Atomic Layer Deposition of Silicon Nitride Thin Films: A Review of Recent Progress, Challenges, and Outlooks

Chemisorption of silicon tetrachloride on silicon nitride: a density functional theory study

We studied the chemisorption of silicon tetrachloride (SiCl4) on the NH2/NH-terminated silicon nitride slab model using density functional theory (DFT) for atomic layer deposition (ALD) of silicon nitride. Initially, two reaction pathways were compared, forming HCl or NH3+Cl- as a byproduct. The NH3+Cl- complex formation was more exothermic than the HCl formation, with an activation energy of 0.26 eV. The -NH2* reaction sites are restored by desorption of HCl from the NH3+Cl- complexes at elevated temperatures of 205 °C or higher. Next, three sequential ligand exchange reactions forming Si-N bonds were modeled and simulated. The reaction energies became progressively less exothermic as the reaction progressed, from -1.31 eV to -0.30 eV to 0.98 eV, due to the stretching of Si-N bonds and the distortion of the N-Si-N bond angles. Also, the activation energies for the second and third reactions were 2.17 eV and 1.55 eV, respectively, significantly higher than the 0.26 eV of the first reaction, mainly due to the additional dissociation of the N-H bond. The third Si-N bond formation is unfavorable due to the endothermic reaction and higher activation energy. Therefore, the chemisorbed species would be -SiCl2* when the surface is exposed to SiCl4.

Conformal High-Aspect-Ratio Solid Electrolyte Thin Films for Li-Ion Batteries by Atomic Layer Deposition

Lithium phosphorus oxynitride (LiPON) is a state-of-the-art solid electrolyte material for thin-film microbatteries. These applications require conformal thin films on challenging 3D surface structures, and among the advanced thin-film deposition techniques, atomic layer deposition (ALD) is believed to stand out in terms of producing appreciably conformal thin films. Here we quantify the conformality (i.e., the evenness of deposition) of thin ALD-grown LiPON films using lateral high-aspect-ratio test structures. Two different lithium precursors, lithium tert-butoxide (LiOtBu) and lithium bis(trimethylsilyl)amide (Li-HMDS), were investigated in combination with diethyl phosphoramidate as the source of oxygen, phosphorus, and nitrogen. The results indicate that the film growth proceeded significantly deeper into the 3D cavities for the films grown from LiOtBu, while the Li-HMDS-based films grew more evenly initially, right after the cavity entrances. These observations can be explained by differences in the precursor diffusion and reactivity. The results open possibilities for the use of LiPON as a solid electrolyte in batteries with high-surface-area electrodes. This could enable faster charging and discharging as well as the use of thin-film technology in fabricating thin-film electrodes of meaningful charge capacity.

Enhancing silicon-nitride formation through ammonolysis of silanes with pseudo-halide substituents

Considering the challenges in reactivity, potential contamination, and substrate selectivity, the ammonolysis of traditional halosilanes in silicon nitride (SiN) thin film processing motivates the exploration of alternative precursors. In this pioneering study, we employed density functional theory calculations at the M06-2X/6-311++G(3df,2p) level to comprehensively screen potential pseudo-halide substituents on silane compounds as substitutes for conventional halosilanes. Initially, we investigated the ammonolysis mechanism of halosilanes, exploring factors influencing activation barriers, with the aid of frontier molecular orbital and charge density analyses. Subsequently, a systematic screening of silane substituents from group 14 to group 16 was conducted to identify pseudo-halides with low reaction barriers. Additionally, we examined the inductive effects on pseudohalide substituents. Using cluster models to represent the silicon surface validates the realistic prediction of ammonolysis barriers with a simplified model. Our findings indicate that pseudo-halide substituents from group 16, particularly those with electron-withdrawing groups, present as practical alternatives to traditional halosilanes in SiN thin film processing, including applications such as low-temperature atomic layer deposition (ALD) techniques.

