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The beam current depends on the acceleration voltage, the spot size, the filament, and on the age of the filament. The emission current as a function of the spot size for three different acceleration voltages.
There have been great technical advances in microprobes and scanning electron microscopes during the last 30 years (mostly in automation and electronics), but all instruments are built around an electron column, which produces a stable electron beam, controls beam current, beam size and beam shape, and rasters the beam for SEM work.
The gas in a vacuum system can be in a viscous state, in a molecular state or in a state that is intermediate between these two.
Thus, MFP of gas molecules is very small at atmospheric pressure (6 x 10-6 cm) and the flow of the gas is limited by its viscosity. The pump-down sequences for a microprobe and scanning electron microscope are very similar.
Water vapor is a major problem in vacuum systems because it desorbs slowly from the internal surfaces of the sample chamber. There are two mechanical (or "sealed-oil") pumps on the Cameca microprobe and JEOL scanning electron microscope.
The outlet valve (F) and moving parts of the pump are bathed in oil that serves to both lubricate the moving parts of the pump but also to trap air molecules. The diffusion pump is used to produce and maintain low pressures in the electron column and the sample chamber (and in the Oxford Spectrometer on the SEM). Sputter ion pumps are a type of capture pump, wherein air molecules are removed from the chamber by plating (gettering) them onto a surface.
Ion pumps consist of short stainless steel cylinders (anodes) sandwiched between two metal (Ti, or Ti and Ta) plates (cathodes), all situated within a strong magnetic field aligned parallel to the cylinder axes. No single gauge can measure pressure from atmosphere to high vacuum, so different gauges are used to monitor the level of vacuum present within the electron column and spectrometers at different pressures. Electron guns provide electrons for an electron beam by allowing them to escape from a cathode material. Thermionic sources, in which electrons are produced by heating a conductive material to the point where the outer orbital electrons gain sufficient energy to overcome the work function barrier and escape. Thermal-field (TF) source, in which the tungsten point in a field emission source is heated, incorporating both thermionic and field emissions; this is also referred to as a "'Schottky cathode".
The electron guns used in NAU's Cameca Camebax MBX microprobe and JEOL-6480LV scanning electron microscope employ tungsten filaments and have a triode (three part) configuration consisting of a cathode, Wehnelt cap and anode. The tungsten filament is a thin (about 0.1 mm) wire bent into an "inverted V" to localize emission at the tip. Electrons are emitted in all directions from the entire heated filament, so a way is needed to localize emission at one spot.
It is important that the filament is properly centered in relation to the opening of the Wehnelt cap and be the proper distance from the opening.
The potential difference between the filament and Wehnelt is maintained using a bias resistor, which allows the gun to be self-regulating.
The electron flux from a tungsten filament is minimal until a temperature of approximately 2500 K. The electrons emitted from the filament are drawn away from the cathode-Wehnelt assembly by the anode plate, which is a circular plate with a hole in its center.
Observation of the absorbed current (that remaining in the sample during electron beam bombardment) allows alignment of the electron gun.
The JEOL-6480LV SEM uses an electromagnetic alignment coil located beneath the anode to align the gun.
The electron beam is divergent after passing through the anode plate and must be refocused.
At present, electrostatic lenses most commonly are used to deflect and focus ion beams in mass spectrometers.
Electron lenses are not as good as optical lenses in terms of defects of focus, called aberrations.
The ultimate electron beam spot size depends on the beam current, which is controlled by the condenser lens, and the type of filament in use. Microprobe electron beam currents are usually from about 10-9 A (10-9 A = 1 nanoamp = 1 nA) to 10-7 A (100 nA). The beam below the condenser lens is again divergent and must be refocused before striking the sample.
The beam regulator is located between the condenser and objective lenses in the electron microprobe column. The Cameca MBX microprobe does not use a true Faraday Cup, although later models (SX-50, SX-100) do.
