Two types of columns are typically used in ultra-high resolution SEM to achieve excellent low voltage topographic and compositional imaging. Using magnetic immersion or electrostatic lenses for improved resolution allows angle-sensitive detection to tune in-lens contrast for a wide variety of materials. Recently, a newly developed compound electrostatic-magnetic final lens has been developed which combines magnetic and electrostatic techniques into a single versatile high resolution low-voltage SEM which enables topographic and compositional information tuning across a wide variety of materials.
Electrostatic and magnetic immersion columns have a couple of differences which define appropriate application space advantages for high resolution and contrast. Electrostatic columns generally have resolution in the range of 1.6 to 1.8nm at 1kV whereas magnetic immersion systems achieve ~1.4nm (without monochromator) or better (0.7nm with monochromated FEG source) at 1kV. Both systems have dedicated in-lens, in column detection options for acquiring high contrast signals. Generally for normal samples the magnetic immersion will produce the highest resolution at the lowest currents, however, some magnetic samples may present challenges due to the strong magnetic field. So working without the immersion lens activated is possible but leads to a slightly lower resolution for such samples.
This new SEM makes use of a combination of a magnetic final lens in the pole piece, a magnetic immersion lens and an electrostatic lens formed by the potential in the bottom of the column. The combination of these lenses focuses the primary electron beam to a very tight spot. The resolution of this new system is specified as 1.0 nm at 1 kV.
The contrast performance of the system benefits from the in-lens backscatter detector located at a position close to the sample where the detector produces high signal contrast due to its position and enables ultra-low beam current BSE imaging. In addition there are higher detectors in the lens and column to collect other low loss signals which travel further up the column. The possibility to sort electrons according to their energies and/or emission angles through detection is usually done by influencing the SE or BSE trajectories with exposure to an electric or magnetic field. With the new compound final lens both are available and the efficient detectors are capable of acquiring an energy-filtered BSE image by tuning the lens strength for selective detection of high- and low-loss BSEs. This enables precise materials contrast on the smallest particles and energy selection works as an effective charge filter, allowing the acquisition of charge-free images on insulating samples.
This new versatile SEM combines a range of technologies including the compound final lens, BSE filtering, and segmented detection all into one tool. The system delivers the resolution and contrast that allows materials researchers to capture the maximum amount of information from their sample, with the right detail, with the least amount of compromises.
