AVS 70 Session TF-ThP: Thin Films Poster Session

Advanced and smart materials nowadays consist of various materials featuring layers and layer systems at the nanoscale. In order to monitor the development process as well as production and customer returns, analytical techniques are required which have an information depth suited for the layered samples.

Starting with information from the outermost atomic layer using Low Energy Ion Scattering (LEIS), adding Time-of-Flight SIMS (ToF-SIMS) for the outermost 3-5 monolayers and ending with X-ray Photoelectron Spectroscopy (XPS) for accessing up to 20 monolayers, a detailed insight can be gained into the composition of layers at the nanoscale. On the other hand, layer systems the structure of which is well known can help to better understand the performance features of the different analytical techniques.

In our ongoing studies we have compared analytical results from LEIS, ToF-SIMS and XPS for various layered systems. We will report on films produced by Atomic Layer Deposition (ALD) as well as core-shell nanoparticles. Amongst the three, XPS is the most often used technique in industry because of its power to obtain quantitative results. Our comparative data will help to put XPS data in a suited information depth perspective.

Power electronics gather many emerging applications such as electrification of transports. In power devices, voltage fluctuations can occur and damage GaN transistors, especially during switching phases. To prevent this, a snubber network can be used. It consists of a series capacitor and resistor connected in parallel to a transistor. Capacitor must have high capacitance, high breakdown voltage and thermal stability up to 150°C (operating temperature for electric vehicles). Furthermore, capacitor and resistor are usually surface-mounted devices and have disadvantages of low heat dissipation and large size. One way to miniaturize these passive components would consist in 3D integration on silicon wafers.

Today, polymer capacitors are widely used because of their low-cost manufacturing and high electric breakdown strength. However, thin films of polymer cannot be conformaly deposited into 3D structures. Also, most of polymers have low operating temperature, lower than the 150°C targeted. Among alternative non-polymeric materials, two groups stand out: high-k (with high dielectric constant but low breakdown voltage (V_bd)) and low-k materials (higher V_bd than high-K materials). A strategy has to be developed to find a compromise between high dielectric constant and high breakdown voltage. A promising way is to integrate high-k/low-k multilayers into a MIM (=Metal-Insulator-Metal) capacitor.

In this work, multilayers combining an organosilicate (SiOCH) as low-k and HfO₂ as high-k were investigated. SiOCH thin films were deposited by plasma-enhanced chemical vapour deposition while HfO₂ thin films were deposited by atomic layer deposition. Chemical and physical properties of the films were studied using FTIR, ellipsometry, X-ray reflectometry and X-ray diffraction. Then, different thicknesses and stacks were fully integrated into MIM capacitors on 200 mm Si wafers for electrical characterizations. Electrical parameters such as dielectric permittivity, dielectric losses, leakage current and breakdown field were measured. It is shown that the introduction of very thin HfO₂ layer (in the 10 nm range) between SiOCH layers (50 nm thick) allow to increase the dielectric constant up to 11%. The use of more complex stack (i.e. up to 5 layers) further improves the C*V_bd parameter. This approach combining low-k and high-k looks promising for the realization of high-voltage 3D capacitors on Si.

In our work, we focused on the investigation of the electrical, photocatalytic and sensory properties of graphene oxide (GO) and polyimide (PI) implanted by gold energetic ions. Gold ions (Au⁺) are favored for ion implantation into polymers due to several key reasons. Primarily, Au⁺ possesses exceptional electrical conductivity, rendering it highly suitable for establishing conductive pathways within the polymer matrix. Additionally, gold exhibits remarkable chemical stability, thereby minimizing undesired reactions with the polymer substrate during the implantation process. The implantation of Au⁺ ions is accompanied by crystallization and carbonization of the modified samples, which leads to the disruption of chemical bonds and the formation of network processes [1]. ]. These processes lead to the formation of conjugated systems that promote electric charge transport [2].

The organic non-conductive materials (GO, PI) were subjected to modification using low-energy Au⁺ ions (20 keV) and medium-energy Au⁺ ions (1.5 MeV). The ion implantation was conducted with three different ion fluences. At the lowest ion fluence (3.75×10¹² cm⁻²), the formation of carbon islands may occur. The second ion fluence (3.75×10¹⁴ cm⁻²) induces the growth of carbon clusters and the formation of conjugated carbon bonds [2]. With the highest ion fluence used (1×10¹⁶ cm⁻²), the formation of metal nanoparticles is expected to take place [3].

