Next-generation Semiconductor Devices Research Articles

Area-selective deposition of germanium on patterned graphene/monolayer molybdenum disulfide stacks via dipole engineering

Heterogeneous integration of two-dimensional materials and the conventional semiconductor has opened opportunities for next-generation semiconductor devices and their processing. Heterogeneous integration has been studied for economical manufacturing by substrate recycling and novel functionalities by a combination of incommensurate materials. However, utilizing the integration requires controlling locations of the integrated architectures. Here, we show area-selective deposition (ASD) of germanium on the graphene/MoS2 stack. Ge nucleation precisely occurred on the surfaces of the patterned graphene/MoS2 stack via dipole engineering. In this study, the growth temperature of ASD of Ge was significantly lower than that based on precursor desorption on SiO2. The first-principles calculations revealed that Ge deposited by ASD on the graphene/MoS2 stack was not affected by charge transfer. This work provides a viable way to utilize atomically thin materials for next-generation semiconductor devices, which can be applicable for “Beyond Moore” and “More Moore” approaches.

Effect of Surface Pretreatment to Reduce the Incubation Period of Iridium Thin Films Grown by ALD on the Oxide Surface

Iridium (Ir) has low figure of merit(×λ) and high melting temperature properties, so it has been recently spotlighted as a very important copper (Cu) alternative interconnect material in next-generation semiconductor devices. In particular, in the case of depositing a thin film using atomic layer deposition (ALD) technology, it can be expected to deposit extremely thin film with conformal, uniform and excellent step coverage characteristics even in a very complex structure or a trench structure of several nm dimension due to the inherent self-limiting characteristic of ALD. In this regard, ALD-Ir is considered as one of the most suitable metallization process for the application of advanced semiconductor interconnects. Therefore, considerable efforts have been conducted to develop a reliable ALD-Ir process having improved film qualities. And, ALD-Ir process with excellent thin film properties such as low electrical resistivity and negligible oxygen impurities was recently reported using Tricarbonyl (1,2,3-η)-1,2,3-tri(tert-butyl)-cyclopropenyl iridium (C18H27IrO3 or TICP) precursor and oxygen [1]. However, in the case of this TICP precursor, it was difficult to deposit extremely thin and continuous Ir films on the hydroxyl-terminated oxide layer due to its long incubation delay.Therefore, in this study, a method for depositing a very thin, uniform and continuous ALD-Ir thin film with low resistivity even on oxide materials was explored by reducing the incubation period and promoting nucleation using various surface pretreatment conditions. In addition, the nucleation behavior as well as film properties of ALD-Ir on the oxide material were systemically compared and analyzed according to the surface pretreatment conditions, and finally, ALD-Ir thin film with excellent properties on the oxide surface was obtained.

SAB-Enabled Room Temperature Hybrid Bonding

On the one hand, direct hybrid bonding is foreseen to revolutionize semiconductor device integration, allowing the seamless 3D interconnection of diverse materials with distinct properties. On the other hand, Surface Activation Bonding (SAB) is a powerful technique within the field of low-temperature bonding, offering enhanced bonding strength and compatibility with a wide range of materials. By dealing with the possibilities of SAB, this paper demonstrates the possibility of realizing a Cu-oxide direct hybrid bonding with a 5 µm pitch interconnections at ambient temperature. This success highlights the potential of SAB as a critical enabler for low-temperature hybrid bonding and its potential impact on next-generation semiconductor devices.

(Invited, Digital Presentation) Low-Temperature Direct Bonding of Wide-Bandgap Semiconductor Substrates

For the next-generation semiconductor devices, our research group has developed direct bonding techniques of wide-bandgap materials, including SiC, Ga2O3, and diamond. It is known that the semiconductor substrates activated by oxygen plasma can form atomic bonds at low temperatures. In this case, a thick oxide layer, which may become a thermal and electrical barrier, is formed at the bonding interface. Meanwhile, our research group demonstrated that the OH-terminated Ga2O3 and diamond substrates were directly bonded without an oxide intermediate layer. In addition, the SiC substrate dipped into the HF acid can be bonded with the Ga2O3 substrate with an ~1-nm-thick amorphous layer. The dissimilar substrates bonded through the ultra-thin intermediate layer would contribute to efficient heat dissipation and future heterojunction devices.

