Mechanical stress is developed in the materials during and after manufacturing of devices. In microelectronics components, like integrated circuits, the stress in the interconnections is a very crucial phenomenon. Consequences are defects, which are more serious with the constantly increasing complexity and miniaturization of the devices. To develop strategies to minimize the stress phenomena, it is essential to analyze the stress morphology. Micro-Raman spectroscopy is an effective technique to measure local patterns with spatial resolution of less than micrometers. Focus of this study is the monitoring of stress induced on blue gallium nitride LEDs after soldering onto copper substrate. Stress values above 1 GPa are observed, which indicate compressive stress, especially in the central area of the LED. The results are discussed in terms of thermal expansion coefficients and soldering process, with the aim to enhance the reliability of the interconnections, optimize the bonding processes and provide crucial data for the improvement of packaging design.
Due to assembly processes in microelectronics packaging, semiconductor materials are under undesired stresses, which become more important at extreme thermo-mechanical operating conditions. Investigations of the performances of the semiconductors is very challenging, because each assembly is a complex structure with many materials with very different physical-chemical properties. Remarkable is the determination of local stresses trough optical techniques, like Raman spectroscopy. Focus of this study is the monitoring of stress induced on silicon chips after bonding. Three bonding processes are investigated: soldering using a eutectic AuSn preform, sintering using a commercial Ag sintering paste, sintering using an in-house developed sintering paste made by surface enhanced etched brass particles dispersed in polyethylene glycol 600. In all cases, the Si-chip is bonded to a Cu substrate. The stress distribution after AuSn-soldering presents a homogeneous pattern, while after Ag- and Cu-sintering an axial distribution along the diagonal is observed. The samples are investigated at three temperatures: -50, 20, and 180 °C. The stress phenomena are higher for the assemblies made via AuSn-soldering, with stress value above 600 MPa. The results are discussed in term of compressive and tensile strain and thermal expansion coefficients.
Integrated circuits constitute a complex mosaic, where materials with different characteristics, grown or deposited in different ways and at different temperatures, are linked together in various geometries. It is well known that during and after processing of these devices, mechanical stresses develop in the layers. These stresses may be due to thermal steps, intrinsic stresses, which are inherent in the formation process of the film, or due to the geometry of the material. For example, high stresses are present in the substrate at film edges. The presence of local residual stress has an important effect on the electrical properties of electronic devices, in particular on the reliability and the lifetime of the semiconductor components.
The present work focuses on the optical investigation of the thermomechanical stress of semiconductor materials used to realize new LED modules for front lighting application. Blue LEDs, based on gallium nitride (GaN) on sapphire, are bonded to a silicon carrier using gold silicon. Afterwards the sapphire is removed. The GaN on silicon devices are soldered by eutectic AuSn soldered on copper substrates, with different thicknesses. In the solder process different AuSn solder layer are achieved by varying the bond force. Raman spectroscopy is used to investigate the influence of the assembly process and assembly material on the local stress in the semiconductor. By that the physical, mechanical and chemical properties of the interconnect material can be analyzed. A model is developed to simulate the thermomechanical stress in the GaN LED assemblies. The Raman results validate the computational model. The phenomena are evaluated at room temperature, at -50°C and at 180°C.
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