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Bottom Electrode Modification of ZrO2 Resistive Switching Memory Device with Au Nanodots
View the table of contents for this issue, or go to the journal homepage for more 2012 Jpn. J. Appl. Phys. 51 02BJ04
(http://iopscience.iop.org/1347-4065/51/2S/02BJ04)
150C are also demonstrated in this device. # 2012 The Japan Society of Applied Physics
1. Introduction
Resistive random access memory (RRAM) device has been considered as a promising candidate to replace the present flash memory for next generation nonvolatile memory applications. RRAM utilizes two distinguishable resistive states (high conductive state, ON state and low conductive state, OFF state) to store digital data in unit cell. Furthermore, RRAM has advantages of long data retention, high-speed operation, thermal robustness, high scalability, and simple structure compatible with the standard comple-mentary metal–oxide–semiconductor (CMOS) process.1,2) However, there are large variations in switching parameters such as the resistance values of ON state (Ron) and OFF state
(Roff), and the required voltages to switch from OFF state to
ON state (Von), andvice versa (Voff) needed to be solved for
possibly practical application. So far, the detailed resistive switching (RS) mechanisms are arguable and unclear, where material properties and fabrication processes have signifi-cantly influence on them. The formation and rupture of conducting filaments with oxygen ions/vacancies migration within RRAM device are the most possible mechanism.3–12) Therefore, based on this mechanism, several techniques were proposed to improve the operation parameters’ variation, including modifying interface by active top electrode,6) lowering effective film thickness by embedded metal technology,7) reducing active device area by plug-bottom electrode,8) stabilize local oxygen migrations by inserting IrO2 buffer layer,9)creating locally strong electric field by process control,10) reducing the effective film thickness by Ni migration,11) and changing structure by lightning rod effect.12)
In this paper, we used nanosphere (NS) lithography to modify Pt bottom electrode with Au nanodots (Au:Pt) and fabricated the Pt/ZrO2/Au:Pt device. NS lithography has merits of low cost and convenience in large-scale self-assembled nano-structure application.13) The conducting filaments in the Pt/ZrO2/Au:Pt device are believed to be easily induced above the Au nanodots on the Pt bottom electrode, where the electric field across the ZrO2 film is the strongest. Hence, the conducting filaments are con-fined here, leading to less RS operation parameters’ variation.
2. Experimental Procedure
Figure 1 depicts the processes of the Pt/ZrO2/Au:Pt device fabrication. First, the polystyrene NSs order pattern was arranged on the Pt/Ti/SiO2/Si substrate immersed with dispersed solution to form the mask. The scanning electron microscope (SEM; Hitachi S4700) image (inset of Fig. 2) shows neat arrangement of NSs on the Pt/Ti/SiO2/Si substrate. After slowly lifting up the substrate from the solution, depositing Au metal by electron beam evaporation and then, removing NSs mask by dissolution in toluene, the Au nanodots formed on the Pt bottom electrode. The Au nanodots prepared by the NS lithography process were observed by SEM image as shown in Fig. 2. Sequentially, a 30-nm-thick ZrO2 film was fabricated on this modified bottom electrode at 200C by a radio frequency magnetron sputtering. The base pressure of the sputtering chamber was below 2 105Torr, and the deposition pressure was maintained at 10 mTorr with a gas mixture of oxygen and argon at a mixing ratio of1 : 2 with a total flow of 18 sccm. A 30-nm-thick ZrO2 film without Au nanodots was also fabricated as a reference sample. Finally, an 50-nm-thick Pt Fig. 1. (Color online) Schematic description of Pt/ZrO2/Au:Pt device’s fabrication procedures. (a) Before depositing Au metal, polystyrene NSs patterning to form the mask. (b) After depositing Au metal and then, removing NSs mask, the Au nanodots formed on the Pt bottom electrode. (c) Stacking ZrO2film on the modified bottom electrode. (d) Completed Pt/ZrO2/Au:Pt device.
top electrode was deposited by electron beam evaporation at ambient temperature with a diameter of 250m patterned by the shadow mask process to complete the Pt/ZrO2/Au:Pt device. In order to confirm the crystalline phase and the orientation of the ZrO2 films, X-ray diffraction (XRD; Rigaku RU-H3R) analyses were carried out. Agilent 4155C semiconductor parameter analyzer was used to measure the current–voltage (I–V) characteristics of the Pt/ZrO2/Au:Pt memory device in ambient atmosphere at room temperature.