Plasma Enhanced Atomic Layer Deposition of Silicon Nitride for Two Different Aminosilane Precursors Using Very High Frequency (162 MHz) Plasma Source

Plasma enhanced atomic layer deposition (PEALD) of silicon nitride (SiN_x) using very high frequency (VHF, 162 MHz) plasma source was investigated at the process temperatures of 100, 200, and 300 °C. Two aminosilane precursors having different numbers of amino ligands, bis(tert-butylamino)silane (BTBAS) and di(sec-butylamino)silane (DSBAS), were used as Si precursors. A comparative study was also conducted to verify the effect of the number of amino ligands on the properties of SiN_x film. At all process temperatures, DSBAS, having one amino ligand, performed better than BTBAS in various aspects. SiN_x films deposited using DSBAS had lower surface roughness, higher film density, lower wet etch rate, improved electrical characteristics, and higher growth rate than those deposited using BTBAS. With the combination of a VHF plasma source and DSBAS with one amino ligand, the SiN_x films grown at 300 °C exhibited low wet etch rates (≤2 nm/min) in a dilute HF solution (100:1 of deionized water:HF) as well as low C content below the XPS detection limit. Also, excellent step coverage close to 100% on high aspect ratio (30:1) trench structures was obtained by using VHF plasma, which could provide sufficient flux of plasma species inside the trenches in conjunction with DSBAS having fewer amino ligands than BTBAS.

Enhanced stimulated Brillouin scattering in the unsuspended silicon waveguide assisted with genetic algorithms

Stimulated Brillouin scattering (SBS), originating from the coupling between optical and acoustic waves, has been widely applied in many fields. Silicon is the most used and important material in micro-electromechanical systems (MEMS) and integrated photonic circuits. However, strong acoustic-optic interaction in silicon requires mechanical release of the silicon core waveguide to avoid acoustic energy leakage into the substrate. This will not only reduce the mechanical stability and thermal conduction, but also increase the difficulties for fabrication and large-area device integration. In this paper, we propose a silicon-aluminium nitride(AlN)-sapphire platform for realizing large SBS gain without suspending the waveguide. AlN is used as a buffer layer to reduce the phonon leakage. This platform can be fabricated via the wafer bonding between silicon and commercial AlN-sapphire wafer. We adopt a full-vectorial model to simulate the SBS gain. Both the material loss and the anchor loss of the silicon are considered. We also apply the genetic algorithm to optimize the waveguide structure. By limiting the maximum etching step number to two, we obtain a simple structure to achieve the SBS gain of 2462 W^-1m^-1 for forward SBS, which is 8 times larger than the recently reported result in unsuspended silicon waveguide. Our platform can enable Brillouin-related phenomena in centimetre-scale waveguides. Our findings could pave the way toward large-area unreleased opto-mechanics on silicon.

Low Loss Vertical TiO2/Polymer Hybrid Nano-Waveguides

This article proposes a novel demonstration of a low-loss polymer channel hybridized with a titania core leading to a nano-waveguide elongated in the normal direction to the substrate. It is aimed at using the quasi-transverse magnetic (TM) mode as the predominant mode in compact photonic circuitry. A detailed design analysis shows how a thin layer of a higher-refractive index material in a trench within the core of the waveguide can increase the confinement and reduce the propagation losses. This thin layer, produced by atomic layer deposition, covers the entire polymer structure in a conformal manner, ensuring both a reduction of the surface roughness and a stronger field confinement. The trench can be made at any place within the polymer channel and therefore its position can be tuned to obtain asymmetric modal distribution. The waveguide is demonstrated at telecom wavelengths, although the material's properties enable operation over a large part of the electromagnetic spectrum. We measured propagation losses as low as 1.75 ± 0.32 dB/cm in a 200 nm × 900 nm section of the waveguide core. All processes being mass-production compatible, this study opens a path towards easier integrated-component manufacture.

Atomic Layer Deposition of Intermetallic Fe4Zn9 Thin Films from Diethyl Zinc

We present a new type of atomic layer deposition (ALD) process for intermetallic thin films, where diethyl zinc (DEZ) serves as a coreactant. In our proof-of-concept study, FeCl₃ is used as the second precursor. The FeCl₃ + DEZ process yields in situ crystalline Fe₄Zn₉ thin films, where the elemental purity and Fe/Zn ratio are confirmed by time-of-flight elastic recoil detection analysis (TOF-ERDA), Rutherford backscattering spectrometry (RBS), atomic absorption spectroscopy (AAS), and energy-dispersive X-ray spectroscopy (EDX) analyses. The film thickness is precisely controlled by the number of precursor supply cycles, as expected for an ALD process. The reaction mechanism is addressed by computational density functional theory (DFT) modeling. We moreover carry out preliminary tests with CuCl₂ and Ni(thd)₂ in combination with DEZ to confirm that these processes yield Cu-Zn and Ni-Zn thin films with DEZ as well. Thus, we envision an opening of a new ALD approach based on DEZ for intermetallic/metal alloy thin films.

