Ultrahigh Vacuum Scanning Electron Research Articles

Apparatus design of operando hydrogen microscope for visualization of time-dependent distribution of hydrogen

The operando hydrogen microscope is an original apparatus that has been developed to visualize the time-dependent permeation of hydrogen through a sample. This apparatus is developed based on an ultrahigh vacuum scanning electron microscope (UHV SEM). The system consists of a lens system to focus ions produced by electron-stimulated desorption (ESD), an ion detector optimized for ESD-signal detection, and a two-dimensional measurement program synchronized with the position information of the electron beam. The developed detectors and electrostatic lenses enable highly sensitive detection of the ions. In this paper, we show the details of the instrumentation of the operando hydrogen microscope. We have succeeded in recording hydrogen and deuterium flowing from the rear of metals to the surface as a series of time-lapse images, showing the time-dependent changes in the surface distribution of hydrogen and deuterium. Deuterium permeation through the metal sample was dynamically visualized at the surface by the two-dimensional mapping of the desorbed ions induced by scanning electron beam irradiation.

High Temperature Equilibrium and Growth Shape of Small Particles: The Case of Thallium

The temperature dependence of the equilibrium shape and growth shape of small thallium particles is studied in situ by ultrahigh vacuum scanning electron microscopy. Pronounced anisotropic surface roughening and surface melting is observed and compared with earlier studies of four metals with different crystal structures illustrating significant differences in disordering behavior. A comparison with theoretical 0 K equilibrium shapes shows that these do not allow in general prediction of the equilibrium and growth shapes at high temperatures.

Revealing the 1 nm/s extensibility of nanoscale amorphous carbon in a scanning electron microscope

In an ultra-high vacuum scanning electron microscope, the edged branches of amorphous carbon film (∼10 nm thickness) can be continuously extended with an eye-identifying speed (on the order of ∼1 nm/s) under electron beam. Such unusual mobility of amorphous carbon may be associated with deformation promoted by the electric field, which resulted from an inner secondary electron potential difference from the main trunk of carbon film to the tip end of branches under electron beam. This result demonstrates importance of applying electrical effects to modify properties of carbon materials. It may have positive implications to explore some amorphous carbon as electron field emission device.

Nanoscale depth-resolved cathodoluminescence spectroscopy of ZnO surfaces and metal interfaces

The electronic properties of ZnO surfaces and interfaces has until recently been relatively unexplored. We have used a complement of ultrahigh vacuum scanning electron microscope (SEM)-based, depth-resolved cathodoluminescence spectroscopy (DRCLS), temperature-dependent charge transport, trap spectroscopy, and surface science techniques to probe the electronic and chemical properties of clean surfaces and interfaces on a nanometer scale. DRCLS reveals remarkable nanoscale correlations of native point defect distributions with surface and sub-surface defects calibrated with capacitance trap spectroscopies, atomic force microscopy, and Kelvin probe force microscopy. The measurement of these near-surface states associated with native point defects in the ZnO bulk and those induced by interface chemical bonding is a powerful extension of cathodoluminescence spectroscopy that provides a guide to understanding and controlling ZnO electronic contacts.

Formation of dense nitroxide radical layers on the Au(1 1 1) substrate for ESN-STM measurement

Ultra high vacuum scanning tunnelling microscopy and electron paramagnetic resonance spectroscopy were used to investigate ISL and TEMPOL piperidine nitroxides molecules deposited on the Au(1 1 1) substrate by the drop-cast method. The STM results suggest that both compounds form dense layers on Au(1 1 1) with high molecular mobility observed during imaging process. High resolution STM topographies of both compounds suggest a well-defined molecular order which can be ascribed to the presence of self-assembly mechanism during layers formation. The EPR results indicated that both compounds deposited on the Au(1 1 1) substrate were not reduced retaining their paramagnetic properties. The significance of the results obtained in the field of single spin detection is briefly outlined.

