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Mark's precise positioning is critical for achieving wafer bonding accuracy. A Mark positioning algorithm is proposed to improve the accuracy and stability of Mark positioning. Shape template matching can effectively reduce the impact of image quality on positioning accuracy, but it can only achieve pixel precision Mark location. On the basis of this method, the algorithm uses the facet model method to extract the sub-pixel edge information for the Mark contour, and combines the sub-pixel traversal strategy to achieve accurate sub-pixel precision Mark location. Simultaneously, the composite template method is proposed for the first time to improve the template information's quality, which effectively improves the robustness of the algorithm to image quality. The simulation results indicate that the algorithm is capable of positioning with an accuracy of approximately 0.04 pixels. The experimental results indicate that the algorithm is capable of positioning with an accuracy of approximately 0.052 pixels and is resistant to image noise and blur, which meets the accuracy and stability requirements for Mark positioning in wafer bonding alignment. • A Mark positioning algorithm based on sub-pixel shape template matching is proposed that enables high-precision mark location during wafer bonding alignment. • The facet model method is used to introduce sub-pixel edge point information for shape template matching, and the sub-pixel positioning is realized combined with the search strategy of sub-pixel step size. • The composite template method is proposed to improve the template quality, which finally improves the reliability of template matching. • Simulation and experiments demonstrate that the algorithm has high positioning accuracy and robustness to image quality. [ABSTRACT FROM AUTHOR]

• Monolithic lithium niobate microfluidic chips are realized by facile fabrication. • Room temperature H 2 plasma proton exchange enhances the bonding performance. • High-density and diverse functional groups on the surface of lithium niobate. • Lithium niobate channel processing with high etching rate and smooth morphology. • The superior performance of full lithium niobate structure in SAW applications. In the quest to enhance the functionality and efficiency of lab-on-a-chip systems, integrating diverse physical phenomena such as acoustics, optics, and electronics into a single platform has emerged as a pivotal strategy. Lithium niobate (LiNbO 3 , or LN) stands out for its exceptional piezoelectric and electro-optic properties, making it an ideal candidate for such integrated systems. However, the full exploitation of LN in microfluidic applications has been hindered by the complex challenges associated with its fabrication. Addressing this gap, our research introduces a facile fabrication method for creating monolithic LN microfluidic chips. By employing a proton exchange-assisted surface-activated bonding technique, we achieve high-strength, flawless bonding at lower temperatures, overcoming the traditional barriers of LN machining. This method eliminates the chemical stability of LN by removing lithium ions and significantly enriches the surface with functional groups, leading to a bonding strength exceeding 10 MPa after annealing at 150 °C. Additionally, we have optimized an argon plasma etching process to ensure the creation of smooth LN channels at room temperature. The development of a fully integrated LN chip through this approach demonstrates superior performance, especially in surface acoustic wave (SAW) applications, compared to conventional PDMS/LN hybrids. This breakthrough not only realizes the fabrication process of LN microfluidic devices but also opens new avenues for the advancement of integrated microsystems across various scientific and technological domains. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bonding interface is modified to a homogenous one with an epitaxial layer. • Epitaxial layer constrains the defects, disperses and reduces the heterogenous mismatch stress. • Epitaxial layer is doped to reduce the carrier recombination leakage current in this defect-rich layer. In the realm of optoelectronic integration, silicon-based Ge bonding has attracted great attention due to its advantages in short-wave infrared response and low cost. However, owing to the inherent lattice mismatch and thermal mismatch between Ge and Si, the bonded Ge-Si heterogenous interfaces encountered problems of thermal instability and high interface states. Herein, an approach of separating the bonding interface from the heterogenous interface is proposed through inserting a Ge epitaxial layer (GeEL) between the Si substrate and Ge film. This separation modifies the bonding interface to a homogenous one, alleviating the massive mismatch stress and achieving film relaxation, enhancing bonding stability in all aspects. Moreover, during 850 °C annealing, GeEL is doped via diffusion hence realizes electric filed modulation, inhibiting the carrier recombination leakage current and trap assisted tunneling in this defect-rich layer. A Ge-Si heterogenous PIN diode prepared by this bonding method has achieved a remarkable low dark current density (1.33 mA/cm2), a low ideality factor (1.11), and a high on–off ratio (1 0 6), confirming the outstanding quality of the bonded heterojunction. This work provides great prospects for higher performance, larger scale and multi-functional Si-based heterogenous material device applications. [ABSTRACT FROM AUTHOR]

• Analytical Method for Wafer Bonding Quality Assessment: The paper introduces an analytical method that connects external conditions such as normal pressure, planarity deviation, and impurity particle counts with key mechanical variables like wafer deflection and strain energy during the direct wafer bonding process. This is a significant advancement as previous research did not account for the impact of impurities on bonding quality. • Influence of Impurities on Bonding Process: The study establishes a mechanical model of the adhesive process that elucidates the relationship between impurity particle count and strain energy. It reveals how impurities affect wafer deflection and strain energy, showing nonlinear variations with changes in particle position. The results indicate that wafer curvature, thickness, and impurity count are critical factors determining deflection under specific pressures. • Theoretical Guidance for Practical Bonding: The proposed mechanical model, verified through finite element simulation, offers theoretical guidance for selecting adhesive materials and evaluating the feasibility of wafer bonding in industrial applications. It provides insights into the effects of initial wafer geometry, adhesive energy, and particle positions on successful bonding outcomes under applied external pressures. Direct wafer bonding technology has been widely used in the manufacturing of microelectromechanical systems (MEMS). The chip deflection and strain energy can be used to evaluate the bonding quality. Current research assumes that the wafer surface is perfectly clean and neglects the influence of impurities, lacking an analytical model to characterize the effects of external conditions (normal pressure, planarity deviation, impurity particle counts) on key mechanical variables (wafer deflection and strain energy) during the bonding process. In this paper, an analytical method is used to establish a bridge between external conditions and bonding variables. A mechanical model of the adhesive process is established, and the relationship between impurity particle count and strain energy is obtained. By considering the bonding pressure, chip geometry, and impurity particle count, analytical expressions for deflection and strain energy are derived, revealing the influence of external conditions and internal geometry on wafer deflection and strain energy. The results show that under certain pressure, the curvature and thickness of the wafer, as well as the impurity particle count, jointly determine the wafer deflection. The wafer deflection and strain energy exhibit nonlinear variations with changes in particle position. The correctness of the proposed model is verified through finite element simulation. This work provides a reference for judging wafer bonding with impurity particles in industrial applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Innovatively employed first-principles calculations to analyze the atomic-level bonding principles of heterogeneous wafer bonding interfaces. • Combines molecular dynamics and reaction kinetics for a multidisciplinary approach. • Presents eight interface models to explore bonding mechanisms. • Investigated annealing and oxygen plasma activation effects for optimizing bonding processes in optoelectronic integration. The direct bonding mechanism of Si/InP is systematically investigated using first-principles calculations. Charge density analysis reveals that Si can be bonded with either In or P and is polar covalent. The single Si-P bond is stronger than the Si-In bond and has a shorter bond length, the Si-P bond length is 2.356 Å compared to 2.590 Å for the Si-In ones, as it involves the sharing of electron pairs and a large transfer and redistribution of electrons. Notably, molecular dynamics simulations show that the interfacial strength of Si bonded to In is greater at room temperature. This is because, after sufficient relaxation, although the average strength of the Si-P bonds is still greater than that of Si-In, the number of Si-In bonds is approximately four times that of the Si-P bonds. The radial distribution function of the atoms in the interfacial layer further confirms that the terminal interface is stronger than the P-terminal interface. Appropriate annealing treatment can effectively increase the number and strength of bonds at the interface. Finally, the bonding mechanism of the oxygen plasma activation is studied using reaction kinetics. These findings contribute to the precise control and optimization of bonding processes, ultimately improving the reliability of optoelectronic integration. [ABSTRACT FROM AUTHOR]

