InP/LiNbO3 Covalent Heterointerface Construction via an Asymmetric Plasma Activation Strategy for Hybrid Integrated Quantum Systems.

Lithium niobate (LiNbO3) is emerging as an appealing candidate for integrated optical applications with enhanced complexity, owing to its inherent abundant optoelectronic properties. To compensate for the inability of LiNbO3 to generate indistinguishable single photons, the evanescent coupling heterointerface constructed between III-V compound semiconductors (e.g., InP) and LiNbO3 through plasma activation provides a feasible solution for balancing the integration efficiency and interfacial stability while achieving sub-50 nm alignment accuracy between devices, thus offering ultracompact on-chip light sources for classical optoelectronics and quantum optics. However, a challenge remains in the formation of the InP/LiNbO3 platform due to the huge mismatch in the coefficient of thermal expansion. Here, we demonstrate the InP/LiNbO3 covalent heterointerface using an asymmetric plasma activation strategy. Different plasmas are used for the activation of InP and LiNbO3 specifically, balancing the enhancement of surface functional group density with the avoidance of defect generation effectively. More importantly, combined with surface comprehensive characterizations and interface performance, we determine that the introduction of ammonia solution enables the surface hydroxyl groups to be "effective" as LiNbO3 surface relaxation increases the chance of -OH groups' contact. Therefore, a robust covalent bond network is established across the InP/LiNbO3 interface at 80 °C with an enhanced bonding strength of 9.7 MPa. Moreover, a hybrid quantum photonic chip based on the InP/LiNbO3 platform is designed to compute the coupling efficiency and the impact of misalignment on it, demonstrating the potential of extending the platform to hybrid integrated quantum systems.

Room temperature wafer bonding through conversion of polysilazane into [Formula: see text].

Room temperature wafer bonding is a desirable approach for the packaging and assembly of diverse electronic devices. The formation of [Formula: see text] layer at the bonding interface is crucial for a reliable wafer bonding as represented by conventional bonding techniques such as hydrophilic bonding and glass frit bonding. This paper reports a novel concept of room temperature wafer bonding based on the conversion of polysilazane to [Formula: see text] at the bonding interface. As polysilazane is converted to [Formula: see text] by hydrolysis, in this work, adsorbed water is introduced to the bonding interface by plasma treatment, thereby facilitating the formation of [Formula: see text] at the wafer bonding interface. The experimental results indicate that the adsorbed water from the plasma treatment diffuses into the polysilazane layer and facilitates its hydrolysis and conversion. The proposed method demonstrates the successful wafer bonding at room temperature with high bond strength without interfacial voids. This technique will provide a new approach of bonding wafers at room temperature for electronics packaging.

Cu-Cu Thermocompression Bonding with a Self-Assembled Monolayer as Oxidation Protection for 3D/2.5D System Integration.

Cu-Cu direct interconnects are highly desirable for the microelectronic industry as they allow for significant reductions in the size and spacing of microcontacts. The main challenge associated with using Cu is its tendency to rapidly oxidize in air. This research paper describes a method of Cu passivation using a self-assembled monolayer (SAM) to protect the surface against oxidation. However, this approach faces two main challenges: the degradation of the SAM at room temperature in the ambient atmosphere and the monolayer desorption technique prior to Cu-Cu bonding. In this paper, the systematic investigation of these challenges and their possible solutions are presented. The methods used in this study include thermocompression (TC) bonding, X-ray photoelectron spectroscopy (XPS), shear strength testing, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The results indicate nearly no Cu oxidation (4 at.%) for samples with SAM passivation in contrast to the bare Cu surface (27 at.%) after the storage at -18 °C in a conventional freezer for three weeks. Significant improvement was observed in the TC bonding with SAM after storage. The mean shear strength of the passivated samples reached 65.5 MPa without storage. The average shear strength values before and after the storage tests were 43% greater for samples with SAM than for the bare Cu surface. In conclusion, this study shows that Cu-Cu bonding technology can be improved by using SAM as an oxidation inhibitor, leading to a higher interconnect quality.

ReS2 on GaN Photodetector Using H+ Ion-Cut Technology.

The wafer-scale single-crystal GaN film was transferred from a commercial bulk GaN wafer onto a Si (100) substrate by combining ion-cut and surface-activated bonding. Well-defined, uniformly thick, and large-scale wafer size ReS2 multilayers were grown on the GaN substrate. Finally, ReS2 photodetectors were fabricated on GaN and sapphire substrates, respectively, and their performances were compared. Due to the polarization effect of GaN, the ReS2/GaN photodetector showed better performance. The ReS2/GaN photodetector has a responsivity of 40.12 A/W, while ReS2/sapphire has a responsivity of 0.17 A/W. In addition, the ReS2/GaN photodetector properties have reached an excellent reasonable level, including a photoconductive gain of 447.30, noise-equivalent power of 1.80 × 10-14 W/Hz1/2, and detectivity of 1.21 × 1010 Jones. This study expands the way to enhance the performance of ReS2 photodetectors.

Microfluidic chip connected to porous microneedle array for continuous ISF sampling.

Minimally invasive biosensing using microneedles (MNs) is a desirable technology for continuous healthcare monitoring. Among a wide range of MNs, porous MNs are expected to be applied for sampling of interstitial fluids (ISF) by connecting the internal tissue to external measurement devices. In order to realize a continuous measurement of biomarkers in ISF through porous MNs, their integration with a microfluidic chip is a promising approach due to its applicability to micro-total analysis system (µTAS) technology. In this study, we developed a fluidic system to directly interface porous MNs to a microfluidic chip consisting of a capillary pump for the continuous sampling of ISF. The porous and flexible MNs made of PDMS are connected to the microfluidic chip fabricated by standard microelectro-mechanical system (MEMS) processes, showing a continuous flow of phosphate buffered saline (PBS). The developed device will lead to the minimally invasive and continuous biosampling for long-term healthcare monitoring.