3. Results and Discussion
Figure 3 indicates the XRD patterns of the ZrO2/Pt (reference sample) and the ZrO2/Au:Pt, where both ZrO2 films are amorphous. The peak of Au(111) is only observed in ZrO2/Au:Pt, showing the existence of the Au nanodots. Although Au nanodots modify the morphology of the Pt bottom electrode, the micro-structure of the ZrO2 film remains amorphous and unchanged. The forming process, similar to the soft breakdown, causes the conducting filaments created in the memory film with the highest electric field.14–16) Hence, a high-voltage forming process with a current compliance of 1 mA is necessary. There is a sudden increase of current occurs near 6.2 V for the Pt/ ZrO2/Pt device before performing any RS as shown in Fig. 4. Different from the Pt/ZrO2/Pt device, the forming voltage of the Pt/ZrO2/Au:Pt device is reduced to about
1.9 V (close toVon). The Au nanodots on Pt bottom electrode
obviously lower the forming voltage. After finishing forming process, we apply a positive voltage to switch them to OFF state and back to ON state with a current compliance by applying another higher positive voltage. It is observed that the Pt/ZrO2/Au:Pt device can be operated with lower voltages (Von andVoff) than the Pt/ZrO2/Pt device.
Figure 5 demonstrates the statistical charts of the RS parameters during continuous RS by dc voltage in both Pt/ ZrO2/Pt and Pt/ZrO2/Au:Pt devices, respectively. BothRon
andRoff of the devices are measured at 0.3 V. The Pt/ZrO2/
Au:Pt device exhibits sharp distributions of Ron,Roff, Von,
and Voff and the corresponding dispersions are reduced
compared with the Pt/ZrO2/Pt device. The stable RS indicates that the regions where the conducting filaments formed and ruptured are controlled and confined to some degree. For our Pt/ZrO2/Au:Pt device, the strongest electric field across the ZrO2 film is believed to exhibit near above the tip of the Au nanodots on the Pt bottom electrode, where the effective thickness of ZrO2 film is reduced and the conducting filaments are easily induced after forming process. Then, the further occurrence of the conducting filaments formed and ruptured is confined here, causing less Fig. 2. SEM image of Au nanodots prepared by the NS lithography
process. The inset shows neat arrangement of NSs on the Pt/Ti/SiO2/Si substrate.
Fig. 3. XRD patterns of Pt/ZrO2/Pt and Pt/ZrO2/Au:Pt devices.
Fig. 4. Typical RSI–V curves of Pt/ZrO2/Pt and Pt/ZrO2/Au:Pt devices.
Fig. 5. Statistics of the RS operation parameters in Pt/ZrO2/Pt and Pt/ZrO2/Au:Pt devices, respectively.RonandRoffmeasured at 0.3 V for both devices.
D.-Y. Lee et al. Jpn. J. Appl. Phys. 51 (2012) 02BJ04
operation parameters’ variation. Such an inner high electric field can also be created by embedding metal nanocrystals17) and embedding Pt nanoparticles18) within memory films, leading to stable RS behaviors.
Table I summarizes the current reported studies and their results of the changes in Ron,Roff,Von,Voff, and resistance
ratio (Roff=Ron). To suppress the RS operation parameters’
variation by reducing and confining the effective RS region, several common phenomena are observed and concluded: (i) a decrease in operation voltage; (ii) a decrease in Roff; (iii) possibly a general increase in Ron; (iv) a total
decrease in Roff=Ron. In spite of the decrease in Roff=Ron,
it still provides an enough memory margin for RRAM application.
For retention characteristics, both ON state and OFF state of the Pt/ZrO2/Pt and the Pt/ZrO2/Au:Pt devices show no data loss for more than 106s at room temperature (RT) and 150C, respectively as shown in Fig. 6(a). There is no significantly different in the retention characteristics be-tween the Pt/ZrO2/Pt and the Pt/ZrO2/Au:Pt devices. In Fig. 6(b), both ON state and OFF state are almost kept at the same value under 0.3 V stress voltage at RT and 150C, respectively, and there are no observable degradation over 104s. Hence, the Pt/ZrO
2/Au:Pt device possesses robust
ON state and OFF state for promising memory application.
4. Conclusions
In this study, the Pt bottom electrode modified with Au nanodots is proposed to suppress the variations in RS operation parameters, where the Au nanodots are prepared
by NS lithography. The high electric field are created on above the tip of Au nanodots within Pt/ZrO2/Au:Pt device. The conducting filaments are believed to be possibly induced near above the tip of Au nanodots after forming process. Then, the conducting filaments are confined to decrease the operation parameters’ variation. The retention characteristics and the nondestructive readout properties are also stable at both RT and 150C. The Pt/ZrO2/Au:Pt device shows a promising potential for next generation nonvolatile memory application.
Acknowledgment
This work was supported by the National Science Council, Taiwan, under project NSC 99-2221-E-009-166-MY3.
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