UHV STEM Research Articles

Atomic Resolution SE Imaging in a 30-200 keV Aberration-corrected UHV STEM.

Atomic Resolution SE Imaging in a 30-200 keV Aberration-corrected UHV STEM.

Contrast Effect of Strain Fields in ADF-STEM Imaging

STEM is an important tool for the study of material by forming Z-contrast ADF images. Studies of strain fields suggest that strain can cause extra contrast in ADF images. Since strain fields exist in many TEM samples, especially interfaces of two different materials, it is necessary to study the contrast effect of strain fields in more detail. in this paper, we study the contrast effect of strain fields by STEM experiments and multislice simulations.Sample thickness and the dimensions of ADF detector can both affect strain contrast. We first study the influence of sample thickness. A cross-section TEM sample of an interface of crystal silicon and amorphous silicon was prepared by typical tripod polishing and was studied with the 100 kV Cornell VG 5.0 UHV STEM. The sample was tilted so that the crystal was on the (110) zone axis and ADF images of the interface were recorded.

Atomic-Scale Study of a Co/AlOx/Co Magnetic Tunnel Junction

Magnetic tunnel junctions (MTJ) which consist of two ferromagnetic electrodes separated by a thin (∼10Å) tunnel barrier can exhibit a magnetoresistance of greater than 40% at 295 K. They hold substantial potential for application as computer memory and magnetic field sensors. Much about the spin-dependent tunneling process is still not understood. in order to identify features that may affect the spin-polarization of the tunnel current, we have used the UHV STEM at Cornell University to perform atomic-scale imaging and EELS on an MTJ system. The MTJ was sputter deposited on (100) Si with layer sequence Co 60Å/Al 12AÅO2 exposure/Co 100Å.The annular dark field (ADF) image (Fig. la) shows the A10x barrier layer has a fairly uniform thickness of about 15 Å. Its profile (Fig. 1c) appears diffuse partly due to layer roughness rather than atomic mixing. in bright field (BF) the appearance of strong Fresnel fringes (Figs. 1b,c), due to a large difference in average electrostatic potential between Co and AlOx, complicates interpretation.

EELS Measurement of the Local Electronic Structure of Copper Atoms Segregated to Aluminum Grain Boundaries

It has long been known that the addition of Cu to Al interconnects improves the resistance to electromigration failure. It is generally accepted that this improvement is the result of Cu segregation to Al grain boundaries. The exact mechanism by which segregated Cu increases service lifetime is not understood, although it has been suggested that the formation of thin layers of θ-CuA12 (or some metastable substoichiometric precursor, θ’ or θ”) at the boundaries may be necessary. This paper reports measurements of the local electronic structure of Cu atoms segregated to Al grain boundaries using spatially resolved EELS in a UHV STEM. It is shown that segregated Cu exists in a chemical environment similar to that of Cu atoms in bulk θ-phase precipitates.Films of 100 nm thickness and nominal composition Al-2.5wt%Cu were deposited by sputtering from alloy targets onto NaCl substrates. The samples were solution heat treated at 748K for 30 min and aged at 523K for 4 h to promote equilibrium grain boundary segregation. EELS measurements were made using a Gatan 666 PEELS spectrometer interfaced to a VG HB501 STEM operating at 100 keV. The probe size was estimated to be 1 nm FWHM. Grain boundaries with the narrowest projected width were chosen for analysis. EDX measurements of Cu segregation were made using a VG HB603 STEM.

Strain measurement in In0.2Ga0.8As/GaAs quantum wires by convergent beam electron diffraction

The positions of high-order Laue zone (HOLZ) lines in the zero order disc of convergent beam electron diffraction (CBED) patterns are extremely sensitive to local lattice parameters. With proper care, these can be measured to a level of one part in 104 in nanometer sized areas. Recent upgrades to the Cornell UHV STEM have made energy filtered CBED possible with a slow scan CCD, and this technique has been applied to the measurement of strain in In0.2Ga0.8 As wires.Semiconductor quantum wire structures have attracted much interest for potential device applications. For example, semiconductor lasers with quantum wires should exhibit an improvement in performance over quantum well counterparts. Strained quantum wires are expected to have even better performance. However, not much is known about the true behavior of strain in actual structures, a parameter critical to their performance.

