Characteristic of Flexible Reram with Al2O3/TiO2 Active Layer By ALD and PDA Process at Low Temperature (2020)

1. Introduction The resistive random access memory (ReRAM), which is a nonvolatile memory device, on flexible plastic substrate have been widely investigated for future wearable device. The flexible ReRAM based on metal-insulator-metal (MIM) structure is generally fabricated at relatively low temperature (~250°C) from the viewpoint of heat resistance of plastics. Numerous active layers, including Al 2 O 3 -TiO 2 , Al 2 O 3 -HfO 2 , and Al 2 O 3 -Ta 2 O 5 bilayers have been studied in a aim of improving resistive swithing reliability. In our previous paper, we found that Pt/Al 2 O 3 /TiO 2 /Pt capacitor fabricated on Si substrate by atomic layer deposition (ALD) and a post-deposition annealing (PDA) process at 200°C, exhibited bipolar resistive swithing charactreistics with a high resistive state (HRS) and a low resisitive state (LRS) in a small switching voltage of ±0.8V. In this paper, we fabricated Pt/Al 2 O 3 /TiO 2 /Pt capacitors on polyimide (PI) substrate by ALD and PDA process at 200°C and examined reliabilities of resistive swithing properties under tensile stress. 2. Experiment MIM capacitors with Al 2 O 3 /TiO 2 active layer were prepared as follows. First, a 200-nm-thick SiO 2 layer as gas barrier was deposited on PI substrate. Pt bottom electrode (BE-Pt) was deposited on PI/SiO 2 layer by DC sputtering. A 5-nm-thick Al 2 O 3 layer was deposited on BE-Pt by ALD at 200°C using Al(CH 3 ) 3 precursor and H 2 O gas and subsquently PDA was carried out at 200°C in N 2 . Next, a 60-nm-thick TiO 2 layer was deposited on Al 2 O 3 layer by ALD at same temperature, using Ti[N(CH 3 ) 2 ] 4 precursor and H 2 O gas and PDA was performed at 200°C in O 2 . Finally, Pt top electrode (TE-Pt) was fabricated on the TiO 2 layer by DC sputtering. The area of the TE-Pt was approximately 1.5 ´ 10 -4 cm -2 . 3. Results and discussion Each of Al 2 O 3 and TiO 2 layer of the flexible Pt/Al 2 O 3 /TiO 2 /Pt capacitor on PI had an amorphous structure because no spotty electron diffraction pattern was observed by TEM and EDS measurement. There were no severe chemical reactions in any of the layers. Fig 1 shows the J-V characterestics of the flexible Pt/Al 2 O 3 /TiO 2 /Pt capacitor. The first forming process was performed to occur the irreversible breakdown of the Al 2 O 3 layer when applied voltage was increased to + 7V. The second forming process was the creation of a TiO 2-x layer due to the oxygen vacancies (V o ) near the TE-Pt and TiO 2 layer when voltage was applied -3V. The set process (switching from the HRS to the LRS) caused at an applied voltage of about +1V and the reset process (switching from the LRS to the HRS) occurred at about -1V. These set and reset processes appeared due to the V o transfer into TiO 2 layer when voltage applied in positive and negative directions. Stable bipolar switching properies between the HRS and the LRS was obtained until the number of swithing cycles of 20. We also observed resistive swithing characteristics of about 2 orders of magnitude between the HRS and the LRS under tensile stress condition. 4. Summary We conclude that flexible Pt/Al 2 O 3 /TiO 2 /Pt capacitors on PI exhibited stable bipolar switching characteristics of the HRS and LRS after two forming steps of breaking Al 2 O 3 layer and creating a TiO 2-x layer due to the V o . Furthermore, a similar resistive switching property was obtained regardless of tensile stress. Acknowledgement This work was partially supported by JSPS KAKENHI Grant Number of JP20H02189. The authors thank all staff members of the Nanofabrication group of the National Institute for Materials Science (NIMS) for their support. Figure 1