Atomic Layer Deposition System Research Articles

Investigation of electrical properties of peald-deposited Ti/Al2O3/Al/Si MIM capacitors

MIM devices fabricated with 10-nm thickness of Al2O3 high-[Formula: see text] thin film deposited using plasma-enhanced atomic layer deposition (PEALD) system on Al-coated Si substrate were investigated. The structural, morphological and electrical properties of Ti/Al2O3/Al/Si MIM capacitors as-deposited and post-deposition annealed (PDA) at different temperatures were studied and compared. Al2O3 thin films were investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD) and Ti/Al2O3/Al/Si MIM capacitors were characterized by current–voltage ([Formula: see text]–[Formula: see text] and capacitance–voltage ([Formula: see text]–[Formula: see text] measurements. The stable phase formation of Ti/Al2O3/Al/Si MIM capacitor provides the lowest leakage current density in the range of nA/cm2 for as-deposited and annealed films.

Investigation of Multiple-Mesa-Nanochannel Array GaN-Based MOSHEMTs with Al2O3 Gate Dielectric Layer

In this work, an atomic layer deposition system was used to deposit Al2O3 high-k dielectric film as the gate insulator of GaN-based metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs). By using the Al2O3 gate dielectric layer, compared to planar channel structure, the direct current, high frequency, and flicker noise performances were improved in the GaN-based MOSHEMTs with fin-nanochannel array. For the GaN-based 80-nm-wide fin-nanochannel array MOSHEMTs, they exhibited superior performances of maximum extrinsic transconductance of 239 mS mm−1, threshold voltage of –0.4 V, unit gain cutoff frequency of 7.3 GHz, maximum oscillation frequency of 14.1 GHz, normalized noise power of 2.5 × 10−14 Hz−1, and Hooge’s coefficient of 1.4 × 10−6. The enhanced performances were attributed to the features of fin-nanochannel array of better gate control capability, enhanced pinch-off effect, and better heat dissipation driven by lateral heat flow within the space between fin-channels.

In situ growth of an opal-like TiO2 electron transport layer by atomic layer deposition for perovskite solar cells

Here, we use an atomic layer deposition system and different sizes of polystyrene colloidal spheres to in situ prepare an opal-like TiO2 mesoporous electron transport layer for perovskite solar cells.

Spectroscopic analysis of ultrathin amorphous ZnO films grown by atomic layer deposition

Ultrathin amorphous Zinc Oxide films were prepared on glass substrates by homemade atomic layer deposition system using Diethyl Zinc and Deionized water as precursors and high pure Argon gas as purging gas. The substrate temperature was set as 125℃ and cycle number was varied. Different spectroscopic techniques were employed to unveil the structural and optical properties of films. The effect of quantum confinement on band gap energy values reduces with increase in thickness. Resonant Raman scattering was observed for all the samples when excited with laser wavelength of 325 nm.

RBS and NRA analysis for films with high growth rate prepared by atomic layer deposition

We present the design and operation of a specialized Atomic Layer Deposition (ALD) system, dedicated to stable isotopic tracing experiments of oxide film growth, using isotopically labelled water as the oxide reactant. A small chamber volume allows operation with only very small quantities of water vapor, minimising the consumption of the isotopically labelled water. The first results for growth of ZnO and TiO2 using Diethlyzinc (DEZ) and Tetrakis(dimethylamino)titanium (TDMAT) as the zinc and titanium precursors, and unlabelled water as reactant, are presented, to establish the growth conditions for stoichiometric ZnO and TiO2 on silicon. Absolute film compositions and thickness are determined by RBS and NRA as a function of vapor pulse duration, number of ALD cycles and substrate temperature. Physical thickness is determined by ellipsometry. The first results obtained for growth of ZnO using water highly enriched in 18O are also presented.

In-situ spatial and temporal electrical characterization of ZnO thin films deposited by atmospheric pressure chemical vapour deposition on flexible polymer substrates

A technique is presented for collecting data on both the spatial and temporal variations in the electrical properties of a film as it is deposited on a flexible substrate. A flexible printed circuit board substrate with parallel electrodes distributed across its surface was designed. Zinc oxide films were then deposited on the flexible substrate at different temperatures via atmospheric pressure chemical vapour deposition (AP-CVD) using a spatial atomic layer deposition system. AP-CVD is a promising high-throughput thin film deposition technique with applications in flexible electronics. Collecting data on the film properties in-situ allows us to directly observe how deposition conditions affect the evolution of those properties in real-time. The spatial uniformity of the growing film was monitored, and the various stages of film nucleation and growth on the polymer substrate were observed. The measured resistance of the films was observed to be very high until a critical amount of material has been deposited, consistent with Volmer–Weber growth. Furthermore, monitoring the film resistance during post-deposition cooling enabled immediate identification of metallic or semiconducting behaviour within the conductive ZnO films. This technique allows for a more complete understanding of metal chalcogen film growth and properties, and the high volume of data generated will be useful for future implementations of machine-learning directed materials science.

