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Searching Ultimate Nanometrology for AlOx Thickness in Magnetic Tunnel Junction by Analytical Electron Microscopy and X-ray Reflectometry

Published online by Cambridge University Press:  28 September 2005

Se Ahn Song ,
Tatsumi Hirano ,
Jong Bong Park ,
Kazutoshi Kaji ,
Ki Hong Kim  and
Shohei Terada
Se Ahn Song
Affiliation:
Samsung Advanced Institute of Technology (SAIT), P.O. Box 111, Suwon, 440-600, Korea
Tatsumi Hirano
Affiliation:
Hitachi Research Lab. (HRL), Hitachi Ltd., Oomika-cho 7-1-1, Hitachi-shi, Ibaraki 319-1292, Japan
Jong Bong Park
Affiliation:
Samsung Advanced Institute of Technology (SAIT), P.O. Box 111, Suwon, 440-600, Korea
Kazutoshi Kaji
Affiliation:
Hitachi Research Lab. (HRL), Hitachi Ltd., Oomika-cho 7-1-1, Hitachi-shi, Ibaraki 319-1292, Japan
Ki Hong Kim
Affiliation:
Samsung Advanced Institute of Technology (SAIT), P.O. Box 111, Suwon, 440-600, Korea
Shohei Terada
Affiliation:
Hitachi Research Lab. (HRL), Hitachi Ltd., Oomika-cho 7-1-1, Hitachi-shi, Ibaraki 319-1292, Japan
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Abstract

Practical analyses of the structures of ultrathin multilayers in tunneling magneto resistance (TMR) and Magnetic Random Access Memory (MRAM) devices have been a challenging task because layers are very thin, just 1–2 nm thick. Particularly, the thinness (∼1 nm) and chemical properties of the AlOx barrier layer are critical to its magnetic tunneling property. We focused on evaluating the current TEM analytical methods by measuring the thickness and composition of an AlOx layer using several TEM instruments, that is, a round robin test, and cross-checked the thickness results with an X-ray reflectometry (XRR) method. The thickness measured by using HRTEM, HAADF-STEM, and zero-loss images was 1.1 nm, which agreed with the results from the XRR method. On the other hand, TEM-EELS measurements showed 1.8 nm for an oxygen 2D-EELS image and 3.0 nm for an oxygen spatially resolved EELS image, whereas the STEM-EDS line profile showed 2.5 nm in thickness. However, after improving the TEM-EELS measurements by acquiring time-resolved images, the measured thickness of the AlOx layer was improved from 1.8 nm to 1.4 nm for the oxygen 2D-EELS image and from 3.0 nm to 2.0 nm for the spatially resolved EELS image, respectively. Also the observed thickness from the EDS line profile was improved to 1.4 nm after more careful optimization of the experimental parameters. We found that EELS and EDS of one-dimensional line scans or two-dimensional elemental mapping gave a larger AlOx thickness even though much care was taken. The reasons for larger measured values can be found from several factors such as sample drift, beam damage, probe size, beam delocalization, and multiple scattering for the EDS images, and chromatic aberration, diffraction limit due to the aperture, delocalization, alignment between layered direction in samples, and energy dispersion direction in the EELS instrument for EELS images. In the case of STEM-EDS mapping with focused nanoprobes, it is always necessary to reduce beam damage and sample drift while trying to maintain the signal-to-noise (S/N) ratio as high as possible. Also we confirmed that the time-resolved TEM-EELS acquisition technique improves S/N ratios of elemental maps without blurring the images.

Keywords

microscopic methodsspecifically for solid interfaces and multilayersnanometrologymagnetic tunnel junctionanalytical electron microscopyX-ray reflectometry
Type
Special Issue: Frontiers of Electron Microscopy in Materials Science
Information
Microscopy and Microanalysis , Volume 11 , Issue 5 , October 2005 , pp. 431 - 445
Copyright
© 2005 Microscopy Society of America

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