Subject index  to publications by Danilatos et al. 

A

ablation, 50

absorbed current, 36, 37, 38, 39

acronym ASEM, 2, 3

acronym ESEM, 3

acronyms ESEM and ASEM, 23

amplification, 36, 52

amplification volume, 36

annular, 53

aperture conductance, 1, 6, 17, 28

applications, 4, 5, 7, 9, 15, 16, 18, 27, 28, 46, 49

argon, 57

artifacts, 51

ASEM, 2, 3, 6, 8, 9, 13, 17, 34

atmospheric SEM, 2, 3

atomic number contrast, 17, 20, 21

Auger electrons, 28

avalanche, 36, 38

B

Bacillus apiarius, 16

beam diameter, 28

beam loss, 56, 54, 57

beam profiles, 28

beam spread, 28

beam transfer, 28, 45, 56, 57

beam-gas interactions, 28

beam-signal interactions, 28

beam-specimen interactions, 28

biological ESEM, 7, 16

botanical specimens, 5

bright halo, 9

BSE, 1, 2, 6, 13, 17, 28, 50, 51

BSE collection angle, 48

BSE detection with GDD, 12

BSE detector (thin), 9

BSE detector (wedge), 9

BSE in colour, 20, 21

bullet, 48, 54

C

cathodoluminescence, 26, 28, 45

charge density, 38

charge distribution, 38

charge neutralization, 45

charging, 28

charging, 51

chemical fixation, 51

chemical treatments, 18

chemistry in ESEM, 21, 31

CL, 45, 50, 51

clearance, 48

collection angle, 17

colour imaging, 20, 21

commercial ESEM, 25, 54, 56

contact angle, 18

contamination, 28

contest inversion, 51

continuum flow (viscous effusion), 28

contrast, 28, 30, 54

contrast and resolution, 51

control specimen, 44

cosine BSE distribution, 38

counters, 36

critical issues, 45

Critical Measurement, 46

critical point drying, 51

critical review, 9

critique, 45

cross-section, 28, 38

cylindrical geometry, 36

D

damage, 28, 51

definition of ESEM, 28

deformation, 51

density, 41

description of ESEM, 51

design & construction of ESEM & ASEM, 6, 13, 17, 34

detection, 52

detection volume, 36

detector efficiency, 45

development of ESEM, 49

differential pumping, 1, 6, 13, 17, 28, 48, 53, 56, 57

diffusion, 36

direct simulation Monte Carlo, 57

discharge, 36, 52

displacement current, 36

distribution, 28, 38

double aperture, 1

double BSE detector, 17

drift velocity, 36

DSMC, 53, 57

dual detectors, 17, 20, 21

E

elastic cross-section, 28, 38

electrode geometry, 36

electrodeless discharge, 52

electron attachment, 36

electron beam loss, 54, 56, 57

electron beam transfer, 51

electron diffusion, 36, 45

electron distribution, 28, 29

electron mobility, 36

electron optics column, 48

electron probe, 28, 38

electron signal, 38

electron scattering, 56 57

electron skirt, 28, 38, 56, 57

electron temperature, 36

electronic devices, 46

electrons, 36

ElectroScan, 40, 49

ElectroScan BSED, 48

ElectroScan ESEM, 32, 48, 49

electrostatic Pinch effect, 28

energy resolution, 36

enhanced detection, 52

equations of charge distribution, 38

ESD, 48

ESEM, 3

ESEM in colour, 21

ESEM operation, 28

E-T detector, 45

F

Faraday cup, 28

fast, 36

fast electrons, 36, 38

fibre extension, 18

field distortion, 1, 9

field emission, 46

field of view, 1, 6, 13

flow field, 41, 43, 45, 55

flow properties, 41, 43, 45, 55

freeze substitution, 51

frequency response, 36

fresh materials, 7

G

gain, 36, 38

gas, 36

gas dynamics, 8, 9, 13, 28, 41, 43, 45, 51, 55

gas equations, 28

gas flow, 28, 53, 55, 57

gas jet, 41, 43, 45, 56, 57

gaseous detection device, 22, 26, 28, 52

gaseous detector device, 11, 12

gaseous ionization, 52

gaseous reactions, 28

gaseous scintillation, 26, 28, 44, 52

gas-specimen interactions, 28

Gaussian distribution, 28, 38

GDD, 12, 22, 26, 34, 36,52

GDD - low bias, 12

GDD above PLA, 34

GDD in colour, 21

Geiger-Muller, 36

generalized GDD, 26

gluteraldehyde, 51

gold coating, 51

grease, 51

greasy wool, 18

Grevillea, 51

grids, 45

gross gain, 38

Gypsophilia paniculata, 51

H

halo, 1

helium, 57

high pressure, 1, 2, 3, 6, 13, 17, 28, 34

hydrogen, 57

hydrolysis, 51

I

image noise, 56, 57

imaging parameters, 36

induction, 36, 37, r3838, 39

inelastic cross-section, 28, 38

inspection, 46

integration, 25, 34

integration ESD/BSE, 32

interaction volume in gas, 28

ion concentration, 28

ion mobility, 36

ion signal, 38

ion temperature, 36

ionization, 12, 28, 36

ionization energy, 36

ions, 36, 52

irradiation, 51

J

jet, 13, 28, 53

jet deflectors, 13

jet length, 13

L

leak rate, 1, 6, 13, 17, 28, 55

Leptospermum flavescens, 9

light, 36

light pipe, 17, 48

