Specimen Chamber Pressure Research Articles

Observation of wet specimens sensitive to evaporation using scanning electron microscopy.

Wet specimens are notoriously difficult to image in scanning electron microscopes (SEM) owing to evaporation from the required vacuum of the specimen chamber. Traditionally, this issue has been addressed by increasing the specimen chamber pressure. Unfortunately, observation under high specimen chamber pressure cannot prevent the initial evaporation effects. The wet cover method, where the original surface water is retained (and, therefore, considered wet), provides a way to introduce and subsequently image specimens that are sensitive to evaporation within a SEM, while preventing evaporation-related damage, and to observe interesting specimen-water interactions.

ESEM modifications to LEO SUPRA 35 VP FESEM

A LEO SUPRA 35VP FESEM has been modified to significantly improve image quality in the low pressure mode of operation. It is further shown that the same conversion can allow the machine to operate also as a fully fledged environmental scanning electron microscope with specimen chamber pressures in excess of 2000Pa. This is achieved by a diamagnetic insert with a thin pressure limiting aperture at the bottom of the pole-piece. The insert allows the use of various size apertures in addition to the existing 1mm differential diaphragm.

Development of a Field Emission VP-SEM and Some Initial Applications

The use of variable pressure SEMs (VP-SEMs) is increasing in various fields of science and industry, allowing microscopy in a variable pressure environment of 1 ∼ 270 Pa utilizing backscattered electrons for imaging. The VP-SEM allows microscopy of insulated samples without the need for sample preparation. Charging artifacts can be minimized as well. When the VP-SEM is operated with a cooling stage, which allows cooling of samples at −20° and above, vaporization of water from samples is reduced. This permits microscopy of wet samples at close to the natural state for extended periods of time.Poor S/N ratio and deterioration of resolution, both of which are due to collisions among residual gas molecules and primary/backscattered electrons, have limited the performance of VP-SEMs. For resolving these limitations, we have completed the development of a new field emission VP-SEM which operates with a stable Schottky field emission source, a new environmental secondary electron detector (ESED), and a multi-stage differential pumping system. Fig. 1 shows a sectional view of the column with the differential pumping system. This design allows stable gun vacuum conditions with variable specimen chamber pressure 10 through 3,000 Pa, permitting a pressure difference from the gun by 1011 Pa without problems.

Space Charge Artifacts in ESEM Images: Shadowing and Contrast Reversal

Abstract The environmental scanning electron microscope (ESEM) employs a series of pressure limiting apertures and a differential pumping system to allow for electron imaging at specimen chamber pressures of up to 50 torr. Images rich in secondary electron (SE) contrast can be obtained using the gaseous secondary electron detector (GSED) or ion current (Iion) signals. The GSED and Iion signals are amplified in a gas cascade. SEs emitted from a sample are accelerated through the gas in the specimen chamber by an electric field, EGSED, produced by a positively biased electrode located in the chamber, above the specimen. The accelerated SEs give rise to a cascade ionization process that can amplify the SE signal by up to three orders of magnitude. Electrons produced in the cascade are rapidly swept to the biased electrode and are efficiently removed from the gas. Positive ions produced in the cascade drift away from the electrode with a velocity that is at least three orders of magnitude lower than that of the electrons.

Effect of humidity on crack healing in glass from in-situ investigations using an ESEM

In-situ healing of Vickers indentation cracks in soda-lime-silicate glass was observed using an environmental scanning electron microscope (ESEM). The change in crack length as a function of time and temperature was studied. The effect of the initial humidity level on the crack healing behavior was also investigated in-situ by varying the initial temperature of the specimen and the ESEM specimen chamber pressure. The crack healing results were fit to an equation previously used to describe thermal and mechanical fatigue crack behavior in ceramics.

Low-voltage backscattered electron imaging of non-coated biological samples in a low-vacuum environment using a variable-pressure scanning electron microscope with a YAG-detector.

