Conditions required for detection of specimen-specific SE-I secondary electrons in an analytical SEM

An analytical SEM equipped with an above-the-lens detector, an in-the-lens specimen stage and a high brightness LaB₆ emitter was used to produce a specimen-specific, secondary electron-I (SE-I) signal for recording edge brightness contrast with high intensity on small particles at high magnification (200,000). The SE-I edge brightness contrast produced from 20–40 nm colloidal gold on silicon wafers was useful for estimating instrument resolution since the edge brightness is the sum of the SE-I signal range (≌ 1 nm) and the beam diameter. LaB₆ crystal saturation and gun conditions were determined in order to minimize the probe diameter at the first cross-over position. Ferritin particles also on the silicon wafers were imaged by adjustments of the gun bias voltage conditions. Establishment of these conditions was useful for high resolution SEM studies of appropriately coated bulk biological specimens.     

Nanotip electron gun for the scanning electron microscope

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.     

Broad beam sources of fast molecules with segmented cold cathodes and emissive grids

Experimental study of fast neutral atom and molecule beam sources with rectangular and circular cross-section of the beam up to 0.8 m² is carried out and the study results are presented. The fast particles are produced as a result of charge exchange collisions between gas molecules and ions accelerated by potential drop between the plasma emitter of the beam source and the secondary plasma inside the processing vacuum chamber. As the emitter is used a glow discharge plasma, whose electrons are confined in an electrostatic trap formed by a cold hollow cathode and an emissive grid, which is negative both to the cathode and to the chamber. In order to prevent from breakdowns between the emitter and the cathode at a current in the cathode circuit up to 10 A as well as between the emitter and the grid at a voltage between them up to 10 kV the cathode and the grid are composed of isolated from each other segments, which are connected to power supplies through resistors. When resistance of the resistorR > U/I ₀, where U is the power supply voltage and I ₀ is the minimal current of stable vacuum arc for a given segment material, then transition from the glow discharge to the steady-state vacuum arc is totally excluded in spite of numerous breakdowns of microsecond duration due to contamination of the source electrodes during its operation with dielectric films and other stimulants of the arc.     

Low voltage scanning electron microscopy

The scanning electron microscope (SEM) is usually operated with a beam voltage, V₀, in the range of 10–30 kV, even though many early workers had suggested the use of lower voltages to increase topographic contrast and to reduce specimen charging and beam damage. The chief reason for this contradiction is poor instrumental performance when V₀=1–3 kV, The problems include low source brightness, greater defocusing due to chromatic aberration greater sensitivity to stray fields, and difficulty in collecting the secondary electron signal. Responding to the needs of the semiconductor industry, which uses low V₀ to reduce beam damage, considerable efforts have been made to overcome these problems. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode. This paper reviews the advantages of low voltage operation, recent progress in instrumentation and describes a prototype instrument designed and built for optimum performance at 1 kV. Other limitations to high resolution topographic imaging such as surface contamination, the de-localized nature of the inelastic scattering event and radiation damage are also discussed.     

Measurement of magnetic domain wall width using energy-filtered Fresnel images

Magnetic domain walls in Nd₂Fe₁₄B have been examined using a series of energy‐filtered Fresnel images in the field emission gun transmission electron microscope (FEGTEM). We describe the changes in the intensity distribution of the convergent wall image as a function of defocus, foil thickness and domain wall width. The effect of tilted domain walls and beam convergence on the fringe pattern is also discussed. A comparison of the experimental intensity profile with that from simulations allows the domain wall width to be determined. Measurement of very narrow walls is made possible only by using a relatively thick foil, which necessitates energy‐filtering to allow quantitative comparison with simulations. The magnetic domain wall width in Nd₂Fe₁₄B was found to be 3 ± 2 nm.     

‘Collapsing rings’ on Schottky electron emitters

The electron beam in systems that use a Schottky emitter as the electron source can display periodic fluctuations when the emitter is operated at an extraction voltage that gives a relatively low field strength at the tip. In the past, these fluctuations have been associated with the so-called “collapsing rings” without much further information. In this paper, the tip’s geometry changes associated with these beam instabilities are investigated in more detail by recording the evolution of the emission pattern of a Schottky emitter showing ‘collapsing rings’ for different operating conditions. Scanning electron microscope (SEM) images of different Schottky emitters have been used to support the interpretation.     

