How will the project be put together?
Components:
The major new components of the TEAM instrument are the aberration corrector for the probe, the sample stage, and the post-specimen corrector. These components will be mounted on a new electron column specifically designed to be compatible with aberration-corrected optics.
The probe corrector, the stage and the post-specimen corrector will all be designed, tested and optimized in different locations and on separate dedicated microscope columns before implementing the results of these tests on the TEAM instrument.
The probe corrector builds on a hexapole design, further developed to meet the increased demands for electronic stability and improved correction of higher order aberrations.
The stage provides five-axis (α, β, x, y, z) sample control, using a concept that is substantially different from the side entry model employed in almost all existing electron microscopes. It consists of two piezo-controlled modules without mechanical links to the atmosphere. The design uses proven scanning tunneling microscope technology to achieve high-precision positioning, effective control of vibration and drift, and accurate position sensing.
The post-specimen corrector is designed to correct both spherical and chromatic aberrations. The device includes two Wien filters made of crossed electric and magnetic quadrupoles and will use symmetry planes for self-cancellation of undesirable aberrations.
Making all the elements work together to provide the required performance has emerged as a key issue in the development of most advanced electron scattering instruments. TEAM will focus closely on column integration and optimization. Integration of all components into a fully functional assembly will be the final step of the TEAM construction project.
Aberration Correction
Electron optical lenses suffer from aberrations similar to those in magnifying glasses or telescopes. Spherical aberration causes an image to be blurred because points are imaged as discs. Chromatic aberration leads to rainbow distortions at the edge of an image because light of different color is refracted at different angles. These two aberrations are illustrated schematically in ray diagrams.
Left: spherical aberration. Right: chromatic aberration.
The same aberrations afflict optical microscopes and even the Hubble space telescope. The difference in an image of outer space taken with the Hubble space telescope before and after correction of spherical aberration shows the increase in resolution and sharpness of the image.
Image of the same region of space taken with the Hubble Space Telescope before and after correction of spherical aberration.
Electromagnetic lenses in an electron microscope are subject to chromatic, spherical and higher-order aberrations. The resolution limit of electron microscopes is described by the Scherzer theorem, which states that chromatic aberration (Cc) and spherical aberration (Cs) are unavoidable for static, rotationally-symmetric electron lenses. As a result, the resolution of conventional electron microscopes is limited to about 100 times the wavelength of the electron.
To surpass this limit is possible only by lifting one of the constraints of the Scherzer theorem, for example by deviating from rotational symmetry. This can be done with multipole elements (quadrupoles, hexapoles, octopoles etc.) used as correctors, much like eye glasses are used as correctors for astigmatism or short-sightedness in the human eye.
Two types of correctors, the hexapole corrector and the quadrupole-octopole corrector, have been proven successful in compensating for spherical aberration. A quadrupole-octopole corrector can also be the basis for a chromatic aberration corrector.
The information limit of a microscope results from mechanical and electromagnetic instabilities. Recent technological advances make it possible to improve mechanical stability by increasing the column’s mechanical stiffness, and to reduce electromagnetic instabilities by stabilizing the fields to an accuracy of about 100 parts per billion. These measures will extend the information limit beyond 0.05 nanometer. To reach the goal of raising the resolution limit to the information limit requires mechanical alignment of the numerous multipole elements via a computer-assisted iterative alignment procedure. The development of rapid autotuning procedures to measure and then correct misalignments and aberrations will be an important step in meeting the challenges of the TEAM project.