Yuanxiang Zhang

Aluminum–copper alloy (AlCu) is commonly utilized as interconnect material in low-power devices. However, as the size of electronic devices continues to decrease and current density increases, electromigration (EM) has emerged as a significant reliability concern for AlCu interconnects in the microelectronics industry. In this study, two-level AlCu interconnect structures with a Ti/TiN barrier layer were fabricated using 0.18 μm technology to perform accelerated EM tests. The test samples were subjected to three current levels (1.45 mA, 2.40 mA and 5.30 mA) at three ambient temperatures (200 °C, 225 °C and 250 °C) to investigate the nucleation and evolution of voids during EM degradation and then obtain the mean time to failure (MTTF). The numerical simulation method of atomic density integral (ADI) was used to simulate experimental observations based on the ANSYS platform, considering the coupled effects of electron wind force, stress gradient, temperature gradient, and atomic density gradient. A comparison of the experimental results and numerical simulations proves that the ADI method can be applied successfully to EM failure prediction of AlCu interconnects. Finite element models with different reservoir lengths were built to demonstrate the mechanism of the reservoir effect. The results show that the reservoir can improve the EM lifetime of the interconnect, but there is a critical extension length beyond which increasing extension sizes have no effect on EM lifetime.

The present study proposed a novel polymer optical fiber sensor design for vibration measuring. The sensor design consists of two fibers; primary and secondary fibers. Where primary fiber leads to optical power loss through bending loss and secondary fiber couples the optical power through phase coupling. The sensor design involves the primary fiber winding on the supportive cylindrical tube to make the sensor helical structure. The winding of the primary fiber makes the eleven bend loops, which are considered as the sensing points. Based on the bend loss theory, each bend loop leads variable amounts of optical power loss. To detect the vibrating motion, the secondary fiber is aligned perpendicularly with the helical structure. Combined primary and secondary fibers generate the proposed vibration sensor. In the experimental setup, the vibration motion continually varies the position of secondary fiber with helical structured fiber to couple variable optical power. The variation in the optical power coupling intensity indicates the vibration motion. The results showed that the proposed sensor design demonstrated its capability to detect the vibration repetitively and different ranges of vibrations effectively. Furthermore, software limitations impose constraints on sampling rates, restricting the proposed sensor's ability to exploit the sensor's high-frequency detection potential fully. Despite this, we have successfully reported the detection of vibrations up to 10 Hz, highlighting the sensor's substantial promise for applications requiring precise vibration monitoring within this frequency range.

Diverse domain patterns significantly influence the nonlinear electromechanical behaviors of ferroelectric nanomaterials, with polarization switching under strong electric fields being inherently a frequency-dependent phenomenon. Nevertheless, research in this area remains limited. In this study, we present a phase-field investigation of frequency-dependent electromechanical dynamics of a polycrystalline BaTiO₃ nanofilm with a core-shell structure, subjected to applied frequencies ranging from 1 to 80 kHz. Our findings elucidate the microstructural mechanisms underlying the electromechanical behaviors observed in these materials. The effect of the grain size and the strains effect are also taken into account. Hysteresis and butterfly loops exhibit a marked change in shape as the frequency changes. We discuss the underlying domain-switching dynamics as a basis for evaluating such frequency-dependent properties. In addition, we examine the scaling behaviors of the dynamic hysteresis and the influence of grain boundaries on the domain structure. We can also observe from hysteresis loops that the remnant polarization and coercive field significantly diminish when grain sizes decrease from 60 to 5 nm. A smaller grain size of the nanofilm yields a larger percentage of the dielectric grain boundary, which “dilutes” the overall ferroelectricity of the film. A vortex domain structure is more likely to form at low frequency and a small grain size.

The development of a high-performance, low-cost, and simply fabricated flexible three-dimensional (3D) force sensor is essential for the future development of electronic skins suitable for the detection of normal and shear forces for several human motions. In this study, a sandwich-structured flexible 3D force tactile sensor based on a polyethylene-carbon composite material (velostat) is presented. The sensor has a large measuring range, namely, 0–12 N in the direction of the normal force and 0–2.6 N in the direction of the shear force. For normal forces, the sensitivity is 0.775 N⁻¹ at 0–1 N, 0.107 N⁻¹ between 1 and 3 N, and 0.003 N⁻¹ at 3 N and above. For shear forces, the measured sensitivity is 0.122 and 0.12 N⁻¹ in x- and y-directions, respectively. Additionally, the sensor exhibits good repeatability and stability after 2500 cycles of loading and releasing. The response and recovery times of the sensor are as fast as 40 and 80 ms, respectively. Furthermore, we prepared a glove-like sensor array. When grasping the object using the tactile glove, the information about the force applied to the sensing unit can be transmitted through a wireless system in real-time and displayed on a personal computer (PC). The prepared flexible 3D force sensor shows broad application prospects in the field of smart wearable devices.

