Direct Comparison of Various Gaseous Secondary Electron Detectors in the Variable Pressure Scanning Electron Microscope

Abstract

The conventional Everhart-Thornely scintillation-photomultiplier secondary electron (SE) detector cannot function at elevated pressures due to the high voltage (~ +12kV) involved in its operation. As a result, SE imaging in the variable pressure scanning electron microscope (VPSEM) has required the development of a new generation of SE detectors that operate under low vacuum conditions. To date, three different methods have been devised to measure the secondary electron (SE) emission signal in a VPSEM. Each of these approaches involves the excitation of the chamber gas by the placement of a low voltage (< +1000V) positively biased electrode in the vicinity of the specimen. A SE image can be obtained by measuring the current induced in either the positive electrode (the gaseous secondary electron detector) or the grounded stage (the ion current detector) or via a photomultiplier that detects light emission from the gas (the gas luminescence detector). In this work, the performance of each of these three low vacuum SE detector types has been compared under identical operating conditions using a Zeiss Supra 55VPSEM and FEI XL30 ESEM.