Multi-temperature deposition scheme for improved resistive switching behavior of Ti/AlOx/Ti MIM structure
Introduction
Resistive random access memories (RRAMs) have emerged as an excellent choice for future non-volatile memory applications [1], [2]. RRAM has demonstrated excellent miniaturization potential, fast switching, low power operation, long retention of data, and capability of multibit storage over the conventional and emerging memory technologies [3], [4], [5], [6], [7], [8]. RRAM has a simple metal-insulator-metal structure with insulating layer sandwiched between two conducting electrodes, working on the resistive switching phenomenon. Information in an RRAM cell is stored in the form of reversible low resistance state (LRS) and high resistance state (HRS) as a result of formation and rupture of conductive filament (CF) [9]. Several materials including metal oxides and nitrides [10], perovskites [11], [12], polymeric dielectrics [13], [14], and chalcogenides [15] have been explored as the insulating material in RRAMs. However, metal oxides have been the main subject of investigation of switching behavior in RRAMs in the recent years. Different metal oxides such as HfOx [16], AlOx [17], NiOx [10], TaOx [16], TiOx [18], and ZnO [19] have been extensively explored in order to obtain the improved RRAM characteristics. Among the different metal oxides investigated, AlOx based RRAM devices have shown promising resistive switching characteristics due to its properties like high dielectric constant, low leakage current, high breakdown voltage, and high thermal stability [17], [20], [21], [22], [23]. In addition, it is used in metal oxide semiconductor field-effect transistors (MOSFET) as the oxide layer and encapsulation layer in organic devices [24], [25]. One of the very widely used AlOx deposition technique is atomic layer deposition (ALD), which produces atomically flat, highly uniform, and stoichiometry-controlled oxide layers, highly desirable for excellent RRAM characteristics. In ALD, the deposition temperature is one of the key parameters to control the above mentioned attributes of the oxide layer. However, there are very few reports available in which the role of deposition temperature has been discussed in detail to improve the RRAM performance [26]. In addition, the deposition temperature has rarely been envisaged as the variable process parameter during the deposition to seek any performance improvement in RRAMs.
In this paper, four types of Ti/AlOx/Ti RRAM devices fabricated on glass substrates are studied for their performance. For first two types of RRAM devices, the deposition temperature of switching layer was 80 °C and 150 °C respectively. This process was named as single temperature deposition (STD). For third and fourth type of devices, multi-layer multi-temperature deposition (MTD) process (80 °C/150 °C/80 °C and 150 °C/80 °C/150 °C respectively) was used to deposit the switching layer stack [27]. All samples were fabricated at low temperatures making the devices compatible with back-end-of-line CMOS process. All the fabricated devices have shown typical bipolar resistive switching behavior, however, the devices fabricated with temperature scheme of 150 °C/80 °C/150 °C have shown improved characteristics with comparatively better repeatability, longer retention time, uniform and lower set and reset voltages over multiple cycles of operation. Conduction mechanism in the RRAM devices has been investigated and it was concluded that ohmic and space charge limited conduction were dominant conduction mechanisms in the devices at lower and higher voltages respectively. Improved performance in devices fabricated with multi-temperature deposition scheme of 150 °C/80 °C/150 °C with lower switching voltage operation was attributed to the localization of switching phenomenon in the region with weaker conductive filament.
Section snippets
Experimentation
RRAM devices were fabricated on glass substrates, which were cleaned thoroughly in isopropyl alcohol, trichloroethylene, acetone and methanol sequentially for 10 min each. A 200 nm thick Ti bottom electrode (BE) was deposited on the glass substrate using e-beam evaporation technique under a high vacuum of 10− 6 Torr. A 40 nm thick AlOx was deposited using Savannah S200 thermal atomic layer deposition (ALD) system from Cambridge Nanotech with tri-methyl-aluminium (TMA) and H2O as precursors at
Results and discussion
The surface morphology of AlOx thin films is shown in Fig. 1(b) and (d). The root mean square (RMS) roughness of AlOx and Ti films are shown in Table 1. Although the AlOx deposition was performed by ALD, which produces smooth film but due to the inherent roughness of glass substrates and that of polycrystalline Ti BE layer, the subsequent layers assumed the morphology of layers beneath. Interestingly, it can be noticed from Table 1 that the roughness of the all four amorphous AlOx thin films
Conclusion
Multi-temperature deposition schemes for RRAM fabrication and their effect on resistive switching behavior of Ti/AlOx/Ti devices were discussed. Under proposed scheme, the middle layer of AlOx was deposited at a lower temperature than that of the outer layers. Among all the explored deposition techniques, MTD2 scheme (with temperature sequence of 150 °C/80 °C/150 °C) has resulted in improved performance in devices with high reliability, higher Ion/Ioff, comparatively low voltage switching
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