Image noise cross-correlation for signal-to-noise ratio estimation in scanning electron microscope images

A new and robust parameter estimation technique, named image noise cross-correlation, is proposed to predict the signal-to-noise ratio (SNR) of scanning electron microscope images. The results of SNR and variance estimation values are tested and compared with nearest neighborhood and first-order interpolation. Overall, the proposed method is best as its estimations for the noise-free peak and SNR are most consistent and accurate to within a certain acceptable degree, compared with the others.     

Gaussian-Taylor signal-to-noise ratio estimation for scanning electron microscope images

A new and robust parameter estimation technique, named Gaussian-Taylor interpolation, is proposed to predict the signal-to-noise ratio (SNR) of scanning electron microscope images. The results of SNR and variance estimation values are tested and compared with piecewise cubic Hermite interpolation, quadratic spline interpolation, autoregressive moving average and moving average. Overall, the proposed estimations for noise-free peak and SNR are most consistent and accurate to within a certain acceptable degree compared with the others.     

New technique for in-situ measurement of backscattered and secondary electron yields for the calculation of signal-to-noise ratio in a SEM

The quality of an image generated by a scanning electron microscope is dependent on secondary emission, which is a strong function of surface condition. Thus, empirical formulae and available databases are unable to take into account actual metrology conditions. This paper introduces a simple and reliable measurement technique to measure secondary electron yield (δ) and backscattered electron yield (η) that is suitable for in-situ measurements on a specimen immediately prior to imaging. The reliability of this technique is validated on a number of homogenous surfaces. The measured electron yields are shown to be within the range of published data and the calculated signal-to-noise ratio compares favourably with that estimated from the image.     

Software acceleration techniques for the simulation of scanning electron microscope images

A scanning electron microscope (SEM) simulator was developed based on the models used in the MONSEL software. This simulator extends earlier work by introducing an object-oriented framework and adding optimization methods based on precomputation of electron trajectories. Several optimizations enable speedup by factors of 5-100 on a single processor over unoptimized simulations without introducing additional approximations. The speedup for a particular surface depends on the self-similarity of the surface at the scale of the electron penetration depth. We further accelerate by parallelizing the calculations for a total speedup of about 100-2000 on 30 processors. The goal of this work was to create a system capable of simulating a quantitatively accurate SEM image of a relatively unconstrained surface. Results of this work include simulation software, optimization algorithms, performance measurements with various optimizations, and examples of simulated images.     

Contrast in high-resolution scanning electron microscope images

Current scanning electron microscopes, equipped with field emission guns and high-performance immersion lenses, can achieve spatial resolutions of the order of 1 nm in both secondary and backscattered imaging modes over a wide range of operating energies. The generation and interpretation of images with nanometre-scale resolution relies on a detailed knowledge, and application, of electron-solid interactions. This paper develops the practical steps required to produce a high-resolution image, and discusses the principles which govern image interpretation. Attention is focused primarily on materials which are low in atomic number and density, such as biological tissue, but the results apply after appropriate scaling of the physical parameters to most other materials.     

Objective comparison of scanning ion and scanning electron microscope images

Common and different aspects of scanning electron microscope (SEM) and scanning ion microscope (SIM) images are discussed from a viewpoint of interaction between ion or electron beams and specimens. The SIM images [mostly using 30 keV Ga focused ion beam (FIB)] are sensitive to the sample surface as well as to low-voltage SEM images. Reasons for the SIM images as follows: (1) no backscattered-electron excitation; (2) low yields of backscattered ions; and (3) short ion ranges of 20–40nm, being of the same order of escape depth of secondary electrons (SE) [=(3–5) times the SE mean free path]. Beam charging, channeling, contamination, and surface sputtering are also commented upon.     