light transmittance, 17

line width measurement, 46

liquid flow, 15, 27

live ants, 9

live plant in atmosphere, 8, 9

live specimens, 5, 7, 8, 9, 28

low bias GDD, 51

low keV, 8, 9, 46

low vacuum SEM, 45

low voltage SEM, 45

M

Macadamia integrifolia, 51

Mach disk, 50, 51

mass spectrometer, 47, 50

mass thickness, 45, 55

mean free path, 28

micro-injector, 15, 27, 51

mobility, 36

molecular beam, 53

molecular flow, 28, 55

Monte Carlo, 41, 43, 45, 53, 55

multi-electrode GDD, 22, 36, 45

multiple scattering, 28

multiple-backscattered electrons (MBSE), 6, 8, 9

multiple-detectors, 47

N

Nikon Corporation, 46

nitrogen, 57

neon, 57

noise,45, 56, 57

noise propagation, 45

O

object, 36

objective aperture, 1, 6, 13

oligo-scattering, 28, 38

open SEM, 2, 3

operational range, 50

optimal ESEM, 51, 56

optimum BSE, 17

optimum clearance, 17

optimum ESEM, 54

optimum operation, 38

orifice, 53, 55, 56

orifice properties, 57

osmium, 51

outline, 9, 10, 18, 32, 33, 40

oxygen, 57

P

parallel electrodes, 37

parallel plates, 36, 38

parameters, 50, 51

Paschen, 52

Paschen law, 36

performance, 54, 56

petroleum, 51

photons, 52

PLA, 1, 6, 13, 17, 28, 41, 43, 45, 53, 56, 56, 57

PLA tilt, 8, 9

PLA-detector, 34

plane electrodes, 36

plant material, 5

plural scattering, 28

pollen, 51

potato, 51

pressure, 41, 43, 45

pressure characteristics, 1, 6, 13, 17, 28

pressure gradients, 41, 43, 45

pressure limiting aperture, 53, 55, 56, 57

pressure stages, 48

primary processes, 36

probe, 28, 36, 38

profile, 29

proportional counters, 36

pump, 53

pumping manifold, 48

R

radiation effects, 9, 18, 24, 28

radiofrequency, 52

radiofrequency gaseous detection device, 52

rat tissue, 9

rays, 36

recombination, 36

reduced variables, 28

resolution, 28, 29, 30, 54

resolution micrograph, 32

resolving power, 28

retarding field, 38

reverse flow, 53

reverse flow pressure limiting aperture, 53

review, 9, 10, 18, 32, 33, 40, 45, 50, 51

review (brief), 10

review of ESEM, 49

RF-GDD, 52

RF-PLA, 53

room temperature, 1, 6, 13

S

salts, 28

scattering, 28, 29, 38, 54

scattering cross-sections, 28, 57

scintillation, 28

scintillation GDD, 36, 44

scouring of wool, 18

SE, 44, 50, 51

SE detection with GDD, 12

SE detector, 45

secondary electrons, 28, 44

secondary processes, 36

sharp PLA, 43, 45, 56

signal separation, 37

signal-gas interactions, 28

signal-to-noise-ratio, 28, 30, 45

simulation, 53, 55

single scattering, 28, 38

skin, 51

skirt, 28, 29, 38, 50, 51

skirt radius, 28, 38

slow, 36

slow electrons, 36, 38

smectite, 51

SNR, 28, 45

solid scintillating BSED, 45

solid state detectors, 23

sparking field, 52

specifications of ESEM, 25

specimen current, 36, 37, 38, 39

specimen preparation, 51

specimen-signal interactions, 28

spectrometry, 50

spectroscopy, 36

speed, 41

spores, 16

spot width, 28

statistics, 36

Stoletow, 38

supersonic, 53

surface charge accumulation, 45

surface chemistry, 31

survey of ESEM, 49

T

temperature, 41

terminology, 23, 28, 36, 38

terms, 36

testing, 46

theory of GDD, 36

time response, 36

tissues, 7

topography, 17

Townsend coefficient, 36

Townsend factors, 36

transition, 50, 51

transition region, 45

TV imaging, 8, 9, 51

TV scan rate, 8, 9, 51

U

uniform BSE distribution, 38

uniform field, 38

universal ESEM, 25, 45, 47, 50

useful gain, 38

uses of ESEM, 49

W

wafers, 46

walls, 36

water condensation, 1

water-clay reactions, 51

water vapour, 57

wedge-shape BSE detector, 8, 9

wet specimen stability, 1

wetting studies, 18

wool, 1, 4, 6, 9, 18, 24, 28, 51

working distance, 48

working pressure loss, 54

X

x-rays, 28, 45, 50, 51

Y

YAG, 45

YAP, 45

Z

Z-contrast, 17, 20, 21

Abstract: First systematic study of vacuum and pressure characteristics of JEOL JSM-2 SEM. The use of a single PLA coinciding with objective aperture does not allow short clearance, because of image distortion and bright halo from the aperture. A double aperture solved the problem. This allowed pressures up to 68.5 mbar with 20 keV with a modified scintillating BSE detector: The small diameter hole in the detector was used for detection at the short working distance as opposed to the "hemispherical or wide angle" geometry used by Robinson. Wet wool fibers under very stable environmental conditions could be examined at room temperature.

