Larry D. Hanke
During the past two decades, the scanning electron microscopy (SEM) has become an indispensable tool for morphological observation in materials and biological sciences. With energy dispersive x-ray spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS), rapid chemical characterization of microscopic features has also become commonplace. As developments in manufacturing methods allow fabrication of smaller and smaller devices, microscopic characterization by SEM has seen increasing demand.
As demand for SEM analysis has grown, computer technology has made SEM/EDS analytical equipment more powerful and easy to operate. The continued development of versatile, easily operated, high-performance SEMs has further increased demand for SEM/EDS evaluations in more and more applications.
Many applications where SEM/EDS evaluation could be useful involve samples that are not electrically conductive. These samples have traditionally required pretreatment, by coating with a conductive film, before SEM examination. Nonconductive samples are subject to a buildup of electrons on the examined surface. This buildup of electrons, or "charging" eventually causes scattering of the incoming electron beam, which interferes with imaging and analysis.
Furthermore, samples that contain substantial water or other materials that volatilize in high vacuum also present challenges for SEM examination. These samples require controlled drying to allow the SEM chamber to reach high vacuum and to prevent deformation of the sample at the SEM vacuum.
Often, sample pretreatment is only partially effective in preparing the samples for SEM examination. For example, wet samples change character when dried, thus appearing different in the SEM examination than in their native state. For porous samples, it is difficult to get the conductive coating into the subsurface spaces. Charge buildup at the pores continues to interfere with the SEM examination.
In addition, coating confuses analysis by the EDS and WDS techniques. Carbon is the typical material for coating when microanalysis is required. Early EDS detectors could not detect carbon. Therefore, carbon coating was not a severe limitation for microanalysis. Today's EDS systems detect carbon and lighter elements, as light as beryllium. Carbon applied to sample surfaces obviously will interfere with detection of carbon in the sample and can inhibit detection any lighter elements present.
As the SEM is increasingly used for routine evaluations, there is increasing demand for examination without pretreatment. Coating is not an alternative for in-process inspection of components to be put into service where the coating affects the components' suitability for service. Coating may also be unacceptable for preparation of samples that should not have their character altered, such as historic materials or evidence in legal actions.
SEM examination can be performed at low accelerating voltages to diminish the effects of charging. Field emission SEMs are especially effective for imaging at sufficient low accelerating voltages to prevent charging. X-ray analysis, however, is not possible at these low accelerating voltages. Minimum voltages of 5 to 10 KV are required for reliable x-ray analysis, which is not sufficiently low to prevent charging on many materials. In addition, field emission SEMs are considerably more expensive than the instruments with the more common tungsten-filament sources.
An answer to the problems of charging and volatile samples is the development of scanning electron microscopes that operate without exposing the sample to high vacuum. These microscopes are referred to alternately as environmental, low-vacuum, high-pressure, or variable-pressure SEMs. Higher pressures in the SEM sample chamber offer two primary benefits compared to traditional high vacuum chambers. First, a higher pressure minimizes the outgassing from volatile samples. Secondly, by allowing a controlled amount of gas into the chamber, charging is diminished on nonconductive samples.
Scanning electron microscopes have traditionally used a vacuum in the column and sample chamber to obtain an energetic, highly focussed electron beam needed for high-resolution imaging. In variable pressure SEMs, the chamber at the electron gun is maintained at high vacuum while a controlled amount of gas is allowed into the sample chamber. A fine aperture separates the gun and sample chambers to prevent excessive gas entrance into the gun chamber. Separate vacuum systems control the vacuum in the sample chamber and at the gun.
The advantages of a higher pressure in the sample chamber are obvious for wet and volatile samples. The higher pressure decreases the rate of volatilization or outgassing. This decreases the drying and deformation of wet samples. Since the sample chamber can tolerate higher pressures, any outgassing does not inhibit operation of the microscope.
For nonconductive samples, the advantage of higher pressure is less obvious. When gas molecules in the sample chamber are struck by the electron beam, the gas is ionized. These positive ions are attracted to and neutralize the negative charge building up on the nonconductive specimens. By controlling the pressure in the sample chamber, the number of gas molecules intercepting the electron beam is maintained at a level that is sufficient to prevent charging, but does not deflect the beam sufficiently to prevent imaging and microanalysis.
At the higher pressures, accelerating voltages up to the maximum capacity of the SEM (typically 30 KV for high-performance SEMs) can be used for imaging and microanalysis of nonconductive and wet samples. No pretreatment, which could interfere with imaging and analysis, is required.
Imaging with a scanning electron microscope is done by either secondary electrons or backscattered electrons emitted from the sample. The gas molecules in the sample chamber inhibit detection of the secondary electrons, so the more energetic backscattered electrons are typically used for imaging during high-pressure operation.
The first scanning electron microscope developed for operation at higher pressure was the ESEM by Electroscan. This microscope was initially offered with only a backscattered electron detector, but now is available with a secondary electron detector that can be used at higher pressures. This instrument is capable of operating at relatively high pressures, up to about 20 torr. It is also set up for operation under a broad range of conditions that make it extremely valuable as a research instrument. The ESEM is quite expensive, however, compared to traditional scanning electron microscopes.
Recently, higher pressure operating capabilities have become available on moderately-priced, easy-to-operate, high-performance SEMs. The best of these microscopes are excellent conventional high-vacuum SEMs, with high-resolution secondary electron imaging, as well as having the capability to operate at variable pressure. These SEMs have all of the features that provide the versatility, usability, and cost-effectiveness on which so many SEM users have come to depend. In addition, there is no adaptation necessary to equip these instruments for microanalysis.
The new generation of variable-pressure scanning electron microscopes have brought the higher-pressure capability out of the research laboratories and into the quality assurance and failure analysis laboratories in manufacturing companies and service organizations. This development will continue the increase in demand for scanning electron microscopy in an ever widening range of applications.
The variety of applications studied in our laboratory using variable pressure SEM include: biological samples, cloth (Figure 2), polymer films, plastic components (Figure 3), printed-circuit boards (Figure 4), ceramics, painted and coated metals, explanted medical devices with attached tissue, metal components encapsulated in polymer, lubricated metals and polymers, and corroded, contaminated parts. In many of these cases, evaporative or sputter coating was either undesirable or not possible. These examples show the value of eliminating sample pretreatment and the utility for performing scanning electron microscopy at variable pressures.