Ultrahigh Vacuum Scanning Electron Research Articles

Development of an Ultrahigh-Vacuum Ultrahigh-Resolution Scanning Electron Microscope

An ultrahigh-vacuum (UHV) ultrahigh-resolution (UHR) 100kV scanning electron microscope with a UHV specimen preparation chamber has been developed for in situ observation of clean specimen surfaces. The measured vacuum in the specmen area was about 2.2 × 10-8 Pa, and a 0.14nm lattice image of Au (220) was observed.Field emission scanning electron microscopes have been developed for the past ten years for sub-nanometer analysis. However, in recent analytical electron microscopy of so-called “new materials” such as ceramics and semiconductors, half nanometer analysis in UHV has become extremely important for materials characterization.Figure 1 shows an external view of the microscope. The microscope column and it’s control console are shown at the left and right, respectively. The seales of microscope column are only metal 0 rings and Cu-gaskets, and automatic bake-out is carried out at about 160°C.Figure 2 shows the electron optics of the system. A thermal assisted field emission gun (TFEG) with a W (100) emitter was used as a high-brightness gun, thus ensuring a brightness of 2 × 108 A/cm2 · str at 100 kV.

Scanning Low-Energy Electron Diffraction Microscopy Combined with Scanning Tunnling Microscopy

A ultra-high vacuum scanning electron microscope (UHV-SEM) with a field emission gun (FEG) has been operated in an energy range of from 100 eV to 3 keV. A new technique of scanning low energy electron diffraction (LEED) microscopy has been added to the other techniques: scanning Auger microscopy (SAM), secondary electron microscopy, electron energy loss microscopy and the others available for the UHV-SEM. In addition to scanning LEED microscopy, a scanning tunneling microscope (STM) has been installed in the UHV-SEM-.The combination of STM with SEM covers a wide magnification range from 105 to 107 and is very effective for observation of surface structures with a high resolution of about 1 Å.A UHV-FEG-SEM is equipped in a chamber in which the vacuum is better than 2×10-10 Torr. A movable cylindrical mirror analyzer (CMA), a two dimensional detector of diffracted LEED beams, an ion gun and a deposition source are installed in this chamber. The concept of the scanning LEED microscope is comprised of two steps: (1) the formation of a selected area LEED pattern and (2) the generation of raster images with information contained in the diffraction pattern. In the present experiment, the LEED detector assembly shown in Fig.l has been used; it consists of two hemisherical grids, a two-stage channel-plate amplifier and a position-sensitive detector. The selection of one (or more) diffracted beam is performed electronically by a window using the two-dimensional analogue comparators. The intensity of a particular beam selected by the window modulates the brightness of the scanning image and a dark field image sensitive to the surface structure is formed. The experimental spatial resolutions of 150 Å and 500 Å have been attained at the primary electron energy 1 keV and 250 eV, respectively.

Ultrahigh-Vacuum Scanning Electron Microscope

Ultrahigh-Vacuum Scanning Electron Microscope

Surface roughening and surface melting the high temperature equilibrium shape of small Pb crystals

The equilibrium shape of Pb crystals of 5–20 microm diameter is studied in situ by ultra high vacuum scanning electron microscopy at temperatures above 560 K. Except for the environments of the 〈111〉 and 〈100〉 directions which are flat the surface is spherical. Above 580 K the {111} planes are surrounded by a ring. It is ascribed to a roughened crystalline region between the flat {111} planes and the surface-molten rest of the surface, the {100} planes excepted. The width of the ring decreases with increasing temperature due to the increase of the {111} planes and the surface-molten region. The results are discussed in terms of the current models of surface roughening and surface melting.

μm-Scale Lateral Growth of Ga-Monolayers Observed In-Situ by Electron Microscopy

ABSTRACTAn ultra high vacuum scanning electron microscope equipped to an MBE system is utilized to study a transient of a surface atomic structure during MBE growth of GaAs and AlGaAs by the alternate supply method. Lateral growth of a Ga-monolayer over microns is realized utilizing Ga droplets. This is confirmed by discriminating the Ga and As top layer by using the secondary electron intensity difference between the Ga and As top layer. The growth mechanism of the Ga monolayer is discussed based on the results.

