(Invited) Role of Oxidant Gas for Atomic Layer Deposition of HfxZr1−XO₂ Thin Films on Ferroelectricity of Metal-Ferroelectric-Metal Capacitors

Since the first report of ferroelectricity in Hf x Zr 1−x O 2 (HZO) in 2011 [1], a lot of attention has been devoted for future non-volatile memory device applications due to its scalability (< 10 nm), low thermal budget (< 400°C) and matured atomic layer deposition (ALD) technique [2]. The thermal ALD process using H 2 O or O 3 is generally employed for the fabrication of ferroelectric HZO thin films. On the other hand, the effect of the plasma-enhanced ALD process using O 2 plasma gas on the crystallinity and ferroelectricity has not been systematically clarified. In this paper, we report the importance of ALD oxidant gases for HZO films using H 2 O or O 2 plasma on the ferroelectricity of metal–ferroelectric–metal (MFM) capacitors. MFM capacitors with the H 2 O- and O 2 plasma-based HZO films were fabricated as follows. A 10-nm-thick HZO films were deposited on a TiN bottom-electrode (BE-TiN) by ALD at 300°C using H 2 O or O 2 plasma as oxidant gases and (Hf/Zr)[N(C 2 H 5 )CH 3 ] 4 (Hf:Zr = 1:1) cocktail precursor. The Hf/Zr ratios in both types of HZO films were estimated to be 0.4/0.6 using X-ray photoelectron spectroscopy (XPS). Next, a post-deposition annealing (PDA) process was carried out at 400 °C for 1 min under a N 2 atmosphere. Finally, a TiN top-electrode was deposited by DC sputtering. First, the crystallinity and remanent polarization (2 P r = P r + − P r − ) of the H 2 O- and O 2 plasma-based HZO films were investigated. Nanocrystals with ferroelectric orthorhombic (O), tetragonal (T), cubic (C) phases partially existed in the O 2 plasma-based film after the ALD process, while an amorphous structure dominantly existed in the H 2 O-based film, analyzed by the grazing-incidence X-ray diffraction analysis and cross-sectional transmission electron microscopy images [3]. This could be because of a strong oxidation power and high energy ion/electron bombardment of O 2 plasma. After the PDA process, the O phase was formed more in the O 2 plasma-based HZO film than the H 2 O-based film. Therefore, the nanocrystal grains in the as-grown O 2 plasma-based HZO film could play an important role as nuclei for the crystallization and formation of O phase [3,4]. From the polarization–electric field hysteresis curves, the PDA-treated O 2 plasma-based capacitor exhibited 2 P r of 20 µC/cm 2 , which was 1.5 times higher than that (13 µC/cm 2 ) of the H 2 O-based capacitor. Thus, these results indicate that the crystallization and O phase formation of HZO films were promoted by using O 2 plasma gas as an ALD oxidant, resulted in a higher 2 P r . Next, endurance properties of these MFM capacitors were evaluated. In the wake-up state, both capacitors showed almost the same increase rate (~8%) of switching polarization ( P sw ). In the fatigued state, on the other hand, the O 2 plasma-based capacitor exhibited less P sw degradation (~33%) after 10 6 cycles, whereas the P sw of H 2 O-based capacitor decreased by ~47%. To clarify these different fatigue properties, the TiO x N y interface reaction layer (IRL) formation at the HZO/BE-TiN interface was analyzed using synchrotron hard X-ray photoelectron spectroscopy (HAXPES). For the HAXPES Ti 2p spectra, the Ti-O peak intensity for the O 2 plasma-based capacitor was larger than that of the H 2 O-based capacitor [5]. Therefore, an oxygen-rich TiON-IRL could be formed at the HZO/BE-TiN interface during ALD process due to the stronger oxidizing power of O 2 plasma. Additional oxygen vacancies (V O ) were reported to formed in HfO 2 -based films during field cycling due to the interface reaction between the HfO 2 -based film and TiN electrode. Moreover, one of the origins of the P sw degradation was thought to be domain pinning caused by V O [6]. Therefore, an oxygen-rich TiON-IRL for the MFM capacitor with the O 2 plasma-based HZO film could prevent the interface reaction and formation of additional V O in HZO films, led to superior fatigue properties. In summary, the MFM capacitor with the O 2 plasma-based HZO film showed superior 2 P r and fatigue resistance compared to those of the H 2 O-based one. Based on these experimental results, it is important to consider the selection of an ALD oxidant for fabrication of HZO-based MFM capacitors. This work was partly supported by JSPS KAKENHI (Nos. JP21J01667, JP20H02189, and JP18J22998) and MEXT Leading Initiative for Excellent Young Researchers (No. JPMXS0320220213). The HAXPES measurements were performed under approval of the NIMS Synchrotron X-ray Station (2020A4602 and 2020A4651). [1] J. Müller et al., Appl. Phys. Lett. 99, 112901 (2011). [2] J.-H. Kim et al., ACS Appl. Electron. Mater. 5, 4726 (2023). [3] T. Onaya et al., APL Mater. 10, 051110 (2022). [4] T. Onaya et al., Microelectron. Eng. 215, 111013 (2019). [5] T. Onaya et al., Solid State Electron. 210, 108801 (2023). [6] M. Pešić et al., Adv. Funct. Mater. 26, 4601 (2016).