A Systematic Approach for Controlling TEM Sample Thicknesses

Historically, TEM prep has been an artisan craft without a systematic workflow that ensures quantified control over TEM sample thickness. Over- and under-thinning is a significant problem in the TEM prep process. A direct measurement process was demonstrated to control the final thickness of the TEM lamella. Final lamella thickness was controlled by directly measuring lamella thickness in real time using a 10–15 kV SEM while performing secondary electron imaging at the mill position, allowing for (human-mediated) closed-loop processing. We demonstrated the utility of this technique by systematically thinning five TEM samples to discretely target thicknesses ranging from 100 nm down to 28 nm. We demonstrated the repeatability and simplicity of the process by fabricating 10 STEM lamella with a targeted thickness of 32 nm. We demonstrate the ability to fabricate an engineered multi-layered structure that acts as a lamella thickness measurement feature that is independent of sample type, allowing a broader implementation of in-line lamella thickness monitoring. We applied our thickness measurement technique to three thin-film materials (Au, Ag, and Cu) to obtain TEM lamella thickness target values needed to achieve electron transparency for each material. Having knowledge of a target thickness parameter prevents the over- and under-thinning problem in TEM prep.