Adaptive tuning piecewise cubic Hermite interpolation with Wiener filter in wavelet domain for scanning electron microscope images

Image processing is introduced to remove or reduce the noise and unwanted signal that deteriorate the quality of an image. Here, a single level two‐dimensional wavelet transform is applied to the image in order to obtain the wavelet transform sub‐band signal of an image. An estimation technique to predict the noise variance in an image is proposed, which is then fed into a Wiener filter to filter away the noise from the sub‐band of the image. The proposed filter is called adaptive tuning piecewise cubic Hermite interpolation with Wiener filter in the wavelet domain. The performance of this filter is compared with four existing filters: median filter, Gaussian smoothing filter, two level wavelet transform with Wiener filter and adaptive noise Wiener filter. Based on the results, the adaptive tuning piecewise cubic Hermite interpolation with Wiener filter in wavelet domain has better performance than the other four methods.     

Monte Carlo simulation of secondary electron images for real sample structures in scanning electron microscopy

Monte Carlo simulation methods for the study of electron beam interaction with solids have been mostly concerned with specimens of simple geometry. In this article, we propose a simulation algorithm for treating arbitrary complex structures in a real sample. The method is based on a finite element triangular mesh modeling of sample geometry and a space subdivision for accelerating simulation. Simulation of secondary electron image in scanning electron microscopy has been performed for gold particles on a carbon substrate. Comparison of the simulation result with an experiment image confirms that this method is effective to model complex morphology of a real sample.     

Imaging deep holes in structures with gaseous secondary electron detection in the environmental scanning electron microscope

The gaseous secondary electron detector (GSED) in the environmental scanning electron microscope (ESEM) permits collection of electron signals from deep inside blind holes in both conducting and insulating materials. The placement of the GSED as the final pressure-limiting aperture of the ESEM creates a situation of apparent illumination along the line of sight of the observer. In principle, any point struck by the primary beam can be imaged. Image quality depends on the depth of the hole. In brass, features at the bottom of a 1.5 mm diameter hole that was 8 mm deep were successfully imaged.     

Secondary electron imaging in the variable pressure scanning electron microscope

Secondary electron imaging is not possible in the variable pressure scanning electron microscope because the mean free path of the secondaries in the gas is too short to permit them to reach the detector. This paper therefore investigates an alternative strategy for producing an image containing significant amounts of secondary electron contrast. This involves modifying the microscope by the addition of a biased electrode above the sample and then collecting a specimen current signal. This system, originally described by Farley and Shah (1988), is found to produce true secondary electron detail over a wide range of conditions.     

Unusual secondary electron emission behavior in carbon nanotube forests

Electron yield was measured from patterned carbon nanotube forests for a wide range of primary beam energies (400–20,000 eV). It was observed that secondary and backscattered electron emission behaviors in these forests are quite different than in bulk materials. This seems to be primarily because of the increased range of electrons due to the porous nature of the forests and dependent on their structural parameters, namely nanotube length, diameter and inter‐nanotube spacing. In addition to providing insight into the electron microscopy of nanotubes, these results have interesting implications on designing novel secondary electron emitters based on the structural degrees of freedom of nanomaterials. SCANNING 31: 221–228, 2009. © 2010 Wiley Periodicals, Inc.     

Application of the scatter diagram technique to the scanning electron microscope: preliminary results from diamond

Using primary beam energies E0 ranging from 0.2 to 15 keV and an in-lens detector, a series of images of the same region of an artificial microstructured diamond sample have been acquired in scanning electron microscopy. Next, the images were analysed by using a scatter diagram technique to underline the topographic contrast change and contrast reversal. The results obtained from 0.5 to 15 keV are discussed with the help of an expression derived from the constant loss model for the secondary electron yield delta of diamond, but including the respective roles of the angle of incidence, i, and of the angle of detection, alpha. More surprising is the quality of images obtained at a beam energy as low as 0.2 keV, and more difficult to explain is the significant contrast change between 0.2 keV and 0.5 keV. For the first time, scatter diagrams are used as a diagnostic tool in scanning electron microscopy, and after some improvements it is hoped that the experimental approach followed here may lead to quantitative estimates of the local tilts of a specimen surface.     

