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SEMs in the PC Era Scanning electron microscopes have moved to a PC platform, and that, as the poet Robert Frost said, has made all the difference. Elizabeth Zubritsky
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COURTESY OF TIM MAUGEL, LABORATORY FOR BIOLOGICAL ULTRASTRUCTURE, UNIVERSITY OF MARYLAND–COLLEGE PARK (COLOR ADDED)
n era has ended for scanning electron microscopes (SEMs). In the old days, you would sit down at the microscope console, and in the flickering light of the cathode ray tube, read dials and turn knobs to obtain the best image of your sample. The sheer number of controls left no doubt that you were operating a complex instrument. And the heft of those knobs told you that they had a purpose. These days, operating an SEM is nearly indistinguishable from other work at your computer. In lieu of the desk-sized console, there is a streamlined computer workstation with the standard Windows-type interface. Most of the adjustments can be dispatched with mouse clicks—swift and simple, yet ethereal. The instrument’s few remaining knobs are neatly packaged in a box that sits discreetly to the side. If some seasoned SEM users rue the passing of the “hands-on” experience, the conveniences offered by the new PC-based systems may make the loss easier to take. This Product Review discusses such changes and surveys other improvements since SEMs were last reviewed by Analytical Chemistry (1997, 69, 749 A–752 A). Tables 1 and 2 list the specifications and features for representative instruments that are available from several manufacturers. This summary is not meant to be a comprehensive review of all SEMs; rather, it aims to provide general information on the capabilities and limitations of current SEM systems.
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Table 1. Selected scanning electron microscopes. M icroscope m odel
PSEM 2000
M X 2500
Quanta series
S-3500 series
JSM -6460 series
LEO 1400 series
M anufacturer
Aspex Instruments 175 Sheffield Dr. Delmont, PA 15626
CamScan Electron Optics, Ltd. CamScan House Pembroke Ave. Waterbeach Cambs CB5 9PY U.K. +44 (0) 1223-861066
FEI Co. 7451 NW Evergreen Pkwy. Hillsboro, OR 97124 503-640-7500
Hitachi (Nissei Sangyo, Co., Ltd.) 5100 Franklin Dr. Pleasanton, CA 95488 925-218-2800
JEOL 11 Dearborn Rd. P.O. Box 6043 Peabody, MA 01961-6043 978-535-5900
LEO Electron Microscopy, Inc. One Zeiss Dr. Thornwood, NY 10594 800-356-1090
URL
www.aspexLLC.com
www.camscan.co.uk www.feico.com
www.nissei.com
www.jeol.com
www.leo-em.com
Resolution1 Tungsten source LaB6 source Low-vacuum mode
8 nm @ 20 kV
3.5 nm @ 30 kV (available)
3.5 nm @ 30 kV
3.0 nm @ 25 kV 3.0 nm @ 25 kV 4.5 nm
3.0 nm N/A 4.0 nm
3.5 nm @ 30 kV 2.5 nm @ 30 kV 5.5 nm @ 30 kV
500 V–50 kV
200 V–30 kV
0.3–30 kV (1171 steps) 0.3–30 kV
200 V–30 kV
Accelerating voltage 2–20 kV
3.5 nm @ 30 kV
M agnification
8–200,000⫻
3–600,000⫻
6–1,000,000⫻
15–300,000⫻
5–300,000⫻
9–900,000⫻
Cham berpressure
0.001–133 Pa
1–200 Pa
Up to 2600 Pa
1–270 Pa
1–270 Pa
1–400 Pa
Features
Industrialized automated analysis; integrated imaging, EDX, and AFA automation; applet for gunshot residue analysis; various chamber and stage sizes available; low operating cost; MacroNavigator sample navigation aid allows point-andclick stage movement; optimized for quick, easy use and training
Chamber design permits simultaneous analysis with multiple detectors; optional X500 Crystal Microprobe for imaging and analysis of crystal microstructures can be used with any electron source; various chamber and stage sizes available
Proprietary environmental SEM technology offers nondestructive imaging of any sample; seamless switching between high-vacuum, low-vacuum, and environmental SEM; patented reduced beam spread for accurate microanalysis; available as high-vacuum-only instrument with future field upgrades to lowvaccum and environmental SEM
Patented dual bias and secondary electron bias plate enhance low-voltage performance; variable pressure with backscattered electron and environmental secondary electron signal collection modes; proprietary PCI information management system with full EDX integration; Windows NT operating system; three chamber sizes available
SmileView software for viewing and editing images; super conical objective lens guarantees 3.0-nm resolution at 8-mm working distance; fully eucentric stage tilts to 60° for 8-in. specimens and 90° for smaller ones; maximum sample height of 80 mm
Highly customizable; packages for fully automated EDX analysis or mineralogical analysis available; choose from several chamber sizes
AFA = automated feature analysis EDX = energy-dispersive X-ray 1 These numbers may have been determined under varying conditions and, therefore, may not be directly comparable.