A variable-pressure scanning electron microscope (VP-SEM) equipped with a high-sensitive backscattered electron (BSE) detector of the YAG type was applied to studies of biological tissue samples. The rat kidney and trachea were fixed, dehydrated in ethanol, critical point-dried and examined in the VP-SEM under a specimen chamber pressure of 1 to 150 Pa. The high-resolution surface images of the non-coated specimens were obtained in a low-vacuum (1-20 Pa) environment at an accelerating voltage of 5 kV, while the images at 10-20 kV contained information beneath the surface of the specimens. The observation in the VP-SEM equipped with YAG detector in a low-vacuum (1-20 Pa) environment at low (3-5 kV) accelerating voltages is useful for the three-dimensional analysis of the surface morphology of biological non-coated samples.

Biological applications of ESEM: Examination of fully hydrated samples

The environmental scanning electron microscope (ESEM) utilises a secondary electron detector capable of operating at pressures in the range of 1 ~ 20 Torr (130 ~ 2700 Pascals). By use of differential pumping apertures the electron gun and column can be maintained at "normal" vacuum levels of 10-5 Torr. A pressure of 10-2 Torr exists within the secondary electron detector separated from the specimen chamber by another differential pumping aperture. The specimen chamber pressure is produced by admitting a gas to the chamber under operator control. The gas can be any ionisable gas. Water vapour is a commonly used chamber gas. By controlling chamber gas pressure and sample temperature, by means of a Peltier effect stage, condensation and evaporation can be produced. The ability to maintain samples in a fully hydrated state in this way singles out the Electroscan ® ESEM as a unique instrument. ESEM presents an opportunity to examine biological samples with their natural water content present.

Detector strategies for environmental scanning electron microscopy

Environmental SEM operate at specimen chamber pressures of ∼20 torr (2.7 kPa) allowing stabilization of liquid water at room temperature, working on rugged insulators, and generation of an environmental secondary electron (ESE) signal. All signals available in conventional high vacuum instruments are also utilized in the environmental SEM, including BSE, SE, absorbed current, CL, and X-ray. In addition, the ESEM allows utilization of the flux of charge carriers as information, providing exciting new signal modes not available to BSE imaging or to conventional high vacuum SEM.In the ESEM, at low vacuum, SE electrons are collected with a “gaseous detector”. This detector collects low energy electrons (and ions) with biased wires or plates similar to those used in early high vacuum SEM for SE detection. The detector electrode can be integrated into the first PLA or positioned at any other place resulting in a versatile system that provides a variety of surface information.

Observations of wet cement using electron microscopy

Advances in scanning electron microscopy have made possible observations of wet specimens at high magnifications. New techniques using the environmental scanning electron microscope allow the study of cement-based materials at critical early stages of the hydration process with no disturbance to the material and its developing microstructure. The microscope provides specimen chamber pressures sufficiently high for water to remain on surfaces and in the pore structure. A cooling stage facilities observation of wet specimens by lowering the temperature of the specimen and therefore the partial pressure necessary for liquid water. A microinjector mechanism permits water to be added during imaging of the specimen. Dissolution and precipitation processes in cement paste are observed uninterrupted and recorded on videotape. The advanced capabilities have contributed new insight into microstructural development in cement paste.

Design and construction of an environmental SEM (part 4)

AbstractA new detection configuration using the gaseous detector device in the environmental scanning electron microscope (ESEM) is demonstrated. First, the pressure‐limiting aperture (PLA1) is used simultaneously as a biased electrode to collect the ionization current produced in the gaseous environment of the microscope. Second, a wire electrode is placed above the pressure‐limiting aperture, and it is shown that enough signal from the specimen escapes through the aperture to produce satisfactory images. These detection configurations allow the use of high specimen chamber pressures.

