Effects of Ti Electrode on the Electrical Properties of Filament within Al2O3 Based Antifuse

Abstract

Metal-insulator-metal antifuse is a kind of one-time programmable device that exhibits a great difference in impedance before and after programming, and it is widely used in the area of nonvolatile memories or programmable array logic devices for radiation-hardened space environment, such as PROM (programmable read-only memory), and FPGA (field programmable gate arrays). In this work, we fabricated a novel antifuse device with an ALD Al2O3 dielectric layer sandwiched between a Tungsten (W) bottom electrode and a top Ti electrode. The MIM stack exhibits a via-hole structure. An Al2O3 film was deposited by atomic layer deposition (ALD), The bottom metal (W) was deposited by chemical vapor deposition (CVD), and a 4000 Å oxide layer was subsequently deposited by plasma enhanced chemical vapor deposition (PECVD). Then, a 0.5 µm via was etched through the oxide, which defined the antifuse area. Afterwards, Al2O3 film was deposited, followed by physical vapor deposition (PVD) of Ti film as the top metal electrode. The Al2O3 dielectric was 7-nm thick, and the device area was 0.5 µm by 0.5 µm. In this paper, we investigated the filament properties after the antifuse is broken down. Extremly low on-state resistance is achieved by taking advantage of current overshoot in our original structure. After the antifuse breaks down, its resistance decreases to approximately 20 Ω. The filament properties are found to be independent on the breakdown polarities, as shown in Fig.1. While the filament properties show strong polarity dependency. If a positive voltage sweep from 0 to 1 V is applied to the top Ti electrode, the resistance would further decrease; if the sweep voltage is negative, the resistance would increase. We have studied the temperature dependency of I-V curves of initial formed filament and find the current decreases with temperature increase, exhibiting a metallic nature. The further decreased resistance filament shows the same temperature dependency, whereas the increased resistance filament after negative sweep voltage shows positive temperature coefficient, indicating a semiconducting characteristic. The negative voltage causes a filament transition from metallic-like nature to semiconducting-like characteristic which is found to be dominated by the oxygen vacancies density at the Ti/Al2O3 interface. A mechanism of the filament formation and transition is proposed that the filament is composed of a metal part (a mixture of metal and insulator reactions) and oxygen vacancy part. The properties of the filament formed in the Al2O3 based antifuse were demonstrated by studying the temperature dependence of the current-voltage characteristics. Fig. 2(a) shows the temperature dependence of I-V curves of the initial formed filament ranges from 25℃ to 125℃. The current reduces with the increasing temperature, indicating a metallic nature of the filament. The log(I)-log(V) curve in the inset of Fig. 2(a) is a straight line with a slope nearly equals to 1, also demonstrating the Ohmic characteristics. The temperature dependence of I-V curves of the filament after negative sweep is also analyzed, as is shown in Fig. 2(b). For the complete temperature range, the current can be fitted using equation , where Reff is the effective resistance of the filament, and the exponent α is close to unity indicating that the conduction is nearly Ohmic. It can be observed that the current increases with the increasing temperature, and the Reff drops from 348 Ω to 326 Ω with the temperature going up from 25℃ to 125℃. Which demonstrates that the reset filament exhibiting semiconducting-like behavior. The process of the filament formation and the filament resistance transition is illustrated in Fig. 3(a-e). (a) Pristine state. (b) The formation of initial oxygen vacancy filament. (c) The initial formed filament causes an abrupt increase of leakage current, leading to the electrochemical reaction driven by large Joule heating and formed mixture metal filament near the bottom W electrode. (d) When a positive voltage is applied to Ti electrode, it would further enhance electrochemical reactions and result in further growth of the filament. (e) When a negative voltage is applied to Ti electrode, some oxygen ions move back to the filament and recombine with oxygen vacancies resulting in rupture of vacancy part filament. The detailed filament transition mechanism would be displayed in the full paper. Figure 1