Design Guidelines of Thermally Stable Hafnia Ferroelectrics for the Fabrication of 3D Memory Devices

We provide a methodology for designing thermally stable hafnia ferroelectric (FE) materials to be taken into account while fabricating 3D memory devices. We reveal the underlying origins for the thermal instability of hafnia FE materials in terms of kinetics and material science. Furthermore, we suggest adopting dopants whose ionic radius is smaller than Hf in the FE matrix as a feasible option to demonstrate a thermally stable hafnia FE material. Using this approach, robust ferroelectricity is achieved even at a subsequent thermal budget (TB) of 750 °C for 30 min. The improved thermal stability stems from the reduced formation of both the m-phase and oxygen vacancy (VO) in hafnia FE materials at high TB. The in-depth electrical and material analysis in this study serves a framework for further research into the thermal stability of FE. This work contributes to the commercialization of FE devices by filling the gap between the functionality of FE materials and their process applicability for 3D devices.

I. Introduction

Today, FE memory devices have drawn enormous attention due to the advantages of hafnia FE materials in CMOS compatibility, functionality and scalability. In particular, vertical FE NAND flash [1] and M3D integrated devices [2] have been extensively researched for low-power and high-density NVMs. However, for the fabrication of 3D devices, the TB of hafnia FE layers dramatically increases with the number of subsequent process steps. Specifically, for 3D FE NAND flash devices, the FE layers are exposed to successive TB (750 °C for 30 min) during the polysilicon channel deposition process using LPCVD (Fig. 1(a)). On the other hand, for M3D integration, the FE materials at the bottom layer are subjected to the following TB (650 °C for 20 min) during the fabrication of the upper layers [3] (Fig. 1(b)). In this regard, the fabrication of 3D FE memory devices requires the high thermal stability of hafnia. However, so far, research efforts have mostly been focused on ‘high-performance 3D devices’ and there aren’t enough reports on the thermal stability of hafnia FE materials.