Technologies for the 3D integration are described within this paper with respect to devices that have to retain a specific minimum wafer thickness for handling purposes (CMOS) and integrity of mechanical elements (MEMS). This implies Through-Silicon Vias (TSVs) with large dimensions and high aspect ratios (HAR). Moreover, as a main objective, the aspired TSV technology had to be universal and scalable with the designated utilization in a MEMS/CMOS foundry. Two TSV approaches are investigated and discussed, in which the TSVs were fabricated either before or after wafer thinning. One distinctive feature is an incomplete TSV Cu-filling, which avoids long processing and complex process control, while minimizing the thermomechanical stress between Cu and Si and related adverse effects in the device. However, the incomplete filling also includes various challenges regarding process integration. A method based on pattern plating is described, in which TSVs are metalized at the same time as the redistribution layer and which eliminates the need for additional planarization and patterning steps. For MEMS, the realization of a protective hermetically sealed capping is crucial, which is addressed in this paper by glass frit wafer level bonding and is discussed for hermetic sealing of MEMS inertial sensors. The TSV based 3D integration technologies are demonstrated on CMOS like test vehicle and on a MEMS device fabricated in Air Gap Insulated Microstructure (AIM) technology.
In this paper, a foundry process for surface micromachined inertial sensors such as accelerometers or gyroscopes is
introduced, with special attention on reliability aspects. Reliability was a major focus during the development phase,
leading to the choice of the single crystalline silicon layer of an SOI device wafer as the mechanically active material.
Glass frit wafer bonding is used for capping and hermetic sealing, but in addition to these fundamental reliability aspects,
many influences on reliability must be considered, such as the risk of sticking, local stress concentration, electrical
effects or the defined limitations of the mechanical movement in the interaction of design and technology. Reliability test
results, as well as measures for improving the reliability and performance, are discussed in this paper.
In this paper, the capabilities and problems if micro-Raman spectroscopy are discussed for measuring local mechanical stress, both in mono- crystalline and poly-crystalline silicon microstructures. The possibilities of this technique are demonstrated for two different MEMS: the crystalline Si membrane of a pressure sensor and a poly-crystalline Si beam. For both MEMS, an auto-focus Raman system was used. The stress in the membrane of the pressure sensor was investigated before and after bonding of the Si wafer containing the sensors to a glass substrate. This bonding resulted in an under-pressure in the sensor, deflecting the membrane inwardly. Raman spectra were measured from the top surface and the bottom surface of the membrane. This resulted in a map of the stress distribution. It indicates, for the top surface, tensile stress near the edges, compressive stress in the center, and hardly any stress at the corners. The stress in the poly-crystalline Si beam was measured using two different wavelengths of the laser beam. The results show a local tensile stress distribution along the length of the beam.
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