A Novel Preparation of Ag Agglomerates Paste with Unique Sintering Behavior at Low Temperature.

A novel bonding process using Ag agglomerates paste prepared by Ag2O reduction has been proposed, which solved the problem of Cu substrate oxidation in the conventional Ag2O sintering process for Cu-Cu bonding. By applying the Ag agglomerate paste to Ag-Ag bonding, a shear strength of 28.3 MPa at 150 °C was obtained. Further studies showed that the optimum sintering temperature was at 225 °C, and a shear strength of 46.4 MPa was obtained. In addition, a shear strength of 20 MPa was obtained at 225 °C for Cu-Cu bonding. Compared to common Ag pastes, the results in this paper revealed that the sintering behavior of Ag agglomerates was unique, and the sintering mechanisms for Ag-Ag and Cu-Cu bonding were also discussed.

Thermal Visualization of Buried Interfaces Enabled by Ratio Signal and Steady-State Heating of Time-Domain Thermoreflectance.

Thermal resistances from interfaces impede heat dissipation in micro/nanoscale electronics, especially for high-power electronics. Despite the growing importance of understanding interfacial thermal transport, advanced thermal characterization techniques that can visualize thermal conductance across buried interfaces, especially for nonmetal-nonmetal interfaces, are still under development. This work reports a dual-modulation-frequency time-domain thermoreflectance (TDTR) mapping technique (1.61 and 9.3 MHz) to visualize the thermal conduction across buried semiconductor interfaces for ß-Ga2O3-SiC samples. Both the ß-Ga2O3 thermal conductivity and the buried ß-Ga2O3-SiC thermal boundary conductance (TBC) are visualized for an area of 200 × 200 µm simultaneously. Areas with low TBC values (≤20 MW/m2·K) are identified on the TBC map, which correspond to weakly bonded interfaces caused by high-temperature annealing. Additionally, the steady-state temperature rise induced by the TDTR laser, usually ignored in TDTR analysis, is found to be able to probe TBC variations of the buried interfaces without the typical limit of thermal penetration depth. This technique can be applied to detect defects/voids in deeply buried heterogeneous interfaces nondestructively and also opens a door for the visualization of thermal conductance in nanoscale nonhomogeneous structures.

Evidence for intermolecular forces involved in ladybird beetle tarsal setae adhesion.

Why can beetles such as the ladybird beetle Coccinella septempunctata walk vertically or upside-down on a smooth glass plane? Intermolecular and/or capillary forces mediated by a secretion fluid on the hairy footpads have commonly been considered the predominant adhesion mechanism. However, the main contribution of physical phenomena to the resulting overall adhesive force has yet to be experimentally proved, because it is difficult to quantitatively analyse the pad secretion which directly affects the adhesion mechanism. We observed beetle secretion fluid by using inverted optical microscopy and cryo-scanning electron microscopy, which showed the fluid secretion layer and revealed that the contact fluid layer between the footpad and substrate was less than 10-20 nm thick, thus indicating the possibility of contribution of intermolecular forces. If intermolecular force is the main physical phenomenon of adhesion, the force will be proportional to the work of adhesion, which can be described by the sum of the square roots of dispersive and polar parts of surface free energy. We measured adhesion forces of ladybird beetle footpads to flat, smooth substrates with known surface free energies. The adhesive force was proportional to the square-root of the dispersive component of the substrate surface free energy and was not affected by the polar component. Therefore, intermolecular forces are the main adhesive component of the overall adhesion force of the ladybird beetle. The footpads adhere more strongly to surfaces with higher dispersive components, such as wax-covered plant leaves found in the natural habitat of ladybird beetles. Based on the present findings, we assume ladybird beetles have developed this improved performance as an adaptation to the variety of plant species in its habitat.

Thermal Transport across Ion-Cut Monocrystalline ß-Ga2O3 Thin Films and Bonded ß-Ga2O3-SiC Interfaces.

The ultrawide band gap, high breakdown electric field, and large-area affordable substrates make ß-Ga2O3 promising for applications of next-generation power electronics, while its thermal conductivity is at least 1 order of magnitude lower than other wide/ultrawide band gap semiconductors. To avoid the degradation of device performance and reliability induced by the localized Joule-heating, proper thermal management strategies are essential, especially for high-power high-frequency applications. This work reports a scalable thermal management strategy to heterogeneously integrate wafer-scale monocrystalline ß-Ga2O3 thin films on high thermal conductivity SiC substrates by the ion-cutting technique and room-temperature surface-activated bonding technique. The thermal boundary conductance (TBC) of the ß-Ga2O3-SiC interfaces and thermal conductivity of the ß-Ga2O3 thin films were measured by time-domain thermoreflectance to evaluate the effects of interlayer thickness and thermal annealing. Materials characterizations were performed to understand the mechanisms of thermal transport in these structures. The results show that the ß-Ga2O3-SiC TBC values are reasonably high and increase with decreasing interlayer thickness. The ß-Ga2O3 thermal conductivity increases more than twice after annealing at 800 °C because of the removal of implantation-induced strain in the films. A Callaway model is built to understand the measured thermal conductivity. Small spot-to-spot variations of both TBC and Ga2O3 thermal conductivity confirm the uniformity and high quality of the bonding and exfoliation. Our work paves the way for thermal management of power electronics and provides a platform for ß-Ga2O3-related semiconductor devices with excellent thermal dissipation.