The objective, or "probe-forming", lens is located at the base of electron column just above sample. The objective lens in the JEOL-6480LV SEM is much simpler than that used in the MBX microprobe, being a solid conical lens with the scanning coils housed inside it. Magnetic lenses do not have perfect symmetry and the electron beam may be elliptical, something termed astigmatism (stigme means "mark" or "spot" in Latin). Both microprobes and scanning electron microscopes have "stigmators" housed with the scanning coils inside objective lens that compensate for imperfections in construction and those not corrected by alignment of column. The alignment of the microprobe column can be tested by changing the objective lens focus and examining the over- and under-focus relationships of the electron beam on a fluorescent sample. Geological microprobe samples are usually petrographic thin sections that have been ground flat and polished. The JEOL SEM chas the added capability to also work with a relatively poor vacuum in the specimen chamber. An environmental SEM utilizes relatively low vacuum pressures (up to 50 torr ~ 6700 Pa) not only to neutralize charges, but to provide signal amplification.
A positively charged detector electrode is placed at the base of the objective lens at the top of the sample chamber. The X, Y, and Z positioning of samples is accomplished by moving the stage with stepper motors. In microprobes, sample changes are accomplished using an airlock to avoid having to vent the entire column. The MBX microprobe is equipped with a light microscope that allows simultaneous examination of electron beam and specimen during operation. Beam position, size and shape are observed using a fluorescent sample such as willemite, or benitoite. These accessories should be available from you local Phenom SEM supplier but if you have any problems with sourcing than please contact us directly.
In close cooperation with its preferred development partner Deben, Phenom-World has developed a Temperature Controlled Sample Holder to study vacuum-sensitive and vulnerable samples. With the use of the Temperature Controlled Sample Holder, the temperature of the sample is manipulated, therefore the humidity around it can be controlled. The Temperature Controlled Sample Holder can be retrofitted to all versions of the Phenom system. The content of this website (content being images, text, sound and video files, programs and scripts) of this website is copyright © Deben UK Limited unless specifically mentioned. This device allows for the precise control of sample temperature over a range from -25° C to +50° C.
This device connects to the sample stage and measures the amount of current passing through a sample to ground. A new product that allows the user of the Scanning Electron Microscope to check the performance of their instrument to make sure that it is working within the manufactures specification.
Is a sample of gold crystals on a carbon background which is useful for checking the performance of the SEM under high-resolution conditions i.e.
1000 mesh and a 2000 mesh grid for magnification calibration of Scanning Electron Microscope. The brass sample holder is 50mm diameter x 8mm thick and comes in MAC's standards presentation box.
A 25mm or 32mm diameter x 5mm thick brass block containing your Analytical Standards, or other smaller sizes if required. Measurements of the beam current as a function of spot size were performed for a particular filament for three different acceleration voltages.
In most microprobes and scanning electron microscopes, vacuum is achieved through a combination of mechanical and diffusion pumps. There is very high voltage between these two components and stray air or gas molecules can cause electrical arcing between them. These collisions can result in spreading or diffusing of the electron beam or, more seriously, can result in volatilization event if the molecule is organic in nature (for example, vacuum oil). The volume around the electron gun must be kept free of gas molecules especially oxygen, which will greatly shorten the filament life. Evacuation of the sample chamber from atmospheric pressure to operating vacuums proceeds from the viscous flow regime into the molecular flow regime. Under these conditions, gas molecules collide more readily with one another than with the chamber walls, and will move as a viscous mass in the general direction of low pressure. The wavelength-dispersive spectrometers are isolated from the electron column by thin windows made of polypropylene or Mylar. Capture pumps, such as ion-getter and cryo, operate by sequestering air molecules onto a surface where they are held it either temporarily (cryo) or permanently (ion-getter).
One is dedicated to pumping on ("backing") the diffusion pump; the other serves to evacuate the column, sample chamber and spectrometers during initial pump-down.
Longitudinal vanes move out of the cylinder and make contact with the sides of the pump housing. An oil diffusion pump requires a initial vacuum of about 50 millitorr (5 to 10 Pa) to operate.
Turbomolecular pumps use no oil and operate like jet engines with multiple, angled blades rotating at very high speed.
