Scintillator-coupled cameras and direct detection cameras, also known as direct electron detectors, are both widely used in transmission electron microscopy (TEM) today. Scintillator-coupled cameras typically have a lower cost, and are widely used as general “workhorse” cameras. Direct detectors generally have a higher cost, but also have a higher sensitivity and detective quantum efficiency (DQE), and are therefore used to perform challenging experiments that are beyond the capabilities of a scintillator-coupled camera.
Scintillator-Coupled TEM Cameras
Scintillator-coupled cameras were introduced to TEM from the 1980s onwards. Initially, these were charge coupled device (CCD) cameras, although, over the past 20 years, complementary metal oxide semiconductor (CMOS) based scintillator-coupled cameras have become more commonly available for TEM. The sensors employed in typical CCD and CMOS detectors can easily be damaged upon direct exposure to an electron beam, so instead electrons from the beam are incident on a scintillator material such as yttrium aluminium garnet in which they are converted to photons. These photons are then transferred via a fibre optic plate to a sensor, which detects the photons. Although this design protects the sensor from radiation damage, it introduces fundamental inefficiencies. Conversion of electrons to light in the scintillator is not 100% efficient, and much energy is lost as heat or scattered photons. Losses also occur in the fibre optic coupling due to imperfect alignment, reflection losses, and a limited range of angles over which light can enter the fibre and be successfully guided to the sensor.
The images show a silicon surface, collected in PEEM mode. The integration time was 300 seconds. The circle marks the same area in both images.
The LV-16 was operated with 2x binning so that both images are 2k x 2k pixels.
Courtesy of Rudolf Tromp, (IBM, Yorktown Heights, NY, USA).
Direct Detection TEM Cameras
The original motivation for introducing scintillator-coupled cameras to TEM was that they offered higher throughput, greater ease of use, and greater dynamic range than the previous technology used to record images – photographic film. However, a significant disadvantage of scintillator coupled detectors was their low DQE relative to film. This drove interest in the development of direct detection cameras, which retain the advantages of scintillator-coupled detectors, whilst having a significantly higher DQE. Unlike scintillator-coupled cameras, direct detection cameras have radiation hardened sensors, which can be directly exposed to the electron beam. This eliminates inefficiencies resulting from electron to photon conversion and photon transport along a fibre coupling, allowing direct detectors to achieve a DQE equaling or surpassing that of film.
With a higher DQE, direct detectors are able to record images with a higher signal to noise ratio (SNR) and resolution at a given beam dose than scintillator-coupled cameras. The adoption of direct detectors has allowed microscopists to capture sharper, more detailed images of specimens at lower beam doses, which has been crucial for the study of a wide range of specimens. This is especially true of radiation sensitive specimens such as biological specimens, battery materials, polymers, and metal organic frameworks, that can damage rapidly when imaged at the higher dose levels required by a scintillator-coupled camera.
