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Scanning Electron Microscopes Expand Fields of View
Hitachi Model SU-3500 is a full-sized SEM system with four-axis motorized stage.
 

Scanning electron microscopes (SEMs) are among the most exotic and, at the same time, invaluable test tools in the electronics industry. Quite simply, they are microscopes that use electrons instead of light to "illuminate" a target. They use an electron gun to produce a beam of electrons that is focused by electromagnetic fields and lenses held in vacuum. The electron beam strikes the sample, which then emits a variety of different electrons and x-ray signals. Different types of detectors are then used to read these signals and feed a display screen for viewing. Although not suited for all materials, SEMs can provide a great deal of insight with the proper targets and materials, and can produce images with resolution of 1nm or better.

While SEMs are often associated with research in physics and medicine, they have increasingly been helping scientists to learn more about electronic materials and circuits. Installations ranging from university research facilities to semiconductor foundries have installed SEM systems to provide high-resolution magnification of different subjects. One advantage that SEMs provides over optical microscopes, in addition to the wide range of magnification, is a large depth of field, which allows a large amount of a specimen to remain in focus at the same time. Of course, SEMs also have their share of limitations. They are designed to work with solid specimens, which must fit within a relatively small SEM vacuum chamber. Samples that are wet or likely to outgas under vacuum conditions are not suitable for use with an SEM.

Preparing Samples
Because a SEM operates in a vacuum, water must be removed from samples, otherwise it will vaporize in the vacuum. SEMs work well with conductive materials, such as metals, but require specimens that are not conductive to be pretreated to render them conductive and suitable for "viewing" in a SEM. Sputter coating is a pretreatment process typically used to prepare a sample for viewing. Although SEMs are radiation-producing systems, they are designed with sufficient shielding materials to make them quite safe under normal operating conditions. However, for safety reasons, the shielding must remain intact and must be properly maintained on any SEM system.
The Hitachi TM3000 Tabletop Microscope is an SEM that is not much larger than an optical microscope.
 


SEMs have traditionally been physically large systems, comprised of an assortment of key components, including, an electron gun, electron lenses, a sample stage, detectors, a display, a vacuum system, and a power supply. For example, one of the leading developers of SEM systems and technology, the Nanotechnology Systems Division (NSD) of Hitachi High Technologies America, Inc. (www.hitachi-hta.com), recently announced that one of its latest and most advanced SEM systems, its model SU-3500, is now available with a four-axis motorized microscope stage, in addition to the initial version of the system, with its five-axis motorized microscope stage. This latest version of the SEM system was introduced to assist users in need of slightly smaller sample applications, with maximum dimensions of about 200mm diameter and 70mm high. Its motorized stage helps to simplify three-dimensional (3D) viewing of samples and saving different vantage points of samples in digital form.

JEOL (www.jeolusa.com), sensitive to the needs of its SEM users for a somewhat wider vacuum operating range to accommodate a wider range of samples, recently announced its model JSM-IT300LV SEM with low-vacuum mode. Capable of operating under vacuum pressure across a range of more than twice that of earlier models, this new system can provide SEM imaging with such samples that are typically wet, oily, suffer excessive outgassing, or are nonconductive, without pretreatment, by operating in low-vacuum mode. The SEM system, with an embedded charge-coupled-device (CCD) camera, uses the firm's InTouchScope touch-screen control as part of its operating system, for simple, intuitive operation. The system's five-axis stage control provides imaging and analysis of samples over a wide range of angles and orientations with high accuracy and speed. The SEM also includes a large vacuum chamber for samples as large as 300mm in diameter and 80mm in high. It provides a magnification range of 5X to 300,000X.

Another supplier of SEM systems, TESCAN (www.tescan.com), is now offering the third generation of its VEGA series of SEM systems, with improvements made in SEM electronics, digital signal processing, and even in software control. These SEM systems perform in both high- and low-vacuum modes, with better than 3-nm resolution in high-vacuum mode and better than 3.5-nm resolution in low-vacuum mode. They provide a maximum field of view of 24mm and magnification ranging from 1X to 1,000,000X. While standard VEGA series SEM systems are supplied with conventional tungsten-heated-cathode electron guns that can be used in either high- or low-vacuum modes, the firm also offers an option for a lanthanum hexaboride (LaB6) electron source which is claimed to provide much improved viewing resolution and brightness compared to the conventional electron sources.

Shrinking Size
Although such room-filling systems offer a great deal of analysis power, in applications as wide ranging as biological research to military semiconductor electronics, a recent trend in SEM systems has been to pack more power into smaller packages. Many of the firms supplying large-scale SEM systems are now also developing more compact systems that are more easily incorporated into research and production facilities. For example, the TM3000 Tabletop Microscope from Hitachi High Technologies America is a compact SEM physically not much larger than an optical microscope. Well suited for semiconductor research, it handles samples as large as 70mm in diameter and 50mm in high, providing stage motion of 35mm in the x and y directions. It provides magnification from 15X to 30,000X, with digital zoom to 120,000X.

Another compact system from a leading SEM supplier, model 8500 from Agilent Technologies (www.agilent.com), is a field-emission (FE) SEM that is not much larger than one of the company's desktop laser printers. It offers several imaging techniques to allow users to study thin-film materials and a variety of substrates, including glass. The SEM system operates with continuously variable imaging voltages from 500 to 2000V for flexibility with a wide range of target materials, and provides better than 10nm resolution at 1000V. It handles samples as large as 100 x 60mm with a viewable area as large as 50 x 30mm.

Among the growing number of suppliers of desktop SEMs, Phenom World (www.phenom-world.com) is fast finding interest in its compact personal-computer (PC) controlled SEM systems for applications not formerly served by SEM technology. The company's Phenom product is a desktop SEM which is designed to be as easy to use as an optical microscope, but providing the high magnification of an SEM. With magnification capabilities to 100,000X, the compact SEM system has found favor among law-enforcement users, for forensic analysis and crime-scene investigations.

As SEM technology expands and is made more accessible, in smaller physical systems that are easily operated under PC control, the number of applications for SEM technology in electronics and other fields will continue to grow. With the capability of many SEM systems to store captured images as digital files, these SEM systems will also play increasingly important roles in the computer modeling of electronic devices and circuits.  

 
 
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