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ETP Semra Pty. Ltd. |
***UNDER CONSTRUCTION***
In a scanning electron microscope (SEM), the performance of any detector is directly related to its signal at a particular accelerating voltage, provided the gain is constant for all detectors. Figure 1 below shows a comparison of detectors at different accelerating voltages, for the same beam current and same gain conditions. (In other words all measurements were performed with the same photo multiplier tube (PMT) and applied PMT voltage.)
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Figure 1: Plot of signal from a detector versus accelerating voltage, for a range of detectors, using the same beam current, PMT and voltage and working distance.
Curve 1 shows the signal response curve for the standard series 5 Robinson Detector. These detectors had approximately the same signal to noise as the secondary electron detector, for a 15 kV beam of electrons vertically incident upon a polished metal, eg copper, stub at a working distance of 10 mm. The Series 5 detector used a 19 mm diameter PMT and the surface of the scintillator was coated with Å 5 - 7 nm of aluminium. Curve 2 shows the results obtained by using a 28 mm Ø PMT, larger scintillator design, but other wise everything else the same. Note that both curves 1 and 2 show very little signal below Å 3 kV. Curve 3 shows the detector in curve 2, with a thinner aluminium coating, Å 1 - 2 nm. This signal drops to very low below Å 2 kV. Curve 4 shows the same material with no surface coating. Note that the signal is linear at accelerating voltages down to Å 2 kV and then drops off below that level.
Previous studies suggest that the performance of YAG type BSE detectors is similar to that shown in curve 4, but that should be independently verified by studies from a YAG detector to see when the shape of the curve varies from linear. Curve 5 shows the results achieved with a new style of detector, using a different scintillation material. It clearly shows the superiority of this material over the old scintillation material used in the Series 5 Robinson detectors.
Theoretical studies of the performance of back scattered electron detectors show that their signal increases with a decrease in the value of any surface work function of the material. The new material did not have any surface coating, significant increases were not expected to be able to be made. However the possibility of surface activation of the material existed and was applied to the material on the completed scintillator. The results, curve 6, show that surface activation of the material produces significantly higher signal than that produced by the normal material. It is the material in curve 6 which constitutes the signal used in the new Series 6 Robinson detectors.
The result of continued research by Dr Robinson and ETP Semra Pty Ltd is still able to give you the highest performance BSE detector available. These results are a signal which is typically an order of magnitude above that of the Series 5 Robinson detector. The curve is so strong that the BSE signal from the Series 5 Robinson detector often has a signal to noise greater than the SE signal at accelerating voltages above 5 kV. Do not be surprised if your BSE image shows much more detail at accelerating voltages above 10 - 15 kV than the SE image.
To show you a result of the new series 6 detector, Figure 2 shows BSE (a) and an SE (b) images, field emission microscope, 1 kV image, original magnification of x 50,000, of gold on carbon. The high resolution of the BSE image is clearly evident. Note that the edge signal associated with the SE image is not evident in the BSE image, clearly indicating that the BSE image is a BSE image and not an SE image.
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The Series 6 Robinson Detector takes you into the Low kV BSE imaging field like no other BSE detector. Typical operating conditions can be 10 pA at 5 kV, with images being able to be formed at beam accelerating voltages below 800 volts.
The Series 6 Robinson detector provides exceptional signal to noise BSE images at all accelerating voltages. It has the uniform illumination charcteristics of the solid state detector, the highest signal to noise of any BSE detector at all accelerating voltages, a topo mode and a thickness of less than 5 mm. It is the ideal BSE detector.
(a) BSE Image