ALD/ALE 2022 Session AA1-WeA: ALD for Memory Applications II

In this work, magnesium oxide (MgO) thin films were grown via atomic layer deposition (ALD) as a buffer layer on titanium nitride (TiN) bottom electrode using bis(cyclopentadienyl) magnesium as the Mg precursor. Subsequently, beryllium oxide (BeO) and hafnium oxide (HfO₂) films were deposited by ALD on top of the MgO buffer layer using diethyl beryllium, tetrakis(ethylmethylamino) hafnium as the Be and Hf precursors, respectively. O₃ was used as the oxygen source for each ALD process. Such stacked films (MgO/BeO, MgO/HfO₂) were used as insulator layers for metal-insulator-metal (MIM) devices with TiN as bottom and top electrodes. The leakage current density (J) levels of the MIMs were significantly suppressed when the MgO buffer layer with a thickness of only ~1 nm was adopted. As a result, the 1 nm MgO buffer layer enabled a smaller total equivalent oxide thickness (EOT) value (defined by J < 1×10^-7 A cm^-2 at an applied voltage of + 0.8 V) for the application of dynamic random access memory capacitor. The electrical performance improvement might be caused by the structural change with the addition of the MgO buffer layer. Therefore, the possible templating effect of in-situ crystallization of ultra-thin (< ~ 3nm) BeO and HfO₂ films on the MgO buffer layer, which was improbable directly on TiN, was investigated through scanning/transmission electron microscopy (S/TEM). Moreover, the intermixing occurring during deposition and post-deposition annealing were discussed based on the depth profiles by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).

As s steep increase of memory capacity has been required, 2-terminal storage class memory (SCM) based on three-dimesional (3D) vertical cross-point (VXP) structure is receiving a lot of attention. With this regard, selector devices are an essential part of minimizing leakage current that can make failures of memory operation. Among them, ovonic threshold switching (OTS) materials consisting of chalcogenide materials (e.g., S, Se, and Te) has been regarded as a promising candidate for 3D X-point owning to its low leakage and high on current (I_on). Nevertheless, research on atomic layer deposition (ALD)-based OTS applications, which is crucial for sophisticated thickness control and conformality, is still in the beginning stage and especially sulfur-based ALD OTS material research is lacking despite its potential to possess superior OTS characteristics.[1]

Herein, we developed a thermal ALD Ge_1-xS_x (Ge-S) process using HGeCl₃ precursor and H₂S reactant. The growth characteristics and film properties of Ge-S film were investigated in detail. Besides, by incorporating thermal ALD Ge-Se designed by W.H Kim et al.[2] to our Ge-S, we fabricated ALD ternary germanium-sulfur-selenium (Ge-S-Se) alloy devices spanning a broad range of compositions by adjusting the ALD super-cycle ratio. The film characteristics and electrical parameters of Ge-S/Ge-Se/Ge-S-Se were evaluated and compared to verify the effect of its composition change. We successfully presented the foundation of the thermal ALD technique of Ge-S and opened the possibility to tune threshold voltage and to apply the Ge-S-Se of various composition ratios for desired purposes suitably.

Acknowledgements

This paper was a result of the research projected supported by SK Hynix Inc.

References

[1] S.Jia et al. Nature Communications 11, 4636 (2020)

[2] W.H Kim et al. Nanotechnology 29, 365202 (2018)

The ferroelectricity of doped HfO₂ thin films have been extensively investigated in many applications such as ferroelectric field effect transistor (FeFET), ferroelectric tunneling junction (FTJ) device, and ferroelectric random-access memory (FeRAM) etc. Previous studies have demonstrated that high-quality ferroelectric doped HfO₂ thin films can be easily obtained with a thickness of 5 nm or more.^1,2 However, it is difficult to achieve good ferroelectric property as the thickness of the ferroelectric layers decreased to sub-5 nm. In addition, the crystallization temperature increases significantly with a down-scaling of the ferroelectric film thickness due to the increase of the surface-area-to-volume ratio.² Therefore, we intensively investigated atomic layer deposition (ALD) process and post annealing conditions to obtain stable ferroelectric property at the Hf_0.5Zr_0.5O₂ (HZO) thickness of sub-5 nm.

In this study, HZO films were deposited using anhydrous H₂O₂ as an ALD oxygen source with the variation of deposition temperature at 250 °C to 350 °C. In addition, annealing temperature was varied from 400 °C to 500 °C. To confirm robust ferroelectric property, we fabricated TiN/HZO/TiN capacitors and characterized electrically (PE hysteresis, Pulse, CV and IV analysis). The crystallinity of HZO films were also analyzed using synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) at the Brookhaven National Laboratory. In addition, the half-cycle study using in-situ reflection absorption infrared spectroscopy (RAIRS) has revealed that anhydrous H₂O₂ forms high surface saturation. It is suspected that this chemical densification gives the advantages of scaling and low thermal budget of ferroelectric HZO film. The detailed results will be presented.

