Investigation of range-energy relationships for low-energy electron beams in silicon and gallium nitride

The electron beam technique of the Scanning Electron Microscopy (SEM) has been widely used for the characterization of bipolar devices and photodiode materials. The resolution of an electron beam technique is affected by the interaction of the beam and the specimen. The size of this interaction volume, commonly termed the generation volume, is usually characterized by what is called the electron penetration range and is measured from the surface. Since there is currently no consensus on the expressions to use in the calculation of the electron range, this paper provides an analysis of the three most commonly used semiempirical expressions. They are the Gruen range, the universal curve of Everhart and Hoff, and the maximum range of Kanaya and Okayama. This analysis is done using data from the statistical method of Monte Carlo simulations. It was found that the Everhart and Hoff universal curve performs better at low beam energies than the equation of Kanaya and Okayama. However, the validity of all the three expressions is questionable below 5 keV. In order to overcome this, fitted expressions based on the extrapolated range are provided for beam energies below 5 keV in the case of Si and GaN materials. The accuracy of these expressions is affected by the physical parameters used in the Monte Carlo simulations.     

High-resolution scanning electron microscopy.

The spatial resolution of the scanning electron microscope is limited by at least three factors: the diameter of the electron probe, the size and shape of the beam/specimen interaction volume with the solid for the mode of imaging employed and the Poisson statistics of the detected signal. Any practical consideration of the high-resolution performance of the SEM must therefore also involve a knowledge of the contrast available from the signal producing the image and the radiation sensitivity of the specimen. With state-of-the-art electron optics, resolutions of the order of 1 nm are now possible. The optimum conditions for achieving such performance with the minimum radiation damage to the specimen correspond to beam energies in the range 1-3 keV. Progress beyond this level may be restricted by the delocalization of SE production and ultimate limits to electron-optical performance.     

Experiments and quantitative analysis of frequency division multiplexing confocal fluorescence microscopy with UV excitation

In this paper, we experimentally demonstrated a two-channel frequency division multiplexing confocal fluorescence microscopy system using a UV laser as the excitation source. In our two-channel frequency division multiplexing confocal fluorescence system, the incoming laser beam was divided into two beams and each beam was modulated with an individual carrier frequency. These two laser beams were then spatially combined with a small angle and focused into two diffraction-limited spots on the targeted cell (rat neural cell) surface to generate fluorescent signal. As a result, the fluorescent signals from two spots of the rat neural cell surface can be demodulated and distinguished during data processing. Furthermore, a quantitative analysis on the cross-talk among different frequencies was provided as well. The experimental results confirm that the two-channel frequency division multiplexing confocal fluorescence technology can not only maintain the high spatial resolution, but also realize the multiple points detection simultaneously with high temporal resolution (within millisecond level range), which benefits the dynamic studies of living biological cells.     

High efficiency beam splitter for multifocal multiphoton microscopy

In this article we present the development of a multibeam two-photon laser scanning microscope. A new type of beam splitter to create the multitude of laser beams is described. This type of beam splitter has higher transmission and generates more uniform beams than can be achieved with the microlens approach used by other groups. No crosstalk exists between the different foci due to small temporal delays between the individual beams. The importance of dispersion compensation to obtain maximum efficiency of the microscope is discussed. With optimum compensation the fluorescence signal was raised by a factor of 14. Different modes of detecting the fluorescence signals and their effect on imaging speed and resolution are discussed.     

High resolution at low beam energy in the SEM: resolution measurement of a monochromated SEM

The resolution of secondary electron low beam energy imaging of a scanning electron microscope equipped with a monochromator is quantitatively measured using the contrast transfer function (CTF) method. High-resolution images, with sub-nm resolutions, were produced using low beam energies. The use of a monochromator is shown to quantitatively improve the resolution of the SEM at low beam energies by limiting the chromatic aberration contribution to the electron probe size as demonstrated with calculations and images of suitable samples. Secondary electron image resolution at low beam energies is ultimately limited by noise in the images as shown by the CTFs.     

