Enabling Capillary Self-Assembly for Microsystem Integration
Efficient and precise assembly of very-large quantities of sub-millimeter-sized devices onto pre-processed substrates is presently a key frontier for microelectronics, in its aspiration to large-scale mass production of devices with new functionalities and applications (e.g. thin dies embedded into flexible substrates, 3D microsystem integration). In this perspective, on the one hand established pick&place; assembly techniques may be unsuitable, due to a trade-off between throughput and placement accuracy and to difficulties in predictably handling very-small devices. On the other hand, self-assembly processes are massively parallel, may run unsupervised and allow contactless manipulation of objects. The convergence between robotic assembly and self-assembly, epitomized by capillarity-enhanced flip-chip assembly, can therefore enable an ideal technology meeting short-to-medium-term electronic packaging and assembly needs. The objective of this thesis is bridging the gap between academic proofs-of- concept of capillary self-assembly and its industrial application. Our work solves several issues relevant to capillary self-assembly of thin dies onto preprocessed substrates. Very-different phenomena and aspects of both scientific and technological interest coexist in such a broad context. They were tackled both experimentally and theoretically. After a critical review of the state-of-the-art in microsystem integration, a complete quasi-static study of lateral capillary meniscus forces is presented. Our experimental setup enables also a novel method to measure the contact angle of liquids. Recessed binding sites are introduced to obtain perfectly-conformal fluid dip-coating of patterned surfaces, which enables the effective and robust coding of geometrical information into binding sites to direct the assembly of parts. A general procedure to establish solder-mediated electro-mechanical interconnections between parts and substrate is validated. Smart surface chemistries are invoked to solve the issue of mutual adhesion between parts during the capillary self-assembly process. Two chemical kinetic-inspired analytic models of fluidic self-assembly are presented and criticized to introduce a novel agent-based model of the process. The latter approach allows realistic simulations by taking into account spatial factors and collision dynamics. Concluding speculations propose envisioned solutions to residual open issues and further perspectives for this field of rapidly-growing importance.