Diffraction patterns of artificial two-dimensional crystals synthesized in situ in an environmental scanning transmission electron microscope

In this study, we demonstrated the use of electron‐beam‐induced deposition for synthesis of artificial two‐dimensional crystals with an in situ scanning transmission electron microscope. The structures were deposited from W(CO)₆ in an environmental scanning transmission electron microscope on a 30‐nm‐thick Si₃N₄ substrate. We present clear electron beam diffraction patterns taken from those structures. The distance between the diffraction peaks corresponded to the dot spacing in the self‐made surface crystal. We propose using these arrays of dots as anchor points for making artificial crystals for diffraction analysis of weakly scattering or beam‐sensitive molecules such as proteins.     

Beam heating in an asymmetrically illuminated thin circular film

The method of images is used to calculate the temperature increase produced in a circular film when the film is illuminated at any point by an electron beam of circular cross-section and constant current density over its cross-section. Formulae are given in dimensionless form for (i) the temperature increase anywhere along the line joining the centres of the beam and of the film, (ii) the temperature increase at the centre of the beam (v_c), and (iii) the position and value of the maximum temperature increase in the film v_m. The relative sizes of v_c and v_m are discussed and it is shown that (v_m – v_c)/v_m ≤ 0.079. The theory applies best to metallic films where power loss by means of thermal conductivity is dominant. A brief discussion is given of the cases where thermal radiation is important, sample calculations are given for carbon and copper films, and the effect of focusing the electron beam at constant current is considered.     

Characteristics of a fast neutral atom source with electrons injected into the source through its emissive grid from the vacuum chamber

In order to increase the equivalent current of a fast neutral atom beam the cold hollow cathode of the beam source is bombarded with electrons extracted from the plasma produced in the vacuum chamber and accelerated in the sheath between the plasma emitter of the source and its emissive grid. The cold cathode bombardment by accelerated electrons raises its electron emission current by an order of magnitude and as a result voltage U _c between the anode and the cathode of the source diminishes more than two times. This allows of increasing several times the beam equivalent current or decreasing the working gas pressure. A slight decrease in the U _c with increasing the accelerating voltage U at an overall cutoff of the electrons from the chamber reveals the influence of secondary electrons emitted by the grid. Measurement of the beam current is discussed.     

Combining Ar ion milling with FIB lift-out techniques to prepare high quality site-specific TEM samples

Focused ion beam (FIB) techniques can prepare site‐specific transmission electron microscopy (TEM) cross‐section samples very quickly but they suffer from beam damage by the high energy Ga⁺ ion beam. An amorphous layer about 20–30 nm thick on each side of the TEM lamella and the supporting carbon film makes FIB‐prepared samples inferior to the traditional Ar⁺ thinned samples for some investigations such as high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). We have developed techniques to combine broad argon ion milling with focused ion beam lift‐out methods to prepare high‐quality site‐specific TEM cross‐section samples. Site‐specific TEM cross‐sections were prepared by FIB and lifted out using a Narishige micromanipulator onto a half copper‐grid coated with carbon film. Pt deposition by FIB was used to bond the lamellae to the Cu grid, then the coating carbon film was removed and the sample on the bare Cu grid was polished by the usual broad beam Ar⁺ milling. By doing so, the thickness of the surface amorphous layers is reduced substantially and the sample quality for TEM observation is as good as the traditional Ar⁺ milled samples.     

A systematic investigation of the charging effect in scanning electron microscopy for metal nanostructures on insulating substrates

