Nm-scale Resolution Research Articles

Performance and characterization of the prototype nm-scale spatial resolution scanning multilayer Laue lenses microscope

Synchrotron based x-ray microscopy established itself as a prominent tool for noninvasive investigations in many areas of science and technology. Many facilities around the world routinely achieve sub-micrometer resolution with a few instruments capable of imaging with the spatial resolution better than 100 nm. With an ongoing effort to push the 2D/3D resolution down to 10 nm in the hard x-ray regime both fabrication of the nano-focusing optics and stability of a microscope become extremely challenging. In this work we present our approach to overcome technical challenges on the path towards high spatial resolution hard x-ray microscopy and demonstrate the performance of a scanning fluorescence microscope equipped with the multilayer Laue lenses focusing optics.

Localization of fluorescently labeled structures in frozen-hydrated samples using integrated light electron microscopy

Correlative light and electron microscopy is an increasingly popular technique to study complex biological systems at various levels of resolution. Fluorescence microscopy can be employed to scan large areas to localize regions of interest which are then analyzed by electron microscopy to obtain morphological and structural information from a selected field of view at nm-scale resolution. Previously, an integrated approach to room temperature correlative microscopy was described. Combined use of light and electron microscopy within one instrument greatly simplifies sample handling, avoids cumbersome experimental overheads, simplifies navigation between the two modalities, and improves the success rate of image correlation. Here, an integrated approach for correlative microscopy under cryogenic conditions is presented. Its advantages over the room temperature approach include safeguarding the native hydrated state of the biological specimen, preservation of the fluorescence signal without risk of quenching due to heavy atom stains, and reduced photo bleaching. The potential of cryo integrated light and electron microscopy is demonstrated for the detection of viable bacteria, the study of in vitro polymerized microtubules, the localization of mitochondria in mouse embryonic fibroblasts, and for a search into virus-induced intracellular membrane modifications within mammalian cells.

Strain mapping with nm-scale resolution for the silicon-on-insulator generation of semiconductor devices by advanced electron microscopy

Strain engineering in the conduction channel is a cost effective method of boosting the performance in state-of-the-art semiconductor devices. However, given the small dimensions of these devices, it is difficult to quantitatively measure the strain with the required spatial resolution. Three different transmission electron microscopy techniques, high-angle annular dark field scanning transmission electron microscopy, dark field electron holography, and nanobeam electron diffraction have been applied to measure the strain in simple bulk and SOI calibration specimens. These techniques are then applied to different gate length SiGe SOI pFET devices in order to measure the strain in the conduction channel. For these devices, improved spatial resolution is required, and strain maps with spatial resolutions as good as 1 nm have been achieved. Finally, we discuss the relative advantages and disadvantages of using these three different techniques when used for strain measurement.

Unified resolution bounds for conventional and stochastic localization fluorescence microscopy.

Superresolution microscopy enables imaging in the optical far field with ~20 nm-scale resolution. However, classical concepts of resolution using point-spread and modulation-transfer functions fail to describe the physical limits of superresolution techniques based on stochastic localization of single emitters. Prior treatments of stochastic localization microscopy have defined how accurately a single emitter's position can be determined, but these bounds are restricted to sparse emitters, do not describe conventional microscopy, and fail to provide unified concepts of resolution for all optical methods. Here we introduce a measure of resolution, the information transfer function (ITF), that gives physical limits for conventional and stochastic localization techniques. The ITF bounds the accuracy of image determination as a function of spatial frequency and for conventional microscopy is proportional to the square of the modulation-transfer function. We use the ITF to describe how emitter density and photon counts affect imaging performance across the continuum from conventional to superresolution microscopy, without assuming emitters are sparse. This unified physical description of resolution facilitates experimental choices and designs of image reconstruction algorithms.

Strain measurement for the semiconductor industry with nm-scale resolution by dark field electron holography and nanobeam electron diffraction

Strain is routinely used in state-of-the-art semiconductor devices in order to improve their electrical performance. Here we present experimental strain measurements obtained by two different Transmission Electron Microscopy (TEM) based techniques. Dark field electron holography and nanobeam electron diffraction (NBED) have been applied to a range of semiconductor devices including recessed SiGe source and drain devices and SiGe pFETs on SOL Here we demonstrate how the spatial resolution and sensitivity of these two techniques has been improved by using a state-of-the-art, probe corrected Titan TEM, and have benchmarked the capabilities of the two different techniques and highlight which method is most appropriate for different types of semiconductor specimens.

