Secondary Electron Count Imaging in a Scanning Electron Microscope*

We have implemented secondary electron (SE) count imaging on a Zeiss LEO 1525 scanning electron microscope (SEM). We established the signal level of single SEs using two methods – image histograms and oscilloscope outcoupling. In both methods, we imaged a uniform sample of aluminum at low beam currents (< 5 pA) and pixel dwell times between 500 ns and 7 μs using both the in-chamber and in-lens SE detectors. In the first method, histograms of the sample images showed distinct peaks corresponding to the number of SEs emitted from each pixel. We used these peaks to establish the single SE signal level and calculated the detective quantum efficiency of our detectors. In the second method, we outcoupled the signal from our SE detectors to a 2 GHz oscilloscope. The output signal trace showed a sequence of pulses of mean duration 180 ns, corresponding to single SEs. Histograms of the total area of the output signal matched the image histograms for the same pixel dwell time. Finally, we synchronized data capture on the oscilloscope with the SEM scan signal to generate SE count images, which showed an improved signal-to-noise ratio compared to conventional images.