Atomic and Molecular Layer Deposition of Alkali Metal Based Thin Films

Atomic layer deposition (ALD) is the fastest growing thin-film technology in microelectronics, but it is also recognized as a promising fabrication strategy for various alkali-metal-based thin films in emerging energy technologies, the spearhead application being the Li-ion battery. Since the pioneering work in 2009 for Li-containing thin films, the field has been rapidly growing and also widened from lithium to other alkali metals. Moreover, alkali-metal-based metal-organic thin films have been successfully grown by combining molecular layer deposition (MLD) cycles of the organic molecules with the ALD cycles of the alkali metal precursor. The current literature describes already around 100 ALD and ALD/MLD processes for alkali-metal-bearing materials. Interestingly, some of these materials cannot even be made by any other synthesis route. In this review, our intention is to present the current state of research in the field by (i) summarizing the ALD and ALD/MLD processes so far developed for the different alkali metals, (ii) highlighting the most intriguing thin-film materials obtained thereof, and (iii) addressing both the advantages and limitations of ALD and MLD in the application space of these materials. Finally, (iv) a brief outlook for the future perspectives and challenges of the field is given.

Characteristics of silicon nitride deposited by very high frequency (162 MHz)-plasma enhanced atomic layer deposition using bis(diethylamino)silane

Silicon nitrides, deposited by capacitively coupled plasma (CCP)-type plasma enhanced atomic layer deposition (PEALD), are generally applied to today's nanoscale semiconductor devices, and are currently being investigated in terms of their potential applications in the context of flexible displays, etc. During the PEALD process, 13.56 MHz rf power is generally employed for the generation of reactive gas plasma. In this study, the effects of a higher plasma generation frequency of 162 MHz on both plasma and silicon nitride film characteristics are investigated for the purpose of silicon nitride PEALD, using bis(diethylamino)silane (BDEAS) as the silicon precursor, and N₂ plasma as the reactant gas. The PEALD silicon nitride film deposited using the 162 MHz CCP exhibited improved film characteristics, such as reduced surface roughness, a lower carbon percentage, a higher N/Si ratio, a lower wet etch rate in a diluted HF solution, lower leakage current, and higher electric breakdown field, and more uniform step coverage of the silicon nitride film deposited in a high aspect ratio trench, as compared to silicon nitride PEALD using 13.56 MHz CCP. These improved PEALD silicon nitride film characteristics are believed to be related to the higher ion density, higher reactive gas dissociation, and lower ion bombardment energy to the substrate observed in N₂ plasma with a 162 MHz CCP.

Atomic Layer Deposition of High-k Insulators on Epitaxial Graphene: A Review

Due to its excellent physical properties and availability directly on a semiconductor substrate, epitaxial graphene (EG) grown on the (0001) face of hexagonal silicon carbide is a material of choice for advanced applications in electronics, metrology and sensing. The deposition of ultrathin high-k insulators on its surface is a key requirement for the fabrication of EG-based devices, and, in this context, atomic layer deposition (ALD) is the most suitable candidate to achieve uniform coating with nanometric thickness control. This paper presents an overview of the research on ALD of high-k insulators on EG, with a special emphasis on the role played by the peculiar electrical/structural properties of the EG/SiC (0001) interface in the nucleation step of the ALD process. The direct deposition of Al2O3 thin films on the pristine EG surface will be first discussed, demonstrating the critical role of monolayer EG uniformity to achieve a homogeneous Al2O3 coverage. Furthermore, the ALD of several high-k materials on EG coated with different seeding layers (oxidized metal films, directly deposited metal-oxides and self-assembled organic monolayers) or subjected to various prefunctionalization treatments (e.g., ozone or fluorine treatments) will be presented. The impact of the pretreatments and of thermal ALD growth on the defectivity and electrical properties (doping and carrier mobility) of the underlying EG will be discussed.

Remote Plasma Atomic Layer Deposition of SiNx Using Cyclosilazane and H2/N2 Plasma

Silicon nitride (SiNx) thin films using 1,3-di-isopropylamino-2,4-dimethylcyclosilazane (CSN-2) and N2 plasma were investigated. The growth rate of SiNx thin films was saturated in the range of 200–500 °C, yielding approximately 0.38 Å/cycle, and featuring a wide process window. The physical and chemical properties of the SiNx films were investigated as a function of deposition temperature. As temperature was increased, transmission electron microscopy (TEM) analysis confirmed that a conformal thin film was obtained. Also, we developed a three-step process in which the H2 plasma step was introduced before the N2 plasma step. In order to investigate the effect of H2 plasma, we evaluated the growth rate, step coverage, and wet etch rate according to H2 plasma exposure time (10–30 s). As a result, the side step coverage increased from 82% to 105% and the bottom step coverages increased from 90% to 110% in the narrow pattern. By increasing the H2 plasma to 30 s, the wet etch rate was 32 Å/min, which is much lower than the case of only N2 plasma (43 Å/min).