The gettering can be accomplished by making the pump wall very cold (cryopumps) or by ionizing gas within a magnetically confined cold cathode discharge (ion pumps). A high voltage is applied between the anodes and cathodes, and the electrons produced from the cathodes move in long helical trajectories through the anode tubes. Microprobes and scanning electron microscopes are usually fitted with thermocouple (1 to 10-3 torr range) and cold cathode gauges (10-3 to 10-10 torr). Cold cathode ionization vacuum gauges contain just two unheated electrodes, a cathode and an anode. However, an electron must be supplied sufficient energy to kick it into a high energy state within the material and additional energy for it to escape the surface. There are two main types of thermionic sources: tungsten metal filaments and LaB6 crystals. Thermionic Emission occurs when enough heat is supplied to the emitter so that electrons can overcome the work-function energy barrier EW of the material and escape from the material. The Schottky effect occurs even at room temperature and depends only very slightly on temperature, indicating that it is not a temperature activated process.
Schematic diagram showing the components of a field emission source: emitter tip (T), first anode (FA) and second anode (SA). The electron source in an SEMs may use either type of thermionic cathode or utilize field emission. This yields a coherent source of electrons emitted from a fairly small area; however, because the filament is bent in a single plane the geometry of the emission region is not perfectly circular. The distance between the Wehnelt and the filament can be adjusted in most SEMs, allowing the shape of the electrostatic field to be changed and optimization of the electron gun (after Goldstein et al. LaB6 has Ew of ~2.5 eV, yielding higher currents at lower cathode temperatures than tungsten.
The distance between the tip of the filament and the Wehnelt aperture is critical in determining the geometry of the lens. Recall from high-school physics that V = I R, where V = voltage, I = current, and R = resistance.

Above 2500 K, the relationship predicts that the electron flux will increase essentially exponentially with increasing temperature, until the filament melts at about 3100 K.
Emission and sample currents as a function of filament voltage in a self-biased gun (think of voltage as analogous to filament temperature). The anode is biased from +1 to +50 keV with respect to the filament (actually, the electron gun is held at a negative voltage relative to the anode, which is grounded). The electron gun is aligned by shifting the position of the filament assembly relative to the anode and the column beneath it, to maximize the absorbed current. A cylinder lens consists of several cylinders aligned along the electron optical axis; there are small gaps between the cylinders. This is accomplished by focusing the electron beam to variable degrees onto a lower aperture. The zoom condenser lens closely maintains beam focus at the sample, without needing operator intervention. Microprobe beam currents are larger (resulting in a larger focus point) because there is no reason for beam diameter to be smaller than diameter of excited region (~2 ?m). The beam current may vary due to shifts in filament emission or changes in the current through the condenser lens.
Schematic of the microprobe electron column showing the electron beam in red with its width greatly expanded. Instead an aperture-capped cylinder of electron absorbing material, through which the beam always passes, is part of the electron column.
The beam is again divergent after passing through the apertures below the condenser lens and must be refocused.
It is an asymmetrical magnet with less of iron casing located below its air gap to minimize the magnetic field at the sample. The light optics (4) and scanning coils (1) are located inside the minicoil probe-forming lens (2), which has a pole piece (7) made of one solid piece of metal.
There is no need for the objective lens to accommodate a light microscope system, allowing it to have a more pointed configuration.
Only the scanning coils are located inside the scanning electron microscope's probe-forming lens.
The location of the scanning coils, beam centering coils and the stigmators is shown in this detailed schematic of the interior of the MBX microprobe's objective lens.
An astigmatic beam will collect signals from a elliptical region that will be rendered as a round spot on viewing screen or final image, producing a reduction in resolution. If the effect of a magnetic lens is slightly tilted (or, equivalently, if the electron beam is tilted off axis), the focusing pattern becomes astigmatic. These consist of eight weak electromagnetic lenses with alternating coils connected in series but wound in opposite directions to achieve different polarities. If the limiting aperture is not centered along the axis of the electron column, the image of the aperture hole will rotate as the setting of the objective lens is changed due to the helicoidal movement of electrons in the beam. In contrast, SEM samples are far more variable ranging from thin sections to actual large pieces of rock. The Camebax microprobe has a sample chamber just large enough to permit movement of two entire thin sections under the electron beam (one usually reserved for standards materials).
