ALD/ALE 2021 Session AA11: Memory Applications: Other Non-Volatile Memories (MRAM, FeRAM, Phase Change,...)
AA11-1 Fabrication of Vertical-Type Phase-Change Memory Leveraging Atomic Layer Deposition
Jeong Woo Jeon, Chanyoung Yoo, Eui-sang Park, Woohyun Kim, Wonho Choi, Byongwoo Park (Seoul National University); Yoon Kyeung Lee (Jeonbuk National University); Cheol Seong Hwang (Seoul National University)
Storage Class Memory (SCM) is a new hybrid storage/memory tier to achieve high speed, low power computing using a nonvolatile and byte-accessible memory denser than DRAM and faster and more durable than flash memory. Intel and Micron Technology recently commercialized the SCM using chalcogenide-based phase-change memory (PCM) that utilizes resistance contrast between the amorphous and crystalline states for data storage. It has stacked memory cell arrays that alternately share either wordlines or bitlines between the different memory layers. This structure inevitably requires lithography and patterning steps proportional to the number of layers, resulting in high production costs. For the commercial success of PCM-based SCMs, it is important to achieve high density and low cost per bit, which requires the development of a novel three-dimensional (3-D) architecture similar to the 3-D vertical-NAND device.
This report demonstrates the vertical-type PCM (V-PCM) enabled by atomic layer deposition (ALD) of Ge2Sb2Te5 (GST-225), as shown in Fig. 1. SiO2 was used for interlayer dielectric (ILD) separating each memory layer, and TiN was used for the bottom electrode (BE). Fig. 1(b) shows an ALD GST-225 films conformally grown on vertically etched sidewalls, in which ILD and BE are alternately stacked. The switching region is defined by the patterned width and thickness of the BE, equivalent to the structure of a mushroom type cell vertically erected. In this work, the contact area was as high as 0.02 mm2 due to the limited lithography capability of university scale research. The electrical characteristics of the fabricated device can be seen in Fig. 2. The SET and RESET characteristics of the V-PCM device are shown with a threshold voltage of 1.4 V and a RESET current of 4 mA, which corresponds to a RESET current density of 20 MA/cm2. The cyclic endurance was more than 108 cycles, which is sufficiently high compared with the planar type PCM devices, showing the feasibility of the ultra-high density V-PCM.
 S. W. Fong et al., IEEE Trans. on Electron Devices, 2017, 64, 11, 4374-4385. E.-S. Park et al., Chem. Mater., 2019, 31, 21, 8752-8763 View Supplemental Document (pdf)
AA11-2 Effect of Ti Scavenging Layer on Ferroelectricity of HfxZr1−xO2 Thin Films Fabricated by Atomic Layer Deposition using Hf/Zr Cocktail Precursor
Takashi Onaya (Meiji University/National Institute for Materials Science/JSPS Research Fellow); Toshihide Nabatame (National Institute for Materials Science); Naomi Sawamoto (Meiji Renewable Energy Laboratory); Akihiko Ohi, Naoki Ikeda, Takahiro Nagata (National Institute for Materials Science); Atsushi Ogura (Meiji University/Meiji Renewable Energy Laboratory)
Ferroelectric HfxZr1−xO2 (HZO) films have attracted a lot of attention for ferroelectric field-effect transistor (FeFET) applications. Numerous papers have reported that an annealing process at > 300°C is required to obtain the ferroelectric orthorhombic (O) phase.  However, an interlayer such as SiOx between an HZO film and a Si substrate was typically formed during the fabrication process of metal-ferroelectric-semiconductor (MFS) structures and an annealing process. To understand how the fabrication process affects the interlayer formation is important because the interlayer can cause reliability problem and reduction in remanent polarization (2Pr). We employed a Ti layer deposited on an HZO film because Ti can scavenge oxygen from a SiOx interlayer.  In this work, we studied the effect of an annealing temperature on the interlayer formation and ferroelectricity of HZO-based MFS capacitors with a Ti layer.
A 10-nm-thick HZO film was deposited on a p+-Si substrate by atomic layer deposition at 300°C using (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor and H2O gas. Next, a 1-nm-thick Ti layer was deposited on an HZO film by DC sputtering. A 100-nm-thick TiN top-electrode was then fabricated by DC sputtering. Finally, a post-metallization annealing (PMA) was performed at 300 or 400°C for 1 min in N2 ambient. TiN/HZO/p+-Si capacitors were also fabricated as references.