The experimental research has been carried out at the CANAM (Centre of Accelerators and Nuclear Analytical Methods) infrastructure LM 2015056. This publication was supported by OP RDE, MEYS Czech Republic under the project CANAM OP CZ.02.1.01/0.0/0.0/16 013/0001812. The scientific results were obtained with the support of the; GACR Project No. 23-06702S and University of J. E. Purkyne student project UJEP-SGS-2023-53-007-2.

References

[1] V. Popok, V. Popok et al., Springer, 2019. p. 69-111

[2] J. Novak, E. Stepanovska, P. Malinsky, et al., Nucl Instrum Meth B 540, 199-209 (2023)

[3] P. Malinsky et al., Polymers 15, 1066 (2023)

We investigated the electrical characteristics of sol-gel processed thin-film transistors (TFTs) and found that they are improved by the addition of UV-irradiated deionized water (DI water). The vulnerability of solution-processed metal-oxide semiconductors is significantly influenced by organic chemical-induced defects such as voids, holes, and organic residues. To address this issue, we introduced hydroxyl radicals (OH•), generated in DI water through an O3/UV process, into the In-Ga-Zn solution mixture to deposit IGZO active layers with fewer defects. The generation of hydroxyl radicals in DI water was confirmed by potassium iodide (KI)/ultraviolet-visible (UV-vis) spectroscopy analysis. The intensity of the absorbance peak at wavelengths of 290 nm and 350 nm increased with longer UV irradiation times on DI water. Additionally, we used the TG-DSC method to determine that organic materials in the IGZO solution mixture with ozonated water began to decompose at a lower temperature (121.6℃) than pristine IGZO solution mixture (144.5℃). Abrupt weight loss was also observed in the IGZO solution with ozonated water compared to pristine IGZO solution. The field-effect mobility and sub-threshold slope of the a-IGZO TFTs made with ozonated water were improved compared to the conventional process, increasing from 0.40 to 0.97 cm^2/V·s and decreasing from 0.34 to 0.29 V/dec, respectively. These results suggest that the addition of ozonated water to the sol-gel mixture is a simple method to achieve high-performance TFTs by reducing organic chemical-induced defects through low-temperature processing.

Graphene (G) and cyclic olefin copolymer (COC) thin films with a thickness of 50 μm were subjected to multi-energy ion implantation of Cu⁺ and Ag⁺ ions. The ions were implanted with energies of (2.8, 2.0, 1.2) MeV and various fluences (10¹², 10¹³, 10¹⁴) ions/cm². Ion energy levels were chosen to create continuous layers within the organic materials, intersecting in a Gaussian distribution. Elemental changes were analyzed using Rutherford backscattering and elastic recoil detection analysis (RBS/ERDA), chemical bonds were monitored using X-ray photoelectron spectroscopy (XPS), infrared (IR) and Raman spectroscopy, and finally electrical properties were investigated using electrochemical impedance spectroscopy (EIS). With increasing ion fluence, the sheet resistance of the implanted layers decreases, indicating an increase in the material's electrical conductivity. This study provides a comprehensive insight into the changes in the microstructure and electrical properties of graphene and cyclic olefin copolymer thin films after ion implantation, which holds significant potential for applications in electronics and sensing.

Polydioxanone (PDO) is an aliphatic polyester with excellent biocompatibility, flexibility, and biodegradability, and has been widely used as a surgical suture. However, lack of functional groups in the backbone of PDO limits its applications to other biomedical aspects. Here, we study vapor-phase oxidative molecular layer deposition (oMLD) of an amine-containing polymer, poly(p-phenylenediamine) (PPDA), onto PDO substrates. Our goal with this is to use vapor-phase infiltration to form PPDA within PDO and provide stable surface amine groups for subsequent grafting of biomolecules. Recent work established oMLD growth of PPDA at 150 °C. Here, a lower deposition temperature is necessary for compatibility with PDO. We study oMLD growth of PDO at 90 °C, both onto flat silicon substrates and within the spin-coated PDO polymer. We employ in situ quartz crystal microbalance (QCM) studies during oMLD growth to benchmark the deposition of PPDA at these lower temperatures. We also report ex situ spectroscopy and chemical analysis on flats to understand the extent of amine incorporation into PDO and the stability of this PPDA polymer in physiologically relevant solution conditions. The incorporation of amine groups onto the PDO surface will facilitate the addition of bioactive components into PDO to control cell-material interactions, therefore expanding its application in regenerative medicine.