Air (or another gas) is admitted into the sample chamber to neutralize the build-up of the negative electrostatic charge on the sample. The electron column is gradually brought up to pressure by a series of pumps and apertures. Secondary electron emitted from the sample are attracted to the positive electrode, undergoing acceleration.
A series of electron micrographs showing the dissolution (a?c) and reprecipitation (d?f) of NaCl as water is condensed on the surface of the salt crystals and then evaporated.
The only requirement is the the sample chamber have sufficient room for the motors and gearing that allow movement of the sample under the electron beam. This allows the operator to locate the beam position prior to analysis of minerals that do not fluoresce. The sample is observed (and illuminated) by a series of mirrors, the latter three of which are coaxial with the electron beam. The transmitted light assembly consists of a right angle mirror positioned beneath the optical microscope and a horizontally oriented light source located outside of the evacuated electron column.
This active sample holder is designed to control the temperature of the sample between -25°C and +50°C. This enables imaging of moistures and water containing samples as well as reducing the effect the electron beam has on beam sensitive samples.
The customer can choose any other five from our lists, with the exception of the SRM 481, SRM 482 and SRM 1872.
The beam of electrons is focused inside the hole by increasing the magnification so that when the hole fills the screen all the electrons are trapped and a true measure of current is achieved. Electron optics are a very close analog to light optics, and most of the principles of an electron beam column can be understood by thinking of the electrons as rays of light and the electron optical components as simply their optical counterparts.
However, when the pressure drops to the point where the MFP exceeds the size of the chamber, the gas molecules do not collide with one another and no longer exhibit viscous flow. When an ultra clean vacuum environment is required, a turbomolecular pump often replaces the oil diffusion pump. The spectrometers are evacuated by a separate mechanical pump and kept at a pressure of about 10-3 torr = 1 Pa, whereas the column is kept at a pressure of about 10-6 torr. Once a vacuum of about 1 torr is achieved the gate valve opens to access the oil diffusion pump (green and red), isolating mechanical pump 1. Momentum transfer pumps, which include oil diffusion, and turbomolecular pumps, move gas by compressing it using some form of mechanical impact, and exhausting it at a higher pressure into another volume at lower pressure. The vane are either spring-loaded (E) or extended by the centripetal force of the spinning cylinder. Gas molecules in the pressure range of a mechanical pump pressure range move via viscous flow. Therefore, it is isolated from the column and sample chamber by a "gate" valve until the mechanical pump has produced a sufficiently good vacuum in the column as described above. Air molecules that have diffused into the vicinity are compressed against the jets and entrained. The ultimate pressure achieved by an ion pump is generally in the region of 2 x 10-11 Torr. The long electron paths increase the probability of collision with and ionization of gas molecules. A "cold" discharge is initiated by an external event (cosmic ray, radioactive decay) and maintained between the cathode and anode by a voltage of ~2 keV. Electrons with a component of velocity at right angles to the surface and kinetic energy at least equal to the work done in passing through the surface will be emitted. Electrons have a range of energies with the highest energy state called the Fermi Level, EF. Voltages applied between the tip and first anode are ~ -3000 V; V0 is the accelerating voltage. Field emission machines are markedly more expensive and are employed for ultrahigh resolution work.
Typically, LaB6 cathodes exhibit 10 times the brightness and more than 10 times the service life of tungsten cathodes. The Wehnelt is biased -200 to -300 V with respect to the filament, producing a repulsive electrostatic field that condenses the cloud of primary electrons produced from the filament.
The Wehnelt cap acts as a convergent electrostatic lens and serves to focus the cloud of electrons. Movement of the filament tip is the major source of beam instability and even a displacement of 1o will produce a significant change. However, in practice, the electron emission reaches a plateau termed saturation due to the self-biasing effects of the Wehnelt cap. The voltage potential between the cathode and the anode plate accelerates the electrons down the column and is known as the accelerating voltage, which is usually given in terms of KeV and abbreviated E0. In the Camebax microprobe this is accomplished by moving the electron gun using two alignment knobs. Although electron lenses in principle behave the same as optical lenses, there are differences and the quality of electron lenses is not nearly as good as optical lenses in terms of aberrations. The resulting change in direction, in turn, yields a direction of force, deflecting the electron toward the the axis. Spherical aberrations, in which the outer zones of a lens focus more strongly than inner zones, are most important in magnetic lenses.
