Combining high mass resolution and velocity imaging in a time-of-flight ion spectrometer using pulsed fields and an electrostatic lens

We describe a momentum resolving time-of-flight ion mass spectrometer that combines a high mass resolution, a velocity focusing condition for improved momentum resolution, and field-free conditions in the source region for high resolution electron detection. It is used in electron-ion coincidence experiments to record multiple ionic fragments produced in breakup reactions of small to medium sized molecules, such as F(3)SiCH(2)CH(2)Si(CH(3))(3). These breakup reactions are caused by soft x rays or intense laser fields. The ion spectrometer uses pulsed extraction fields, an electrostatic lens, and a delay line detector to resolve the position. Additionally, we describe a simple analytical method for calculating the momentum from the measured hit position and the time of flight of the ions.     

First tests of a dipole lens for a scanning electron microscope

We have previously shown that a dipole lens has superior properties that are particularly suited for use in a low voltage scanning electron microscope (SEM) ( Tsai & Crewe, 1996 ). The aberrations are lower than for any other type of lens and lead to a prediction of high resolution. We describe the construction details of a microscope based on this principle and present some early results.     

A high-resolution add-on in-lens attachment for scanning electron microscopes

A compact add-on objective lens for the scanning electron microscope (SEM) has been designed and tested. The lens is < 35 mm high and can be fitted on to the specimen stage as an easy-to-use attachment. Initial results show that it typically improves the spatial resolution of the SEM by a factor of three. The add-on unit is based upon a permanent magnet immersion lens design. Apart from the extra attachment to the specimen stage, the SEM with the add-on lens functions in the normal way. The in-lens unit can comfortably accommodate specimen heights up to 10 mm. The new add-on lens unit opens up the possibility of operating existing SEMs in the high-resolution in-lens mode. By using a deflector at the top of the add-on lens unit, it can also operate as a quantitative multichannel voltage contrast spectrometer, capable of recording the energy spectrum of the emitted secondary electrons. Initial experiments confirm that a significant amount of voltage contrast can be obtained.     

X-ray analysis and mapping by wavelength dispersive X-ray spectroscopy in an electron microscope

A compact and easy-to-use wavelength dispersive X-ray spectrometer using a multi-capillary X-ray lens attached to a scanning (transmission) electron microscope has been tested for thin-film analysis. B-K spectra from thin-film boron compounds (B4C, h-BN, and B2O3) samples showed prominent peak shifts and detailed structural differences. Mapping images of a thin W/Si double-layer sample resolved each element clearly. Additionally, a thin SiO2 film grown on a Si substrate was imaged with O-K X-rays. Energy and spatial resolution of the system is also discussed.     

A sub-50meV spectrometer and energy filter for use in combination with 200kV monochromated (S)TEMs

A high-energy resolution post-column spectrometer for the purpose of electron energy loss spectroscopy (EELS) and energy-filtered TEM in combination with a monochromated (S)TEM is presented. The prism aberrations were corrected up to fourth order using multipole elements improving the electron optical energy resolution and increasing the acceptance of the spectrometer for a combination of object area and collection angles. Electronics supplying the prism, drift tube, high-tension reference and critical lenses have been newly designed such that, in combination with the new electron optics, a sub-50 meV energy resolution has been realized, a 10-fold improvement over past post-column spectrometer designs. The first system has been installed on a 200 kV monochromated TEM at the Delft University of Technology. Total system energy resolution of sub-100 meV has been demonstrated. For a 1s exposure the resolution degraded to 110 meV as a result of noise. No further degradation in energy resolution was measured for exposures up to 1 min at 120 kV. Spectral resolution measurements, performed on the pi* peak of the BN K-edge, demonstrated a 350 meV (FWHM) peak width at 200 kV. This measure is predominantly determined by the natural line width of the BN K-edge.     