Resolution in low voltage scanning electron microscopy

As the energy of an electron beam is reduced, the range falls and the secondary electron yield rises. A low voltage scanning electron microscope can therefore, in principle, examine without damage or charging samples such as insulators, dielectrics or beam sensitive materials. This paper investigates the way in which the choice of beam energy affects the spatial resolution of a secondary electron image. It is shown that for samples which are thin compared to the electron range, the edge resolution and contrast in the image improve with increasing beam energy. In samples that are thicker than the electron range, the resolution can be optimized at either high or low energies, but low energy operation will produce images of higher contrast. At an energy of 2 keV or less beam interaction limited resolutions of the order of 3 nm should be possible.     

The use of backscattered electrons for imaging purposes in a scanning electron microscope.

It is shown that the use of a very large detector for backscattered electrons can provide a signal comparable to that obtained in the secondary emission mode and that the resolution available is also comparable. A technique is described whereby the difference is taken between these two signals, and the use of this difference signal can significantly enhance surface details. It is believed that the use of such a signal should ultimately prove advantageous in increasing resolution.     

Microchannel plate detector for low voltage scanning electron microscopes

A microchannel plate detector has advantages over conventional Everhart-Thornley detectors for low voltage SEM applications. A microchannel plate can provide symmetric SEM signal detection at the low beam energies needed for non-destructive examination of integrated circuits. Microchannel plate detectors are effective for both secondary electron and backscattered electron imaging. Their relatively small size and ability to be mounted directly below the final pole piece give them improved performance relative to conventional detectors in applications requiring short working distances. A significant amount of laboratory and applications experience has shown that microchannel plates are reliable and sufficiently resistant to contamination for use in high volume, production environments which require low voltage SEM imaging.     

Double-pulse fluorescence lifetime measurements

It is demonstrated that fluorescence lifetimes in the nanosecond and picosecond time-scale range can be observed with the recently proposed double-pulse fluorescence lifetime imaging technique (Müller et al. , 1995, Double-pulse fluorescence lifetime imaging in confocal microscopy. J. Microsc 177, 171–179).
A laser source with an optical parametric amplifier (OPA) system is used to obtain short pulse durations needed for high time resolution, wavelength tunability for selective excitation of specific fluorophores and high pulse energies to obtain (partial) saturation of the optical transition.
It is shown that fluorescence lifetimes can be determined correctly also with nonuniform saturation conditions over the observation area.
A correction scheme for the effect on the measurements of laser power fluctuations, which are inherently present in OPA systems, is presented. Measurements on bulk solutions of Rhodamine B and Rhodamine 6G in different solvents confirm the experimental feasibility of accessing short fluorescence lifetimes with this technique.
Because signal detection does not require fast electronics, the technique can be readily used for fluorescence lifetime imaging in confocal microscopy, especially when using bilateral scanning and cooled CCD detection.     

Image correlation microscopy for uniform illumination

Image cross-correlation microscopy is a technique that quantifies the motion of fluorescent features in an image by measuring the temporal autocorrelation function decay in a time-lapse image sequence. Image cross-correlation microscopy has traditionally employed laser-scanning microscopes because the technique emerged as an extension of laser-based fluorescence correlation spectroscopy. In this work, we show that image correlation can also be used to measure fluorescence dynamics in uniform illumination or wide-field imaging systems and we call our new approach uniform illumination image correlation microscopy. Wide-field microscopy is not only a simpler, less expensive imaging modality, but it offers the capability of greater temporal resolution over laser-scanning systems. In traditional laser-scanning image cross-correlation microscopy, lateral mobility is calculated from the temporal de-correlation of an image, where the characteristic length is the illuminating laser beam width. In wide-field microscopy, the diffusion length is defined by the feature size using the spatial autocorrelation function. Correlation function decay in time occurs as an object diffuses from its original position. We show that theoretical and simulated comparisons between Gaussian and uniform features indicate the temporal autocorrelation function depends strongly on particle size and not particle shape. In this report, we establish the relationships between the spatial autocorrelation function feature size, temporal autocorrelation function characteristic time and the diffusion coefficient for uniform illumination image correlation microscopy using analytical, Monte Carlo and experimental validation with particle tracking algorithms. Additionally, we demonstrate uniform illumination image correlation microscopy analysis of adhesion molecule domain aggregation and diffusion on the surface of human neutrophils.     

