STEM detector serves up structural detail


Rebecca Pool

Wednesday, April 5, 2017 - 13:00
Image of pixel array detector pioneers: Professor Sol Gruner, left, and Professor David Muller from Cornell.
US-based physicists have developed a pixel array detector to boost the performance of electron microscopy.
Now licensed by FEI, part of Thermo Fisher Scientific, the electron microscope pixel array detector (EMPAD) provides a wealth of information about the electrons that create the image, delivering extraordinary detail on sample structure.
“We can extract local strains, tilts, rotations, polarity and even electric and magnetic fields,” explains Professor David Muller, from applied and engineering physics at Cornell.
EMPAD comprises a 128x128 array of electron-sensitive pixels, bonded to an integrated circuit.
As Muller highlights: "The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit."
"The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity," he adds.
A STEM, left, fires an electron beam through a sample, scanning back and forth to produce an image. The Pixel Array Detector, right, reads the landing point and from that the scattering angle of each electron, giving information about the atomic structure of the sample.
Designed to detect the angles at which electrons emerge as each charge hits a different pixel, EMPAD is a spinoff of the X-ray detectors physicists built for X-ray crystallography at the Cornell High Energy Synchrotron Source (CHESS).
Combined with the focused beam of the electron microscope, the detector allows researchers to build up a “four-dimensional” map of both position and momentum of the electrons passing through a sample, to reveal the atomic structure and forces inside.
"By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyse the full scattering distribution, allowing true centre of mass imaging," says Muller. 
According to the researchers, EMPAD is unusual in its speed, sensitivity and wide range of intensities it can record; from detecting a single electron to intense beams containing hundreds of thousands or even a million electrons.
“The EMPAD records an image frame in less than a millisecond and can detect from one to a million primary electrons per pixel, per image frame,” explains Muller. “This is 1000 times the dynamic range, and 100 times the speed of conventional electron image sensors.”
“Now we can get a better look at processes inside intact cells,” adds Professor Lena Kourkoutis, also from applied and engineering physics.
The low dose of radiation allows multiple exposures, to take time-lapse “movies” of cellular processes or to view the same specimen from different angles to get a clearer 3-D image. 
Kourkoutis plans to use these techniques in work with the new Cornell Center for the Physics of Cancer Metabolism, looking at how cancer progresses from cell to cell.
The researchers tested their first EMPAD by installing it in a spare port in a state-of-the-art FEI microscope. The prototype is now used intensively for experiments in the Cornell Center for Materials Research.
FEI expects to complete the commercialisation of the design and offer the detector for new and retrofitted electron microscopes this year.
Research is published in Microscopy and Microanalysis.
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