The perqs of PCs The PC platform brings substantial digital imaging capabilities. Gone are the cathode ray tube display for viewing the sample and, if you like, the Polaroid camera for taking micrographs. The computer’s monitor and hard drive fulfill these roles. For some users, this change may be especially welcome since Polaroid filed for bankruptcy in October 2001, says Doug Yates of the Materials Characterization Facility at the University of Pennsylvania. “If you look at the microscopy list server, . . . most of the discussion [lately] has revolved around coming up with possible digital imaging solutions” in case Polaroid film disappears from the market, he says. Granted, some SEMs dating back to
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the mid-1980s could store images digitally, and users have added digital scan control and storage to other instruments for about one-quarter of the price of a new system. But the new systems offer complete integration and better image resolution—up to 3000 ⫻ 2000 pixels. In fact, the SEM has nearly become a computer peripheral, say experts such as Michael Postek of the National Institute of Standards and Technology. Instead of a console, the user is now presented with the standard PC interface. This makes the system much easier for beginners to operate, which is particularly helpful in labs that have multiple users, say the experts. It also adds convenience. For example, some systems can store
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each user’s preferred settings in profiles that can be recalled at every session, says Robert Kubic of General Motors’ Global Research & Development Operations. And with the newer software, a user can simply position the cursor and click to bring a particular feature to the center of the screen, he says. Digital systems also offer “real” automated focusing and astigmatism correction, Postek adds, so beginners can obtain acceptable images by just pressing a button or two. In addition, computer control of the sample stage means that a user can specify points to revisit later in the session. A user might view a sample using a relatively low spot size to get good picture quality and store points of interest along the way, ex-
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plains Kubic. Once the imaging is complete, the user can return to the stored waypoints to perform X-ray analysis. “It’s more efficient than changing the machine and refocusing and so on for each point,” he says. With some stages, it is also possible to reload a sample at a later date and get very close to the original location by recalling the stored waypoint, he adds. Having an integrated computer also increases the throughput, says Yates. Previously, Polaroid snapshots were scanned and stored digitally, he explains. “In a typical microscopy session, you may shoot 25 or 30 photos,” he says. “The time involved in scanning every one of those is quite extensive.” With a newer system, the images are standard PC files, which can be transferred to removable media, sent over network connections, opened in image processing programs to extract information, or put directly into reports or presentations.
Sources and resolution When choosing the source, the main consideration should be what types of samples you’re going to use, say the experts. For high-magnification work, many people immediately think of field-emission sources. If you’re working at or above 100,000⫻, it will be easier to get a good image with a field-emission gun (FEG), and the image will be sharper; but it is possible to get good images at reasonably high magnifications on many modern tungsten and LaB6 instruments, says Postek. “You’re just going to have to work a little harder.” FEGs certainly offer the best resolution at 20- to 30-kV accelerating voltage, ~1 nm as compared with ~2.5–4 nm for the tungsten or LaB6 sources. But for some users, the more important consideration may be that FEGs maintain good resolution at lower accelerating voltages—from
