in cases where conventional approaches do not suffice.

A Small Spot, Inert Gas, Ion Milling Process as a Complementary Technique to Focused Ion Beam Specimen Preparation.

This paper reports on the substantial improvement of specimen quality by use of a low voltage (0.05 to ~1 keV), small diameter (~1 μm), argon ion beam following initial preparation using conventional broad-beam ion milling or focused ion beam. The specimens show significant reductions in the amorphous layer thickness and implanted artifacts. The targeted ion milling controls the specimen thickness according to the needs of advanced aberration-corrected and/or analytical transmission electron microscopy applications.

Optimisation of sample preparation and analysis conditions for atom probe tomography characterisation of low concentration surface species

The practicalities for atom probe tomography (APT) analysis of near-surface chemistry, particularly the distribution of low concentration elements, are presented in detail. Specifically, the challenges of surface analysis using APT are described through the characterisation of near-surface implantation profiles of low concentration phosphorus into single crystal silicon. This material system was chosen to illustrate this surface specific approach as low concentration phosphorus has significant mass spectra overlaps with silicon species and the near surface location requires particular attention to focused ion beam specimen preparation and deposition of various capping layers. Required changes to standard sample preparation procedure are described and the effects of changes in APT analysis parameters are discussed with regards to this specific material system. Implantation profiles of 14 kV phosphorus ions with a predicted peak concentration of 0.2 at .% were successfully analysed using APT using pulsed laser assisted evaporation. It is demonstrated that the most important factor in obtaining the most accurate implantation profile was to ensure all phosphorus mass peaks were as free of background noise as possible, with thermal tails from the Si2+ ions obscuring the P2+ ions being the major overlap in the mass spectrum. The false positive contribution to the phosphorus profiles from hydride species appears minimal at the capping layer/substrate interface. The initial capping layer selection of nickel was successful in allowing the analysis of the majority of the phosphorus profile but nickel and phosphorus mass spectra overlaps prevent optimum quantification of phosphorus at the surface.

Atom probe study of radiation induced grain boundary segregation/depletion in a Fe-12%Cr alloy

Ferritic steels are important candidate structural materials for future fission and fusion reactors. However, the effect of radiation induced segregation/depletion (RIS/RID) of Cr in grain boundaries and its effect on microstructures and properties are still not clear. Therefore, a systematic approach is shown in this paper, combining electron backscattered diffraction, focused ion beam specimen preparation and atom probe tomography for analysing a single grain boundary in a Fe-12wt%Cr from the point of grain boundary orientation, microchemistry and impurities. Several samples have been prepared from the same grain boundary and consistent 3D reconstruction with quantitative calculation of segregation demonstrates the high reliability and repeatability of this approach.

A systematic approach for the study of radiation-induced segregation/depletion at grain boundaries in steels

High Cr ferritic steels are candidate materials for structural applications in Gen-IV and fusion nuclear reactors. However, the relative contributions of irradiation conditions and materials microstructures on radiation-induced segregation or depletion of Cr at grain boundaries in ferritic steels are unclear. Here, the possibility of systematically analyzing the chemistry of the same grain boundary of known character during irradiation is demonstrated using a combination of electron back-scattered diffraction, atom-probe tomography and focused ion beam specimen preparation. This method provides a dynamic evolution of grain boundary chemistry as function of dose, spatial variations within the grain boundary plane, and quantification of minor solute elements such as carbon otherwise difficult to obtain experimentally.

Focused ion beam specimen preparation for off-axis electron holography using Si, Ga, and Au ions

Silicon specimens containing p-n junctions have been prepared by focused ion beam milling using gallium, silicon, and gold ion sources. The specimens have been examined using off-axis electron holography and the total thickness of the electrically ‘inactive’ layer has been measured as 80, 140, and 410±15nm for the Au, Ga, and Si ions, respectively. We also show that the thickness of this damaged layer can be predicted using simulations.

