Sample Drift Research Articles

Automated spatial drift correction for EFTEM image series

Energy filtering transmission electron microscopy (EFTEM) is a widely used technique in many areas of scientific research. Image contrast in energy-filtered images arises from specific scattering events such as the ionization of atoms. By combining a set of two or more images, relative sample thickness maps or elemental distribution maps can be easily created. It is also possible to acquire a whole series of energy-filtered images to do more complex data analysis. However, whenever several images are combined to extract certain information, problems are introduced due to sample drift between the exposures. In order to obtain artifact-free information, this spatial drift has to be taken care of. Manual alignment by overlaying and shifting the images to find the best overlap is usually very accurate but extremely time consuming for larger data sets. When large amounts of images are recorded in an EFTEM series, manual correction is no longer a reasonable option. Hence, automatic routines have been developed that are mostly based on the cross-correlation algorithm. Existing routines, however, sometimes fail and again make time consuming manual adjustments necessary. In this paper we describe a new approach to the drift correction problem by incorporating a statistical treatment of the data and we present our statistically determined spatial drift (SDSD) correction program. We show its improved performance by applying it to a typical EFTEM series data block.

Exit wave reconstruction at atomic resolution

An iterative method for exit wave function reconstruction based on wave function propagation in free space is presented. The method, which has the potential for application to many forms of microscopy, has been tailored to work with a through focal series of images measured in a high-resolution transmission electron microscope. Practical difficulties for exit wave reconstruction which are pertinent in this experimental environment are the slight incoherence of the electron beam, sample drift and its effect upon the defocus step size that can be utilised, and the number of image measurements that need to be made. To gauge the effectiveness of the method it is applied to experimental data that has been analysed previously using a maximum likelihood formalism (the MAL method).

Current Issues Affecting Utility Bankruptcies

Single-molecule localization microscopy (SMLM) promises to provide truly molecular scale images of biological specimens^1–5. However, mechanical instabilities in the instrument, readout errors and sample drift constitute significant challenges and severely limit both the useable data acquisition length and the localization accuracy of single molecule emitters⁶. Here, we developed an actively stabilized total internal fluorescence (TIRF) microscope that performs 3D real-time drift corrections and achieves a stability of ≤1 nm. Self-alignment of the emission light path and corrections of readout errors of the camera automate channel alignment and ensure localization precisions of 1-4 nm in DNA origami structures and cells for different labels. We used Feedback SMLM to measure the separation distance of signaling receptors and phosphatases in T cells. Thus, an improved SMLM enables direct distance measurements between molecules in intact cells on the scale between 1-20 nm, potentially replacing Förster resonance energy transfer (FRET) to quantify molecular interactions⁷. In summary, by overcoming the major bottlenecks in SMLM imaging, it is possible to generate molecular images with nanometer accuracy and conduct distance measurements on the biological relevant length scales.

A Novel Approach to Process Modeling for Instrument Surveillance and Calibration Verification

This work presents an empirical modeling approach combining a bilinear modeling technique, partial least squares, with the universal function approximation abilities of single hidden layer nonlinear artificial neural networks. This approach, referred to as neural network partial least squares (NNPLS), is compared to the common autoassociative artificial neural network. The NNPLS model is embedded into a graphical user interface and implemented at the Electrical Power Research Institute’s Instrumentation and Control Center located at Tennessee Valley Authority’s Kingston fossil power plant. Results are presented for 51 process signals with an average absolute estimation error of ~1.7% of the mean value, and sample drift detection performances are shown.

In situ scanning tunneling microscopy of individual supported metal clusters at reactive gas pressures from 10−8 to 104 Pa

An experimental apparatus has been designed and an approach developed for imaging individual oxide supported nanoparticles with scanning tunneling microscopy (STM) during their nucleation, growth, alloying, and “real world” chemical, thermal, and other in situ treatments. By careful selection of the tunneling conditions and using the STM tip to index the surface, it is demonstrated that preselected individual particles can be imaged at elevated temperatures while changing the reactive gas pressure over 12 orders of magnitude. The experimental challenges due mainly to tunnel junction instabilities, a relatively weak cluster-support interaction, and sample drift are considered and strategies to overcome these obstacles proposed. Using Au and Ag clusters deposited on TiO2(110) as a model system, the potential of the method is demonstrated for exploring on a particle-by-particle basis cluster growth, alloying, thermal coarsening, and the evolution of particle morphology in a reactive gas environment. An additional advantage of the approach is that a wide cluster size distribution can be synthesized and surveyed on the same substrate. The size effects on cluster morphology therefore can be probed in situ for a wide variety of treatments on preselected clusters.

