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edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 2 38 MASTER FA TECHNIQUE Travis Mitchell, Brian Popielarski, and Frieder Baumann, GlobalFoundries, Malta, N.Y. travis.mitchell@globalfoundries.com THE PROBLEM TEM sample preparation of etched wafers with large aspect ratio vias has been a problem for years. The lithog- raphy layers,whichoftenconsist of ultra-low-k (ULK)mate- rials, are sensitive to the electron beam which can cause shrinking and distortion. Therefore, only low energy elec- tron beams can be used during TEM lamella preparation. Figure 1 illustrates an example of the problemat hand. The high aspect ratio vias or trenches are to be filledwith a suitable material to ensure homogeneous thinning in the focused ion beam(FIB) tool. When using lowvoltage in the SEM in order to preserve these ULK layers, the deposition of any film, metal or compound, does not fill the etched cavities from the bottomup. Instead, the films pinch off at the topof the vias and tend to forma bridge over the cavity creating a void as shown on the right in Fig. 1. This void often changes the critical dimensions of the cavity during subsequent thinningor electron imaging (out-bowing) and can cause curtaining during sample preparation. THE SOLUTION We found that the vias and cavities can be filled homogeneously and reliably by employing a fast-drying carbon-based liquid. By using a commercially available felt-tip pen, one can fill the cavity while protecting the critical dimensions from warping due to electron beam exposure. The deposited ink fills the cavity completely and dries up in amatter of seconds to build a pure carbon film. We use a three-step process to prepare these samples before loading them into a FIB/SEM (Fig. 2). 1. Deposit sputtered chromium (optional) • This step is only needed if the top layers of the sample are carbon based and you’d like to differentiate the ink from the top layers in subsequent imaging. ARTIFACT-FREE CROSS SECTIONING OF HIGH ASPECT RATIO ETCH CAVITIES USING INK • A nanometer or two of Cr will suffice. 2. Apply ink • We’ve found that using a felt-tip pen delivers the best results. • Using a light stroke apply the ink gently adjacent to your area of interest, trying to leave some recognizable features nearby for alignment once the sample is loaded into the FIB/SEM. You want the ink to bleed into your area of interest from nearby so that you can control how much is applied. Fig. 1 Deposition of a film pinches off at the top of the vias and forms a bridge over the cavity, creating a void. Orange is underlayer, yellow is etched ULK layer(s), blue is protection layer to be deposited. Fig. 2 Three-stepprocess of depositing chromium, applying ink, and depositing chromium. Gray is chromium, blue is ink.