A special feature of ferroelectric HfO₂ films is that they have large coercive fields (E_C), as large as 1 MV/cm and more. A merit of the large coercive field is that a sufficient memory window is attainable even in ultrathin films. For example, a 2-V-memory window is available by 10-nm-thick Hf-Zr-O ferroelectric film. Compared with the conventional ferroelectric oxide capacitors using Pb(Zr, Ti)O₃ and SrBi₂Ta₂O₉ that applies around 100-nm-thick films, the ferroelectric HfO₂ films contribute to the scaling of film thickness about one order. It is attractive for the development of high-density non-volatile memories integrated on LSI. On the other hand, the large coercive fields of ferroelectric HfO₂ films bring serious concerns such as breakdown and endurance in capacitors. In this work, we studied the influence of large coercive fields on the endurance properties of Hf-Zr-O capacitors.

Metal-ferroelectric-metal (MFM) capacitors that consist of TaN/Hf-Zr-O/TaN were prepared on heavily doped Si substrates. TaN electrode films were deposited by DC sputtering, using Ta target and mixture gas of Ar and N₂. Hf-Zr-O films were prepared by RF sputtering, using HfO₂ and ZrO₂ targets and Ar gas. The metal composition was adjusted to Hf : Zr=50 : 50, and the thickness is 10 nm. After deposition of the top TaN film, crystallization anneal was processed in a vacuum condition at 700 degree Celsius. Finally, top electrodes were patterned by photolithography and dry etching.

Ferroelectric properties of 10-nm-thick Hf-Zr-O capacitors were investigated by the polarization-voltage measurements. The E_Cvalue of this capacitor was evaluated to be 1 MV/cm. The electrical breakdown by the DC mode was measured to be about 4 MV/cm. Remnant polarization (Pr) and endurance properties of the capacitors are summarized in the figure. The Pr is faint when the applied electric field is less than the E_C. The Pr value increases drastically when the electric fields are set between E_C and 2xE_C, and then gradually saturates over 2xEc. Thus a larger electric field is favorable for the achievement of large and stable polarization. In contrast, the endurance characteristics show a negative trend with the electric field. The endurance cycle as large as 10⁸ was observed when the electric field is 2xE_C, but it decreases drastically with the increment of electric field over 2xE_C. It indicates that the capacitors are damaged even when the electric field is much lower than the breakdown field. Although a better endurance is expected at electric field lower than 2xE_C, it makes a sacrifice of the Pr value and the stability. This trade-off relationship is considered to be a critical hurdle for the application of ferroelectric HfO₂ capacitors.

This work was supported by JST CREST Grant Number JPMJCR14F2, Japan.