Feature Article - Electron Gun Configurations for Scanning Electron Microscopes - By Christopher Henderson

The Scanning Electron Microscope is a basic instrument for analysis and characterization. We will cover the basic configuration of the electron guns in this article. Scanning Electron Microscopes (SEMs) fall into three basic configurations, Tungsten, Lanthanum Hexaboride or LaB6 and field emission. Within the field emission category, there are two basic configurations: the cold cathode configuration and the Schottky Emitter configuration.

Figure 1 shows the basics of a tungsten-based system. In a standard tungsten system, a bias is placed across the filament; the current through the filament heats it. At high temperatures the material emits electrons, which can then be accelerated down the column. The high voltage power supply between the Wehnelt cylinder, filament, and anode plate determines the primary electron beam energy.

Figure 2 shows the basics of a lanthanum hexaboride system. Lanthanum hexaboride, also known as LaB₆, emits a higher number of electrons than a tungsten filament, permitting higher quality images. In this gun configuration, a heating coil encompasses the LaB₆ rod to heat it. As with the tungsten system, the high voltage power supply lies between the Wehnelt cylinder and anode plate to determine the primary electron beam energy.

Another method for generating electrons is the field emission gun. A schematic of a field emission tip is shown in Figure 3. When the cathode forms a very sharp tip (typically 100 nm or less) and the cathode is placed at a negative potential with respect to the first anode so that the local field at the tip is very strong (greater than 10 to the 7 Volts per centimeter), electrons can tunnel through the potential barrier and become free. Although the total current is lower than either the tungsten or the LaB₆ emitters, the current density is between 10 to the 3 and 10 to the 6 Amps per centimeter. Thus, the field emission gun is hundreds of times brighter than a thermionic emission source. Furthermore, since the electrons are field generated rather than thermally generated, the tip remains at room temperature. Tips are usually made from tungsten etched in the <111> plane to generate the lowest work function. Because a native oxide will quickly form on the tip even at moderate vacuum levels (10 µPa), a high vacuum system (10 nPa) is needed. To keep the tip diameter sufficiently small, the cathode is warmed to 800-1000 °C or rapidly heated to approximately 2000 °C for a few seconds to blow off material.

This table summarizes the basic capabilities of the four basic configurations, where we break the cold field emission and Schottky field emission systems into their own separate groups. The highest performers are the field emission systems, which include cold cathode and Schottky. The high brightness and sharp tip leads to high resolution and longer source lifetimes. Notice that both tungsten and LaB₆ have lower brightness, lower resolution, and reduced source lifetimes. However, the lower vacuum requirements can facilitate more rapid sample exchange, especially when venting the column is necessary. For more information on the Cold Cathode and Schottky field emission systems, please see the Technical Tidbit on this topic elsewhere in this newsletter.