Nano-field mapping for the semiconductor industry

There is a need to measure the dopant potentials and strain fields in semiconductor materials with nm-scale resolution. Here we show that off-axis electron holography is a powerful technique that can be used to measure the fields present in a high-k metal gate 28-nm node nMOS device with a contact etch stop liner stressor. Off-axis electron holography has been used to map the positions of the active dopants with a spatial resolution of 1 nm. The experimental results have been compared to electron energy loss spectroscopy maps. Finally, dark field electron holography has also been used to provide strain maps and the experimental results have been verified using nanobeam electron diffraction.

Spatially-Resolved Structure and Electronic Properties of Graphene on Polycrystalline Ni

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the π-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the π-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure.

Strain mapping for the semiconductor industry by dark-field electron holography and nanobeam electron diffraction with nm resolution

There is a requirement of the semiconductor industry to measure strain in semiconductor devices with nm-scale resolution. Here we show that dark-field electron holography and nanobeam electron diffraction (NBED) are both complementary techniques that can be used to determine the strain in these devices. We show two-dimensional strain maps acquired by dark holography and line profiles that have been acquired by NBED of recessed SiGe sources and drains with a variety of different gate lengths and Ge concentrations. We have also used dark-field electron holography to measure the evolution in strain during the silicidation process, showing that this can reduce the applied uniaxial compressive strain in the conduction channel by up to a factor of 3.

Nm-scale resolution studies of the bond interface between ultrasonically welded Al-alloys by an analytical TEM: a path to comprehend bonding phenomena?

The analysis of the ultrasonically welded zone between aluminum alloys is taken as an example of the possibilities of analytical TEM to gain an insight into the phenomena occurred at the inner boundary surfaces of contacting metals during the ultrasonic welding. It is shown that combined TEM studies (imaging, diffraction, chemical analysis) of the welded seam allow to thoroughly characterize such a “buried” layer as the ultrasonic welded zone and to reconstruct, thereby, the physical and chemical processes responsible for its formation.

Near-field diffraction of fs and sub-fs pulses: super resolution of NSOM in space and time

The diffraction of fs and sub-fs pulses by a nm-size slit-type aperture in a thick perfectly conducting screen and its implication for near-field scanning optical microscopy (NSOM) is analyzed. The results theoretically demonstrate the feasibility of a NSOM with nm-scale resolution in space together with fs-scale resolution in time. The further increase of simultaneous resolution in space and time is limited by the spatial and temporal broadening of the diffracted pulse and the decrease of the screen opacity.

A Novel Method of Data Collection for Automated Electron Tomography Based upon Pre-cal1bration of Image Shifts and Defocus Changes

Electron tomography is a three-dimensional (3D) imaging method with transmission electron microscopy (TEM) that provides high-resolution 3D images of structural arrangements. with electron tomography a series of images is acquired of a sample that is tilted over a large angular range (±70°) with small angular tilt increments. For the 3D-reconstruction, the images of the tilt series are aligned relative to each other and the 3D-reconstruction is computed. Electron tomography is the only technique that can provide 3D information with nm-scale resolution of individual and unique samples. Routine application of electron tomography will comprise a major step forward in the characterization of complex materials and cellular arrangements. When collecting tilt series for electron tomography image shifts and defocus changes have to be corrected for by the human operator. The repetitive correction of these changes is highly time consuming, error prone and very hard to carry out under low-dose imaging conditions.Many practical problems are overcome when electron tomography data collection is performed in an automated fashion. Automation includes the (a) image acquisition on a (digital) CCD camera, which implies that (b) changes in image position and defocus can be detected by on-line image processing and (c) immediately be corrected for by computer control of the microscope, (d) Finally, tilt series are directly available in digital format for subsequent processing. Typically, carrying out such an experiment would take a day, and the actual data collection 2-4 hours. in spite of the enormous progress made in terms of data collection speed during the last few years, the current status of automated tomography still does not meet the requirements that would make it a routinely applicable tool. For a great number of biological assays and research projects, results obtained under different experimental conditions have to be compared, and series of experiments have to be carried out. Therefore, we propose a novel approach for recording a tilt series that significantly increases data collection speed, and widens the applicability of the technique.