Abstract: A new detection configuration for the SEM has been devised, which allows the imaging of the surface of a specimen in the open room, i.e., at atmospheric pressure. Such a device gives rise to a new microscope: the Atmospheric Scanning Electron Microscope (ASEM). In this configuration, a backscattered electron detector is placed between the pressure limiting aperture and the electron column. The electron beam passes through the final aperture, reaches the sample in the open room and the backscattered electrons passing through the same final aperture react the detector. This principle has been tested and the result reported. The size of aperture used was 22 mm. The acronym ASEM has been introduced for first time.

Abstract: Further advances on the development of ASEM and ESEM are reported. The acronym ESEM has been introduced for the first time. These two terms correspond to two different detection configurations, both of which allow the examination of specimens at any pressure up to one atmosphere. In ASEM the detector is placed above the PLA, whereas in ESEM the detector is placed below or is contiguous or integral with PLA. First time two-stage differential pumping is introduced in the SEM, via two apertures and an additional rotary pump. The PLA diameter for ASEM was increased to 30 mm. Scintillating BSE detectors are used. The concept of a portable electron optics column to examine specimens at room conditions is introduced. Specimens were no more limited by size, portability or nature: The idea is to take the microscope to the specimen and examine its natural surface.

Abstract: An overview of ESEM with particular emphasis on its application to studies of wool fibers.

Abstract: An environmental scanning electron microscope (ESEM) has been developed which allows the examinations of live and wet plant specimens. The results are compared with those obtained using similar material that has been dehydrated or prepared by conventional techniques. The plant materials can survive the hypobaric pressure and beam irradiation, especially if the latter is carefully controlled.

Abstract: This is the first of a series of reports on the design and construction of an atmospheric or environmental SEM. The introduction of better vacuum pumping between the objective and pressure limiting aperture (PLA) has allowed the use of relatively large pressure limiting apertures, i.e. up to 57 mm for operation at atmospheric pressure or up to 400 mm for operation at saturation water vapour pressure and room temperature. The imaging obtained has been considerably improved by these developments and high quality BSE images of specimens at saturation water vapour pressure and at one atmosphere, at room temperature are presented. The first part of experimentation and analysis on the vacuum characteristics of the new system together with different detection configuration is also presented. An integrated detector/PLA system is proposed. By a multiple-backscattered electron (MBSE) imaging mode, it is shown that the surface of a specimen can be imaged although the BSE detector is placed below an (opaque) specimen.

Abstract: Fresh, fixed and critical point dried specimens of rat trachea, stomach and skin were examined with ESEM. Results showed that in tissues with fine surface detail such as trachea and stomach there was some loss of contrast and resolution in the image when they were viewed in the fresh state. In a 'harder' tissue such as skin, quite clear images could be obtained. A comparison of fresh and critical point dried skin showed some distortion in the form of increased desquamation, and shrinkage in the critical point dried tissue. At present, the advantages on this technique seem to lie in the fact that some features of biological tissues that may be masked by processing could be revealed. Living tissue can be examined. It is time saving as tissues do not need to be fixed, dehydrated and critical point dried.

Abstract: Advances in ASEM and ESEM are presented. The PLA is tilted to avoid the effects of a gas jet forming above PLA. A wedge shaped BSE detector is used. The multiple-backscattered electrons contribute to the image. Low (7) keV at TV scan rate is used to image salt crystal formation. Wool and live plant specimen is imaged at one atmosphere with 15 keV. The PLA size of ASEM has been dramatically increased to 140 m m for operation at one atmosphere pressure.

Abstract: A critical review of ESEM and its applications to date is presented. Wool fibers subjected to various treatments, wet (fresh) rat tissues, crystallisation and rewetting of salts, and some radiation effects have been examined. A wedge shaped BSE detector together with a tilted aperture allowed the use of relatively low (7 keV) at TV scan rate.

Abstract: Brief review of ESEM is presented.

Abstract: First disclosure of gaseous detector device (GDD). This is a novel method in electron microscopy, whereby the gaseous ionization produced by the signal-gas interactions is used for imaging.

Abstract: This is the first paper announcing the gaseous detector device, by which the ionization produced in the gas by the signal-gas interactions is used for imaging. Ionizing radiations such as BSE and SE electrons produce positive ions and free electrons in the gas. These charge carriers can be collected by electrodes placed in various positions in the specimen chamber. The contrast varies with gas pressure, electrode positioning and electrode or specimen bias level and polarity. The variation of ionization current measured with a Faraday cup and with a ring electrode was measured. An inversion of contrast corresponds to a "cross-over" point of the ionization current collected by the Faraday cup as we raise the pressure. Various contrast phenomena are recorded.

