Transmission Electron Microscopy Lamella Research Articles

Multi-Stimuli Operando Transmission Electron Microscopy for Two-Terminal Oxide-Based Devices.

The integration of microelectromechanical systems (MEMS)-based chips for in situ transmission electron microscopy (TEM) has emerged as a highly promising technique in the study of nanoelectronic devices within their operational parameters. This innovative approach facilitates the comprehensive exploration of electrical properties resulting from the simultaneous exposure of these devices to a diverse range of stimuli. However, the control of each individual stimulus within the confined environment of an electron microscope is challenging. In this study, we present novel findings on the effect of a multi-stimuli application on the electrical performance of TEM lamella devices. To approximate the leakage current measurements of macroscale electronic devices in TEM lamellae, we have developed a postfocused ion beam (FIB) healing technique. This technique combines dedicated MEMS-based chips and in situ TEM gas cells, enabling biasing experiments under environmental conditions. Notably, our observations reveal a reoxidation process that leads to a decrease in leakage current for SrTiO3-based memristors and BaSrTiO3-based tunable capacitor devices following ion and electron bombardment in oxygen-rich environments. These findings represent a significant step toward the realization of multi-stimuli TEM experiments on metal-insulator-metal devices, offering the potential for further exploration and a deeper understanding of their intricate behavior.

Weld-free mounting of lamellae for electrical biasing operando TEM

Recent advances in microelectromechanical systems (MEMS)-based substrates and sample holders for in situ transmission electron microscopy (TEM) are currently enabling exciting new opportunities for the nanoscale investigation of materials and devices. The ability to perform electrical testing while simultaneously capturing the wide spectrum of signals detectable in a TEM, including structural, chemical, and even electronic contrast, represents a significant milestone in the realm of nanoelectronics. In situ studies hold particular promise for the development of Metal-Insulator-Metal (MIM) devices for use in next-generation computing. However, achieving successful device operation in the TEM typically necessitates meticulous sample preparation involving focused ion beam (FIB) systems. Conducting contamination introduced during the FIB thinning process and subsequent attachment of the sample onto a MEMS-based chip remains a formidable challenge. This article delineates an improved FIB-based sample preparation methodology that results in good electrical connectivity and operational functionality across various MIM devices. To exemplify the efficacy of the sample preparation technique, we demonstrate preparation of a clean cross section extracted from a Au/Pt/BaSrTiO3/SrMoO3 tunable capacitor (varactor). The FIB-prepared TEM lamella mounted on a MEMS-based chip showed current levels in the tens of picoamperes range at 0.1 V. Furthermore, the electric response and current density of the TEM lamella device closely align with macro-scale devices. These samples exhibit comparable current densities to their macro-sized counterparts thus validating the sample preparation process and confirming device connectivity. The simultaneous operation and TEM characterization of electronic devices enabled by this process enables direct correlation between device structure and function, which could prove pivotal in the development of new MIM systems.

An evaluation of Ar ion milling in TEM sample preparation by energy-filtered TEM technique

Irradiated materials impose unique challenges when it comes to sample preparation for transmission electron microscopy (TEM) analysis. After the preparation of TEM samples by electrolytic polish, Ar ion milling was usually used to remove the surface contamination, as well as to thin the samples to desired thickness. Therefore, it is necessary to evaluate the advantages and limitations of Ar ion milling technique. In this work, the optimum milling condition with minimal surface artifacts was determined by Ar ion milling with various incident angles and beam energies, and the high-resolution TEM (HR-TEM) examinations confirmed the optimum condition to be 4.0keV Ar ions with incident angles of ±4°. In addition, Ar ion milling and subsequent thickness measurement by energy-filtered TEM (EF-TEM) were conducted to determine the thinning rate. It is found that electrolytic polished TEM lamella has a wedge-shape structure with a very sharp edge, and the thinning rates varied from 9.7±1.4nm/minute to 5.0±2.3nm/minute across the TEM lamella for 4.0keV Ar ions. However, the thinning rates for 2keV and 1keV Ar ions were under the examination limit, and they were supposed not sufficient to thin the tungsten samples. Even though the exact thinning rate varies for different materials, the general trends for the Ar ion milling observed in this study can be used as a guidance.

Stability of the Interface Between LIPON and LCO During TEM Sample Preparation by FIB.