For the MFS capacitor without a Ti layer, the SiOx interlayer could be formed between an HZO film and a Si substrate after the PMA at 400°C while the formation of the interlayer was found to be negligible, evaluated by X-ray photoelectron spectroscopy. The 300°C-PMA-treated MFS capacitors showed almost the same capacitance (C) of 0.8 µF/cm2 regardless of the presence of a Ti layer. After the PMA at 400°C, on the other hand, the higher C was obtained because HZO films were crystallized with the ferroelectric O phase. Moreover, the MFS capacitors with a Ti layer exhibited slightly higher C of 1.5 µF/cm2 than that (1.3 µF/cm2) without a Ti layer. This might be attributed to the reduction of the interfacial SiOx layer due to the scavenging effect of a Ti layer.  Therefore, the higher 2Pr value (33 µC/cm2) of the MFS capacitor with a Ti layer was achieved compared to that (26 µC/cm2) without a Ti layer. Based on these results, inserting a Ti layer could be one of the pathways to improve ferroelectricity of HZO films in MFS structures.
This work was partially supported by JSPS KAKENHI (JP18J22998 and JP20H02189).
 T. Onaya et al, Microelectron. Eng. 215, 111013 (2019).
 H. Kim et al., J. Appl. Phys. 96, 3467 (2004).View Supplemental Document (pdf)
AA11-3 Atomic Layer Deposition of Antiferroelectric La-Doped Hf0.5Zr0.5O2 Thin Film and Its Electrical Behaviors
Yong Chan Jung, Jin-Hyun Kim, Su Min Hwang, Jaidah Mohan, Heber Hernendez-Arriaga (University of Texas at Dallas); WanJoo Maeng, Kivin Im (SK hynix Inc); Jiyoung Kim (University of Texas at Dallas)
Recently, the ferroelectricity and antiferroelectricity of doped Hf-based fluorite-structured ferroelectric thin films have been extensively investigated. In particular, it has been reported that La-doping for Hf0.5Zr0.5O2 (HZO) thin films can be applied to stabilize the ferroelectric orthorhombic (O) phase (Pca21) if the low doping concentration is precisely controlled.1 On the other hand, ferroelectric-antiferroelectric transition is shown when the amount of La doping is relatively large due to its amorphizing characteristic for HfO2,i.e. increase of crystallization temperature.2 For the antiferroelectricity of HZO, it is known as the nonpolar tetragonal (P42/nmc) is contributed, however, it is less clear than the ferroelectric O phase.
In this study, we investigated the doping effects of La on the antiferroelectric properties of the HZO film. The HZO film as a reference was deposited on the TiN bottom electrode by atomic layer deposition (ALD) using TDMA-Hf, TDMA-Zr, and O3 as the precursors of Hf, Zr, and oxidant, respectively. To dope the HZO film with La, La(iPrfAMD)3 and O3 were used as the La precursor and oxygen source, respectively. The 10-nm thick La-doped HZO film (LHZO) growth was proceeded with 6 super cycles consisting of 8 (Hf-purge-O3-purge-Zr-purge-O3-purge) and 1 (La(iPrfAMD)3-purge-O3-purge). In Figure 1, the Hf, Zr, and La concentration of the LHZO film is 48, 48, and 4 at. % as confirmed by XPS depth profiling. After the TiN top electrode was deposited on HZO and LHZO films, rapid thermal annealing was done, and metal-insulator-metal capacitors were fabricated using a Pd/Au hard mask and wet etch process.
In Figure 2(a) and 2(b), the small-signal dielectric constant of LHZO film at 0 MV/cm was increased to 69 compared to 48 of HZO film and the leakage current density of LHZO film at 1 MV/cm was approximately 2 order magnitude lower than HZO film, respectively. We suspected that the higher dielectric constant and lower leakage current is caused by tetragonal phase in the LHZO film. In Figure 3(a), the ferroelectric-antiferroelectric transition of the LHZO film was proved by the polarization-electric field curves, the remnant polarization (2Pr) of HZO and LHZO devices is 56 and 5 µC/cm2, respectively. As shown in Figure 3(b), interestingly, after 108 and 109 endurance cycling, the 2Pr of the LHZO film is recovered to 13 and 26 µC/cm2, respectively. It is plausible to suggest that this phenomenon is occurred due to the field-induced ferroelectric phase transition3 or the effect of domain unpinning after longer switching cycles with high electric field (2.5 MV/cm).This work is supported by SK hynix Inc. View Supplemental Document (pdf)
AA11-4 Metal-insulator Transition in ALD VO2 using VCl4 and H2O as Precursors
Jeya Prakash Ganesan, Durjoy Dev, Adithi Krishnaprasad (University of Central Florida); Daniel Moser, Ravindra Kanjolia (EMD Electronics); Tania Roy (Nanoscience Technology Center, University of Central Florida); Parag Banerjee (University of Central Florida)
Vanadium dioxide (VO2) undergoes a reversible transition between the semiconducting (monoclinic) and metallic (tetragonal) state at 68 °C, thus making VO2 a perfect candidate for electrical/optical switches, thermal sensors, metamaterials, and oscillators. Atomic Layer Deposition (ALD) of VO2 has been reported with different metalorganic and halide-based vanadium precursors. Out of these, the halide-based precursors have the advantage of a simpler chemistry, high vapor pressure and ease of delivery, little or no potential carbon residue and use of milder oxidants such as, H2O.