Abstract: This is a continuation of reports on the design and construction of an ESEM and ASEM. It presents a thorough experimental investigation of the gas dynamics in the system. Experiments specifically aimed to establish how the vacuum in the electron optics system was affected by the relative positioning of the objective and pressure limiting aperture, as well as the pumping speeds employed, specimen chamber pressure, geometry and size of apertures, and by other means. Further, the influence 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 one 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 mm 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 video cassette dynamic phenomena, including wetting or recrystalizing salt solutions, etc.

Abstract: Short review of gaseous detection

Abstract: Some applications of ESEM required the invention of a device for the injection of microdroplets of liquid onto the specimen in situ. The practical problem of transferring liquid from ambient pressure into the hypobaric environment has been solved by the following system: A small cavity behind a capillary needle in the specimen chamber is filled or emptied with a liquid by means of two tubes leading outside the microscope. The internal diameter of the needle (20 mm) was chosen so as to conduct sufficient liquid when externally applied under pressure, whilst at other times allowing a negligible air leak. The needle can be moved in three dimensions with controls independent from the microscope stage. To accommodate the moving needle and specimen within 1 mm from the pressure limiting aperture a BSE detector integrated with PLA was constructed. This arrangement has been used to obtain video recordings of various systems.

Abstract: Bacillus apiarius spires have been examined at relative humidities between 99% and 12% in an ESEM. The spores have also been studied both wet and dry with an interference microscope. Their shape remains rectangular whether wet or dry. A calculation of the effect of an osmotic pressure change of about 200 atm upon the maximum deflection of the longest side of the spire shows that the deflection is less than 4.5 nm. The shape of the spore therefore is not markedly affected by a change from dry to wet and the shape will remain as it was when the coat was initially formed, unless the coat is weakened by some chemical attack. The refractive index of the material is 1.532-1.536.

Abstract: The continuation of the advances in the design and construction of ESEM and ASEM is reported. A pair of scintillator backscattered electron detectors have been designed and made so that signal processing to enhance topography and Z-contrast can be performed. Optimum shapes and positioning of the detectors have been determined. The light transmittance in the light pipe has been mapped and measured to be 51%. Satisfactory signal-to-noise-ratio images of a carbon specimen have been obtained with 5 kV, 2 pA and 50 s scan time at 0.1 mbar. With a higher intensity beam, clear images of wool fibres have been achieved at one atmosphere. The ASEM and ESEM have been shown to operate both under conditions of a gaseous environment and with detection modes used in other microscopes. Changes and phenomena occurring as the relative humidity varies between 0 and 100% have been recorded. With ASEM, imaging was possible with a 150 mm PLA at 10 keV.

Abstract: The new technique of ESEM has been developed to a highly efficient operational state. It has been used to study greasy wool fibres treated and untreated, wet and dry, to study mechanical properties in situ and to observe liquid spreading on the fibre surface. An examination of the effect of the electron beam has been undertaken under various irradiation and environmental conditions.

Abstract: Short review

Abstract: The backscattered electron (BSE) signals detected by a pair of detectors in the ESEM can be used for producing colour micrographs. The image corresponding to any of these signals, or to a mixture of these signals, is assigned one primary colour, and two or more of these images are superimposed onto the same colour frame. In addition, the mixing of signals from the gaseous detector device together with their use for colour imaging is also examined.

Abstract: The environmental SEM has been developed to a stage where colour can be introduced during imaging. A new approach to production of colour micrographs is demonstrated together with some typical uses of the environmental mode of SEM. A combination of the outputs of two backscattered electron (BSE) detectors alone or in combination with the outputs from a gaseous detector device can form images corresponding to particular aspects of a given specimen. Two or more of these images can be superimposed on to the same colour print to produce a colour micrograph.

Abstract: Two wires are used for the gaseous detection device (GDD). Directionality contrast is produced. By adding the outputs from GDD, atomic number contrast is produced equivalent to that obtained with a pair of scintillating BSE detectors. By subtracting the outputs, topography contrast is produced. By inverting the electrode bias, the contrast is inverted.

Abstract: Brief review of ESEM. Proposal to unify the terms of ASEM and ESEM to one only, namely, ESEM. Design of integrated solid state detectors for the general ESEM.

Abstract: Various beam irradiation effects on wool fibers are reported. The type and amount of beam effects depend on (a) the electron beam: accelerating voltage, current intensity, scanning mode (raster or other, line density, magnification), (b) the environment: nature, pressure and temperature of gas and (c) the specimen: composition, structure, texture and orientation.

Abstract: A summary of ESEM specifications is presented with a view to designing a commercial ESEM. The concept of integrating various fundamental components is introduced: For a commercial instrument we should integrate (a) objective lens and scanning coils with (b) differential pumping and with (c) detection systems, all in a new design. The concept of a universal ESEM is introduced, whereby ESEM can perform both as a conventional SEM and as an ESEM.

Abstract: A novel detection means for the environmental SEM (ESEM) is described. Certain gases, apart from being the environmental conditioning medium, can also act as a scintillator detector. All signals, such as secondary and backscattered electrons, which can cause a particular gas to luminesce, can be detected. It is further concluded that the gas can act as a generalized detector device for all signal-gas reactions provided some suitable parameter could be monitored. New possibilities in the detection of specimen cathodoluminescence created by the ESEM are demonstrated.

