Ultimate Lateral Resolution Research Articles

The NanoSIMS-HR: The Next Generation of High Spatial Resolution Dynamic SIMS.

The high lateral resolution and sensitivity of the NanoSIMS 50 and 50L series of dynamic SIMS instruments have enabled numerous scientific advances over the past 25 years. Here, we report on the NanoSIMS-HR, the first major upgrade to the series, and analytical tests in a suite of sample types, including an aluminum sample containing silicon crystals, microalgae, and plant roots colonized with a symbiotic fungus. Significant improvements have been made in the Cs+ ion source, high voltage (HV) control, stage reproducibility, and other aspects of the instrument that affect performance. The modified design of the NanoSIMS-HR thermal-ionization Cs+ source enables a 5 pA primary ion beam to be focused into a 100 nm spot, a ∼2.5-fold increase compared to Cs+ sources on previous instruments (∼2 pA at 100 nm). The brightness of the new Cs+ source enables an ultimate lateral resolution as high as 30 nm and improved detection limits for a given analysis area. Sample stage movement accuracy is higher than 500 nm, enabling many-fold higher throughput automated analyses. With the new HV control, the primary ion beam impact energy can be reduced from 16 to 2 keV, which enables higher depth resolution during depth profiling (a 2-fold improvement), albeit with a 5-fold decrease in lateral resolution. In the NanoSIMS-HR, the secondary ion column and detection system are identical to those used in the previous series, and the isotopic analysis performance is as precise as in previous NanoSIMS instruments.

Locating atoms with higher precision

Electron Microscopy Two major problems that limit the resolution and interpretation of electron microscopy images are lens aberrations and multiple scattering. Chen et al. overcame these issues with ptychography, a technique that uses coherent scattering and multiple overlapping illumination spots to reconstruct an image from far-field diffraction patterns. This method works at a resolution that is limited, not by optics, but rather by the scattering strength of the sample, so it can work better with thicker samples. The authors achieved ultimate lateral resolution better than the thermal vibration of atoms in a PrScO3 sample and showed that it is theoretically possible to identify single dopant atoms. Science , abg2533, this issue p. [826][1] [1]: /lookup/doi/10.1126/science.abg2533

Electron ptychography achieves atomic-resolution limits set by lattice vibrations.

Transmission electron microscopes use electrons with wavelengths of a few picometers, potentially capable of imaging individual atoms in solids at a resolution ultimately set by the intrinsic size of an atom. However, owing to lens aberrations and multiple scattering of electrons in the sample, the image resolution is reduced by a factor of 3 to 10. By inversely solving the multiple scattering problem and overcoming the electron-probe aberrations using electron ptychography, we demonstrate an instrumental blurring of less than 20 picometers and a linear phase response in thick samples. The measured widths of atomic columns are limited by thermal fluctuations of the atoms. Our method is also capable of locating embedded atomic dopant atoms in all three dimensions with subnanometer precision from only a single projection measurement.

Real-space imaging of atomic-scale spin textures at nanometer distances

Spin-polarized scanning tunneling microscopy (SP-STM) experiments on ultrathin films with non-collinear spin textures demonstrate that resonant tunneling allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm. Spin-polarized resonance states evolving between the foremost atom of a magnetic probe tip and the opposed magnetic surface atom are found to provide a loophole from the hitherto existing dilemma of losing spatial resolution when increasing the tip-sample distance in a scanning probe setup. Bias-dependent series of SP-STM images recorded via resonant tunneling reveal spin sensitivity at resonance conditions, indicating that the spin-polarized resonance states act as mediators for the spin contrast across the nm-spaced vacuum gap. With technically feasible distances in the nm regime, resonant tunneling in SP-STM qualifies for a spin-sensitive read-write technique with ultimate lateral resolution in future spintronic applications.

Perfect digital holographic imaging with high resolution using a submillimeter-dimension CCD sensor

In order to improve the resolution of digital holography with a common-dimension charge-coupled device (CCD) sensor, the point spread functions are briefly derived for the commonly used and practical post-magnification, pre-magnification, and image-plane digital holographic microscopic systems. The ultimate resolutions of these systems are analyzed and compared. The results show that the ultimate lateral resolution of pre-magnification digital holography is superior to that of post-magnification digital holography in the same conditions. We also demonstrate that the ultimate lateral resolution of image-plane digital holography has no correlation with the photosensitive dimension of the CCD sensor, and it is not significantly related to the pixel size of the sensor. Moreover, both the ultimate resolution and the imaging quality of image-plane digital holography are superior to that of pre- and post-magnification digital holographic microscopy. High-resolution imaging, whose resolution is close to the ultimate resolution of the microscope objective, can be achieved by image-plane digital holography even with a submillimeter-dimension sensor. This system, by which perfect imaging can be achieved, is optimal for commonly used digital holographic microscopy. Experimental results demonstrate the correctness of the theoretical analysis.

Spatial resolution in prism-based surface plasmon resonance microscopy.

