Electrons may be emitted from a conducting cathode material either by heating it to the point where outer orbital electrons gain sufficient energy to overcome the work function barrier of the conductor (thermionic sources) or by applying an electric field sufficiently strong that electrons "tunnel" through the barrier (field emission sources). Electron guns used in microprobes employ the former method, in which electrons are effectively evaporated from a resistively-heated tungsten filament; some alternative names for the filament include cathode or emitter.
Electrons leave the filament with an average energy of E = kT, where k = Boltzmann's constant (8.617398 x 10-5 eV/K), and T = filament temperature (K). At 2700 K, the electrons have energies of about 0.23 eV. To escape from the cathode, electrons must have a component of velocity at right angles to the surface and their corresponding kinetic energy must be at least equal to the work done in passing through the surface. The energy required to for a material to give up electrons is related to its work function, Ew. The work function of a material is given by:
E = Ew + Ef
where, E is the total amount of energy needed to remove an electron to infinity from the lowest free energy state, Ef is the highest free energy state of an electron in the material, and Ew is the work function or work required to achieve the difference.
The emission flux can be expressed by the ‘Richardson- Dushman’ equation, dating from 1923, which describes the current density emitted by a heated filament:
The electron flux from a tungsten filament is minimal until a temperature of approximately 2500 K. 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. However, in practice, the electron emission reaches a plateau termed saturation due to the self-biasing effects of the Wehnelt cap (see below). Proper saturation is achieved at the edge of the plateau (Figure 3.1.1a); higher emission currents serve only to reduce filament life.
The electron source in microprobes has a triode configuration, consisting of an emitter/cathode (filament), grid cylinder (Wehnelt) and anode. The filament is usually a thin (about 0.1 mm) tungsten wire bent into an "inverted V." Electrons are preferentially emitted from the bent tip and produce a coherent source of electrons in a fairly small area; however, because the filament is bent in a single plane the geometry of this region is not perfectly circular. Tungsten is used because it withstands high temperatures without melting or evaporating. Unfortunately, as noted above, it has a very high operating temperature (2700 K). Heating is accomplished by running a 3- to 4-amp current through the filament. Higher temperatures can deliver greater beam current, but the tradeoff is an exponentially decreasing lifetime due to thermal evaporation of the cathode material.
Figure 3.1.1a. Emission and sample currents as a function of filament voltage in a self-biased gun (think of voltage as analogous to filament temperature). The operating voltage was 20 keV. Approximate saturation voltages (operating values) are indicated. Notice the false peak at about 3.4 volts caused by region of filament that reaches emission temperature before tip. After Heinrich (1981).
Emission currents range from about 50 to 200 mA (1 mA = 10-for electron microprobe, whereas they are much higher (15 to 25 mA) for X-ray fluorescence and much lower (100s pA) for SEM work. The life expectancy, t, of a 0.125-mm-diameter filament (in hours) is approximately:
t = 50 / J,
where J = emission flux (A/cm2).
The cloud of primary electrons is condensed by the Wehnelt cap that surrounds the filament and is biased -200 to -300 V with respect to the filament. The Wehnelt cap has an aperture located below the filament tip and suppresses electron emission from the filament except at the tip.. It is important that the filament be properly centered in relation to the opening of the Wehnelt cap and be the proper distance from the opening. Otherwise, an off center beam that is either weak/condensed or bright/diffuse will be produced. The Wehnelt cap acts as a convergent electrostatic lens and serves to focus the cloud of electrons (Figure 3.1.1b). The electrons converge at a point (10-100 µm in diameter) located between the base of the Wehnelt cap and the anode plate, This point is called the "cross-over" and is the location of the effective electron source. The distance between the tip of the filament and the Wehnelt aperture is critical in determining the geometry of the lens.
||Figure 3.1.1b. Configuration of self-biased electron gun (after Goldstein et al. 1981). The distance between the Wehnelt and the filament can be adjusted in most microprobes, allowing the shape of the electrostatic field to be changed and optimization of the electron gun.|
The potential difference between the filament and Wehnelt is maintained using a bias resistor, which allows the gun to be self-regulating. Recall from high-school physics that V = I R, where V = voltage, I = current, and R = resistance. As the filament emits electrons, an emission current (I) flows from filament to Wehnelt. 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. As the emission increases, so does the voltage difference between Wehnelt and filament, causing the emission to plateau. Proper bias voltage also optimizes the electron beam brightness (current density per solid unit angle) providing the the most focused electron beam (Figure 3.1.1c).
|Figure 3.1.1c. Schematic relationship between bias voltage, emission current, and beam brightness. (after Goldstein and Yakowitz, 1975, p. 25.).|
The electrons emitted from the filament are drawn away from the cathode by the positively charged anode plate, which is a large circular plate with a central aperture. The anode has a hole in its center and is biased from +1 to +50 keV with respect to the filament-Wehnelt. (Actually the electron gun is held at a negative voltage relative to the anode.). The voltage potential between the cathode and the anode plate accelerates the electrons down the column and is known as the "accelerating voltage" and is given in terms of KeV. Together the Wehnelt cylinder and anode plate serve to condense and roughly focus the beam of primary electrons.
Movement of the filament tip is the major source of beam instability and even a displacement of 1o will produce a significant change. The electron gun is aligned by shifting the position of the filament relative to the anode and column beneath it. This position may have to be periodically checked. Most machines (including the Cameca MBX) are aligned mechanically by moving the filament with setscrews or knobs. However, the most modern Cameca microprobes use electromagnets to align the gun. This is accomplished with alignment coils consisting of two sets of four radially oriented magnets one above the other.
Copyright 1997-2003, James H. Wittke
Last update: 01/18/2006 01:47 PM.