Quantification in dynamic SIMS: Current status and future needs

Dynamic SIMS has made great strides in quantification. The use of ion-implanted standards has made quantification routine for any element in a wide range of matrices. Both contaminant and matrix measurements can be reliably quantified in the same depth profile for selected multi-element substrates, such as SiGe and AlGaN. Dose measurements with less than 1% reproducibility have been demonstrated for magnetic sector, quadrupole, and time-of-flight instruments. Progress has been made in quantification at the surface, at interfaces, in thin layers, in insulators, within small areas, and for two and three dimensions. Current challenges include quantification in a proliferation of layers, interfaces, and elements, and obtaining quantitative information from the smallest possible area. Analyses using complementary techniques will be useful for these difficult SIMS measurements. Focused ion beam specimen preparation has shown promise for small area and three-dimensional analyses. Developments in ion sources show potential for small area analysis using focused ion beam SIMS.

High-Quality Sample Preparation by Low kV FIB Thinning for Analytical TEM Measurements

Focused ion beam specimen preparation has been used for NiTi samples and SrTiO3/SrRuO3 multilayers with prevention of surface amorphization and Ga implantation by a 2-kV cleaning procedure. Transmission electron microscopy techniques show that the samples are of high quality with a controlled thickness over large scales. Furthermore, preferential thinning effects in multicompounds are avoided, which is important when analytical transmission electron microscopy measurements need to be interpreted in a quantitative manner. The results are compared to similar measurements acquired for samples obtained using conventional preparation techniques such as electropolishing for alloys and ion milling for oxides.

Application of Multivariate Statistical Analysis to STEM X-ray Spectral Images: Interfacial Analysis in Microelectronics

Multivariate statistical analysis methods have been applied to scanning transmission electron microscopy (STEM) energy-dispersive X-ray spectral images. The particular application of the multivariate curve resolution (MCR) technique provides a high spectral contrast view of the raw spectral image. The power of this approach is demonstrated with a microelectronics failure analysis. Specifically, an unexpected component describing a chemical contaminant was found, as well as a component consistent with a foil thickness change associated with the focused ion beam specimen preparation process. The MCR solution is compared with a conventional analysis of the same spectral image data set.

Depth Profile Analysis on the Nanometer Scale by a Combination of Electron Probe Microanalysis (EPMA) and Focused Ion Beam Specimen Preparation (FIB)

Electron probe microanalysis (EPMA) offers high sensitivity and high accuracy in quantitative measurements of chemical compositions and mass coverages. Owing to the low detection limits of the wavelength-dispersive technique, monolayers with mass coverages of about 0.05 µg cm−2 can be detected. Assuming a density of 5 g cm−3 this corresponds to a thickness of 0.1 nm. With these advantages in mind, EPMA was extended to depth profile analysis in the sub-micron range using a surface removal technique.

Local composition analysis of SiC microstructures formed by ion projection in silicon using energy filtered TEM in combination with FIB specimen preparation

Buried silicon carbide (SiC) microstructures with lateral dimensions in the μm range were formed by high-dose projection of 1.5 MeV C 2+ ions in Si(100) at different doses and temperatures and subsequent annealing for 10 h at 1250 °C. Sections of individual SiC microstructures were prepared for cross-sectional transmission electron microscopy (TEM) analysis using a focused ion beam (FIB). Besides the possibility to select an individual microstructure, the FIB technique has the advantage of producing specimen foils of uniform thickness. Therefore, it was possible to map the carbon concentration of microstructures by energy filtered TEM (EFTEM) using the C K absorption edge without the need of any sample thickness correction. Local overstoichiometric (>50%) carbon concentrations are shown to be correlated to the formation of an amorphous phase in the SiC and to significant swelling visible at the Si wafer surface 2 μm above.

New Transmission Electron Microscope Characterization Techniques from Precision Focused Ion Beam Membranes

Abstract The combination of focused ion beam (FIB) sputtering with transmission electron microscopy (TEM) offers new opportunities for the nanoscale characterization of materials. The FIB may be used to prepare membranes for TEM imaging which are: (i) Site selective, i.e. the membranes may be placed with sub-micron precision in all three dimensions, (ii) Largely free of differential sputtering artifacts, such that membranes may be prepared which are of constant thickness from structures with very dissimilar materials, and (iii) Of precisely known geometry. The challenges associated with FIB specimen preparation will also be discussed and are summarized in Figure 1: (a) Surface amorphization damage, (b) Residual differential sputtering effects, (c) Redeposition of sputtered material and (d) Membrane bowing due to internal or beaminduced stresses. It will be demonstrated that each of these effects can be sufficiently controlled to allow high quality diffraction contrast imaging in a wide range of materials.

