Thin-Film Measurements in the Scanning Electron Microscope

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is < lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along <100> directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

The low-loss electron (LLE) image in the scanning electron microscope (SEM) is useful for the study of uncoated photoresist and some other poorly conducting specimens because it is less sensitive to specimen charging than is the secondary electron (SE) image. A second advantage can arise from a significant reduction in the width of the “penetration fringe” close to a sharp edge. Although both of these problems can also be solved by operating with a beam energy of about 1 keV, the LLE image has the advantage that it permits the use of a higher beam energy and therefore (for a given SEM) a smaller beam diameter. It is an additional attraction of the LLE image that it can be obtained simultaneously with the SE image, and this gives additional information in many cases. This paper shows the reduction in penetration effects given by the use of the LLE image.

Surface organic residues inhibit the extraordinary electronic properties of graphene, hindering the development of graphene electronics. However, fundamental understanding of the residue morphology is still absent due to a lack of high-throughput and high-resolution surface characterization methods. Here, we demonstrate that secondary electron (SE) imaging in the scanning electron microscope (SEM) and helium ion microscope (HIM) can provide sub-nanometer information of a graphene surface and reveal the morphology of surface contaminants. Nanoscale polymethyl methacrylate (PMMA) residues are visible in the SE imaging, but their contrast, i.e. the apparent lateral dimension, varies with the imaging conditions. We have demonstrated a quantitative approach to readily obtain the physical size of the surface features regardless of the contrast variation. The fidelity of SE imaging is ultimately determined by the probe size of the primary beam. HIM is thus evaluated to be a superior SE imaging technique in terms of surface sensitivity and image fidelity. A highly efficient method to reveal the residues on a graphene surface has therefore been established.

Secondary electron (SE) imaging in a scanning electron microscope is used to map electrically active dopant distributions of B-doped superlattices in Si. By comparing SE contrast profiles with secondary ion mass spectroscopy data, it is shown that B is electrically deactivated when the damage caused during Si implantation falls onto a doped region. Following a 450 °C anneal, the effect of the implantation damage is severely reduced in the SE profiles and the B is partially reactivated. An 815 °C anneal results in transient enhanced diffusion of some of the B with the remainder trapped in an inactive immobile peak.

Secondary electron (SE) imaging techniques, such as scanning electron microscopy and helium ion microscopy (HIM), use electrons emitted by a sample in response to a focused beam of charged particles incident at a grid of raster scan positions. Spot size—the diameter of the incident beam's spatial profile—is one of the limiting factors for resolution, along with various sources of noise in the SE signal. The effect of the beam spatial profile is commonly understood as convolutional. We show that under a simple and plausible physical abstraction for the beam, though convolution describes the mean of the SE counts, the full distribution of SE counts is a mixture. We demonstrate that this more detailed modeling can enable resolution improvements over conventional estimators through a stylized application inspired by semiconductor inspection: localizing the edge in a two-valued sample. We derive Fisher information about edge location in conventional and time-resolved measurements (TRM) and also derive the maximum likelihood estimate (MLE) from the latter. Empirically, the MLE computed from TRM is approximately efficient except at very low beam diameter, so Fisher information comparisons are predictive of performance and can be used to optimize the beam diameter relative to the raster scan spacing. Monte Carlo simulations provide an example of the MLE giving a 5-fold reduction in root mean-squared error (RMSE) of edge localization as compared to conventional interpolation-based estimation. The RMSE is substantially below both the beam diameter and the raster scan spacing and thus sub-pixel localization is demonstrated. Applied to three real HIM datasets, the average RMSE reduction factor is 5.4.

Focused Ion Beams (FIBs) provide a cross-sectioning tool for submicron dissection of organs, tissues, cells and subcellular structures. In combination with a Scanning Electron Microscope (SEM), FIB makes electron and ion microscopy available at the same time, for complementary morphological information, that moreover can be completed by EDS (Energy Dispersive X-ray Spectroscopy). In a previous paper [1] we have shown that Secondary Electron Imaging (SEI) and Back Scattered Electron Imaging (BEI) allow to recognize sample morphology; these techniques can rely on preliminary FIB operations (such as FIB sectioning, smoothing and lamellae formation) that provide specific manipulation of the sample in order to enhance the out-coming results. Here we discuss X ray analysis (EDS) to check the chemical composition of FIB exposed digestive gland cells of a terrestrial isopod Porcellio scaber.