Observation of the evolution of 55-atom icosahedral Ag clusters by coherent electron nanodiffraction in a UHV stem

The morphology of metallic nano particles fluctuates and rapidly changes under the influence of electron beam irradiation. Structural transformations of small Au particles and disordering of small Pt clusters have been reported. Although the existence of a quasimolten state in nano particles has been experimentally observed there are still many unanswered questions concerning the onset of this unstable state. The previous experiments were carried out in non-UHV microscopes and the particles were prepared ex situ. We report preliminary observations of the evolution of clean Ag nano particles in a UHV scanning transmission electron microscope (STEM).The experiments were performed on a UHV STEM (VG HB-501S), known by the acronym MIDAS (Microscope for Imaging, Diffraction and Analysis of Surfaces). The probe size used for nanodiffraction was approximately 1 nm in diameter with a beam current density of 104 A/cm2. Silver nano particles were formed by evaporating Ag onto a pre-cleaned thin (< 5 nm in thickness) carbon film inside the UHV specimen preparation chamber attached to the MIDAS column.x

Local Electronic Structure and Cohesion of Grain Boundaries in Ni3Al

AbstractOne of the fundamental questions concerning Ni3Al is why doping with boron improves the room temperature ductility of the polycrystalline material. Boron is thought to prevent environmental embrittlement and increase the cohesive strength of grain boundaries since it changes the fracture mode from intergranular to transgranular. This change in cohesive energy must be reflected in the bonding changes at the grain boundary which can be probed using spatially resolved electron energy loss spectroscopy (EELS). We have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24using EELS, EDX and ADF imaging in a UHV STEM. Ni-enrichment is seen in a 0.5–1 nm wide region at large angle grain boundaries, both in the absence and presence of B. EELS shows that B segregation can vary along the interface. The Ni L2, 3core edge fine structure which is sensitive to the filling of the Ni d-band, shows only the boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. These results demonstrate that boron segregation increases the cohesive energy and hence improves the fracture resistance of the grain boundary, by making the bonding at boundaries similar to that in the bulk. The measured changes in d band filling may also affect the local solubility of hydrogen.

Electronic Structure and Bonding at Interfaces Between cvd Diamond and Silicon

ABSTRACTThe interfacial structure of CVD diamond grown on silicon was studied using spatially resolved electron energy loss spectroscopy (EELS) in a UHV STEM with a subnanometer probe size. Both the plasmon and core excitations in the bulk appear to be localized on this scale. Spatial maps of the different bonding configurations of carbon were obtained by forming images from transmitted electrons that had undergone energy losses characteristic of threefold and fourfold coordinated carbon. Films grown on both prescratched silicon and intermediate amorphous carbon layers were examined. In the latter case, diamond nucleation on a narrow sp2 a-C occurred. For diamond grown directly on silicon, at some regions of the interface, threefold coordinated defect states smaller than 1 nm are observed on the diamond side of the interface while at other regions along the interface the presence of an intermediate 2nm thick SiC layer preserves the fourfold coordination of the carbon.

Coherent electron nanodiffraction from clean silver nano particles in a UHV STEM

Small metal particles have peculiar chemical and physical properties as compared to bulk materials. They are especially important in catalysis since metal particles are common constituents of supported catalysts. The structural characterization of small particles is of primary importance for the understanding of structure-catalytic activity relationships. The shape and size of metal particles larger than approximately 5 nm in diameter can be determined by several imaging techniques. It is difficult, however, to deduce the shape of smaller metal particles. Coherent electron nanodiffraction (CEND) patterns from nano particles contain information about the particle size, shape, structure and defects etc. As part of an on-going program of STEM characterization of supported catalysts we report some preliminary results of CEND study of Ag nano particles, deposited in situ in a UHV STEM instrument, and compare the experimental results with full dynamical simulations in order to extract information about the shape of Ag nano particles.

Observation of a supported metal catalyst in an ultra-high-resolution field-emission SEM

Zeolites loaded with noble-metal particles are interesting catalytic systems. The relative positions of small metal particles on and within the support structure may have significant consequences for catalytic activity. Small metal particles dispersed on model and commercial catalyst supports have been observed by using secondary and Auger electrons in a UHV STEM. In this paper we further report preliminary results of observing a metal/zeolite catalyst in a commercial high-resolution field-emission SEM and investigate the contrast variations of SEM images with the change of incident beam energies.Metal-loaded zeolite samples were ground to fine powders and were dispersed onto a thin holey carbon film coated on a copper grid. The samples were directly observed without being coated with conducting materials. Such coating would obscure fine-scale surface details and make imaging of small metal particles impossible. The experiments were performed in a field-emission Hitachi S-5000 SEM, operating from 0.5 to 30 kV.

