Mirror Electron Microscopy for Inspection of Flat Surfaces
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1088-W04-02
Mirror Electron Microscopy for Inspection of Flat Surfaces Masaki Hasegawa, and Tomokazu Shimakura Advanced Technology Research Department, Hitachi, Ltd., Central Research Laboratory, 1-280, Higashi-koigakubo Kokubunji-shi, Tokyo, 185-8601, Japan ABSTRACT We have developed a mirror electron microscope (MEM) for inspecting fine processed flat surfaces. This apparatus can detect nanometer-sized particles and scratches on very flat surfaces. Its sensitivity for electric charge distribution is useful for detecting failures in microdevice patterns. In observation of an insulator surface with the MEM, complex features caused by small amount of charge distribution were observed in the images. We believe that the MEM can provide different methods for characterization of electrical behavior on insulator surfaces. INTRODUCTION The mirror electron microscope (MEM) was developed in the early stage of the development history of electron microscopes [1]. Its low spatial resolution and its image property, which is far different from a real surface structure, prevent it from being widely used for surface characterization [2]. In the MEM, the sample has a slightly more negative bias than the accelerating voltage of the illuminating electrons. The incoming electrons are decelerated near the sample surface and return at a potential surface where the kinetic energy of the electrons is zero. The MEM produces images of the returned electrons on a screen through an electrical optical system. The images describe the topographical structure of the electrical potential surfaces. If a completely flat sample surface is illuminated by a perfectly parallel electron beam, the reflected electrons are also parallel. If there is a tiny protrusion on the sample, the equipotential surface is slightly distorted and deflects the electrons. The reflected electron trajectories come close to each other far from the surface and can form caustics. Thus, the MEM can provide a highly sensitive method for detecting ‘singularities’ such as particles and scratches on the flat surface. Flat surfaces are important in the recent manufacturing of various high-tech devices. For example, in manufacturing hard disk drives, protrusions higher than 10 nm on magnetic disks are not permitted. That is because the gap between the magnetic head and the magnetic disk has been continuously decreasing with increasing storage capacity. At present, it is at about 10 nm. In semiconductor device manufacturing, electrical circuit patterns are made on a silicon wafer with an extremely flat surface. Shrinkage of the minimum feature size of the patterns requires such a clean surface that about 20-nm sized particles on the wafer may lead to severe circuit failure. Because these singularities have to be detected within a very short period to maintain high-yield production, high-resolution scanning microscopy for inspection is not adequate. Recent work has been done by using MEM in a low energy electron microscope (LEEM) [3] which can acquire a large field images simultaneously
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