Direct measurement of beam size in a spectroscopic ellipsometry setup

Spectroscopic ellipsometry signals used in thin film analysis are dependent on the beam probe size. In this work, we report a technique to determine the beam size that uses the existing detection facilities in a spectroscopic ellipsometry setup without the need to rearrange the optical components. The intensity signal recorded with the technique comprises a coupled boundary diffraction and knife edge wave that can be isolated using nonlinear fitting. This then permitted an accurate measurement of the beam size with the stronger knife edge component. The technique has the added advantage of picking up chromatic aberration in the probing lens which may be a factor in ellipsometry measurement.     

CARS microscopy with folded BoxCARS phasematching

Three-dimensional microscopy based on coherent anti-Stokes Raman scattering (CARS) is a powerful new imaging technique, in which the contrast arises from molecular vibrations. Based on a simple numerical model, it is shown how the CARS interaction volume depends on the focusing parameters and the type of phasematching used. Collinear phasematching yields an ellipsoidal interaction volume, with lateral dimensions that readily cause vignetting of the CARS signal emission at the collection microscope objective. A folded BoxCARS phasematching geometry, on the other hand, results in an almost cylindrical interaction volume — at the cost of a reduced resolution, for which the possible vignetting of the CARS emission is much reduced. In addition, this type of phasematching provides spatial separation of the signal from the input laser beams, permitting simple signal detection of low frequency vibrational modes. Calculations show that when CARS is performed in a microscopic volume, the phasematching restraint on tuning over the vibrational band is strongly relaxed. A first example of CARS imaging using a folded BoxCARS imaging geometry is shown.     

A beam-position monitor system at the VEPP-4M electron−positron collider

The VEPP-4M storage ring has been designed for experiments with colliding electron?positron beams, a synchrotron radiation beam, and extracted γ-ray beams at energies of 1?5 GeV. A correct equilibrium beam orbit must be kept in the storage ring to maintain the required experimental conditions. A beam position monitor system is used for this purpose. It includes 54 beam position monitors, monitor signal-processing modules, and synch signals distributors. In 2013–2016, new signal processing modules with a high time resolution were developed and installed at the VEPP-4M storage ring. These have two main advantages over the old modules. First, the new modules allow separate position measurements of electron and positron bunches at an interval between the bunch signals as long as 18 ns. Second, they are able to perform fullfledged turn-by-turn beam position measurements, which is very important for adjustment of the storagering parameters. Turn-by-turn beam position measurements can be performed during injection of a new beam, beam kicking, beam loss, or at an arbitrary instant of time. The structures and main features of the new modules are described, the measurement accuracy is analyzed, and some results of their operation at the storage ring are presented.     

Preservation of protein fluorescence in embedded human dendritic cells for targeted 3D light and electron microscopy

In this study, we present a correlative microscopy workflow to combine detailed 3D fluorescence light microscopy data with ultrastructural information gained by 3D focused ion beam assisted scanning electron microscopy. The workflow is based on an optimized high pressure freezing/freeze substitution protocol that preserves good ultrastructural detail along with retaining the fluorescence signal in the resin embedded specimens. Consequently, cellular structures of interest can readily be identified and imaged by state of the art 3D confocal fluorescence microscopy and are precisely referenced with respect to an imprinted coordinate system on the surface of the resin block. This allows precise guidance of the focused ion beam assisted scanning electron microscopy and limits the volume to be imaged to the structure of interest. This, in turn, minimizes the total acquisition time necessary to conduct the time consuming ultrastructural scanning electron microscope imaging while eliminating the risk to miss parts of the target structure. We illustrate the value of this workflow for targeting virus compartments, which are formed in HIV‐pulsed mature human dendritic cells.     