Formation of Fe–Pt intermetallic phase in nanostructures by electron‐beam‐induced deposition and postdeposition alloying processes

AbstractIt was found that Fe and Pt can be successfully alloyed in rod‐like nanostructures by electron‐beam‐induced deposition (EBID) using a Fe(CO)5 precursor in an ultrahigh vacuum scanning electron microscope (SEM), followed by a 600 °C vacuum heating of the deposit and a subsequent surface platinum coating and a further 600–700 °C annealing treatment. By using high‐resolution transmission electron microscopy (HRTEM), image simulation and energy dispersion X‐ray spectra analysis, L12‐type intermetallic FePt3 phase occurred in the nanostructure formed by the aforementioned EBID and postdeposition alloying processes. Copyright © 2006 John Wiley & Sons, Ltd.

Carbon Nanostructure on a Tungsten Needle Tip Formed by Electron Beam Induced Deposition and Its Graphitization through Resistive Heating

Using ferrocene as a precursor, an amorphous carbon nanorod was fabricated on a tungsten needle tip by electron beam induced deposition inside an ultrahigh-vacuum scanning electron microscope at room temperature. This needle tip was driven by a stepping motor and pizeo-driving device to make contact with a molybdenum substrate inside a specially designed transmission electron microscope (TEM) specimen holder. A 100 µA current flowed through this nanorod and resistance heating was induced. Within 120 s, the original amorphous carbon nanorod was transformed into graphite nanostructures by this thermal energy. This phase transformation process was confirmed by high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). After applying a current of 120 s, graphite sheets with 0.34 nm lattice spacing in HRTEM image and π* bonding peak in EELS spectra were detected from the nanorod, which were not detected before that treatment.

Two-stage gas amplifier for ultrahigh resolution low vacuum scanning electron microscopy

We describe a magnetic field assisted, two-stage secondary electron gas amplification process for low vacuum scanning electron microscopy. The field of an ultrahigh resolution magnetic immersion objective lens and the electric field of an annular electrode configuration partition the amplification volume into two regions in which the electric and magnetic fields are parallel and crossed, respectively. The fields confine secondary electrons to axial and radial oscillations within the detector volume, until all of the kinetic energy imparted by an anode is dissipated through inelastic collisions with gas molecules. The electron confinement yields high gas amplification efficiency at short working distances and low gas pressures, facilitating high resolution imaging at low electron beam energies. Charging of insulating specimens is stabilized by positive ions produced in the gas ionization cascade. Furthermore, the signal to background level and bandwidth of this detector are superior to those of earlier generations of environmental secondary electron detectors. The combination of low vacuum, short working distance, and low beam energy is attractive to the semiconductor metrology industry, in particular, for critical dimension measurements on photolithographic masks.

Fabrication and electron holography characterization of FePt alloy nanorods

Well-aligned, ferromagnetic FePt nanorods have been fabricated by electron beam-induced deposition using an ultrahigh-vacuum scanning electron microscope. A mixture gas of iron pentacarbonyl and cyclopentadienylplatinum (IV) trimethyl was used as a precursor and post-annealing at 600 °C for 2 h was performed to accomplish the crystallization process. Each nanorod was composed of a chain of crystalline Fe-Pt alloy nanoparticles encapsulated within a carbon-containing sheath. The nanoparticles were identified to be face centered tetragonal (fct) FePt phase (L10,P4∕mmm) by electron diffraction and high-resolution transmission electron microscopy (HRTEM). The residual magnetic flux density Br of the nanorods was evaluated to be about 1.53 T via off-axis electron holography, showing a strong ferromagnetic character.

Ultrahigh vacuum scanning electron microscope system combined with wide-movable scanning tunneling microscope

A surface analysis system has been newly developed with combination of ultrahigh vacuum scanning electron microscope (SEM) and wide-movable scanning tunneling microscope (STM). The basic performance is experimentally demonstrated. These SEM and STM images are clear enough to obtain details of surface structures. The STM unit moves horizontally over several millimeters by sliding motion of PZT actuators. The motion resolution is proved to be submicrometers. The STM tip mounted on another PZT scanner can be guided to a specific object on the sample surface during SEM observation. In the observation of a Si(111) surface rapidly cooled from high temperature, the STM tip was accurately guided to an isolated atomic step and slightly moved along it during SEM observation. The STM observation shows an asymmetry of the (7×7)-transformed region along the step between the upper and lower terraces. (7×7) bands continuously formed along the edge of terraces, while (7×7) domains distributed on the terraces slightly far from the step. These experiments show the wide-movable STM unit resolves a gap of observation area between SEM and STM and the system enables a specific object found in the SEM image to be observed easily by STM.

