High-performance HfO x /AlO y -based resistive switching memory cross-point array fabricated by atomic layer deposition
© Chen et al.; licensee Springer. 2015
Received: 15 November 2014
Accepted: 7 January 2015
Published: 18 February 2015
Resistive switching memory cross-point arrays with TiN/HfO x /AlO y /Pt structure were fabricated. The bi-layered resistive switching films of 5-nm HfO x and 3-nm AlO y were deposited by atomic layer deposition (ALD). Excellent device performances such as low switching voltage, large resistance ratio, good cycle-to-cycle and device-to-device uniformity, and high yield were demonstrated in the fabricated 24 by 24 arrays. In addition, multi-level data storage capability and robust reliability characteristics were also presented. The achievements demonstrated the great potential of ALD-fabricated HfO x /AlO y bi-layers for the application of next-generation nonvolatile memory.
Metal oxide-based resistive random access memory (RRAM) has been extensively studied as one of the most promising candidates for next-generation nonvolatile memory due to the great performances such as fast switching speed, low operating voltage, 3D integration, and good compatibility with CMOS fabrication processes [1-5]. For high-density integration of RRAM array, a cross-point structure with the smallest cell area of 4 F 2 is needed [6,7]. However, the metal oxide-based RRAM devices usually have a large variability [8-10], which hinders application in industries. Thus, it is imperative to seek an effectively technical solution to minimize the variability of RRAM devices.
Various transitional metal oxides such as HfO x [11-13], TaO x [14-16], TiO x [17-19], and ZrO x [20-22] have been reported as resistive switching materials. Among them, HfO x is a superior resistive switching material, which has stable electrical properties, good process repeatability, and small leakage current [23,24]. Based on a previous work , an additional buffer oxide layer of AlO y which has a larger oxygen ion migration barrier (E m) will confine the switching in the active oxide, which can improve the uniformity in HfO x -based RRAM devices. Both physical vapor deposition (PVD) and atomic layer deposition (ALD) have been applied to fabricate resistive switching layers. Compared to PVD, the ALD technique has more advantages at constructing uniform, conformal, and ultrathin films, which is a central component for high-density and 3D integration.
In this paper, the bi-layered HfO x /AlO y films are deposited by ALD as the resistive switching layer of cross-point RRAM array, which shows the precise control of the resistive switching layer in thickness, uniformity, and conformity. The fabricated TiN/HfO x /AlO y /Pt RRAM devices in the cross-point array show excellent performances including low operation voltage (+2/−2 V), sufficient resistance ratio (>10), smaller cycle-to-cycle and device-to-device variations, and high yield (>95%). Meanwhile, multi-level data storage capability, good direct current (DC) endurance (>1,000 cycles), and retention (>104 s at 85°C) properties are demonstrated in the devices.
Electrical characterizations were performed using an Agilent B1500A semiconductor parameter analyzer (Agilent Technologies, Inc., Santa Clara, CA, USA). During the measurements, voltage was applied on the TE, while the BE was grounded.
Results and discussion
Here, εHfO x /εAlO y refers to the dielectric constant of HfO x /AlO y , EHfO x /EAlO y is the electric field intensity in the HfO x /AlO y layer, dHfO x /dAlO y is the thickness of the HfO x /AlO y layer, and V is the value of the applied voltage. By calculating the above equations, the electric field intensity in the AlO y layer is found to be stronger than that in the HfO x layer. Therefore, the dielectric breakdown happens firstly in the AlO y layer at a lower voltage, and then it happens in the HfO x layer at a higher voltage.
In order to confirm the nonvolatility of the devices, time-dependent evolution of the resistance values of both HRS and LRS was monitored at 85°C. The resistance was read every second with a read voltage of 0.1 V. As shown in Figure 7b, both LRS and HRS show no signs of degradation for 104 s.
Excellent resistive switching characteristics of TiN/HfO x /AlO y /Pt RRAM devices in a cross-point array structure have been demonstrated in this work. The devices in the array show excellent cycle-to-cycle and device-to-device switching uniformity, which can be attributed to the precisely controlled HfO x /AlO y bi-layered resistive switching layer by ALD and the effect on the resistive switching behaviors. These superior characteristics of the cross-point RRAM array could be useful for future nonvolatile memory applications.
This work is supported in part by the 973 Program (2011CBA00600) and NSFC Program (61334007, 61421005, 61376084, 61404006).
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