Several optical surface sensing techniques, such as Surface Plasmon Resonance (SPR), work by imaging the base of a prism by one of its faces. However, such a fundamental optical concern has not been fully analyzed and understood so far, and spatial resolution remains a critical and controversial issue. In SPR, the propagation length L(x) of the surface plasmon waves has been considered as the limiting factor. Here, we demonstrate that for unoptimized systems geometrical aberrations caused by the prism can be more limiting than the propagation length. By combining line-scan imaging mode with optimized prisms, we access the ultimate lateral resolution which is diffraction-limited by the object light diffusion. We describe several optimized configurations in water and discuss the trade-off between L(x) and sensitivity. The improvement of resolution is confirmed by imaging micro-structured PDMS stamps and individual living eukaryote cells and bacteria on field-of-view from 0.1 to 20 mm(2).

Spatial resolution of confocal XRF technique using capillary optics

XRF (X-ray fluorescence) is a powerful technique for elemental analysis with a high sensitivity. The resolution is presently limited by the size of the primary excitation X-ray beam. A test-bed for confocal-type XRF has been developed to estimate the ultimate lateral resolution which could be reached in chemical mapping using this technique. A polycapillary lens is used to tightly focus the primary X-ray beam of a low power rhodium X-ray source, while the fluorescence signal is collected by a SDD detector through a cylindrical monocapillary. This system was used to characterize the geometry of the fluorescent zone. Capillary radii ranging from 50 μm down to 5 μm were used to investigate the fluorescence signal maximum level This study allows to estimate the ultimate resolution which could be reached in-lab or on a synchrotron beamline. A new tool combining local XRF and scanning probe microscopy is finally proposed.

Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

On the road to high-resolution 3D molecular imaging

This contribution reviews the state-of-the-art in the domains of molecular imaging and depth profiling, the two methodological platforms required for 3D molecular imaging by secondary ion mass spectrometric (SIMS). Using molecular dynamics calculations, it also describes some of the mechanisms that make cluster projectiles such as C 60 so different for organic sample analysis. The discussion addresses issues that deserve proper attention on the way to 3D molecular imaging in SIMS, such as ultimate lateral resolution, limited molecular yields, chemical effects and damage, and highlights solutions currently in embryo in the many research teams concerned by 3D molecular imaging.

High sensitivity mapping of Ti distributions in Hadean zircons

Detrital zircons as old as nearly 4.4 Ga from the Jack Hills, Western Australia, offer possible insights into a phase of Earth history for which there exists no known rock record. Ti concentrations of Hadean zircons indicate a spectrum of crystallization temperatures that range from a cluster at ca. 680 °C to apparent values exceeding 1200 °C. The low temperature peak has been interpreted to indicate the existence of ‘wet’ melting conditions during the Hadean, but alternate views have been advanced. We have developed methods for quantitative ion imaging of titanium in zircons using positive and negative secondary ions, produced respectively under bombardment of O − and Cs +, that permit detailed insights regarding Ti concentration distributions. Each approach has particular advantages that tradeoff in terms of sensitivity, ultimate lateral resolution, and reproducibility. Coupled with high resolution spot analyses, these ion images show that Ti contents greater than about 20 ppm in the Jack Hills zircons are associated with cracks or other crystal imperfections and that virtually all of the high apparent temperatures (i.e., > 800 °C) yet obtained are suspect for contamination by Ti extraneous to the zircon. Removing these from consideration strengthens the previous conclusion that the Hadean Jack Hills population is dominated by zircons crystallized at 680–720 °C.

Improving the spatial resolution of a magnetic force microscope tip via focused ion beam modification and magnetic film coating

To fabricate high-resolution magnetic tips by a more convenient method, this study applied focused ion beam milling and magnetic film coating techniques to manufacture high aspect ratio (HAR) magnetic force microscope (MFM) tips. The results showed that a HAR MFM tip with a ratio of 10:1 was successfully fabricated. MFM measurements demonstrated that this probe provided clear high-resolution MFM images. The ultimate lateral resolution of the HAR MFM tip was reduced to a minimum of 20 nm.

Evaluation of focused O+ ion beams as a tool for making resist masks by reactive etching

A method of reactive etching is proposed in which focused 100-eV O+ ion beams are used for making organic-resist masks 12–24 nm thick with nanometer-sized apertures. Focused-ion-beam and ion-projection-lithography systems with an electrostatic immersion decelerating objective are considered. They are shown to provide an ultimate lateral resolution of ∼14 or ∼21 nm, respectively. Attainable etch rate is estimated. The potential usefulness of resist masks thus obtained is discussed in the context of quantum-computing devices containing quantum dots, quantum wires, and nanoscale conducting tracks.

CoFe-Coated Carbon Nanotube Probes for Magnetic Force Microscope

CoFe-coated carbon nanotube (CNT) probes have been successfully fabricated by a radio-frequency sputtering method. The sputtering conditions for obtaining a uniform ferromagnetic film on the CNT probe were investigated. The optimal sputtering conditions were determined from the observations using a scanning electron microscope and an atomic force microscope. CNT magnetic force microscope (MFM) probes prepared under optimized conditions provided us clear images with an ultimate lateral resolution of approximately 10 nm.