Integration of a high‐NA light microscope in a scanning electron microscope

We present an integrated light‐electron microscope in which an inverted high‐NA objective lens is positioned inside a scanning electron microscope (SEM). The SEM objective lens and the light objective lens have a common axis and focal plane, allowing high‐resolution optical microscopy and scanning electron microscopy on the same area of a sample simultaneously. Components for light illumination and detection can be mounted outside the vacuum, enabling flexibility in the construction of the light microscope. The light objective lens can be positioned underneath the SEM objective lens during operation for sub‐10 μm alignment of the fields of view of the light and electron microscopes. We demonstrate in situ epifluorescence microscopy in the SEM with a numerical aperture of 1.4 using vacuum‐compatible immersion oil. For a 40‐nm‐diameter fluorescent polymer nanoparticle, an intensity profile with a FWHM of 380 nm is measured whereas the SEM performance is uncompromised. The integrated instrument may offer new possibilities for correlative light and electron microscopy in the life sciences as well as in physics and chemistry.     

An optical configuration for fastidious STEM detector calibration and the effect of the objective‐lens pre‐field

In the scanning transmission electron microscope, an accurate knowledge of detector collection angles is paramount in order to quantify signals on an absolute scale. Here we present an optical configuration designed for the accurate measurement of collection angles for both image‐detectors and energy‐loss spectrometers. By deflecting a parallel electron beam, carefully calibrated using a diffraction pattern from a known material, we can directly observe the projection‐distortion in the post‐specimen lenses of probe‐corrected instruments, the 3‐fold caustic when an image‐corrector is fitted, and any misalignment of imaging detectors or spectrometer apertures. We also discuss for the first time, the effect that higher‐order aberrations in the objective‐lens pre‐field has on such an angle‐based detector mapping procedure.     

Accurate stigmating of a high voltage electron microscope

To reach the 0·2 nm point-to-point resolution possible with some high voltage electron microscopes, the astigmatism of the objective lens must be compensated to within 5 nm. Due to a number of factors the resolution of the image seen on the viewing screen of the high voltage microscope is, however, quite poor and does not permit compensation of such accuracy. We describe a technique for evaluating and correcting the astigmatism that starts from a recorded micrograph of a thin amorphous specimen. The astigmatism is determined from the optical diffraction pattern using a variation of the Thon method. This variation avoids any direct measurement of the radii of the contrast transfer zones, and is extremely rapid and convenient. Adjusting the stigmator coil currents, calibrated in terms of their stigmating power, for zero astigmatism completes the correction in less than 10 min after the recording of the micrograph.     

A new aberration-corrected,energy-filtered LEEM/PEEM instrument. I. Principles and design

We describe a new design for an aberration-corrected low energy electron microscope (LEEM) and photo electron emission microscope (PEEM), equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control over the chromatic and spherical aberration coefficients C_c and C₃, as well as the mirror focal length, to match and correct the aberrations of the objective lens. For LEEM (PEEM) the theoretical resolution is calculated to be ∼1.5 nm (∼4 nm). Unlike previous designs, this instrument makes use of two magnetic prism arrays to guide the electron beam from the sample to the electron mirror, removing chromatic dispersion in front of the mirror by symmetry. The aberration correction optics was retrofitted to an uncorrected instrument with a base resolution of 4.1 nm in LEEM. Initial results in LEEM show an improvement in resolution to ∼2 nm.     

Transfer-matrix-based method for an analytical description of velocity-map-imaging spectrometers

We propose a simple and general analytical model describing the operation of a velocity-map-imaging spectrometer. We show that such a spectrometer, possibly equipped with a magnifying lens, can be efficiently modeled by combining analytical expressions for the axial potential distributions along with a transfer matrix method. The model leads transparently to the prediction of the instrument's operating conditions as well as to its resolution. A photoelectron velocity-map-imaging spectrometer with a magnifying lens, built and operated along the lines suggested by the model has been successfully employed for recording images at threshold photoionization of atomic lithium. The model's reliability is demonstrated by the fairly good agreement between experimental results and calculations. Finally, the limitations of the analytical method along with possible generalizations, extensions, and potential applications are also discussed. The model may serve as a guide for users interested in building and operating such spectrometers as well as a tutorial tool.     