Scanning electron microscopy is perhaps the most important method for investigating and characterizing nanostructures. A well‐known challenge in scanning electron microscopy is the investigation of insulating materials. As insulating materials do not provide a path to ground they accumulate charge, evident as image drift and image distortions. In previous work, we have seen that sample charging in arrays of metal nanoparticles on glass substrates leads to a shrinkage effect, resulting in a measurement error in the nanoparticle dimension of up to 15% at 10 kV and a probe current of 80 ± 10 pA. In order to investigate this effect in detail, we have fabricated metal nanostructures on insulating borosilicate glass using electron beam lithography. Electron beam lithography allows us to tailor the design of our metal nanostructures and the area coverage. The measurements are carried out using two commonly available secondary electron detectors in scanning electron microscopes, namely, an InLens‐ and an Everhart–Thornley detector. We identify and discriminate several contributions to the effect by varying microscope settings, including the size of the aperture, the beam current, the working distance and the acceleration voltage. We image metal nanostructures of various sizes and geometries, investigating the influence of scan‐direction of the electron beam and secondary electron detector used for imaging. The relative measurement error, which we measure as high as 20% for some settings, is found to depend on the acceleration voltage and the type of secondary electron detector used for imaging. In particular, the Everhart–Thornley detectors lower sensitivity to SE₁ electrons increase the magnitude of the shrinkage of up to 10% relative to the InLens measurements. Finally, a method for estimating charge balance in insulating samples is presented.     

Calculation of the variable-profile electron beam for electron coolers

Interaction of an electron beam with a cooled ion beam makes it possible to reduce its phase volume, perform accumulation of particles, and suppress various “heating” effects. The electron beam can also be used as a target for an electron-ion recombination reaction, which offers a chance to carry out atomic physics experiments and ensure slow uniform extraction of the ion beam from the storage ring. A high-perveance electron beam with a variable profile is required for effective cooling, while a high current density and a low energy of transverse motion of electrons in the beam is needed for extraction by means of recombination. It is shown that a convex cathode placed in a magnetic field can be used to form such a beam. A high current density can be attained with this shape of the cathode, but additional efforts must be focused on optimizing the gun’s optics in order to obtain a low energy of transverse motion of particles. Since ions repeatedly pass through the cooling section during their lifetime at different values of the betatron oscillation phase, the rates of recombination and cooling are dependent on the rms electron velocity averaged over the volume in which the beam interaction occurs. The proposed design of the gun with a convex cathode 10.2 mm in diameter ensures formation of a variable-profile electron beam with a nominal current of 1 A and a current density of 1.2 A/cm². The rms energy of Larmor gyration of electrons at the exit from the gun, averaged over the beam cross section (the “transverse” temperature) is 1 eV. A focusing electrode that forms the Pierce optics near the edge of the cathode, an electrode controlling the beam profile, and an anode are included in the optics of the electron gun.     

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.     

Emitters based on semiconductor Bragg microcavities

This paper presents a brief review of the progress in the development of two types of semiconductor emitters based on semiconductor Bragg microcavities. The first type of these emitters is a vertical-cavity laser based on Al _x Ga_1−x As. The laser demonstrates stable single-mode lasing at a wavelength of 795 nm, which opens prospects for its use in miniature atomic frequency standards on the basis of Rb⁸⁷. The second type is a single-photon emitter based on semiconductor quantum dots. The design of such an emitter was developed on the basis of a semiconductor Bragg microcavity providing a high level of external quantum efficiency of the emitter (up to 80%) and high performance due to the Purcell effect.     

Magnetic field emission gun with zirconiated emitter

A magnetic-field-superimposed field emission gun with low aberrations and equipped with a zirconiated tungsten emitter has been developed for applications where very stable high probe currents are required. It has been tested on a conventional electron microscope at 10 kV and on an electron beam testing system at 1 kV. Probe current i = 250 nA in a probe size d = 0.4 μm is obtained at 10 kV; at 1 kV the resolution is 0.1 μm with i = 5 nA, and 0.4 μm with i = 30 nA. For these probe currents, the spatial broadening effect due to electron–electron interactions in the beam is the preponderant factor limiting the probe size.     

An apparatus based on a plasma emitter for electron-beam transportation to air

An apparatus for electron-beam transportation to air has been developed on the basis of an electron gun with a plasma emitter. A pressure drop from values of (1–3) × 10^?4 Torr in an accelerating gap to an air pressure of 760 Torr is produced by the differential pumping system. Usually, no less than three pumping stages are used in apparatuses based on hot-cathode guns for electron-beam transportation to air because of the need to maintain the pressure in the hot-cathode area at a level of 10^?5 Torr. It is possible to simplify the differential pumping system by reducing the number of stages to two via to the use of a gun with a plasma emitter. The construction of an apparatus for electron-beam transportation to air is described and its key characteristics are presented.     

