Atomic Layer Deposition of HfAlOx for High-k Gate Dielectrics
Authors: Bedjaoui Messaoud , Dartois Mélanie , Jousseaume Vincent , Nolot Emmanuel , Mazel Yann , Richy Jérôme , Bout Céline 
Published: February 2024

Nowadays, the downscaling of CMOS devices requires continued effort to propose high performance components with extremely compact size. Surely, this trend is not achievable without extremely enhanced High-k gate stack materials, inter alia. For years, HfO2-based dielectrics deposited using ALD (Atomic Layer Deposition) technique are perfect dielectrics of choice for various devices and applications (Memristor, resistive random access memory, ferroelectric random access memory, antiferrolectric capacitances, ferroelectric FET...).
As such, HfAlOx films present a good compromise between HfO2 and Al2O3 in term of coupling properties and insulating capabilities in comparison with both single HfO2 and Al2O3 films. The purpose of this work is to evaluate binary high-k HfAlOx dielectrics deposited by ALD technique. ALD is well suited for deposition of ultrathin films with exceptional capabilities enabling precise thickness, stoichiometry control and conformal growth at atomic scale. In this work, we focused on Al2O3/HfO2 films fabricated using two approaches: (i) repeatedly alternating the deposition of two different and separate extremely thin oxides (Al2O3 and HfO2; particular focus on HfO2/Al2O3/HfO2/Al2O3/HfO2 multilayer stack with equivalent Al fraction around 30%); (ii) combination of ALD super cycles allowing the formation HfAlOx layers. We analyze the general performance and the behavior of the binary high-k HfAlOx dielectrics (10 nm thin layers deposited at 300°C using TiCl4-TMA-H2O precursors) using physico-chemical (X-ray photoelectron spectroscopy XPS, X-ray reflectometry XRR, wavelength dispersive X-ray fluorescence WDXRF) and electrical measurements (Hg probe measurements, MIM capacitance).
The incorporation of Al2O3 in the composition of the HfO2 gate dielectrics allows a perfect monitoring of physico-chemical properties varying from HfO2 layers to Al2O3 layers by controlling the number of Al2O3 and HfO2 ALD cycles. In this respect, we were able to monitor the Al fraction in the range of 5% to ~86% in the mixed binary dielectrics (Figure 1: XRR density of mixed binary layers vs Al/[Al+Hf] ratio, Figure 2: Al and Hf fractions in the mixed layer determined by WDXRF and XPS analysis).
The electrical characteristics of the HfAlOx dielectrics confirm the influence of the Al fraction in the HfO2 layer. The leakage currents of the mixed layers decreases with increasing the Al fraction (Figure 3: J/E characteristics of mixed and stacked HfAlOx layers). The breakdown voltages for all mixed HfAlOx layers are greater than that of reference HfO2 layer. On the other hand, the shown multilayer presents an intermediate leakage currents profiles between the two mixed HfAlOx (25%Al and 40%Al). Thus, the incorporation of Al in the construction of HfAlOx system is likely to decrease the number of defects. Furthermore, the fully integrated MIM structures exhibit areal capacitance in the range 10-15 nF/mm2 of ALD-based high-k HfAlOx (10 to 12 nm) depending on the Al fraction. Thus, ALD-HfAlOx with various composition highlight the potential tremendous for high-κ capacitances stack for CMOS semiconductor devices.

Figure 1