Phase-formation diagram for precursors to epitaxial growth of NiSi2 on Si(111).

Using reflection high-energy electron diffraction and Auger microanalysis in a custom-built ultrahigh vacuum scanning electron microscope we have identified six ordered structures that precede epitaxial ${\mathrm{NiSi}}_{2}$ during the annealing of nickel overlayers on Si(111). A novel technique of depositing through holes in a mask positioned near the sample allows us to obtain the complete coverage dependence of these structures in a single annealing cycle. The role of these structures in relation to the ${\mathrm{NiSi}}_{2}$ (B) (twinned) and ${\mathrm{NiSi}}_{2}$ (A) (nontwinned) structures is described.

Plasmons: quanta for micro-region temperature measurement

An analysis is made of the shifts in the plasmon loss energies for a 1 keV electron beam scattered from pure tin as a function of temperature in ultra-high vacuum. The plasmons in tin are free-electron like and the thermal shifts are shown to arise from the volume expansion and consequent reduction in electron density as the temperature rises. The shifts for the volume plasmon of 0.49 meVK−1 in the solid state, and 0.60 meVK−1 in the liquid state, may be measured fairly readily to an accuracy better than 10 meV and hence provide a temperature measurement to better than 20 K. In the study of solid surfaces in an ultra-high vacuum scanning electron microscope, by a choice of electron beam energy, this method may be used to define the temperature in the outermost 1 nm at a solid surface with a lateral region limited only by the electron probe size. This may be less than 100 nm in modern surface analysis instruments and involve power of less than 10 nW. In the study of thin films by transmission electron microscopy, the temperature may be determined with a spatial resolution close to that of the imaging and involve even lower power inputs than for solid surfaces.

Visualization of submonolayers and surface topography by biassed secondary electron imaging: Application to Ag layers on Si and W surfaces

Abstract An ultra high vacuum-scanning electron microscope (UHV-SEM), equipped with micro-Auger electron spectroscopy, RHEED, and a deposition source has been used to study submpnolayers of Ag on Si(111), Si(100) and W(110) surfaces. A new SEM technique has been developed to visualize these submonolayers directly by applying a high negative bias (−500 V) to the sample. The dependence of the visibility on bias voltage, layer thickness and angle of incidence of the electron beam has been studied. The sensitivity was found to be better than 0.1 ML in the case of Ag on Si surfaces. Some applications to surface diffusion have been made and preliminary results for Ag on Si(111) are noted. Contrast at the 1 ML level can be > 10% in certain cases. The understanding of this contrast in terms of work function and ionization energy (band-bending) changes is discussed.

Visualization of submonolayers and surface topography by biassed secondary electron imaging: Application to Ag layers on Si and W surfaces

Abstract An ultra high vacuum-scanning electron microscope (UHV-SEM), equipped with micro-Auger electron spectroscopy, RHEED, and a deposition source has been used to study submpnolayers of Ag on Si(111), Si(100) and W(110) surfaces. A new SEM technique has been developed to visualize these submonolayers directly by applying a high negative bias (−500 V) to the sample. The dependence of the visibility on bias voltage, layer thickness and angle of incidence of the electron beam has been studied. The sensitivity was found to be better than 0.1 ML in the case of Ag on Si surfaces. Some applications to surface diffusion have been made and preliminary results for Ag on Si(111) are noted. Contrast at the 1 ML level can be > 10% in certain cases. The understanding of this contrast in terms of work function and ionization energy (band-bending) changes is discussed.

Electron spectroscopy in the scanning electron microscope

Abstract The combination of ultra high vacuum scanning electron microscopy with spectroscopy of the emitted electrons gives new possibilities for surface analysis. The paper surveys recent results in secondary, Auger, ionization loss, and elastic peak electron spectroscopy.