Titanium dioxide (TiO₂) is a widespread semiconductor material present in numerous applications such as photovoltaic panels, various cosmetic products and batteries.Nowadays it is intensively studied for its depolluting action thanks to its photocatalytic properties. However, generating crystalline thin films using a cold plasma operating at atmospheric pressure is not trivial. This work focuses on the crystallization of anatase thin films, using a Plasma Enhanced Chemical Vapor Deposition method (PECVD). Different TiO₂ films are synthesized in an atmospheric pressure dielectric barrier discharge system, enhanced with an original heating device [1,2], and analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-rays photoelectron spectroscopy (XPS) and Infrared reflection-absorption spectroscopy (IRRAS). The ultimate goal of this research is to better control the crystal size and the crystalline structure which are decisive parameters in the efficiency of the photocatalytic effect. To do so, different plasma depositions were done with a total flow of 10 slm (15% of argon flow going in the bubbler, 15 % of O₂ and 70 % of Argon carrier flow) and a deposition time of 20 minutes. By changing different parameters such as substrate temperature, power and frequency, a constant crystal size could be obtained and by increasing precursor temperature, crystal sizes ranges from 10 to 20 nm. At a larger scale, by tuning the frequency of the plasma generator, the coatings deposited exhibit different mean cauliflower-like size ranging from 570 to 975 nm, which influence the specific surface area. These preliminary results open a door on controlling more efficiently the crystal size, the crystal growth and the coating texture and density, which could help improving further photocatalytic applications.

Acknowledgments:
This project is funded by the FNRS PER Virusurf project and by the Fonds de la Recherche Scientifique – FNRS under the Synthesis of crystalline N-TiO₂ via NTP project.

References:

[1] A. Remy, et al. Thin Solid Films, 688, 137437 (2019).

[2] A. Remy, F. Reniers. patent EP3768048A1 (2019).

Keywords: anatase, atmospheric pressure plasma, crystal size, bubbler temperature, photocatalytic application.

Area-selective atomic layer deposition (AS-ALD) has emerged as a promising technique for precisely controlling thin film deposition on desired areas. When combined with other fabrication methods, AS-ALD can address existing challenges in nanopatterning fabrication. In this study, ruthenium AS-ALD was integrated with an area-selective etching (ASE) process to achieve self-aligned nanopatterns. Tricarbonyl (η4-2-methylene-1,3-propanediyl)Ruthenium(II) (TRuST) and O₂ were used as as novel Ruthenium precursor and reactant, respectively, for Ru AS-ALD. The bis(N,N-dimethylamino)dimethylsilane (DMADMS) served dual roles: as a precursor in SiO₂ ALD and as an inhibitor in the Ru AS-ALD process. During Ru AS-ALD, DMADMS selectively adsorbed on SiO₂ surfaces, effectively blocking Ru film growth, but not on the Ru substrate surface. Subsequently, O₃ was introduced as a counter reactant for SiO₂ ALD, reacting with DMADMS-adsorbed SiO₂ surfaces to grow ALD SiO₂ and simultaneously etching Ru from the Ru substrate surface. By controlling the selectivity of Ru using DMADMS and the O₃ exposure time, desired thicknesses of Ru and SiO₂ films were achieved, enabling controlled Ru/SiO₂ nanopatterning in a single combined AS-ALD and ASE process. The adsorption of the DMADMS inhibitor, the selectivity and etching of Ru, and the growth of SiO₂ films were investigated using several analytical techniques including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), in-situ spectroscopic ellipsometry (SE), and in-situ quadrupole mass spectrometry (QMS).