Wetting of Si surfaces by Au–Si liquid alloys

The behavior of liquid Au–Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650 °C and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid–solid–vapor phase line. In addition, the behavior of Au–Si droplets on vicinal and patterned surfaces of Si has been examined. The droplets cause step bunching and modify the local surface structure. Solidification of the droplets on all surfaces leads to phase separation.

Direct observation of growth motion of elements by ultrahigh vacuum scanning electron microscopy combined with total reflection angle x‐ray spectroscopy

AbstractBy using ultrahigh vacuum scanning electron microscopy (UHV‐SEM) combined with characteristic x‐ray spectroscopy under a grazing detection‐angle condition, the behaviours of elements on surfaces and at interfaces during the growth of Ga on an Si(111)‐√3 × √3‐Ag surface and of Au on Si(111)‐√3 × √3‐Ag were studied. Site exchange and other surrounding elements were observed directly. When a 2/3 monolayer of Ga was deposited, Ag was substituted by Ga and dissociated from the √3 × √3‐Ag structure, resulting in the formation of Ag particles 10–20 nm in size. During further Ga deposition, Ga surrounded the Ag particles. Substitution of Ag by Au also was observed during the growth of Au. Compared with the resolution of conventional SEM energy‐dispersive x‐ray spectrometry (EDX), which is several micrometres, resolution in elementary analysis has been improved significantly due to a small beam size and cut‐off of x‐rays from deep regions. Using the present method, local elementary analysis of surface structures of only ∼5 nm in size is possible. Copyright © 2003 John Wiley & Sons, Ltd.

Field electron emission from carbon nanotubes grown by plasma-enhanced chemical vapor deposition

Densely distributed, aligned carbon nanotubes were grown on Co-covered W wires by dc-plasma-enhanced chemical vapor deposition with a mixture of acetylene and ammonia as precursors. Each nanotube was capped with a Co carbide particle. The diameter of the nanotubes ranged from 20 to 130 nm, and their average length was about 5 μm. Their field emission characteristics, measured in a modified ultra-high vacuum scanning electron microscope, were analyzed by a statistical field emission model developed for aligned carbon nanotube film cathodes. An indirect method was thus established to obtain the average electrical parameters of the film using only a limited amount of experimental data.

Step dynamics on growing silicon surfaces observed by ultrahigh vacuum scanning electron microscopy

The motion of atomic steps on Si(1 1 1) “1×1” is studied using in situ scanning electron microscopy to observe ultra-flat Si(1 1 1) substrates that have flat terraces of 100 μm width. Atomic steps are revealed by (7×7)-domain decoration. Step-flow growth and sublimation processes are compared for single-step-high islands and holes. The step motion is highly symmetric for growth in comparison with sublimation, and for islands in comparison with holes. The step motion fits well with the simple model by Burton, Cabrera and Frank. No clear asymmetry in step kinetics is observed for upper terraces in comparison with lower terraces. However, by introducing an asymmetry in the relative terrace sizes on either side of a step, the adatom flux balance is tilted, and the result is a morphological instability in step flow.

Electromigration and phase transformation of Ag on a Cu-precovered Si([formula omitted]) surface

Electromigration and phase transformation of an ultrathin-Ag film patch deposited on a Cu-precovered incommensurate “5×5” phase on Si(111) surface were investigated by in situ ultrahigh-vacuum scanning electron microscopy and microprobe reflection high-energy electron diffraction, and compared with the previous results on the clean and Au-precovered Si(111) surfaces. The film patch spread quite slowly towards the cathode by current apply (and resultant heating), at the initial stage of which the patch area showed a 3×3 phase, consisted of Ag–Cu surface alloy, with three-dimensional (3D) islands in it. Later, a phase transformation from the 3×3 into a 21×21 structure proceeded, accompanied with the 3D islands dissolving. These surface alloy formations play a key role in the migration, which is similar to the case of on Au-precovered Si(111) surfaces.