Recent developments and new strategies in scanning electron microscopy*

In addition to improvements in lateral resolution in scanning electron microscopy, recent developments of interest here concern extension of the incident beam energy, E₀, over two decades, from ≈ 20 keV to ≈ 0.1–0.5 keV and the possibility of changing the take-off emission, α, of detected secondary electrons. These two degrees of freedom for image acquisition permit a series of images of the same field of view of a specimen to be obtained, each image of the series differing from the others in some aspect. The origins of these differences are explored in detail and they are tentatively interpreted in terms of the change in the secondary electron emission yield δ vs. E₀, δ = f(E₀), and also of the change in δ vs. α, ∂δ/∂α. Various origins for the chemical contrast and topographic contrast have been identified. Illustrated by correlating a secondary electron image and a backscattered electron image, use of the scatter diagram technique facilitates image comparison. The difference between the lateral resolution and the size of the minimum detectable detail is outlined to avoid possible errors in nanometrology. Some aspects related to charging are also considered and possible causes of contrast reversal are suggested. Finally, the suggested strategy consists of the acquisition of various images of a given specimen by changing one parameter: primary beam energy and take-off angle for conductive specimens; working distance or beam intensity for high-resolution experiments; scanning frequency for insulating specimens.     

Dynamic growth of carbon nanopillars and microrings in electron beam induced dissociation of residual hydrocarbons

Electron beam induced dissociation (EBID) of residual hydrocarbons is a common contamination problem in electron microscopy. Formation of amorphous carbon thin films, dots, rings, and complex three-dimensional (3D) structures has been documented. In recent years there has been a renewed interest in utilization EBID of residual hydrocarbons for nanomanufacturing and metrology. With the increase and the diversification of EBID applications, ability for an a priori prediction of the appropriate deposition settings (such as electron beam current, diameter, and energy) that would result in a desired deposit size and geometry is becoming increasingly important. Toward this end, we report how simulations can be used to quantitatively predict the complex shape and non-linear dynamics of secondary ring-type microstructures formed around nanopillar deposits. The deposition experiments were performed on a Si(100) substrate for different electron beam energies and currents. Deposit shape and transient evolution were characterized using atomic force microscopy and critically compared against simulations results.     

Spectrally resolved cathodoluminescence analyses in the optical near-field

By implementing a scanning near-field optical microscope into the specimen chamber of a scanning electron microscope, cathodoluminescence can be locally detected in the optical near-field. The achievable spatial resolution in this set-up is only limited by the size of the aperture in a coated fibre probe and its separation from the sample, rather than by the energy dissipation volume of the primary electrons and diffusion processes of excess carriers inside the specimen. We demonstrate how electronically active defects in polycrystalline diamond can be distinguished and localized with sub-wavelength lateral resolution by spectral filtering of the cathodoluminescence signal.     

基于米散射激光雷达的大气气溶胶检测系统设计

设计了一种基于米散射激光雷达的大气气溶胶检测系统。依据米散射激光雷达的原理,研究了激光光束与大气气溶胶的相互作用,设计并搭建了米散射激光雷达大气气溶胶检测实验平台,利用Zemax设计了一套具有滤波聚焦功能的光学望远系统。通过对光束进行扩束并利用可调谐式固定装置来增强回波信号强度。实验结果表明,设计的系统具有高的回波信号强度和低的杂散光干扰,能获得超高信噪比的激光回波信号。由此大大降低了后续算法的难度,使测得的气溶胶数据更加准确。     

Spectrally resolved cathodoluminescence analyses in the optical near-field

By implementing a scanning near-field optical microscope into the specimen chamber of a scanning electron microscope, cathodoluminescence can be locally detected in the optical near-field. The achievable spatial resolution in this set-up is only limited by the size of the aperture in a coated fibre probe and its separation from the sample, rather than by the energy dissipation volume of the primary electrons and diffusion processes of excess carriers inside the specimen. We demonstrate how electronically active defects in polycrystalline diamond can be distinguished and localized with sub-wavelength lateral resolution by spectral filtering of the cathodoluminescence signal.     

Progress in quantitative X-ray microanalysis of frozen-hydrated bulk biological samples

The analysis of bulk frozen-hydrated biological samples has developed now to a level where practical application of the technique is possible. Provided the sample is carefully coated with a conductive metal, the development of a space charge capable of causing a significant distortion of the electron diffusion volume does not seem to occur, and analytical resolution can be conveniently held to approximately 2 μm (both depth and lateral resolution). Two valid quantitative methods are available, and two methods of determining dry weight fractions are also available. An area where further research could lead to improvement in analysis of frozen-hydrated bulk samples is in the investigation of fracturing methods. If fracture planes that were flat and reproducible could be easily obtained, some of the difficulties of analysing frozen-hydrated bulk samples would be considerably reduced.