The focused ion beam – scanning electron microscope (FIB-SEM) combined a focused ion beam column with a electron column in a single instrument vacuum chamber. The columns are typically offset by a known angle, α, which is typically 52° or 90°, so that when one beam is orthogonal to the sample surface, the other beam can be used to image the side of the sample. Both columns are directed to focus at the same point in the instrument chamber, with the beam co-incidence point also known as the ‘eucentric’ point (Figure 1). In most FIB-SEMs, the sample stage is designed to tilt in the same plane as the beams and to pivot around the eucentric point.
Columns #
The columns in the FIB-SEM system include both the electron column and the ion column. Each column type has their our source and optic systems.
FIB Column #
cross-section.
A FIB column is comprised of the ion source with extractor and suppressors, lenses, octopoles, beam defining aperture strip(s), a beam blanking device. A schematic of a FIB column is shown in Figure 2, but the FIB column arrangement may vary by vendor and model.
Ion Sources #
Currently, there are three main classes of ion sources: gas field ion sources (GFIS), liquid metal ion sources (LMIS), and inductively couple plasma sources (pFIB). Each source type has advantages and drawbacks and are used for different applications. See the FIB source page for greater details.
Ion Beam Optics #
Typical FIB columns consist of two electromagnetic lenses: the condenser and objective lenses. The condenser lens (Lens 1 in Figure 2) is nearest the ion source and is used change the size and shape the electron beam. The objective lens (Lens 2 in Figure 2) is used to change the focal point of the beam onto the sample.
Magnetic octopoles are also used to shape the electron beam and to steer the beam. One octopole (or sometimes a quadrapole) is used as a stigmator to make the beam as round as possible. The second octopole is used to deflect the beam for scan rastering and patterning.
Beam Blanker #
The beam blanker consists of electrodes that are able to deflect the ion beam at high frequencies for patterning. When the electrodes are activated, the beam is diverted into the beam blanking aperture rather than down the column. The high frequency switching of the beam blanker enables precise patterning to ensure that there are not scan artifacts when the beam travels between spot positions. Many beam blankers also have an integrated Faraday cup to measure the beam current when the ion beam is not being directed at the sample.
Aperture Strip #
The aperture strip in the ion column sets the beam current as well as the beam spot size. Larger holes in the aperture strip allow more ions through the column, resulting in larger beam currents. The larger aperture holes result in larger beam spot sizes.
SEM Column #
The SEM column typically consists of an electron source, focusing optics, and various apertures.
Electron Sources #
There are two standard types of SEM sources: thermionic emission guns (W, LaB6, or CeB6), and field emission guns (FEGs).
Thermionic emission sources heat a filament to the point where electrons can are freely emitted from the source tip. The least expensive thermionic sources can be made from tungsten (W) hairpins, but suffer from low overall resolution. Lanthanum or cerium hexaboride (LaB6 or CeB6) tips offer improved resolution and brightness as compared to W tips, but are significantly more expensive.
Field emission guns use a strong electric field to lower the barrier to emission from the source tip. FEGs come in two varieties: cold cathode and Schottky. Cold cathode FEGs do not heat the tip and therefore the tip lifetime is very high. Schottky FEGs do a combination of source heating and high electric field to create electron emission from the source. In general, FEGs sources have better resolution than thermionic sources, but require higher vacuum to maintain the source and are more expensive to maintain.
Vacuum System #
The vacuum system in a FIB-SEM is composed of three different sub-systems: the chamber vacuum, the electron column vacuum, and the ion column vacuum.
In most SEMs, the chamber vacuum is maintained by a turbomolecular pump that is backed by a scroll pump. Typical chamber vacuum levels are between 10-2 to 10-4 Pa (10-4 to 10-6 torr), but higher vacuum levels can be achieved. Increased vacuum levels are typically beneficial to prevent deposition of carbon on a sample surface during electron-beam imaging.
The electron beam column vacuum system will be different depending on the electron source type. Typical thermionic sources use the chamber vacuum system to maintain the electron column vacuum levels. FEG sources require much higher vacuums to operate, and rely on ion-getter pumps (IGPs). As a result, FEG columns can achieve vacuum levels approaching 10-9 Pa.
Ion column vacuum systems are also usually pumped by IGPs and can reach vacuum levels as high as 10-6 Pa (10-8 torr). The high vacuum levels are needed to prevent discharging of the ion source in the column.
Chamber #
Detectors #
Detectors in FIB instruments are usually similar to those in standard SEMs.