Effect of microgroove structures on the fluid dynamics and film conformality in spatial atomic layer deposition of Al2O3

With the development of semiconductors, efficient ultrathin coatings on large areas is of great significance to the commercial needs for the encapsulation of materials with micro- or nano-structures. Spatial Atomic Layer Deposition (ALD) is an emerging high throughput and economic technique which can deposit nano-scale films on substrates with different topographies. However, the complex fluid dynamics and diffusion kinetics in the spatial ALD system bring great challenge to the investigation and optimization of the film conformality. In this work, a two-dimensional model coupling Computational Fluid Dynamics (CFD) coupled with chemical kinetics details is built to investigate the hydrodynamics and film growth characteristics in spatial ALD system with different substrates. The effects of the microgroove structures and the microgroove heights on the velocity flow fields and the conformality of alumina films are studied, as shown in the Figure. With numerical simulations, our results show that higher aspect ratio of groove structure leads to more vortex flow at the corner of the microstructures. The motion of the substrates exacerbates the formation of vortex, which will definitely cause the precursor intermixing. Besides, the conformality of the alumina film decreases as the aspect ratio of microgroove structures increases. In order to ensure the self-limited ALD reaction and improve the film conformality, process parameters of total gap size, separate gas flow rate, the carrier gas flow rate as well as the motion velocity of the substrate are investigated. It is found that low carrier gas flow rate can not only reduce the chemical vapor deposition (CVD) growth, but also increase precursor utilization. This work is important for the packaging of micro structure devices and is rather instructive to the further design of spatial ALD system. Figure 1

Ultrathin Quasibinary Heterojunctioned ReS2/MoS2 Film with Controlled Adhesion from a Bimetallic Co-Feeding Atomic Layer Deposition.

Heterojunctioned transition-metal dichalcogenide (TMD) films with regulatable interface adhesion have shown broad application prospects in the design of advanced materials and the manufacturing of novel functional devices. To date, the controlled fabrication of TMD heterojunctions or heterojunction-rich films with tailorable thickness and composition has proved challenging. Herein, a bimetallic co-feeding atomic layer deposition (ALD) system was developed capable of fulfilling these requirements. In the co-feeding ALD fabrication, by adjusting the Re/Mo ratio, 3-layered quasibinary heterojunctioned ReS2/MoS2 films with adjustable composition and grain size were prepared. Moreover, the measurements between atomic force microscopy Si tip coated with the ReS2/MoS2 films and films on the substrate indicate that the adhesion force can be regulated from 13.5 to 136.3 nN. Further experimental data and theoretical analysis show that the adhesion force between the coated tip and films possesses a positive correlation with the "tip-film unanimity" in composition.

Low cycles pulsed chemical vapor deposition of polycrystalline anatase TiO2

In this work, an atomic layer deposition (ALD) system is used to identify deposition conditions resulting in anisotropic growth and the formation of highly defined polycrystalline anatase titanium dioxide. FESEM, Raman Spectroscopy, and XRD were extensively used to characterize the deposited titania. The ALD parameter refinement resulted in the attainment of a polycrystalline anatase phase titania in as low as 30 cycles, although the final parameter resulted in a pulsed-chemical vapor deposition (pulsed-CVD). This work suggests that the anatase crystal phase’s development is more dependent on deposition process parameters such as precursor pulse, waiting time, and vacuum times than on the number of cycles. Moreover, the developed pulsed-CVD procedure to deposit anatase titania was capable of coating rough aluminum and titanium substrates with polycrystalline anatase titania, highly increasing the potential to be used in other biomedical implants made of different metals such as stainless steel or in other applications such as dielectrics

Neuro-Inspired-in-Memory Computing Using Charge-Trapping MemTransistor on Germanium as Synaptic Device