Analytical electron microscopy of InP/(In, Ga)As heterogeneous structures

Aspects of processing raw analytical data to correct for a finite probe size and beam−specimen interactions are discussed and demonstrated for the case of interdiffusion in lattice-matched InP/(In, Ga)As heterostructures. At first, the probe size of the microscope was evaluated following the method of Michael and Williams. Then the two principal factors, affecting the results of composition profile measurement, were taken into account, namely the sample drift during prolonged data acquisition and the beam broadening by elastic scattering inside the foil. A method of quantification of the sample drift effect was proposed, as well as a method of final scaling of the composition profiles. Using this approach, we can get closer to measuring the true changes of composition profiles caused by interdiffusion at the nanometre level.

A Nanoscratch Method for Assessing Wear of Metal Carbide-Metal Nacreous Nanostructures

AbstractThe wear resistance of metal carbide-metal nacreous nanostructures is compared to that of a solid metal carbide hard coating. A reciprocating nanoscratch method has been developed which allows the assessment of wear resistance of the film independent of substrate type or film-substrate adhesion. A series of nominally 1 micron thick TiC-based multilayer films on Si substrates were produced using RF magnetron sputtering. A scratch protocol consisting of 70 reciprocal scratches each 9.5 m m in length was applied. Data reduction techniques were used to remove the effects of sample tilt and drift. The resulting normal displacement as a function of scratch cycles is used to compare the wear of different films on Si <111> substrates. Specific wear characteristics related to film morphology are examined. This investigation showed that this nanoscratch technique holds promise as a thin film wear characterization technique.

Learning Changing Concepts by Exploiting the Structure of Change

This paper examines learning problems in which the target function is allowed to change. The learner sees a sequence of random examples, labelled according to a sequence of functions, and must provide an accurate estimate of the target function sequence. We consider a variety of restrictions on how the target function is allowed to change, including infrequent but arbitrary changes, sequences that correspond to slow walks on a graph whose nodes are functions, and changes that are small on average, as measured by the probability of disagreements between consecutive functions. We first study estimation, in which the learner sees a batch of examples and is then required to give an accurate estimate of the function sequence. Our results provide bounds on the sample complexity and allowable drift rate for these problems. We also study prediction, in which the learner must produce online a hypothesis after each labelled example and the average misclassification probability over this hypothesis sequence should be small. Using a deterministic analysis in a general metric space setting, we provide a technique for constructing a successful prediction algorithm, given a successful estimation algorithm. This leads to sample complexity and drift rate bounds for the prediction of changing concepts.

Packed-column SFC in the pharmaceutical industry: cGMP aspects

General aspects of drug development are discussed in the context of current Good Manufacturing Practice (cGMP) specified by various administrations. The potential of packedcolumn SFC (pSFC) instrumentation in the pharmaceutical industry is demonstrated along with different qualification parameters influencing chromatographic results, methods for their determination, and specifications defined in light of the ICH (International Conference on Harmonisation) Guideline for industry. Standard Operation Procedures (SOPs) are described for measuring flow rate, pressure, temperature, linearity and precision, sample carry-over, noise and drift, composition gradient, and wavelength accuracy of the pSFC instrumentation. The strong requirements for impurity and/or assay methods lead to a relative high effort of validation: beside selectivity, linearity and precision must be demonstrated over a wide range of concentrations for several repeated injections; limit of detection (LOD) and limit of quantitation (LOQ) must be in compliance with the specifications for impurity methods. A system suitability test (SST), filed together with the testing instructions of the drug, ensures that the pSFC instrumentation can be used for the analysis. Finally, method transfer between different systems and further instrumental improvements are discussed.

A Comparison of Stem-EDX and Auger Analysis of Segregation: The Plaudits and the Pitfalls.

