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In this paper, the low temperature direct wafer-bonding technique under 350 °C is used to fabricate the capacitive micromachined ultrasonic transducers (CMUTs) and the first illustration of CMUTs with a 15 × 15 array is obtained. In comparison with other wafer-bonding technology under high temperature beyond 400 °C, the thermal stress and thermal deformation of CMUTs produced in the wafer-bonding process can be decreased largely because of low bonding temperature. What's more, the low temperature direct wafer-bonding technique is beneficial to combine the CMUTs chip with integrated circuits (ICs) as well as reduce the parasitic capacitance. Based on the low temperature direct wafer-bonding process, a silicon-on-insulator (SOI) wafer with the silicon device layer used for transducer membrane and a low-resistivity silicon substrate with the structured SiO 2 layer are bonded to fabricate the main structure of CMUTs. Therefore, the parasitic capacitance of CMUTs can be further reduced since there is no metal auxiliary layer existing during the bonding process. The structure morphology and electrical characterization of fabricated CMUTs are tested in this paper. Then, the impedance-frequency curve and the corresponding phase-frequency curve of the CMUTs are obtained perfectly, which demonstrate that the fabricated CMUTs based on the low-temperature direct wafer-bonding technique present the fine mechanical and electrical characteristics. [ABSTRACT FROM AUTHOR]

Wafer level bonding and stacking process for 3D stacked IC was proposed and technical issues were discussed. Cu bumps surrounded by recessed SiO 2 were thermo-compression bonded and electrically evaluated using Kelvin structures. Defects at the bonding interface such as voids were responsible for electrical resistance distribution. Wafer alignment during stacking process was found to be affected by several factors like non-uniform bump height, the spacers in a bonding fixture and wafer warpage. Step by step warpage measurement revealed that the wafer warpage was dependent on process steps and worsened at bonding step. [ABSTRACT FROM AUTHOR]