High-performance bilayer flexible resistive random access memory based on low-temperature thermal atomic layer deposition
© Fang et al.; licensee Springer. 2013
Received: 1 November 2012
Accepted: 27 January 2013
Published: 19 February 2013
We demonstrated a flexible resistive random access memory device through a low-temperature atomic layer deposition process. The device is composed of an HfO2/Al2O3-based functional stack on an indium tin oxide-coated polyethylene terephthalate substrate. After the initial reset operation, the device exhibits a typical bipolar, reliable, and reproducible resistive switching behavior. After a 104-s retention time, the memory window of the device is still in accordance with excellent thermal stability, and a 10-year usage is still possible with the resistance ratio larger than 10 at room temperature and at 85°C. In addition, the operation speed of the device was estimated to be 500 ns for the reset operation and 800 ns for the set operation, which is fast enough for the usage of the memories in flexible circuits. Considering the excellent performance of the device fabricated by low-temperature atomic layer deposition, the process may promote the potential applications of oxide-based resistive random access memory in flexible integrated circuits.
KeywordsFlexible memory Atomic layer deposition Low temperature
Since flexible electronic system (FES) appeals to be light, convenient, has conformal contingence with the crooked surface, and excellent interfaces with humans, it ought to be a prospective existing form of electronic product to substitute its clumsy predecessors manufactured and packaged by traditional bulk silicon technology[1, 2]. Up to now, multifarious electronic components, such as integrated circuits (ICs)[3, 4], active matrix organic light-emitting diodes, sensors, radiofrequency identification antennas, and solar cells[8, 9], have been fabricated on flexible substrates and are delved by many researchers. As we know, among all the components used in ICs, good and reliable memories[10, 11] will maximize the functionality of ICs, and it is also important for the FES.
Among all the memories, nonvolatile resistive random access memory (RRAM) is the most promising candidate because of its low power consumption, high speed, simple structure, and high packaging density, compared with its counterparts such as flash memory and DRAM[12–14]. Currently, oxides, such as STO, HfO2, NiO, Al2O3, ZnO, and GO, have received much interest in resistive switching research. Among the oxides mentioned, HfO2 has been profoundly studied and contains great potentiality to be put into applications. However, the application of HfO2-based RRAM on flexible substrate is still rare.
In recent years, atomic layer deposition (ALD) has emerged as a new technique for depositing films, particularly for fabricating oxide films. Owing to its self-limiting mechanism during the process, excellent step coverage and conformal thickness of the film can be achieved. Although the deposition of oxide film by ALD on bulk silicon is very mature, seldom had researchers used this method to deposit films on flexible substrate. The main reason is that the flexible substrate could not undergo high-temperature processing above 200°C, except in some cases such as depositing films using plasma-enhanced atomic layer deposition under low temperature where plasma damage and degradation of the step coverage is unavoidable.
In this letter, we fabricated a bilayer flexible RRAM device based on HfO2/Al2O3 films under low temperature, with resistive layers deposited using a low-temperature ALD process at 120°C and the electrodes sputtered by direct current (DC) magnetron reactive sputtering at room temperature. The devices fabricated by these methods exhibit impressive resistive switching characteristics with reliable data retention properties under room temperature and elevated temperature up to 85°C.
Flexible RRAM was fabricated on polyethylene terephthalate (PET) substrate coated by indium tin oxide (ITO) conducting film, and ITO serves as the bottom electrode in our devices. During the process, the substrate was fixed on a 3-in wafer with polyimide tapes in order to maintain sufficient mechanical support. The Al2O3 layer was deposited by 41 cycles of low-temperature ALD at 120°C with trimethyl aluminum (TMA) and water as precursors. Subsequently, the HfO2 layer was deposited by 67 cycles within the same framework using tetrakis(ethylmethylamino)hafnium (TEMAH) and water as precursors. TMA was pulsed at room temperature, and TEMAH was heated to 85°C to offer enough evaporation pressure. Al2O3 film was deposited with a pulse time of 0.1 and 0.2 s for TMA and water, and the purging time for TMA and water was 5 and 20 s, respectively. The deposition method of HfO2 was derived from our previous work. Finally, a 50-nm TiN top electrode was sputtered on the resistive layer by DC magnetron reactive sputtering through a metal shadow mask with a diameter of 400 μm.