As current silicon-based transistor devices begin to approach the fundamental material scaling limits of silicon, new designs and material integration methods are required to meet the ever-increasing demand for greater computing power and memory storage densities. Molybdenum disulfide (MoS₂) is one material in the two-dimensional materials class which is a potential replacement for silicon within next generation microelectronic devices to enable further device scaling due to its high electron mobility, even when in monolayer form. High quality MoS₂ is required for integration into microelectronic devices as even small concentrations of defects can significantly impact the electrical properties of the monolayer film. Synthesis of monolayer MoS₂ which is of sufficient quality for integration into microelectronic devices while maintaining processing temperatures that are within the allowable thermal budget of back end of line processing has proven difficult. In this work we demonstrate a method of growing crystalline MoS₂ directly from contact metals via atomic layer deposition (ALD) using molybdenum hexafluoride and hydrogen sulfide as the reactants. Several different contact metals including tungsten, molybdenum, nickel, and platinum were used for blanket studies to determine the ability to deposit crystalline MoS₂ at low temperatures. Raman spectroscopy was used to examine the crystallinity of the deposited films and x-ray photoelectron spectroscopy was utilized to determine the chemical composition of the deposited films. To obtain high quality MoS₂ test structures, contact metals were patterned to template direct ALD of MoS₂. Following MoS₂ deposition the samples were again characterized using Raman spectroscopy and atomic force microscopy to determine the crystallinity and morphology of the resulting MoS₂ film.

Aluminum thin films are an ideal broadband reflectors for application including space observatories. However, oxide forms on freshly-deposited Al when it is exposed to oxygen or water vapor. This oxide layer absorbs a large portion of far ultraviolet (FUV) wavelengths. In order to prevent oxidation a barrier layer layer is deposited on top of Al mirrors. To preserve high FUV reflectance it is usually a low Z metal fluoride, such as MgF₂, AlF₃ or LiF. Of the commonly used metal fluorides, LiF has the best FUV transparency. However, it is susceptible to radiation damage. Many studies have shown that the electron beam in an electron microscope (SEM or TEM) can damage halide salts such as sodium chlorides and bromides. One such process termed radiolysis. Alkali halide salts are known to be most sensitive. The halide anion disappears as does the cation. Since mirrors on space telescopes will also be exposed to the solar wind and coronal mass ejection (CME) understanding the degradation mechanism and kinetics was deemed a high priority. We have studied the radiolysis of fluoride films important to FUV optics, mainly LiF films. The first damage is a roughening of the surface and a decrease of the fluorine peak in energy dispersive x-ray analysis (EDX). Note that, lithium (Z = 3) cannot be detected by x-ray florescent techniques since its electrons are not sufficiently bound. In some cases, LiF is fully removed from the scanned region after beam exposure.

The indium gallium zinc oxide (IGZO) has been widely investigated as the active layer for the display backplane thin-film-transistor (TFT). In ultra-high-resolution displays, reducing transistor size per pixel results in submicron-scale channel lengths.[1] Consequently, the short-channel effects become a concern as TFT channel lengths decrease to the submicron scale. Novel architectures such as 3-dimensional vertical or FinFET structures would be developed. The fast-operation speed and low-voltage operation of TFTs are made possible by using the gate insulator (GI) with a high-dielectric-constant (k) value. Among many high-k materials, ZrO₂ has a high-k value (28) and a suitable band gap (5.8 eV).[2] Atomic layer deposition (ALD) has the advantages of conformal depositions, thickness/composition controllability of thin films, and thin film quality due to its self-limiting growth behavior.

High-k materials exhibit higher dielectric constants compared to amorphous-phase SiO₂ due to their crystallinity.[3] However, high-k materials used as GI result in TFT performance degradation due to increased surface roughness caused by crystallinity, increasing surface scattering, and coulomb scattering. In the previous study, our research group controlled the crystallinity of ZrO₂ by varying the deposition temperature and optimized ZrO₂ ALD processes in Mo/ZrO₂/IGZO metal-insulator-semiconductor structures.[4] In this study, we investigated ALD-derived IGZO TFTs using an optimized ZrO₂ ALD process.

The ZrO₂ film was deposited by the thermal ALD process using Cyclopentadienyl Tris(dimethylamino)Zirconium (CpZr), and ozone (O₃). The IGZO film was deposited by thermal ALD process using STIn-7-7, trimethyl gallium (TMGa), and diethyl zinc (DEZ) precursors. O₃ was used as a reactant. The deposited film characteristics were evaluated using glancing incidence X-ray diffraction, atomic force microscope, and X-ray fluorescence spectroscopy. The ALD-derived IGZO TFTs with a bottom gate staggered structure were fabricated. The electrical properties of the TFTs were measured using a 4156C precision semiconductor parameter analyzer.

AcknowledgmentsThis work was supported by the BK21 Plus program.The authors would thank SK Trichem for their support and permission to publish this collaborative work.

