Scintillator-Photomultiplier Detector System

The detector most commonly used in scanning electron microscopy is the scintillator-photomultiplier syster, developed into its current configuration by Everhart and Thornley(1960). This detector operates in the following manner. An energetic electron strikes the scintillator material, which may be a doped plastic or glass target, or a compound such as CaF2 doped with europium. The electron produces photons which are conducted by a light pipe (a solid plastic or glass rod with total internal reflection) to a photomultiplier. Since the signal is now in the form of light, the signal can pass through a quartz window into a photomultiplier which is permanently isolated from the vacuum of the SEM. The photons strike the first electrode causing it to emit electrons, which then cascade through the remaining electrode stages eventually producing an output pulse of electrons with a gain of 10⁵ to 10⁶. This gain is obtained with very little noise degradation and a wide frequency bandwidth. In order to make use of the low-energy secondary electron signal, the scintillator is covered with a thin (10-50nm) layer of aluminum and biased to approximately +10kV, which serves to accelerate the low-energy electrons. Note that for typical beam energies, e.g., 20keV, most of the backscattered electrons can excite the scintillator without the aid of the high-voltage bias. In order to prevent the 10kV bias from displacing the incident beam or introducing astigmatism, the biased scintillator is surrounded by a Faraday cage near ground potential. The Faraday cage has a mesh opening to permit the entrance of electrons. To improve the collection of secondary electrons, a positive potential of as much as +300 V can be placed on the cage. This voltage will not cause significant degradation of the incident beam. To reject secondary electrons, the potential on the Faraday cage can be set to -50 V or else the high voltage on the scintillator can be removed.

The take-off angle is another factor need to be taken into account. Because of the cosine angular distribution over which the backscattered electrons are emitted at normal beam incidence, at low take-off angles only a small fraction of the backscattered electrons are collected. For a typical SEM scintillator-photomultiplier detector, the take-off angle for backscattered electrons is on the order of 30°, and the solid angle of collection is about 0.05 sr (1 cm diameter scintillator, 4 cm from specimen).

The positive bias on the Faraday cage causes a substantial deviation of the secondary electron trajectories toward the detector. From a flat surface, the collection efficiency can approach 100%. Only by applying a negative biase to the Faraday cage can the secondary electrons be rejected from the scintillator.