Three-Dimensional Heterogeneous Integration for RF/Microwave Applications
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The first proposed approach involves an interlocking mechanical structure that supports flip-chip assembled Monolithic Microwave Integrated Circuits (MMICs). Photolithographically patterned thick-film SU-8 structures are applied to both the chip and the carrier such that the chip self-aligns into place and mates with the carrier. Gold bumps embedded within the structures electrically connect the chip pads to the carrier pads. This method is demonstrated through the assembly of a SiGe power amplifier MMIC onto a high resistivity silicon carrier.
The second proposed approach involves vertical interconnects consisting of room temperature liquid-state metals. The fluid nature of the liquid bumps allows them to be robust in the presence of thermo-mechanical stresses, such as Coefficient of Thermal Expansion (CTE) mismatch between the interconnected chips. SU-8 structures are used to form a shaping mold on the bottom carrier that contains the liquid metal. Gold posts are electroplated on the top chip, then mated with the SU-8 mold, thereby making contact with the liquid metal to form the electrical continuity.
For each of these proposed methods, design and fabrication considerations are discussed in detail. RF measurements on prototype structures up to Ka band are performed to verify the functionality of the proposed methods. Given the results of these proof-of-concept efforts, electrical characteristics of the materials used in these methods are determined, and recommendations are provided for future improvements and refinements to these two techniques.
- Masters Theses