Abstract: The micro-injector previously developed and reported has been used in various applications. It is possible to form a droplet standing at the tip of the needle of the micro-injector and move around the object under examination to make contact with the liquid. The subsequent wetting, absorption or reaction of the liquid can be viewed at TV scanning rates and a video recording can be made for further analysis. With this system, it has been possible to observe the wetting and removal of different components from the surface of greasy (raw) wool fibers. Apart from the specific wool applications, other industrial and scientific investigations can benefit. Video recordings of various liquids and liquid transport are shown. The flow and absorption of water by paper tissue at various "instants" is captured. The fast changes of configuration of liquids can be captured in real time for subsequent study. The ESEM equipped with this system has created new possibilities for surface physics and chemistry.

Abstract: A comprehensive survey on the fundamentals of ESEM is presented. A formal definition of ESEM is proposed. The state of gas in ESEM is given, namely, the basic equations frequently needed, an analysis of the gas flow, calculation of conductance and experimental assessment of gas flow. The general interactions in ESEM are outlined by way of pointing to all possible combinations of dual component systems. Above all, the main thrust of this survey concentrates around the fundamental question of electron beam scattering and distribution in gas. Until this time, there have been conflicting reports on whether the useful beam spot spreads or not. Applying complex mathematical formulas that describe the electron scattering and distribution in a plural scattering regime, a definitive answer was found for the first time: Fraction of electrons is removed from the original beam in vacuum and is redistributed in a very broad "skirt" surrounding the remaining intact fraction at the center. This result is further confirmed by careful experimentation. This finding is extremely significant, because it means that the resolving power of ESEM can be maintained in the presence of gas. The preservation of a core electron beam with the same distribution as in vacuum occurs over a finite beam travel distance at a given gas pressure. This regime is characterized by the condition that the average number of collisions per electron is less than three, and it has been termed "oligo-scattering" regime. In the course of this study, calculations of scattering cross-sections for atomic and molecular gases and profiles of skirts for a "point" beam and for a Gaussian distribution beam have been found. Furthermore, the beam-gas interaction volume, ionization of gases, ion concentration, and electrostatic effects are analysed. All detection modes, namely, BSE, SE, CL, x-ray and Auger electrons are discussed in detail. The multipurpose gaseous detection device is reviewed. Analytical equations of signal-to-noise-ratio and a thorough examination of contrast and resolution are undertaken. The beam radiation effects, namely, charging, contamination and damage are discussed. Some basic considerations for the operation and application of ESEM are outlined. This survey is an up-date of the state of the art in ESEM at present.

Abstract: This is a summary on the electron beam profiles and resolution based on a previous extended survey.

Abstract: This is a summary on contrast and resolution based on a previous survey.

Abstract: A brief review of ESEM with emphasis on chemistry in the system.

Abstract: A concise review of ESEM with early micrographs from the ElectroScan ESEM. Efficient scintillator design in conjunction with a sharp tip ESD. The fundamental aspects of ESEM are outlined in the four page extended abstract.

Abstract: A four-page exposition of the basic features of ESEM.

Abstract: A new detection configuration using the gaseous device in the environmental scanning electron microscope (ESEM) is demonstrated. First, the pressure-limiting aperture (PLA) 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, namely, well above 200 mbar. Above this pressure level, life is fully sustainable. This report presents one example of unifying the detection of signals both below and above the PLA1 by use of the gaseous detector device. This unification further confirms the need for unifying also the terms ASEM and ESEM into one, namely, ESEM, as proposed earlier.

Abstract: Review abstract.

Abstract: A comprehensive theoretical survey and analysis of the gaseous detector device is presented. It is established that the true and correct nature of signal generated on various electrodes is induction. As long as there are moving charges in the inter-electrode space, a signal current flows in the external circuit. The theory of induced signals in general and in the ESEM, in particular, is given. It is shown that the conventional notion of "specimen absorbed current" is misleading and can lead to erroneous results in ESEM. An image can be made even if no "absorbed" current by the specimen is present. The electrical conductivity of a specimen is responsible for the after-effects of charging and image distortion in the vacuum SEM. The magnitudes of imaging parameters in ESEM are calculated, and a realistic picture is conveyed. This survey includes a detailed collection of and analysis of various physical parameters such as electron and ion temperature, electron and ion mobilities, electron diffusion, recombination, electron attachment and effective ionization energy. To describe the new complex physical phenomena in electron microscopy, a new terminology is necessitated and proposed. The discharge characteristics with regard to electrode geometry, nature of gas, and electrode bias are explained. The amplification or gain characteristics of the GDD are thoroughly analyzed for various electrode geometries The limits and advantages are determined. Apart from the ionization GDD, the scintillation GDD is shown to have some unique advantages in performance. Considering the spectroscopy, statistics and energy resolution it is proposed that nuclear methods and instruments can be transferred and properly adapted to electron microscopy in general and, in particular, to environmental scanning transmission electron microscopy and to ESEM. Practical tips and construction details are gathered for efficient designs of GDD.