In this work, we fabricated charge-trapping MemTransistors (CTMTs) on a germanium (Ge) substrate with a single-charge-trapping-layer gate-stack or a double-charge-trapping-layer gate-stack. We first constructed the energy band diagram of two gate stacks using transmission electron microscope (TEM) images and by X-ray photoelectron spectroscopy analysis. We deposited Al2O3 as a tunneling layer and a barrier layer using an atomic layer deposition (ALD) system while depositing HfO2 by ALD as the charge-trapping layer whose conduction band offset with respect to Al2O3 is 1.74 eV. Next, we demonstrated the memory characteristics of the CTMTs. By implementing the double-charge-trapping-layer gate-stack on the CTMT, we were able to enlarge the memory windows by 372 mV, improve the retention by 2.7%, and reduce the read disturbance. Furthermore, we demonstrated the synaptic device characteristics of the CTMTs. With the optimization of pulse schemes, we reduced the nonlinearity of potentiation ( $\alpha _{\text {p}}$ ) and depression ( $\alpha _{\text {d}}$ ) from 8.62 and −6.01 to 0.71 and 0.01, respectively, enlarged the ON/OFF ratio from 10.2 to 66.2, and increased the recognition accuracy from 24.5% to 82.1% simultaneously. With the implementation of the double-charge-trapping-layer gate-stack, we could further enlarge the ON/OFF ratio to 75.3 and increase the recognition accuracy to 86.5% simultaneously.

In-situ observation of nucleation and property evolution in films grown with an atmospheric pressure spatial atomic layer deposition system

Atmospheric pressure—spatial atomic layer deposition (AP-SALD) is a promising open-air deposition technique for high-throughput manufacturing of nanoscale films, yet the nucleation and property evolution in these films has not been studied in detail. In this work, in situ reflectance spectroscopy was implemented in an AP-SALD system to measure the properties of Zinc oxide (ZnO) and Aluminum oxide (Al2O3) films during their deposition. For the first time, this revealed a substrate nucleation period for this technique, where the length of the nucleation time was sensitive to the deposition parameters. The in situ characterization of thickness showed that varying the deposition parameters can achieve a wide range of growth rates (0.1–3 nm/cycle), and the evolution of optical properties throughout film growth was observed. For ZnO, the initial bandgap increased when deposited at lower temperatures and subsequently decreased as the film thickness increased. Similarly, for Al2O3 the refractive index was lower for films deposited at a lower temperature and subsequently increased as the film thickness increased. Notably, where other implementations of reflectance spectroscopy require previous knowledge of the film’s optical properties to fit the spectra to optical dispersion models, the approach developed here utilizes a large range of initial guesses that are inputted into a Levenberg-Marquardt fitting algorithm in parallel to accurately determine both the film thickness and complex refractive index.

Performance of H-diamond MOSFETs with high temperature ALD grown HfO2 dielectric

Hydrogen-terminated diamond (H-diamond) metal-oxide-semiconductor field effect transistors (MOSFETs) with different structures were fabricated on the same polycrystalline diamond plate and their performances were compared. A 28-nm-thick HfO2 layer grown at 300 °C by atomic layer deposition (ALD) system was used as both gate dielectric and passivation layer. Devices A and B have the same source-to-drain distance of 6 μm but different gate lengths of 2 μm (device A) and 6 μm (device B), and in device B the gate extends over the source/drain electrodes with the HfO2 film separating them. Device C has a similar structure with device A but a larger gate length (6 μm). Among these devices, device A shows the highest saturation drain current of 190.6 mA/mm at VGS = −8 V, and device B shows the lowest on-resistance of 61.61 Ω·mm. All the devices show a high on/off ratio of about 109 and a gate leakage current lower than 10−10 A. It is also extracted a constant carrier mobility of 37.1 cm2/Vs in the gated channel in a large VGS range of −2 V ≤ VGS ≤ −8 V, which indicates the good interface characteristics between H-diamond and HfO2 and contributes to the high device performances. These results indicate that high conductivity 2DHG and high performance MOSFET devices can also be achieved by depositing high temperature grown HfO2 on the H-diamond surface.

Flexible 3D Electrodes of Free-Standing TiN Nanotube Arrays Grown by Atomic Layer Deposition with a Ti Interlayer as an Adhesion Promoter.