The design and implementation of a high-fidelity Raman imaging microscope

We describe a Raman imaging microscope that produces high-fidelity, large format Raman images and Raman spectra from samples as small as 1 μm in size. Laser illumination is delivered to the object by means of an infinity corrected microscope objective, either by a galvanometer scanning system or a widefield fibre optic. Wavelength selection of Raman scattered emission is achieved by an acousto-optic tunable filter (AOTF), which maintains image fidelity and provides either continuous or random wavelength selection. The collimated AOTF output is imaged first by a tube lens and then by a projection lens onto a cooled silicon CCD array. Instrument features, including factors that determine the system's spatial and spectral resolution, and design considerations are discussed in detail. Images and spectra of test objects and samples that demonstrate the capability of this imaging spectrometer are presented. The potential of intrinsic chemical imaging is discussed in terms of its use in the analyses of a variety of chemical and biological samples.     

Scanning reflection image from a solid specimen in the scanning electron microscope with a condenser-objective lens

To achieve the highest resolution in the scanning electron microscope (SEM) or in the scanning transmission electron microscope (STEM), the sample must be mounted in the high-field region of a condenser-objective lens. A secondary electron (SE) image can then be obtained using a collector before the lens. It is also possible to obtain a scanning reflection image by tilting the specimen so that the second half of the condenser-objective lens field deflects the forward-scattered electrons onto the transmission detector beyond the specimen. Experiments were made with an unmodified commercial SEM fitted with a condenser-objective in the upper stage and with a transmission detector, and it was found that the scanning reflection image from a solid sample can provide additional useful information when used in conjunction with the SE image.     

Zero-loss electron microscopy with the Zeiss EM902

The recently introduced Zeiss EM902 is the first commercially available electron microscope to incorporate a magnetic spectrometer capable of forming directly energy-filtered images and diffraction patterns. We have briefly assessed the potential usefulness of this microscope for structural studies of thin biological specimens, through removal of inelastically scattered electrons. The improvements we find in image contrast, without significant loss of resolution, suggest energy-filtering may prove particularly worthwhile with unstained specimens.     

High-resolution,energy-dispersive microcalorimeter spectrometer for X-ray microanalysis

We have developed a prototype X-ray microcalorimeter spectrometer with high energy resolution for use in X-ray microanalysis. The microcalorimeter spectrometer system consists of a superconducting transition-edge sensor X-ray microcalorimeter cooled to an operating temperature near 100 mK by a compact adiabatic demagnetization refrigerator, a superconducting quantum interference device current amplifier followed by pulse-shaping amplifiers and pileup rejection circuitry, and a multichannel analyser with computer interface for the real-time acquisition of X-ray spectra. With the spectrometer mounted on a scanning electron microscope, we have achieved an instrument response energy resolution of better than 10 eV full width at half-maximum (FWHM) over a broad energy range at real-time output count rates up to 150 s^?1. Careful analysis of digitized X-ray pulses yields an instrument-response energy resolution of 7.2 ± 0.4 eV FWHM at 5.89 keV for Mn Kα_1,2 X-rays from a radioactive ⁵⁵Fe source, the best reported energy resolution for any energy-dispersive detector.     

Initial resolution measurements of an improved magnetic-electrostatic detector objective lens for LVSEM

In this paper we present some initial resolution measurements of an improved magnetic-electrostatic detector objective lens for a low-voltage scanning electron microscope. The electron optical design of the lens was already proposed by the authors [G. Knell, E. Plies, Nucl. Instr. & Meth. A 427 (1999) 99]. The magnetic circuit of this lens has a radially arranged pole-piece gap. Thus, the specimen is immersed in a strong magnetic field of 106 mT (working distance: 1 mm, primary electron energy: 200 eV). The electrostatic field strength of our optimized lens variant amounts to a moderate value of 100 V/mm for a working distance of 1 mm. At a final beam energy of 1 keV a resolution of 3 nm, at 260 eV a resolution of 5 nm was obtained.     