Digital data acquisition, display and analysis of signals from surfaces

Abstract A digital data acquisition and display system has been interfaced to an ultra high vacuum scanning electron microscope (UHV-SEM); the sample chamber contains detection systems for Auger and secondary electron spectroscopy and many in-situ surface preparation facilities. The acquisition system is being used for collection and display of this data in the form of spectra, line-scans and images. Subsequent analysis of stored data is done both on this system and on a similar off-line computer system. This paper describes the computer systems chosen and the detection and acquisition hardware developed. Program structures for on-line and off-line analysis are discussed. Examples of applications to surface science are given, including the collection and analysis of micro-Auger spectra, SEM picture analysis of crystals grown in-situ, and Auger and biased secondary electron imaging from sub-monolayer deposits.

Nucleation, growth and the intermediate layer in Ag/Si(100) and Ag/Si(111)

The growth of Ag on Si(100)2 × 1 and Si (111)7 × 7 surfaces has been examined as a function of deposition time and substrate temperature, using an ultra high vacuum-scanning electron microscope (UHV-SEM), equipped with micro-Auger electron spectroscopy and RHEED. Ag grows on both Si surface in the layer plus island or Stranski-Krastanov growth mode. The thickness of the two-dimensional layer has been measured as 0.27 ± 0.03 monolayer on Si(100) in comparison with 0.63 ± 0.04 monolayer on Si(111). The Ag island shapes and sizes, and density dependencies N ( T ) on substrate temperature were found to be very different in the two orientations. The N ( T ) data are interpreted in terms of nucleation theory, and values for the critical nucleus sizes, binding, diffusion and adsorption energies are estimated. Biassing the Si samples made it possible to visualize the intermediate Ag layer by SEM, and examples for the two systems are shown.

Nucleation and growth of thin films: recent progress

Progress in understanding the three growth modes of crystal growth and surfaces is briefly reviewed. Island growth typically has small critical nucleus sizes (i=1 frequently) and adsorption, diffusion and cluster binding energies can be abstracted from comparison of kinetic models with electron microscope experiments. Recent work which extends this approach to include cluster mobility, defect induced nucleation and alloy films is cited. Recent work on the layer-plus-island, or Stranski-Krastanov growth mode, using an ultra-high vacuum scanning electron microscope and several related surface techniques to study the growth of silver on several metal and semiconductor surfaces, is highlighted. It is shown that typically i ∼ 10 and binding energies of small clusters on metal surfaces are much less than expected on simple pair-bond models. These ideas can be extended to layer growth, but experimental work has been hampered by lack of suitable surface sensitive microscopic techniques. Some comments on the potential use of RHEED and reflection microscopy, and predictions for the nucleation densities expected in layer growth of semiconductors and metals, are made.

Characterized Metal Microcontacts

Abstract This paper describes a study of the contact between a fine tungsten point and a flat single crystal of nickel over a load range of approximately 1 to 1000/JN. The experiments are carried out in an ultrahigh vacuum (10−10 torr) scanning electron microscope with an Auger facility for characterising the nickel surface in situ. Electrical resistance measurements provide a reasonable estimate of the area of contact both on loading and unloading. The results show that even for zero joining load finite adhesion is observed, the unloading involving plastic deformation. A partial monolayer of oxygen has no effect on the adhesion: one or two monolayers produce some reduction: an oxide film about 5 nm thick produces a marked reduction in adhesion until, at a critical load, the oxide is penetrated. The results are discussed in terms of surface energy, ductility and fracture mechanics.

Variation of relative intensities between surface and bulk plasmon losses due to crystal orientations for aluminium in low energy electron reflection loss spectroscopy

The contrast change of secondary electron images due to the crystal orientations is observed by the ultra high vacuum scanning electron microscope (UHV-SEM) for crystal grains of clean surface of polycrystalline Al in the primary energy E p of 200 eV to 5 keV. The low energy electron loss spectra are measured by the cylindrical mirror analyzer. The relative intensity ratio between surface and bulk plasmon loss spectra was dependent on the crystal orientations. The SEM images taken by the surface and bulk plasmon signals at E p = 230 eV show the inverse contrast depending on the grains. The inversion of the relative intensities between the surface and bulk plasmon losses is explained qualitatively by taking into account of variation of the penetration depth of the incident beam caused by the electron channeling.