We report a low-temperature and low-cost wafer-bonding method for silicon (Si) and indium phosphide (InP) based on a microcrystalline germanium (Ge) interlayer. The sputtered amorphous Ge (a-Ge) annealed at different temperatures exhibited different crystallinities. It is fully crystallized when the sputtering is carried out at 400 °C. For the microcrystalline Ge sputtered on the Si and InP, the Ge crystallizes along the crystal orientation of the Si, and exhibits a polycrystalline state on the InP. The crystallization evolution, bubble evolution, and bonding strength evolution at the wafer-bonded interface are investigated. The mechanism by which the bubbles are eliminated is revealed. In the case of samples featuring microcrystalline Ge grown on Si, the bonding interface consists of single crystal Ge. Bubbles can only escape from the bonding interface through atomic migration. Due to lattice mismatch and lack of gas transport channels, bubbles and dislocations form at the bonding interface. The samples with microcrystalline Ge grown simultaneously on Si substrate and InP wafer have a polycrystalline Ge layer (poly-Ge) at the bonding interface, which acts as a lattice barrier and gas transport channel. The combined action of lattice blocking and channeling effectively removes bubbles and dislocations at the bonding interface. After post-annealing, the bonding strength of the sample with fully crystallized intermediate layer can reach the bulk fracture strength of InP wafers. In addition, the carrier transport mechanism at the InP/Si bonding interface is also clearly elucidated. The above findings provide a direction for the preparation of high-performance Si-based III-V compound semiconductor photodetectors. • Microcrystalline Ge semiconductor interlayer is proposed to realize InP/Si bonding. • The InP/Si lattice is isolated by polycrystalline Ge and the dislocation nucleation is eliminated. • The crystallization status of microcrystalline Ge on Si and InP is clarified. • The process unveils bubble fade out at bonded interfaces via gas removal by atomic recombination and polycrystalline Ge. • A microcrystalline Ge layer is introduced to eliminate the influence of the oxide layer. [ABSTRACT FROM AUTHOR]

This paper proposes a novel method for friction-induced chemical reaction machining of single-crystal 4H-SiC wafers using pure iron. Material Removal Rate (MRR) and surface quality of single-crystal 4H-SiC were studied, and the results demonstrate that both high surface quality and high MRR (Ra: 2.2 nm, MRR: 375 nm/min) were obtained. The mechanism of material removal during friction was investigated by examining the critical roles of friction speed. Solid-state chemical reactions between metal and SiC surfaces can be induced by friction, and the subsequent removal of reactants, resulting in a near-damage-free machined surface. A removal mechanism model of friction was established, which reveals that both the rates of solid-state reaction and reactant removal significantly impact the final removal rate of friction. [Display omitted] • A novel friction-induced chemical removal method single crystal 4H-SiC wafer was proposed. • High surface quality and high MRR (Ra: 2.2 nm, MRR: 375 nm/min) were obtained for a 4-inch wafer. • 4H-SiC with near-damage-free surface/subsurface were obtained. • Solid-state chemical reaction occurred at low speed, pressure, and temperature. [ABSTRACT FROM AUTHOR]

Aluminum nitride (AlN) is a ceramic material with excellent dielectric and thermal properties, and it is used in the microelectron and energy fields. Despite polishing, the roughness (rms >10 nm) of AlN wafers fabricated by sintering fails to meet the surface requirements (rms <0.5 nm) for direct wafer bonding. However, rough-surface wafer bonding of paired AlN and AlN can be achieved with the following procedure: (1) rough surfaces are activated by oxygen plasma to introduce strong capillary action, (2) reactions of AlN and bonding species (OH−) are promoted by clamping during storage and annealing and (3) low-temperature annealing (<150 °C) joins the two surfaces of AlN/AlN by synthesizing Al 2 O 3 at the bonding interface. TEM and XPS results show that a transition layer exists between bonding surfaces wherein the concentration of nitrogen decreases while that of oxygen significantly increases. [ABSTRACT FROM AUTHOR]

Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III–N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stresses are investigated using an analytical approach, a numerical model and experimental validation. The thermal stresses are caused by the mismatch in the coefficient of thermal expansion (CTE) between the GaN/III-N stack, silicon (Si) and the diamond from room temperature to CVD growth temperatures. Simplified analytical wafer bow models underestimate the membrane bow for small sizes while numerical models replicate the stresses and bows with increased accuracy using temperature gradients. The largest tensile stress measured using Raman spectroscopy at room temperature was approximately 1.0 ± 0.2 GPa while surface profilometry shows membrane bows as large as 58 μm. This large bow is caused by additional stresses from the Si frame in the initial heating phase which are held in place by the diamond and highlights challenges for any device fabrication using contact lithography. However, the bow can be reduced if the membrane is pre-stressed to become flat at CVD temperatures. In this way, a sufficient platform to grow diamond on GaN/III-N structures without wafer bonding can be realised. Image 1 [ABSTRACT FROM AUTHOR]

A miniaturized lead zirconate titanate (PZT)-actuated Fabry-Perot interferometer (FPI) was developed for an 850 nm light beam selection by modulating the air gap between two refractive mirrors. The developed FPI consists of mirrors, a PZT actuator for air gap modulation, metal electrodes, and interface electronics capable of applying a square voltage at a frequency of 0.1 Hz. Eutectic wafer bonding method was employed to bond two mirrors through rapid thermal annealing (RTA) and applying mechanical pressure for a short time to minimize the interdiffusion between layers. The PZT actuator and metal electrodes serve as a thickness spacer to create an air gap between the two mirror plates. Alternating high and low dielectric layers (TiO 2 and SiO 2) with precise thickness and high crystallization quality were utilized for the formation of the refractive mirror because these layers offer high reflectivity and low absorption loss, enabling effective transmission of a specific wavelength in the incident light beam. By applying 0.1 Hz square pulses to the PZT actuator, the air gap was modulated, resulting in significant shifts in the center wavelength of the filtered beam at the desired wavelength. The study demonstrates the successful modulation and optimization of transmission peaks, ensuring a sharp full width at half maximum (FWHM) and high transmission rate. It also provides comprehensive experimental process factors for optimizing heat treatment conditions for PZT and refractive mirrors, minimizing metal interdiffusion between the dielectric layers, and ensuring mirror plate flatness. The device parameter effects such as cavity length orders, sequence of refractive indices for dielectric layers, and the extent of elongation of PZT with applied voltage were systematically studied to achieve optimal performance of FPI filter. The developed interface electronics allow voltage modulation up to 21.9 V at a frequency of 0.1 Hz, while preserving the elasticity of the PZT actuator. [Display omitted] • A miniaturized PZT-controlled Fabry-Perot Interferometer (FPI) designed for selecting light beam at 850 nm was developed. • Interface electronics was implemented that allow voltage modulation up to 53.8 V in DC voltage. • Transmission peaks were effectively modulated, maintaining both a sharp FWHM and a high transmission rate. • Comprehensive insights for bonding, minimal metal interdiffusion between layers, and the flatness of mirror were provided. [ABSTRACT FROM AUTHOR]

Fully-epitaxial magnetoresistance (MR) devices with half-metallic Heusler alloys has attracted considerable attention for years due to their excellent spin-dependent transport properties such as high MR ratio and ultra-low resistance-area product. However, their poor manufacturability due to the necessity of epitaxial growth on a special single crystalline substrate such as MgO(001) hinders their practical applications. To overcome this issue, in this study, we fabricated Heusler-based epitaxial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) film grown on Si substrate and directly bonded it to poly-crystalline electrode wafer by using three-dimensional (3D) integration processes such as direct wafer bonding and removal of backside Si substrate. First we explored suitable seed/buffer layers for the (001)-oriented epitaxial growth of Heusler CPP-GMR on Si(001) substrate. Si-subs/NiAl/CoFe seed/buffer structure was found to induce (001)-oriented epitaxial growth and to suppress an Al diffusion from NiAl to the Heusler electrode even after annealing at 500 °C, resulting in large MR ratio comparable to that with MgO substrate. After direct wafer bonding process, the microstructure analysis revealed clean damage-free bonded interface between the epitaxial Heusler GMR and poly-crystalline electrode films with Au capping layers. After the 3D integration processes, high MR performance has been successfully reproduced. This unique processing method enables the integration of high performance Heusler-based epitaxial spintronic devices for various practical applications. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Reactive bonding using nanoscale multilayers based on the high specific surface/interface energy is a promising low-temperature and low-pressure wafer-bonding process. Herein, Si wafers were bonded using nanoscale Ag/Cu multilayers in N 2 or the ambient atmosphere. A flawless joint composed of Ag-rich and Cu-rich face-centered cubic (fcc) phases was achieved in N 2 with 22.2 MPa shear strength. However, a peculiar "fcc-(Ag)+voids/Cu 2 O/fcc-(Ag)+voids" three-layer sandwich structure with 11.1 MPa shear strength was formed due to significant out segregation of Cu toward the bonding-interface in the ambient atmosphere. The bonding mechanism and the role of oxygen were unveiled based on CALPHAD (CALculation-of-PHAse-Diagram) thermodynamic modeling. Image, graphical abstract [ABSTRACT FROM AUTHOR]