EELS study of bulk Co and CoSi2

CoSi2 can grow epitaxially on Si and the Si/CoSi2/Si heterostructure has potential uses in metal-based transistors. To fully utilize the device potential of CoSi2, it is important to develop a thorough understanding of its electronic properties. By comparing the Co L2,3 spectra of pure Co and CoSi2, the bonding between Co and Si atoms can be studied.A Si (111) wafer was treated by Shiraki cleaning before 500 Å of Co was deposited in an E-beam evaporator with a base pressure of 10-7 Torr. The wafer was then annealed in an UHV oven with a base pressure of 10-10 Torr at 950°C for 2 hrs. A standard TEM plan view sample was then prepared from the wafer by dimpling and ion-milling, both from the Si side to preserve the silicide. The stochiometry of CoSi2 was confirmed by SAD and EDS. EELS measurements were carried out in a VG-HB501A UHV STEM operating at 100 keV equipped with a serial EELS detector.

Surface studies in UHV SEM and STEM

SummarySome recent advances in surface analyses with scanning electron microscopy techniques are reviewed. It is shown that secondary electron microscopy can image monatomic steps, emergent dislocations and the early growth of thin films and oxide nuclei. The chemical composition of inhomogeneous surfaces can, in some cases, be determined with nanometre resolution with Auger signals. Overlayer coverages can occasionally be monitored with the secondary electron signal if the change of work function with coverage has been determined previously. The interpretation of these secondary and Auger signals is discussed. The combination of angular electron spectroscopy with electron microscopy is potentially a powerful technique for determining atomic positions of surface atoms and one method for obtaining the experimental data is described. Several substrate/overlayer systems are used to illustrate the amount of information that can be obtained about surface processes with scanning electron microscopy.

Diffusion of Ge Along Grain Boundaries During Oxidation of Polycrystalline Silicon-Germanium Films

AbstractPolycrystalline silicon-germanium alloys with compositions between 5 and 30% Ge were wet oxidized in pyrogenic steam with TCA at 700°C. TEM showed that the columnar structure and grain size, ∼500Å, were independent of composition and did not change during oxidation. Both energy dispersive x-ray spectroscopy (EDS) in a UHV STEM and RBS indicate that for low Ge containing films (≤20% Ge), Ge is rejected from the growing oxide, whereas for films with a greater Ge concentration, some Ge is incorporated in the oxide. In low Ge containing alloys, the average Ge profile was determined by RBS and the local Ge distribution was determined by EDS from the oxide interface to about 500Å deep into the alloy. A Ge rich layer was observed at the oxide/poly-SiGe interface. The Ge concentration at the grain boundaries was found to be at least three times higher than that in the grains. The mean diffusion length of Ge along the grain boundaries, measured in cross-section by EDS, was about 300Å, corresponding to an effective diffusivity of about 6×10−16 cm2/s. Analysis of RBS profiles yielded an average effective diffusivity of ∼1.3×10−14 cm2/s. Both diffusivity values suggest that Ge rejected from the oxide growth front diffuses along grain boundaries since the bulk diffusivity of Ge in Si at 700°C is on the order of 10−22 cm2s/s.

Experimental Measurement of the Local Electronic Structure of Grain Boundaries in Ni3Al

AbstractWe have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24using electron energy loss spectroscopy (EELS), x-ray fluorescence (EDX) and annular dark field (ADF) imaging in a UHV STEM. A detailed study of a high angle grain boundary in nickel rich Ni3Al doped with 1000 ppm boron shows nickel enrichment occurring in a 5Å wide region. Boron segregation to the boundary is observed with EELS and is seen to vary along the boundary, coinciding with ADF contrast changes in the surrounding grains that may be due to local strain fields. Spatially resolved EELS of the Ni L2,3core edge, which is sensitive to changes in the hole density in the nickel d band, shows boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. Boundary regions without boron have an electronic structure similar to that of the undoped grain boundaries where the Fermi level lies deeper in the nickel d band. In addition to studying boron segregation, EELS provides a unique opportunity to examine the changes in bonding that control the local properties of the material.

High-resolution Auger Electron Microscopy and spectroscopy of supported metal particles

A detailed understanding of catalytic processes requires structural knowledge of the catalyst system. The surface properties of small metal particles which are highly dispersed on insulating supports must play a dominant role in determining this system's catalytic behavior. Therefore characterization of surface topography and composition of heterogeneous catalysts is important. Surface topography of the carrier materials can be obtained by high resolution secondary electron imaging (SEI) in a STEM. The small metal particles can also be located and their topographic relationship with the support morphology determined. However, an elementally specific signal such as the Auger electrons must be used to extract chemical information about the surface species. High resolution Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM) have recently been incorporated into a UHV STEM. In this paper we report some results from the application of this instrumentation to the study of supported metal clusters.