In situ electron beam induced current measurements of the local thickness in semiconductor devices

A quantitative electron beam induced current method is shown, applicable in situ for electron beam current measurement on a semiconductor sample without the need for a Faraday cup. As a validation technique, the measurement of top overlayer thickness in the semiconductor structure was chosen for two reasons. First, the measured thickness is easily verified using the same electron microscope in the secondary electron mode by measuring the layer thickness at the layer edges. Second, the measurement of a layer thickness and its local variations constitute an important issue in semiconductor processing and characterization. The proposed method is used in the planar view of the sample, and also for locations far from the layer edges. Quasi‐three‐dimensional maps of the thickness spatial distribution are presented.     

Limitations of beam damage in electron spectroscopic tomography of embedded cells

Elemental mapping in the energy filtering transmission electron microscope (EFTEM) can be extended into three dimensions (3D) by acquiring a series of two‐dimensional (2D) core‐edge images from a specimen oriented over a range of tilt angles, and then reconstructing the volume using tomographic methods. EFTEM has been applied to imaging the distribution of biological molecules in 2D, e.g. nucleic acid and protein, in sections of plastic‐embedded cells, but no systematic study has been undertaken to assess the extent to which beam damage limits the available information in 3D. To address this question, 2D elemental maps of phosphorus and nitrogen were acquired from unstained sections of plastic‐embedded isolated mouse thymocytes. The variation in elemental composition, residual specimen mass and changes in the specimen morphology were measured as a function of electron dose. Whereas 40% of the total specimen mass was lost at doses above 10⁶ e^?/nm², no significant loss of phosphorus or nitrogen was observed for doses as high as 10⁸ e^?/nm². The oxygen content decreased from 25 ± 2 to 9 ± 2 atomic percent at an electron dose of 10⁴ e^?/nm², which accounted for a major component of the total mass loss. The specimen thickness decreased by 50% after a dose of 10⁸ e^?/nm², and a lateral shrinkage of 9.5 ± 2.0% occurred from 2 × 10⁴ to 10⁸ e^?/nm². At doses above 10⁷ e^?/nm², damage could be observed in the bright field as well in the core edge images, which is attributed to further loss of oxygen and carbon atoms. Despite these artefacts, electron tomograms obtained from high‐pressure frozen and freeze‐substituted sections of C. elegans showed that it is feasible to obtain useful 3D phosphorus and nitrogen maps, and thus to reveal quantitative information about the subcellular distributions of nucleic acids and proteins.     

Construction and characterization of the fringe field monochromator for a field emission gun

Although some microscopes have shown stabilities sufficient to attain below 0.1 eV spectral resolution in high-resolution electron energy loss spectroscopy, the intrinsic energy width of the high brightness source (0.3-0.6 eV) has been limiting the resolution. To lower the energy width of the source to 50 meV without unnecessary loss of brightness, a monochromator has been designed consisting of a short (4 mm) fringe field Wien filter and a 150 nm energy selection slit (nanoslit) both to be incorporated in the gun area of the microscope. A prototype has been built and tested in an ultra-high-vacuum setup (10(-9) mbar). The monochromator, operating on a Schottky field emission gun, showed stable and reproducible operation. The nanoslits did not contaminate and the structure remained stable. By measuring the current through the slit structure a direct image of the beam in the monochromator could be attained and the monochromator could be aligned without the use of a microscope. Good dispersed imaging conditions were found indicating an ultimate resolution of 55 meV. A Mark II fringe field monochromator (FFM) was designed and constructed compatible with the cold tungsten field emitter of the VG scanning transmission microscope. The monochromator was incorporated in the gun area of the microscope at IBM T.J. Watson research center, New York. The monochromator was aligned on 100 kV and the energy distribution measured using the monochromator displayed a below 50 meV filtering capability. The retarding Wien filter spectrometer was used to show a 61 meV EELS system resolution. The FFM is shown to be a monochromator which can be aligned without the use of the electron microscope. This makes it directly applicable for scanning transmission microscopy and low-voltage scanning electron microscopy, where it can lower the resolution loss which is caused by chromatic blur of the spot.