(Invited) Visualization of Conductive Filament of ReRAM during Resistive Switching by In-Situ TEM
In this work, we used two types of Cu-based ReRAM (CBRAM: Conductive Bridge RAM) memories for in-situ TEM observation. One has Cu/MoOx structure, where MoOx is used as a solid electrolyte. The TEM sample was prepared using the ion-shadow method , which is an ion milling technique with carbon mask particles. This method is simple and effective to make thin samples for TEM observation because many cone-shaped needles with miniaturized ReRAM devices were formed. The other is Cu-Te based ReRAM that has a structure similar to previously reported cells [1-3]. The 30- or 70-nm Cu-Te ReRAM cell was thinned by conventional FIB techniques to 100 nm thick for TEM observation . The method is suitable to cut up the small ReRAM devices from a patterned memory chip.
In-situ TEM observation was performed with a homemade TEM sample holder with a piezo actuator that was used to control a Pt-Ir probe needle so as to contact the needle to the top electrode of the ReRAM to be measured. Near the TEM specimen, a MOSFET was serially connected to control the current compliance. Together with the I-V characteristics, real-time TEM videos were also collected by the CCD Camera.
The clear formation and rupture of Cu filament were observed in TEM corresponding to SET and RESET resistive switching for the Cu/MoOx samples. The size of the filament increased as the increase of compliance current. An interesting results is that the positions of the filament changed. The phenomena was thought to be caused by the over-reset where the filament seeds were completely removed.
In the Cu-Te based sample, filament formation was not detected at low current compliance though the SET and RESET resistive switching were clearly observed. When the current compliance increased higher than 100 mA, filament formation corresponding to the SET process was detected in the TEM images by enhancing the contrast. In this device, the filament formed almost the same position. In addition, we achieved 100k pulse switching cycles of SET and RESET inside the TEM without any damage.
These results clearly show that the in-situ TEM will be a powerful tool to analyze the mechanisms and the reliability of ReRAM.
The authors wish to thank Drs. K. Ohba, M. Shimuta, I. Fujiwara, S. Yasuda, A. Maesaka, S. Kusanagi, and K. Aratani from SONY Corporation, and Drs. T. Fujii, K. Hamada, and N. Sakaguchi from Hokkaido University for their technical supports and meaningful discussions. This work was partly supported by Grants-in-Aid from the Japan Society for the Promotion of Science (JSPS KAKENHI 24360128, 25420279, and 2663014104) and the Nanotechnology Platform Program organized by MEXT.
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 M. Kudo, M. Arita, Y. Takahashi, K. Ohba, M. Shimuta, and I. Fujiwara: “Visualization of conductive filament during write and erase cycles on nanometer-scale ReRAM achieved by in-situ TEM,” IMW (2015).