Imaging bulk insulators with secondary electrons in an UHV STEM

When an insulator is bombarded by electrons a surface potential will build up if the total number of electrons entering the sample is not equal to that coming out. This potential can be positive or negative depending on the energy of the incident electrons and the target material. The effects of charging will limit, or at least perturb, the use of electron beam techniques for examining the surface properties of insulators. Various methods have been developed to avoid insulator charging. However none of these methods can be applied to high spatial resolution electron beam studies of clean insulator surfaces. At the electron beam energies typically used in STEM instruments the surface of bulk insulators will always acquire a negative potential. Secondary electron imaging (SEI) and Auger electron spectroscopy (AES) would be possible if the surface potential were stable under electron beam illumination and was small compared with the incident beam potential.

Biassed secondary electron imaging of monatomic surface steps on vicinal Si(100) in a UHV STEM

Biassed secondary electron (b-SE) SEM images of steps on Si(100) surfaces misoriented 1° towards 〈110〉 have been obtained using a number of imaging geometries including normal incidence. The periodicity of the regularly spaced steps unambiguously identifies these as single-atom-high steps. High-efficiency secondary electron collection from the input surface of the sample makes it possible to obtain images of low contrast features on low-atomic-number samples in UHV. Simple topography is shown to be a likely mechanism for the contrast observed in the digitally acquired b-SE images. Images of step pinning at refractory particles during surface sublimation identify the up-step direction and are similar to those obtained by other surface microscopic techniques.

Design of the VG HB603 300kV Field-Emission STEM

A new generation of UHV field-emission STEMs operating at up to 300 kV has been designed by VG Microscopes. The design philosophy of these instruments has been to improve further the analytical performance achieved by 100 kV cold field-emission STEMs, such as the VG HB501 series.There are three types of instrument with a common basic design:HB603: an analytical STEM with optimised X-ray microanalysis (0.3srad collection angle per detector) and parallel/serial electron energy-loss facilities;HB603U: a high-resolution STEM with optimised high-angle dark-field detection and < 0.13 nm resolution;HB603S: a full UHV STEM with Auger analyser and specimen preparation facilities at 3 × 10-10 mbar pressure throughout the instrument.

Heteroepitaxy of Ge on Vicinal Si(100)

ABSTRACTThe initial stages of germanium heteroepitaxy on vicinal Si(100) have been studied using in-situ deposition in a UHV STEM. Germanium was deposited using molecular beam techniques onto substrates misoriented 1° and 5* toward <110> held at room temperature, 375°C and 525°C. Film thicknesses were in the range 4-6 ML, just greater than the stable intermediate layer of 3-4ML (1ML = 0.14nm). The Ge clusters were observed using biassed secondary electron (b-SE) imaging with nanometer resolution. Comparisons were made between deposition at the elevated temperatures, and room temperature deposition followed by anneals at the same temperatures.Annealing the low temperature deposits produces coarsening of the islands which is similar on the 1° and 5° samples. Island size distributions and other film growth parameters obtained from the 375°C and 525°C anneals indicate that the coarsening is different at these temperatures and is possibly affected by instabilities in the intermediate layer. Results of the high temperature depositions indicate that neither surface steps nor the edges of islands act as perfect sinks, and that diffusion distances are of the order of several microns. The nucleation density and size distributions are markedly different for deposition at 375°C and 525°C possibly due to competitive capture at strong sinks.In a parallel set of experiments in a standard UHV chamber, macroscopic wafer samples were analyzed with RHEED, Auger and secondary electron spectroscopy. These correlate well with the intermediate layer thicknesses previously reported in the literature, and the large contrast observed in the b-SE images. Ex situ TEM studies of samples grown in this chamber show islands with various contrast features including those of coherent strain.

Secondary electron imaging of MoO3 reduction in a UHV STEM

Recently, a number of people have studied the ability of focussed electron beams to form lines and holes in metal oxides. Many metal oxides decompose when irradiated under the high intensity focussed probes used in field emission STEMS. Initially, many of these oxides reduce to the metal and in some cases it is possible to drill holes in the material. Understanding the nature of the interaction between the electron beam and the sample is important for materials characterisation. These studies may also lead to the development of new electron beam resists and masks. Here we present some preliminary results on the effect of electron irradiation on MoO3 in a field emission UHV STEM. We show, for the first time, secondary electron (SE)images of the entrance and exit surface of the sample after radiation damage has occurred.The experiments described here were performed on the VG HB501S; a dedicated STEM in which vacuums better than 10-10 torr are routinely available in the column.