For overcoming the heat dissipation problem of GaN-based high-power semiconductor devices, GaN wafers were bonded on polycrystalline and single-crystalline diamond wafers by using Mo/Au nano-layer at room temperature. Mo/Au double layers (~5 nm/11 nm) were deposited on the surfaces of GaN and diamond wafers. The scanning acoustic microscope (SAM) analyzation and tensile strength testing results show that a low voidage of ≤1.5% can be obtained for Mo/Au nano-layer, along with a bonding strength of 6.8 MPa. In addition, the thermal-impact-resistance performance was evaluated with thermal cycling testing. Schematic illustration of the bonding process of GaN/diamond wafers performed at room temperature by using Mo/Au nano-layer. Unlabelled Image [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a hydrophilic sapphire wafer bonding method combining plasma-activation and high-temperature bonding process. It is successfully employed in two sapphire bonding structures, including sapphire direct bonding and sapphire indirect bonding based on an intermediate amorphous Al 2 O 3 (a-Al 2 O 3). Transmission electron microscopy (TEM) results show that the a-Al 2 O 3 turns to be the single crystalline phase due to the induction of the sapphire substrate after high temperature bonding at 1000 °C. Moreover, high bonding strength is achieved for sapphire/sapphire (9.91 MPa) and sapphire/Al 2 O 3 /sapphire (17.96 MPa) structures and exceeds that of the bulk sapphire. This sapphire bonded structure with cavity can be applied to fabricate optical devices and pressure and temperature sensors for extreme high temperature environments. • A hydrophilic sapphire wafer bonding method combining plasma-activation and high-temperature bonding process is demonstrated. • For sapphire/sapphire bonding, the interface achieves a seamless bond on the atomic-scale. • For sapphire/Al 2 O 3 /sapphire bonding, the amorphous Al 2 O 3 turns to be the single crystalline phase. • This sapphire bonded structure with cavity can be applied in extreme high temperature environments. [ABSTRACT FROM AUTHOR]

As a micro-component for detecting and acquiring pressure data, piezoresistive pressure sensors are widely used in the microelectronic industry. Enhancing the sensitivity and reliability of piezoresistive pressure sensors can be achieved with optimized fabrication process and advanced techniques. However, in previous studies, there is limited information and knowledge of the relationship between the sensor performances and their fabrication process. In this study, we further optimized the fabrication process for piezoresistive pressure sensors, achieving a significantly higher stability of the pressure sensor compared with that of the commercial products obtained based on the standard test of "Pressure & High temperature Operating Life Test." A detailed analysis revealed that both the designed structure of the piezoresistive pressure sensor and the preliminary thermal treatment conditions for tetraethylorthosilicate (TEOS) oxide substrate are critical factors that contribute to the improved performance of the pressure sensors. After low-temperature annealing, the atomic scale characterization of the interfaces between different material layers of the sensor showed well-proportioned interfaces. Furthermore, reducing water content in the sensors through high-temperature annealing is crucial for improving the stability of the piezoresistive pressure sensors. [Display omitted] • The pressure sensor was fabricated base on room temperature Si-SiO2 fusion bonding and annealed at 300 °C. • Optimized structure will suppress H 2 O diffusion to decrease the pressure of cavity after bonding. • The hysteresis/nonlinearity and long-term stability of the pressure sensor are both below 0.1 % full scale output (FSO). [ABSTRACT FROM AUTHOR]

Circuit Under Pad (CUP), also known as Bond Pad Over Active (BPOA), has been gaining popularity in the semiconductor industry. However, such a design concept amplifies the effect of stress-inducing processes, which are detrimental to the chip's electrical performance and mechanical reliability. The said processes are wire bonding and wafer probing, and they are unavoidable in the manufacturing of a semiconductor. Although the Finite Element Analysis (FEA) of these processes has been carried out in previous research, the focus was not on the polysilicon resistor or metallization layers in CUP, but on the bond pad only. On the other hand, wire bonding is carried out after wafer probing in the manufacturing line. However previous studies, either numerically or experimentally, investigated the processes separately. Specifically, those studies did not link the stress and deformation history of the wafer-CUP structure resulting from the wafer probing to the stress and deformation induced in the subsequent wire bonding process. Therefore, this project scoped both simulations in a continuous manner and using a CUP design as the chip's structure. ANSYS Mechanical was utilized to simulate the dynamic wafer probing and thermosonic wire bonding. The result showed that copper bonding exerted more stress compared to gold bonding. Meanwhile, the Cu/low- k chip is more sensitive to stress-inducing factors than the Al/SiO 2 chip. • Circuit Under Pad (CUP) layout inclusive of the polysilicon, metallization, bond pad and dielectric layers were modeled using FEM. • Copper wire bonding and the use of Cu/low-k interconnect were more susceptible to detrimental stress. • Microchip consisting of two dielectric layers is less effective in shielding the polysilicon in a wire bonding process. [ABSTRACT FROM AUTHOR]

High-quality Si/Si and Ge/Si wafer bonding is achieved by introducing an ultra-thin microcrystalline Ge interlayer on Si substrate. The bonded wafer with a high-crystallinity Ge interlayer can effectively transfer and absorb bubbles, achieving high bonding strength of Ge/Si and Si/Si bonded wafers with bubble-free and oxide-free bonded interface after low-temperature annealing. Different from the tunneling model appropriated for the bonded wafers with low-crystallinity interlayer, a carrier recombination model is constructed for these with high-crystallinity interlayer to describe the carrier migration behaviors at the bonded interface. Furthermore, five carrier transport models are established to illustrate the effect of different crystallinity of the Ge interlayer on the carrier transport. This work provides a practical and simple method to fabricate high-quality bonding materials, which is of great significance for the development of micro-electronics devices. • Hydrophilic bonding with an interlayer is used to fabricate high-quality Ge/Si and Si/Si films. • The evolution of bubbles and oxide layers at the bonding interface is studied. • Annealing at low temperature to obtain high bond strength was investigated. • Oxide-free, bubble-free bonding at room temperature is achieved. • The carrier transport mode between Ge layers with different crystallinity is established. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposed a highly sensitive lock-in thermography method for internal bubbles. • Enhanced bubbles detectability by optimizing LLT parameters. • Visualize the bubbles larger than 500 μm at depth of 725 μm. • The detection speed reaches 130 mm2/10 s, which is 38% faster than that of SAM. • The ratio between defect depth and width reaches 1.45, 2-fold of latest researches. Online and non-destructive detection of interfacial bubbles in wafer bonding is a critical approach to enhance the yield of advanced packaging. However, the commonly used scanning acoustic microscopy (SAM) is time-consuming and may cause additional risk due to soaking in water. The conventional thermography is suffering from low sensitivity and resolution. In this work, we demonstrate for the first time an effective method for highly sensitive and online imaging of real interfacial bubbles in wafer bonding using transmissive laser lock-in thermography (LLT). The transmissive LLT system projects a modulated laser beam onto target wafer surface and records the thermal image sequence on the other side. The bubbles distinctly emerge with clear edges in the extracted lock-in phase images. To enhance the detectivity, the relationship between phase image contrast and LLT parameters was explored. As a result, we can visualize and detect the bubbles larger than 500 μm at depth of 725 μm even under the interference of metal pattern with a speed of 130 mm2/10 s, which is 38% faster than that of SAM. Besides, the LLT phase images contain both bubbles and metal patterns information. The ratio between bubble depth and width reaches 1.45, which is about 2-fold of the latest infrared researches. Above all, this work paves the way for high-sensitive imaging of various tiny internal defects in advanced packaging using infrared thermography. [ABSTRACT FROM AUTHOR]

