PRACTICAL ADVANTAGES OF A CASCADE DIFFUSION PUMP SYSTEM OF A SCANNING ELECTRON-MICROSCOPE

A cascade diffusion pump (DP) system shows a superior performance in dynamic evacuation when switching from roughing to fine pumping, as well as in clean static evacuation. This is particularly true when the DPs have a relatively low tolerable forepressure. Moreover, a cascade DP system shows stable performance during variations of the DP heater voltage. A scanning electron microscope (SEM) with a large specimen chamber was evacuated by a cascade DP system with a buffer tank. With a cascade DP system, the requirements for the vacuum system of a SEM, such as rapid pumping down and a clean vacuum, were almost completely satisfied. This report presents experimental data on the cascade DP system of a SEM, which shows practical advantages over the single DP system.