An electrode material with high performance, long durability, and low cost for supercapacitors has long been desired in academia and industry. Among all the factors that affect the electrochemical performance and cycling stability of electrode materials, the morphology and intrinsic structure characteristics are the most important. In this study, a novel 3D flower-like Ce(COOH)₃ electrode material was designed by taking advantage of the Ce³⁺ and -COOH groups and fabricated by a one-pot microwave-assisted method. The morphology and structure characteristics of the sample were examined by SEM, EDS, TEM, XRD, FT-IR, XPS, N₂ adsorption/desorption techniques, and the electrochemical behaviors were investigated in a three-electrode configuration. The Ce(COOH)₃ electrode presents an excellent specific capacitance of 140 F g⁻¹ at 1 A g⁻¹, higher than many other previously reported Ce-based electrodes. In addition, it delivers high rate capability that retains 60% of its initial capacitance when the current density is magnified 20 times. Dramatically, the Ce(COOH)₃ electrode exhibits an ultra-high cycling stability with capacitance retention of 107.9% after 60,000 cycles, which is the highest durability among reported Ce–organic compound electrodes to the best of our knowledge. The excellent electrochemical performance is ascribed to its intrinsic crystal structure and unique morphology. This work indicates that the 3D flower-like Ce(COOH)₃ has significant potential for supercapacitor applications and the facile and scalable synthesis strategy can be extended to produce other metal–organic composite electrodes.

The development of advanced electronic devices leads to highly miniaturized interconnect circuits (ICs), which significantly increases the electromigration (EM) phenomenon of solder and circuits due to higher current density. The electromigration of solder joints under high current density has become a severe reliability concern in terms of microelectronic product reliability. The microstructure of the solder plays an important role in the electromigration induced degradation. In this study, Sn-3.0Ag-0.5Cu solder bumps with Ni/Au under bump metallization (UBM) layer were fabricated and electromigration acceleration tests were conducted under current density of 1.4 × 10⁴ A/cm² and 120 °C to investigate the effect of grain structure and Ni/Au-UBM layer on EM-induced failure. Grain structures of solder bumps were determined by utilizing the Electron Backscatter Diffraction (EBSD) technique, and single-crystal solder, single-crystal dominated solder, and polycrystalline solder are observed in different test samples. According to the Scanning Electron Microscope (SEM) images, it is observed that the Ni/Au-UBM layer of the Cu pad can inhibit atom diffusion between solder bump and Cu pad, which reduces the consumption of Cu pad but causes a large void and crack at the interface. The EM lifetime of single crystal solder bumps is lower than that of polycrystalline solder bumps when the c-axis of single crystal solder bumps is perpendicular to the electron flow direction. Additionally, the single crystal structure will increase the brittleness of the solder bump, and cracks are easily generated and expanded under the stress caused by the mismatch of thermal expansion coefficients between the solder bump and Ni/Au-UBM layer near Cu pad. Polycrystalline solder bumps with a higher misorientation angle (15–55°) have a higher atom diffusion rate, which will result in the acceleration of the EM-induced failure.

The grain structure has a great influence on the mechanical reliability of miniaturized interconnect circuits. Under the working conditions, Cu₃Sn will be the main intermetallic compound that forms during electromigration and the thermal ageing process, which can easily induce cracks between Cu₃Sn and the copper pad. This paper investigated the effect of the Cu grain structure on the Cu₃Sn/Cu interface reliability via molecular dynamic simulations. Single-crystal Cu, twin-crystal Cu, and polycrystalline Cu models were implemented to form the interface structure, and uniaxial tension simulations were performed at strain rates of 0.01, 0.1, and 0.5% ps⁻¹ to evaluate the interfacial strength and interface failure mechanism. Results show that the polycrystalline Cu models always exhibits a lower degree of stress and shows a great ductile character. The existence of twin grain boundaries makes Cu layer more stable, and their failure processes are dominated by stair-rod type dislocations with a Burgers vector of 1/6 [110] instead of Shockley dislocations with a Burgers vector of 1/6 [112], which induce their interface models fail in Cu₃Sn layer near the interface. The strain rate dependence mechanical character of both interface and grains would be the main reason of phenomenon for the difference failure character affected by grain structure and strain rate.

In this and subsequent papers, we consider the effects of the microstructure, in particular grain orientations, and the associated anisotropy of Sn grains on electromigration induced degradation in lead-free solder bumps. This paper investigates the effect of the anisotropy of Sn grains on currentdriven self diffusion and the resulting atomic flux divergence that is associated with material depletion at the cathode side of the bump and ultimately failure. Finite element thermoelectric simulations of a section of a chip scale package (CSP) and a refined sub-model are carried out assuming single crystal and bi-crystal solder bumps. Anisotropic material properties of βSn are used in the simulations. By varying crystal orientation of the solder bumps, the effect of grain orientation on the current density, temperature, and atomic flux divergence is studied and discussed. In future papers, we will consider the coupled mechanical response including stress-driven diffusion.