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Journal of Electron Microscopy 47(4): 335-343 (1998)
© 1998 Oxford University Press

EDS elemental mapping of a DRAM with an FE-TEM

Masahiro Kawasaki1, Tetsuo Oikawa1, Katsuhiko Ibe1, Kyung-ho Park2 and Makoto Shiojiri3

1JEOL Ltd Akishima, Tokyo 196–8558, Japan
2Texas Instruments Inc. 13570 North Central Expressway, MS3740, Dallas, TX 75243, USA
3Kyoto Institute of Technology Matsugasaki, Sakyo, Kyoto 606–8585, Japan

The elemental distribution in an O-N-O dielectric multi-layer within a dynamic random access memory (DRAM) device was investigated by X-ray elemental mapping using a field-emission (scanning) transmission electron microscope (FE-(S)TEM). The O-N-O layer is designed to be an insulator between a single-crystal silicon substrate and a poly-crystalline silicon in a capacitor region. The nominal structure of the layer is SiO2-Si3N4-SiO2. A small amount of phosphorus is doped in the poly-crystalline silicon to make the poly-crystalline silicon conductive. Transmission electron microscopy (TEM) was first used to view the microstructure in the layer. A bright field image gave clear contrast in the O-N-O layer, however, TEM images do not provide direct information about the elemental distribution. X-ray mapping was thus used to show this distribution. Initial observation of the oxygen map and the nitrogen map confirmed that the layer composition was not constructed as fabricated but oxygen and nitrogen migrated into each other layer. The phosphorus map showed phosphorus segregation along the interface between the poly-crystalline silicon and the neighbouring oxygen layer, which is of importance to understand the device electrical property. This phosphorus layer was not visible in the TEM image. Since the elemental distributions across the structure were presumed to be uniform, X-ray intensity profiles of Si, O, N and P could be obtained by compressing the original X-ray images. The X-ray intensity of each column pixel was sufficient to compute the concentration and thus elemental concentration line profiles were obtained to characterize the structure quantitatively from the previously obtained elemental maps.

Keywords     EDS, elemental mapping, DRAM, FE-(S)TEM, quantitative analysis

Received     26 November 1997, accepted 21 April 1998


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