1. Field of the Invention
This invention relates to charged particle beam systems and more particularly to a detector for detecting back scattered and secondary particles in such a system.
2. Description of the Prior Art
Electron beam lithography and inspection systems, scanning electron microscopes and other particle optics systems usually use a magnetic objective "lens" to focus a charged particle beam upon a substrate. Magnetic deflection systems are often used to position (deflect) these beams on the substrate. A static magnetic lens focus field and time varying magnetic deflection field are often superimposed and located as near as possible to the substrate in order to minimize axial lens aberrations and deflection distortions.
The same particle optics systems often include annular shaped detectors that detect secondary particles and x-rays coming from the substrate area irradiated by the primary beam. In conjunction with a raster or vector scanned particle beam, these secondary particles are used to form images or to identify the composition of the substrate. In the case of electron beam lithography, inspection or ion beam milling tools, secondary or backscattered electron or ion detected images are also used for registering the beam position with respect to the substrate. To maximize these detection signals, it is important for the detector to subtend a large solid angle when viewed from the substrate. Thus the detector as well as the lens and deflection coils should be close to the substrate.
Thus the region between an objective lens/deflection system and substrate must include lens, deflection, and detector elements. It is particularly desirable that the lens, deflection and detection functions are superimposed to minimize the space that they occupy.
Annular shaped solid state detectors are known in particle beam systems. Sometimes it is necessary to separately record signals from different azimuthal angles to gain more information about the substrate geometry, so such annular detectors with several individual detectors (active areas) on a single detector substrate are also used. Typically such a detectors consist of planar diode structures on a thin substrate of high purity doped silicon. Each detector segment (active area) is a thin doped layer covered with an even thinner conductive coating. A bias voltage is applied to the opposing back surface, creating a depletion layer in the silicon. High energy particles penetrate into a depletion region, forming many electron hole pairs that, when extracted, are an amplified version of the incident current signal.