Low-Profile Microchannel-Plate Electron Detector System for SEM

Abstract

One main impetus of present-day scanning electron microscopy is in the low accelerating voltage mode. This mode of operation is useful for nondestructive inspection especially in the on-line inspection and metrology of semiconductor samples. Today, the majority of the scanning electron microscopes used in nondestructive inspection utilize the standard Ever-hart/Thornley (E/T) detector or a modification of this detector as the main detection system. The E/T detector although extremely efficient, suffers from poor signal to noise ratio at low accelerating voltages. This type of detector also suffers from alignment difficulties especially where linewidth measurement for semiconductor applications is concerned because of the uneven distribution of the collection field which is possible, especially if the detector is not located in a plane of symmetry of the specimen and electron beam. These limitations and others have recently led investigators to reconsider the design of secondary electron detection systems especially for low accelerating voltage and metrological applications.A unique situation developed at the National Institute of Standards and Technology (NIST) where a high efficiency electron detector with an exceptionally low profile was needed for the highly customized scanning electron microscope based metrology instrument under development. This instrument incorporates a large laser interferometer stage in the sample chamber which is used for traceability to the national length standard. The large interferometer stage and the restrictive size and shape of the specimen chamber required the development of a lower-profile electron detector than the standard E/T detector and one which does not interfere with the stage motions and interferometry. It was decided that, since the microchan-nel-plate-type electron detector fulfilled the fundamental requirements imposed by the space restrictions, and since this type of detector is also highly efficient at low accelerating voltages, this type of detector system would be used.