First demonstration of Channel-All-Around TiO₂ Ferroelectric FET for high speed memory application

September 24, 2024

We propose a 3D stackable 4F² vertical Ferroelectric FET (FeFET) array concept (Figure 1). The key component is TiO₂ channel FeFET with Channel-All-Around (CAA) structure. FeFETs with oxide semiconductor channel are known to have better endurance than Si-channel FeFET owing to their interface-layer-free nature^[2]. We have demonstrated an excellent endurance over 1e11 cycles with planar TiO₂ FeFET^[1]. TiO₂ channel also has advantages in 3D stackability compared to other OS channels, thanks to its thermal stability.

Figure 2 shows TEM images of our CAA FeFET devices. The TiO₂/Fe-HfO/TiN stack was formed uniformly on the sidewall of 30nm-diameter hole. It is found that not only Fe-HfO but also TiO₂ channel were crystallized, which leads to high thermal stability of the channel against subsequent processes.

The I_d-V_g curve in Figure 3(a) exhibited counter-clockwise hysteresis after PMA, indicating ferroelectric switching. We focus on the change in drain current by polarization reversal (ΔI_on) at V_gs=0V, because read operation with Word Line=0V is desirable from the view point of high speed and low power operation. Endurance properties in Figure 3(b) showed stable ΔI_on over 1e6 cycles regardless of the TiO₂ thickness.

 In particular, high drive current (ΔI_on > 2μA/cell) and stable endurance were achieved simultaneously in 5-nm-thick TiO₂ devices, which has not been achieved in other vertical FeFETs. This work opens a pathway to future 3D stackable 4F² high speed memory.