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Journal of Electron Microscopy Advance Access originally published online on January 28, 2008
Journal of Electron Microscopy 2008 57(2):47-52; doi:10.1093/jmicro/dfm041
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Back-etch method for plan view transmission electron microscopy sample preparation of optically opaque films

Bo Yao* and Kevin R. Coffey

Advanced Materials Processing and Analysis Center, and Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA

* To whom correspondence should be addressed. E-mail: bo555252{at}pegasus.cc.ucf.edu

Back-etch methods have been widely used to prepare plan view transmission electron microscopy (TEM) samples of thin films on membranes by removal of the Si substrate below the membrane by backside etching. The conventional means to determine when to stop the etch process is to observe the color of the light transmitted through the sample, which is sensitive to the remaining Si thickness. However, most metallic films thicker than 75 nm are opaque, and there is no detectable color change prior to film perforation. In this paper, a back-etch method based on the observation of an abrupt change of optical reflection contrast is introduced as a means to determine the etch endpoint to prepare TEM samples for these films. As the acid etchant removes the Si substrate material a rough interface is generated. This interface becomes a relatively smooth and featureless region when the etchant reaches the membrane (film/SiO2). This featureless region is caused by the mirror reflection of the film plane (film/SiO2 interface) through the optically transparent SiO2 layer. The lower etch rate of SiO2 (compared with Si) gives the operator enough time to stop the etching without perforating the film. A clear view of the morphology and control of Si roughness during etching are critical to this method, which are discussed in detail. The procedures of mounting wax removal and sample rinsing are also described in detail, as during these steps damage to the membrane may easily occur without appropriate consideration. As examples, the preparation of 100-nm-thick Fe-based amorphous alloy thin film and 160-nm-thick Cu-thin film samples for TEM imaging is described.

Keywords     back-etch method, TEM, sample preparation, Si, opaque films

Received     24 April 2007, accepted 25 December 2007


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