New development of atomic layer deposition: processes, methods and applications

Atomic layer deposition (ALD) is an ultra-thin film deposition technique that has found many applications owing to its distinct abilities. They include uniform deposition of conformal films with controllable thickness, even on complex three-dimensional surfaces, and can improve the efficiency of electronic devices. This technology has attracted significant interest both for fundamental understanding how the new functional materials can be synthesized by ALD and for numerous practical applications, particularly in advanced nanopatterning for microelectronics, energy storage systems, desalinations, catalysis and medical fields. This review introduces the progress made in ALD, both for computational and experimental methodologies, and provides an outlook of this emerging technology in comparison with other film deposition methods. It discusses experimental approaches and factors that affect the deposition and presents simulation methods, such as molecular dynamics and computational fluid dynamics, which help determine and predict effective ways to optimize ALD processes, hence enabling the reduction in cost, energy waste and adverse environmental impacts. Specific examples are chosen to illustrate the progress in ALD processes and applications that showed a considerable impact on other technologies.

Investigation of the Physical Properties of Plasma Enhanced Atomic Layer Deposited Silicon Nitride as Etch Stopper

Correlations between physical properties linking film quality with wet etch rate (WER), one of the leading figures of merit, in plasma-enhanced atomic layer deposition (PEALD) grown silicon nitride (SiN _x) films remain largely unresearched. Achieving a low WER of a SiN _x film is especially significant in its use as an etch stopper for technology beyond 7 nm node semiconductor processing. Herein, we explore the correlation between the hydrogen concentration, hydrogen bonding states, bulk film density, residual impurity concentration, and the WERs of PEALD SiN _x using Fourier transform infrared spectrometry, X-ray reflectivity, and spectroscopic ellipsometry, etc. PEALD SiN _x films for this study were deposited using hexachlorodisilane and hollow cathode plasma source under a range of process temperatures (270-360 °C) and plasma gas compositions (N₂/NH₃ or Ar/NH₃) to understand the influence of hydrogen concentration, hydrogen bonding states, bulk film density, and residual impurity concentration on the WER. Varying hydrogen concentration and differences in the hydrogen bonding states resulted in different bulk film densities and, accordingly, a variation in WER. We observe a linear relationship between hydrogen bonding concentration and WER as well as a reciprocal relationship between bulk film density and WER. Analogous to the PECVD SiN _x processes, a reduction in hydrogen bonding concentration arises from either (1) thermal activation or (2) plasma excited species. However, unlike the case with silane (SiH₄)-based PECVD SiN _x, PEALD SiN _x WERs are affected by residual impurities of Si precursors (i.e., chlorine impurity). Thus, possible wet etching mechanisms in HF in which the WER is affected by hydrogen bonding states or residual impurities are proposed. The shifts of amine basicity in SiN _x due to different hydrogen bonding states and the changes in Si electrophilicity due to Cl impurity content are suggested as the main mechanisms that influence WER in the PEALD processes.

A Three-Step Atomic Layer Deposition Process for SiN x Using Si2Cl6, CH3NH2, and N2 Plasma

We report a novel three-step SiN _x atomic layer deposition (ALD) process using Si₂Cl₆, CH₃NH₂, and N₂ plasma. In a two-step process, nonhydrogenated chlorosilanes such as Si₂Cl₆ with N₂ plasmas lead to poor-quality SiN _x films that oxidize rapidly. The intermediate CH₃NH₂ step was therefore introduced in the ALD cycle to replace the NH₃ plasma step with a N₂ plasma, while using Si₂Cl₆ as the Si precursor. This three-step process lowers the atomic H content and improves the film conformality on high-aspect-ratio nanostructures as Si-N-Si bonds are formed during a thermal CH₃NH₂ step in addition to the N₂ plasma step. During ALD, the reactive surface sites were monitored using in situ surface infrared spectroscopy. Our infrared spectra show that, on the post-N₂ plasma-treated SiN _x surface, Si₂Cl₆ reacts primarily with the surface -NH₂ species to form surface -SiCl _x ( x = 1, 2, or 3) bonds, which are the reactive sites during the CH₃NH₂ cycle. In the N₂ plasma step, reactive -NH₂ surface species are created because of the surface H available from the -CH₃ groups. At 400 °C, the SiN _x films have a growth per cycle of ∼0.9 Å with ∼12 atomic percent H. The films grown on high-aspect-ratio nanostructures have a conformality of ∼90%.