A magnetic force microscope using CoFe-coated carbon nanotube probes

Ferromagnetic-film-coated carbon nanotube (CNT) probes were employed in a magneticforce microscope (MFM) observation. We succeeded in making a uniform ferromagneticfilm on the CNT probes by improving the coating process and selecting materials. Theperformance of the CoFe-coated CNT probe was evaluated in ultra-high-density perpendicularmagnetic storage media with densities from 600 to 1100 k flux changes per inch (FCI). Themagnetic domain structure of the magnetic storage media was clearly observed up to1100 kFCI. The ultimate lateral resolution of the newly developed MFM probes is down toabout 10 nm, which exceeds the bit length of a magnetic recording with a density ofTbit inch−2.

Resolution deterioration in emission electron microscopy due to object roughness

In the present paper a general analytic expression has been obtained and confirmed by a computer simulation which links the surface roughness of an object under study in an emission electron microscope and it's resolution. A quantitative derivation was made for the model case when there is a step on the object surface. It was shown that the resolution is deteriorated asymmetrically relative to the step. The effect sets a practical limit to the ultimate lateral resolution obtainable in an emission electron microscope.

Resolution deterioration in emission electron microscopy due to object roughness

AbstractIn the present paper a general analytic expression has been obtained and confirmed by a computer simulation which links the surface roughness of an object under study in an emission electron microscope and it's resolution. A quantitative derivation was made for the model case when there is a step on the object surface. It was shown that the resolution is deteriorated asymmetrically relative to the step. The effect sets a practical limit to the ultimate lateral resolution obtainable in an emission electron microscope.

Secondary ion emission from molecular overlayers: Thiols on gold

The secondary ion emission behavior of 10 different thiols (8 alkanethiols, with n=4–18 C atoms, TSA, and MCP) was investigated. In particular, we determined the generated negative secondary ion species Xi−, their yields Y, the corresponding damage cross section σ(Xi−), and the activation energy for desorption ED(Xi−) (from the temperature behavior) of these secondary ions. The results supply important information on the analytical capabilities of secondary ion mass spectrometry (e.g., sensitivity and ultimate lateral resolution), as well as on the capabilities and limits of ion beam induced modification of these molecular overlayers. By comparing spectra and yields for different thiols bonded in the same way to the same substrate, these results give some additional insight into the ion formation process. A comparison of the activation energy for desorption ED(Xi−) with the corresponding damage cross section σ(Xi−) supplies insight into the energy distribution within the impact cascade.

Recent instrumental developments in magnetic sector SIMS

The recent developments in magnetic sector SIMS are reviewed. It is not aimed at a detailed description of these instruments which, for several of them, can be found elsewhere [1–8] but reviews the driving forces making necessary the development of magnetic sector SIMS for all types of analysis (single instrument alternative) as well as instruments dedicated to ultimate lateral resolution (50 nm), to isotopic ratio measurements with multicollection of all isotopes, to depth profiling at high depth resolution using an impact energy of 250 eV, and instruments allowing better flexibility about the amount of cesium incorporated in the specimen to optimize the formation of MCs clusters for the analysis of complex materials.

Imaging of Silicon Carrier Dynamics with Near-Field Scanning Optical Microscopy

AbstractA contactless high spatial resolution technique has been developed to characterize semiconductor materials using the Near-Field Scanning Optical Microscope. The technique can be used to non-invasively measure: surface topography, defect content, and carrier lifetime variations in silicon. The success of the technique relies on the sensitive detection of changes in infrared transmission induced by local generation of free carriers using pulsed visible radiation. Here we extend the application of this technique to characterize silicon on insulator materials. We also include computer simulation results to address the role played by diffusion in the ultimate lateral resolution that can be achieved using this technique.

Resolution in surface plasmon microscopy

In this article we demonstrate how to obtain the ultimate lateral resolution in surface plasmon microscopy (SPM) (diffraction limited by the objective). Surface plasmon decay lengths are determined theoretically and experimentally, for wavelengths ranging from 531 to 676 nm, and are in good agreement. Using these values we can determine for each particular situation which wavelength should be used to obtain an optimal lateral resolution, i.e., where the plasmon decay length does not limit the resolution anymore. However, there is a trade-off between thickness resolution and lateral resolution in SPM. Because of the non-optimal thickness resolution, we use several techniques to enhance the image acquisition and processing. Without these techniques the use of short wavelengths results in images where the contrast has vanished almost completely. In an example given, a 2.5 nm SiO2 layer on a gold layer is imaged with a lateral resolution of 2 μm, and local reflectance curves are measured to determine the layer thickness. The SPM image is compared with an atomic force microscopy image of the same object. We obtain a 3 μm resolution when thickness differences within a lipid monolayer are imaged and measured.