Heterogeneous integration of LiNbO 3 and GaAs at room temperature has been achieved by using an activated Si atom interlayer, which demonstrate effectiveness in the fabrication of GaAs based LiNbO 3 wafer. By using the activated Si atom interlayer as the adhesive layer, 4-inch LiNbO 3 wafer is successfully directly bonded to the surface of GaAs wafer. The wafer bonding rate reaches over 95%. After bonding, the LiNbO 3 /GaAs bonding pair is cut into 10 × 10 mm2 square bonded chips. After annealing, the maximum bonding strength of square bonded chips exceeds 22 MPa. This means that the bonding strength of LiNbO 3 /GaAs wafer can be comparable to that of bulk materials. The characterization results of the bonding interface confirm that the intermediate amorphous Si layer is effective in improving the bonding strength and quality of LiNbO 3 /GaAs heterojunction. Heterogeneous integration of LiNbO 3 with GaAs and other Ⅲ-Ⅴ compound semiconductor materials is conducive to reducing the area occupied by optical communication platforms and improving the overall efficiency of devices. • 4 inches high-quality LiNbO 3 /GaAs heterostructure was prepared by Si atom intermediate layer. • The bonding mechanism of LiNbO 3 /GaAs was analyzed. • The bonding strength of LiNbO 3 /GaAs is 22 MPa respectively. This is the best known result. • The surface chemical states of LiNbO 3 , GaAs, Si deposition and debonding surface were characterized by XPS spectrum. [ABSTRACT FROM AUTHOR]

We report a first demonstration of GaAs HBT-OI fabricated using epitaxy growth, wafer bonding and layer transfer technique. Excellent bonding yield was found on the 200 mm GaAs HBT-OI wafer with wafer bowing of less than 90 µm. A dc gain of 20, and cutoff frequency (fT) of 6.5 GHz was measured for a device with emitter finger of 6 × 18 µm2. Its ideality factors for collector and base were 1.19 and 1.44, respectively. These results showed that the method of GaAs HBT-OI fabrication can be used for future integration of III-V on a common Si platform with low RF power dielectric attenuation. [ABSTRACT FROM AUTHOR]

• Highly sensitive CMUTs were fabricated with nitride-to-oxide wafer bonding technique. • The technique provided outstanding device uniformity with reduced fabrication complexity. • CMUTs functionalized by graphene oxide showed high relative humidity sensitivity. In this paper, we present highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based gravimetric humidity sensors that are fabricated with nitride-to-oxide wafer bonding technology. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the proposed process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. The proposed process provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Furthermore, additional benefits such as reduced fabrication complexity and more controllable membrane thickness are possible. Ten MHz CMUTs were designed, fabricated and functionalized by graphene oxide (GO) with three different concentrations. The device uniformities and resonance frequencies were characterized by a laser vibrometer and a vector network analyzer. Humidity sensing performance of the sensors was analyzed through the resonance shifts in a relative humidity (RH) range of 11.3%–93.6%. The CMUT coated with the highest concentration of GO (0.5 mg/ml) exhibited the highest sensitivity of up to 15.3 kHz/%RH. Decent repeatability and a short response/recovery time were achieved by all the CMUT sensors. This study demonstrates that the CMUT humidity sensors that are realized with nitride-to-oxide bonding technology can be an excellent candidate for humidity sensing due to their high sensitivity and CMOS-compatible fabrication process. [ABSTRACT FROM AUTHOR]

• A criterion for normal operation of PHPs in a horizontal orientation is developed. • The PHP is modeled as a vibration system based on the experimental observation. • It is found a large-amplitude oscillation in PHPs is induced by negative damping. In this study, a criterion for normal operation of micro pulsating heat pipes (MPHPs) even in a horizontal orientation is developed through experimental and theoretical investigations. Using MEMS techniques, a ten-turn closed-loop MPHP with overall dimensions of 50 × 33.5 × 1.7 mm3 is fabricated. The rectangular micro-channels engraved on a silicon wafer have a width of 1 mm and a depth of 0.5 mm. Ethanol is used as the working fluid and the filling ratio is 47% on the average. To allow flow visualization, a glass wafer is bonded to the silicon wafer. The temporal variations in the temperature and pressure of two neighboring vapor plugs in the MPHP are measured using micro-thermocouples and pressure transducers. The flow behavior in the MPHP is recorded using a high-speed camera and fully synchronized in real time with the measured temperature and pressure data. To find a criterion for normal operation of PHPs, the MPHP is treated as a mass-spring-damper system, and it is postulated that the transition from (to) a stopover to (from) a large-amplitude oscillation in the MPHP occurs with negative (positive) damping. This postulate is experimentally confirmed with the MPHP operating in a horizontal orientation. Finally, a figure of merit for normal operation of PHPs is proposed. The ratio of the minimum thermal resistance in a vertical orientation to that in a horizontal orientation (R th, V, min / R th, H, min ) is proportional to (D h / L eff)2, the square of the ratio of the hydraulic diameter to the effective length of PHPs. For PHPs filled with ethanol, it is experimentally confirmed that D h / L eff has to be greater than approximately 0.03 for normal operation without any stopovers in a horizontal orientation. [ABSTRACT FROM AUTHOR]

Highlights • The multi-junction solar cells were grown by MBE. • A higher growth temperature is good for GaInP and InGaAsP. • Multi-junction solar cells were fabricated by wafer bonding technique. • The optimiztion process of four-junction solar cell was discussed. Abstract We report on the room-temperature wafer-bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid-source molecular beam epitaxy (MBE). The material growth and structure optimization of multi-junction solar cell were investigated. The effect of growth temperature on phosphide-related material of the solar cell was discussed. The lattice-matched InGaP/GaAs tandem cell on GaAs substrate and InGaAsP- or InGaAsP/InGaAs-tandem cell on InP substrate were grown separately by MBE. Room-temperature wafer bonding technique was used to integrate the subcells into multi-junction solar cells. The current limiting of multi-junction cells was analysed and the device structure of four-junction solar cell was optimized. The best wafer-bonded four-junction solar cell reached a conversion efficiency of 42% under concentration. [ABSTRACT FROM AUTHOR]

In this paper, we employ an electron beam writer as metrology tool to investigate distortion of an exposed pattern of alignment marks in heterogeneously bonded InP on silicon. After experimental study of three different bonding and processing configurations which represent typical on-chip photonic device fabrication conditions, the smallest degree of linearly-corrected distortion errors is obtained for the directly bonded wafer, with the alignment marks both formed and measured on the same InP layer side after bonding (equivalent to single-sided processing of the bonded layer). Under these conditions, multilayer exposure alignment accuracy is limited by the InP layer deformation after the initial pattern exposure mainly due to the mechanical wafer clamping in the e-beam cassette. Bonding-induced InP layer deformations dominate in cases of direct and BCB bonding when the alignment marks are formed on one InP wafer side, and measured after bonding and substrate removal from another (equivalent to double-sided processing of the bonded layer). The findings of this paper provide valuable insight into the origin of the multilayer exposure misalignment errors for the bonded III-V on Si wafers, and identify important measures that need to be taken to optimize the fabrication procedures for demonstration of efficient and high-performance on-chip photonic integrated devices. Unlabelled Image • E -beam metrology study of processing-induced deformations for 2" InP bonded to Si. • Bonding-induced pattern deformation dominant for the BCB bonding case. • Processing-induced pattern deformation dominant for the direct bonding case. [ABSTRACT FROM AUTHOR]

