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We have developed an ion-assisted gas etching (IAGE) technique which enhances existing focused ion beam (FIB) failure analysis (FA) cross-sectioning techniques. Preliminary results show enhanced sputter removal rate of certain materials, decoration of material interfaces or boundaries, and preferential etching of materials particularly difficult to image with standard secondary imaging systems. In particular we show two examples of IAGE using XeF2. The first sample is a multilayer of resist on oxides through which contact holes have been made by reactive ion etching (RIE) to an implanted silicon layer. A typical FIB cross-section made without IAGE shows some delineation of boundaries between oxides and a slight cascade of the resist at the top edge of the section. An FIB section made while etching with XeF2 shows improvement in delineation of the boundaries between the oxides and in particular, decorates the implanted layer. To verify the preliminary evidence that implanted material can be selectively etched with XeF2, we used a silicon wafer implanted with As+ to a depth of 850 A, and the resist was stripped. Typical FIB sectioning could not distinguish between the implanted and nonimplanted regions on the wafer, either with scanning electron (SEM) images or scanning ion (SIM) images. However, there was a slight difference in the SIM image of the top surface of the wafer. A very low ion dose combined with XeF2 selectively etched the implanted and nonimplanted regions enough to distinguish them clearly in an SEM image. Thus we have preliminary evidence that IAGE with XeF2 complements existing FIB FA cross-sectioning techniques and has particular potential for defining boundaries of materials which typically have low contrast such as oxides, and as a way of imaging implanted regions.
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