Study of SiO2 Interfacial Layer Growth during Fabrication Process of Ferroelectric HfxZr1−XO2-Based Metal-Ferroelectric Semiconductor

Abstract

Recently, ferroelectric HfxZr1−xO2 (HZO) thin films have attracted much attention towards high-density and low-power ferroelectric memories such as ferroelectric field-effect transistors (FeFETs) for artificial intelligence (AI). [1] An annealing process at ≥ 300 °C is typically required to form the ferroelectric orthorhombic (O) phase of HZO films. [2, 3] It remains a big issue of an abnormal SiO2 interfacial layer (SiO2-IL) growth at the HZO/Si interface during metal-ferroelectric-semiconductor (MFS) fabrication process. It has been reported that a SiO2-IL growth led to degradation of remanent polarization (2P r) and reliability of MFS capacitors. [4] However, it is unclear how the process conditions of MFS structures, such as an atomic layer deposition (ALD) of HZO films and annealing process, affect the SiO2-IL formation in detail. In this work, we investigated a SiO2-IL growth as a function of the process temperature of ALD and annealing processes and a SiO2-IL growth of TiN/HZO/Si structures.P+-Si substrate was first cleaned using a buffered hydrofluoric acid to remove native oxide, and then an HZO film was deposited by ALD using H2O gas and (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor. The ALD growth temperature was varied from 200 to 300 °C. A 100-nm-thick TiN film was then fabricated as gate electrode by DC sputtering. Finally, the post-metallization annealing (PMA) was carried out at 300–600 °C in an N2 atmosphere. A 3-nm-thick HZO film was used to evaluate the SiO2-IL growth using XPS analysis. On the other hand, TiN-gated MFS capacitor with a 10-nm-thick HZO film was prepared to examine electrical properties.First, the formation of the SiO2-IL was found to be negligible small at HZO (3 nm)/Si interface regardless of the ALD growth temperatures because no significant peak originating from Si-O bonds was observed in XPS spectra of Si 2p. Therefore, we employed 300 °C as an ALD temperature of HZO films for the fabrication of MFS capacitors. Next, the SiO2-IL growth as a function of PMA temperature was evaluated. The SiO2-IL growth increased as PMA temperature increased from 400 to 600 °C. Therefore, the annealing process at a lower temperature suppressed a SiO2-IL growth for the fabrication of MFS structures.Finally, polarization–electric field (P– E) measurements were conducted to evaluate the ferroelectricity of TiN/HZO (10 nm)/p+-Si MFS capacitors after the PMA process at 300–600 °C. The MFS capacitors after the PMA process at 500 and 600 °C showed breakdown during the P– E measurement due to a large leakage current. The MFS capacitor after the PMA process at 400 °C showed significantly higher 2P r value of 26 µC/cm2 compared to that (7.6 µC/cm2) after the PMA process at 300 °C. This is because the HZO films of the MFS capacitor after the PMA process at 400 °C were crystallized with the ferroelectric O phase while the HZO films had an amorphous-like structure after the PMA process at 300 °C, evaluated by X-ray diffraction (XRD). These experimental results indicated that the PMA temperature significantly affects the growth of the SiO2-IL at the HZO/Si interface while almost no difference was observed between the ALD temperature conditions.This work was partially supported by JSPS KAKENHI (JP18J22998 and JP20H02189).[1] J. Y. Kim et al., APL Mater. 9, 021102 (2021).[2] T. Onaya et al., Microelectron. Eng. 215, 111013 (2019).[3] S. J. Kim, et al., S. J. Kim et al., Appl. Phys. Lett. 111, 242901 (2017).[4] J. Mohan et al., Appl. Phys. Lett. 118, 102903 (2021).