Secondary electron energy filtration: methods and applications

The Secondary Electron signal is traditionally used in Scanning Electron Microscopy to provide topographical information from the surface of a sample. The physics of Secondary Electron signal generation and emission is dependent on further mechanisms, beyond surface topography, that will alter the energy profile of the generated and emitted Secondary Electrons. Selectively viewing portions of the Secondary Electron’s spectrum of energies can be used to reveal hidden contrast mechanisms and may be used to image voltage differences and chemical contrasts. This imaging method has gained some traction in recent years but is not a commonly advertised technique by microscope manufactures. This work uses the mirror electrodes housed within the columns of an FEI Helios and a JEOL 7800F in conjunction with their Secondary Electron In-column detectors to understand how these two systems can be used to gather Secondary Electron energy spectra and energy filtered Secondary Electron images. The tests seek to reveal the intricacies of Secondary Electron energy filtering by this native method and determine whether these systems can be used for quantitative Secondary Electron energy filtration applications. The results indicate that the mirror electrodes in the FEI Helios and JEOL 7800F operate with similar energy filtration mechanisms but with different levels of reliability and precision. Both systems show multiple regions across their range of filter voltage in which a Secondary Electron spectrum can be collected. After experimentally calibrating the filter voltage to Secondary Electron energy, it is found that within the positive range of filter voltages the FEI Helios is capable of producing reliable Secondary Electron spectrum with a 1.0 eV energy resolution. The same tests for the JEOL 7800F reveal that Secondary Electron energy filtration is not statistically reliable within a 95% confidence interval and has a coarse energy resolution, estimated at 3.2 eV. However, calibration by experimental means is shown to be an unreliable way of calibrating the mirror electrodes in both systems. These results extend the list of reasons why Secondary Electron energy filtration is not a frequently used technique and why the applications attempted within this work, namely: PN contrast in a Cr2O3 Oxide; graphene layer work function quantification; and Secondary Electron chemical contrast are only somewhat achievable, with restricted usability and precision.