Determination of conditions for optimum resolution of a high angle thin window energy dispersive spectrometer.

The energy resolution of an energy dispersive spectrometer (EDS) equipped with an ultrathin window (UTW) and mounted at a high take-off angle (72 degrees) on a transmission electron microscope has been studied under a variety of operating conditions. The spectrometer resolution is close to that specified by the manufacturer, up to count rates of 400 cps. Above 400 cps the resolution deteriorates rapidly, and the MCA dead time and zero width increase. Above 10 keV, the height of the background is much greater than expected for bremsstrahlung and shows the shape which has previously been attributed to backscattered electron flux into the detector. It is postulated that the deterioration in resolution with count rate is caused by backscattered electrons reaching the detector through the UTW.     

Upper limits for the residual aberrations of a high-resolution aberration-corrected STEM

The development of correctors for electron optical systems has already brought the improvement of resolution for a low-voltage scanning electron microscope and a commercially available transmission electron microscope and is anticipated in the near future for a dedicated scanning transmission electron microscope (STEM). The resolution attainable especially of a probe-forming system at 200 kV cannot only be estimated from calculations ignoring all non-rotationally symmetric axial aberrations in an electron optical system. For a certain resolution, one would like to attain, the influence of the deviations from the ideal, aberration-free system has to be investigated. Therefore, in the following we have carried out the evaluation of the required accuracy for the compensation of the various residual aberrations in order to achieve a resolution in the sub-Angstrom regime with a probe-forming system.     

The electron microscope: The British contribution

The unpredictable development of the electron microscope is reviewed in the context of the 150th Anniversary of the Royal Microscopical Society. Abbe convinced himself that an electron microscope could never be constructed. Later, J. J. Thomson established the minute corpuscular nature of the electron and Rutherford realized that such beams can reveal the nature of the atom; his student H. G. Moseley invented electron probe analysis, while looking for X-ray spectra, but the technique did not seem very practical. G. P. Thomson invented the electron diffraction camera, but it was of limited use without an electron lens. In 1931, Knoll and Ruska built a two-stage TEM with magnetic lenses and understood where Abbe went wrong. Although Ernst Ruska surpassed the resolving power of the light microscope in 1933, the first commercial TEM was manufactured in the U.K., leading to many further commercial developments. The realization of a digital computer with a stored program, now indispensible in microscopy, was also achieved first in the U.K. Gabor's electron beam holography, invented in Rugby in 1949, was not a challenge to the conventional TEM at first, but recently atomic resolution holography has been achieved in Abbe's native country, using a digital computer to process the hologram, ironically making use of the very principles laid down by Abbe in the last century for the light microscope.     

Design and fabrication of a scanning electron microscope using a finite element analysis for electron optical system

In this paper, we describe the design and fabrication of a thermionic scanning electron microscope (SEM) and examine its characteristics analytically. In the design process, the dimensions and capacity of the SEM components, such as the electron column, lenses, and apertures, were determined using finite element analysis. All components were integrated systematically during fabrication in order to achieve the maximum performance by adjusting the lens parameters, high voltage source, and image calibration methods. As a result, a thermionic SEM image with high resolution was achieved. We discuss the primary considerations required to achieve a high-performance image.     

Early results using high-resolution,low-voltage,low-temperature SEM

Recent advances in the design of the scanning electron microscope (SEM) column, such as the coupling of a field-emission gun to a low-aberration immersion lens and the availability of a high-stability cryo-transfer stage, make low-temperature, low-voltage SEM (LTLVSEM) possible at very high resolution. We have used this combination to obtain results with uncoated biological specimens. The trichocyst from a Paramecium was used as a test specimen to observe the shrinkage of this structure as the temperature is raised from 170 K to room temperature following freeze-drying. High-magnification stereo images were obtained of trichocysts that had been prepared by freezing, freeze-substitution and critical-point drying and which were subsequently viewed by LTLVSEM to reduce beam damage and contamination.