Nanostructured electrodes and their flexible integrated systems have great potential for many applications, including electrochemical energy storage, electrocatalysis and solid-state memory devices, given their ability to improve faradaic reaction sites by large surface area. Although many processing techniques have been employed to fabricate nanostructured electrodes onto flexible substrates, these present limitations in terms of achieving flexible electrodes with high mechanical stability. In this study, the adhesion, mechanical properties and flexibility of TiN nanotube arrays on a Pt substrate were improved using a Ti interlayer. Highly ordered and well-aligned TiN nanotube arrays were fabricated on a Pt substrate using a template-assisted method with an anodic aluminum oxide (AAO) template and atomic layer deposition (ALD) system. We show that with the use of a Ti interlayer between the TiN nanotube arrays and Pt substrate, the TiN nanotube arrays could perfectly attach to the Pt substrate without delamination and faceted phenomena. Furthermore, the I-V curve measurements confirmed that the electric contact between the TiN nanotube arrays and substrate for use as an electrode was excellent, and its flexibility was also good for use in flexible electronic devices. Future efforts will be directed toward the fabrication of embedded electrodes in flexible plastic substrates by employing the concepts demonstrated in this study.

Atomic layer deposition of zirconium oxide thin film on an optical fiber for cladding light strippers

Cladding light strippers are essential components in high-power fiber lasers used for removal of unwanted cladding light that can distort the beam quality or even damage the whole fiber laser system. In this study, an Atomic Layer Deposition system was used for the first time to prepare the cladding light stripper devices using a 40 nm thick zirconia layer grown on optical fiber. The thickness of the zirconia coating was confirmed using the Scanning Electron Microscopy (SEM) and the Ellipsometry techniques. The elemental analysis was also performed using the wavelength dispersive X-ray spectroscopy technique. The Raman spectroscopy and XRD data confirm the structure of the atomic layer deposition-grown zirconia thin films to be predominantly amorphous. The cladding light stripper devices formed using the zirconia thin films with the lengths of 8.5 and 15.5 cm were able to strip approximately 30% (~1.5 dB) and 40% (~2.3 dB) of the unwanted cladding light.

Transient analysis and process optimization of the spatial atomic layer deposition using the dynamic mesh method

Spatial atomic layer deposition (ALD) is acknowledged as a high-throughput thin film preparation technique. Since the films are obtained at a high relative velocity of the substrate, the dynamic flow field distribution is more intricate compared to the traditional ALD. In this paper, by adopting the dynamic mesh method, a mathematical model combing fluid dynamics with chemical kinetics is established. Quantitative analysis is carried out to evaluate how different parameters influence precursor intermixing through this dynamic model. The viscous dragging effect caused by the relative movement of the substrate is also investigated. The good agreement between the mathematical results and the experimental data demonstrates the effectiveness of the dynamic model. To obtain a uniform thin film with a relatively high deposition rate and precursor usage, the motion speed of the substrate is also optimized. This numerical model has momentous guidance to the further process optimization and the spatial ALD system design.

Direct fabrication of two-dimensional ReS2 on SiO2/Si substrate by a wide-temperature-range atomic layer deposition

The building of high-quality, especially thickness-controllable ReS2, is the crux of researching it and developing its wider application. In this work, ultrathin (1–5 layers) ReS2 with controllable thickness and improved quality is obtained on SiO2/Si substrates, using wide-temperature-range (WTR) atomic layer deposition (ALD). First, a WTR ALD system for building thin films and in situ annealing is constructed. ReS2 of precise thicknesses can be achieved by regulating the number of ALD cycles, controlling the reaction temperature and plasma treatment. In particular, a method of in situ H2S annealing is used to reduce S defects, which improves the quality of ReS2. After annealing, the atomic ratio of S/Re in ReS2 increases from 1.74 to 1.92, considering the presence of Re–O bond at the SiO2–ReS2 interface, which indicates that the S defects in ReS2 films are completely eliminated at annealing temperature of 850 °C and 900 °C. In particular, at an annealing temperature of 900 °C, ReS2 recrystallizes to form about 120 nm triangular grains, and its frictional force is reduced by 27.5% compared with the as-grown ReS2.

Analog switching characteristics in TiW/Al2O3/Ta2O5/Ta RRAM devices

In this letter, we report analog switching characteristics in an analog resistive random access memory device based on a TiW/Al2O3/Ta2O5/Ta stack. For this device, both oxides were grown by using an atomic layer deposition system and the oxygen vacancies were found to exist at the interface of these oxides by using angle-resolved X-ray Photoelectron Spectroscopy. The device exhibits analog switching behaviors. Multiple states were achieved by applying 128 consecutive identical pulses of <20 μs duration and stable for at least 104 s. These characteristics show that the TiW/Al2O3/Ta2O5/Ta device is a promising candidate for synaptic applications.