References

[1] M. Cho et al., ACS Appl. Mater. Interfaces, 13, 16628-16640 (2021)

[2] J. C. Garcia et al., J. Appl. Phys., 100, 104103 (2006)

[3] L. Manchanda et al., Microelectron. Eng., 59, 351-359 (2001)

[4] M. Nam et al., Adv. Mater. Interfaces, 11, 11, 2300883(2024)

Semiconductor devices, especially dynamic random-access memory (DRAM), are achieving high-density integration and remarkable technological development continuously. At the forefront of advancement in the DRAM device was the introduction of high dielectric constant (high-k) materials, which significantly increased its memory capacity. Especially, since the introduction of ZrO₂ as a high-k material, TiN electrode has been widely used as the electrode for DRAM capacitor application, owing to its crystallinity coherent with ZrO₂ [1][2]. However, due to the oxygen scavenging effect of TiN, low-k TiO_xN_y interfacial layer is formed between the TiN electrode and oxide. The oxygen scavenging effect generates oxygen vacancies in ZrO₂, leading to increase in leakage current. This is because the leakage current mechanism of ZrO₂ is attributed to the presence of oxygen vacancy [3]. Additionally, meeting the requirement of low leakage current for practical DRAM applications in aggressively scaled devices becomes challenging since the TiN electrode has a low work function (~4.3 eV), which reduces conduction band offset [3]. Therefore, in this work, we investigated suppressing the oxygen scavenging effect of TiN electrode using molybdenum dioxide (MoO₂) electrodes, aiming to reduce the concentration of oxygen vacancy in the ZrO₂ and leakage current.

MoO₂ is proposed as a novel oxide electrode for DRAM capacitor, owing to its high work function and excellent chemical stability [2]. To mitigate the oxygen scavenging effect of TiN and reduce leakage current, we compared TiN and MoO₂ electrodes in ZrO₂-based metal-insulator-metal (MIM) capacitors and analyzed the oxygen vacancy defects and electrical properties. Oxygen vacancy defects in ZrO₂ were investigated through AC nonlinearity in the C/C₀–V_ac (where C₀represents capacitance at zero V_ac) [4] and XPS analysis. This revealed a reduction of defect sites induced by oxygen vacancy within ZrO₂ when introducing MoO₂ electrodes. Moreover, it should be noted that the decrease in defect sites induces a reduction in leakage current.

Reference

[1] Hwang C. S., Adv. Electron. Mater., 1, 1400056 (2015).

[2] Kim, Y. W., et al, J. Mater. Chem. C., 10, 12957 (2022).

[3] Jeon W, J. Mater. Res., 35, 775 (2020).

[4] Han, D. H., et al. IEEE Trans. Electron Devices, 68, 5753 (2021).

Dynamic random-access memory (DRAM) is extensively employed in various industrial sectors as a prominent semiconductor component. Among them, there is a trend toward scaling down DRAM capacitors with MIM (metal-insulator-metal) structure to achieve high capacitance and low leakage current (<10^-7A/cm²) at operating voltage. However, scaling down also resulted in decreased the thickness, thereby reducing the area of capacitor and subsequently decreasing the amount of charge that can be stored. Consequently, high dielectric constant materials such as ZrO₂ and HfO₂ are employed as insulators to mitigate this issue. Hence, the conventional DRAM MIM capacitor commonly employs TiN for both top, bottom electrodes, while utilizing ZrO₂ as the insulator. ZrO₂ exists in three phases: monoclinic, tetragonal, and cubic. Among them, ZrO₂commonly used in DRAM is manufactured via an atomic layer deposition process and exhibits a high dielectric constant (~40) in its tetragonal phase. However, there are several problems. Crystallized ZrO₂ monolayers exhibit high leakage currents due to carrier conduction paths formed by grain boundaries.[1]

Therefore, in this presentation, we demonstrate the results of employing Sc₂O₃ to improve the electrical properties of ZrO₂ dielectric film. Sc₂O₃has a high dielectric constant (ε = 13) and a large band gap (Eg=~6 eV). [2]We expect that the wide band gap of Sc₂O₃ will serve as a barrier in the carrier conduction path. Using these properties, we conducted two experiments via the ALD process. First, we examined the effectiveness of Sc doping into ZrO₂. Second, we precisely inserted a Sc₂O₃ monolayer between ZrO₂ insulating layers to see how it affects the performance of ZrO₂ based MIM capacitor. Moreover, we performed crystal phase analysis and evaluation of electrical properties to investigate the effect of Sc incorporation.

