Abstract:

HfO₂-based ferroelectric capacitors, particularly TiN/Hf_xZr_1-xO₂/TiN metal insulator metal (MIM) capacitors, have attracted considerable attention as promising candidates in the new generation of nonvolatile memory applications, because of their excellent stability, high performance, and complementary metal oxide semiconductor (CMOS) compatibility. At the electrode interface of TiN/Hf_xZr_1-xO₂/TiN MIM ferroelectric devices, the existence of the TiO_xN_y layer, which was formed during Hf_xZr_1-xO₂ film crystallization and TiN oxidization, can affect interface/grain boundary energy, film stress, and conduction band offset at the TiN/Hf_xZr_1-xO₂ interface, thereby affecting the ferroelectric device performance. Because the electrical performance of TiN/Hf_xZr_1-xO₂/TiN capacitors depends on both the ferroelectric Hf_xZr_1-xO₂ thin films and electrode TiN/insulator Hf_xZr_1-xO₂ interface, it is essential to control the fabrication of the TiN/Hf_xZr_1-xO₂/TiN heterostructure. Herein, we report a new method for preparing Hf_xZr_1-xO₂ ferroelectric thin films, sandwiched between TiN electrodes, by atomic layer deposition (ALD) and using ultra high vacuum (UHV) sputtering equipment interconnected with an ultra-high vacuum system. The quasi in situ characterization by transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and other analytical methods conducted in our study indicates that the surface of the bottom TiN electrode does not contain oxygen. Moreover, a flat signal for impurities at the interface suggests that the superior ferroelectric performance of Hf_xZr_1-xO₂-based device is mainly attributed to the pristine Hf_xZr_1-xO₂/TiN interface. Furthermore, the ferroelectric properties of TiN/Hf_xZr_1-xO₂/TiN heterostructures on silicon can be modulated by varying ZrO₂ doping concentration and rapid thermal annealing (RTA) temperature, which can be well monitored and controlled by the interconnected system. We also investigate the ferroelectric properties of TiN/Hf_xZr_1-xO₂/TiN capacitors with different ZrO₂ doping concentrations (30%–60% (x)) at room temperature by changing the ALD pulsing ratio within the vacuum interconnected system. Three identical 10 nm-thick Hf_0.5Zr_0.5O₂ samples sandwiched between TiN electrodes are annealed in N₂ ambient at 400, 450 and 600 ℃ for 5 min to investigate the effect of RTA on device performance. The evolution of P-E hysteresis at different applied voltages and RTA temperatures reveals that the saturation of P-E hysteresis and remanent polarization increase with RTA temperature. This increase is especially evident at low applied voltages such as 1.5 V. A higher remanent polarization of 21.5 μC·cm^-2 than the previously reported value and a low coercive voltage of 1.35 V were achieved for the electric field of 3 MV·cm^-1 by doping 50% (molar fraction, x) ZrO₂ in HfO₂ through RTA at 600 ℃ for film crystallization.

Key words: Ferroelectrics, Surface, Interface, HfO₂, Vacuum interconnection, In situ