Environmental scanning electron microscope

The environmental scanning electron microscope or ESEM is a scanning electron microscope (SEM) that allows for the option of collecting electron micrographs of specimens that are "wet," uncoated, or both by allowing for a gaseous environment in the specimen chamber.

Although there were earlier successes at viewing wet specimens in internal chambers in modified SEMs, the ESEM with its specialized electron detectors (rather than the standard Everhart-Thornley detector) and its differential pumping systems, to allow for the transfer of the electron beam from the high vacuums in the gun area to the high pressures attainable in its specimen chamber, make it a complete and unique instrument designed for the purpose of imaging specimens in their natural state. The instrument was designed originally by Gerasimos Danilatos while working at the University of New South Wales.

Starting with Manfred von Ardenne, early attempts have been reported on the examination of specimens inside "environmental" cells with water or atmospheric gas, in conjunction with conventional and scanning transmission types of electron microscopes. However, the first images of wet specimens in an SEM were reported by Lane in 1970 when he injected a fine jet of water vapor over the point of observation at the specimen surface; the gas diffused away into the vacuum of the specimen chamber without any modification to the instrument. Further, Shah and Beckett reported the use of differentially pumped cells or chambers to presumably maintain botanical specimens conductive in order to allow the use of the absorbed specimen current mode for signal detection in 1977 and in 1979. Spivak et al. reported the design and use of various environmental cell detection configurations in an SEM including differential pumping, or the use of electron transparent films to maintain the specimens in their wet state in 1977. Those cells, by their nature, had only limited application use and no further development was done. In 1974, an improved approach was reported by Robinson with the use of a backscattered electron detector and differential vacuum pumping with a single aperture and the introduction of water vapor around 600 Pa pressure at the freezing point of temperature. However, neither of those approaches produced a stable enough instrument for routine operation. Starting work with Robinson in 1978 at the University of New South Wales in Sydney, Danilatos undertook a thorough quantitative study and experimentation that resulted in a stable operation of the microscope at room temperature and high pressures up to 7000 Pa, as reported in 1979. In the following years, Danilatos, working independently, reported a series of works on the design and construction of an environmental or atmospheric scanning electron microscope (ASEM) capable of working at any pressure from vacuum up to one atmosphere. These early works involved the optimization of the differential pumping system together with backscattered electron (BSE) detectors until 1983, when he invented the use of the environmental gas itself as a detection medium. The decade of 1980 closed with the publication of two major works comprehensively dealing with the foundations of ESEM and the theory of the gaseous detection device (GDD). Furthermore, in 1988, the first commercial ESEM was exhibited in New Orleans by ElectroScan Corporation, a venture capital company wishing to commercialize the Danilatos ESEM. The company placed an emphasis on the secondary electron (SE) mode of the GDD and secured the monopoly of the commercial ESEM with a series of additional key patents. Philips and FEI companies succeeded ElectroScan in providing commercial ESEM instruments. With the expiration of key patents and assistance by Danilatos, new commercial instruments have been recently added to the market by LEO (succeeded by Carl Zeiss SMT). Further improvements have been reported to date from work on the original experimental prototype ESEM in Sydney and from numerous other workers using the commercial ESEM in a wide variety of applications worldwide. An early comprehensive bibliography was compiled in 1993 by Danilatos, whilst a more recent survey can be found in a Ph.D. Thesis by Morgan (2005).

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