Comprehensive Assessments in Bonding Energy of Plasma Assisted Si-SiO2 Direct Wafer Bonding after Low Temperature Rapid Thermal Annealing

Direct wafer bonding is one of the most attractive techniques for next-generation semiconductor devices, and plasma has been playing an indispensable role in the wider adoption of the wafer bonding technique by lowering its process temperature. Although numerous studies on plasma-assisted direct wafer bonding have been reported, there is still a lack of deep investigations focusing on the plasma itself. Other than the plasma surface treatment, the wafer bonding process includes multiple steps such as surface cleaning and annealing that require comprehensive studies to maximize the bonding strengths. In this work, we evaluate the various process steps of Si-SiO2 wafer bonding through case-by-case experimental studies, covering factors including the plasma conditions for surface treatment and secondary factors such as the time intervals between some process steps. The results show that plasma treatment with increasing input power has a trade-off between bonding strengths and interfacial voids, requiring the optimization of the plasma conditions. It is also noticeable that the effects of plasma treatment on wafer bonding can be improved when the plasma-treated wafers are stored in ambient atmosphere before the subsequent process step, which may suggest that wafer exposure to air during the bonding process is advantageous compared to processing entirely in vacuum. The results are expected to allow plasma-assisted direct wafer bonding technology to play a bigger role in the packaging process of semiconductor device manufacturing.

Zeolite-like molecules: Promising dielectrics for two-dimensional semiconductors

Two-dimensional (2D) semiconductors have opened opportunities for the development of next-generation semiconductor devices and the continuation of Moore’s law. The absence of qualified dielectrics hinders the scalable application of 2D semiconductors. Herein, we present a joint theory-experiment study shedding light on the exciting prospect of zeolite-like molecules (ZLMs) as the dielectrics for 2D semiconductors. Specifically, ZLMs possess bandgaps and dielectric constants comparable to those of SiO2 (the commonly used dielectric in the Si-based semiconductor industry), and ZLMs and 2D semiconductors can form ideal interfaces, in which ZLMs are in van der Waals (vdW) contact with 2D semiconductors and preserve well the intrinsic semiconducting properties of 2D semiconductors. Furthermore, we demonstrated that molecules with a zeolite-like backbone can exhibit remarkable environmental stability. This work offers a promising new family of vdW dielectrics with important implications for the field of 2D semiconductors.

Analytical approach to metal oxide resist modeling: exposure, bake, and network formation

Metal oxide photoresists (MORs) have received attention for use in extreme ultraviolet (EUV) lithographic patterning, in which more established chemically amplified resists suffer from long blur lengths and low EUV absorptivity. To meet the demand for next-generation semiconductor devices, a thorough understanding of the physics behind MOR patterning is required to predict the outcome of arbitrary patterning schemes and to determine optimal process conditions. However, the ability to model and predict MORs through bake and development is still in early stages. We provide a modeling framework and associated analytical expressions that describe the behavior of MORs that have so far been described by kinetic Monte Carlo simulation approaches. We further extend the aforementioned analytical expressions to directly predict dose-to-gel transition energies for wet-developed MORs for arbitrary bake temperature and bake duration. Finally, we briefly compare our methodology using an in-house simulation program to patterning results from an experiment using Fractilia MetroLER™.

Scanning Deposition Method for Large-Area Diamond Film Synthesis Using Multiple Microwave Plasma Sources.

