Focused Ion Beam Milling Technique for Plan-View TEM Sampling of DRAM Capacitor
Figure 1-(a) shows the vertical TEM result of two bits failure caused by DRAM storage node’s bending. In the conventional plan-view TEM sampling process, the target height for plan-view TEM sampling is marked with ‘l’ shape marker  after realizing the height of node bridge as shown in figure 1-(b). It is important to know the end point during FIB milling . And then, the top (red area) and bottom (yellow area) parts are removed to get the plan-view TEM sample. Figure 1-(c) is a TEM image after this plan-view TEM sampling process. The nodes do not stand in a line as you see in figure 1-(c). The nodes seem to move during the 3rd step milling (mill a bottom part) because of the void between nodes. The node may fall off a TEM thin foil sample in the worst case. In order to overcome this problem, we add a carbon deposition step right after the 2nd step of the top part milling as shown in figure 2-(a). As a result, we are able to get a plan-view TEM image where the nodes stand in a line as shown in figure 2-(b). Four major steps are,
Step 1 : ‘1’ shape marking at the target height
Step 2 : milling a top part of capacitor
Step 3 : carbon deposition to fill the space between nodes
Step 4 : milling a bottom part of capacitor
We adopted this modified milling method to the TEM sampling of a real failure case and obtained a PFA result of a 2 bit failure using plan-view TEM image as shown in figure 2-(c). The new modified milling method turned out to be very effective and can be adopted in a mass production.
 D. Kim, J. Kim, M. Huh, Y. Hwang, J. Park, D. Han, D. Kim, M. Cho, B. Lee and H. Hwang, in Electron Devices Meeting, 2004. IEDM Technical Digest. IEEE International, p. 69 (2004).
 Moon-Yee Wang, S.X. Lee, S. Peri, “The Effect of FIB Technology on Design Methodology”, Circuits and Systems, 1993, 725-728 vol1.