Atomic Layer Deposition of High-Quality Al2O3 Thin Films on MoS2 with Water Plasma Treatment.

Atomic layer deposition (ALD) of ultrathin dielectric films on two-dimensional (2D) materials for electronic device applications remains one of the key challenges because of the lack of dangling bonds on the 2D material surface. In this work, a new technique to deposit uniform and high-quality Al2O3 films with thickness down to 1.5 nm on MoS2 is introduced. By treating the surface using water plasma prior to the ALD process, hydroxyl groups are introduced to the MoS2 surface, facilitating the chemisorption of trimethylaluminum in a conventional water-based ALD system. Raman and X-ray photoelectron spectroscopy measurements show that the water plasma treatment does not induce noticeable material degradation. The deposited Al2O3 films show excellent device-related electrical performance characteristics, including low interface trap density and outstanding gate controllability.

(Invited) Phonon Engineering for Improved Seebeck Coefficient and Thermal Conductivity of Thermoelectric Nanolaminate Films Synthesized on Porous Templates

Recently promising advances have been achieved with phononic crystal (PnC) nanostructures in thermoelectric materials, because the thermal conductivity of PnC samples is lower compared to non-patterned thermoelectric samples due to phonon-boundary scattering. In this study we report on the successful synthesis of PbTe and PbSe thermoelectric layers on patterned porous silicon substrates by a thermal ALD system. PbTe/PbSe nanolaminates grown on porous silicon membranes have higher Seebeck coefficients than the ones grown on regular planar silicon wafers. The higher Seebeck values result from the lower thermal conductivity k in porous structures, which in turn lead to a higher figure of merit ZT. We demonstrate the feasibility to enhance the figure of merit ZT further by modulating the size and periodicity of the pattern and the thickness of the thermoelectric film in relation to the mean free path (MFP) of the phonons of the thermoelectric material. In our study PbTe and PbSe thin films and nanolaminates were synthesized by Atomic Layer Deposition (ALD) technology on regular planar silicon wafers and on macro-porous silicon templates. ALD is a surface saturating, self-limiting, absolutely conformal thin film synthesis technique reacting two pulsed precursors that enable film growth of one atomic layer per growth cycle. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2), (trimethylsilyl) telluride ((Me3Si)2Te) and (trimethylsilyl) selenide ((Me3Si)2Se) were employed as the chemical ALD precursors for lead, tellurium and selenium, respectively. The surface morphology observations indicate Volmer–Weber growth mechanism of PbTe/PbSe nanolaminates during the thermal ALD deposition process rather than classic layer-by-layer growth. The ALD PbTe/PbSe nanolaminates exhibited complete surface coverage on planar bulk silicon wafers and also inside the pores of microporous silicon membranes. For the case of porous Si templates, the Seebeck measurements in both the horizontal and vertical directions revealed significant enhancement in Seebeck coefficients for PbTe/PbSe nanolaminates synthesized inside strictly periodic lithographically defined porous silicon membranes.

Numerical study on the effectiveness of precursor isolation using N2 as gas barrier in spatial atomic layer deposition

Spatial atomic layer deposition (ALD) is capable to increase film deposition rate dramatically by eliminating the need of traditional pulse/purge steps in ALD process. Numerical simulations offer valuable information on spatial ALD system design and development. In this paper, a numerical study of inline spatial ALD process with a large gap (36 mm) is presented to investigate the effectiveness of precursor isolation using N2 flow as gas barrier in two different geometric designs, namely, top and bottom purge configurations. Bottom purge configuration is shown more effective in preventing precursor intermixing than top design. Higher N2 injecting flow rate and higher relative purging pressure are beneficial to construct an effective gas barrier in spatial ALD system. Evenly assigned precursor dosing pressure is advantageous to further prevent material intermixing. Transient material distributions in the flow field, contour plots of species, velocity field, and material tracelines have confirmed the effective gas barrier built by N2. Precursor intermixing molar fraction is shown as low as 0.0045%, and the intermixing molar concentration is as low as 9.105 × 10−5 mol/m3. The study provides invaluable information in determining geometric and process parameters for system design, and the effectiveness of precursor isolation using N2 as gas barrier in spatial ALD with a large gap is validated.