The sharper the focus, the less of the beam intercepted by the aperture and the higher the current. Tungsten filaments yield the largest final diameters, reflecting the larger initial size of the beam produced from the filament. In contrast, SEM beam size is not limited by excitation area because SE are produced from just the upper ~100 pm of the sample. Emission from a tungsten filament can vary by up to 1% in 10 minutes; although this seems a small change, it will result in unacceptable analytical results. The regulator senses changes in the current and adjusts the condenser lens power accordingly. The effects of the condenser lens (1) and objective or probe-forming lens (PFL) are shown schematically by divergent black arrows; the sample is located at (6).
The beam is measured by swinging a "trap door" to block the bottom of the cylinder, blocking the beam.
The objective lens focuses the electron beam onto the sample and controls final size and position.
It is designed to accommodate the light optical system and scanning coils inside it and has apertures that allow X-rays from the sample to pass to the spectrometers at a 40? take-off angle. Consequently, there is no need for apertures for X-ray spectrometers to achieve comparable take-off angles. The coils consist of four magnets oriented radially that produce fields perpendicular to optical axis of the electron beam. Astigmatism results from machining errors, asymmetry in magnetic windings, inhomogeneous magnetic fields in the iron, the buildup of contamination within electron column, and misalignment of the electron column. The beam first focuses along a horizontal segment (tangential focus), has a more or less circular shape at the focal plane, and focuses again along a vertical segment (sagittal focus) behind the focal plane. Stigmators apply a correcting magnetic field to produce symmetrical electron beam at the sample. The aperture should be centered using the limiting aperture knobs to eliminate this movement. Perfect alignment: the beam changes diameter symmetrically when changing objective lens focus. In contrast, the JEOL-6480LV scanning electron microscope has a sample chamber that permits large and complex movements of the sample stage; for example, the SEM can accommodate samples up to 200 mm in diameter and ~80 mm high.
In "low-vacuum (LV) mode," the sample chamber pressure, typically 20-30 Pa, is maintained at a specifiable value by a separate extra pump with a large foreline trap. As they travel through the gaseous environment, collisions occur between electrons and a gas particles, producing more electrons and ionization of the gas molecules and effectively amplifying the original secondary electron signal (electron cascade). Most microprobes can accommodate standard polished thin sections or 1" polished round sections and most permit examination of the sample with either reflected or transmitted light (see below). This changer is evacuated to a pressure of about 2 Torr, whereupon the airlock door can be opened and the sample inserted. There is only one magnification (400x) available for observation on the MBX probe; this magnification is higher than that used during standard petrographic examination, making it sometimes difficult to locate points of interest for analysis. To maintain a voltage of 20 keV between the Wehnelt and anode plate at a pressure of 10-4 torr, requires a gap of about 2 mm; higher voltages require better vacuums.
At this pressure, gas movement is independent of any pressure gradient and molecules may "backstream" from the pump into the sample chamber, moving against the pressure gradient.

A mechanical pump is used to evacuate the sample chamber from atmospheric pressure to a moderate vacuum. Polypropylene windows are used on the light-element spectrometer(s) because they absorb fewer X-rays; however, they are more fragile than Mylar windows. These pumps require initial pumping (?roughing?) to reach operating vacuum and continued pumping to maintain a low exhaust pressure (?backing?).
It may take many hours to reach operating pressures after an electron column has been vented to atmosphere because of the attendant admission of water vapor into the instrument. Diffusion pumps are so named because rather than viscously "pull" air molecules out of a volume, they wait for the molecules to diffuse into the active part of the pump where they are then trapped and removed. The upper part of the pump wall is cooled by circulating cold water through coils wrapped around it. As gas molecules enter through the inlet, the rotor, consisting of a number of angled blades, strikes the molecules, imparting energy to them.
