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6.1.2 Scanning Electron Microscopy
Optical microscopes quickly reach their limitations if the feature sizes drop
into the nm range.
Things like step coverage or the quality of the insulating
barrier or even surface roughness at more detailed levels cannot be resolved with
an optical microscope. The reason for this is the finite wavelength of light. This
finite wavelength gives photons a “size” of several hundred nanometers, making it
impossible to resolve features smaller than this size without distortion to the image
due to interference effect. To circumvent this limit, a different type of microscope
can be used that is known as an SEM (Scanning Electron Microscope). As the
name suggests, this microscope does not use optical photons to probe the surface,
but a beam of electrons that is reflected off the sample. The small size of the
electrons allows for magnifications up to 250, 000×, which is sufficient to, for
example, look at the profile of the 2 nm oxide barrier inside the qubit junction.
The use of electrons requires the sample to be conductive so that it can reflect the
electrons efficiently. Commonly, non-conductive samples are therefore first covered
with a thin layer of gold. The images generated by the SEM can naturally not
preserve the color of the sample.
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