The demand for synthetic diamonds and research on their use in next-generation semiconductor devices have recently increased. Microwave plasma chemical vapor deposition (MPCVD) is considered one of the most promising techniques for the mass production of large-sized and high-quality single-, micro- and nanocrystalline diamond films. Although the low-pressure resonant cavity MPCVD method can synthesize high-quality diamonds, improvements are needed in terms of the resulting area. In this study, a large-area diamond synthesis method was developed by arranging several point plasma sources capable of processing a small area and scanning a wafer. A unit combination of three plasma sources afforded a diamond film thickness uniformity of ±6.25% at a wafer width of 70 mm with a power of 700 W for each plasma source. Even distribution of the diamond grains in a size range of 0.1–1 μm on the thin-film surface was verified using field-emission scanning electron microscopy. Therefore, the proposed novel diamond synthesis method can be theoretically expanded to achieve large-area films.

Fundamental study on the mechanical strength of soft magnetic composite prepared by ultra-high aspect ratio flake iron powder using ball-milling process

ABSTRACT Soft magnetic composites (SMCs) are consolidated ferromagnetic powders covered with polymers. Ultra-thin flat powder can effectively reduce eddy current loss of SMCs, even at the frequency at which next-generation semiconductor devices are driven. In this study, pure iron powders were processed with ball-milling to obtain the ultra-thin powder and prepare the SMC. The effect of aspect ratio of the powder on the mechanical strength of SMC was investigated. Although the maximum bending stress of SMC made of iron particles with an aspect ratio of 1140 reached 60% of the strength of sintered iron powder, the increase in the maximum bending stress with increasing aspect ratio slowed down when the aspect ratio exceeded 100. The reason for the slowdown was assumed to be a decrease in insulating film coverage on the particle at high aspect ratio ranges due to the stacking and welding of particles during the ball milling process.

Growth of MoS2–Nb-doped MoS2 lateral homojunctions: A monolayer p–n diode by substitutional doping

Monolayer transition metal dichalcogenides (TMDs) have been considered as promising materials for various next-generation semiconductor devices. However, carrier doping techniques for TMDs, which are important for device fabrication, have not been completely established yet. Here, we report a monolayer p–n junction formed using in situ substitutional doping during chemical vapor deposition (CVD). We synthesized monolayer MoS2–Nb-doped MoS2 lateral homojunctions using CVD and then characterized their physical and electrical properties. The optimized growth condition enabled us to obtain spatially selective and heavy Nb doping in the edge region of a single-crystalline MoS2, thus resulting in an obvious work function difference between the inner and edge regions of the crystal. The obtained monolayer crystal demonstrated n-type and degenerate p-type semiconducting behaviors in each region, and a clear rectifying behavior across the n-type and p-type interface was observed. We believe that the results obtained can expand the research field of exploring two-dimensional homo p–n junctions, which can be important for realizing various TMD-based devices, such as diodes and field-effect transistors, with low-contact resistance.

A review of WBG and Si devices hybrid applications

In recent years, next-generation power semiconductor devices, represented by silicon carbide (SiC) and gallium nitride (GaN), have gradually emerged. Because wide-bandgap (WBG) devices have better electrical characteristics than those of silicon (Si) based devices, they have attracted increased attention both from academic researchers and industrial engineers. Employing WBG devices will further improve the efficiency and power density of power converters. However, the current price of WBG devices remains extremely high. Thus, some researches have focused on the hybrid utilization of WBG devices and Si-based devices to achieve a tradeoff between the performance and cost. To summarize the current research on WBG/Si hybrid applications, the issues mentioned above with representative research approaches, results, and characteristics, are systematically reviewed. Finally, the current research on WBG/Si hybrid applications and their future trends are discussed.

Electrocaloric Effect in Emerging Fluorite-Structure Ferroelectrics

The electrocaloric effect can be observed in pyroelectric materials based on conversion between electrical and thermal energy, and can be utilized for the future environment-friendly refrigeration technology. Especially, a strong electrocaloric effect is expected in materials in which field-induced phase transition can be achieved. Emerging fluoritestructure ferroelectrics such as doped hafnia and zirconia, first discovered in 2011, are considered the most promising materials for next-generation semiconductor devices. Besides application of fluorite-structure ferroelectrics for semiconductor devices based on their scalability and CMOS-compatibility, field-induced phase transition has been suggested as another interesting phenomenon for various energy-related applications such as solid-state cooling with electrocaloric effect as well as energy conversion/storage and IR/piezoelectric sensors. Especially, their giant electrocaloric effect is considered promising for solid-state-cooling. However, the electrocaloric effect of fluorite-structure oxides based on field-induced phase transition has not been reviewed to date. In this review, therefore, the electrocaloric effect accompanied by field-induced phase transition in fluorite-structure ferroelectrics is comprehensively reviewed from fundamentals to potential applications.

