Environmental scanning electron microscope
The environmental scanning electron microscope is a scanning electron microscope 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 and its differential pumping systems, to allow for the transfer of the electron beam from the high vacuum in the gun area to the high pressure attainable in its specimen chamber, make it a versatile instrument for imaging specimens in their natural state. The instrument was designed originally by Gerasimos Danilatos while working at the University of New South Wales.
History
Starting with Manfred von Ardenne, early attempts were reported of 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. 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 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 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 works dealing with the foundations of ESEM and the theory of the gaseous detection device. 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 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 later added to the market by LEO. 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 bibliography was compiled in 1993 by Danilatos, whilst a more recent survey can be found in a Ph.D. Thesis by Morgan.Microscope
An ESEM employs a scanned electron beam and electromagnetic lenses to focus and direct the beam on the specimen surface in an identical way as a conventional SEM. A very small focused electron spot is scanned in a raster form over a small specimen area. The beam electrons interact with the specimen surface layer and produce various signals that are collected with appropriate detectors. The output of these detectors modulates, via appropriate electronics, the screen of a monitor to form an image that corresponds to the small raster and information, pixel by pixel, emanating from the specimen surface. Beyond these common principles, the ESEM deviates substantially from an SEM in several respects, all of which are important in the correct design and operation of the instrument. The outline below highlights these requirements and how the system works.Differential pumping
The specimen chamber sustaining the high-pressure gaseous environment is separated from the high vacuum of the electron optics column with at least two small orifices customarily referred to as pressure-limiting apertures. The gas leaking through the first aperture is quickly removed from the system with a pump that maintains a much lower pressure in the downstream region. This is called differential pumping. Some gas escapes further from the low pressure region through a second pressure limiting aperture into the vacuum region of the column above, which constitutes a second stage differential pumping. A schematic diagram shows the basic ESEM gas pressure stages including the specimen chamber, intermediate cavity and upper electron optics column. The corresponding pressures achieved are p0>>p1>>p2, which is a sufficient condition for a microscope employing a tungsten type of electron gun. Additional pumping stages may be added to achieve an even higher vacuum as required for a LaB6 and field emission type electron guns. The design and shape of a pressure limiting aperture are critical in obtaining the sharpest possible pressure gradient through it. This is achieved with an orifice made on a thin plate and tapered in the downstream direction as shown in the accompanying isodensity contours of a gas flowing through the PLA1. This was done with a computer simulation of the gas molecule collisions and movement through space in real time. We can immediately see in the figure of the isodensity contours of gas through aperture that the gas density decreases by about two orders of magnitude over the length of a few aperture radii. This is a quantitatively vivid demonstration of a first principle that enables the separation of the high-pressure specimen chamber from the low pressure and vacuum regions above.By such means, the gas flow fields have been studied in a variety of instrument situations, in which subsequently the electron beam transfer has been quantified.
Electron beam transfer
By the use of differential pumping, an electron beam is generated and propagated freely in the vacuum of the upper column, from the electron gun down to PLA2, from which point onwards the electron beam gradually loses electrons due to electron scattering by gas molecules. Initially, the amount of electron scattering is negligible inside the intermediate cavity, but as the beam encounters an increasingly denser gas jet formed by the PLA1, the losses become significant. After the beam enters the specimen chamber, the electron losses increase exponentially at a rate depending on the prevailing pressure, the nature of gas and the acceleration voltage of the beam. The fraction of beam transmitted along the PLA1 axis can be seen by a set of characteristic curves for a given product p0D, where D is the aperture diameter. Eventually, the electron beam becomes totally scattered and lost, but before this happens, a useful amount of electrons is retained in the original focused spot over a finite distance, which can still be used for imaging. This is possible because the removed electrons are scattered and distributed over a broad area like a skirt surrounding the focused spot. Because the electron skirt width is orders of magnitude greater than the spot width, with orders of magnitude less current density, the skirt contributes only background noise without partaking in the contrast generated by the central spot. The particular conditions of pressure, distance and beam voltage over which the electron beam remains useful for imaging purposes has been termed oligo-scattering regime in distinction from single-, plural- and multiple-scattering regimes used in prior literature.For a given beam accelerating voltage and gas, the distance L from PLA1, over which useful imaging is possible, is inversely proportional to the chamber pressure p0. As a rule of thumb, for a 5 kV beam in air, it is required that the product p0L = 1 Pa·m or less. By this second principle of electron beam transfer, the design and operation of an ESEM is centered on refining and miniaturizing all the devices controlling the specimen movement and manipulation, and signal detection. The problem then reduces to achieving sufficient engineering precision for the instrument to operate close to its physical limit, corresponding to optimum performance and range of capabilities. A figure of merit has been introduced to account for any deviation by a given machine from the optimum performance capability.