Millisecond Crystallization of Ferroelectric Al:HfO2 Thin Films with Different Al Concentrations by Flash Lamp Annealing

Al-doped HfO₂ (HAO), which exhibits ferroelectric orthorhombic phase in narrow Al concentration range (2~4 at%) [1, 2], is one of promising candidates for 3D device applications because of its high thermal stability [3]. On the other hand, HAO requires a higher crystallization temperature than other HfO₂-based ferroelectrics. Therefore, we have developed flash lamp annealing (FLA) process for HAO films, which offers low-thermal-budget processing on a millisecond timescale. We found that FLA can crystallize 6%-doped HAO films and the films showed 2Pr of 17.6 μC/cm². In this study, we investigated the dependence of the crystallization and ferroelectric properties of HAO films on the doping concentration, with the aim of optimizing the Al concentration for FLA.
We fabricated TiN/10-nm-thick HAO (Al:0-12%)/TiN capacitors. HAO films were prepared by atomic layer deposition (ALD) and subsequently crystallized by FLA at temperature ranging from 800 to 1000 °C for durations of 2 to 10 ms. As a comparison, rapid thermal annealing (RTA) between 400 and 800 °C for 1 min was also used for crystallization. Both FLA and RTA were performed in an N₂ atmosphere. Crystalline and ferroelectric characterization were characterized by grazing incidence X-ray diffraction (GIXRD) and ferroelectric test system (FCE10).
GIXRD measurements revealed that HAO film with higher Al concentrations required a higher crystallization temperature in both RTA and FLA. In addition, the orthorhombic (o)/tetragonal (t)-phase HAO peak around 30° shifted to a higher angle with increasing Al concentration, suggesting that Al was incorporated into HfO₂ and/or contributed to an increase in the residual tensile stress in the HAO films. At the same Al concentration, the intensity and diffraction angle of FLA-derived HAO films (FLA-HAO) were larger and higher than those of the films treated by RTA (RTA-HAO). Furthermore, at Al concentration of 2%, a monoclinic phase was observed for RTA-HAO, whereas FLA-HAO exhibit no such phase. These XRD results indicate that FLA can suppress the transition to monoclinic phase and promote the formation of orthorhombic phase more effectively than RTA. This enhancement is attributed to the reduced stress relaxation during heat treatment of only a few milliseconds, which reinforces the tensile stress in HAO films induced by compressive stress in TiN electrodes.
P-E measurements demonstrated that polarization was dependent on Al concentration; 2Pr showed maximum values of 40 and 17 µC/cm² at an Al concentration of 2% for FLA-HAO and RTA-HAO, respectively. Taking into account for larger XRD peak intensity for FLA-HAO, the larger 2Pr can be attributed to the increased volume fraction of ferroelectric phase. These results indicate that 2Pr as high as 40 µC/cm² can be achieved for HAO films crystallized by a low-thermal-budget heating process.
[1] J. Mueller et al., Adv. Funct. Mater. 22, 2412-2417(2011), [2] M. H. Park et al., Nano Energy 36, 381-389(2017), [3] G. Kim et al.,IEDM,5.4.1-5.4.4(2022), [4] H. Tanimura et al., Jpn. J. Appl. Phys. 63, 09SP10 (2024)