New Instrument Boosts Capability Of Scanning Electron Microscopy

With the scanning electron microscope (SEM), scientists in a range of fields—from biology to materials science to microelectronics—can analyze the surface of objects with a resolution approaching molecular dimensions. Although conceptually developed in the 1940s, the SEM was not put into practical use until the 1960s. Improvements in SEM technology continue with the introduction this year of a device that can image unprepared specimens without contaminating the microscope or charging or degrading the specimen. Secondary Electrons Using a scanning electron beam of extremely small diameter, the SEM generates low-energy, secondary electrons in the upper 100 A of the specimen. These secondary electrons are bent and collected by a detection system that is scanned in synchronization with a cathode ray tube (CRT). The strength of the detected signal determines the CRT brightness, thereby producing an image of the specimen. Specimen detail can be examined with a resolution as low as 20 A and a depth of field many times greater than a light microscope. Magnification ranges employed vary so greatly that an entire macromolecular object, such as an insect or experimental animal, can be viewed with clarity at lOX, or an extremely fine surface structure, such as microvilli on a cell surface, can be resolved at 200,OOOX. In addition to structural analysis, compositional analysis of specimens is possible through the use of detectors for high-energy back-scattered electrons or X-rays. Elements differing by as little as one in atomic number can be distinguished using such methods. Today, most commercially available SEMs are of uniform high quality, but vary in imaging systems, degree of automation, physical design, and cost. The four SEMs described below depict the range of features that are currently available. These companies, as well as others, also offer a variety of models tailored to individual needs. Image processing is a recently added feature of several SEMs now on the market and provides an advance in the quality of images that can be obtained over the standard unprocessed CRT image. The DSM 960 marketed by Zeiss Inc., located in Thornwood, N.Y., uses an integrated framestore system in which the incoming signal is put into digital memory and subsequently transferred at TV scan rates. The signal-to-noise ratio is optimized on a pixel-by-pixel basis. The degree to which this can be accomplished is dependent on the pixel dwell time, which can be varied with the scanning rate. This system results in a crisp, noise-free image that can be transferred to a camera for hard copy that automatically corrects for contrast, brightness, and gamma response. An additional feature of the DSM 960 is its ability to pseudocolor images such as color-coded compositional maps of specimens. Imaging modes are optimized automatically as the microscope’s operating parameters (such as transition from high to low voltage) change. The DSM 960 is pricey for an SEM, reflecting its sophistication in design. Similar degrees of sophistication in design, however, are now available for substantially less money. Bedford, Mass.-based Amray’s model 1810 SEM offers many features seen in the Zeiss instrument for less than half the price. The 1810 takes advantage of computer control to both simplify microscope operation and enhance performance. The instrument will automatically adjust video contrast and brightness levels as column operating parameters are changed. A digital framestore imaging system is used to produce high-quality images that can be stored via VCR or laser disk. Image focusing and stigmation can be achieved automatically. Like the DSM 960, a variety of scanning formats are possible with the 1810, allowing the scientist to differentially scan the specimen. Scientists interested in achieving the upper limits of SEM resolution may wish to consider an SEM developed by Milpitas, Calif.-headquartered International Scientific Instruments Inc. The DS 130 features a unique dual-stage design. Specimens in the smaller top chamber can be examined with a resolution of 20 A, while the large bottom chamber permits a 35 A resolution. This microscope uses a lanthanum hexaboride (LaB6) filament that provides a larger beam current density than the more commonly used tungsten filament. The maximum operating voltage of the DS 130 is 10 kv higher than conventional models. Although the DS 130 does not offer image processing, automated functions including focus and stigmation are provided. It has been a long-standing belief that an extremely low (10^-5 Torr) vacuum must be maintained in order to obtain satisfactory electron microscopic images. This requirement has placed severe constraints on the conditions under which the sample can be viewed. ElectroScan, of Danvers, Mass., has culminated 10 years of research and development in the production of an Environmental Scanning Electron Microscope (ESEM). With the ESEM scientists can now visualize specimens under saturated water vapor conditions (20 Torr and above) or other gas environments. A differential pumping system in conjunction with a series of pressure-limiting apertures allows the specimen chamber to be maintained at 20 Torr without degradation of gun or column vacuum levels. The electron detectors are capable of 100 A resolution at 5 Torr. It is now possible to view such dynamic processes as wetting, drying, absorption, and crystallization with SEM resolution. “With the ESEM, people are seeing things they have never seen before. This represents the most significant advance I’ve seen in scanning microscopy in the last 16 years,” says Neil Baumgarten, vice president and director of engineering at ElectroScan. A second and equally notable breakthrough is that insulators such as ceramics, plastics, or textiles can now be viewed with high beam voltages. The usual beam-distorting surface charge build-up is prevented effectively by the relatively high gas pressures in the ESEM specimen chamber. As a result, uncoated specimens of any type can now be visualized without risk of artifact. In addition, elemental composition analysis is greatly facilitated. The ESEM incorporates all the advanced features available with other SEMs. The system is controlled centrally by a VME-bus-based 68000 series computer, operating in real time. As with other SEMs, optional backscattered electron detectors and x-ray detectors are available that can be used in conjunction with the secondary electron detector. Heating, cooling, and micromanipulation devices are also available with the new ElectroScan ESEM. The SEMs described above, as well as others on the market today, clearly demonstrate advances in their ease of operation (via automation) and the quality of images obtained (via image processing). In addition, an exciting technical breakthrough now makes it possible to visualize dynamic processes with EM resolution. The continued development of the ESEM promises a bright future for scanning microscopy. V. Richard Sheridan is a postdoctoral associate in pharmacology at Fox Chase Cancer Center, Philadelphia.

New Instrument Boosts Capability Of Scanning Electron Microscopy

Secondary Electrons

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