Transmission electron microscopes primarily employ indirect cameras (IDC) for electron detection in imaging, diffraction and EELS modes. Such cameras convert incident electrons to photons which, through a fiber optic network or lens, are coupled to a light sensitive camera. This indirect detection method typically has a negative impact on the point spread function (PSF) and detective quantum efficiency (DQE) of the camera. Over the last decade, radiation tolerant CMOS active pixel sensors, which directly detect high-energy incident electrons and have the speed to count individual electrons events, have been developed. These detectors result in greatly improved PSF and DQE in comparison to conventional IDCs. Such direct detection cameras (DDCs) have revolutionized the cryo-TEM field as well as have strong advantages for in-situ TEM in both imaging and diffraction applications. EELS applications can benefit from the improved PSF and the ability to count electrons. The improved PSF allows spectra to be acquired over larger energy ranges while maintaining sharp features and greatly reduced spectral tails. The ability to count electrons nearly eliminates the noise associated with detector readout and greatly reduces the proportional noise associated with detector gain variations. This effectively leaves the shot noise as the limiting noise source present. The implication for EELS acquisition is that fine structure analysis becomes more straightforward for typical conditions and even possible for the case of low signal levels.
Very high-energy edges above 3000eV have always been very hard or almost impossible to acquire using EELS due to the very limited amount of signal. With the introduction of DD detectors the amount of noise has been enormously reduced and as result low intensity signals can now be observed and detected. EELS spectra of Cu K and Ni K-edges at about 9keV and 8.3keV can now be collected and easily observed and the quality is such that high contrast elemental maps can be generated. Until now, such high energy edges have been collected using synchrotron based techniques such as XAS with very limited spatial resolution. Now, by acquiring EELS data in counting mode using DD detectors, high energy edges can be collected and their signal mapped out with high spatial resolution. A new world is about to open up.
In this presentation, we will review the current state of electrons counting detectors for electron microscopy with an emphasis on system for EELS measurements.