Highlights • Heterojunction bipolar transistor solar cells with non-ideal effects are studied. • Simpler structure and fabrication compared with dual-junction solar cells. • High conversion efficiency can be achieved at low solar concentrations. • Low injection efficiency and working temperature provide better performance. • Paving a route towards the development of low-cost high-performance solar cells. Abstract Without fabricating intermediate tunnel junctions or wafer bonding schemes for interconnecting the subcells, heterojunction bipolar transistor solar cells offer a promising new route in solar energy conversion. In this work, an improved theory for the three-terminal heterojunction bipolar transistor solar cell is presented with inclusion of non-ideal effects missing from the previous treatment, namely the non-radiative recombination and the thermal conduction losses that are inevitably present in realistic devices. Following detailed balance theory, the revised analytical formula for the cell conversion efficiency is derived, and the maximum efficiencies under different conditions are further calculated. Under the condition of 100 sun irradiance and 50% injection efficiency, a Gallium arsenide/Gallium antimonide-based solar cell operating at 465 K yields a maximum efficiency of 46.4%. Moreover, the effects of solar concentration, injection efficiency, and other key parameters on the cell performance are analyzed, and, consequently, optimal operating conditions and limiting factors on the conversion efficiency are determined. Simulation results show that such a solar cell operating with low injection efficiency under moderate concentration factor and low cell temperature can significantly boost its conversion performance. This work provides new physical insights for optimal designs, thus paving a route towards the development of low-cost high-performance solar cells. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we present a highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based resonant gravimetric humidity sensor using graphene oxide as the sensing material. The CMUT transducer with a diameter of 2 mm and an operating frequency of 12 MHz was fabricated by the direct wafer bonding technique. To validate the design, both the laser vibrometer and the vector network analyzer were adopted to characterize the resonant properties of the fabricated device. A graphene oxide thin film was then coated as the water vapor absorbing layer on the resonating membranes of the CMUT. The humidity sensing performance of the CMUT humidity sensor was evaluated through the electrical impedance analysis over a relative humidity (RH) range of 22.5% to 93.6%. Frequency shifts caused by the added mass of the absorbed water molecules was observed in the measurements. The CMUT humidity sensor exhibited a sensitivity of up to 11.5 kHz/%RH in the tested humidity range and showed a good repeatability and a rapid response/recovery. Benefit from the microelectromechanical (MEMS) technology and the water-sensitive graphene oxide, the developed CMUT humidity sensor is expected to be a strong candidate for miniaturized, highly sensitive, and reliable humidity sensors. [ABSTRACT FROM AUTHOR]

Abstract As a powerful enabling technology, three-dimensional (3D) integration, which uses wafer bonding to integrate multiple wafers in the vertical direction and uses through‑silicon-vias (TSV) to electrically connect the wafers, have greatly promoted the technological advances and market development of MEMS in the past ten years. 3D integration enables heterogeneous integration of MEMS and IC chips fabricated with different technologies and materials, and thus permits the realization of integrated, sophisticated, and multifunctional microsystems with high performance, low cost, and compact size. This paper first introduces the fundamental fabrication technologies of 3D integration, and then reviews the recent progresses of MEMS and microsystems using 3D integration and TSV technologies. MEMS-CMOS integration, TSV-based wafer level packaging, 2.5D interposer integration of MEMS are illustrated with academic achievements and commercial products by functional categorization. Finally the conclusions are made and the future trends are discussed. © 2008 Elsevier B.V. All rights reserved. Highlights • This paper introduces the fundamental fabrication technologies of 3D integration. • Recent advances in 3D MEMS integration, TSV wafer level packaging, 2.5D MEMS integration are reviewed. • Both academic achievements and commercial products are presented. [ABSTRACT FROM AUTHOR]

Abstract Integration of Ga 2 O 3 on SiC substrate with a high thermal conductivity is one of the promising solutions to reduce the self-heating of Ga 2 O 3 devices. Direct wafer bonding of Ga 2 O 3 –SiC at room temperature was achieved by surface activated bonding (SAB) using a Si-containing Ar ion beam. An average bonding energy of ~2.31 J/m2 was achieved. Both the structure and the composition of the interface were investigated to understand the bonding mechanism. According to the interface analysis, a ~2.2 nm amorphous SiC layer and a ~1.8 nm amorphous β-Ga 2 O 3 layer originating from the ion beam bombardment for surface activation were found at the interface. A slight diffusion at the interface might already happen at room temperature, which should contribute to the strong bonding. To confirm the diffusion at a low temperature and investigate the possible interfacial variation during device operation, an annealing process was carried out at 473 K. The same analysis was applied on the annealed bonding interface. The interfacial layer shrank by ~0.5 nm after annealing. The further diffusion of Ga and Si at the interface caused by the annealing was confirmed. Besides, the position of the Ar count peak inside the amorphous Ga 2 O 3 layer shifted by ~0.5 nm toward SiC. [ABSTRACT FROM AUTHOR]

Abstract Solar cell wafer industry is classified as one of the most complex electronic industries that produces a significant proportion of waste in the form of broken/damaged cells or cells having some defects in their chemical composition that can generally be called Rejected Solar Cell Wafers (RSCWs). Although these wastes contain valuable metals (e.g. Silver (Ag), Aluminum (Al), and Silicon (Si)), unfortunately there is currently no obvious strategy for recovery of these metals and landfilling is considered the primary option for their disposal what can lead to serious soil contamination and human health risks if the waste is not treated properly. This research aims to develop an integrated technology to recover all these metals as high added value nano/micro products with total recovery rate > 98%, thus achieving the sustainability and goals of circular economy. Milling process was used as a pretreatment to break the chemical and mechanical bonds between Ag electrodes, Al electrode, and Si layer of RSCW samples to increase the surface area for reaction. The pretreatment was followed by leaching process using Nitric Acid (HNO 3) with concentration > 60% to dissolve Ag and break the chemical bonds between spherical Al microparticles in the Al paste layer. Thus the particles were separated and only a small amount of the Al was dissolved from the surface of the particles since due to the high concentration of HNO 3 passivation of Al occurred, forming a thin layer from Aluminum Oxide (Al 2 O 3) that prevented the further dissolution of Al. Under the effect of soundwaves, the separated particles began splitting further and exfoliate into Al nanocrystals. After that, micro-filtration process was used to extract Si microparticles from the leached solution and the extracted particles were purified by etching process using Hydrofluoric acid (HF) to remove Silicon Nitride contamination. Centrifugal process was used to separate Al nanocrystals from the leached solution. Finally, Hydrochloric acid (HCl) was added to the remaining saturated solution to recover AgCl. SEM-EDS, FTIR, ICP, XRD, and TEM were used to analyze and examine the selected wafers and recovered materials. Also, the economic performance, sustainability, and CO 2 emissions of the advanced technology were evaluated. Highlights • Milling, leaching, and etching process was used to recover Si, Al, and Ag form Rejected Solar Cell Wafers. • Structural and chemical analysis of Rejected Solar Cell Wafers was performed. • Si was recovered in form micro powder with average size ~50 µm and recycling rate > 98%. • Al was recovered in form nanocrystals with average size 25 nm and yield > 91%. • The carbon footprint has been reduced by − 9581 kg CO2-eq/ton by the developed technology. [ABSTRACT FROM AUTHOR]

Abstract Novel vertical light-emitting diodes (VLEDs) decorated with N-doped graphene quantum dots (N-GQDs) have been fabricated, based on the metal organic chemical vapor deposition method (MOCVD), wafer bonding technique, laser lift-off (LLO) technique, inductively coupled plasma (ICP) etching and impregnation processing. There is an obvious luminescence enhancement and resistance reduction for VLEDs after coating N-GQDs. The results show an improvement of light output power (LOP), dependent on the improvement of optical extraction efficiency and current spreading. The improvement of external quantum efficiency (EQE) is mainly owing to the roughened surfaces. Meanwhile, an enhancement of electroluminescence (EL) intensity and photoluminescence (PL) intensity has been achieved when the VLEDs were immersed in 0.9 mg/mL of N-GQDs solution, 4 times that of bare VLEDs. After coating N-GQDs, the performance of VLEDs is greatly improved, which could be attributed to the photons recovered by extracting light from waveguide modes. Highlights • Novel vertical light-emitting diodes (VLEDs) were fabricated by coating N-doped graphene quantum dots (N-GQDs) on the surface. • The existence of N-GQDs on the surface of VLEDs can significantly improve the current spreading. • The VLEDs after being coated with N-GQDs exhibited a 25% improvement in EQE. [ABSTRACT FROM AUTHOR]

