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Don't Sweat the Small Stuff: See It
New nano-imaging techniques provide sub-wavelength spatial resolution images for applications like electronics and biomedicine
STRUCTURE OF THINGS
On the analytical chemistry front, a collaboration led by David Muller at Cornell University (Ithaca, N.Y.) has leveraged aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) to generate 2D elemental and valence-sensitive images with spatial resolutions of about 0.1 nm.
In EELS, a high energy electrons from a narrowly focused e-beam are scattered to the detector plane by atoms in the material of interest. The inelastically scattered electrons lose energy in the collision to a degree characteristic of the atom, and that absorption data can be processed to not only identify the materials but determine electronic structure or chemical bonding. The resolution of the technique depends on the beam current and the spatial resolution of the e-beam probe, which is where the scanning transmission electron microscope (STEM) comes in.
The better the signal-to-noise ratio (SNR) of the system, the higher the spatial resolution of the images. SNR can be impacted by the numerical aperture (NA) of the system and by the fact that the inelastically scattered electrons can undergo secondary elastic scattering from other atoms, which sends them out of the detector plane.
The Cornell group first worked with collaborators from Nion Co. (Kirkland, Wash.) to correct the STEM for fifth-order aberrations using magnetic multipole optics, allowing them to increase the NA of the system by a factor of four over that of an uncorrected system. This, in turn, yielded a 16-fold increase in beam current. They also used a cold-field emitter instead of a Schottky emitter, for another order of magnitude increase.
The obvious solution to the elastic scattering issue was to capture scatter over a wider collection angle, but the tradeoff of increased angle is lower energy resolution. The group compromised by increasing the angle by a factor of three to six, and collimating the scattered electrons using a quartet of round coupling lenses and a corrective quadrupole/octupole module. The combination of higher NA and increased collection levels allowed them to achieve energy resolutions as high as 0.5 eV for up to 60 mrad collection angles. The finished system exhibited a signal over 100 times better than an uncorrected system.