Electrical characterization of semiconductor materials and devices using scanning probe microscopy

Several scanning probe microscopy (SPM) modes exist for the electrical characterization of semiconductor materials and devices with nm-scale resolution. The most important electrical SPM modes are: scanning capacitance microscopy (SCM), scanning spreading resistance microscopy (SSRM), and tunneling-AFM (TUNA). SCM and SSRM are primarily used for 2-D carrier profiling and resistivity mapping on cross-sectioned devices. The spatial resolution is on the order of 20nm, while the dynamic range goes from 1015 to 1020atoms/cm3. Imaging examples are shown for sub-quarter micron MOSFETs, ferro-electric material, and compound semiconductor structures. The TUNA method allows one to perform thickness mapping and defect imaging of thin dielectric films using a tunneling current flowing between the SPM tip and the sample. Spatial resolution is on the nm-scale, while the current range is from 50fA to 120pA. Examples are shown of gate oxide breakdown and defect localization in thin gate oxides.

A Novel Method for Automated Acquisition of Tilt Series for Electron Tomography Based on Pre-Calibration of the Specimen Stage

Abstract Electron tomography is a three-dimensional (3D) imaging method with transmission electron microscopy (TEM) that provides high-resolution 3D images of structural arrangements. Conventional TEM images are in first approximation mere 2D-projections of a 3D sample under investigation. With electron tomographya series of images is acquired of a sample that is tilted over a large angular range (±70°) with small angular tilt increments (so called tilt-series). For the subsequent 3D-reconstruction, the images of the tilt series are aligned relative to each other and the 3D-reconstruction is computed. Electron tomography is the only technique that can provide true 3D information with nm-scale resolution of individual and unique samples. For (cell) biology and material science applications the availability of high-resolution 3D images of structural arrangements within individual samples provides unique architectural information that cannot be obtained otherwise. Routine application of electron tomography will comprise a major revolutionary step forward in the characterization of complex materials and cellular arrangements.

UHV-AFM study of MBE-grown 10 nm scale ridge quantum wires

We evaluate the shape of ridge quantum wires (RQWIs) with nm-scale resolution by using SEM and ultra-high-vacuum (UHV) AFM system which is connected to MBE chamber. This MBE-AFM system provides us a detailed information about the evolution of size and uniformity of ridges grown under various conditions. In this report, we investigate systematically how the growth temperature T s and As flux affect the width and morphology of GaAs ridge structure. The results show that a very sharp and uniform GaAs ridge structures (W < 10 nm) can be obtained. On the basis of this understanding, we fabricated the ridge quantum wire of 10 nm width.

Fluctuations of the Au-Si(100) Schottky barrier height.

Schottky barrier height fluctuations of Au films on Si(100) are directly imaged with nm-scale resolution by ballistic electron emission. Fluctuations are made visible by using a highly doped (${\mathit{N}}_{\mathit{d}}$\ensuremath{\approxeq}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) substrate. Randomly distributed (approximately ${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) spots (about 2 nm in diameter) of reduced barrier height (typical \ensuremath{\Delta}\ensuremath{\Phi}=20--50 meV) are observed. The microscopic distribution of barrier heights effective in emission is consistent with mean barrier height values measured by standard techniques.

Performance and applications of a field-emission gun TEM/STEM

A Field Emission Gun (FEG) on a TEM/STEM instrument provides a major improvement in performance relative to one equipped with a LaB6 emitter. The improvement is particularly notable for small-probe techniques: EDX and EELS microanalysis, convergent beam diffraction and scanning. The high brightness of the FEG (108 to 109 A/cm2srad), compared with that of LaB6 (∼106), makes it possible to achieve high probe currents (∼1 nA) in probes of about 1 nm, whilst the currents for similar probes with LaB6 are about 100 to 500x lower. Accordingly the small, high-intensity FEG probes make it possible, e.g., to analyse precipitates and monolayer amounts of segregation on grain boundaries in metals or ceramics (Fig. 1); obtain high-quality convergent beam patterns from heavily dislocated materials; reliably detect 1 nm immuno-gold labels in biological specimens; and perform EDX mapping at nm-scale resolution even in difficult specimens like biological tissue.The high brightness and small energy spread of the FEG also bring an advantage in high-resolution imaging by significantly improving both spatial and temporal coherence.