ALD/ALE 2021 Session AA8: Memory Applications: DRAM

This research mainly studied the dissimilarities of MgO thin film’s growth mechanisms in the atomic layer deposition (ALD) process depending on the oxygen sources, such as O₂ plasma, O₃, and H₂O (Fig. 1a, b). The bis(cyclopentadienyl)magnesium (Cp₂Mg) was adopted as the Mg-precursor. The effects of the oxidizing power and the temperature from 230℃ to 390℃ on the Cp-ring rupture reactions in the ALD process were analyzed by ex-situ techniques, such as X-ray diffraction/reflectometry, X-ray photoelectron spectroscopy (Fig. 1c and d), time of flight secondary ion mass spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy and transmission electron microscopy (Fig. 2a, b). It was intended to examine the crystallization, impurities and their depth profiles, stoichiometric ratio, surface/interface bonding, relative volumetric density. At relatively low substrate temperature (~ 250℃), albeit the similar crystallization and surface morphology were measured by glanced incidence X-ray diffraction and the field emission scanning electron microscope, O₂ plasma-enhanced ALD MgO showed the least impurity level and highest density. At the same time, the O₃-based thermal ALD-MgO exhibited the lowest density and highest impurity level among those three co-reactants. Unlike the overgrowth and void formation (Fig. 2b) at the growth temperature of 290℃, the O₃ ALD showed a comparably lower impurity level than the plasma-enhanced ALD at 335℃. Figure 1e shows the suggested ALD reaction mechanism. This was corroborated by the electrical characteristics using the planar metal-insulator-metal capacitors (metal layer was TiN). Furthermore, the pre-, post-annealing conditions were optimized for the optimal leakage current and dielectric constant of ALD-MgO.

ZrO₂ / Al₂O₃ / ZrO₂ (ZAZ) thin films have been applied as a representative dielectric for metal-insulator-metal (MIM) capacitors in mass-produced dynamic random-access memory (DRAM) devices. However, the inserted Al₂O₃ layer required for the leakage current suppression degraded the overall crystallinity of the dielectric film, resulting in a much higher equivalent oxide thickness (EOT) compared with the undoped ZrO₂ film as the films became thinner (< 6 nm).

In this work, ZrO₂ / Y₂O₃ / ZrO₂ (ZYZ) thin films were grown on TiN (bottom) electrode by atomic layer deposition (ALD) using Zr[N(CH₃)C₂H₅]₄ and Y(EtCp)₂(ⁱPr-amd) as precursors. ZAZ films were also deposited in the same manner for comparison. The thicknesses of both the Y₂O₃ and Al₂O₃ layers were fixed at ~ 0.3 nm, and they were deposited on top of the ~ 2.1 nm thick ZrO₂ bottom layer. Only the top ZrO₂ layer thickness was varied between 0 and 8 nm. All samples were subjected to post-metallization annealing (PMA) at 600 °C, after TiN/Pt (top) electrode deposition.

As can be seen from Figure 1, the EOT of the ZrO₂ and ZYZ films showed a sudden drop at a similar physical oxide thickness (POT) of ~ 3.5 nm, whereas that of the ZAZ curve showed a similar decrease at a much greater POT (~ 6.0 nm). The result evidently indicates that the minimum POT required for the crystallization of the top ZrO₂ layer during the PMA increased significantly on the Al₂O₃ layer, while the impact of the Y₂O₃ layer on the crystallization was almost negligible.

It is noteworthy that there was another transition point in the case of the ZAZ at ~ 7 nm, which might indicate the crystallization of the bottom ZrO₂ layer. This implies that even the bottom ZrO₂ layer had not been crystallized in the thinner ZAZ case due to the presence of the Al-doping, which was not the case in ZYZ. Although the Y₂O₃ insertion layer in ZrO₂ based dielectric did not inhibit the crystallization, the leakage current was still well suppressed (2.9 × 10^-8 A cm^-2 at 0.8 V, POT ~ 6.8 nm). ZYZYZ thin films exhibited the most stable and improved leakage current control among the tested candidates.