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PublicationEffect of the loop forming process on the lifetime of aluminum heavy wire bonds under accelerated mechanical testing( 2024-02-21)Heavy wire bonding is one of the most common interconnection technologies in manufacturing of high-power electronics. For industrial applications, the long-term reliability of these connections is crucial. Besides the selection of the wire material and the loop geometry itself, the loop forming process parameters also have an influence on the reliability of the wire bond. In this work, the influence of the backward bond head movement during wire bonding process on the quality of wire bond connections was systematically investigated and qualified by cyclic mechanical lifetime tests, surface roughness measurements of the heel area by laser confocal microscopy and static pull tests. The wire bond loops were fabricated with 300 μm aluminum H11 and H14CR wires with different hardness values. The lifetime at low frequency cycle and high frequency cycle regime was determined by means of two different mechanical cyclic test methods operating at 5 Hz and at 60 kHz respectively. The results have shown, that the surface topology of the heel region caused by the initial plastic deformation during the loop forming process has a significant effect on the wire bond failure due to heel cracking. The number of loading cycles to failure shows an inverse correlation with the degree of surface roughness in a so called wrinkling analysis in the low and high frequency cycle regime. The soft wire exhibits different lifetimes compared to the hard ones depending on the testing conditions, while a significant decrease of the lifetime is observed with >30 % reverse movement during bonding in all cases.
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PublicationForced Motion Activated Self-Alignment of Micro-CPV Solar Cells( 2024)
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PublicationLaboratory X-ray Microscopy of 3D Nanostructures in the Hard X-ray Regime Enabled by a Combination of Multilayer X-ray Optics( 2024)High-resolution imaging of buried metal interconnect structures in advanced microelectronic products with full-field X-ray microscopy is demonstrated in the hard X-ray regime, i.e., at photon energies > 10 keV. The combination of two multilayer optics—a side-by-side Montel (or nested Kirkpatrick–Baez) condenser optic and a high aspect-ratio multilayer Laue lens—results in an asymmetric optical path in the transmission X-ray microscope. This optics arrangement allows the imaging of 3D nanostructures in opaque objects at a photon energy of 24.2 keV (In-Kα X-ray line). Using a Siemens star test pattern with a minimal feature size of 150 nm, it was proven that features < 150 nm can be resolved. In-Kα radiation is generated from a Ga-In alloy target using a laboratory X-ray source that employs the liquid-metal-jet technology. Since the penetration depth of X-rays into the samples is significantly larger compared to 8 keV photons used in state-of-the-art laboratory X-ray microscopes (Cu-Kα radiation), 3D-nanopattered materials and structures can be imaged nondestructively in mm to cm thick samples. This means that destructive de-processing, thinning or cross-sectioning of the samples are not needed for the visualization of interconnect structures in microelectronic products manufactured using advanced packaging technologies. The application of laboratory transmission X-ray microscopy in the hard X-ray regime is demonstrated for Cu/Cu6Sn5/Cu microbump interconnects fabricated using solid–liquid interdiffusion (SLID) bonding.
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PublicationFine-Pitch Copper Nanowire Interconnects for 2.5/3D System Integration( 2024)Heterogeneous integration is a key driver within the field of advanced electronic packaging. The realization of tomorrow’s highly integrated electronic systems depends on the combination and compatibility of various integration technologies at the same hierarchy level. The adoption of novel bonding technologies for a cost-effective realization of multi-chiplet systems is a key aspect. Cu nanowire (NW) interconnects exhibit distinct advantages in terms of their scalability down to a few micrometers, the resulting joint properties and moderate demands with respect to the surface preparation, and the cleanliness of the bonding environment. No solder or flux is required for the bonding process, but the NW bumps still can compensate low height differences. The bonding process can be carried out near room temperature under ambient conditions. We demonstrate the technological possibility to integrate the Cu-NWs for a bump processing scheme including the Cu seed etching on 300 mm wafer for the first time. This paper focuses on the microstructure evaluation and the shear test of the formed Cu-NW interconnects fabricated under ambient conditions within a few seconds. The microstructure analysis shows the intact bonded interconnects and reveals high-resolution details of Cu-NWs. The shear strength of the formed interconnects varies between 4.6 MPa and 90.5 MPa depending on the bonding and annealing conditions. Overall, the results of this study highlight the potential of Cu-NW interconnects for future 3D heterogeneous system integration.
