Engineered Solder-Directed Self-Assembly Across Length Scales
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Engineered Solder-Directed Self-Assembly Across Length Scales Robert Knuesel, Shameek Bose, Wei Zheng, and Heiko O. Jacobs Department of Electrical and Computer Engineering, University of Minnesota, 200 Union Street SE, Minneapolis, MN, 55455
ABSTRACT We report on recent progress in the directed self-assembly of discrete inorganic semiconductor device components. Different from prior research, the goal is to enable the integration of increasingly small dies while supporting unique-angle orientation and contact pad registration. The process is based on the reduction of surface free energy between liquid solder coated areas on the substrate and metal-coated binding sites on the semiconductor dies. Recent advances include flip-chip assembly with unique angular orientation accomplished using “twoelement” docking sites that contain pedestals that act as chaperones for the solder directed assembly to take place. The scale reduction to 20 µm sized components involves the use of a liquid-liquid interface to concentrate component delivery and speed up the self-assembly process to prevent oxidative dissolution of the solder sites prior to completion.
INTRODUCTION The construction of man-made artifacts such as cell phones and computers relies on robotic assembly lines that place, package, and interconnect a variety of devices that have macroscopic (>1 mm) dimensions [1]. The key to the realization of these systems is our ability to integrate/assemble components in 2D/3D as well as link/interconnect the components to transport materials, energy, and information. The majority of these systems that are on the market today are heterogeneous in nature. Heterogeneous systems can be characterized as systems that contain at least two separate parts, thereby prohibiting monolithic integration. Such systems are typically fabricated using robotic pick and place. The size of the existing systems could be reduced by orders of magnitudes if microscopic building blocks could be assembled and interconnected effectively [2]. The difficulty is not the fabrication of smaller parts, but their assembly into an interconnected system. For components with dimensions less than 100 µm, adhesive capillary forces often dominate gravitational forces, making it difficult to release the components from a robotic manipulator [3]. As a direct result, heterogeneous integration using an extension of serial robotic pick and place and wire-bonding has not proven a viable solution. At the other extreme, nature forms materials, structures, and living systems by selfassembly on a molecular length scale [4, 5]. As a result, self-assembly based fabrication strategies are widely recognized as inevitable tools in nanotechnology and an increasing number of studies are being carried out to “scale-up” these concepts to close the assembly gap between nanoscopic and macroscopic systems. Recent demonstrations of processes that can assemble micrometer to millimeter-sized components include: shape-directed fluidic methods that
assemble trapezoidal parts on plan
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