In this talk, we demonstrate the ALD of VO2 using VCl4 and H2O in a VEECO® FIJI Gen2 ALD system. The as-deposited films are amorphous and turn crystalline VO2 only after a post-deposition anneal at 550 oC, 60-minute using forming gas. Raman spectroscopy is used to confirm the amorphous nature of the film pre-anneal, and its conversion to monoclinic VO2 post-anneal. X-ray photoelectron spectroscopy suggests that the as-deposited film and the annealed film show vanadium oxides with mixed valence states on the surface and VO2 in the bulk. Thus, despite using a V4+ precursor significant surface oxidation takes place during deposition to produce a multivalent oxygen-rich surface. The excess surface oxygen could result in an amorphous film. Temperature-dependent Raman spectroscopy and ellipsometric studies reveal the semiconducting to metallic transition (SMT) of annealed and crystallized VO2 thin film. The transition temperature is recorded at 68 °C for a 30 nm film. Optical constants (n, k) from ellipsometry suggests that beyond 68 °C, significant free carrier absorption in the near infrared results in higher k. Electrical measurements performed on a fabricated device showed SMT behavior at 68 °C with a resistance high (semiconducting) : low (metallic) ratio of 66.
In conclusion, we have deposited 30 nm VO2 via ALD using VCl4 and H2O at 350 oC. Contrary to a past report,1 the VO2 deposited in the current work is amorphous and must be annealed at 550 oC for 60 minutes in forming gas to obtain VO2 films with SMT properties. Experimental investigations are currently underway to understand the synthesis-structure-property relationship in this promising ALD chemistry such that as-deposited, crystalline VO2 films can be reliably obtained.
AA11-7 Study of the Impact of Annealing and Doping Element on the Crystallinity of Hafnium Oxide: Application for FeFET Memories
Ivane BOTTALA-GAMBETTA, Mickael GROS-JEAN (STMicroelectronics); Arnaud MANTOUX (SIMaP, Grenoble-INP, CNRS); Nicolas VAXELAIRE (CEA/LETI-University Grenoble Alpes, France); Stéphane COINDEAU (SIMAP, Grenoble-INP, CNRS); Thierry ENCINAS (CMTC University Grenoble Alpes CNRS, Grenoble INP); Jean COIGNUS (CEA/LETI-University Grenoble Alpes, France); Elisabeth BLANQUET (SIMAP, Grenoble-INP, CNRS)
The development of ferroelectric memories is a promising way to obtain low power consumption and efficient memories. This was particularly motivated by the discovery of ferroelectric behavior in thin films of pure hafnium oxide and doped hafnium oxide (~10 nm). This led to the possibility of constructing 1T ferroelectric field effect transistors (FeFETs). Indeed, hafnium oxide is commonly used as gate oxide in nodes smaller than 45 nm. Contrary to other well-known ferroelectric materials such as PZT or SBT oxide, it is therefore easier to integrate into manufacturing processes and does not have such harmful effects as lead for example. However, only the orthorhombic phase (Pca21) of HfO2 has ferroelectric properties and must be stabilized by heat treatment or doping.
To fabricate a FeFET with a gate first approach, a final integration budget close to 1000°C is mandatory. However, HfO2 or HZO experience a transition to the tetragonal structure with such high temperature. It is why, we investigate the effect of doping on the structure in the goal to conserve the ferroelectric properties of our layer after annealing representative of a FeFET process. Especially, this work focuses on the study of crystalline phase obtained in hafnium oxide doped with different cations: Al, Ti, La and Si. Atomic Layer Deposition (ALD) has been used for hafnia deposition.
For each of these samples, impact of annealing conditions on crystallization has be studied (Fig 1 and 2). Indeed, most of sample are amorphous after deposition. To crystallize the material, an annealing step is therefore necessary. Different annealing conditions have been evaluated: temperature was varied from 600°C to 1065°C and time of annealing from 1s (RTA) to 2h (oven). Another challenge due to the gate first integration is the thermal budget reaching 1000°C. Therefore, the HfO2 final material needs to keep the orthorhombic crystal structure after the complete fabrication process.
To analyze our films, structural information extracted for X-ray diffraction (XRD) is correlated the ferroelectric properties obtained by the so-called Positive Up Negative Down (PUND) method (Fig 3, 4 and 5).
Grazing incidence and In-plane XRD measurements has been implemented to reinforce Rietveld refinement and decorrelate orthorhombic phase from the tetragonal one (Fig 6).
Concerning PUND characterizations, both Metal / Insulator / Metal (MIM) and Metal / Oxide / Semiconductor (MOS) have been measured to address influence of electrodes.
Finally, we will show how the Si and La doping seem the most promising way to functional FeFET using HfO2 oxide.View Supplemental Document (pdf)