Current Trends in the Development of Normally-OFF GaN-on-Si Power Transistors and Power Modules: A Review

Gallium nitride (GaN) power transistors have attracted significant interest in the power electronics industry over the past decade as the next-generation power semiconductor devices. GaN power transistors are suitable for high power and high frequency applications due to their higher electron mobility, temperature tolerance, electrical conductivity, critical breakdown electric field, and breakdown voltage compared to the conventional silicon-based transistors and other wide bandgap (WBG) power transistors. In particular, GaN-on-silicon (GaN-on-Si) technology has opened up the possibility of manufacturing high-performance, low-cost WBG power devices in silicon-compatible fabrication facilities. The first GaN power transistor structure to be developed was the normally-ON depletion mode (D-mode) device. It relies on the highly mobile two-dimension electron gas (2DEG) at the GaN/AlGaN epitaxial layers’ interface to provide very low on-resistance. The normally-OFF enhancement mode (E-mode) GaN power transistor soon became available by controlling the 2DEG using various gate structures. This paper provides a review of the developments of GaN power transistors followed by a survey on current state-of-the-art GaN power technologies and applications, including comparisons between GaN growth substrates and developments of enhancement mode (E-mode) device structures and their process techniques. Moreover, developments of power module designs are also addressed, including gate driver designs and their requirements, and packaging techniques for power transistors and power modules.

Quasi-Fermi level splitting in nanoscale junctions from ab initio

The splitting of quasi-Fermi levels (QFLs) represents a key concept utilized to describe finite-bias operations of semiconductor devices, but its atomic-scale characterization remains a significant challenge. Herein, the nonequilibrium QFL or electrochemical potential profiles within single-molecule junctions obtained from the first-principles multispace constrained-search density-functional formalism are presented. Benchmarking the standard nonequilibrium Green's function calculation results, it is first established that algorithmically the notion of separate electrode-originated nonlocal QFLs should be maintained within the channel region during self-consistent finite-bias electronic structure calculations. For the insulating hexandithiolate junction, the QFL profiles exhibit discontinuities at the left and right electrode interfaces and across the molecule the accompanying electrostatic potential drops linearly and Landauer residual-resistivity dipoles are uniformly distributed. For the conducting hexatrienedithiolate junction, on the other hand, the electrode QFLs penetrate into the channel region and produce split QFLs. With the highest occupied molecular orbital entering the bias window and becoming a good transport channel, the split QFLs are accompanied by the nonlinear electrostatic potential drop and asymmetric Landauer residual-resistivity dipole formation. Our findings underscore the importance of the first-principles extraction of QFLs in nanoscale junctions and point to a future direction for the computational design of next-generation semiconductor devices.

DFT investigations of linear Zn3-type complex with compartmental N/O-donor Schiff base: Synthesis, characterizations, crystal structure, fluorescence and molecular docking