An amorphous interphase between lithium phosphorus oxynitride (LIPON) solid electrolyte and lithium cobalt oxide (LCO) has been reported recently in several electron microscopy studies of Li ion thin-film micro-batteries (TFMB), along with its implications to battery operation. However, the origin of the observed interphase at the as-made LIPON/LCO interface remained obscure. In this work, this interface has been characterized comprehensively by scanning electron microscope (SEM) imaging at all steps of the in situ focused ion beam (FIB) lift-out procedure for transmission electron microscopy (TEM) sample preparation. It was found that the interphase is formed during TEM lamella preparation when the portion of LIPON layer contained within the lamella is physically disconnected from the rest of the LIPON layer by FIB. Therefore, it was demonstrated that a disordered interphase can form in LCO at its interface with LIPON during TEM sample preparation by the FIB lift-out procedure and that subtle nature of the interphase formation makes it likely to go unnoticed during the preparation. This interphase was not produced even after galvanostatic charging of a battery with a Li metal anode but inevitably appeared after the FIB lift-out of that sample.

Improvement of Quantitative STEM/EDXS Analyses for Chemical Analysis of Cu(In,Ga)Se2 Solar Cells with Zn(O,S) Buffer Layers

Abstract Energy-dispersive X-ray spectroscopy (EDXS) in a transmission electron microscope is frequently used for the chemical analysis of Cu(In,Ga)Se2 (CIGS) solar cells with high spatial resolution. However, the quantification of EDXS data is complicated due to quantification errors and artifacts. This work shows how quantitative EDXS analyses of CIGS-based solar cells with Zn(O,S) buffer and ZnO-based window layers can be significantly improved. For this purpose, CIGS-based solar cells and a reference sample with a stack of Zn(O,S) layers with different [O]/[S] ratios were analyzed. For Zn(O,S), the correction of sample-thickness-dependent absorption of low-energy O–Kα X-rays significantly improves the results of quantitative EDXS. Absorption of characteristic X-rays in CIGS is less relevant. However, for small transmission electron microscopy (TEM) sample thicknesses, artifacts can occur due to material changes by focused-ion-beam (FIB)-based preparation of TEM samples, electron-beam-induced damage, and oxidation of the sample surface. We also show that a Pt-protection layer, deposited on the sample surface before FIB preparation of TEM lamellae, can induce artifacts that can be avoided by first depositing a carbon layer.

Warp-Free TEM Sample Preparation Methods Using FIB/SEM Systems.

Warping is a limiting factor when preparing transmission electron microscopy (TEM) samples using focused ion beam (FIB)/scanning electron microscope (SEM) systems. The conventional FIB sputtering process leaves at least one side of the lamella too thin to provide structural support to offset inherent stresses. As a result, warping can occur impacting imagining and reducing the potential size of lamellae. For example, capturing more than a few back-end metal layers in a 3 μm wide cross-section lamella can be difficult. Frequently, TEM analysts must perform multiple stage adjustments to analyze such a sample. In this paper, two methods are presented that enable FIB/SEM operators to prepare TEM samples where the thinned region of interest is surrounded by thick structures. As a result, these methods reduce warping and enable the fabrication of TEM lamellae not possible by conventional means. For example, these methods have been used to produce a 10 μm wide by 8 μm tall cross-section TEM sample that captured front-end transistors and 14 back-end metal layers. Furthermore, warping was so limited that only one alignment was needed by the TEM analyst to complete the imaging of the sample. The methods, called the horizontal bracing and window methods, make use of the deposition of low-Z amorphous films that are electron transparent in the TEM.

A transmission electron microscopy study of low-strain epitaxial BaTiO3 grown onto NdScO3

Ferroelectric materials exhibit a strong coupling between strain and electrical polarization. In epitaxial thin films, the strain induced by the substrate can be used to tune the domain structure. Substrates of rare-earth scandates are sometimes selected for the growth of ferroelectric oxides because of their close lattice match, which allows the growth of low-strain dislocation-free layers. Transmission electron microscopy (TEM) is a frequently used technique for investigating ferroelectric domains at the nanometer-scale. However, it requires to thin the specimen down to electron transparency, which can modify the strain and the electrostatic boundary conditions. Here, we have investigated a 320 nm thick epitaxial layer of BaTiO3 grown onto an orthorhombic substrate of NdScO3 with interfacial lattice strains of −0.45% and −0.05% along the two in-plane directions. We show that the domain structure of the layer can be significantly altered by TEM sample preparation depending on the orientation and the geometry of the lamella. In the as-grown state, the sample shows an anisotropic a/c ferroelastic domain pattern in the direction of largest strain. If a TEM lamella is cut perpendicular to this direction so that strain is released, a new domain pattern is obtained, which consists of bundles of thin horizontal stripes parallel to the interfaces. These stripe domains correspond to a sheared crystalline structure (orthorhombic or monoclinic) with inclined polarization vectors and with at least four variants of polarization. The stripe domains are distributed in triangular-shaped 180° domains where the average polarization is parallel to the growth direction. The influence of external electric fields on this domain structure was investigated using in situ biasing and dark-field imaging in TEM.