We demonstrate the surface potential sensitivity of secondary electron (SE) imaging of titania thin film by scanning electron microscopy (SEM), and discuss the image formation mechanism. The 1‐nm titania film on Si substrate was imaged as bright region against the substrate. The SE image of the film varied with SE detector, but did not with primary electron energy. The contrast of the film was reversed by only 1‐nm carbon deposition. This result suggests that a slight change of surface potential altered the efficiency of secondary electron emission. This high sensitivity of secondary electrons to the surface potential is promising for improvement of thin‐film observation by SEM. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characteristic energy losses on carbon contaminated layers correlated with secondary electron, auger electron emission and contrasts in scanning microscope

Freeze-cracked taste buds from rabbit foliate papillaes were studied using both the secondary electron (SE) image and the back-scattered electron (BSE) image. Zinc-Iodide-Osmium (ZIO) and Ruthenium red (RR) staining were used for SEM observation of the BSE image.With the SE image, the three-dimensional architecture of taste buds was relatively easy to visualize. However, individual cells could not be identified clearly. Basal cells were recognized easily from their position in the taste bud. The dense substance (DS) in the taste pore and the dark granules (DG) of the type I taste cell could be seen in the SE image. However, it was found that the BSE image produced by the ZIO method provided a strong contrast for both DS and DG that was superior to that of the conventional technique. Also a distinct BSE image could be obtained from the Golgi vesicles and DG near the Golgi apparatus at the perinuclear area in only the type I cell. It is of interest that DS and DG exhibit the same strong BSE image. It could be strongly suggested that the DG originated from the Golgi apparatus. RR, Which has an affinity for acid glycoconjugate, strongly stained the substance present along the membrane of cytoplasmic processes and some of the pore vesicles but DS and DG were only faintly stained or not at all in the BSE image. This implies that DS and DG contains only a small amount of polysaccharides. It appears then, that utilizing the BSE image in conjunction with the SE image is very useful in determinating the three-dimensional localization of histocytochemical components in taste buds.

Alternant-channel chaotic laser secure communication system and its chaos alternant coding (CAC) are presented and deeply studied via two coupled lasers synchronizing or desynchronizing with another single laser. The coupled lasers are used as two chaotic transmitters and transmit two chaotic carriers to mask two information signals, respectively. One conventional encoding scheme is that the single laser is used as a receiver and obtain synchronization with one of two chaotic transmitters for chaos decoding. And another CAC scheme as a non-conventional coding is presented and its coding principle is defined for secure communication. CAC coding and scheme is that one laser of chaotic transmitters transmits a chaotic carrier to mask an information signal, where the carrier is modulated by an information signal, while the decoding of CAC is performed via synchronization or de-synchronization between another laser of two chaotic transmitters and a receiver. A physical model of the synchronous system is presented using a drive-feedback technique while a chaotic synchronization is obtained between a chaotic transmitter and a receiver. And alternant-channel chaotic secure communication and CAC are successfully implemented while a novel CAC technique is validated. The system has the characteristics of high dimension, many degrees of freedom in phase-space, a high degree of nonlinearity, a nonconventional CAC, and many secret keys. So it has high security and can transmit securely two chaotic carries masking two information signals, respectively.

