Ole Gronenberg (Kiel / DE), Richard Marquardt (Kiel / DE), Ulrich Schürmann (Kiel / DE), Hermann Kohlstedt (Kiel / DE), Lorenz Kienle (Kiel / DE)
Abstract text (incl. figure legends and references)
Introduction
HfO2 and related compounds attracted considerable interest for different non-volatile storage elements. The functionality and device performance of memory related applications is strongly affected by oxygen vacancies, which can stabilize the metastable ferroelectric phase of HfO21. The wake-up effect of ferroelectric HfO2 was attributed to defect related changes2, phase transitions3 and structural transformations4.
Objectives
Ferroelectric capacitors consisting of TiN electrodes and sputtered HfO2 on an 1 cm2 Si substrate showed different ferroelectric properties due to a temperature gradient during rapid thermal annealing (RTA). Devices from the hot corner had low remnant polarization of 10 µC/cm2 without changing during field cycling, while devices from the cold corner showed a strong wake-up effect with 0 µC/cm2 in pristine devices and up to 26 µC/cm2 after 106 cycles. TEM analysis of devices from both positions was applied to correlate the electrical characteristics with microstructural differences.
Materials & methods
These devices were prepared by reactive sputtered TiN electrodes and RF sputtered HfO2. TiN top electrodes were deposited after RTA and patterned with UV lithography. In the RTA set-up, approximately 600 °C were applied for 10 s in N2 atmosphere. IR thermal imaging after RTA revealed a thermal gradient due to a disaligned filament inside the projector lamp used for heating.
Plan-view TEM samples from the hot and the cold corner were prepared by PIPS after the devices in the center were trained for 105 cycles with +/- 3 V (see Fig. 1). Additionally, cross-sectional TEM samples were prepared by FIB. TEM analysis in plan-view and in cross-section via electron diffraction (ED), HRTEM and EELS was correlated with the electrical characteristics.
Fig. 1: The Si substrate with TiN/HfO2/TiN capacitors is shown on the left with trained devices marked with blue circles. On the right the PIPS sample is shown with two trained devices close to the hole.
Results
The combination of ED in plan-view and cross-sectional TEM analysis, revealed different microstructures and interfaces to the TiN top electrodes. At the interface to the TiN top electrode a TiOxNy interlayer was confirmed with EELS, which was thicker in the cold corner. Regarding the microstructure, the cold corner shows a stronger [110] texture while in the hot corner the <111> oriented grains became dominating due to Ostwald ripening. The in-plane polarization of the [110] textured HfO2 and the observed structural transformation of the ex situ trained devices hints towards a ferroelastic switching during wake-up.
Conclusion
In-depth structural characterization in close correlation with the electrical response of ferroelectric TiN/HfO2/TiN capacitors revealed a strong structural influence on the wake-up effect induced by a temperature gradient during RTA. The hot corner showed wake-up free remnent polarization due to increased <111> texture, while the cold corner has a strong wake-up effect due to [110] oriented grains with a pristine in-plane polarization. Gradual ferroelastic switching to [011] and [101] orientations increases the remnent polarization with field cycling.
References
1 T. Mittmann et al., Adv. Mater. Interfaces 1900042 (2019).
2 S. Starschich et al., Appl. Phys. Lett. 108, 032903 (2016).
3 E.D. Grimley et al., Adv. Electron. Mater. 2, 1600173 (2016).
4 T. Shimizu et al., Appl. Phys. Lett. 113, 212901 (2018).