Highlights • Arbutus unedo L. fruit extract rich in anthocyanins was used as a colorant. • The extract revealed good antioxidant and antifungal activity. • The stability of the extract was studied for the development of new pastry products. • The extract was incorporated in wafers as a natural colorant additive. Abstract An extract from Arbutus uned o fruits, rich in anthocyanins, was studied as a natural colorant with bioactive properties (antioxidant, antimicrobial and cytotoxic). The aqueous stability of the extract was monitored using the anthocyanins' content as response (determined by HPLC-DAD) in function of time, temperature and pH. Aided by mechanistic/phenomenological models, the conditions that favours the stabilization of the extract were provided, highlighting the suitability of the colorant for pastry/bakery products. As a case study, the extract was incorporated into wafers and the changes on the nutritional profile, free sugars, fatty acids and antioxidant properties were monitored during 6 days of storage. The results provide information for: i) potential application of the rich extract in anthocyanins for producing a natural colorant with bioactive properties; and ii) shelf-life predictions. The extract incorporation did not cause changes in the nutritional components of wafers but added colorant and antioxidant properties. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A de-bondable SiC SiC wafer bonding, compatible with RTA at ∼1273 K, has been accomplished via a Ni nano-film. • The SiC SiC bonding by Ni nano-film achieved at room temperature without any pressure is seamless and robust. • The interfacial precipitation of layered carbon material parallel to the SiC surfaces is considered as the reason of interface weakening and de-bonding. Abstract In this study, a de-bondable wafer bonding method for silicon carbide (SiC) that can sustain rapid thermal annealing (RTA) at ∼1273 K has been realized. Two SiC wafers were bonded via an intermediate nickel (Ni) nano-film at room temperature without any pressure, which was characterized as a seamless and robust bonding. After the RTA process, the strength of the bonding interface was dramatically decreased and the de-bonding could happen at the interface during pulling test. Both of the mechanisms of bonding and de-bonding have been investigated through interface analyses. The sufficient atomic diffusion between two deposited Ni nano-films together with the interfacial mixing between amorphous SiC and the Ni nano-film contribute to the strong bonding of SiC SiC. The interfacial precipitation of layered carbon material parallel to the SiC substrates is assumed to be the reason of the interface weakening and de-bonding after annealing. It is believed that the further development of this bonding and de-bonding technology will advance thin SiC device fabrication, where the RTA process at ∼1273 K is widely used. [ABSTRACT FROM AUTHOR]

In this study, Ni particle was validated as an effective grain refiner for the Cu-Sn solid-liquid interdiffusion (SLID) wafer bonding joints, which induced a dispersive non-interfacial nucleation of intermetallic grains, effectively preventing the grain mergence phenomenon. With increasing the Ni particle addition, there were significant reductions in both the (Cu, Ni) 6 Sn 5 grain size and the Cu 3 Sn layer thickness, 6 wt% addition of Ni particles even led a formation of the intermetallic joints only consisted of (Cu, Ni) 6 Sn 5 grains with an average grain size of 1.3 μm. (Cu, Ni) 6 Sn 5 grain refinement could notably enhanced the shear strength of the intermetallic joints. [ABSTRACT FROM AUTHOR]

Highlights • Bonding and de-bonding of thin Si layer was processed at low temperature (<250 °C). • Unlike conventional wafer bonding such as smart-cut based SOI or similar substrate, wafer flip-up/down is not needed. • Hydrophilic surface was formed via plasma treatment instead of high temperature bonding annealing. Abstract We studied low temperature (<250 °C) transfer of 8 in. full sized thin Si wafer layer on the SiO 2 /Si substrate without any wafer flip up/down and subsequent high temperature process. This method includes temporary bonding of carrier wafer with bonding material at 200 °C, grinding or etching substrate, and transfer layer at 250 °C. Thickness values of transferred thin Si layer using bulk Si and silicon-on-insulator (SOI) wafer substrates are 87 μm and 216 nm on the SiO 2 /Si substrate, respectively. Plasma treatment under N 2 and O 2 mixture ambient assisting to form hydrophilic surface was carried out during bonding process and enhanced bonding strength was confirmed by contact angle measurement. Our wafer bonding process can be feasible to form various monolithic 3D devices due to thin Si layer transfer and low temperature process. [ABSTRACT FROM AUTHOR]

Highlights • We optimized the adhesive wafer bonding process for the fabrication of CMUTs. • A void-free and deformation-free adhesive wafer bonding process was demonstrated. • CMUTs with in-air resonance frequency of 6.44 MHz was fabricated and characterized. • An inverse CMUT structure is demonstrated enabling arbitrary cavity height control. Abstract It has been demonstrated that the adhesive wafer bonding technique benefits the fabrication of capacitive micromachined ultrasonic transducers (CMUTs) in the aspects of structural flexibility, CMOS process compatibility, and fabrication yield. Recently, the feasibility of fabricating CMUTs using benzocyclobutene (BCB) based polymer as both the adhesive and structural material was reported. However, the issues of voids at the wafer bonding interface, material deformation and electrode distance control need to be addressed. In this paper, we present an improved fabrication process that allows a void-free device with reduced polymer deformation. The chemical reactions during the BCB curing process were revealed using Fourier transform infrared spectroscopy. Microscopy and cross-sectional SEM images were employed to illustrate the structure improvement in terms of voids and polymer deformation. In addition, an inverse structure that allows an accurate control of the effective electrode distance was presented. The cross-sectional SEM images and the impedance measurement showing the dynamic response of the CMUT demonstrated the feasibility of this inverse structure. The differences between the inverse structure and the conventional structure were also discussed and compared. [ABSTRACT FROM AUTHOR]

• Two types of transducers for generating unidirectional-propagation SH 0 wave are designed. • No extra time delay is required for driving the unidirectional SH 0 wave transducers. • Simulations and experiments validate the proposed unidirectional transducers. Excitation of a pure guided wave with a controllable wavefield is essential in structural health monitoring (SHM). For example, a unidirectional-propagation guided wave can significantly reduce the complexity of signal interpretations by avoiding unwanted reflections. However, few transducers are currently capable of exciting a pure unidirectional-propagation guided wave, which cannot satisfy the emerging demands from the field of SHM. In this work, the thickness-shear vibration characteristics of the piezoelectric PZT wafer bonded on a waveguide are investigated by theoretical modeling and numerical simulations. It is found that there is a phase difference between the electric-excitation signal applied on the PZT wafer and the mechanical response signal of the bottom surface of the viscoelastic adhesive layer that connects the PZT wafer and waveguide. Moreover, such a phase difference can be adjusted by changing the equivalent width of the PZT wafer. Based on this finding, two piezoelectric transducers with different shape configurations are proposed to excite the unidirectional-propagation SH 0 wave (the fundamental shear horizontal wave). Finite element simulations and experiments are conducted to verify the performances of the two unidirectional transducers. Results show that the two transducers can excite a pure SH 0 wave and enhance the wave energy along a single direction. No time delay is required to excite the proposed transducers. Due to their simple configurations, the developed unidirectional SH 0 wave transducers will have great potential applications in SHM. [ABSTRACT FROM AUTHOR]