Dislocations at coalescence boundaries in heteroepitaxial GaN/sapphire studied after the epitaxial layer has completely coalesced

We have performed cross-sectional scanning capacitance microscopy (SCM), cathodoluminescence (CL) microscopy in the scanning electron microscope (SEM) and transmission electron microscopy (TEM) all on the same few-micron region of a GaN/sapphire sample. To achieve this, it was necessary to develop a process flow which allowed the same features viewed in a cleaved cross-section to be traced from one microscope to the next and to adapt the focused ion beam preparation of the TEM lamella to allow preparation of a site-specific sample on a pre-cleaved cross-section. Growth of our GaN/sapphire samples involved coalescence of three-dimensional islands to form a continuous film. Highly doped marker layers were included in the sample so that coalescence boundaries formed late in the film growth process could be identified in SCM and CL. Using TEM, we then identified one or more dislocations associated with each of several such late-coalescing boundaries. In contrast, previous studies have addressed coalescence boundaries formed earlier in the growth process and have shown that early-stage island coalescence does not lead to dislocation formation.

Interplay of anomalous strain relaxation and minimization of polarization changes at nitride semiconductor heterointerfaces

We present a methodology to quantify polarization and electron affinity changes at interfaces by combining scanning tunneling spectroscopy, off-axis electron holography in transmission electron microscopy (TEM), and self-consistent calculations of the electrostatic potential and electron phase change. We use a precisely known grown-in doping structure to calibrate the surface potential of the TEM lamella and thereby achieve a quantitative analysis of electron phase changes measured by off-axis electron holography. Using this calibration, we deduce quantitatively polarization and electron affinity changes for ${\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}/\mathrm{GaN}$ and ${\mathrm{In}}_{\text{0.05}}{\mathrm{Ga}}_{\text{0.95}}\mathrm{N}/{\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}$ interfaces. The latter interface reveals, as expected, biaxial relaxation as well as polarization and electron affinity changes. However, at the ${\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}/\mathrm{GaN}$ interface anomalous lattice relaxations and vanishing polarization and electron affinity changes occur, whose underlying physical origin is anticipated to be total energy minimization by the minimization of Coulomb interactions between the polarization-induced interface charges.

Wet-chemical etching of FIB lift-out TEM lamellae for damage-free analysis of 3-D nanostructures

Reducing ion beam damage from the focused ion beam (FIB) during fabrication of cross sections is a well-known challenge for materials characterization, especially cross sectional characterization of nanostructures. To address this, a new method has been developed for cross section fabrication enabling high resolution transmission electron microscopy (TEM) analysis of 3-D nanostructures free of surrounding material and free of damage detectable by TEM analysis. Before FIB processing, nanopillars are encapsulated in a sacrificial oxide which acts as a protective layer during FIB milling. The cross sectional TEM lamella containing the nanopillars is then mounted and thinned with some modifications to conventional FIB sample preparation that provide stability for the lamella during the following wet-chemical dip etch. The wet-chemical etch of the TEM lamella removes the sacrificial oxide layer, freeing the nanopillars from any material that would obscure TEM imaging. Both high resolution TEM and aberration corrected scanning TEM images of Si/SiGe pillars with diameters down to 30 nm demonstrate the successful application of this approach.

TEM investigations of gate-all-around nanowire devices

Vertically stacked gate-all-around nanowires (GAA NWs) are considered a promising architecture for ultimately scaled complementary metal oxide semiconductor devices. These are the natural evolution of the fin-shaped field effect transistor (finFET) design and enable a better electrostatic control and a higher drive current per footprint w.r.t. previous architectures. Transmission electron microscopy (TEM) analysis is employed in the development stages of these devices to investigate morphology, material diffusion, oxidation and strain in order to achieve the desired nanowires shape and size and the required performances. Nano beam diffraction and geometric phase analysis of high-resolution scanning TEM (STEM) images are used in this work to evaluate strain at the nm-scale along the nanowires at different steps of the fabrication process. Initially strained Ge layers, in the early stages of the GAA NWs fabrication, relax after the fin-reveal and source/drain etching process steps. Strain is then restored after source/drain epitaxial deposition and maintained till the NWs release. TEM analyses of these structures are particularly challenging due to the dimensions of the GAA NWs which are smaller than the thickness of a typical TEM specimen. This generates artifacts due to different materials and multiple structures overlapping in projection in TEM images. To avoid these issues, several TEM lamellae at different positions in the device and/or 3D imaging STEM/energy dispersive spectroscopy tomography are employed.