This work was supported by the Technology Innovation Program (20010806) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). This work was also partially supported by GRC-LMD program (#3001.001) of SRC, and the National Research Foundation of Korea (NRF) grant funded by MSIT (NRF-2019R1F1A1059972).

¹ Y. C. Jung et al., Phys. status Solidi-RRL15, 2100053 (2021).

² S. J. Kim et al., Appl. Phys. Lett.112, 172902 (2018).

The presence of ferroelectricity in hafnium oxide thin films can be controlled via doping. Since those layers are usually deposited via atomic layer deposition, the doping element (like Zr, Al, Si, La) is supplied via monolayers. This way, the metastable ferroelectric phase can be stabilized. However, often wake-up effects and asymmetries like imprints are present in the produced films, impairing optimal device performance in e.g. embedded non-volatile memory devices like ferroelectric field effect transistors. These imperfections will also affect the device behavior in piezo- and pyroelectric sensors and actuators.

We report on a novel method to control the phase and the switching behavior in hafnium oxide thin films by atomic layer deposition on 300 mm wafers. By utilizing two different doping elements with strong differences in their ionic properties, especially in their ionic radius and charge with respect to Hf, the here presented co-doping process enables to tune local stresses or electric fields. This is implemented by varying the doping concentration homogeneously in the layer and implementing concentration differences within the layer. For the hafnium oxide deposition two different precursors are explored, namely hafniumtetrachloride (HfCl₄) and tetrakis(ethylmethylamino)hafnium(IV) (TEMAHf). Water and ozone are used as oxidizers, respectively. For the electrical characterization, the ferroelectric films are implemented in MIM stacks with TiN as electrode material. These MIM stacks are annealedat different temperatures between 650 and 1050 °C to achieve the improved ferroelectric properties.

Depending on the used processing method, alternating or block wise dopant deposition, the phase stabilization of the ferroelectric orthorhombic Pca2₁ phase can be influenced, as indicated by the grazing incident X-ray diffraction results.

The polarization-voltage hysteresis shape can be modified by defining favored polarization axis orientations or influencing the domain wall movement. Moreover, effects like imprint can be counteracted. This is reflected in the displacement current by the current peak position, amplitude, shape and width. Consequently, this will affect the mechanical displacement and the pyroelectric properties as well, since they are dependent on the polarization behavior.

Kristjan Kalam¹, Markus Otsus¹, Raul Rammula¹, Joosep Link², Raivo Stern², Guillermo Vinuesa³, Salvador Dueñas³, Helena Castán³, Kaupo Kukli¹, Aile Tamm¹

¹ Institute of Physics, University of Tartu, W. Ostwald 1, 50411 Tartu, Estonia.

² National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia.

³ Department of Electronics, University of Valladolid. Paseo Belén, 15. 47011 Valladolid, Spain.

E-mail of presenting author: kristjan.kalam@ut.ee

Atomic layer deposited thin films have had a role in memory technology for quite some time, mostly as high-k dielectrics [1]. In recent years, however, such thin films have become a subject of interest as having intrinsic memory properties themselves, such as ferromagnetism and resistive switching [2-3]. HfO₂ thin films have been found to exhibit resistive switching properties [2]. Fe₂O₃ thin films have exhibited ferromagnetic properties [3]. Therefore, it is of interest to investigate if HfO₂ coupled with Fe₂O₃ would exhibit both resistive switching and ferromagnetic hysteresis in the same material sample.

Precursors to the films were FeCp₂ and HfCl₄, whereby O₃ was the oxidizer. Film thickness, elemental composition and crystal structure were evaluated. Most films exhibited ferromagnetic and/or superparamagnetic properties. Even un-doped HfO₂ could be magnetized, provided that the cubic phase was stabilized and present in the sample. Some films exhibited an electrical nonvolatile memory effect, unipolar resistive switching, where the resistance of a film can be switched between two distinct values. For example, a layered structure HfO₂+ Fe₂O₃+ HfO₂+ Fe₂O₃ was found to have both ferromagnetic and resistive switching properties.

[1]Niinistö, Jaakko, et al. Atomic layer deposition of high‐k oxides of the group 4 metals for memory applications. Adv. Eng. Mater. 11 (2009) 223-234.