Characterization of FIB Damage in Silicon

Abstract Focused ion beam (FIB) instruments are useful for high spatial resolution milling, deposition, and imaging capabilities. As a result, FIB specimen preparation techniques have been widely accepted within the semiconductor community as a means to rapidly prepare high quality, site-specific specimens for transmission electron microscopy (TEM) [1]. In spite of the excellent results that have been observed for both high resolution (HREM) and standard TEM specimen preparation applications, a degree of structural modification is inherent to FIB milled surfaces [2,3]. The magnitude of the damage region that results from Ga+ ion bombardment is dependent on the operating parameters of the FIB (e.g., beam current, beam voltage, milling time, and the use of reactive gas assisted etching). Lattice defects occur as a consequence of FIB milling because the incident ions transfer energy to the atoms of the target material. Momentum transferred from the incident ions to the target atoms can result in the creation of point defects (e.g., vacancies, self interstitials, and interstitial and substitutional ion implantation), the generation of phonons, and plasmon excitation in the case of metal targets.

The Effect of Focused Ion Beam (Fib) Specimen Geometry on X-Ray Fluorescence During Energy Dispersive X-Ray Spectroscopy (EDS) Analysis in the Transmission Electron Microscope (TEM)

Abstract The focused ion beam (FIB) has become an indispensable tool for a variety of applications in materials science, including that of specimen preparation for the transmission electron microscope (TEM). Several FIB specimen preparation techniques have been developed, but some problems result when FIB specimens are analyzed in the TEM. One of these is X-ray fluorescence from bulk material surrounding the thin membrane in FIB-prepared samples. This paper reports on a new FIB specimen preparation method which was devised for the reduction of X-ray fluorescence during energy dispersive X-ray spectroscopy (EDS) in the TEM. Figure 1 shows three membrane geometries that were investigated in this study on a single-crystal Si substrate with a RF sputter-deposited 50 nm Ni film. Membrane 1 is the most commonly reported geometry in the literature, with an approximately 20 urn wide trench and a membrane having a single wedge with a 1.5° incline.

Proposals for exact-point transmission-electron microscopy using focused ion beam specimen-preparation technique

A focused ion beam (FIB) has been actively applied for preparation of about 0.1-μm-thick specimens for transmission electron microscopes (TEMs). For device failure analyses, however, it is mostly difficult to prepare the exact-point TEM specimens. The reason is that the failures are mostly beneath the surface and their exact locations are unknown. Then, even step-by-step FIB cross sectioning may sputter away the failures in the TEM specimen preparation. In the present study, we review two proposals for exact-point TEM microscopy using FIB specimen-preparation technique: (1) high-voltage scanning electron microscopes (HV-SEMs) imaging in TEM and (2) energy filtering TEM (EF-TEM) imaging. The HV-SEM imaging provides information on not only the sample surface but also the inner structure up to about 1 μm deep. The EF-TEM imaging is applicable even for 0.5-μm-thick specimens at 100 kV in the accelerating voltage, in contrast with about 0.1-μm-thick specimens for conventional TEM imaging. Preliminary experiments have supported that either proposal presumably improves the yield of the exact-point TEM inspection.

Focused Ion Beam Milling for Site Specific Scanning and Transmission Electron Microscopy Specimen Preparation

It has been shown that a focused ion beam (FIB) instrument may be used to prepare site specific cross-sectioned specimens to within < 0.1 μm for both scanning and transmission electron microscopy (SEM and TEM, respectively). FIB specimen preparation has been used almost exclusively in the microelectronics industry. Recently, FIB specimen preparation has been utilized for other materials systems and applications.A cross-sectioned SEM specimen is produced by sputtering away a trench of material from near the region of interest. Large amounts of material are sputtered using large ion beam diameters (e.g., l00’s nm) and high beam current (e.g., l000’s pA), while the final sputtering operations are achieved using smaller beam diameters (e.g., < 10 nm) and lower beam current (e.g., 10’s of pA). The SEM specimen may then be etched to reveal particular microstructural features of interest. A low magnification SEM image of a multi-layered device prepared for cross-section analysis by the FIB method is shown in FIG. 1.