The thicknesses of the HfO2 and the Al2O3 layer were estimated to be 10.1 and 4.9 nm by Sopra GES5E spectroscopic ellipsometry. X-ray photoelectron spectroscopy (XPS) of HfO2 and Al2O3 on the PET substrate was performed using a Kratos Axis Ultra DLD XPS (Kratos Analytical, Ltd., Manchester, UK). Electrical properties at room temperature and at 85°C of the device were assayed using an Agilent B1500A (Agilent Technologies, Inc., Santa Clara, CA, USA) semiconductor parameter analyzer and an Agilent B1525A high-voltage semiconductor pulse generator. Impedance of high and low resistance states was analyzed by an Agilent 4294A precision impedance analyzer. The device was tested with top biased and grounded bottom electrodes.
Results and discussion
In conclusion, a highly reliable and uniform flexible RRAM based on the TiN/HfO2/Al2O3/ITO structure, fabricated by a low-temperature process, was investigated. The fresh cell shows an ultra-low resistance state, and after the initial reset operation, a typical bipolar reliable and reproducible resistive switching behavior was demonstrated. It is found that the memory window is still in accordance with excellent thermal stability after a 104-s retention time, and a 10-year usage is still possible with the resistance ratio larger than 10 at room temperature and at 85°C. The resistance of the LRS and HRS exhibits a very concentrated distribution with almost 90% of the LRS around 0.6 kΩ and 80% of the HRS around 10 kΩ. The developed low-temperature process for the memories may promote the potential applications of oxide-based RRAM in flexible ICs.
RCF received his B.S. degree in Physics and Electronics from Nanjing Information Engineering University, Nanjing, China in 2010. He is currently studying at the School of Microelectronics, Fudan University for his Master's degree. His research interests include flexible memory and device design. QQS received his B.S. degree in Physics, his M.S. and Ph.D. degrees in Microelectronics and Solid state Electronics from Fudan University, Shanghai, China in 2004 and 2009, respectively. He is currently an associate professor at the School of Microelectronics in Fudan University. His research interests include fabrication and characterization of advanced metal oxide semiconductor field effect transistors, mainly high-k dielectric-based devices. He is also interested in design, fabrication, and characterization of advanced memory devices, such as resistive switching memory devices and Flash. PZ received his B.S. degree in Physics and Ph.D. degree in Optics from Fudan University, Shanghai, China in 2000 and 2005, respectively. He is currently an associate professor at the School of Microelectronics, Fudan University. His research interests include fabrication and characterization of advanced metal oxide semiconductor field effect transistors, advanced memory devices, and graphene device. WY received her B.S. degree in Physics and Electronics from Henan University, Henan, China in 2010. She is currently studying at the School of Microelectronics, Fudan University for her Ph.D. degree. Her research interests include low-power circuit, memory and device design, and theoretical and experimental investigations of two dimensional materials. PFW received his B.S. and M.S. degrees from Fudan University, Shanghai, China in 1998 and 2001, respectively, and his Ph.D. degree from the Technical University of Munich, München, Germany in 2003. Until 2004, he was with the head of the Memory Division of Infineon Technologies in Germany on the development and process integration of novel memory devices. Since 2009, he has been a professor at Fudan University. His research interests include design and fabrication of semiconductor devices and development of semiconductor fabrication technologies such as high-k gate dielectrics and copper/low-k integration. DWZ received his B.S., M.S., and Ph.D. degrees in Electrical Engineering from Xi’an Jiaotong University, Xi’an, China in 1988, 1991, and 1995, respectively. In 1997, he was an associate professor at Fudan University, Shanghai, China, where he has been a full professor since 1999. He is currently the Dean of the Department of Microelectronics and the Director of the Fudan-Novellus Interconnect Research Center. He has authored more than 200 referred archival publications and is the holder of 15 patents. More than 50 students have received their M.S. or Ph.D. degrees under his supervision. His research interests include integrated-circuit processing and technology, such as copper interconnect technology, atomic layer deposition of high-k materials; semiconductor materials and thin-film technology; new structure dynamic random access memory (RAM), Flash memory, and resistive RAM; and metal oxide semiconductor FET based on nanowire and nanotube and tunneling FET.
This work was supported by NSFC (grant nos. 61076114 and 61106108), the Shanghai Educational Development Foundation (10CG04), SRFDP (20100071120027), the Fundamental Research Funds for the Central Universities, and the S&T Committee of Shanghai (10520704200).
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