Equilibrium segregation to grain boundaries has been studied for many years, but only recently have the techniques to quantify such segregation at high spatial resolution become widely available. These include, for example, the Auger surface analysis of fractured samples, or the study of boundaries edge-on in a field emission gun STEM equipped with energy dispersive X-ray analysis or electron energy loss spectroscopy. The key question, however, is whether these different techniques yield the same results when applied to a specific set of samples, and this paper reviews some of the successes and problems associated with such studies.For STEM analysis of grain boundary segregation, several instruments now routinely give probe sizes of ∼1nm. Hence line profiles across grain boundaries at this level of resolution are quite feasible. However a larger probe with greater probe current and better X-ray counting statistics is often more useful, especially if sample drift is a potential problem.

Development of EXELFS for Nanoscale Atomic Structure Determination

EXELFS spectroscopy contains the same local atomic structural information as XAFS; furthermore, it is readily applied to low Z elements, has high spatial resolution, and the capacity of combining other local WM measurements. Due to hitherto relatively poor quality of the EELS data, the EXELFS technique has not been developed to its full advantage until recently. We introduced various new methods to improve the data acquisition including real-time alignment and accumulation of virtually unlimited number of spectra while monitoring sample drift, radiation damage, and changes in energy resolution. We also developed a systematic data analysis procedure which corrects for the differences between EXELFS and XAFS, and adopts the UWXAFS data analysis software package to perform EXELFS data analysis with the same level of sophistication. The technique is demonstrated for SiC.

EXELFS χ-data renormalization

Extended Electron Energy Loss Fine Structure (EXELFS) spectroscopy is useful not only because it contains the same local atomic structure information as XAFS (X-ray Absorption Fine Structure), but also it has good low Z element sensitivity, up to nanoscale spatial resolution, and the capability of combining other high spatial resolution TEM measurements together with EXELFS. Until presently, due to poor quality of the EELS data, however, the EXELFS technique has not been developed to its full advantage. We have introduced various new methods to improve the data acquisition technique which includes removal of channel to channel gain variation and correction of dark-current background under real conditions; on-line aligning and accumulation, of virtually unlimited number of spectra while monitoring the thickness change (sample drift), radiation damage, electron beam energy drifts, and change in energy resolution during measurements. We have also developed a systematic data analysis procedure which utilizes the sophisticated UWXAFS data analysis software package after correction of the differences between EXELFS and XAFS data.

The Time-of-Flight Technique Applied to Amorphous Silicon Pin Solar Cells

ABSTRACTFast transient photocurrent measurements have been performed on a-Si:H based pin diodes with thicknesses down to 0.5 μm. On a thick sample drift mobilities derived in either the small-signal or the space-charge limited current mode are compared. On real solar cell devices in SCL voltage (integral) Mode the electron lifetimes τ are determined from the extraction time of the photo-generated charge in dependence on the degradation state of the sample. A clear and reproducible correlation between lifetime and degradation state has experimentally been established. Lifetimes τ, for which an intensity dependence has been observed in the SCL voltage Mode, are compared to the μτ-product from a small-signal charge collection Measurement. The influence of the real absorption profile on the measurements is discussed.

Electron-beam induced current determination of shallow junction depth

Electron-beam induced current (EBIC) was investigated as a possible method of determining shallow junction depth. Molecular-beam epitaxy Si n+/p junctions 200–1800 Å deep were explored with both cross-sectional and plan-view EBIC geometry. Cross-sectional EBIC analysis proves to be accurate and reproducible in determining the center of the depletion region for the deep junctions (1800 Å) when using a conventional scanning electron microscopy (SEM) with a LaB6 filament. Confidence in the location of shallow junctions decreases due to sample drift and the resolution limits of the SEM and EBIC techniques. In the plan-view geometry, in which the EBIC current is recorded as a function of electron beam energy, we can distinguish between shallow junctions with greater precision than deeper junctions. Collection efficiency versus electron-beam energy curves reveal junction depth through shifts in both peak position and height. The collection efficiency versus electron-beam energy curves were modeled assuming the n+ layer is equivalent to a Schottky barrier. The model agrees well with peak shifts, but carrier loss due to the n+ layer must be implemented to sufficiently describe the change in peak height.