Measurement of SiGe composition in 3-D semiconductor Fin Field Effect Transistor devices

A Transmission Electron Microscopy (TEM)-based method is suggested to measure the composition of SiGe in 3-D structures using Electron Energy Loss Spectroscopy (EELS). The method accounts for the presence of films other than SiGe within the TEM lamella in the electron beam direction. The partial cross section of inelastic scattering of Ge was calibrated using a reference structure, which was earlier analyzed by Electron Energy Dispersive X-ray Analysis (EDX) and Secondary Ion Mass Spectrometry. The composition of SiGe in the p-FET Fin Field Effect Transistor devices with the overlaying Si oxynitride films was measured as a demonstration of the method. We show that the application of EELS yields smaller measurement errors of the SiGe composition as compared to EDX. The effect of beam damage in thin SiGe films surrounded by Si oxynitride is evaluated and compared to the blanket Si/SiGe structures. The method can be applied to the development of novel devices and state-of-the-art processes where the composition of SiGe plays a critical role.

Flash electropolishing of TEM lamellas of irradiated tungsten

The study in the transmission electron microscopy (TEM) of the irradiation-induced microstructure of tungsten, a material promising for its applications at high temperatures in harsh radiation environments, is rendered difficult by the high level of radioactivity. To circumvent this problem, it is nowadays a routine to reduce the TEM sample size with the focused ion beam (FIB) technique. In this way, one extracts an electron transparent thin lamella that is only a few microns in size. However, FIB induces additional radiation damage in the surface layer, which has to be removed by the so-called flash polishing technique in order to detect the initial damage structure in an irradiated sample. In this study, we evaluate the technique of flash electropolishing in an aqueous solution of NaOH as a method to remove this surface damage from tungsten lamella samples. The conditions of flash electropolishing are optimized as a function of temperature, voltage, FIB grid and weld material. Anisotropic etching dependence on the crystallographic direction was evaluated, with the highest polishing resistance in the region between (5 2 1) and (1 1 0).

Towards 3D crystal orientation reconstruction using automated crystal orientation mapping transmission electron microscopy (ACOM-TEM).

To relate the internal structure of a volume (crystallite and phase boundaries) to properties (electrical, magnetic, mechanical, thermal), a full 3D reconstruction in combination with in situ testing is desirable. In situ testing allows the crystallographic changes in a material to be followed by tracking and comparing the individual crystals and phases. Standard transmission electron microscopy (TEM) delivers a projection image through the 3D volume of an electron-transparent TEM sample lamella. Only with the help of a dedicated TEM tomography sample holder is an accurate 3D reconstruction of the TEM lamella currently possible. 2D crystal orientation mapping has become a standard method for crystal orientation and phase determination while 3D crystal orientation mapping have been reported only a few times. The combination of in situ testing with 3D crystal orientation mapping remains a challenge in terms of stability and accuracy. Here, we outline a method to 3D reconstruct the crystal orientation from a superimposed diffraction pattern of overlapping crystals without sample tilt. Avoiding the typically required tilt series for 3D reconstruction enables not only faster in situ tests but also opens the possibility for more stable and more accurate in situ mechanical testing. The approach laid out here should serve as an inspiration for further research and does not make a claim to be complete.

Mitigating Curtaining Artifacts During Ga FIB TEM Lamella Preparation of a 14 nm FinFET Device.

We report on the mitigation of curtaining artifacts during transmission electron microscopy (TEM) lamella preparation by means of a modified ion beam milling approach, which involves altering the incident angle of the Ga ions by rocking of the sample on a special stage. We applied this technique to TEM sample preparation of a state-of-the-art integrated circuit based on a 14-nm technology node. Site-specific lamellae with a thickness <15 nm were prepared by top-down Ga focused ion beam polishing through upper metal contacts. The lamellae were analyzed by means of high-resolution TEM, which showed a clear transistor structure and confirmed minimal curtaining artifacts. The results are compared with a standard inverted thinning preparation technique.

Interphase analysis of hierarchical composites via transmission electron microscopy

Observation and analysis of the interphase are essential for a detailed understanding of the global composite properties when nanofillers are incorporated as interfacial agents. Techniques such as atomic force microscopy and nano-indentation provide valuable information on interfacial properties associated with the viscoelastic behavior of each phase. However, when the morphology of this region is observed in detail, instrumental errors may regularly appear, decreasing the accuracy of measurements. In this work, the use of transmission electron microscopy (TEM) was explored to image the glass fiber-reinforced polymer GFRP interphase containing interfacial nanocellulose. TEM lamellas were prepared via a focused ion beam to observe the phases disposed within the composite arrangement. Energy dispersive X-ray spectroscopy was also performed to determine the elemental composition in each sample phase. Interphase sizes between 25 and 50 nm thick were found, highlighting the ability of this characterization route to give accurate interfacial measurements. This kind of measurement will open new routes for getting rich information on hierarchically structured composites containing a nanostructure as an interfacial agent.