A linear Zn(II) complex, [Zn3(LOMe-pn)2(ƞ1-NCS)2] (1) containing a N2O2O′2 donor Schiff base, (H2LOMe-pn = N, N-bis(3-methoxysalicylidene)-2, 2-dimethylpropane-1, 3 diamine) has been synthesized and structurally well characterized. SCXRD study reveals that in 1, the asymmetric section contains two identical discrete unit and each discrete unit contains three zinc metal ions (Zn1, Zn2 and Zn3) which are crystallographic independent, two deprotonated ligands [LOMe-pn]2- and [SCN−] ions which are linked with only terminal zinc ion in a η1 mode where terminal zinc atoms (Zn1/Zn3) are distorted square pyramidal while Zn2 is square antiprismatic. All computational calculations were performed by the DFT/M06 functional level and the 6-31G * basis set in the ground state. Complex optimized structure, HOMO-LUMO energy gap, Hirshfeld surface, MEP and NLO property was nicely explained with the help of DFT. Total dipole moment, average polarizability and first hyperpolarizability were calculated where α and βhyp values are well demonstrate the NLO property of complex (1). In addition, Bader’s “atoms-in-molecules” was analysed to delineate complex non-covalent interactions using the same level of theory. Experimental electronic spectra were explained using TD-DFT level of calculations with an IEFPCM solvent model. DMF solvent explore complex (1) is a fluorescent material with maximum emission at 479 nm at an excitation wavelength of 300 nm. Finally, molecular docking was executed with cytochrome P450 from B. megaterium and M. tuberculosis. Results of molecular docking and HOMO-LUMO energy gap can provide new insights in the development of antimycobacterial drugs and next-generation semiconductor devices.

Synthesis of Semiconducting Graphene Nanoribbons on Ge and Ge/Si via Chemical Vapor Deposition

Graphene nanoribbons are promising materials for semiconductor electronics due to their atomic thickness and their potential to be exceptional charge and thermal conductors. However, challenges in nanoribbon production have limited their applicability. We have recently developed a scalable, bottom-up technique to synthesize semiconducting graphene nanoribbons directly on Ge and Ge/Si wafers via chemical vapor deposition. The nanoribbons exhibit high-performance charge transport properties and can be grown into dense, large-area arrays with controlled placement and alignment via seed-mediated growth. Therefore, this synthesis may provide a high-throughput pathway towards the practical realization of next-generation semiconductor devices based on graphene.

Effects of Microwave Annealing on High-k Dielectric HfO₂ Thin Films.

In this study, the effect of microwave annealing (MWA), which has been attracting considerable attention as a low-thermal-budget method, on the high-k materials required for next-generation ultrahigh-integration semiconductor devices is investigated. Metal-oxide-semiconductor (MOS) capacitor-structure devices are fabricated using HfO₂ films, which are typical high-k materials; post deposition annealing (PDA) is then performed by MWA and compared to conventional thermal annealing (CTA). The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the fabricated HfO₂ MOS capacitors are measured to evaluate the leakage current, time-zero dielectric breakdown (TZDB), time-dependent dielectric breakdown (TDDB), and frequency dispersion. Compared to the as-deposited state, the PDA-treated HfO₂ dielectric film is found to have reduced trap density, improved dielectric breakdown, frequency dispersion, and lifetime. In particular, the high-k HfO₂ thin films treated with MWA exhibit excellent surface properties, including a low root-mean square roughness (Rq) of 0.141 nm, thickness of 49.3 nm, and low inter face trap density (Dit) of 7.82 × 1012 cm-2eV-1 and electrical properties such as a high breakdown electric field (EBD) of 10 MV/cm, and high breakdown to charge (QBD) of 1,000 C/cm². As a result, the MWA-treated high-k HfO₂ thin films are smoother and denser than the CTA-treated thin films, and show better electrical properties and reliability, suggesting that the defects at Si the interface as well as in the high-k films are effectively removed.

Atomic layer deposition of silicon-based dielectrics for semiconductor manufacturing: Current status and future outlook