In the main pumping mechanism, the ionized molecules are accelerated toward one of the cathodes where they are buried by Ti atoms. As the pressure decreases, the filament becomes hotter because the number of gas molecules hitting the wire and conducting heat away from the wire decreases. A strong magnetic field makes the electron paths long enough so that they collide with enough gas molecules to maintain the discharge.
This total energy required to for a material to give up electrons is related to its work function, Ew. The energy required to place an electron in the vacuum from the lowest energy state in the metal is E.
Most geological SEM applications, however, require relatively low magnifications and use thermionic guns.
Additionally, the smaller, circular, emission region improves the final resolution of the SEM. The Vogel design heats the LaB6 rod by passing a current perpendicular to the length of the rod and uses graphite spacers between the rod and contacts to limit chemical reactivity between them.
The electrons converge at a point (10-100 μm in diameter) located between the base of the Wehnelt cap and the anode plate.
Any increase the emission current causes a larger voltage drop (V) across the bias resistor and a larger negative voltage is applied to the Wehnelt, reducing the current. Emission currents, those that flow between the filament and anode, generally are on the order of 200 pA (1 pA = 10-12 A), in SEMs. Notice the false peak at about 3.4 volts caused by region of filament that reaches emission temperature before tip. Together the Wehnelt cylinder and anode plate serve to condense and roughly focus the beam of primary electrons.
Their action is similar in principle to optical lenses, but electron lenses can be made only to converge, not diverge. The result is that electrons along beam axis are deflected less than electrons passing through beam periphery, yielding more than one focal point.
The Cameca MBX microprobe has a second condenser lens that is used to provide a better focus for SEM work, but during normal operation, this lower condenser lens is "decoupled" (not used). The condenser lens should be operated to keep the focus point above the lower limiting aperture (currents along BC). Scanning electron microscopes typically use beam diameters much less than 1 ?m and use smaller final apertures to limit beam diameter.
The beam regulation system, located below the cross-over from the condenser has two apertures: a limiting aperture (2) and lower aperture with a smaller diameter hole (3). This device consists of a cup of electron absorbing material, such as carbon, capped by a 25 to 100 ?m aperture. Hosted within it are scanning coils that allow the beam to be rastered across the sample surface, the astigmation correction coils, the beam shift coils, and the visible light microscope optics.
The apertures have electron traps, which use negative voltages to prevent backscattered electrons from entering X-ray spectrometers.
Only misalignment of the column is easily addressed because cleaning the column is a difficult and prolonged exercise. The operator can change both the strength and orientation (angle) of the magnetic field produced by stigmators to control the final beam shape.
The electron column is separated from the chamber by a vacuum orifice (aperture) that permits the column and objective lens apertures to be maintained at a higher pressure. This form of signal amplification is identical to that which occurs in a gas-flow X-ray detector. The number of sections and the maximum amount of movement depend upon the individual instrument. In the MBX microprobe the sample slides into a narrow forked holder that allows a transmitted light source to be positioned beneath them.
The optical system consists of a series of mirrors with holes in them through which electron beam passes. Once the chamber pressure low enough (~10-3 torr), a gate valve is opened, and high vacuum achieved using an oil diffusion pump.
This arrangement using windows allows a smaller volume (the electron column) to be kept at high vacuum. Once the vacuum is sufficiently good, the "gate" valve that isolates an oil diffusion pump from the column is opened.
The ballast tank serves as a vacuum reservoir and allows the diffusion pump to function for short intervals with pumps 1 and 2 turned off.
When the oil vapor strikes the upper pump wall, it condenses into liquid and sinks to the bottom of the pump taking the adsorbed air molecules with it. Gas molecules are propelled into gas transfer holes in a plate below the blades called a ?stator?.
Ion impacts also sputter titanium from the cathode and the resulting Ti atoms acts as a getter for reactive gases (O, N, CO and H) and are deposited as stable oxides, carbides, nitrides and hydrides elsewhere in the pump. As temperature rises, the thermocouple voltage increases and is measured by a sensitive meter that has previously been calibrated against a manometer to determine the pressure.