Variation of relative intensities between surface and bulk plasmon losses due to crystal orientations for aluminium in low energy electron reflection loss spectroscopy

The contrast change of secondary electron images due to the crystal orientations is observed by the ultra high vacuum scanning electron microscope (UHV-SEM) for crystal grains of clean surface of polycrystalline Al in the primary energy E p of 200 eV to 5 KeV. The low energy electron loss spectra are measured by the cylindrical mirror analyzer. The relative intensity ratio between surface and bulk plasmon loss spectra was dependent on the crystal orientations. The SEM images taken by the surface and bulk plasmon signals at E p = 230 eV show the inverse contrast depending on the grains. The inversion of the relative intensities between the surface and bulk plasmon losses is explained qualitatively by taking into account of variation of the penetration depth of the incident beam caused by the electron channeling.

Direct observation of the nucleation and growth modes of Ag/Si(111)

Abstract Direct observation in an ultrahigh vacuum scanning electron microscope (SEM) has revealed the growth modes of Ag on Si(111), previously deduced indirectly from analysis of Auger electron spectra, and the form and number of crystallites as a function of deposition time and temperature. At high temperatures, T s > 200°C, the silver grows definitively in a Stranski-Krastanov (layer plus island) growth mode, with a very strong dependence of the island density on the deposition temperature, varying from ≲ l0 6 to ∼ 10 10 cm −2 between T s = 500 and 200°C. The crystals have more or less regular hexagonal forms with (111) and {1 1 1} faces predominating with a height to width ratio which decreases with deposition time in the range 0.1–0.6. At temperatures below 200°C, the island density is too high to be observed directly by SEM, and silver appears to grow in a Frank-van der Merwe (layer by layer) mode. However, this uniform deposit is unstable on heating above 200°C, and annealed deposits recreate rather similar islands. An analysis of published Auger amplitude-time curves is made to estimate the initial island density produced by depositions below T s = 200° C, and it is shown that these densities constitute a very reasonable extrapolation of the higher temperature SEM values. A model of Stranski-Krastanov growth is given in outline and applied to Ag/Si(111). It is argued that the island density is determined largely by island instability at high temperatures, even though condensation is complete. At low temperatures it is argued that Stranski-Krastanov growth becomes essentially equivalent to Frank-van der Merwe growth for high enough island density, for kinetic rather than thermodynamic reasons, without any change in the basic Stranski-Krastanov mechanism.

High spatial resolution surface potential measurements using secondary electrons

Abstract High spatial resolution surface potential and work function measurements have been made by observing the onset energy of secondary electron emission using a band-pass analyser (CMA) in an ultra high vacuum scanning electron microscope (UHV-SEM). Multiple secondary electron spectra can be displayed for closely spaced positions on the sample. Alternatively a feedback loop is used to maintain a constant surface potential as the SEM beam is scanned across the sample. Both methods have been shown to yield linear measures of surface potential, and of work function differences, with a sensitivity of

High spatial resolution surface potential measurements using secondary electrons

Abstract High spatial resolution surface potential and work function measurements have been made by observing the onset energy of secondary electron emission using a band-pass analyser (CMA) in an ultra high vacuum scanning electron microscope (UHV-SEM). Multiple secondary electron spectra can be displayed for closely spaced positions on the sample. Alternatively a feedback loop is used to maintain a constant surface potential as the SEM beam is scanned across the sample. Both methods have been shown to yield linear measures of surface potential, and of work function differences, with a sensitivity of

Scanning electron diffraction and its application to the oxidation of iron

Abstract The merits of scanning electron diffraction as a method of studying thin films and solid surfaces are discussed. An ultra-high vacuum scanning electron diffraction system designed for the study of surfaces and surface reactions is described. Results of experiments on the oxidation of the (110) surface of iron single crystals using this apparatus are presented. Epitaxial growths of oxide phases were observed, some of these phases exhibiting orientations different from those normally reported. The proportions of the various phases present were found to change as the oxidation proceeded.