[2] Lin, Kuan-Liang, et al. Electrode dependence of filament formation in HfO2 resistive-switching memory. J. Appl. Phys. 109 (2011) 084104.

[3] Kalam, Kristjan et al. Atomic layer deposition and properties of ZrO2/Fe2O3 thin films, Beilstein J Nanotech. 9 (2018) 119.

ALD-grown ferroelectrics have garnered much attention over the past decade due in large part to the discovery of ferroelectric doped-hafnia which has renewed interest in scalable, non-volatile, and neuromorphic memory. Nanolaminates consisting of ferroelectric, dielectric, and antiferroelectric thin films may be engineered to exhibit neuromorphic-enabling multi-state read/write capabilities, however, there are comparably few ALD processes available for archetypal antiferroelectric thin films compared to their ferroelectric and dielectric counterparts. Here, we present an ALD process to grow perovskite, antiferroelectric lead hafnate (PbHfO₃, PHO) using the commonly-used amide hafnium precursor tetrakis dimethylamino hafnium (TDMAH), lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) (Pb(DMAMP)₂) and O₂ gas as the co-precursor for both metalorganic compounds. We show that the composition may be controlled using a super-cycle consisting of n PbO cycles and one HfO₂ cycle. The films were deposited in an initially amorphous state with an interspersed polycrystalline PbO phase and crystallized into the perovskite state upon either furnace or rapid thermal anneal in an oxygen atmosphere. We observed dose saturation of the O₂ gas co-precursor only for extremely large exposures in excess of 3x10⁹ L. The growth per cycle (GPC) of the PHO, averaged across the supercycle, is shown to be approximately 0.5 Å. We investigate the chemical distribution and phase of the ALD PHO films before and after annealing using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS). Quantitative electrical characterization was performed on fabricated capacitor structures using 60 nm-thick PHO on a platinized substrate, which showed double-hysteresis polarization versus voltage loops with max/min polarization values of ±50 μC/cm² at ±16 V.

Recently, we have reported the ALD characteristics and film properties of Hf_0.5Zr_0.5O₂ (HZO) using high purity H₂O₂ and O₃. H₂O₂-based HZO showed higher GPC, lower wet-etch rate (WER), and higher film density than O₃-based HZO.¹ In comparison to H₂O, high purity H₂O₂ delivers 50% higher GPC, improved WER, and denser film from H₂O (SFig.1). H₂O₂ has low oxygen dissociation energy and high oxidation power comparable to O₃ as well as hydroxyl groups for ligand exchange reactions like H₂O, making it an ideal candidate for the oxide ALD process. Thus, an extensive study of H₂O₂ surface reactions is necessary to further investigate the reasons behind observed improvements. The conventional H₂O₂ precursor is commonly dissolved in a high content H₂O. Therefore, identifying the effects of H₂O₂ as an oxidant of ALD process from those of H₂O is a significant challenge.² In this study, we have implemented high purity anhydrous H₂O₂ to better understand ALD growth mechanism, interface formation, and film properties attributed by H₂O₂ while prohibiting the effects of H₂O.

It is expected that different oxidants (H₂O₂, H₂O, and O₃) can significantly impact the overall film growth and interfacial growth behaviors. ALD processes of HfO₂ using various oxidants are monitored to examine the surface pathways of H₂O₂, H₂O, and O₃. Comprehensive surface studies of ALD-HfO₂, deposited using TDMA-Hf and oxidants on TiN substrate, are studied using an in-situ reflection absorption infrared spectroscopy (RAIRS) system.³ Half-cycle study with differential and accumulated FTIR spectra will be investigated to identify the growth mechanism of different oxidants. This will allow us to observe the TiN and oxide interface formation and ligand exchange reaction while ALD process. Moreover, the FTIR spectra of the oxide bonding region will provide a better understanding of bonding density and oxide growth. Initial in-situ RAIRS study indicates that the surface absorption rate of H₂O₂ is significantly faster than H₂O, providing additional reaction sites during subsequent TDMA-Hf exposure steps. Eventually, the additional Hf–O bonds may increase film density, which can potentially provide enhanced film properties. Furthermore, interface formation is expected to be also observable using in-situ spectra of full ALD cycles by comparing the initial and bulk cycles (SFig.2).

This work was supported by Tech. Innovation Program (20010806) funded by MOTIE and GRC-LMD program (task#3001.001) through SRC. We thank RASIRC Inc. for providing the H₂O₂ source.

¹ J. H Kim et al., AVS, ALD 2021.

² D. Alvarez Jr. et al., Proc. SPIE 11326, 113260S (2020).

³ S. M. Hwang et al., ECS Trans., 92, 265 (2019).