Microscopy is the art and science of visually observing small objects. Electrons are electrically charged subatomic particles typically responsible for the flow of electrical current. Scanning is a method of observation where the focus of attention is moved in a chosen pattern and desired information is gained by assembling observations in sequence. In a scanning electron microscope (SEM), a beam of electrons is scanned in a regular pattern over a small object, and the image of the object appears on a screen by displaying the image points in synchrony with the movements of the electron beam on the object.A familiar example of image scanning is a television receiver, where a picture is created on a screen by rapidly scanning an electron beam across the phosphor face of the television tube. The beam striking the phosphor produces light, which is seen as a picture. The signal that modulates the electron beam in a television set is created in a television camera, which is scanned in the same way that the television screen is scanned. In a SEM, the signals come from special detectors in the microscope, not from an optical camera. Otherwise, the image display in a SEM is very similar to television. Although it is typical for SEM images to be scanned at a slower rate than are television signals, some SEMs display images at the same speed as television.The most common signal for detection in a SEM is the secondary electron signal. Secondary electrons are low‐speed electrons (having energies less than about 50 eV) that result from the interaction of the primary (i.e. beam) electrons with the sample. The secondary electrons are detected, usually by a scintillator/photomultiplier detector, and the resulting signal modulates the display screen.Secondary electrons are popular because they provide a kind of shading that gives a three‐dimensional appearance to an image, making it easy to visualize the structural relationships of parts of the sample (Figure 1).Other signals provide information of a more analytical nature. The signal produced by backscattered electrons is directly (although not exactly proportionally) related to the atomic number of the material being struck by the primary beam. Diffracted electrons provide information about crystal spacings and orientations in the sample. Induced‐current signals are particularly interesting for studying the response of active electronic components.The resolution of a SEM image depends on the quality of the optical system that forms the electron beam, including the nature of the electron source. Most SEMs can typically resolve objects as small as 10 nm. The resolution of specialized SEMs is better than 1 nm. In contrast, the resolution of light microscopes is limited by the wavelength of light to about 500 nm.The magnification of a SEM is not directly related to the optical system, very unlike a light microscope. The magnifying power of a SEM is related to the ratio between the size of the scan on the object compared to the size of the scan on the image display. In this way a SEM functions like a mechanical pantograph that was used, typically in older days, for copying drawings. If the arms of the pantograph were of equal length, the drawing would be copied the same size. If the arms were adjusted to different lengths, the drawing could be magnified or reduced.The primary advantage of a SEM is that it provides familiar‐looking images of surfaces with higher resolution and greater depth‐of‐field than an optical microscope. Although the images are typically familiar looking, the contrast mechanism in a SEM is not the same as the contrast provided by light, and an observer expecting always to interpret a SEM image the same way as a light image could easily be fooled.As the electron beam consists of electrically charged particles, the sample for SEM observation must either be inherently electrically conducting or must be made electrically conducting to yield good images. Otherwise, electrical charge builds up on the sample, the yield of secondary electrons is changed, and the scanning beam can become erratic. Low‐voltage or low‐vacuum SEMs can be used to reduce the charging problem.

Biological ultrastructures have been extensively studied with the scanning electron microscope (SEM) for the past 12 years mainly because this instrument offers accurate and reproducible high resolution images of cell shapes, provided the cells are dried in ways which will spare them the damage which would be caused by air drying. This can be achieved by several techniques among which the critical point drying technique of T. Anderson has been, by far, the most reproducibly successful. Many biologists, however, have been interpreting SEM micrographs in terms of an exclusive secondary electron imaging (SEI) process in which the resolution is primarily limited by the spot size of the primary incident beam. in fact, this is not the case since it appears that high resolution, even on uncoated samples, is probably compromised by the emission of secondary electrons of much more complex origin.When an incident primary electron beam interacts with the surface of most biological samples, a large percentage of the electrons penetrate below the surface of the exposed cells.

The proliferation of laser technologies has profoundly increased the demand for high-quality optical thin films whose physical properties are tunable and well defined. Such films are frequently deposited in thicknesses much shorter than the wavelengths of visible light and consequently present challenges for characterization by traditional microscopy. Metal films in particular exemplify these challenges, due to their broad range of refractive indices, optical absorption and often near-complete reflectivity in the visible spectrum. However, due to their relatively consistent crystalline structure, the bulk optical properties of metal thin films are chiefly dependent on their thickness. This review therefore presents a compendium of viable alternative characterization techniques to highlight their respective utilities, limitations and resolutions, specifically with regard to the characterization of the thickness of metal films. Furthermore, this review explicitly addresses the operating theories, methods and analyses relating to the five most predominantly utilized techniques: X-ray Reflectivity (XRR), Spectroscopic Ellipsometry (SE), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). This work is intended as an introductory guide to thin film characterization modalities and their applicability for metal and optically-absorptive films, while also identifying AFM and SEM/EDS as being amongst the more reliable of the techniques.

A mechanism is described for changing the signal detector in the Cambridge ``Stereoscan'' scanning electron microscope without breaking the vacuum. This has three main advantages. First, it makes it easier to compare the back scattered electron image, the secondary electron image, and the luminescent image, which can help the interpretation in some cases. Second, a spare secondary electron detector can be kept in reserve for high resolution work, thus avoiding the degradation of scintillator performance that occurs after prolonged use. Finally, it makes it possible to optimize detectors by comparing different versions of the same basic design.

Basically, the term “Filtering” is referred to a technique to extract information (signal in this context) from noise contaminated observations (measurements). If the signal and noise spectra are essentially non-overlapping, the design of a frequency domain filter that allows the desired signal to pass while attenuating the unwanted noise would be a possibility. A classical filter could be either low pass, band pass/stop or high pass. However, when the noise and information signals are overlapped in spectrum, then the design of a filter to completely separate the two signals would not be possible. In such a situation the information has to be retrieved through estimation, smoothing or prediction. Figure 2.1 shows a general diagram of an open-loop system (plant) subject to noise contamination at the output end.