Acknowledgments: This work was supported by the BK21 Plus program. The authors would like to thank SK Trichem for their support and permission to publish this collaborative work.

References:

[1] W. Jeon, J. Mater. Res, 35, 7 (2020)

[2] M. Pachecka et. al., AIP Adv.7, 105324 (2017)

Hafnia (HfO₂) stands as a promising option for substituting SiO₂ on transistors due to its high dielectric constant. Furthermore, its high band gap provides transparency over a wide spectral range, which makes it applicable for optical coatings. Hafnia also presents a high index of refraction and excellent thermodynamic stability, and its orthorhombic phase exhibits a ferroelectric response. Among the different HfO₂ fabrication strategies, atomic layer deposition (ALD) is a reliable method for obtaining high-quality conformal hafnia films. Adjusting the process parameters and using different precursors can lead to variations in the resulting film properties. This work evaluates how the deposition temperature and the ALD oxygen source change the hafnia films deposited on Si(100). We use tetrakis(dimethylamido)hafnium (TDMAH) as the metal precursor, and we compare the properties of the films for two distinct oxygen precursors: H₂O and O₃. We measure the deposition rate and index of refraction using ellipsometry and correlate the results with the ALD oxygen source and deposition temperature. The results of this work provide insights for adjusting ALD deposition conditions to tune hafnia properties.

ZnSe as Window Layer for n-CdTe Solar Cells

To overcome the V_OC bottleneck for traditional p-CdTe solar cells, polycrystalline n-type CdTe thin films were used as the absorber layer. Polycrystalline n-type CdTe films were deposited by the elemental vapor transport (EVT) process. The EVT process can be used to deposit CdTe films under Cd- or Te-rich conditions to facilitate extrinsic doping. Indium as n-type dopant is used to increase the CdTe conductivity. A proper p-type partner is critical to form a p-n junction with the n-CdTe absorber. A ZnSe layer was used as the p-type partner of n-CdTe solar cells. The device structure includes Glass/ITO/CdS/n-CdTe/p-ZnSe:Cu/ITO. The ZnSe layer was deposited by RF sputtering followed by the deposition of a thin Cu layer. This work investigated the effect of ZnSe as a window layer for n-CdTe solar cell, specifically: (1) the substrate temperature during ZnSe deposition; (2) Cu thickness. In addition, the effect of ZnSe as an interfacial layer for n-CdTe/p-ZnTe solar cell was studied as well. The device structures were characterized by current-voltage (J-V), spectral response (SR), and capacitance-voltage (C-V) measurements.

ZnSe thin film as an interfacial layer for n-CdTe/p-ZnTe devices causes a “kink” in the J-V curve, which is due to the large valence band offset at CdTe/ZnSe. Two substrate temperatures of depositing ZnSe thin films were studied (i.e., 250℃ and 350℃), when it was used as p-type partner of n-CdTe solar cell. The devices where the ZnSe films was deposited at 350℃ showed higher carrier collection from the SR data. Cu layers with thicknesses of 5Å and 15Å were deposited after ZnSe. Devices with larger amount of Cu exhibit lower carrier collection resulting in lower device performance.

One of the overwhelming obstacles to improving device performance is the high carrier concentration originating from Sn vacancies, resulting in self p-doping and affecting the optoelectronic properties of the material. Simultaneously, the facile oxidation of Sn²⁺ to Sn⁴⁺ further contributes to increasing the p-doping in the bulk and non-radiative recombination centers on the surface.

One effective approach to reduce the presence of oxidized species within the material is to add reducing agents directly within the precursor solution in the form of additives. The most used and effective is SnF₂, which promotes a slower crystallization and improved film quality and effectively limits the self-p doping effect.

Ferroelectricity, characterized by spontaneous polarization that an external electric field can reverse, originates from the formation of a non-centrosymmetric crystallographic phase. Hf_0.5Zr_0.5O₂(HZO) thin films are recognized as promising components for next-generation semiconductor applications. Various strategies have been explored to enhance the ferroelectric properties of HZO films, including cation and anion doping and the optimization of process conditions. Particularly, it has been reported that the stress/strain applied to the HZO films significantly impacted the ferroelectric properties of HZO. Therefore, this study aimed to enhance the ferroelectricity of HZO films by controlling the stress/strain applied to the HZO films through the bilayer electrode stacking methods.