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PublicationSensor Systems for Extremely Harsh Environments( 2023-10-31)Sensors are key elements for capturing environmental properties and are today indispensable in the industry for monitoring and control of industrial processes. Many applications are demanding for highly integrated intelligent sensors to meet the requirements on safety, clean and energy efficient operation or to gain process information in the context of industry 4.0. While in many everyday objects highly integrated sensor systems are already state of the art, the situation in an industrial environment is clearly different. Frequently the use of sensor systems is impossible, due to the fact that the extreme ambient conditions of industrial processes like high operating temperatures or strong mechanical loads do not allow a reliable operation of sensitive electronic components. Eight Fraunhofer Institutes have bundled their competencies and have run the Fraunhofer Lighthouse Project ‘eHarsh’ to overcome this situation. The project goal was to realize sensor systems for extremely harsh environments, whereby sensor systems are not only pure sensor elements, rather containing one or multiple sensor elements and integrated readout electronics. Various technologies which are necessary for the realization of such sensor systems have been identified, developed and finally bundled in a technology platform. These technologies are e. g. MEMS and ceramic based sensors, SOI-CMOS based integrated electronics, board assembly and laser based joining technologies. All these developments have been accompanied by comprehensive tests, material characterization and reliability simulations. Based on the platform a pressure sensor for turbine applications has been realized to prove the performance of the eHarsh technology platform.
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PublicationUse of Rotary Ultrasonic Plastic Welding as a Continuous Interconnection Technology for Large-Area e-Textiles( 2023-01-28)
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PublicationInvestigation of failure mechanism of aluminum-scandium wire bond contact under active power cycle test( 2023)The failure mechanism of aluminum-scandium (Al-Sc) wire bond in comparison with pure Al wire bond under active power cycle (APC) test was investigated. To monitor the degradation of the bond quality, bond shear forces (shear force) were measured over the testing cycles at different level of temperature swing (ΔT). The results show that the Al-Sc wire bond was much more robust than the pure Al wire bond when the ΔT (Tmax) was mild. On the other hand, as the ΔT was increased, the shear force reduction in the Al-Sc wire bond became more and more remarkable. In the cross-sectional analysis of bond contact after the APC tests, a significant crack propagation was observed even for low ΔT conditions for the pure Al wire, and the crack progressed primarily on the wire side of the bond area. As for the Al-Sc wire, while the bond crack was efficiently suppressed for low ΔT conditions, a significant progress of crack was observed at high ΔT conditions, and the crack grew mainly through the Al pad side of the bond area. A detailed analysis at the bond interface after the APC test at high ΔT condition revealed three phenomena at the Al pad side of the bond area: grain coarsening, precipitation of Cu particles, and large void formation. As the material strength is compromised by these effects, the crack grew more easily through the Al pad side of the contact area, resulting in a rapid reduction of shear force at high ΔT. The bond failure mechanism obtained in this study will provide a valuable insight for the future development of high reliability power semiconductor device.
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PublicationCu-Cu Thermocompression Bonding with a Self-Assembled Monolayer as Oxidation Protection for 3D/2.5D System Integration( 2023)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.
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PublicationLovewear: Haptic Clothing that Allows Intimate Exploration for Movement-Impaired People( 2023)
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Publication6 Core fiber and VCSEL based interferometer sensor for motion or vibration monitoring( 2023)We demonstrate a XYZ motion / vibration sensor based on a 6-core fiber interferometer and a VCSEL lasers array. The sensor can detect changes in movement as low as 500 um and frequencies ranging from 1 Hz to 100 kHz with potential to extend this range further.
