Laboratory Management and Services: Scanning Electron Microscopy Laboratory

A Scanning Electron Microscope (SEM), unlike an optical microscope, uses an electron beam rather than light. SEM imaging has both disadvantages and advantages compared to optical microscopy.

Three distinct disadvantages are: the inability to reproduce color (electrons have none), the specimen must be stable under vacuum, and (in most cases) the specimen must be electrically conductive. Special SEMs and conductive thin film coatings can reduce vacuum and conductivity problems.

Some advantages of a SEM are: A very large depth of focus allows crisp images of very irregular surfaces, a large magnification range (<20x to >80,000x) is achievable, stereo (3D) images can be acquired and compositional data can often be acquired. Not only can a SEM produce images that are analogous to an optical microscope, but it can produce images whose contrast is based on a specimen's composition variations. Characteristic x-rays produced when the electron beam hits the sample can be used to identify and image specific elemental distributions (from boron to plutonium) in a specimen.

The BWXS SEM is a Hitachi model S-3500H. This state of the art SEM does not have Polaroid film capability to record images. Instead, digital images are acquired and saved. Customers can be provided with inkjet prints, CD-Rs, and/or DVDs. Images are normally stored in TIFF format. Other formats are available.

Hitachi S-3500H SEM Specimen Sizes

Specimen Preparation

The Hitachi S-3500H is a high vacuum only SEM. This means specimens for the S-3500H must not be degraded by exposure to high vacuum. A high vacuum SEM cannot be used to examine specimens with a low vapor pressure, e.g. most liquids, wet specimens, etc.

Specimens must exhibit at least slight electrical conductivity. Many insulating specimens can be coated with a conductive thin film for examination; some may be imaged using low beam energy without coating. Equipment is available to evaporate carbon or metals, and to sputter coat a specimen surface. Each method will provide conductivity. Sample characteristics and analysis goals will determine the appropriate technique.

The SEM Lab currently has a pure gold (Au) sputtering target. Other targets can be used as well. Each method will provide a conductive film, but each has its merits. Since the coating could impede an examination, discuss your examination goals with the analyst to select the appropriate coating method.

Magnification

Magnification can range from less than *18x to over 80,000x . A number of factors influence the magnification range and image quality. Not all specimens can be imaged over the full range. Exact limits must be determined on a case-by-case basis, but almost any specimen can be imaged from 50x to over 5,000x.

Software installed early in 2004, is specified to reduce the minimum magnification to 5x. Special conditions must be met, however, to reduce magnification to the lowest values. Tall samples and high beam voltages raise the minimum achievable magnification.

Imaging Modes

The SEM is equipped for both secondary electron (SE) and backscattered electron (BSE) imaging. Images can also be constructed from x-ray data (mapping). X-ray mapping will be discussed later.

Secondary electron imaging most closely approximates what would be seen using a conventional light microscope. It is most sensitive to topography and fine detail.

The next figures show SE and BSE images of the same location on a heterogeneous mineral specimen, illustrating the difference between SE and BSE modes.

Backscatter mode images can also be acquired using differential detectors. This imaging variation tends to suppress composition contrast and enhance topography contrast. The mode is often known as Topographic BSE (T-BSE) imaging. An image of the same location as above was collected using T-BSE and is shown below.

X-Ray Analysis for Composition

The SEM is equipped with both an Energy Dispersive x-ray Spectrometer (EDS) and a Wavelength Dispersive x-ray Spectrometer (WDS). These accessories provide compositional information about a specimen. The elemental identification function of these spectrometers is similar, but they have different strengths and weaknesses.

EDS uses a detector that is sensitive to x-ray energy from almost all elements. Software combines this data into a composite spectrum. It acquires information about all elements in a specimen (essentially) simultaneously. Data collection is typically faster than WDS. Minimum Detectable Concentration Levels (MDL) for EDS typically range from 0.1% to 1%, depending on the overall specimen composition and data collection parameters. The EDS mounted to the SEM is configured to provide both qualitative and quantitative results.

Using a diffracting crystal, WDS acquires data at only one x-ray wavelength at a time. This means it can analyze only to one element at a time. The diffracting crystal can be mechanically swept through its diffracting range to produce a (partial) spectrum similar to that presented by EDS. Four (4) or more crystals are required to span the elemental range of EDS. While slower than EDS, WDS analysis can yield MDLs in the range of 10s to 100s of ppm. Like EDS, the exact MDL will vary with the specimen and analysis conditions. The WDS at BWXS is configured to provide only qualitative information.

BWXS also has an Electron MicroProbe Analyzer (EMPA). The EMPA has limited imaging capability, but is equipped with six wavelength spectrometers (WDS) and is configured for standards based quantitative analysis.

EDS Details

The Hitachi S-3500H is equipped with an IXRF EDS (Energy Dispersive Spectrometer) with a Gresham light element detector. The system can detect x-ray energy from less than 200 eV. to over 20, 000 eV. This range covers the elements boron to plutonium.

Minimum detectable concentration levels (MDL) will vary with SEM operating conditions and the overall composition of the unknown. MDLs greater than =10% can be the case for light elements in a heavy matrix, to better than 0.1% heavy elements in a low atomic matrix.

Since operating conditions also play a significant role in "trace" element detectability, it is important to discuss your analysis expectations with the SEM operator prior to beginning an examination.

EDS can quickly identify the major constituents of a material. The next figure is an example of the most common qualitative data presentation. This example is actually two spectra (compared overlay), that highlight the composition difference between two areas of a specimen.

If certain conditions are satisfied, the data from spectra can be quantified. Quantitation can be either "standards based" (if standards available), or "standardless" (sometimes called semi-quant). Accuracy will vary depending on a number of parameters including which elements are involved, peak overlap interference, homogeneity, surface condition (polish), etc. Most often, standardized quantitation accuracy is better than ±0.5%. Standardless accuracy is usually within 1%, but can error by several percent in some cases. Discuss your samples and needs with the analyst. An example of a standardless quantitation is shown. Typically EDS quantitations are normalized to 100%.

Other forms of x-ray data presentation such as "dot maps" (x-ray maps) and line scans (line profiles) are also available. These forms are often used to illustrate the distribution of elements within a field of view.

Below is an example of an EDS x-ray (dot) map showing the distribution of carbon, oxygen, and silicon in a field of view of a polished metallographic specimen.

Below is an example of the same area, but an EDS line scan for the same three elements.

X-ray maps provide information about element concentration within an area, while line scans provide information only along a line within a field of view. Mapping is most useful when searching for elements in a field of view. Map acquisition can be slow compared to an equally sensitive line scan. Small concentration variations are easier to perceive using line scans, and for the same time investment, sensitivity is higher than mapping. This is especially useful in diffusion studies.

WDS Specifics

The Hitachi S-3500H is equipped with a Microspec WDX-2A spectrometer, controlled by a Kevex Sesame24 system. Typical applications for the WDS are resolving overlapped element peaks found using EDS, and detection of elements present in a specimen in very low concentration.

Both "line scans" and "x-ray maps" can be acquired (for a single element) or raw intensity data can be stored. Data files saved from the WDS are in ascii format and suitable for entry into MS Excel for presentation.

For more information, please contact us.