Lattice reorientation in tetragonal PMN-PT thin film induced by focused ion beam preparation for transmission electron microscopy

Focused ion beam sample preparation for transmission electron microscopy (TEM) can induce relaxation mechanisms in epitaxial thin films. Here, we describe a relaxation mechanism that can occur in materials having a tetragonal structure. We investigated the lattice structure of a 600 nm thick 0.4[Pb(Mg1/3Nb2/3)O3]−0.6[PbTiO3] layer grown by epitaxy on (110) GdScO3 substrate using geometrical phase analysis applied to high resolution TEM images. The lattice mismatch at the interface is expected to favor the formation of c-domains. However, it was measured that the out-of-plane lattice parameter can decrease abruptly along the growth direction and the transition depends on the thickness of the TEM lamella. Different observations indicate that the crystal flipped by 90° following the preparation of the sample, so that the c-axis is oriented in the thinning direction. Such a mechanism can easily lead to misinterpretations and might happen in other materials with a similar structure.

3D Analytical TEM Approach to Effectively Characterize 3D-FinFET Device Features in Semiconductor Wafer-foundries

Introductions of 3-Dimentional FinFET transistors in semiconductor device open a new era for continuous shrinkage of semiconductor transistor nodes. One big challenge for physical failure analysis (PFA) and transmission electron microscopy (TEM) is that, for device of 20nm, 14nm, an beyond, features of interests (FOI) are mostly thinner than a thickness of a TEM lamella (say, around 50~150nm), depending on the orientation. This adds in ambiguity in interpretation of the TEM imaging straightforward. Combined with intrinsic complexity from the 3D-FinFET structures themselves, PFA/TEM teams at waferfoundries truly have to constantly think out of box for novel approaches to effectively and timely isolate faults and conclusively identify root-causes of yield detractors [1~4].

Microstructure and Direct Measured Micro-Strain by TEM of Hot Iso-Static Pressed Alumina-Titanium Carbide (Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-TiC) Composite

Alumina-titanium carbide composite (Al2O3-TiC) is one of advance ceramic matrix composites (CMC) that have been used in machining tool and tribology application because of its excellent mechanical properties. Powder processing is often a chosen method to produce this family of material, i.e. hot pressed (HP), hot iso-static pressed (HIP), and pressure-less sintering (PS). These manufacturing techniques convert sub-micron powder into dense bulk component. Al2O3-TiC composite has been prepared by HIP process, containing 65%vol Al2O3and 35%vol TiC. The powders were pressured and sintered at temperature between 1250˚C - 1650˚C. Thermal residual stress from Al2O3-TiC manufacturing process is conventionally unavoidable. The aim of this study is to investigate the microstructure of HIPped Al2O3-TiC composite and its residual micro-strain using transmission electron microscopy (TEM) with selected area electron diffraction pattern (SADP) analysis. The Al2O3-TiC composite was prepared by high-precision machining/grinding processes then focus ion beam milling process was used to section the TEM lamella. Microstructures, grain size and phases were determined by TEM and X-ray diffractometry (XRD). Micro-strains of {200}TiC, {111}TiC , {220}TiC and {012}Al2O3were investigated by SADP (Selected Area Diffraction Pattern). Our result revealed there is approximately 0.01 compressive strains distributed in Al2O3-TiC composite.

Influence of TEM specimen preparation on chemical composition of Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

The influences of different transmission electron microscopy (TEM) specimen preparation techniques on the chemical composition of Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) single crystals was studied. Ion-milled samples where no cooling with liquid nitrogen (L-N2) was applied show permanently changed composition also deep inside the bulk material. When the PMN–PT samples were cooled to L-N2 temperature during the ion-milling process and in addition lower accelerating voltages were used, the chemical composition was altered only in the thinnest parts close to the specimen edge. Samples prepared using only tripod polishing technique show compositional irregularities close to the specimen edge. For the preparation of lead-containing samples, such as PMN–PT single crystals, a combination of tripod polishing and short Ar-ion-milling at low accelerating voltages while cooling the samples to liquid nitrogen temperature proved to be the most suitable to obtain artefact-free electron-transparent TEM lamellae.