The magnetic field is oriented so that the field lines cross the electric field lines, confining electrons to a helical path. When the emitter material is heated to a high temperature, a small fraction of the electrons at the Fermi level acquire enough energy to overcome EW and escape into the vacuum. Higher temperatures can deliver greater beam current, but the tradeoff is an exponentially decreasing lifetime due to thermal evaporation of the cathode material.
The Ferris design uses a short LaB6 rod is supported by a ribbon or strip through which an electrical current is passed for heating. As the emission increases, so does the voltage difference between Wehnelt and filament, causing the emission to plateau. Proper saturation is achieved at the edge of the plateau; higher emission currents serve only to reduce filament life. Magnetic lens consist of two circularly symmetric iron pole-pieces with copper windings with a hole in center through which beam passes. Chromatic aberrations, in which electrons of slightly different energies are focused differently, are relatively minor because the electron gun produces electrons with essentially uniform velocities.
In essence, for all filaments beams with higher currents, having more electrons, spread due to repulsive electrostatic forces.
We tend to operate the JEOL-6480LV SEM as if it were a microprobe, using high sample currents.
Such variations can be ignored in a scanning electron microscope, where the primary function is imaging; it takes no longer than about 2 minutes to acquire an image.
The amount of current impinging on the lower aperture (3) is amplified (5) and used to control the condenser lens power supply (4). Deflecting the beam off-axis introduces aberrations that limit the maximum deflection that can be used and still keep the sample in focus.
Positively charged gas ions are attracted to the negatively biased specimen and offset charging effects.
Usually, water-cooled plates or baffles are positioned above the diffusion pump to promote condensation of pump oil before it enters the electron column. Cameca?s SX-50 and SX-100 designs use an additional ion pump to help maintain vacuum around the electron gun. Additionally, they help keep the column clean by limiting out-gasses from gear and fitting oils and detector gas leaking from the spectrometers. At the bottom, the heater reboils the oil, releasing the air molecules, producing a build up of air molecules in the lower region. The ribbon is made of a material that does not react with LaB6, such as graphite or tantalum.
Proper bias voltage also optimizes the electron beam brightness (current density per solid unit angle) providing the most focused electron beam. Spherical aberrations are minimized by placing a spray aperture in front of the magnetic lens, confining electrons to the center.
Increased current through the condenser causes the beam to widen and more of it is blocked by the limiting aperture.
Additionally, deviations from linearity in X and Y increase as amount of deflection increases.
The worm gears on the stage drives and spectrometers (see below) are subject to mechanical "slop", even with very high tolerance machining. There is a water-cooled baffle located above the pump to prevent oil vapor from entering the electron column. The lower aperture senses the small beam current and decreases the condenser lens current, allowing more beam to pass through it. Those backscattered hit the bottom of the aperture; similarly secondary and Auger electrons are unable to escape. As a result, peak positions appear to shift when approached from different directions (up or down the drive screw) and stage positions are not reproducible. A cold-cathode (ion) vacuum gauge, which operates at pressures down to 10-6 torr, is used to monitor the pressure at higher vacuums.
Unfortunately, the calibration of an ion gauge changes over time as the result of gradual contamination of the components. If the beam current decreases and narrows, less is blocked by the limiting aperture, more strikes the lower aperture. Note that rather than sweeping, the beam seems to expand and contract from one side as focus is changed; r2 is the distance from the focused beam to the electron column axis.
Normally, these materials would be either coated with an electrically conductive coating, which takes time and hides the true surface, or studied at low voltages, which do not give backscatter or X-ray information (E0 < Ec for most elements of interest).
The MBX microprobe analytical programs use a "backlash" to compensate for this slop in which all peaks and stage positions bypassed by a certain number of steps and always approached from the same direction. Unfortunately, because of the high rotation speeds (10-20,000 rpm), turbomolecular pumps have shorter life spans than oil diffusion or sealed-oil mechanical pumps.
Additionally, samples containing volatile substances (water) can often be studied directly. The newest Cameca microprobes (SX-50, SX-100) use optically encoded light bars to control stage (and spectrometer) movement.
The direction of movement does not matter, since the positions are located using absolution positions on the encoders. The stage and spectrometers are moved using DC motors rather than AC stepper motors, and movement is very rapid and precise.

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