Imaging methods and 2D sparse arrays have been extensively researched to realize real-time 3D volumetric imaging with higher frame rates. In this study, we developed a 3D plane-wave imaging method using an annular capacitive micromachined ultrasonic transducer (CMUT) array as a breakthrough technique for realizing 3D imaging with a limited number of elements. We adopted the double oxidation and wafer bonding processes for creating the cavity of a CMUT with a 2 µm silicon membrane. To ensure surface uniformity before the bonding process, we implemented an additional reactive ion etching process after the second oxidation. The characterization results showed that the resonant frequency of the proposed device was 6.2 MHz with 65 V collapse voltage. The demonstration of real-time 3D volumetric imaging was conducted in oil using a Vantage 64 ultrasound research system. A plane-wave imaging scheme was used to detect wire phantoms immersed in the oil, with 289 plane waves transmitted over a range of 20° from the center of the annular array. The reconstructed data were used to create three orthogonal cross sections of the target region. The − 6 dB lateral and axial resolutions were 0.56 mm and 0.35 mm for the wires positioned at 18 mm from the proposed device, which were narrow enough to detect a wire phantom of 380 µm diameter. The normalized frame rate was calculated and compared with that in a previous study. We verified that plane-wave imaging has advantages in real-time imaging when compared with the classic phased array and synthetic phased array. Thus, the annular CMUT array is a promising mechanism for real-time 3D imaging with higher frame rate than that possible with previous methods. [Display omitted] • A real-time 3D plane-wave imaging method using an annular CMUT array is proposed. • 3D imaging is achieved using a limited number of elements. • The method requires low number of transmissions and delivers high frame rate. • The imaging quality is superior to that achieved with conventional methods such as CPA and SPA imaging. [ABSTRACT FROM AUTHOR]

Micro-LED is a new type of display technology. Its advantages are high resolution, high brightness, long life, low power consumption and so on. However, due to the reduction in size, the proportion of surface damage increases, resulting in a decrease in device performance, which affects the development of Micro-LEDs. In this paper, ion implantation technology and resonant cavity structure are used to reduce device sidewall damage and improve device characteristics. The photoelectric properties of GaN-based blue micro light emitting resonant cavity diodes were studied. Pixel-to-pixel isolation is achieved by fluorine ion implantation. The resonant cavity structure is fabricated by wafer bonding technique. The research shows that the electrical characteristics of the device have realized good stability. Under medium reflectivity conditions, the device enhancement effect is best when the reflectivity of the top mirror is 40%. With the decrease of the size, the enhancement of optical characteristics weakens due to the size effect, and the spontaneous emission intensity of the 10 μm device increases by 212.6%. The FWHM of devices with different sizes narrowed by about 10 nm. The stability of the device peak wavelength and full width at half maximum is improved. Therefore, this paper provides research value for the application of resonant cavity structures in Micro-LEDs. • Silicon substrate removal technology and wafer bonding technology. • Ion implantation technology completes the pixel isolation of the device. • The resonant cavity structure enhances the performance of devices of different sizes. • The resonant cavity structure makes the peak wavelength and full width at half maxima of the device more stable. [ABSTRACT FROM AUTHOR]

Despite a significant number of new structures in the past few years, MEMS loudspeaker still are not competitive in terms of performance compared to non-MEMS loudspeakers for free field applications. For industrial perspectives, a high sound pressure level on a wide frequency band is required, as well as a low total harmonic distortion. To widen the frequency range of MEMS loudspeakers, we propose to separate the actuating element from the radiating one, in order to separate design constraints to reach an optimal structure. In this paper, the lumped element model of the loudspeaker in presented, as well as the innovative manufacturing process. Finally, the computed frequency response is compared to the measured one. At the resonance, pressures as high as 110 d B S P L at 1 kHz and at 10 mm are reported for an active surface of 36 mm2, which is above the known state of the art for a loudspeaker with similar dimensions. Also, the flatness of the radiated sound pressure in a wide frequency range is closer to the ideal frequency response of loudspeakers than other MEMS loudspeakers, due to the piston mode of the moving rigid plate of the loudspeaker. The total harmonic distortion, mainly due to the nonlinearity of the piezoelectric transduction, is below 5% for reasonable sound pressure levels in the usable frequency band. The use of digital signal processing and of a dedicated packaging will allow our loudspeaker to advantageously replace the main or secondary one in smartphones. [Display omitted] • Simulation of a piezoelectric MEMS loudspeaker using a lumped element model and the two-port formulation. • Measurement of the frequency response of the manufactured loudspeaker. • Pressures of 110 dB SPL at 10 mm and at 1 kHz are reported. • Pressure response is within the tolerances of the simulated one. [ABSTRACT FROM AUTHOR]

[Display omitted] • A polycrystalline Si layer is introduced to eradicate the influence of lattice misfit. • Ar Plasma treatment highly enhanced the bonding strength of InGaAs/Si bonded pair at low temperatures. • A defect-rich region near the epitaxial interface of InGaAs and InP is revealed by depth-dependent and power-dependent photoluminescence (PL) measurements. • A high-quality Si-based InGaAs film is obtained by heterogeneous bonding and wet etching techniques. We report the fabrication of high-quality InGaAs films on Si by wafer bonding at low temperatures (300 ℃). A thin polycrystalline Si (poly-Si) interlayer is introduced between InGaAs and Si to block the lattice mismatch and absorb the bubble by-products produced at the bonded interface using its disordered crystal orientation and gain boundaries. To achieve a high bonding strength, argon plasma treatment was employed on InGaAs and poly-Si surfaces before the pre-bonding process. By using post-wet etching, a defect-rich region near the surface of exfoliated InGaAs film that was initially caused by the thermal mismatch between InGaAs and InP during epitaxy growth can be removed. As a result, a Si-based InGaAs film with a 1.05 MPa bonding strength and bubble-free bonding interface was produced. The defective region is 1.2 μm-thick. After etching the defective area, a high-quality, 1.8 μm-thick Si-based InGaAs film was obtained with an XRD FWHM of 94 arcsec. The results reveal that this method is prospective for the fabrication of high-quality III-V materials on Si, proving that a prospective approach for the fabrication of low-noise InGaAs/Si PIN photodiodes and InGaAs/Si avalanche photodiodes with ultralow dark currents. [ABSTRACT FROM AUTHOR]

We report a high-quality germanium (Ge) and silicon (Si) wafer bonding based on an intermediate layer of 5 nm-thick microcrystalline Ge. The bubble evolution, crystalline state evolution, and oxide layer evolution at the wafer-bonded interface are investigated. The mechanisms of bubble elimination and oxide layer disappearance are systematically analyzed. The sputtered microcrystalline Ge film consists of an amorphous Ge (a-Ge) layer and a low-quality single-crystal Ge (c-Ge) layer. The bubbles can be eliminated by the loose structure of the low-quality c-Ge at the bonded interface. Thus, the bubble-free bonded interface can be obtained. Besides, the bonding strength decreases with the increase of the sputtering temperature. The pressure on the bonded wafers before post-annealing can eliminate large bubbles and improve bonding strength. It is worth mentioning that the oxide layer and the threading dislocations are not observed at the bonded interface. This study can give a guidance for the subsequent manufacture of high-performance Si-based Ge optoelectronic devices. [Display omitted] • High-quality Ge/Si bonding with microcrystalline Ge interlayer is proposed. • The crystallization of a-Ge layer improves bonding strength after post-annealing. • The oxide layer at bonded interface is destroyed when a-Ge layer becomes c-Ge. • The low-quality c-Ge interlayer is a gas transporter to eliminate the bubbles. • Above three results lead to the improvement of the Ge/Si bonding quality. [ABSTRACT FROM AUTHOR]

The effect of temperature on fatigue crack growth in epoxy adhesive bonds was investigated for a range of temperatures from −55 to 80 °C. The fatigue crack growth behaviour was characterised using both strain energy release rate (SERR) and by measurements of energy dissipation. It was found that for a given maximum SERR, or a given energy dissipation per cycle, crack growth rate was higher at higher temperatures. The resistance to crack growth (in terms of energy dissipation per unit crack growth) was linearly related to the maximum SERR, and this relationship was not affected by temperature. A number of tests did show anomalous behaviour, which could be linked to differences on the fracture surfaces. Previous work had found a power-law relationship between the amount of available energy and the applied cyclic work. This relationship was found to be insensitive to temperature changes in the range of 0 °C to 40 °C, but at −55 °C and −20 °C, as well as at 60 °C and 80 °C, the behaviour was affected by temperature. This could again be linked to differences on the fracture surfaces. It was concluded that temperature does not affect crack growth by directly affecting the failure mechanisms themselves, but rather by affecting which mechanisms are active. [ABSTRACT FROM AUTHOR]

