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|Dr. Bert Freitag
Marketing Manager Nanoresearch and Industry
There are a number of factors that limit TEM resolution, but there has been recent progress toward overcoming them. Two years ago, FEI introduced the Titan™ 80-300, the first commercially available S/TEM (combining transmission and scanning transmission modes) capable of sub-Ångström resolution. FEI continues to push the limits of TEM performance with its newest creation, the Titan3. With the broadest range of applications, the Titan3 is unquestionably the world's most powerful and flexible S/TEM.
October 29th, 2007
FEI's new Titan3—Pushing the Limits of TEM Performance
It is a bit of a chicken and egg question. Did the demand created by the nanotech revolution drive the development of the needed tools, or did the availability of tools with nanoscale capability drive the revolution? Of course, it was not one or the other, but both. In the same speech in which Richard Feynman made his famous observation that, "There is plenty of room at the bottom," arguably the kick-off event of the revolution, he also remarked on the need for a microscope a hundred times more powerful than those available at the time. It is now almost fifty years since he made that speech in 1959. How are we doing?
At the time of Feynman's speech, scientists had just broken the nanometer barrier for transmission electron microscopes (TEM). Two years ago, FEI introduced the Titan™ 80-300, the first commercially available S/TEM (combining transmission and scanning transmission modes) capable of sub-Ångström resolution. So we are better by at least ten times—halfway to the goal on an exponential scale. Let's take a look at factors that limit TEM resolution and recent progress toward overcoming them.
Light microscopists describe limits to resolution in terms of the wavelength of light, the light-gathering power of the lens system (numerical aperture), and the transmission properties of the medium (refractive index). Following the same argument, TEMs, which use electrons with wavelengths 100,000 times shorter than light, certainly ought to be able to do the job. A primary limitation for TEMs is the magnetic lenses used to focus electrons, or rather in our ability to manufacture them. Significant progress has been made in electron optics. In particular, we have improved the spherical aberration that is inherent in round magnetic lenses, and it is this ability that enables the Titan to deliver sub-Ångström resolution.
TEM scientists had long recognized the limit imposed by spherical aberration, and had even developed two distinct specifications for describing resolution. The image or point resolution is that which can be directly interpreted from an image like in astronomy, where you can resolve two stars as separate objects in the image (Rayleigh criterion), limited primarily by spherical aberration. The information limit describes the finest spatial information (highest spatial frequency) that can be transferred by the lens system. When spherical aberrations are corrected, either with physical correctors in the lens system, or through post processing of the images, the Image resolution equals the information limit.
The information limit is determined by many considerations, optical and non-optical, but the most significant (those that must be addressed to improve resolution) relate to the mechanical, electrical and environmental stability of the microscope and the chromatic aberration of the imaging lens. To return to the light microscope analogy, think of it as trying to achieve the ultimate resolution with a perfect lens held in your hand, but with the flickering light of a candle. In a TEM, trying to resolve sub-Ångström features requires extreme stability.
Aberration corrector technology has been known for years. Unfortunately, the correctors are necessarily bulky and add significant length and mass to the electron column. The great achievement of the Titan was the design of a system with the mechanical, electrical and thermal stability needed to realize the benefits of the aberration correctors. The new Titan3™ 80-300 continues this evolution by attacking the principal remaining sources of instability.
Internally, the Titan3 includes a newly-designed column and base stable enough to permit two correctors, one for TEM imaging and one for STEM probe formation, and a monochromator to reduce the energy spread of the beam, all in the same column. Researchers can now switch between TEM and STEM techniques on the same region of a sample to study the structure on the atomic level with these complementary techniques. The monochromator enables ultra high spectral resolution for EELS analysis to obtain information of the bonding and the electronic structure of the same region. The system's unique constant power lenses eliminate thermal shifts caused by changes in lens settings in other designs. Ultra stable electronics and power supplies exhibit extremely low drift, ripple and noise. A larger pole piece gap, permitted by the corrected optics, brings new flexibility to high-resolution TEM applications such as tomography, cryo TEM, environmental TEM and dynamic experiments. Field upgrades that permit the addition of correctors or a monochromator to an existing column provide budgetary flexibility as well.
Externally, the Titan3 features a specially-designed environmental enclosure ("the cube") to isolate the column from external interferences. A high-speed remote control interface removes the operator from the instrument environment, a change that will be welcomed. Less apparent is the potential cost savings that result from the addition of the enclosure—the relaxation of environmental requirements eliminates the need of costly facility upgrades.
The Titan3 continues to push the limits of TEM performance. Double correctors, monochromator, 80-300 kV accelerating voltage, larger specimen space, environmental isolation, field upgrades, and much more, extend Titan performance to the broadest range of applications and make the Titan3 unquestionably the world's most powerful and flexible S/TEM.