Bottom electrodes with relatively low thermal expansion coefficients, such as Tungsten (W) and Hafnium Nitride (HfN), were employed to apply tensile stress to the HZO thin films, thereby improving remanent polarization (P_r) value. TiN is usually employed as the bottom electrode for HZO films to prevent endurance degradation. When using the TiN/W, TiN/HfN bilayer as the bottom electrode, the 2P_r value increased by approximately 20~30 % compared to the TiN bottom electrode and demonstrated high reliability.

Recently, the demand for high-performance, low-power memory has increased with the growth of artificial intelligence and deep learning technology. The dynamic random access memory (DRAM) industry maintains the scaling trend through cell size shrink technologies to satisfy this demand. However, the two-dimensional scaling of the DRAM below 10 nm design rule is expected to be challenging due to many technical and performance challenges. Thus, it is required to adopt a stacked structure that can overcome these structural limitations. Amorphous InGaZnO (a-IGZO) has emerged as a feasible channel material for stackable DRAM due to its very low off-current and higher electron mobility compared to polycrystalline silicon.

Previous studies have mainly reported the improvement of the electrical characteristics of a-IGZO channel as a single thin film transistor (TFT). However, few reports have been made on a-IGZO TFTs' output current, threshold voltage (V_th) variation, and instability related to charging operation when TFT is directly connected to the capacitor.

This study implemented a planar 1T-1C DRAM structure to simulate the write/read operation of the conventional DRAM and investigated the electrical characteristics of a-IGZO TFTs during the DRAM operation process, where the TFTs have bottom-gate staggered structure using the sputtered a-IGZO channel. The TFT adopted a 20 nm-thick bottom gate with different metals and a 10 nm-thick high dielectric constant (k) dielectric film (HfO₂ and Al₂O₃) as the gate insulator. The drain electrode of the transistor was connected to the top electrode of the capacitor, and the gate insulator thin film of the transistor shared the dielectric film of the capacitor. Al₂O₃ was selected as the passivation layer, which also contributed to achieving the controllability of V_th^[1] and stability of V_th under bias stress.

This work investigates whether these improvements of a-IGZO TFTs have comparable impacts under bias stress conditions caused by the charge stored in the capacitor of 1T-1C DRAM application. The findings of this study will contribute to the understanding of a-IGZO as an alternative channel material for the next DRAM transistor.

References

[1] Rha et al. “Variation in the threshold voltage of amorphous-In₂Ga₂ZnO₇ thin-film transistors by ultrathin Al₂O₃ passivation layer.” Journal of Vacuum Science Technology B, 31, 061205 (2013).

As dynamic random access memory (DRAM) capacitors continue to undergo aggressive scaling, increasing capacitance is paramount for securing sufficient sensing margins^[1]. The ZrO₂/Al₂O₃/ZrO₂ (ZAZ) dielectric thin film prevalent in DRAM is favored for its low leakage current. However, its low dielectric constant presents a significant limitation, prompting extensive research into higher-k (dielectric constant) materials. Among these, BaTiO₃ (BTO) is a perovskite compound with a high k value because of the softening of phonon associated with the titanium ion motion in the crystal lattice, making the BTO highly promising for future DRAM capacitors.

BTO has traditionally been deposited using physical and chemical vapor deposition techniques, which lack sufficient conformality over the extreme three-dimensional DRAM capacitor structure. In contrast, atomic layer deposition (ALD) offers a significant advantage through its self-limiting mechanism at the atomic level, enabling the deposition of BTO thin films with excellent step coverage. This characteristic makes ALD particularly well-suited for producing BTO films that can be effectively utilized in smaller dimensions and complex three-dimensional structures.

This research examined the optimal ALD conditions for the BTO film growth on Ru substrate using the Ba(ⁱPr₃Cp)₂ and Ti(Me₅Cp)(OMe)₃ as the Ba- and Ti-precursors, respectively. Ru substrate was also employed to mitigate the susceptibility of high-k materials to leakage current because of its high work function. Also, this study examines the degree of crystallization according to the rapid thermal annealing (RTA) conditions. A comprehensive analysis of the dielectric constant and leakage current characteristics of the BTO thin films was conducted to evaluate their viability as dielectric materials for DRAM capacitors.

References

[1] S. W. Lee et al. "Atomic layer deposition of SrTiO3 thin films with highly enhanced growth rate for ultrahigh density capacitors." Chemistry of Materials 23.8 (2011): 2227-2236.