ALD/ALE 2021 Session AA8: Memory Applications: DRAM
AA8-1 Influences of Oxygen Source and Substrate Temperature on Growth Mechanism of Atomic Layer Deposited Magnesium Oxide Using Bis(Cyclopentadienyl)Magnesium Precursor
Bowen Wang, Jinwoo Choi, Hyoung Gyun Kim, Seung Dam Hyun, Hoin Lee, Cheol Seong Hwang (Seoul National University)
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.View Supplemental Document (pdf)
AA8-2 Atomic Layer Deposition of Y-Doped TiO2 Thin Films to Decrease the Leakage Current for DRAM Capacitor Applications
Tae Kyun Kim, Cheol Seong Hwang (Seoul National University, South Korea)
This work reports the electrical behavior of Y-doped TiO2 (YTO) thin film as a dielectric layer for a capacitor of dynamic random access memory. The electrical performance of YTO thin films is comparable to that of Al-doped TiO2 (ATO) thin films (Fig. 1(a)), which is well-known for its excellent leakage current property among other series of doped rutile TiO2 thin films . Nonetheless, YTO thin films appear to suppress the leakage current via a different mechanism from that of the ATO thin films. The large ionic size of Y ion compared with the Ti inhibits the efficient diffusion of them into the lattice of the rutile TiO2 thin film, which results in a nano-laminate structure (Fig. 1b). In contrast, the small ionic size of Al in ATO fluently diffused into the TiO2 film and form acceptor states. Therefore, it could be guessed that the leakage current suppression mechanism in YTO film is similar to that in the ZrO2/Al2O3/ZrO2 case, where the interrupted columnar grain structure of the ZrO2 layer has contributed to the leakage current control. View Supplemental Document (pdf)
AA8-3 Improved Properties of the Atomic Layer Deposited Ru Electrode for Dynamic Random-Access Memory Capacitor Using Discrete Feeding Method
Dae Seon Kwon (Seoul National University, Korea); Woojin Jeon (Kyung Hee University, Korea); Dong Gun Kim, Tae Kyun Kim, Haengha Seo, Junil Lim, Cheol Seong Hwang (Seoul National University, Korea)
A ruthenium (Ru) thin film as a bottom electrode of dynamic random-access memory (DRAM) capacitor was grown via atomic layer deposition (ALD) with a discrete feeding method (DFM). The DFM-ALD was conducted by dividing the Ru source feeding and purge steps of the conventional ALD process into four steps (shorter feeding + purge times). The DFM-ALD could efficiently eliminate the adverse effects of the Ru precursors' physisorption and byproducts during the ALD process, resulting in a more fluent ALD reaction. The physicochemical properties of the Ru films deposited via conventional ALD, DFM-ALD, and sputtering were investigated and compared in detail. The Ru films deposited via DFM-ALD showed a higher density, smoother surface morphology (Fig.1), and preferred orientation of a (101) phase compared to the conventional ALD process. Also, the Ru films grown via DFM-ALD showed higher susceptibility of oxygen atoms into the Ru films, resulting in a higher proportion of the RuO2 layer on the Ru film surface, which is formed during the subsequent ALD process of the high-k TiO2 thin film. Such a higher RuO2 portion led to more fluent lattice-matched local epitaxial growth of the TiO2 films with a rutile structure as well as the smoother surface morphology without local protrusions. The possible mechanism of the improved TiO2 film growth mechanism was suggested (Fig. 2). These improvements in the physical performances also improved the electrical properties of the Pt/TiO2/Ru capacitors. A decrease in the interfacial component of equivalent oxide thickness by ~ 0.1 nm could be achieved by the DFM-ALD process compared with the cases on the sputtered Ru film. Consequently, minimum EOT values of ~0.76 nm and ~0.48 nm were obtained with enough low leakage current density (<10-7 A/cm2 at 0.8 V) for TiO2 and Al-doped TiO2 (ATO) dielectric films, respectively. The minimum EOT of 0.48 nm was comparable to the most remarkable results from the ATO/RuO2 layers, which would have induced integration issues caused by the reduction of RuO2. This finding indicated that the electrical performance of the high-k dielectric film was determined by not only the surface morphology but also the chemical properties of the underlying Ru substrate. View Supplemental Document (pdf)
AA8-6 A Comparative Study on the Crystallization Behavior of Atomic Layer Deposited ZrO2 / Y2O3 / ZrO2 Dielectric Thin Films for DRAM Capacitors
Haengha Seo, Dong Gun Kim, Dae Seon Kwon, Junil Lim, Tae Kyun Kim, Heewon Paik, Cheol Seong Hwang (Seoul National University)
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.View Supplemental Document (pdf)