Abstract: For proper understanding of image formation using charge carriers, it is shown that signal detection by means of induction must be considered. This explains the possibility of imaging insulators as well as other phenomena especially in the conditions of the environmental scanning electron microscope. In addition, a basic principle and method to separate the secondary and backscattered electrons is demonstrated. A system of concentric electrodes placed either above or below the specimen is used, and BSE and SE are effectively separated. It is also shown the equivalence of signal detected by either of two parallel electrodes. Images of insulating specimens, or of conducting specimens separated from the detecting electrode via an insulating material can now be readily understood and explained.

Abstract: In the environmental scanning electron microscope (ESEM), the electron beam together with various signals emanating from the beam-specimen interaction ionize the gaseous medium in the specimen chamber. A detailed derivation of equations describing the charge density and current flow in the system is presented. It is shown that the various causes of ionization operate over distinct regions, which can be separated out by suitable electrode configuration. The electron probe retains a fraction of electrons with the original charge distribution; this is surrounded by a widespread electron skirt, which, in turn, is surrounded by charge created by the secondary electrons, beyond which extends the action of backscattered electrons.

Abstract: Further demonstration and review of the mechanism of detection by induction is presented. Salt crystals resting on a glass plate were imaged both by a flat electrode above the specimen and a flat electrode below the glass plate. The images were equivalent (the same) after inverting one of them. This proves that the conventional concept of "absorbed" specimen current, is both redundant and non-existent. Instead, detection by induction is the ever-present mechanism for all specimens, conducting and insulating.

Abstract: An up-date of ESEM is presented. Applications and imaging by use of the ElectroScan ESEM are included.

Abstract: The flow properties, namely, gas density, temperature and speed are presented for the case of a flat pressure limiting aperture (PLA). It is important to know the variation of these properties in the immediate neighborhood of the PLA, through which the electron beam passes and near which the specimen may be placed. The direct simulation Monte Carlo method was used. The gas jet forming through an aperture was actually imaged with the gaseous detection device.

Abstract: Short review of flow field properties.

Abstract: Further results from a study on gas dynamics in ESEM are presented. The flow field around a conical-sharp pressure limiting aperture is analyzed by the direct simulation Monte Carlo method. The variation of flow field properties with different specimen clearances is presented. One conclusion is that the specimen surface pressure is practically unaffected by the gas flow when the specimen is placed further away than one PLA diameter. The calculation of gas density gradients along the axis of the system is needed for the determination of mass density and electron scattering.

Abstract: An alternative way to detect secondary electrons in a gaseous environment is by use of the gaseous scintillation that accompanies an electron avalanche. Generally, apart from ionization we also have electron excitation as the electrons collide with gas molecules in the multiplication process. The gaseous scintillation can be detected with a suitable light pipe/PMT (photomultiplier) system. Results showing definite SE images are presented. It was found that the SE images could also be obtained at TV scanning rates, which shows that the GDD frequency response is very broad. This is consistent with short electron transit times previously calculated.

45 Environmental scanning electron microscope-some critical issues.
Danilatos GD, Scanning Microscopy International, Supplement 7, 1993, 57-80 (1993a).

Abstract: This is both a review and a survey into some critical issues of the environmental scanning electron microscope (ESEM). Some new concepts and designs are also presented. An attempt to unify various detection modes is made. In ESEM, the gas flow around the main pressure limiting aperture establishes a density gradient through which the electron beam passes. Electron beam losses occur in this transition region and in the uniform gas layer above the specimen surface. In the oligo-scattering regime, the electron distribution consists of a widely scattered fraction of electrons surrounding an intact focussed probe. The secondary electrons are multiplied by means of gaseous ionization and detected by both the ionization current and the accompanying gaseous scintillation. The distribution of secondary electrons is governed by the applied external electric and magnetic fields and by electron diffusion in the gas. The backscattered electrons are detected both by means of the gaseous detection device and by solid scintillating detectors. Uncoated solid detectors offer the lowest signal to noise ratio especially under low beam accelerating voltages. The lowest pressure of operation with uncoated detectors has been expanded by the deliberate introduction of a gaseous discharge near the detector. The gaseous scintillation also offers the possibility of low noise detection and signal discrimination. The "absorbed specimen current" mode is re-examined in the conditions of ESEM and it is found that the current flowing through the specimen is not the contrast forming mechanism: It is all the electric carriers in motion that induce signals on the surrounding electrodes. The electric conductivity of the specimen may affect the contrast only indirectly, i.e. as a secondary, not a primary process. The ESEM can operate under any environment including high and low pressure, low or rough vacuum and high vacuum; it also operates at both high and low beam accelerating voltage, so that it may be considered as the universal instrument for virtually any application previously accessible or not to the conventional SEM.

