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Scanning Electron Microscope

Scanning Electron Microscope

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

The Hitachi S-3500H incorporates a large specimen chamber. Maximum specimen size will vary depending on analysis requirements. In many cases, specimens can be greater than 100 mm x 100 mm x 25 mm tall. Specimens approaching this size, however, can limit movement options as well as limit minimum magnification to 30x or higher. Weight should be less than 1 kg.

The specimen stage (x,y) travel is 80 mm x 40 mm. Examination of larger specimens is possible, but may require interrupting the examination and remounting to the stage.

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.