STEM in SEM has been evolving for a number of years and the latest designs to achieve resolution down to 3Å at 30kV. Moving from the initial holders where the transmitted signal was reflected to the ETD detector once SE and BSE were created on the target below the sample, the introduction and refinement of solid-state STEM has been enhanced by the addition of a new retractable lower objective "projection" lens to concentrate signals. This innovation is allowing access to lower voltages and high resolution STEM and changing the limits of what we believed was possible.
The first inexpensive STEM in SEM were based on sample holders which operated by reflecting transmitted electrons towards the Everhart-Thornley detector for collection. This was accomplished by placing a slanted surface below the TEM grid and provided an inexpensive STEM in SEM solution. This was however low resolution and improvement came with the introduction of solid state detector crystals.
Solid State STEM revolutionized STEM in SEM by providing an easy to connect, sensitive and robust detector platform. The first iterations used the standard 2 segment BSE detector mounted under a TEM grid to collect BF/DF signal. This was quite inefficient and was replaced by an annular STEM diode specially made to collect signals - which included a DF ring placed outside of this area followed by segments in a ring to collect high angle dark-field signals which are scattered further during transmission. With the later diode designs additional DF rings were added to segment four angular DF signals outside of the BF area. Typical resolution volues are under 0.8nm at 30KeV beam energy with this setup.
In order to increase the resolution, FEI incorporated an in-lens STEM mode into the DualBeam FIB/SEM. Since this extra lens must sit between the TEM grid and the STEM detector there were challenges for integration. To solve the challenges this lens has been incorporated into a retractable mechanism which is combined with a special TEM-type holder to more easily change the configuration. This modification is quite special and allows higher resolution double tilt mounting/manipulation for both thinning and analysis in SEM mode. With the new design, resolution below 3Å is routinely achievable in the Helios G4 FX which often eliminates the need to move samples to a TEM for critical dimension measurements.
The contrast and resolution on soft materials at extreme low voltages is opening the door to new analysis. Though this technology is only available on the dedicated Helios model, it is bringing performance of STEM in SEM to a completely new level and ideas for how to use and extend this new capability are multiplying.
