Scanning electron microscope, its system design is from top to bottom, the electron beam is emitted by the electron gun, and after being focused by a set of magnetic lenses, the size of the electron beam, through a set of scanning coils that control the electron beam, and then through the objective lens to focus, hit on the sample, and a signal receiver is installed on the upper side of the sample to select the secondary electron or backscattered electron imaging.
The necessary characteristics of an electron gun are high brightness and small electron energy spread. There are three commonly used types at present, tungsten (W) filament, lanthanum hexaboride (LaB6) filament, and field emission There are differences in the size of the electron source, the amount of current, the current stability, and the life of the electron source.
The thermal ionization electron gun
There are two types of thermal ionization electron guns: tungsten (W) filament and lanthanum hexaboride (LaB6) filament. It uses high temperatures to make electrons have enough energy to overcome the work function barrier of the electron gun material and escape.
The variables that have a significant impact on the emission current density are temperature and work function. However, when operating the electron gun, it is hoped to operate at the lowest temperature to reduce the volatilization of materials. Therefore, when the operating temperature does not increase, it is necessary to use low Work function materials to increase the emission current density.
The cheapest and most commonly used is the tungsten filament, which emits electrons in the form of thermal dissociation. The electron energy distribution is 2 eV, and the work function of tungsten is about 4.5eV. The tungsten filament is about 100 μm in diameter and bent into a V shape. The thin wire, the operating temperature is about 2700K, the current density is 1.75A/cm2, the diameter of the filament becomes smaller with the evaporation of the tungsten wire during use, and the service life is about 40~80 hours.
The work function of the lanthanum hexaboride (LaB6) filament is 2.4eV, which is lower than that of the tungsten filament. Therefore, the same current density can be achieved by using LaB6 at 1500K, and the brightness is higher, so the service life is longer than that of the tungsten filament. Many, electron energy spread is 1 eV, better than tungsten wire.
However, because LaB6 is very active when heated, it must be operated in a good vacuum environment, so the purchase cost of the instrument is relatively high.
Field emission electron gun
Field emission electron guns are 10-100 times brighter than tungsten filaments and lanthanum hexaboride filaments, and the electron energy spread is only 0.2-0.3eV, so the high-resolution scanning electron microscopes currently on the market all use Field emission electron guns, its resolution can be as high as below 1nm.
There are two common field emission electron guns:
Cold field emission (FE), thermal field emission (TF)
When the metal surface in a vacuum is subjected to an electron-accelerating electric field of 108V/cm, a considerable number of electrons will be emitted. This process is called field emission. The principle is that the high electric field causes the potential barrier of electrons to produce the Schottky effect, that is The barrier is narrowed in width and low in height, so electrons can directly “tunnel” through this narrow energy barrier and leave the cathode.
Field emission electrons are emitted from a very sharp cathode tip, so an extremely thin electron beam with high current density can be obtained, and its brightness can reach hundreds or even thousands of times that of a thermal ionization electron gun.
The cathode material selected for the field emission electron gun must be a high-strength material to withstand the high mechanical stress imposed on the tip of the cathode by a high electric field. Tungsten is a better cathode material because of its high strength.
A field emission gun usually has a set of upper and lower anodes to produce functions such as absorbing electrons, focusing, and accelerating electrons. The electrostatic field generated by the special shape of the anode can produce a focusing effect on the electrons, so there is no need for a Weiss mask or a grid.
The first (upper) anode is mainly to change the field emission extraction voltage to control the current intensity of the needle tip field emission, while the second (lower) anode is mainly to determine the acceleration voltage to accelerate electrons to the required energy.
To field-emit electrons from a very fine tungsten tip, the metal surface must be completely clean, without any atoms or molecules of foreign materials on the surface, even if only one foreign atom falls on the surface, it will reduce the field emission of electrons, so the field emission electron gun must Maintain an ultra-high vacuum to prevent the accumulation of atoms on the surface of the tungsten cathode. Due to the extremely high price of ultra-high vacuum equipment, field emission electron guns are rarely used unless high-resolution SEMs are required.
Cold field emission
The biggest advantage of the cold field emission type is that the diameter of the electron beam is the smallest and the brightness is the highest, so the image resolution is the best. The energy spread is minimized, which improves the effect of operating at low voltage.
In order to prevent the needle tip from being adsorbed by foreign gas, reduce the field emission current, and make the emission current unstable, the cold field emission electron gun must be operated at a vacuum of 10-10 torr. Even so, it is necessary to periodically heat the needle tip to 2500K (this process is called flashing) to remove the adsorbed gas atoms. Another disadvantage is that the total current emitted is minimal.
Thermal field emission
The thermal field electron gun operates at a temperature of 1800K, which avoids the adsorption of most gas molecules on the surface of the needle tip, thus eliminating the need for needle tip flashing.
The thermal type can maintain better emission current stability and can operate under poor vacuum (10-9 torr).
Although the brightness is similar to the cold type, its electron energy distribution is 3 to 5 times larger than that of the cold type, and the image resolution is poor, so it is usually used less frequently.