eTraining Introduction

Specimen Preparation

Leica Ultracut UCT

Histology

JEOL JSM-6400 SEM

Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB

Microanalysis

Veeco Dim 3000 AFM

Fluorescence Microscopes

Support

Signal Source and Detection

Electron Imaging Signal

Fruit Fly ImageSince the SEM is, in effect, an electrical circuit, the specimen must be conductive. Any nonconductive specimen is coated with a very thin layer of conductive material in order to provide a pathway for the negatively charged electrons to escape the beam impact site. Gold, carbon and other pure metals or alloys are used as conductive coating. This will prevent a charge from forming on the specimen.

If charging does occur, image resolution is degraded and imaging can become impossible. As the beam electrons interact with the specimen, they are scattered elastically (changing their trajectory but carrying almost the same kinetic energy) and inelastically (losing energy but remaining at almost the same trajectory). Some of the electrons are carried to the ground circuit but some of them penetrate the sample and after losing only a small amount of energy escape the sample as backscatter electrons, BSE.

During inelastic scattering, loosely bound electrons located in the outer atomic shells are knocked out of the sample by the incident (beam) electrons. These ejected electrons are known as secondary electrons, SE. Secondary electrons are useful in imaging of the surface topography. Number of backscatter electrons produced in the beam impact site is a function of elemental composition of the sample, and these electrons are collected to provide compositional information about the sample. In BSE mode the topographic character of the image is suppressed.

The signals produced during the electron beam-specimen interactions (SE, BSE, and X-rays) are picked up by detectors. The detectors are specifically designed to specialize in and detect one or more types of signals. One of the most commonly known and used electron detectors is the Everhart-Thornley detector, or E-T detector. It has the ability to collect SEs and BSEs, both together and/or separately. SEMs may also be equipped with a dedicated Backscatter Electron detector, which is specialized in the amount of signal it produces and its position in the chamber to effectively collect only backscatter electrons.

Chemical Analysis (X-ray) Signal

X-rays are generated by the interaction of the electrons with the sample. X-ray signals provide valuable information about the elemental composition of the sample, and they are generated from a micro volume size sample of material: ~1-5 μm3. X-rays are formed when the high-energy beam electrons eject inner shell electrons from the atom. An electron from an outer shell fills up the vacancy and a difference in its binding energy at two different orbits is emitted as an X-ray photon. Each atom has a unique X-ray spectral "fingerprint" comprised of emission lines. These lines can be identified with already published emission line tables for all elements. Software programs are also available and widely used to help with emission line identification.

Collecting X-rays is more difficult than collecting SEs and BSEs. X-ray signals may be collected using either a crystal-based detector known as the Wavelength Dispersive Spectrometer (WDS) or a semi-conductor based detector known as an Energy Dispersive Spectrometer (EDS). EDS is the most common X-ray detector found on SEMs. Physically, the EDS detector has a shaft that penetrates the SEM chamber wall to place the sensor very close to the sample. It is easily recognizable with its liquid nitrogen tank. Liquid nitrogen is used to reduce electronic noise in the detected X-ray signal. This type of detector is relatively less expensive than the WDS, and it senses the entire spectrum of X-ray signals of varying energy from the sample at once. A disadvantage is that it is not sensitive enough to detect trace amounts of elements in the sample. Another disadvantage of EDS systems is that they will not separate closely spaced emission lines, which overlap on the X-ray EDS spectrum. Both of these common problems disappear when the WDS detector is used.

WDS detectors are more complicated, more expensive, and more sensitive than EDS systems. A WD spectrometer utilizes a coupled arrangement consisting of an analyzing crystal and a detector, called a counter in WD nomenclature. Analyzing crystals are a plate with known atomic spacing and are ideally chosen to measure a specific X-ray emission line. The coupling of the crystal/detector pair has to be correctly positioned for that measurement.

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