Abstract: This is a review with particular emphasis on the application of ESEM technology to the examination of electronic devices. Because of its universal capabilities, ESEM is ideally suited for inspection and testing, in general. In particular, microelectronic devices can now be studied faster, better and more reliably, or even in ways not previously feasible. For wafer pattern measurements, the high resolutions required need not be compromized by the use of a very low keV incident beam. It is much better to use rather a compromise voltage of, say, 3 keV to avoid specimen damage and yet to allow a high resolution. At the same time, specimen charging is avoided by maintaining a gaseous pressure around 100-200 Pa. A dedicated instrument with a field emission gun has been developed for linewidth measurements (Critical Dimension Measuring SEM, Nikon Corporation). Apart from inspection and testing, ESEM has the potential for a wide range of applications in microprocess industry and research, because the natural surface of a specimen can be placed directly under the beam. There seems only the need to control the amount and nature of gas for each particular application. In fact, the very presence of a controlled gaseous environment has opened many new possibilities for beam-specimen and gas-specimen interactions. Electron beam-induced chemical reactions and applications to direct writing and associated processes can be given a new impetus. Resist materials and processing techniques can also be studied in a new way. Both etching and carbonaceous depositions have been observed and it is envisaged that these processes can be used in a controlled way during imaging to monitor those processes and to put them to practical use. Electron lithography and microfabrication are yet to see the benefits of ESEM. Hot and cold stages can be incorporated in the microscope for studies of materials in situ at high, intermediate and low temperatures. Already, several studies on soldering techniques with this technology have appeared in the literature, and transition phases of materials from very low to very high temperatures can be readily monitored. The range of ESEM uses can only be underestimated at this early stage of development.

Abstract: A universal system of detectors can now be incorporated in ESEM so that the complete pressure range from high pressure to high vacuum can be used. The GDD is integrated with solid scintillating materials together with an optimized gas dynamics system. An array of electrodes (grids and apertures) serves in the detection, separation and control of various signals. They are combined with highly efficient scintillating materials and/or light pipes. This system should be incorporated at the lower part of an electron optics column. Thus, all main modes of detection can be represented. Secondary (SE) and backscattered (BSE) electron signals, cathodoluminescence (CL) and x-ray microanalysis can be practised at any pressure. In addition, a mass spectrometer can be interfaced for analysis of the gas flowing through PLA1. By directing the electron beam at the feature of interest, ESEM produces an ablation mass spectrometer with very high resolution and sensitivity, in an equivalent but much better system than that used with a laser beam.

Abstract: An outline is presented of the first commercial environmental scanning electron microscope (ESEM) made by ElectroScan Corporation. A concise description of this instrument and its operation, from a user's perspective, is given. More specifically, the description includes the electron optics, pressure stages and control, detection modes, resolution and ancillary equipment

Abstract: Two comprehensive and updated lists of publications on environmental scanning electron microscopy are compiled. One list contains mainly those papers dealing with the development and instrumentation, while the other deals mainly with the applications of the technique. A brief introductory summary of the field is presented.

Abstract: This is a review of ESEM whereby the main principles and instrument design considerations are unified in order to define the range of various operational parameter as we vary pressure. It is shown that the environmental scanning electron microscope is the natural extension of the scanning electron microscope. The former incorporates all of the conventional functions of the latter and, in addition, it opens many new ways of looking at virtually any specimen, wet or dry, insulating or conducting. The environmental scanning electron microscope is characterized by the possibility of maintaining a gaseous pressure in the specimen chamber. All operational parameters can be varied within a range, which is a function of pressure. It can be used with all types of gun and all basic modes of detection and, hence, it can be applied both to morphological and to microanalytical studies. It has opened many novel ways of looking at specimens and phenomena not previous accessible with scanning electron microscopy. A model for specimen charge distribution and dissipation is proposed. The interface of a mass spectrometer by sampling the gas flowing through the PLA is suggested; this would give rise to high resolution ablation mass spectrometry. An outline of present approaches to the problem of electron skirt in microanalysis is presented.

Abstract: This is a chapter from a book on in-situ microscopy where the state of the art is summarised with some new material and . Following early works on in-situ transmission electron microscopy by using environmental cells, the environmental scanning electron microscope (ESEM) has formed the counterpart for the examination of specimen surfaces in a gaseous environment at pressures up to one atmosphere. As accelerating voltages are relatively low in ESEM, it has been necessary to establish the optimum electron beam transfer conditions from a high vacuum to a high pressure region by using windowless apertures. Studies on the gas and electron dynamics of the system have determined that it is possible to use tungsten, LaB6 and field emission guns without compromising the useful probe size in the presence of gas. The backscattered electron, cathodoluminescence and x-ray detection modes are preserved with proper modification of the detectors. A new method for detection of the secondary and backscattered electron signal has been introduced by the use of the ionisation and scintillation of the environmental gas by corresponding signals. Further, the ionised gaseous environment substitutes the conventional conductive coating or treatment techniques necessary for insulators in vacuum SEM. The high pressure also allows a fully or partially moist environment for the examination of biological or wet specimens, or of chemical reactions in the gas/liquid/solid phases. The possibility of examining the natural or true surface of practically any specimen has added a new dimension to electron microscopy. New contrast mechanisms reveal information not previously possible to see. It has greatly facilitated the examination of specimens by eliminating or reducing the specimen preparation procedures and the specimen exchange time. Based on the success of an experimental ESEM, new commercial instruments are now available making this technology accessible to all. Published scientific literature demonstrates that ESEM has been applied to the most diverse disciplines. A future prospect is to integrate and jointly develop the scanning transmission electron microscope towards a universal kind of environmental EM..

