The method of maintaining two regions (chambers) at different gasous pressure while they communicate only via an aperture (orifice) and are evacuated by independent pumping means of different pumping speed. Additional regions may be cascaded to create extreme pressure differences between the first and last region (two- or multi-stage differential pumping). No pumping means is needed to apply to the first region which may be supplied with a gas from ambient pressure at a controlled rate to achieve a desired pressure level. In ESEM, the apertures are collimated along the electron optics column to allow the electron beam to travel from the gun region to the specimen region.
For a given mechanical system, a performance factor is generally defined as the ratio of the value of a parameter attained by the system over the value the same parameter should have under ideal conditions of design. Often, physical limits can be established by theoretical calculations but in practice, the implementation usually entails compromises due to objective construction limitations that cannot easily be overcome. However, often the latter limitations are also due to oversight, ignorance, or incompetence of an engineer. The amount of deviation due to such a human error expressed as a ratio deviation from an objectively achievable physical parameter is defined as "engineering crudeness factor".
It is the distance of the specimen from the pressure limiting aperture.
The fraction of electrons lost (scattered out) of the primary electron beam as it travels from the electron gun to the last pressure limiting aperture just prior to its entry in the specimen chamber of an ESEM.
The fraction of electrons in the primary electron beam that survives totally un-scattered at the point of the last pressure limiting aperture just prior to its entry in the specimen chamber of an ESEM.
The transfer of the electron beam from the high vacuum electron gun chamber to the high pressure specimen chamber of an ESEM
The fraction of electrons that is scattered out of the primary electron beam travelling to the specimen though a gas layer in the ESEM. The diameter of electron skirt is orders of magnitude greater than the diameter of the electron beam (probe) so that it adds only a constant d-c noise level that is subtracted from the useful signal of the probe by d-c electronic suppression.
A reasoned definition of an environmental scanning electron microscope (ESEM) has been provided in Foundations of Environmental Scanning Electron Microscopy, as a scanning electron microscope (SEM) which can operate at the usual vacuum level of a conventional SEM through to, at least, the pressure required to observe liquid distilled water, namely, 609 Pa of saturation water vapor pressure at 0 degrees Celsius temperature.
Environmental secondary detector (ESD) is a commercial version of the "ionization GDD" purported to detect secondary electrons emanating from the beam-specimen interaction
Gaseous detection device (GDD) is a generalized term for when the environmental gas in an ESEM is used for detection of various signals emanating from the beam-specimen interaction. It is subdivided to two main detection modes, namely, the ionization GDD and the scintillation GDD.
Gaseous secondary electron detector (GSED) is a commercial version of the "ionization GDD" purported to detect "pure" secondary electrons emanating from the beam-specimen interaction
Ionization GDD denotes the use of the environmental gas in an ESEM as a detector by way of the ionization created by the signals emanating from the beam-specimen interactions. This is subdivided in secondary electron and backscattered electron detection by appropriate electrode configuration and electrode bias. It can be distinguished further by low or high electrode bias.
The total mass per unit area contained in a column of a gaseous layer. The gaseous layer may be homogeneous or with variable density along the column height.
The number density thickness divided by the stagnation number density of the gas divided by the diameter of the pressure limiting aperture in a ESEM. It is a dimensionless number.
It is the normalised particle thickness of the gaseous column downstream and aft the throat of the pressure limiting aperture in an ESEM. It is a dimensionless characteristic of the aperture for a given stagnation pressure of a gas flowing through the aperture in a differential pumping system.
The gaseous pressure-distance regime in which the primary electron beam travelling through a gas layer to the specimen surface retains a sufficient fraction of un-scattered electrons in the original spot to form an image in the usual manner as in a scanning electron microscope.
The specifications of an ESEM with zero "engineering crudeness factor", i.e. when the instrument is free from artificial defects and runs at the physical limits of operation in an optimum way.
The total number of particles (atoms or molecules) per unit area contained in a column of a gaseous layer. The gaseous layer may be homogeneous or with variable particle number density along the column height.
Any of the apertures (orifices) used in differential pumping
Scintillation GDD denotes the use of the environmental gas in an ESEM as a detector by way of the scintillation created by the signals emanating from the beam-specimen interactions. This is subdivided in secondary electron and backscattered electron detection by appropriate light pipes and electrode configuration and electrode bias. It can be distinguished further by low or high electrode bias.
It is the thickness of a gas layer at specimen chamber pressure (stagnation pressure) equivalent to the total gaseous beam path length (GBPL).
It is the distance of the specimen from the bottom of the objective lens pole-piece.