Signal components in the environmental scanning electron microscope

We demonstrate that the gas-amplified secondary electron signal obtained in the environmental scanning electron microscope has both desired and spurious components. In order to isolate the contributions of backscattered and secondary electrons, two sets of samples were examined. One sample consisted of a pair of materials having similar secondary emission coefficients but different backscatter coefficients, while the other sample had a pair with similar backscatter but different secondary emission coefficients. Our results show how the contribution of the two electron signals varies according to the pressure of the amplifying gas. Backscatter contributions, as well as background due to gas ionization from the primary beam, become significant at higher pressure. Furthermore, we demonstrate that the relative amplification efficiencies of various electron signals are dependent upon the chemistry of the gas.     

Monte Carlo simulation of secondary electron and backscattered electron images in scanning electron microscopy for specimen with complex geometric structure

A new Monte Carlo technique for the simulation of secondary electron (SE) and backscattered electron (BSE) of scanning electron microscopy (SEM) images for an inhomogeneous specimen with a complex geometric structure has been developed. The simulation is based on structure construction modeling with simple geometric structures, as well as on the ray-tracing technique for correction of electron flight-step-length sampling when an electron trajectory crosses the interface of the inhomogeneous structures. This correction is important for the simulation of nanoscale structures of a size comparable with or even less than the electron scattering mean free paths. The physical model for electron transport in solids combines the use of the Mott cross section for electron elastic scattering and a dielectric function approach for electron inelastic scattering, and the cascade SE production is also included.     

Amplification and noise in high-pressure scanning electron microscopy

High-pressure scanning electron microscopy (HPSEM) is a promising new family of techniques. The present knowledge of these techniques is reviewed and a new set of criteria developed for optimizing signal detection in HPSEM with a view to preserving specimen integrity. For this purpose, amplification of contrast signals generated in HPSEM was examined by computing the effect of ionization over a range of pressures and biasing fields, routinely used for this technique. The influence of secondary ionization due to ion impact was included in the calculations. To check the calculated results, the experiments were performed in the HPSEM apparatus in a nitrogen atmosphere. A divergence between the experimental values and the calculated values was found. This was removed by taking into account the effect of recombination of charge carriers. Inductive currents generated in the HPSEM environment are transient and do not affect the conclusions of this study. The gas pressure, biasing fields and beam current for preserving specimen integrity and obtaining good micrographs were selected from the data. Experimental measurements of noise are reported, and criteria for optimizing the signal-to-noise ratio for performing HPSEM are discussed. The application of these criteria has shown that specimen current detection can be more advantageous than other methods and configurations of detection and was capable of obtaining high/medium-resolution micrographs.     

Tilt dependence of the secondary electron emission at low excitation energy

Influence of the specimen's slope on the secondary electron emission has been experimentally studied. Strong deviations from the inverse cos law have been observed and corresponding phenomenological equation (taking into account this deviation) is suggested. The consequences of the dependence on the topography contrast of low- and high-voltage scanning electron microscopy (SEM) image, especially for three-dimensional (3-D) reconstruction, are considered.     

The trajectories of secondary electrons in the scanning electron microscope

Three-dimensional simulations of the trajectories of secondary electrons (SE) in the scanning electron microscope have been performed for plenty of real configurations of the specimen chamber, including all its basic components. The primary purpose was to evaluate the collection efficiency of the Everhart-Thornley detector of SE and to reveal fundamental rules for tailoring the set-ups in which efficient signal acquisition can be expected. Intuitive realizations about the easiness of attracting the SEs towards the biased front grid of the detector have shown themselves likely as false, and all grounded objects in the chamber have been proven to influence the spatial distribution of the signal-extracting field. The role of the magnetic field penetrating from inside the objective lens is shown to play an ambiguous role regarding possible support for the signal collection.