The fabrication of next-generation semiconductor devices has created a need for low-temperature (≤400 °C) deposition of highly-conformal (>95%) SiO2, SiNx, and SiC films on high-aspect-ratio nanostructures. To enable the growth of these Si-based dielectric films, semiconductor manufacturers are transitioning from chemical vapor deposition to atomic layer deposition (ALD). Currently, SiO2 films deposited using ALD are already being integrated into semiconductor device manufacturing. However, substantial processing challenges remain for the complete integration of SiNx films deposited by ALD, and there are no known processes for ALD of SiC at temperatures that are compatible with semiconductor device manufacturing. In this focused review, the authors look at the status of thermal and plasma-assisted ALD of these three Si-based dielectric films. For SiO2 ALD, since low-temperature processes that deposit high-quality films are known, the authors focus primarily on the identification of surface reaction mechanisms using chlorosilane and aminosilane precursors, as this provides a foundation for the ALD of SiNx and SiC, two material systems where substantial processing challenges still exist. Using an understanding of the surface reaction mechanisms, the authors describe the underlying reasons for the processing challenges during ALD of SiNx and SiC and suggest methodologies for process improvement. While both thermal and plasma-assisted SiNx ALD processes have been reported in the literature, the thermal NH3-based ALD processes require processing temperatures >500 °C and large NH3 doses. On the other hand, plasma-assisted SiNx ALD processes suffer from nonuniform film properties or low conformality when deposited on high-aspect-ratio nanostructures. In the SiNx section, the authors provide a broad overview of the currently known thermal and plasma-assisted SiNx ALD processes using chlorosilane, trisilylamine, and aminosilane precursors, describe the process shortcomings, and review the literature on precursor reaction pathways. The authors close this section with suggestions for improving the film properties and conformality. In the case of SiC, the authors first outline the limitations of previously reported SiC ALD processes and highlight that unlike SiO2 and SiNx plasma-assisted ALD, no straightforward pathway for low-temperature plasma-assisted growth is currently apparent. The authors speculate that low-temperature ALD of SiC may require the design of completely new precursors. Finally, they summarize the progress made in the ALD of C-containing SiNx and SiO2 films, which may provide many of the benefits of SiC ALD in semiconductor manufacturing. In closing, through this review, the authors hope to provide the readers with a comprehensive knowledge of the surface reactions mechanisms during ALD of Si-based dielectrics, which would provide a foundation for future precursor and process development.

Effect of Excitation Wavelength on Optical Performances of Quantum-Dot-Converted Light-Emitting Diode.

Light-emitting diode (LED) combined with quantum dots (QDs) is an important candidate for next-generation high-quality semiconductor devices. However, the effect of the excitation wavelength on their optical performance has not been fully explored. In this study, green and red QDs are applied to LEDs of different excitation wavelengths from 365 to 455 nm. The blue light is recommended for exciting QDs from the perspective of energy utilization. However, QD LEDs excited at 365 nm have unique advantages in eliminating the original peaks from the LED chip. Moreover, the green or red light excited by ultraviolet light has an advantage in colorimetry. Even for the 455 nm LED with the highest QD concentration at 7.0 wt%, the color quality could not compete with the 365 nm LED with the lowest QD concentration at 0.2 wt%. A 117.5% (NTSC1953) color gamut could be obtained by the 365 nm-excited RGB system, which is 32.6% higher than by the 455 nm-excited solution, and this can help expand the color gamut of LED devices. Consequently, this study provides an understanding of the properties of QD-converted LEDs under different wavelength excitations, and offers a general guide to selecting a pumping source for QDs.

Common-Mode Voltage Attenuation of an Active Common-Mode Filter in a Motor Drive System Fed by a PWM Inverter

The switching speeds of next-generation power semiconductor devices, such as those made of silicon carbide and gallium nitride are roughly ten times those of conventional devices (e.g., silicon insulated-gate bipolar transistors). This increases the frequency range of the electromagnetic noise accompanying the switching operations of pulsewidth modulated (PWM) converters and worsens the influence of radiated noise. The authors have previously proposed an active common-mode filter (ACF) that reduces the radiated noise from the power cables connected to a PWM converter and evaluated its effect in reducing radiated noise by using a function generator as a common-mode (CM) noise source. In this paper, the ACF is applied to a motor drive system fed by a three-phase PWM inverter, and the attenuation characteristics of the CM voltage are evaluated. To avoid saturating the ACF, the combination of an active common-noise canceller and the ACF is discussed. The experimental results show that the system constructed in this paper can suppress the CM voltage produced by the PWM inverter over a wide frequency range from 100 to 100 MHz.