Abstract: A radiofrequency gaseous detection device is proposed for use with instruments employing charged particle beams, such as electron microscopes and ion beam technologies, as well as for detection of ionising radiations as in proportional counters. An alternating (oscillating) electromagnetic field in the radiofrequency range is applied in a gaseous environment of the instrument. Both the frequency and amplitude of oscillation are adjustable. The electron or ion beam interacts with a specimen and releases free electrons in the gas. Similarly, an ionising radiation source releases free electrons in the gas. The free electrons are acted upon by the alternating electromagnetic field and undergo an oscillatory motion resulting in multiple collisions with the gas molecules, or atoms. At sufficiently low pressures, the oscillating electrons also collide with surrounding walls. These processes result in an amplified electron signal and an amplified photon signal in a controlled discharge. The amplified signals, which are proportional to the initial number of free electrons, are collected by suitable means for further processing and analysis.

Abstract: The reverse flow pressure limiting aperture is a device that creates and sustains a substantial gas pressure difference between two chambers connected via an aperture. The aperture is surrounded by an annular orifice leading to a third chamber. The third chamber is maintained at a relatively high pressure that forces gas to flow through the annular aperture into the first of said two chambers. The ensuing gas flow develops into a supersonic annular gas jet, the core of which is coaxial with the central aperture. A pumping action is created at the core of the jet and any gas molecules leaking through the aperture from the second chamber are entrained and forced into the first chamber, thus creating a substantial pressure difference between the first and second chamber.

Abstract: Conventional differential pumping is used on present day commercial ESEM instruments to separate the high vacuum of the electron optics column from the high pressure specimen chamber. A transition region of intermediate pressures is established between the two extreme regions of the microscope. The electron beam is transferred from the high vacuum to the high pressure via a transition region in which the beam undergoes collisions with the gas and suffers the first electron losses. The loss of electrons in the transition region is an inevitable consequence of the differential pumping method. However, the magnitude of electron loss is determined by the design of a given instrument and can be greater or equal to some physical limit imposed by the formation of a supersonic gas jet downstream of the pressure-limiting aperture.

The electron beam loss can be found from knowledge of the gas density variation along the axis of the system. The latter is difficult to find by experiment. Analytical formulations do not exist because the flow varies from free molecular to continuum condition. However, the direct simulation Monte Carlo method by some millions of simulated molecules in physical space has become practical with the advent of fast personal computers. While this method is time consuming, it provides unique possibilities for studies of gas flows of complicated geometry inaccessible by other methods.

The gas density variation is computed for two commercial ESEM instruments with different pressure limiting aperture holders (also known as "bullets"), where the transition region occurs. While a superficial observation of the two bullets might not reveal any substantial difference, a detailed measurement of the internal geometry, where the gas flow takes place, and subsequent gas flow computation reveal that the mass thickness can be significantly different in the two instruments.

The optimum (i.e. minimum) mass thickness of the transition region for an ideal system is also computed and is shown that a significant improvement can be effected in practice. The existing commercial instruments can be modified to a certain extent to decrease the electron losses, which will result in better contrast and resolution, and higher specimen chamber operation. Future designs can be made to approach the natural minimum limit of electron loss when the conventional differential pumping method is used. Alternatively, the novel method of the reverse flow pressure limiting aperture offers the advantage of practically eliminating the gas jet and the accompanying electron beam losses in it.

Abstract. A study of the flow properties of argon through an orifice has been performed with the direct simulation Monte Carlo method. The study covered the full extent of the transition regime between free-molecule and continuum flow, both in the upstream and downstream regions. Results for Mach number, number density, velocity and temperature are shown for three representative cases for a specified geometry with argon gas. The variation of molecule flow rate through the orifice and the variation of mass-thickness of the gas downstream of the orifice are given in the complete transition range. The molecule flow rates computed herewith show good agreement with previously published experimental measurements. The isentropic equations of a perfect gas are shown to reproduce the expected relationship between properties as a function of the computed Mach number in the continuum regime, but they clearly deviate elsewhere, as expected. The theoretical density function and flow rate agree well with the computed values in the free-molecule flow. However, the computed flow rate is less than the flow rate of a nozzle in the continuum flow. Empirical equations for both the characteristic speed and normalized number thickness have been derived and shown to predict well the values of other gases, for all practical purposes.

Abstract. The electron beam in an environmental scanning electron microscope can suffer significant losses even before it enters the specimen chamber of the instrument. These losses result from electron scattering in gaseous jet formed inside and downstream (above) of the pressure-limiting aperture (PLA), which separates the high pressure and high vacuum regions of the microscope. In this work, the electron beam loss above the PLA has been calculated for three different environmental scanning electron microscopes each with a different PLA geometry; namely an ElectroScan E3, a Philips XL30 ESEM and a prototype instrument. The mass-thickness of gas above the PLA in each case has been determined by computing the gas density variation in the gas jet using the direct simulation Monte Carlo method. It has been found that PLA configurations utilized in the commercial instruments produce significant electron beam loss that dramatically degrades their performance at high chamber pressure and low accelerating voltage conditions. Conversely, these detrimental effects are minimal with the prototype instrument operating with the optimum thin foil PLA design specification.

ESEM Home ||ESEM Research Laboratory||History ||Publications||Patents || Danilatos ||Commercial ESEM||Subject index||Public Images ||contact