16 July 2002 Low-voltage-point source microscope for interferometry
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Abstract
Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we have designed, and are presently testing, a low voltage point source microscope operated with a nanotip field emitter and without any electron optical lenses. The microscope is designed such that can be operated in the transmission mode as well as in a reflection mode. The ultra-sharp field emitter delivers emission currents of several nanoamps at energies less than 100 eV. The magnification of the object wave is achieved by placing the specimen in the divergent electron beam from the nanotip and observing the object wave using a microchannel plate (MCP) at a great distance from the sample. Images obtained that way are out of focus images. As no lenses are present a special procedure for scaling the magnification has been developed. Since electrons from a point source are highly coherent the out of focus images of the sample are interferograms. Electrons diffracted at an edge of the specimen cause Fresnel fringes in the image plane. An electrically charged holey carbon foil acts in the same way on the electrons as the Youngs double slit experiment and results in an interference pattern consisting of parallel fringes. A comparison between the transmission mode and the reflection mode shows great similarities with respect to the magnification and the interference pattern. An electron gun needed in the transmission mode is the most important difference between the two modes of operation. The experimental results at a reflection of 45 degrees are in good agreement with our simulation. Following our simulations a reflection angle of 90 degrees is most promising for easiest image interpretation.
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Bernhard G. Frost, Bernhard G. Frost, David C. Joy, David C. Joy, Alexander Thesen, Alexander Thesen, } "Low-voltage-point source microscope for interferometry", Proc. SPIE 4689, Metrology, Inspection, and Process Control for Microlithography XVI, (16 July 2002); doi: 10.1117/12.473498; https://doi.org/10.1117/12.473498
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