Abstract:
A significant loss in electron probe current can occur before the electron beam enters the specimen
chamber of an environmental scanning electron microscope (ESEM). This loss results from electron scattering
in a gaseous jet formed inside and downstream (above) the pressure-limiting aperture (PLA), which separates
the high-pressure and high-vacuum regions of the microscope. The electron beam loss above the PLA has been
calculated for three different ESEMs, each with a different PLA geometry: an ElectroScan E3, a Philips XL30
ESEM, and a prototype instrument. The mass thickness of gas above the PLA in each case has been determined
by Monte Carlo simulation of the gas density variation in the gas jet. It has been found that the PLA
configurations used in the commercial instruments produce considerable loss in the electron probe current that
dramatically degrades their performance at high chamber pressure and low accelerating voltage. These detrimental
effects are minimized in the prototype instrument, which has an optimized thin-foil PLA design.