Time-Dependent Deformation in Room-Temperature Indentation Experiments using a Nanoindenter

ABSTRACTTime-dependent deformation in room-temperature indentation is a well known phenomenon which is frequently overlooked in depth-sensing indentation experiments. In this study, the time-dependent displacements which occur during depth-sensing indentation experiments using a Nanoindenter were investigated. Time-dependent displacements were found to result both from plastic deformation of the sample and experimental drift. A simple correction for experimental drift was applied in experiments on sputtered copper thin films and reasonable strain rate sensitivities were obtained. The sources of experimental drift were identified and measured. It appears to be possible to measure the rate-dependence of plastic deformation in a variety of materials using this instrument.

Detection of Subpopulations in Near-Infrared Reflectance Analysis

In typical near-infrared multivariate statistical analyses, samples with similar spectra produce points that cluster in a certain region of spectral hyperspace. These clusters can vary significantly in shape and size due to variation in sample packings, particle-size distributions, component concentrations, and drift with time. These factors, when combined with discriminant analysis using simple distance metrics, produce a test in which a result that places a particular point inside a particular cluster does not necessarily mean that the point is actually a member of the cluster. Instead, the point may be a member of a new, slightly different cluster that overlaps the first. A new cluster can be created by factors like low-level contamination or instrumental drift. An extention added to part of the BEAST (Bootstrap Error-Adjusted Single-sample Technique) can be used to set nonparametric probability-density contours inside spectral clusters as well as outside, and when multiple points begin to appear in a certain region of cluster-hyperspace the perturbation of these density contours can be detected at an assigned significance level. The detection of false samples both within and beyond 3 SDs of the center of the training set is possible with this method. This procedure is shown to be effective for contaminant levels of a few hundred ppm in an over-the-counter drug capsule, and is shown to function with as few as one or two wavelengths, suggesting its application to very simple process sensors.

High resolution auger electron spectroscopy for materials characterization

High resolution Auger microanalysis has become a widely applied technique in various fields of materials research. Its submicrometer spatial resolution is shown as being advantageous for surface, interface and thin film analysis. Limitations of the lateral resolution are outlined with respect to influences of electron backscattering, detection sensitivity, sample drift, beam heating and other electron induced processes. High in-depth resolution is linked with high spatial resolution for optimized depth profiling by sputtering. Crater edge profiling with scanning Auger microscopy is particularly useful for obtaining a three-dimensional microanalysis. Some examples demonstrate the capabilities and limitations in the analysis of precipitates, fracture surfaces and multilayer structures.

Miniature ion mobility spectrometer cell

A miniature ion mobility spectrometer call using resistive coated ceramic electrodes is described. Its construction, performance and results of parametric studies are reported. The cell has a total ion current in the range 1 to 8*10-10 A and a minimum baseline ion current of 1.9*10-11 A. The total ion current less baseline ion current difference affects sensitivity through scaling relationships with the reactant and product ion currents. Because a membrane inlet is used for sample introduction, the carrier and drift gas flow rates both affect sensitivity. Although protonated quasi-molecular ions of sample molecules are found to be stable in the cell, protonated dimer ions dissociate in the higher electric fields which can be applied to the cell. Resolution of ion mobility peaks scales as the two-thirds power of the drift length squared divided by the gate width applied to the shutter grid. A microprocessor-controlled environmental monitoring system has been assembled using the cell as a sensor.

Effects of a magnetic field on the surface conductance of germanium

A.C. field effect measurements on etched 40 Ω cm n-type germanium are reported. The surface potential was varied by means of the capacitively applied electric field over the range −2.25 to +1.25 (in dimensionless units of eφ kT ); the corresponding values of the density of charge trapped in surface states ranged from approximately +0.9 to −3.2 (in units of 10 −9C cm −2). The surface was slightly n-type and the field effect mobility was found to be 200 cm 2 V −1 sec −1. Measurements were also made in the presence of a magnetic field transverse to both the sample drift current and the oscillating electric field. In fields ranging from zero to 9.00 kOe, in both normal and reverse senses, the field effect mobility varied from 1.9 × 10 3 ( p-type) to 2.2 × 10 3 ( n-type) cm 2 V −1 sec t1̄.