Low Energy Electron Microscopy for Semiconductor Applications
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1088-W03-01
Low Energy Electron Microscopy for Semiconductor Applications Marian Mankos, Vassil Spasov, Liqun Han, Shinichi Kojima, Ximan Jiang, Salam Harb, Luca Grella, and Cory Czarnik EBI, KLA-Tencor, 160 Rio Robles, San Jose, CA, 95134 ABSTRACT A novel low energy electron microscope (LEEM) aimed at improving the throughput and extending the applications for semiconductor devices has been developed. A dual beam approach, where two beams with different landing energies illuminate the field of view, is used to mitigate the charging effects when the LEEM is used to image semiconductor substrates with insulating or composite (insulator, semiconductor, metal) surfaces. We have experimentally demonstrated this phenomenon by imaging a variety of semiconductor device wafers without deleterious charging effects. Results from several important semiconductor device layers will be illustrated in detail.
INTRODUCTION The continuing trend toward smaller features in the semiconductor industry poses a formidable problem for scanning electron beam tools due to their relatively low throughput. As the pixel size gets smaller, the number of pixels to be examined on a wafer rises, resulting in inspection times that exceed practical limits. One of the possible approaches to circumvent this problem is to replace the serial acquisition process of scanning electron microscopes with a parallel scheme, where all the image pixels of interest on the surface are acquired in parallel on a scintillating screen and further processed on a computer. A LEEM [1,2], optimized for high throughput, i.e. large beam currents and field sizes, is ideally suited for this application. In a LEEM, a flood beam illuminates the sample with electrons having energies ranging from a few hundred eV to near zero eV, depending on the substrate bias. When the wafer is biased positively with respect to the electron source, the electrons scatter at or near the surface, and either reflect back from the sample, undergoing low energy electron diffraction, or generate secondary electrons, provided the bias and therefore kinetic energy of the illuminating electrons is large enough (few tens to hundreds eV). When the substrate is biased slightly negative (~1V) with respect to the electron source, the illuminating electrons are reflected above the surface, without hitting the surface. This imaging mode is also known as mirror electron microscopy (MEM). However, when a LEEM instrument is used to image substrates with a variety of insulating materials at the surface, the imbalance of the electron flux results in charging effects that can significantly reduce the imaging quality. In this paper we show how the need for high throughput and imaging of insulating surfaces impacts the electron-optical design of a LEEM and present experimental results from several types of substrates used in the semiconductor industry.
A HIGH THROUGHPUT, DUAL BEAM LEEM A LEEM suitable for semiconductor applications must be capable of imaging insulating surfaces at high throughput. These requirements
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