Air-Gaps via Thermally Decomposable Polymers and Their Application to Compliant Wafer Level Packaging (CWLP)

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Kelleher, Hollie Anne
Bidstrup Allen, Sue Ann
Kohl, Paul A.
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A method was proposed for the fabrication of air-gaps embedded in dielectric layers using thermally decomposable sacrificial polymers. The research had two main objectives: (1) the development and characterization of air-gap fabrication for use in a wide spectrum of applications; and (2) the integration of air-gaps into a specific application: air-gaps in an integrated circuit compliant wafer level packaging technology, Sea of Leads. Polynorbornene and polycarbonate sacrificial materials were used to form air-gaps at temperatures of 200, 300, and 400oC. Fabrication results of air-gaps encapsulated by both inorganic and organic dielectric materials indicated that the thermal and mechanical properties of the dielectric materials at the decomposition temperature of the sacrificial material resulted in success or failure of the process. Multi-layered encapsulating materials enabled the use of a dielectric material which does not successfully form air-gaps on its own. Thermal decomposition of the sacrificial materials with alteration in the polymer chemistry was studied. Polynorbornene containing 90 mol% butyl and 10 mol% triethoxysilyl side groups was selected as an optimum 400oC decomposition temperature material. The decomposition of this polynobornene composition in an open nitrogen atmosphere was contrasted to decomposition of the polynorbornene while completely encapsulated in a dielectric material. Thermogravimetric analysis and examination of residual surfaces following the decomposition, combined with comparison of the overall kinetic parameters of the decomposition reaction, indicated differences in the two overall processes. The design concept of Sea of Leads three-dimensionally compliant packaging technology with embedded air-gaps is presented. The critical issues resulting from the addition of air-gaps into the process are the compatibility of materials, lithography on topographical features, and yield and uniformity. Factors influencing the z-axis mechanical performance of the air-gap were determined to be the air-gap shape and size, the encapsulating material dielectric properties and thickness, and the decomposition conditions. Model calculations combined with the known limitations of fabrication provided a design space for maximum out-of-plane mechanical movement and compliance of the air-gaps. The results demonstrated that the incorporation of an embedded air-gap in Sea of Leads technology can achieve the necessary z-axis compliance goals for future applications.
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