Plan to Vertical Milling Technique By Focused Ion Beam

The result revealed that vertical milling technique by Focused Ion Beam (FIB) [1] [2] was very effective when analyzing a single bit or twin bits. However, in case of multi-bits, the cause of failure remains unknown. Vertical milling lamella by FIB was usually created approximately in the center of the multi-bits. In this case, the visualization success rate of a failure analysis has dropped.  To improve the success rate, we, thus, made a new failure analysis technique. The first step was to create a lamella with a widely planned milling technique by FIB. Secondly, the lamella with Transmission Electron Microscopy (TEM) [3] was carefully observed to determine the cause of failure. Next, we made a specific target lamella as the center of a source with vertical milling technique by FIB and observed a specific target lamella with TEM. The fundamental concept was very simple. However, it is possible to make a new technique after several trial and errors. In this paper, we have introduced the new failure analysis technique step by step and each step’s precaution. At first, when making a plan lamella, milling and deposition should be simultaneously done. If we milled a lamella without a deposition, a part of the lamella’s pattern would often drop during the milling process. Creating a very thin lamella with a plan milling technique would often result in a bended or torn lamella. On the contrary, creating a thick lamella with a plan milling technique would result to an unknown cause of failure. Therefore, the proper thickness of the lamella is very crucial when using a plan milling technique. We arrived at a conclusion that the proper thickness was over 100 nanometer. After finding out the source of fault by observing a plan lamella through TEM, we marked a specific target in a plan lamella to make a vertical milling lamella by FIB. However, it is important to note that without a horizontal marking, the process would not allow us to make a vertical milling lamella as the end point will remain unknown. Therefore, it is very important to have a horizontal marking. Below summarizes the five major steps and its precautions:

1. Marking a plan milling lamella by FIB (Precaution : milling and deposition should be done simultaneously)
                                                         (Precaution : the thickness should be over 100 nanometer)
2. Observing a plan lamella with TEM
3. Marking a specific target in a plan lamella (Precaution: horizontal marking)
4. Making a vertical milling lamella by FIB
5. Observing a vertical lamella with TEM

Figure 3 is the result of the new failure analysis technique. We could determine the source of fault by observing a plan lamella with TEM. We could, then, create a specific target lamella by vertical milling. The result of the new failure analysis technique was very effective and was applied in a present mass production.

References

[1] Semiconductor Material and Device Characterization 3rd, by Dieter K. Schroder, Page 723
[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] Transmission Electron Microscopy, by David B. Williams and C. Barry Carter