Independently driven four-tip probes for conductivity measurements in ultrahigh vacuum

To measure electrical conductivity of materials in scales ranging from nanometer to millimeter, a four-point probe system was developed and installed in an ultrahigh-vacuum scanning electron microscope (UHV-SEM). Each probe, made of a W tip, was independently driven with piezoelectric actuators and a scanner in XYZ directions to achieve precise positioning in nanometer scales. The SEM was used for observing the tips for positioning, as well as the sample surface together with scanning reflection-high-energy electron diffraction capability. This four-point probe system has two kinds of special devices. One is octapole tube-type scanners for tip scanning parallel to the sample surface with negligible displacements normal to the surface. Another is a pre-amplifier which can be switched in current measurement mode between tunnel contact for scanning tunneling microscopy and direct contact for four-point probe method. The electrical resistance of a silicon crystal with a Si(111)-7×7 clean surface was measured with this machine as a function of probe spacing between 1 mm and 1μm. The result clearly showed an enhancement of surface sensitivity in resistance measurement by reducing the probe spacing.

High-resolution observation of the growth motion of elements on surfaces and at interfaces

The results of high-resolution, real-space observations of the behaviors of elements on surfaces and at interfaces during growth of Ga on Si(111)-∛×∛-Ag are presented. Site exchange and surrounding other elements were directly observed using a high-resolution ultrahigh vacuum scanning electron microscope (SEM) newly combined with characteristic x-ray spectroscopy under a grazing detection-angle condition. Compared with the resolution of conventional SEM-energy dispersive x-ray spectrometry (several μm), resolution in elementary analysis has been significantly improved due to a small beam size and cut off of x-rays from deep regions, and local elementary analysis of surface structures of only about 10–20 nm in size is possible by the present method.

Micro-Four-Point Probes in a UHV Scanning Electron Microscope for In-Situ Surface-Conductivity Measurements

For in-situ measurements of surface conductivity in ultrahigh vacuum (UHV), we have installed micro-four-point probes (probe spacings down to 4 μm) in a UHV scanning electron microscope (SEM) combined with scanning reflection–high-energy electron diffraction (RHEED). With the aid of piezoactuators for precise positioning of the probes, local conductivity of selected surface domains of well-defined superstructures could be measured during SEM and RHEED observations. It was found that the surface sensitivity of the conductivity measurements was enhanced by reducing the probe spacing, enabling the unambiguous detection of surface-state conductivity and the influence of surface defects on the electrical conduction.

MICRO-FOUR-POINT PROBES IN A UHV SCANNING ELECTRON MICROSCOPE FOR IN-SITU SURFACE-CONDUCTIVITY MEASUREMENTS

For in-situ measurements of surface conductivity in ultrahigh vacuum (UHV), we have installed micro-four-point probes (probe spacings down to 4 μm) in a UHV scanning electron microscope (SEM) combined with scanning reflection–high-energy electron diffraction (RHEED). With the aid of piezoactuators for precise positioning of the probes, local conductivity of selected surface domains of well-defined superstructures could be measured during SEM and RHEED observations. It was found that the surface sensitivity of the conductivity measurements was enhanced by reducing the probe spacing, enabling the unambiguous detection of surface-state conductivity and the influence of surface defects on the electrical conduction.

Effect of energy on direct ion beam deposition of carbon thin films: induced defects and graphitization

The direct deposition of ions at low energy enables fine control of the fundamental processes of thin film formation. Using a source of carbon and nickel ions with energy ranging from a few tens to a few hundreds of electron volts, combined with an ultra-high vacuum scanning electron tunneling microscope, we have studied the formation of thin films for thicknesses equivalent to less than one atomic layer. We have revealed the influence of ion-induced defects on the formation mechanisms of the thin film as a function of ion energy using the dynamic scaling analysis of the roughness of the thin film terminal surfaces. In the case of carbon ions we acknowledge the formation of an sp2 bonded layer at 300°C, that is above the critical temperature where the formation of a diamond-like carbon sp3 layer recess as described by the subimplantation model [4].