Focused Ion Beam Milling Technique for Plan-View TEM Sampling of DRAM Capacitor

As the semiconductor device feature size becomes smaller, the aspect ratio of DRAM capacitor gets higher. The higher the height of capacitor, the more storage node bent. Therefore, we adopt the Mechanically Enhanced Storage node for virtually unlimited Height (MESH) [1] process to prevent storage node’s bending nowadays. Nevertheless, the bending failure still exists, and thus we need to make a sample for physical failure analysis (PFA). Normally PFA needs a site specific transmission electron microscopy (TEM) sample which is vertically cross-sectioned by using focused ion beam (FIB) , but sometimes a plan-view TEM sample is also required to clarify the exact cause of failure.

 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 [2] after realizing the height of node bridge as shown in figure 1-(b). It is important to know the end point during FIB milling [3]. 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 3^rd 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 2^nd 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.

References

[1] 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).

[2] Moon-Yee Wang, S.X. Lee, S. Peri, “The Effect of FIB Technology on Design Methodology”, Circuits and Systems, 1993, 725-728 vol1.

[3] S. Lee, C.-W. Yang, T. J. Park, J. K. Kim, J. H. Ahn, J. Choi and G. Y. Song, in Meeting Abstracts, p. 1608 (2014).