Design and construction of an atmospheric or environmental SEM—2

This paper is a continuation of a series of reports on the design and construction of an atmospheric or environmental SEM. The work described is an extended study of the gas jet developed above the pressure limiting aperture (Bell, 1974; Lacaze et al., 1977). Experiments specifically aimed to establish how the vacuum in the electron optics system was affected by the relative positioning of the objective and pressure limited aperture, as well as the pumping speeds employed, specimen chamber pressure, geometry and size of apertures, and by other means. Further, the nfluence of the jet deflectors, to control the effects of this jet on the microscope system were studied quantitatively using a specifically designed apparatus. In addition, the study of the pressure gradients below the pressure limiting aperture revealed that specimens can be placed as close as radius from the aperture and still experience an almost saturated vapour pressure environment. The results of the present study are currently being used in the design of an optimum detection configuration. A preliminary result has allowed the use of 140 μm pressure limiting aperture to observe specimens at atmospheric pressures as well as the use of low accelerating voltages (e.g. 7 kV) at TV scanning rates to record on video cassette dynamic phenomena, including wetting or recrystallizing salt solutions, etc.

Dynamic hydration studies in an SEM

AbstractA Cambridge Mark II Stereoscan SEM equipped with TV scan capabilities has been modified to study both static and dynamic interactions of water with water sensitive materials. The design and operation of this instrument to permit it to be used at 10°C temperature and 6.5 mbar specimen chamber pressure are included.A study of the continuous interaction of water with a natural sponge is included to demonstrate the usefulness of the dynamic capability of this technology. A time‐lapse study of this hydration process shows how a sponge absorbs water. Micrographs of water freezing are also included. This instrument, which has a resolution of about 150 nm, can also be used for static studies of hydrated materials. We anticipate that this technology will be used to study many other dynamic and static liquid‐solid and liquid‐liquid interactions such as emulsions, detergency processes, crystallization, solubilization, and perhaps even living biological specimens.

Principles of scanning electron microscopy at high specimen chamber pressures

Principles of scanning electron microscopy at high specimen chamber pressures

A simple technique for examining frozen hydrated specimens in the scanning electron microscope.

The use of a wide angle backscattered electron detector in a scanning electron microscope, which has the capability of the specimen chamber pressure being controlled independently of the column pressure, provides a simple technique for examining frozen hydrated specimens. Large specimens have been examined within 1 min of being placed on the stub and have been examined for many hours without charging artefacts or distortion due to dehydration.

Improvements in an Electron Microscope for Clean, Quiet, Vibrationless Operation

A new vacuum system has been designed and constructed for a Siemens Elmiskop I electron microscope in order to provide a cleaner environment about the specimen during normal use of the instrument, and to prepare the instrument for the addition of an ultrahigh vacuum specimen chamber for in situ film growth studies. Liquid nitrogen sorption pumps are used on the upper and lower parts of the microscope column, and an Orb-Ion pump is used to pump differentially on the large Siemens specimen chamber (with airlock). An impedance tube at the top and an impedance aperture at the bottom of the specimen chamber restrict the gas flow rate into the chamber to 7.04×10−3 liter/sec. Under operating conditions the pressure in the upper and lower parts of the microscope is <2×10−5 Torr, the (large Siemens) specimen chamber pressure is <2×10−6 Torr, the hydrocarbon contamination rate is ∼0.7 Å/min, the specimen changing time is 2 min, and the plate changing time is 4 min. The corresponding figures in the original unmodified microscope are ∼2×10−4 Torr, ∼2×10−4 Torr,∼ 500 Å/min, 1 min, and 4 min. Further advantages are noiseless and vibrationless operation.

Residual Gas Reaction in the Electron Microscope: II The Design of a Gas-Control Chamber for Quantitative Observations

Quantitative studies of the phenomenon associated with reactions induced by the electron beam between specimens and gases present in the electron microscope require precise knowledge and control of the local environment experienced by the portion of the specimen in the electron beam. Because of outgassing phenomena, the environment at the irradiated portion of the specimen is very different from that in any place where gas pressures and compositions can be measured. We have found that differential pumping of the specimen chamber by a 4" Orb-Ion pump, following roughing by a zeolite sorption pump, can produce a specimen-chamber pressure 100- to 1000-fold less than that in the region below the objective lens.