Capacitive micromachined ultrasonic transducer (CMUT) has shown the potential for the air-coupled application due to its good acoustic match with air. Therefore, air-coupled guided wave detection technology based on CMUT array has a good application prospect in structure detection. In this paper, a 16-element CMUT array for air-coupled guided wave detection is designed and fabricated by the silicon-on-insulator (SOI) wafer bonding process. All the CMUT cells have the same geometric parameters with circular and sealed membrane. Each element has good consistency in impedance and transceiver performance. In addition, the transmit performance was improved by a third through impedance matching. The resonant frequency is 207 kHz at 150 DC voltage and − 6 dB main lobe width of the sound beam is 9.4°. The emission beam can be steered by programmable driving circuit. So different modes of guided wave can be excited without adjusting the tilt angle of the transducer. This will reduce the interference caused by the movement of device in the experiment. After that, the air-coupled guided wave detection experiment was carried out by two CMUT arrays. The results show that the CMUT array can well excite the single mode Lamb wave in aluminum plate and composite plate, and receive the leaky Lamb wave. Besides, the defect can be identified by the variation of the received signal amplitude. Thus, the position of the defect can be located and the approximate size of the defect can be predicted. In general, it is proved that the CMUT has ability for the air-coupled Lamb wave detection by the non-contact guided wave detection experiment and the comparison experiment with commercial air coupling probe. [Display omitted] • A 16-element CMUT array for air-coupled guided wave detection is designed and fabricated by SOI wafer bonding process. • Each element has good consistency by acoustic and electrical properties test and the performance is improved by impedance matching. • The CMUT can excite the single mode Lamb wave in plate and receive the leaky Lamb wave,。Besides, the defect can be identified. [ABSTRACT FROM AUTHOR]

[Display omitted] • To overcome serious CTE mismatch, we proposed a room-temperature wafer-bonding method using activated Si atomic-layer for the fabrication of glass- or sapphire-based heterogeneous LiNbO 3 -on-insulator (LNOI) wafer. • We demonstrate the effectiveness of the activated Si atomic-layer as an adhesive for both LiNbO 3 /glass and LiNbO 3 /sapphire. • The tensile strength of the fabricated LNOI wafers exceeded 23 MPa, which indicates that the bond between the LiNbO 3 and glass or sapphire wafers was strong. • The TEM, STEM-EDX, XPS, and TOF SIMS analysis clearly showed the nanostructure and atomic behavior near the bonding interface. We proposed a room-temperature wafer-bonding method using activated Si atomic layer and verify its effectiveness for the fabrication of glass- or sapphire-based LiNbO 3 -on-insulator (LNOI) devices for radio-frequency photonic applications. Four-inch LiNbO 3 was successfully fabricated on glass or sapphire wafers using the proposed method. The tensile strength of the fabricated wafers exceeded 23 MPa, which indicates that the bond between the LiNbO 3 and glass or sapphire wafers was strong. Atomic-structure analysis of bonding interfaces confirmed the effectiveness of the activated Si atomic layer as an adhesive for both LiNbO 3 /glass and LiNbO 3 /sapphire. In addition, the amorphous Si atomic layer existing at the bonding interface showed high transmittance over a wide range of wavelength, including the near-infrared region used for communication. The results demonstrate the potential of the proposed wafer-bonding method for the fabrication of LNOI devices used in optical communications, including broadband traveling-wave modulators. [ABSTRACT FROM AUTHOR]

• A novel radial-torsional hollow ultrasonic motor is designed, optimized and tested. • The rotary motor has a novel hollow design working by converting the radial in-plane mode into torsional motion of the inner ring. • A new method of coating Si 3 N 4 film on contact interface by magnetron sputtering is presented. • The maximum torque and speed of the prototype are 0.27 mN m and 600 r/min respectively before coating. • After Si 3 N 4 film was coated on the contact surface of the motor, the maximum torque of the motor can be increased by 55.56% to 0.42 mN m. A novel miniature radial-torsional hollow ultrasonic motor is proposed in this work, which is mainly composed of a base, a hollow rotor, a stator. One piece of piezoelectric wafer bonded on the bottom face of the stator is used to excite the radial in-plane vibration mode of the outer ring, which is converted into the revolved motion of the inner ring through the connection beams. The revolved motion of the inner ring can drive the rotor pressed on the inner ring to rotate through friction. The finite-element method was performed to verify the working principle of the motor and optimize the key dimensions of the motor. One prototype motor with the appearance size of Φ35 mm × 12 mm and weight of 20.1 g is fabricated and characterized. In addition, Si 3 N 4 thin film is coated on the contact interface of the stator by the magnetron sputtering coating machine to improve the output performance of the ultrasonic motor. The experiment results show that the locked-rotor torque of the motor with coated stator can reach 0.42 mN m, which is 55.56 % higher than that of the uncoated motor. [ABSTRACT FROM AUTHOR]

[Display omitted] • A deterministic mechanic model to evaluate the strain energy under an arbitrary pressure for wafer bonding is proposed. • The gap between external conditions and evolutions of contact area, wafer deflection as well as strain energy across the bonding process is filled. • Analytical expression of strain energy coupling the effects of bonding pressure and wafer bow is derived. • Predictions of strain energy agree with FE simulations and the previous classical model. Direct wafer bonding is widely employed in the fabrication of micro-electromechanical systems. Wafer deflection and strain energy can be used to evaluate the bonding quality. Since the complexity of the mechanic process of contact interface, researches mainly concentrate on the final state of wafer bonding. The gap between external conditions (include normal pressure and flatness deviation) and evolutions of key variables (include contact area, wafer deflection and strain energy) across the bonding process remains unfilled. In this paper, the bridge between external conditions and bonding variables is established analytically. A mechanic deterministic model across the bonding process is proposed and the dependence of contact radius on the bonding pressure is obtained. Considering the coupling effects of bonding pressure and wafer geometry, analytical expressions for the contact area, deflection and strain energy are derived. Accordingly, the effects of both external conditions and internal geometry on the wafer deflection as well as strain energy are revealed. Results demonstrate that the wafer curvature and thickness jointly determine the deflection of the wafer at a given pressure. Wafer deflection and strain energy vary with critical contact position nonlinearly. The proposed model of wafer bonding is verified by the finite element simulations. The current work may provide a reference for the bonding pressure selection in industrial applications. [ABSTRACT FROM AUTHOR]

Room temperature GaN-Si direct wafer bonding was done by surface activated bonding (SAB). At first, a feasibility study using GaN template has been done. Then, crystal-face dependence of the bonding results for freestanding GaN substrate has been investigated between Ga-face and N-face. The results of Ga-face to Si bonding are better than that of N-face to Si bonding such as higher bonding energy and larger bonded area. This difference should be caused by their different surface roughnesses after chemical-mechanical polishing (CMP). Besides, both of the structure and composition of the two kinds of interfaces were investigated to understand the bonding mechanisms. The phenomenon of Ga-enrichment during surface activation and Ga-diffusion into Si at room temperature for both Ga-face bonding and N-face bonding has been confirmed. [ABSTRACT FROM AUTHOR]

This paper introduces a room-temperature transfer bonding method for a future integration of LiNbO 3 thin-film device with a micromachined Si platform. A single-crystal LiNbO 3 thin film (5 μm thickness) prepared by mechanical polishing was successfully transfer-bonded onto a micromachined Si substrate with Au microbumps in ambient air using surface activated bonding. Tensile testing showed the strong bond strength between the Au thin film and the Au microbumps, which was sufficient for device applications. An air/LiNbO 3 thin film/air structure was demonstrated on a Si substrate using the proposed method. In addition, our simulation and experimental results showed that room-temperature bonding was essential to overcome the large coefficient of thermal expansion mismatch between LiNbO 3 and Si. We expect that this technology can be utilized to realize new configurations of highly-functional integrated microelectromechanical systems, including Si-based high-density integrated photonic devices. [ABSTRACT FROM AUTHOR]