Modern FIB-Based Specimen Preparation for APT 201
Figure 10. FinFET analysis options alongside the resulting simplified, evaporation field variation expected for four considered orientations with the fin as the ROI (blue=high field, gray=low field). (Note the TEM images used to illustrate the fin in various analysis directions are courtesy of Chipworks.)
along the gate, and cross-section along the fin. A successful analysis will capture the fin, but the quality of the recon- struction and potential for specimen survival may be very different. Although top-down and backside preparation will have most of the interfaces oriented parallel to the analysis direction, which should positively affect yield, the differences in evaporation field between the fin (low, gray) and gate or silicon oxide regions (high, blue) will degrade lateral resolution, and may even result in ion crossing (Birdseye et al., 1974) in extreme cases. This may result in fin reconstructions where the entire region is mixed with material from adjacent high field regions. Using the cross-section along the gate approach may result in lower yield, because the interfaces are mostly orthogonal to the analysis direction; however, the planar presentation of the high and low field layers should provide higher spatial resolution of these neighboring layered regions, without the potential for ion crossing. In addition, any remaining reconstruction distortions observed from this orientation have been well studied and have published methods for correction (Vurpillot et al., 2004). Besides options for general orientation of the ROI,
additional options exist for improving both data quality and yield. Although capturing a full device in a single analysis seems enticing, this approach may result in low yield with only some areas having good data quality. Narrowing the full device down to a single ROI by removing, reorienting, and rebuilding material around a critical sub-volume may be the only way to both to improve yield and data quality at the same time (Larson et al., 2011a). Another option is to build test structures (Moore et al., 2008; Gilbert et al., 2011; Kambham et al., 2011), where problematic materials have not been included in the device and/or replaced with more compatible atom probe materials (good adhesion, similar evaporation field, high yield).
Targeted Preparation
In order to properly target and center a single transistor, the location must be identified and tracked throughout the specimen preparation process and properly centered in three dimensions. This can take some creativity, as the location of the device may not be visible from any or all orientations during the process steps. One such example is shown in
viously, allows for orienting the device structures (fins) parallel to the analysis direction and possibly centering the features within the tip along one direction (e.g., Fig. 11a y axis), but centering along the other orthogonal direction (e.g., Fig. 11a x axis) might prove difficult. The device features that are exposed along the orthogonal direction (if visible in the SEM image) may not provide clear indica- tion as to the relative location of the targeted fin. Rotating the specimen stage back-and-forth to monitor the shaping process and make adjustments so that the fin is properly centered, is both time consuming and requires multiple re-alignments of the annular milling pattern. In addition, the view from the top of the specimen is not ideal for centering because that view may be obscured by capping/protection layers, or if unprotected, affected by differential sputtering and deep gallium damage, which may result in poorly formed and damaged specimens. Targeted backside (Lawrence et al., 2014) and targeted
Figure 11. Here, a series of 22-nm device fins are targeted for cross-section preparation with the goal of capturing the key regions at the top of the fin (which define the key gate and source/drain contacts). The standard cross-section method, described pre-
cross-section (Lawrence et al., 2015) preparation utilize ion-beam-milled markers to enable accurate centering of the ROI during the annular milling process, as viewed from the top of the specimen. First a wedge is removed, rotated, and deprocessed so that the top-most surface is close to (within ~100nm) the targeted analysis volume of the feature (in this case the targeted location along the fin direction). Before markers aremilled into this surface, the ROIs are first protected with electron-beam-deposited Pt (Figs. 11a, 11b). Then 30kV gallium milling is used to make marker trenches, enabling alignment during the annular milling steps (Fig. 11c). Afterward, the wedge is rotated and the Pt regions are removed, Figure 11d, and replaced with a more atom-probe- friendly capping material. This allows for the annular milling steps to proceed normally, Figure 11e, where some capping material is left on the final specimen to allow the user to better control tip size and shape, with minimal interference from differential milling
effects.The final specimen is then ready for analysis. Figure 11f shows an example of a fin-ROI centered within a tip that was successfully analyzed.
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