- Nano Express
- Open Access
Impact of program/erase operation on the performances of oxide-based resistive switching memory
© Wang et al.; licensee Springer. 2015
Received: 14 November 2014
Accepted: 29 December 2014
Published: 5 February 2015
Further performance improvement is necessary for resistive random access memory (RRAM) to realize its commercialization. In this work, a novel pulse operation method is proposed to improve the performance of RRAM based on Ti/HfO2/Pt structure. In the DC voltage sweep of the RRAM device, the SET transition is abrupt under positive bias. If current sweep with positive bias is utilized in SET process, the SET switching will become gradual, so SET is current controlled. In the negative voltage sweep for RESET process, the change of current with applied voltage is gradual, so RESET is voltage controlled. Current sweep SET and voltage sweep RESET shows better controllability on the parameter variation. Considering the SET/RESET characteristics in DC sweep, in the corresponding pulse operation, the width and height of the pulse series can be adjusted to control the SET and RESET process, respectively. Our new method is different from the traditional pulse operation in which both the width and height of program/erase pulse are simply kept constant which would lead to unnecessary damage to the device. In our new method, in each program or erase operation, a series of pulses with the width/height gradually increased are made use of to fully finish the SET/RESET switching but no excessive stress is generated at the same time, so width/height-controlled accurate SET/RESET can be achieved. Through the operation, the uniformity and endurance of the RRAM device has been significantly improved.
Thanks to the increasing demand from portable electronic products like smartphones, cameras, and laptops, the demand for solid-state memories has been increasing rapidly in recent years. However, in the further scaling down, the traditional flash memory is facing more and more problems due to its physical limitations. Although innovations in cell structure and device materials may help extend flash memory for another couple of technology nodes, alternative candidates must be explored for future non-volatile memory (NVM) applications. Among various candidates, resistive random access memory (RRAM) is the most promising one for future high-density NVM application, owning to its characteristics of simple cell structure, fast program/erase (P/E) speed, excellent scalability, low operation power consumption, and good compatibility with the standard complementary metal-oxide-semiconductor (CMOS) process [1-5]. However, in order to meet the practical application requirements, the performances of RRAM demonstrated to date still need improvements in the following areas: (1) effective control of high and low resistance state; (2) minimization of the variations of resistive switching parameters. Some approaches have been proposed to improve the operation test method of RRAM. Nevertheless, few works systematically studied the detailed influence of DC and pulse program/erase operations on the performances of oxide-based RRAM. In this work, aiming at addressing the above challenges, we try to elucidate the impact of P/E operation on the performances through comprehensive device characterizations and to explore possible solutions through innovations in test and operation methods.
For the necessity of plenty of DC and pulse measurement, stable valence change mechanism (VCM) devices with Ti/HfO2/Pt structure [5-12] is made use of in this work. In the DC positive voltage sweep the SET transition is abrupt, but it becomes gradual under current sweep. The RESET process is gradual in negative voltage sweep. So SET and RESET are current and voltage controlled, respectively. Combining positive current sweep SET and corresponding negative voltage sweep RESET operation, stable and uniform distributions of on-state and off-state resistance can be obtained. Gaining inspiration from the DC SET/RESET characteristics, we proposed a novel pulse operation scheme, i.e. the width and height of the pulse series are adjusted to control the SET and RESET process, respectively. Our new method is different from the traditional pulse operation with single constant program/erase pulse. Thus, accurate SET/RESET controlled by pulse width/height can be achieved through our new method. As a result of the new method, the uniformity and endurance of the RRAM device has been significantly improved.
Resistive switching memory devices with Ti/HfO2/Pt structure were fabricated as follows. First, after the standard chemical cleaning of the silicon substrate, a SiO2 film with a thickness of 100 nm was thermally grown through dry oxidation method. Then, Ti/Pt bilayer with thickness of 30/70 nm was sequentially deposited by e-beam evaporation to act as the bottom electrode (BE). Next, a high-quality 8-nm-thickness HfO2 resistive switching layer was grown by atom layer deposition (ALD) technology, which has the advantage of well controlling on the deposition parameters and excellent deposition uniformity. Finally, the 10/70-nm-thickness Ti/Pt bilayer or 70-nm-thickness Cu film was prepared by e-beam evaporation and then patterned by lift-off process to form the top electrode (TE). The area of TE is defined as 100 × 100 μm2. The DC electrical characteristics of the devices were measured by Keithley 4200-SCS semiconductor characterization system, where the Pt BE was grounded while the bias voltage was applied on the Ti/Pt or Cu TE. In the traditional pulse measurement, a single pulse was usually employed to fulfill the SET/RESET operation, and a small read pulse or a visual tool was used to verify if the SET/RESET is completed and to measure the switching time. In the height/width-adjusting pulse operation measurement, Keithley 4205-PG2 pulse generator was used to generate program/erase pulse series by an automatic procedure, and the device states were read by Keithley 4200-SCS. A matrix Keithley 707A is used to carry out the switching between the pulse program/erase operation and DC read operation.
Results and discussion
The distributions of R on and R off under different operation modes
VS_SET and VS_RESET
CS_SET and CS_RESET
VS_SET and CS_RESET
CS_SET and VS_RESET
SP_P and SP_E
AP_P and AP_E
We have investigated the impact of DC and pulse program/erase operation on the uniformity and endurance performances of Ti/HfO2/Pt-based RRAM device. Appropriate program/erase conditions are necessary to acquire the uniform resistive switching. A width-adjusting program and the height-adjusting erase pulse operation method are proposed. Our new method is advantageous to obtain the moderate program/erase operation in each cycle, without any inadequate operation and over operation. Thus, the endurance performance of the device is greatly improved. Based on our method, some technical solutions to improve the endurance of the RRAM can be developed.
This work was supported by the National Natural Science Foundation of China under Grant Nos. 61322408, 61221004, 61422407, 61334007, and 61274091, the National Basic Research Program of China under Grant No. 2011CBA00602, and the National High Technology Research Development Program under Grant No. 2014AA032900.
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