Recently, Hf_xZr_1−xO₂ (HZO) thin films have been widely investigated to achieve low power device applications such as ferroelectric random access memory (FeRAM) and ferroelectric field effect transistor (FeFET), due to its stable ferroelectricity even in extremely thin region (~10 nm) and CMOS applicability [1]. Moreover, matured atomic layer deposition (ALD) techniques of HfO₂ and ZrO₂ enable the fabrication of three-dimensional structure required for future memory devices. TiN top- and bottom-electrodes (TE- and BE-TiN) are typically used as stressor layers to fabricate metal-ferroelectric-metal (MFM) capacitors with HZO films because TiN electrodes provide the mechanical stress required for the formation of the unstable ferroelectric orthorhombic phase [2]. However, this method is suitable only for MFM capacitors, cannot be applied to FET structures. On the other hand, in our previous research, the HZO film fabricated using a polycrystalline ZrO₂ nucleation layer, inserted between the HZO film and the BE-TiN, results in the large remanent polarization (2P_r = P_r⁺ − P_r⁻) [3]. Therefore, the HZO film with top- and bottom-ZrO₂ nucleation layers is expected to improve the ferroelectricity. In this study, we fabricated two types of MFM capacitors with HZO films crystallized using top- and bottom-TiN stressor layers and ZrO₂ nucleation layers, and discuss the ferroelectricity and the leakage current (J) properties of two types of MFM capacitors.

The TiN/ZrO₂/HZO/ZrO₂/TiN capacitors with top- and bottom-ZrO₂ nucleation layers (D-ZrO₂) were fabricated as follows: A 2-nm-thick bottom-ZrO₂ layer was deposited on the BE-TiN by ALD at 300°C using (C₅H₅)Zr[N(CH₃)₂]₃ precursor and H₂O gas. Next, a 10-nm-thick HZO film was deposited on the bottom-ZrO₂ layer by ALD at 300°C using (Hf/Zr)[N(C₂H₅)CH₃]₄ (Hf/Zr = 1/1) cocktail precursor and H₂O gas. A 2-nm-thick top-ZrO₂ layer was then deposited on the HZO film using the same ALD conditions. After that, post-deposition-annealing (PDA) was carried out at 600°C for 1 min in a N₂ atmosphere. Finally, TE-TiN was fabricated on the top-ZrO₂ layer by DC sputtering. Two types of TiN/HZO/TiN capacitors with 10-nm-thick HZO films were also prepared as references. One is the capacitor annealed before TE-TiN deposition (w/o), the other is the capacitor annealed after TE-TiN deposition under the same annealing process (D-TiN).

The Hf:Zr ratio in the HZO film was estimated to be 0.43:0.57 by energy dispersive x-ray spectroscopy analysis. Fig. 1 shows the polarization-electric field (P-E) hysteresis curves of w/o, D-TiN, and D-ZrO₂. All of the capacitors exhibited hysteresis loops in the range from −3.0 to 3.0 MV/cm. The 2P_r value was increased in the following order: w/o (12 µC/cm²) < D-TiN (24 µC/cm²) < D-ZrO₂ (29 µC/cm²). The ratio of ferroelectric orthorhombic phase was increased in the following order: w/o < D-TiN < D-ZrO₂, from x-ray diffraction patterns (not shown). Therefore, we determined that the top- and bottom-ZrO₂ nucleation layers play an important role for the ferroelectric orthorhombic phase formation of HZO films. Fig. 2 shows the relationship between the J value at 1.0 V and the 2P_r value of w/o, D-TiN, and D-ZrO₂. For D-TiN case, the 2P_r value was approximately 2.0 times larger than that of w/o probably due to the mechanical stress from TE-TiN. However, the J value of D-TiN was increased in the low voltage region compared to w/o. On the other hand, for D-ZrO₂ case, the 2P_r value was about 2.4 times larger than that of w/o. Moreover, the J value of D-ZrO₂ was lower by one order of magnitude than that of D-TiN due to the larger thickness of D-ZrO₂ than that of D-TiN. Based on these experimental results, the HZO films exhibited significantly superior ferroelectricity using double ZrO₂ nucleation layers compared to that fabricated using double TiN stressor layers.

This work was supported partly by Grant-in-Aid for JSPS Research Fellow and CREST, JST.

[1] J. Müller et al., Appl. Phys. Lett. 99, 112901 (2011).

[2] S. J. Kim et al., Appl. Phys. Lett. 111, 242901 (2017).

[3] T. Onaya et al., Appl. Phys. Express 10, 081501 (2017).