Hollow Cathode Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using Pentachlorodisilane

In this work, a novel chlorodisilane precursor, pentachlorodisilane (PCDS, HSi₂Cl₅), was investigated for the growth of silicon nitride (SiN _x) via hollow cathode plasma-enhanced atomic layer deposition (PEALD). A well-defined self-limiting growth behavior was successfully demonstrated over the growth temperature range of 270-360 °C. At identical process conditions, PCDS not only demonstrated approximately >20% higher growth per cycle than that of a commercially available chlorodisilane precursor, hexachlorodisilane (Si₂Cl₆), but also delivered a better or at least comparable film quality determined by characterizing the refractive index, wet etch rate, and density of the films. The composition of the SiN _x films grown at 360 °C using PCDS, as determined by X-ray photoelectron spectroscopy, showed low O content (∼2 at. %) and Cl content (<1 at. %; below the detection limit). Fourier transform infrared spectroscopy spectra suggested that N-H bonds were the dominant hydrogen-containing bonds in the SiN _x films without a significant amount of Si-H bonds originating from the precursor molecules. The possible surface reaction pathways of the PEALD SiN _x using PCDS on the surface terminated with amine groups (-NH₂ and -NH-) are proposed. The PEALD SiN _x films grown using PCDS also exhibited a leakage current density as low as 1-2 nA/cm² at 2 MV/cm and a breakdown electric field as high as ∼12 MV/cm.

Novel Cyclosilazane-Type Silicon Precursor and Two-Step Plasma for Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride

We designed cyclosilazane-type silicon precursors and proposed a three-step plasma-enhanced atomic layer deposition (PEALD) process to prepare silicon nitride films with high quality and excellent step coverage. The cyclosilazane-type precursor, 1,3-di-isopropylamino-2,4-dimethylcyclosilazane (CSN-2), has a closed ring structure for good thermal stability and high reactivity. CSN-2 showed thermal stability up to 450 °C and a sufficient vapor pressure of 4 Torr at 60 °C. The energy for the chemisorption of CSN-2 on the undercoordinated silicon nitride surface as calculated by density functional theory method was -7.38 eV. The PEALD process window was between 200 and 500 °C, with a growth rate of 0.43 Å/cycle. The best film quality was obtained at 500 °C, with hydrogen impurity of ∼7 atom %, oxygen impurity less than 2 atom %, low wet etching rate, and excellent step coverage of ∼95%. At 300 °C and lower temperatures, the wet etching rate was high especially at the lower sidewall of the trench pattern. We introduced the three-step PEALD process to improve the film quality and the step coverage on the lower sidewall. The sequence of the three-step PEALD process consists of the CSN-2 feeding step, the NH₃/N₂ plasma step, and the N₂ plasma step. The H radicals in NH₃/N₂ plasma efficiently remove the ligands from the precursor, and the N₂ plasma after the NH₃ plasma removes the surface hydrogen atoms to activate the adsorption of the precursor. The films deposited at 300 °C using the novel precursor and the three-step PEALD process showed a significantly improved step coverage of ∼95% and an excellent wet etching resistance at the lower sidewall, which is only twice as high as that of the blanket film prepared by low-pressure chemical vapor deposition.

Advanced Silicon-on-Insulator: Crystalline Silicon on Atomic Layer Deposited Beryllium Oxide

Silicon-on-insulator (SOI) technology improves the performance of devices by reducing parasitic capacitance. Devices based on SOI or silicon-on-sapphire technology are primarily used in high-performance radio frequency (RF) and radiation sensitive applications as well as for reducing the short channel effects in microelectronic devices. Despite their advantages, the high substrate cost and overheating problems associated with complexities in substrate fabrication as well as the low thermal conductivity of silicon oxide prevent broad applications of this technology. To overcome these challenges, we describe a new approach of using beryllium oxide (BeO). The use of atomic layer deposition (ALD) for producing this material results in lowering the SOI wafer production cost. Furthermore, the use of BeO exhibiting a high thermal conductivity might minimize the self-heating issues. We show that crystalline Si can be grown on ALD BeO and the resultant devices exhibit potential for use in advanced SOI technology applications.